NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Comparison of API & IEC 

Standards for Offshore Wind 

Turbine Applications in the 

U.S. Atlantic Ocean: Phase II

March 9, 2009 – September 9, 2009 

A. Jha, D. Dolan, T. Gur, S. Soyoz, and 

C. Alpdogan

MMI Engineering

Houston, Texas

NREL Technical Monitor: Walt Musial

Subcontract Report

NREL/SR-5000-49688

January 2013
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Comparison of API & IEC 

Standards for Offshore Wind 

Turbine Applications in the 

U.S. Atlantic Ocean: Phase II 

March 9, 2009 – September 9, 2009 

A. Jha, D. Dolan, T. Gur, S. Soyoz, and 

C. Alpdogan

MMI Engineering

Houston, Texas

NREL Technical Monitor: Walt Musial

Prepared under Subcontract No. LEE-7-66518-01

Subcontract Report

NREL/SR-5000-49688

January 2013
This publication received minimal editorial review at NREL.

NOTICE

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iv

List of Acronyms and Abbreviations

API American Petroleum Institute

BSEE Bureau of Safety and Environmental Enforcement (formerly Minerals Management 

Service)

CAP Capacity Analysis Program

FAST Fatigue, Aerodynamics, Structures, and Turbulence (software) 

IEC International Electrotechnical Commission

JIP Joint Industry Project 

NREL National Renewable Energy Laboratory 

OCS U.S. Outer Continental Shelf 

OTM overturning moment

OWT offshore wind turbine 
v 

List of Symbols

A Area (projected or cross section)

c Shear strength of clay

C Wave celerity

CD Viscous drag coefficients

Cds Steady state flow drag coefficient at post-critical Reynolds numbers

CM Inertia coefficient

CS Shape coefficient

CX Water height above mean still water at numerical limit for Stream Function theory 

CY Water height above mean still water at physical limit for wave breaking condition 

D Diameter

Dmp Monopile diameter

E Young’s modulus 

G Shear modulus or limit state function

Hb Wave height at the breaking location 

Hmax Maximum wave height 

Hs Significant wave height

HX Wave height at numerical limit for Stream Function wave theory 

HY Wave height at physical limit for wave breaking condition 

I Moment Inertia

K Keulegan-Carpenter Number

L Load 

LBW Aerodynamic blade load and the hydrodynamic wave load 

LCorr Additional load where a correction to Stream function is warranted

LSlam Wave slam load
vi 

Ltower Wind load on the tower 

M Bending moment 

Mb Base over-turning moment 

Mt Tower top over-turning moment

N Axial load

Rmp Radius of the monopile 

R Resistance (or capacity)

t Thickness 

Tmax Zero crossing wave period corresponding to maximum wave height in a storm

Tz Average zero crossing wave period

u Velocity (water particle or wind)

Um Maximum water particle velocity normal to the cylinder axis 

V Displaced volume of the cylinder per unit length 

V Shear force

Vb Base shear force

Vt Tower top shear force

Ws Average wind speed

XBW Model uncertainty and variability for aerodynamic load on blade and hydrodynamic load 

from drag and inertia from wave particle kinematics acting on the foundation. 

XCorr Model uncertainty for the load correction applied to Stream Wave theory 

XSlam Model uncertainty for slam loads from breaking waves

XTower Model uncertainty on tower wind load 

ηb Maximum elevation of the free water surface above the still water level

λ Curling factor 

ρ Density (water or air) 
vii 

Executive Summary

The National Renewable Energy Laboratory (NREL) contracted MMI Engineering to conduct 

Phase II of the Joint Industry Project on Comparison of two design standards for offshore wind 

turbines (OWTs) in the United States Outer Continental Shelf waters. The two standards were

the American Petroleum Institute (API) RP – 2A Recommended Practice for Planning, 

Designing, and Constructing Fixed Offshore Platforms and the International Electrotechnical 

Commission (IEC) 61400-3 Design Requirements for Offshore Wind Turbines. 

In Phase I, two structure types (a monopile and a tripod) were studied at two sites – offshore 

Texas at 24-m water depth and offshore Massachusetts at 15-m water depth. This study provided 

insights into the loads imposed on the structures caused by wind and ocean forces, and also 

provided insights into the strength (capacity) of the structures subject to these loads. The 

resulting reliability (annual failure probability for ultimate strength) for the two structures at the 

two sites were generally governed by turbine and blade resonance avoidance for designing the 

structure, and not by ultimate strength needs to resist extreme loads. The conclusions drawn from 

Phase I were based on these limited numbers of case studies. This limitation prompted the need 

to conduct this second phase to study two new designs, the monopile and a jacket (space frame)

substructure, at the same site where ultimate strength would likely also influence design and 

resonance avoidance. 

The site selected for Phase II was the offshore Massachusetts site, but with a water depth of 25 m 

(instead of 15 m as in Phase I). This selection permitted the use of the same meteorological and 

oceanographic (metocean) parameters, with the breaking wave criteria altered as a result of the 

increased water depth. 

Preliminary designs for the monopile and the jacket structure were developed based on insights 

into 50-year loads (for the IEC based design; loads exceeding on average once every 50 years) 

and 100-year loads (for the API-based design) from Phase I. These two structures were then 

analyzed carefully to include coupled aerodynamic blade loads and hydrodynamic loads on the 

support structure, and breaking wave loads in extreme storms. The design was then finalized 

using these loads to ensure the utilization of the members remained below allowable stress 

thresholds, according to API and IEC design codes. Once the design was finalized, which 

resulted in few changes to the preliminary design, the structure was analyzed to calculate its

capacity to resist strength failure for operating loads, as well as for loads during 50- and 100-year 

storm conditions. 

The final step of the study was to conduct a reliability analysis to find the annual probability of 

ultimate strength failure. This step was done by calculating the annual probability of the imposed 

loads on OWTs that exceed the ultimate strength capacity, including the probability distribution 

of the key parameters to model their randomness.

The key conclusion upon comparison of API and IEC results reinforced the observations in 

Phase I, in which the resulting reliabilities from the API and IEC methods were similar for both 

the monopile and the tripod substructure for ultimate strength failure. Note the reliability against 

fatigue failure was not the focus of this study or of Phase I. The monopile design was largely 

driven by resonance avoidance conditions; however, the tripod design was also impacted by 

consideration of ultimate strength failure.
viii

Further study is recommended on overall system reliability to include fatigue consideration, 

especially from breaking waves. Another critical recommended step is the development of an 

acceptable target reliability level for OWTs in U.S. waters, and inclusion of this target reliability

via specific partial safety factors for the design guidelines that would include considerations of 

turbine size, regional hurricane risk, and water depth. 
ix

Table of Contents

List of Acronyms and Abbreviations ........................................................................................................ v

List of Symbols ..........................................................................................................................................vi

Executive Summary................................................................................................................................. viii

List of Tables .............................................................................................................................................xii

List of Figures ..........................................................................................................................................xiii

1 Introduction........................................................................................................................................... 1

1.1 Overview.......................................................................................................................................... 1

1.2 Report Structure............................................................................................................................... 1

1.3 Background...................................................................................................................................... 1

1.4 Objective.......................................................................................................................................... 2

2 Methodology ......................................................................................................................................... 4

2.1 Structure Type.................................................................................................................................. 4

3 Turbine and Site Characterization ...................................................................................................... 6

3.1 Turbine Specifications..................................................................................................................... 6

3.1.1 Turbine Size........................................................................................................................ 6

3.1.2 Tower Properties................................................................................................................. 7

3.1.3 Turbine Operation Requirements........................................................................................ 8

3.2 Site Location and Water Depth........................................................................................................ 9

3.3 Support Structure Configuration.................................................................................................... 10

3.3.1 Monopile........................................................................................................................... 10

3.3.2 Jacket ................................................................................................................................ 11

3.4 Meteorological and Oceanographic Data....................................................................................... 13

3.4.1 Breaking Waves................................................................................................................ 16

3.4.2 Metocean Data for the Site ............................................................................................... 16

3.5 Other Site Assumptions ................................................................................................................. 17

4 Analysis Methodology .......................................................................................................................18

4.1 Coupled Wave and Wind Load Analyses...................................................................................... 20

4.1.1 Wind Load on Blades ....................................................................................................... 20

4.1.2 Wave Load on Monopile .................................................................................................. 21

4.1.3 Wind Load on Tower........................................................................................................ 22

4.2 Equivalent Monopile Properties for FAST Analyses .................................................................... 22

4.3 Coupled Wave and Wind Load Analyses with FAST ................................................................... 23

4.4 Breaking Wave Forces................................................................................................................... 24

4.5 Additional Drag and Inertia ........................................................................................................... 28

5 Monopile Substructure Analysis.......................................................................................................30

5.1 Wind and Wave Load Analyses with FAST.................................................................................. 30

5.2 Total Wind and Wave Load Demand ............................................................................................ 34

5.3 Strength Checks............................................................................................................................. 36

5.4 Capacity Analysis.......................................................................................................................... 38

5.5 Monopile Reliability Analysis....................................................................................................... 40

6 Jacket Substructure Analysis ...........................................................................................................48

6.1 Design Requirements for Resonance Avoidance........................................................................... 48

6.2 Wind and Wave Load Analyses with FAST.................................................................................. 48

6.3 Total Wind and Wave Load Demand ............................................................................................ 50

6.4 Strength Checks............................................................................................................................. 52

6.5 Capacity Analysis.......................................................................................................................... 55

6.6 Jacket Reliability Analysis............................................................................................................. 58

7 Comparison of Monopile and Jacket................................................................................................64

7.1 Monopile Results, 25-m versus 15-m water depth ........................................................................ 64
x 

7.2 Jacket versus Monopile at 25-m water depth site .......................................................................... 66

7.3 Overall Comparison....................................................................................................................... 68

8 Summary and Conclusions ...............................................................................................................69

8.1 Key Findings.................................................................................................................................. 69

8.1.1 Effect of Breaking Waves................................................................................................. 69

8.1.2 Effect of Change in Water Depth on Monopiles............................................................... 69

8.1.3 Effect of Structure Type on Loads, Design, and Reliability............................................. 69

8.2 Recommendations.......................................................................................................................... 70

References.................................................................................................................................................71

Appendix A: Soil Springs in the Analyses .............................................................................................72

Appendix B: Drag and Inertia Coefficients for Large Diameter Turbine Components ......................75

Appendix C: Study on Sensitivity of Load Results to the Number of Simulations............................77
xi 

List of Tables

Table 1. Properties of the NREL 5-MW Baseline Wind Turbine............................................................. 7

Table 2. Undistributed Blade Structural Properties ............................................................................... 7

Table 3. Tower Dimensions....................................................................................................................... 8

Table 4. Metocean criteria for site (offshore Massachusetts) .............................................................17

Table 5. FAST input data for monopile ..................................................................................................34

Table 6. Monopile design loads at mudline section .............................................................................36

Table 7. Monopile Design Loads at Critical Section (Highest Bending Stress) Below Mudline .......37

Table 8. Member Utilization Ratios for Monopile..................................................................................38

Table 9. Lateral Load and Overturning Capacity for the Monopile.....................................................39

Table 10. FAST Input Data for Jacket Structure ...................................................................................50

Table 11. Jacket Design Loads at Mudline Section..............................................................................52

Table 12. Jacket Design Loads per IEC/ISO..........................................................................................53

Table 13. Jacket Design Loads per API ................................................................................................. 54

Table 14. Utilization Ratio Summary for Jacket....................................................................................55

Table 15. Capacity Analysis Results for Jacket....................................................................................58

Table 16. Design Load Comparison for Monopile in 25-m Depth to 15-m Depth (from Phase I).....66

Table 17. Utilization Ratio Comparison for Monopile in 25-m Depth to 15-m Depth (from Phase I)66
xii 

List of Figures

Figure 1. Types of support structures: three-legged space frame structure on left, and a monopile 

structure on the right. (Illustration from MMI Engineering) ............................................................. 5

Figure 2. Campbell diagram...................................................................................................................... 8

Figure 3. Location of site........................................................................................................................... 9

Figure 4. Soil-strength profile.................................................................................................................10

Figure 5. Monopile with 4-second period. (Illustration from MMI Engineering) ................................11

Figure 6. Jacket with 4-second period. (Illustration from MMI Engineering).....................................12

Figure 7. Correlation of wind speed Ws and significant wave height Hs for tropical storms..........14

Figure 8. Relation of maximum wave height to significant wave height Hs in a tropical storm......14

Figure 9. Relation of average zero-crossing period Tz to significant wave height Hs in a storm ...15

Figure 10. Surge height as a function of significant wave height for MA site...................................16

Figure 11. Methodology flowchart..........................................................................................................19

Figure 12. Grid points for the wind velocity data and force components acting on a blade. 

(Illustrations from NREL) ..................................................................................................................21

Figure 13. Tower and monopile model properties. (Illustration from MMI Engineering)..................23

Figure 14. Breaking waves at the site. (Tapp is the apparent wave period.).......................................25

Figure 15. Impact load from a breaking wave. (Illustration from MMI Engineering) .........................26

Figure 16. Impulse force on structure from a breaking wave of height 17.4 m................................. 27

Figure 17. Response of the monopile to the impulse force from a breaking wave of height 17.4 m

..............................................................................................................................................................27

Figure 18. Comparison of impact forces from breaking waves on monopile and jacket structures 

(member diameters shown in parenthesis) .....................................................................................28

Figure 19. Additional drag and inertia forces for modeling missing effects in wave theory near a 

breaking wave. (Illustration from MMI Engineering).......................................................................29

Figure 20. Comparison of CAP and FAST results in the frequency domain for the monopile ........31

Figure 21. Effect of damping on monopile structure (“target” is from CAP; numbers in legend 

denote FAST analyses with different overall damping values) .....................................................33

Figure 22. Coupled wind and wave loads for monopile for blade loads and hydrodynamic loads. 

(Illustration from MMI Engineering)..................................................................................................35

Figure 23. Monopile model in CAP.........................................................................................................37

Figure 24. Results of a typical capacity analysis. (Left: Load-displacement curve; Right: deflected 

shape with nonlinear events.) ...........................................................................................................39

Figure 25. Mudline overturning moment (OTM) versus wind speed for the smallest storm 

analyzed. (There is no breaking wave phenomenon for this small storm.) ................................. 41

Figure 26. Mudline OTM vs. wind speed for the largest storm analyzed. (Includes breaking wave 

effect.)..................................................................................................................................................42

Figure 27. Loads on blade and wave loads only as a function of Ws, shown for all Hs values 

analyzed...............................................................................................................................................43

Figure 28. Loads on blade and wave load as a function of Hs............................................................44

Figure 29. Total loads (including all applicable components) vs. wind speed. (1-hour average, at 

10-m reference height.)......................................................................................................................45

Figure 30. Mudline overturning moment (including all load components) of monopile as a function 

of Hs.....................................................................................................................................................46

Figure 31. Total load and capacity for monopile as a function of wind speed. (Failure mechanism 

is different for operating vs. parked modes.) ..................................................................................47

Figure 32. Total load and capacity for monopile as a function of significant wave height..............47

Figure 33. Effect of damping on jacket structure .................................................................................49

Figure 34. Coupled wind and wave loads for a jacket-type structure for blade (top) and 

hydrodynamic (bottom) loads...........................................................................................................51

Figure 35. Jacket model in CAP. (Illustration from MMI Engineering) ...............................................53

Figure 36. Results of a typical capacity analysis under operating storms (Bottom: loaddisplacement curve; Top: deflected shape with nonlinear events at important steps.).............56

Figure 37. Results of a typical capacity analysis for a 50-year wave (Bottom: load-displacement 

curve; Top: deflected shape with nonlinear events at important steps.) .....................................57
xiii

Figure 38. Mudline overturning moment (OTM) versus wind speed for the smallest storm 

analyzed...............................................................................................................................................59

Figure 39. Mudline OTM vs. wind speed for the largest storm analyzed. (Includes breaking wave 

effect.)..................................................................................................................................................59

Figure 40. Mudline overturning moment due to aerodynamic wind load and hydrodynamic wave 

loads. (No breaking wave effect included yet.) ...............................................................................60

Figure 41. Loads from aerodynamic and hydrodynamic effects (excluding breaking wave effects) 

vs. Hs....................................................................................................................................................60

Figure 42. Total load (including all effects) vs wind speed for different Hs values ..........................61

Figure 43. Total load (including all effects) vs Hs for different wind speed values ..........................61

Figure 44. Total load and capacity for jacket as a function of wind speed........................................62

Figure 45. Total load and capacity of jacket as a function of Hs ........................................................63

Figure 46. Monopile loads for 15-m and 25-m depth as a function of wind speed and significant 

wave height .........................................................................................................................................65

Figure 47. Jacket vs. monopile loads for 25-m water depth for key return periods as a function of 

wind speed ..........................................................................................................................................67

Figure 48. Jacket vs. monopile loads for 25-m water depth for key return periods as a function of 

significant wave height......................................................................................................................68
1 

1 Introduction

1.1 Overview

This report documents the results of the project that was undertaken to address design standards 

needed for offshore wind turbines (OWTs) in the United States. This is Phase II of the joint 

industry project (JIP) that was funded by several organizations and executed by MMI 

Engineering. This second phase was sponsored by the National Renewable Energy Laboratory 

(NREL). It will help the reader to first review the JIP results as background to this report.

1.2 Report Structure

The report is structured as follows: 

• Section 2 provides a summary of the general approach to the work. 

• Section 3 details the turbine specification, provides a description of the substructure types 

studied, and summarizes the meteorological, oceanographic, and site characteristics of 

the location for the OWTs.

• Section 4 provides the methodology for calculating the loads imposed on the structure 

from wind and hydrodynamic loads. 

• Section 5 presents load, capacity, and reliability results for the monopile. 

• Section 6 presents load, capacity, and reliability results for the jacket. 

• Section 7 compares the monopile results to the jacket results at the site. 

• Section 8 offers the key findings and recommendations for further work. 

1.3 Background

The Bureau of Safety and Environmental Enforcement (BSEE), formerly known as the Minerals 

Management Service or MMS, has regulatory authority over offshore wind power developments 

in the U.S. Outer Continental Shelf (OCS). There are codes and standards that have been 

developed for overseas offshore wind power development; however, no standards have yet been 

adopted for use in the U.S. OCS by the BSEE or other local U.S. regulatory agencies. The 

standards that are being considered include the: 

• American Petroleum Institute (API) recommended practice, which is currently used for 

the design and regulatory review of structures used for oil and gas development in the 

U.S. OCS, titled Recommended Practice for Planning, Designing, and Constructing 

Fixed Offshore Platforms, Working Stress Design, RP-2A. 

• International Electrotechnical Commission (IEC) 61400 requirements, which have been 

developed specifically for the design of OWT generators, titled Part 3: Design 

Requirements for Offshore Wind Turbines.

NREL and BSEE have a common interest in the definition of a standard that can be applied for 

design of OWT support structures in U.S. OCS waters. This new standard could include an 

adapted form of API RP2A [4], the IEC 61400-3 [3], or some other standard that has been 
2 

developed specifically for OWT design. In the selection of any standard, both the BSEE and the 

U.S. wind power industry need to have confirmation that the standards applied will fulfill the 

requirements for both safety and reliably over the intended service life of any facility.

Under a separate JIP (referred to as Phase I in this report), that was partially sponsored by NREL 

and conducted over the past two years, MMI investigated the relative level of safety provided by 

each of these standards using two sites and two different support structures for a 5-MW reference 

wind turbine developed by NREL. Under Phase I it was revealed that the IEC and API designs 

have comparable levels of safety, but the conditions upon which this conclusion was reached 

were limited to the two cases investigated under the JIP. Those investigations were insufficient 

to characterize all the possible differences between the standards. To increase the number of case 

studies to validate these conclusions, Phase II looked at two additional cases – a monopile and a 

jacket (space frame) substructure at a relatively deeper water depth at one of the Phase I sites.

More details of Phase I can be found in the MMI Engineering report [5]. The reader can also 

refer to a conference paper based on this report [6]. 

1.4 Objective

The previous study indicated that the levels of reliability generated with the use of the API and 

IEC codes are dependent upon region (tropical or nontropical), wind load, wave load, structure 

type, and water depth. Specifically, the API refers to the American Petroleum Institute 

Recommended Practice 2A, Working Stress Design, 21st Edition [4]. The IEC refers to the 

International Electrotechnical Commission (IEC), IEC 61400-3 Ed.1, Wind Turbines – Part 3: 

Design requirements of OWTs, IEC TC 88 WG3 Committee Draft, December 2005 [3]. This 

Phase I study provided specific calibration data for two very specific conditions (e.g., a shallowwater monopile at a Northeast Atlantic site and a medium-water depth tripod at a Gulf of Mexico 

site). In deep open ocean, the height of the extreme storm-generated waves generally grow 

increasingly larger with storm intensity. However, when the wave height and the water depth are 

near the same magnitude, the waves begin to break, but are simultaneously attenuated by 

interaction with the sea bed. These shallow-water effects have first-order impact on the 

magnitude of the extreme loads. The breaking waves impart a much different and generally more 

severe force on the support structures, but this effect is offset by reduced wave height. As a 

result, the turbine reliability index computed in shallow water (15 m) off the Massachusetts coast 

for a monopile foundation was found to be virtually unaffected by the difference between the 50-

year and the 100-year return period that distinguishes the API and IEC standards. This result 

occurred because the wave heights could not grow beyond about 10 m in height due to the 

shallow-water depth limitation. Therefore, extreme loading on this structure is dominated by 

wind-driven operational loads (blades rotating and generating power) rather than the extreme 

wave loading (blades parked, no power generation) from rare tropical storms.

The objective of this current study is to expand the matrix of conditions analyzed in order to 

provide a more complete representation of actual applications to U.S. wind farms. 

The study will compare API and IEC guidelines for OWTs with jacket and monopile foundations 

at the same Massachusetts site analyzed under the previous study. The same metocean data were

applied for the Phase II study, with the assumption that the water depth at this site is now 25 m 
3 

versus 15 m, which was considered under the previous study. This change in water depth will 

provide an understanding of the following: 

• Effect of change in water depth on the monopile loads, design, and reliability for API and 

IEC calculations

• Effect of structure type (a monopile versus a jacket structure, which is more transparent 

in a wave action zone) on the load, design, and reliability for API and IEC calculations

• Shallow-water effects on the most critical load components.
4 

2 Methodology

Monopile and jacket-type structures were compared to establish both their similarities and 

differences with respect to their applicability to OWT support structures in U.S. OCS waters, 

based on API and IEC guidelines. The IEC guideline has been developed specifically for OWT

support structures (safety levels seem to reflect European metocean conditions). This guideline 

addresses many design requirements that are specific to OWT support structures, which are not 

addressed within API RP2A, given API’s focus on offshore oil and gas platforms. The API 

design guideline suggests use of 100-year storm specifications for developing the design of a 

structure. The IEC guideline does not specifically address the requirements for OWT support 

structures in regions subject to hurricanes. Also, the IEC guideline utilizes a 50-year storm 

condition to define the loads that are required to establish the minimum strength requirements for 

the support structure. 

The approach in executing this study was the following: 

• Utilize for this site, the same metocean data directly from the previous Phase I study, but 

assume a new water depth of 25 meters for the Massachusetts site and apply appropriate 

consideration of the shallow-water effects for this 25-m water depth.

• Develop designs for a jacket-type and a monopile foundation for the turbine at this site, 

applying the necessary resonance-avoidance constraints and the design guidelines, as 

suggested in API and IEC documents. 

• Compare the loads, capacities, and reliability of the jacket and monopile at this site.

• Address the shallow-water effects and consequent loads on structure. A representative 

soil profile modeling variation of soil properties with depth below mudline was assumed 

for this site (similar to Phase I).

• Investigate the impact of API and IEC guidelines on overall reliability (probability of 

failure).

2.1 Structure Type 

There are a number of different types of support structures that may be used for OWTs. These

include monopiles, gravity base structures, tripods, jackets, and various floating concepts. 

Different design standards will very likely produce levels of reliability that vary for these 

different concept types. The study focused on the structure types that will most likely be used in 

the near-term development of offshore wind farms in the United States. These types included a 

monopile and a jacket (a three-legged space frame structure), as illustrated in Figure 1. 
5 

Figure 1. Types of support structures: three-legged space frame structure on left, and a monopile 

structure on the right. (Illustration from MMI Engineering)

Sea Bed Sea Bed

Rotor Blade

Nacelle

Tower

Monopile

Foundation

Multi-pile Jacket

Foundation
6 

3 Turbine and Site Characterization

This section summarizes the turbine characteristics, the site parameters, and the metocean 

characteristics at the site. The turbine type utilized is the same as in Phase I, and the details of the 

section below are largely repeated from the Phase I report for convenience.

The designs presented herein are representative of a monopile and a three-legged jacket-type 

structure. The wind turbine utilized is an upwind rotor or a horizontal axis turbine with the blade 

turned towards the incoming winds. The results presented in this document may not directly 

apply to other turbine configurations, such as a downwind rotor or a vertical axis wind turbine. 

3.1 Turbine Specifications

3.1.1 Turbine Size

The size of the wind turbine (i.e., the megawatt rating of the turbine system) predominantly 

determines both the magnitude of the wind load and the elevation of the centroid of effective 

wind pressure. Together, these two factors have a significant effect on the base overturning 

moment (OTM), which may control the design of many of the components of any support 

structure configuration. The demand (predominantly the wind load) on the support structure 

increases with increasing turbine size; however, it is not clear what effect turbine size may have 

on the relative levels of reliability that are achieved with API and IEC. The reason for this 

complexity relates to the relative significance of wind and wave loading and how these forces 

vary for both operating and extreme loading conditions. It is intuitive, however, to assume that 

the 100-year wind and wave load would generally be higher than the 50-year wind and wave 

load for the same structural system. The specific loads and resulting reliability are discussed in 

more detail in the subsequent sections of this report.

A 5-MW turbine was used as the basis of the study, as in Phase I, to permit comparisons of the 

results of this phase to prior phase results. The most significant factor in selecting the 5-MW 

turbine was the availability of models that were needed to define wind and wave loads for all of 

the conditions required during the case study analysis. Wind and wave force simulations were 

performed using the FAST1

 software provided by NREL. NREL also provided the model data 

for its reference-level turbine and this became the basis for all wind and wave force calculations 

performed for this study. 

The properties of the turbine and blades used in the analysis are summarized in Table 1 and 

Table 2. These specifications are obtained from the NREL Offshore 5-MW baseline wind turbine 

as described in NREL/TP-500-41958 technical report [1]. 

 

1 Fatigue, Aerodynamics, Structures, and Turbulence. An Aeroelastic Design Code for Horizontal Axis Wind 

Turbines. Jason Jonkman, National Wind Technology Center. http://wind.nrel.gov/designcodes/simulators/fast/. 
7 

Table 1. Properties of the NREL 5-MW Baseline Wind Turbine 

Rating 5 MW

Rotor Orientation, Configuration Upwind, 3 Blades

Control Variable-Speed, Collective Pitch

Drivetrain High-Speed, Multiple-Stage Gearbox

Rotor, Hub Diameters 126 m, 3 m

Hub Height 90 m

Cut-In, Rated, Cut-Out Wind Speed 3 m/s, 11.4 m/s, 25 m/s (10-minute 

average, at hub height)

Cut-In, Rated Rotor Speed 6.9 rpm, 12.1 rpm

Rated Tip Speed 80 m/s

Overhang, Shaft Tilt, Precone 5 m, 5o, 2.5o

Rotor Mass 110,000 kg

Nacelle Mass 240,000 kg

Tower Mass 347,460 kg

Coordinate Location of Overall Center of 

Mass (-0.2 m, 0.0 m, 64.0 m)

Table 2. Undistributed Blade Structural Properties 

Length (w.r.t Root Along Preconed Axis) 61.5 m

Overall (Integrated) Mass 17,740 kg

Second Mass Moment of Inertia (w.r.t. Root) 11,776,047 kg-m2

First Mass Moment of Inertia (w.r.t. Root) 363,231 kg-m

CM Location (w.r.t. Root along Preconed Axis) 20.475 m

Structural Damping Ratio (All Modes) 0.477465 %

3.1.2 Tower Properties

The tower, which is the column that spans the height from the top of the support structure to the 

nacelle, was included in all of the wind and wave load response analyses; however, the design of 

the tower was not varied for the IEC and API design conditions. The tower used in this study is 

identical with the one used by NREL for the analyses of a 5-MW baseline wind turbine [1]. 

Some of the key dimensions of the tower are provided in Table 3. 
8 

Table 3. Tower Dimensions 

Tower base outer diameter (m) 6.0

Tower base wall thickness (m) 0.03

Tower top outer diameter (m) 3.87

Tower top wall thickness (m) 0.02

Tower length (from tower base to yaw bearing) (m) 77.6

3.1.3 Turbine Operation Requirements

The support structure must be designed to avoid resonant response with the rotor. If a resonance

condition happens, the amplification of motion will likely cause significant damage to the rotor 

and the blades and would also lead to premature fatigue distress in the support structure. Turbine 

manufacturers provide frequency resonance criteria for their specific turbines that are based on 

the operating speed of the rotor and the number of blades. These data are typically represented in 

a Campbell diagram, such as that shown in Figure 2 for the reference 5-MW turbine. The 

Campbell diagram defines both rotor and blade passing frequencies (i.e., the frequency with 

which any of the three blades pass the central support column) for a range of rotor speeds up to 

the operating speed of the turbine (i.e., the range between the two vertical lines). The normal 

range of rotor operating speeds can thus be used to establish a range of structure frequencies that 

will avoid resonant behavior for either of these two inputs. As seen in the diagram, resonance 

conditions would occur for structural frequencies of 0.1 to 0.2 (Hertz) Hz; and 0.34 to 0.6 HZ as 

shown in the shaded areas of the figure. A frequency of 0.25 Hz (4-second period) was defined 

as the target frequency for the first structural mode frequency for all support structure 

configurations at both sites. 

Avoid Shaded 

Areas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 5 10 15

Structural frequency, Hz

Rotor speed, rpm Cut-in rotor speed

Rated rotor speed

Operating Range

Figure 2. Campbell diagram
9 

3.2 Site Location and Water Depth

The location of the site is shown in Figure 3. The site is located south of Massachusetts and 

Rhode Island between Martha’s Vineyard and Block Island, which is at 41°15’ N 71°15’ W. The 

water depth at the site was assumed to be 25 m, instead of the actual water depth of 15 m used in 

Phase I. The water depth was increased artificially to permit the study of an increased water 

depth effect (especially wave breaking effects) on structure design and resulting reliability. This 

change in depth would also provide for an understanding of results with variation of water depth 

for monopile design when compared to Phase I results.

Figure 3. Location of site2

 

2 Note that these maps are publicly available on the Internet and were simply annotated with the site labels to 

indicate approximate location http://www.windpoweringamerica.gov/wind_maps.asp. 

Phase II Site

Phase II Site
10 

3.3 Support Structure Configuration

3.3.1 Monopile

The monopile is the most basic of configurations. It includes a large single pile that is driven to a 

penetration depth that provides the necessary mudline fixity to resist the large OTMs caused by 

wind and wave loads. The monopile is assumed to extend to a distance of 10 m above water line. 

At this point, there is a transition to the tower, which is connected to the monopile either by a 

bolted flange or with a grouted sleeve connection, which accounts for any transition in pile 

diameter to tower diameter. It is assumed that the tower tapers in diameter from this transition 

point to the upper flange that supports the nacelle. The yaw bearing that provides support to the 

nacelle is supported on the tower and located 87.6 m above the water line. 

Phase I monopile section properties were used to establish an initial definition of the gross 

properties of the monopile. In Phase I, a parametric study was performed to evaluate the 

sensitivity of the structural period to the monopile diameter. In this analysis, the monopile was 

assumed to have a constant diameter-to-section-thickness (D/t) ratio. Given this assumption, all 

of the key properties of the monopile could be defined as a function of monopile diameter. This 

includes the variables that control pile structural mass and stiffness and soil-pile interaction. 

Figure 4 shows that a uniform soil-strength profile was considered. Soil-pile interaction was 

represented explicitly using soil springs to model the lateral bearing (p-y), shaft friction (t-z) and 

end bearing (q-z) reaction of the soil. The soil spring properties were developed based on the 

formulations recommended by API RP2A, as shown in Appendix A. 

Figure 4. Soil-strength Profile

A monopile diameter Dmp of 6.5 m was selected because this generated the 4-second target 

maximum period. The corresponding wall thickness is 65 mm. 

0

10

20

30

40

50

60

70

80

0 50 100 150

Undrained Shear Strength (kPa)

Depth Below Mudline (m)
11 

The basic properties of the monopile are illustrated in Figure 5. The fixed attributes of the 

monopile configuration included the flange elevation, nacelle height, and tower top diameter. 

The attributes of the monopile that were considered to be primary variables include its diameter, 

wall thickness, and penetration depth. Note that the pile wall thickness was assumed to be 

uniform through the depth of the pile for simplicity. 

Figure 5. Monopile with 4-second period. (Illustration from MMI Engineering)

3.3.2 Jacket

As a second substructure type, a jacket configuration was used. The jacket concept is one of 

several multi-piled configurations. The basic difference between the multi-pile concepts and the 

monopile is their ability to resist overturning forces through the couple of pile axial tension and 

compression forces, rather than bending as in the case of a monopile. These configurations can 

El. +10 m

El. 0 m

El. +87.6 m

Soil:

c= 12 kPa (0.25 ksf) at 0 m

c= 120 kPa (2.5 ksf) at 4 m 

c= 120 kPa (2.5 ksf) at 61m

Constant submerged unit weight profile: 

8000 kN/m3

Tower 

Top: 

Dtop= 3.87 m; ttop = 0.019 m 

Base: 

Dbase = 6.0 m; tbase = 0.03 m 

Length: 77.6 m 

Monopile 

Dmp= 6.5 m 

tmp = 0.065 m 

Nacelle

Mass: 350 tons

Rotational Inertia: 

Ixx= 43,700 t-m2

 

Iyy= 23,530 t-m2

Izz= 25,420 t-m2

 

El. -25 m

El. -85 m

x

z

y

(into 

paper)
12 

provide greater strength and stiffness for systems in deeper water where the large monopile 

diameter may become prohibitive, both in terms of material and installation costs. For this study, 

a three-leg jacket structure, illustrated in Figure 6, was considered as sufficient for the water 

depth at this site.

Figure 6. Jacket with 4-second period. (Illustration from MMI Engineering)3

 

3 Tower length is 5 m shorter than the monopile case because the top of the jacket is up to an elevation of 15 m to 

keep the junction of the top of the jacket and the base of the tower clear of the extreme storm crests.

El. -85 m

Soil:

c= 12 kPa (0.25 ksf) at 0 m

c= 120 kPa (2.5 ksf) at 4 m

c= 120 kPa (2.5 ksf) at 61m

Constant submerged unit weight profile: 

8000 kN/m3

Tower 

Top: 

Dtop= 3.87 m; ttop = 0.019 m 

Base: 

Dbase = 6.0 m; tbase = 0.03 m 

Length: 72.6 m 

Nacelle

Mass: 350 tons

Rotational Inertia: 

Ixx= 43,700 t-m2

 

Iyy= 23,530 t-m2

Izz= 25,420 t-m2

 

El. 0 m

El. -25 m

El. +87.6 m

Legs

1.0 m x 0.05 m below El. -16 m 

0.9 m x 0.045 m above El. -16 m 

Vertical braces

0.4 m x 0.02 m 

 Horizontal braces

0.3 m x 0.01 m 

Piles 

1.2 m x 0.045 m above El. -31 m 

1.2 m x 0.040 m below El. -31 m 

El. +15 m

El. +8 m

El. +0.5 m

El. -7 m

El. -16 m
13 

3.4 Meteorological and Oceanographic Data

Site-specific wind, wave, and current data from Phase I were used for this study. A 25-m water 

depth was imposed on this data set (instead of 15 m in Phase I) to account for wave breaking and 

shallow-water depth effects.

The raw data were available to MMI Engineering from Phase I study. These data are referred to 

as hindcast data and similar data have also been used by the oil and gas industry for much of the 

reliability work for offshore structures in the Gulf of Mexico. The hindcast data are generated by 

metocean models that have been calibrated to observed data from past storms. 

The metocean data included the following: 

• Wind speed for 1-hour-average duration at 10 m above sea level

• Significant wave height, Hs 

• Average zero-crossing wave period, Tz

• Current velocity

• Surge height. 

The data that were available to define the metocean conditions are as follows: 

• Tropical: covering tropical storms from 1900 to 2005 

• Extratropical and Continuous storms: metocean statistics provided from analysis of 

Extratropical storms from 1957 to 2000, and Continuous storms from 1990 to 2005. 

The key metocean parameters for OWTs are wind speed and significant wave height, Hs. The 

remaining parameters are defined based on Hs. For example, given a storm with a specific Hs

value, the data regression provides the associated wave period, maximum wave height, current 

velocity, and surge height. Given an Hs value, the remaining ocean parameters are modeled as 

deterministic, with the value obtained through regression functions that define the parameter as a 

function of Hs. 

Figure 7 conveys the level of correlation seen in wind speed and Hs values. The correlation for 

these data was about 85%. This correlation is included4 in the reliability analysis to calculate the 

reliability index.

 

4 The annual maximum wind speed (1 hour average at 10-m reference height) and the annual maximum significant 

wave height are each specified by a Gumbel probability distribution; a correlation coefficient of 0.85 is additionally 

specified to model the observed correlation in the hindcast data. For the design storms (i.e., 50-year storm for the 

IEC and 100-year storm for the API), the associated 50-year and 100-year estimates are used to define the storm. 

This is a minor enhancement in the approach to Phase I to now account for the joint occurrences of metocean 

parameters that define a 50-year or a 100-year design storm.
14 

Figure 7. Correlation of wind speed Ws and significant wave height Hs for tropical storms

Figures 8 and 9 convey the deterministic regression functions used to relate the associated ocean 

parameters to Hs. Although not relevant, in these figures, the GP number refers to the grid point 

used in the hindcast models to generate the metocean parameters. The Massachusetts site is 

referred to as GP 3468. 

0 1 2 3 4 5 6 7 8 9

0

5

10

15

20

25

30

35

Hs (m)

Ws (m/s) 1-hour average at 10m

 0

 2

 4

 6

 8

 10

 12

 14

 16

 0 1 2 3 4 5 6 7 8 9

Hmax (m)

Hs (m)

Tropical Storm GP 3468

Data

2.14419 x (Hs^0.87186)

Figure 8. Relation of maximum wave height to significant wave 

height Hs in a tropical storm
15 

The hindcast data from Oceanweather does not account for breaking wave consideration. The 

large Hmax values represent “unbroken” waves. Later we discuss the breaking wave limit and 

how the wave heights were reduced to account for this effect. 

The scatter observed in Tz values for small Hs values (Figure 9) causes only a nominal change in 

the wave load on the OWT structure. The load also generally decreases as the wave period 

increases. Therefore, a conservative simplification was adopted in which the median Tz value 

was used as the associated Tz (and hence the associated Tmax) for small values of Hs. Small wave 

heights do not generally cause ultimate strength failure, so this assumption was considered

reasonable. The data for large Hs values are limited and does not show a large scatter around the 

median Tz value. Tmax is assumed to 1.2×Tz for this study. 

Figure 9. Relation of average zero-crossing period Tz to significant wave height Hs in a storm

Figure 10 shows the relationship of surge height to significant wave height. The hindcast data 

indicate scatter in the surge height; however, as the regression line indicates, the surge height 

generally increases with Hs. This regression line was used to calculate the surge height to be 

used for storms with different Hs values. 

 2

 3

 4

 5

 6

 7

 8

 9

 10

 0 1 2 3 4 5 6 7 8 9

Tz (s)

Hs (m)

Tropical Storm GP 3468

Data

4.28982 x (Hs^0.351236)
16 

Figure 10. Surge height as a function of significant wave height for MA site

3.4.1 Breaking Waves 

The wave data provided by Oceanweather (data vendor for Phase I of the study) do not 

specifically address breaking waves; therefore, wave heights defined using this model for 

shallow-water conditions may be significantly greater than the breaking wave limits. To address 

this limitation in the data, wave heights were reduced when required to equal the associated 

breaking wave limit corresponding to the specific water depth. This correction also required 

special attention to the wave slam forces associated with breaking waves. Additional details on 

this issue are presented in Section 4.4. 

3.4.2 Metocean Data for the Site

The wave and wind data that were developed for the Massachusetts site are summarized in Table 

4. This summary includes the conditions used for the Power Generation (Operating), 50-year, 

and 100-year storm conditions. 

-100

-50

 0

 50

 100

 150

 200

 0 1 2 3 4 5 6 7 8 9

Surge Height (cm)

Hs (m)

Tropical Storm GP 3468

Data

1.1033 x (Hs^1.92955)
17 

Table 4. Metocean criteria for site (offshore Massachusetts)

Return Period Ws 

10m,1hr 

(m/s) 

Ws 

90m,10 

min 

(m/s) 

Hs (m) Tz (sec) Hmax

(m) 

Tmax

(sec) 

Surge 

(m) 

Current 

(m/sec) Max. 

Wave 

Wind 

Speed 

1 <1 10.99 11.4 3.99 6.97 7.17 8.37 0.159 0.146

A 50 46.3 49.47 7.48 8.7 12.4 10.4 1.346 0.393

50 A 40.42 42.98 8.67 9.16 14.1 10.9 0.945 0.381

A 100 51.85 55.62 8.32 9.03 13.6 10.8 1.644 0.447

100 A 45.29 48.35 9.81 9.57 15.7 11.4 1.172 0.441

Note: “A” stands for associated. For example, for a 50-year storm defined as the one with largest maximum wave 

height (referred to as wave with 50-year return period), the wind speed chosen is the associated wind speed for the 

50-year storm with the largest wave height. Also, for wind speed columns, the two quantities qualifying the definition 

are elevation above sea level and duration, over which the average wind speed is provided. So, Ws 10 m, 1 hr (m/s) 

refers to average wind speed meters per second observed at an elevation of 10 m above sea level, with the 

averaging performed over a 1-hour duration. The water current on the right-most column is assumed to be a uniform 

current through the water depth.

3.5 Other Site Assumptions

To permit easier comparison of results, a stiff clay profile was selected to represent the site. The 

shear strength profile selected for the analyses is shown in Figure 4. Additional soil details are in 

Appendix A. 

No special site conditions, including seafloor slope, seafloor irregularity, and scour, were part of 

the study. 
18 

4 Analysis Methodology

The methodology adopted for the analyses in this study is summarized in Figure 11. Due to the 

complexity of the formulation of steep and breaking wave forces and some limitations of the 

FAST simulation software in this regard, a sequential process was developed to determine loads 

and structural response. The gross dimensions of each structure were developed based on the 

dynamic performance considerations described in Section 3.3. Using FAST to define the wind 

and wave force time-histories and to specifically represent the change in blade wind forces 

caused by motions in the system generated by wave loading, a coupled model of the turbine, 

tower, and support structure was developed. The FAST analysis does not determine the 

impulsive forces caused by breaking wave slam on the tower. The FAST analysis is also 

restricted to some extent by the limits in particle kinematic theories that are used to define wave 

drag and inertia forces. Lastly, FAST does not determine the wind force applied directly to the 

tower shaft through drag. These additional forces were estimated using CAP5 and other 

calculations, and were added to the FAST results to define the complete set of physical loads. 

Once the total wind and wave forces were defined, a second CAP model was developed and used 

for both the structural design calculations (e.g., member utilization checks) and nonlinear 

capacity analysis. Member sizes were evaluated and modified as required to meet the minimum 

IEC or API requirements for each case study.

 

5 CAP is the Capacity Analysis Program, which provides the capability for the rapid assessment of the ultimate 

strength and for nonlinear dynamic (wave and earthquake) analysis of the offshore structure. This program permits 

nonlinear modeling of the structure and the ability to capture soil-structure interaction. This tool is licensed by MMI, 

and has been in use in the offshore industry for more than a decade.
19 

Figure 11. Methodology flowchart

Target Period 

(Campbell Diagram)

Coupled Wind & Wave 

Analysis using FAST

Site-specific Metocean 

Study

Equivalent Monopile

for Wind/Wave Loads 

Preliminary Design for 

Resonance Avoidance

Slam Force due to 

Breaking Wave

Additional Drag and 

Inertia Forces for

missing effects 

Wind Drag 

Force on Tower

Total Demand on the 

Substructure (Base 

shear and OTM)

Design Checks

all Utilization

Ratios <1.0?

No

Yes

Capacity Analysis

Reliability Calculation 

for Structure
20 

4.1 Coupled Wave and Wind Load Analyses

In order to evaluate the demand for a range of site-specific environmental conditions, a series of 

dynamic, coupled wind and wave analyses were performed. The FAST code developed by the 

NREL was used for the analysis. FAST is a comprehensive, aerodynamic/hydrodynamic 

simulator that is capable of calculating the response of an onshore or OWT under operational and 

extreme metocean conditions, with some limitations as mentioned earlier. 

As in Phase I, a coupled analysis for aerodynamic and hydrodynamic loads was targeted to 

model any cross effects of the two loads in a time-history analysis of the loads in different storm 

conditions. A coupled analysis seeks to include the dynamic effects of simultaneous application 

of aerodynamic and hydrodynamic loads. This analysis includes effects of damping and any 

phasing (in time) associated with the two loads, especially if the phasing were to reinforce the 

two loads' components, and results in higher than simple addition of loads. 

Each load case was defined with a combination of maximum wave height and average wind 

speed for storms of different return periods with associated current and surge. Each case was 

examined with a number of different wind/wave simulations to assess the effect of variation of 

wind turbulence and to provide a converged estimate of maximum structural response in these 

storms. A sensitivity study was performed to assess the number of simulations that are necessary 

to obtain a converged estimate of the maximum demand for different turbulent wind flow 

models. (Appendix C presents these results.) Each analysis that was performed included 10-

minute time-history simulations for 10 different stochastic simulations of turbulent wind flow. 

From each 10-minute simulation, the maximum OTM and base shear values were extracted. The 

mean and standard deviation of these maximum values were then calculated for design and 

capacity analysis and for reliability calculations. 

The following subsections categorize the wind turbine loads and describe how each are modeled 

and analyzed. 

4.1.1 Wind Load on Blades

The calculation of blade wind load requires the definition of full-field, wind-flow time histories

that are defined at grid points that cover the vertical plane of turbine blades (Figure 12). 

TurbSim6 was used to develop wind turbulence time histories. A stochastic turbulence model 

(extreme turbulence model for wind-turbine class 1, according to IEC 61400-1 Edition 3 

standard) was applied for a given mean wind velocity at hub-height. Due to the stochastic nature 

of the turbulence model, 10 different simulations were performed for a single mean wind 

velocity specified at hub-height. The vertical mean wind profile was obtained by applying a 

power law over the rotor disk and a logarithmic profile below the disk. The power law exponent 

as stated in the IEC guidelines is 0.14 for normal wind conditions and 0.11 for extreme winds. 

Below the rotor disk, API suggests use of a logarithmic vertical-wind profile that is consistent 

with IEC.

 

6 A stochastic, full-field, turbulent-wind simulator for use with the AeroDyn-based design codes (YawDyn, FAST, 

and MSC.ADAMS®) http://wind.nrel.gov/designcodes/preprocessors/turbsim/. 
21 

The aerodynamic calculations were based on two-dimensional airfoil-data coefficients (pitch, 

drag, and pitching moment coefficients), with corrections for three-dimensional behavior. These 

calculations were performed within the AeroDyn module of FAST software. The structural 

properties (summarized in Table 1) and aerodynamic properties of blades were obtained from an 

NREL report by Jonkman [1]. 

In the aerodynamic load calculation for extreme storms, it is assumed that the blades are able to 

yaw into the incoming winds. IEC refers to this as the “normal” condition. An “abnormal” 

condition would be one where the yaw mechanism is ineffective during severe weather, leaving 

the blades in some other orientation. This abnormal condition would result in potentially greater 

loads than when the blades are facing the incoming winds. The IEC permits use of a lower safety 

factor (of 1.1) for the 50-year load in an abnormal extreme load condition. A safety factor of 1.35 

is used for a normal extreme condition. 

Figure 12. Grid points for the wind velocity data and force components acting on a blade. 

(Illustrations from NREL) 7

4.1.2 Wave Load on Monopile

A coupled dynamic wave and wind analysis requires the time-history of wave forces to be 

applied along the depth of the structure. Incident wave kinematics were modeled with Stream 

function wave theory. To calculate the drag and inertia forces on the monopile, the relative

velocity form of Morison’s equation (Eq. 1) was used: 

||)( 2

1 )1( ququAC

t

u

A A VCqVCF D  ++−= + ρ −−  δ

δ

ρ ρ (1) 

where 

A is the projected area 

V is the displaced volume of the cylinder per unit length 

q is the displacement degree of freedom of tower/monopile node 

u is the water particle velocity

CA is the normalized hydrodynamic-added-mass coefficient

CM is the inertia coefficient given by 1+CA

 

7 The figure showing the grid points is from Turbsim User’s Guide for version 1.40, NREL, September 12, 2008. 

The figure showing the forces on the blade is from AeroDyn Theory Manual, NREL/EL-500-36881, Dec. 2005
22 

CD is the normalized viscous drag coefficient

q is the particle velocity

q is the particle acceleration

The drag and inertia coefficients used for each type of substructure are listed in Table 5 and Table 

10, and the calculation of these coefficients is provided in Appendix B. 

Morison’s equation becomes less accurate for member diameters that are large relative to the 

length of the wave. A typical wave length of the incident storm waves used in the analysis is 

about 100 meters. For the monopile diameter of 6.5 meters, the ratio of wave length to the 

member diameter is well above the limiting value of 5 as suggested by API RP2A for Morison’s

equation applicability.

8

 

A series of analyses were performed to verify the wave force formulation included in FAST. 

Various comparisons were completed to assess time-histories of monopile base shear and 

overturning to those generated with CAP and through other independent wave force calculations. 

4.1.3 Wind Load on Tower

The FAST program does not represent the drag load on the tower caused by wind. This drag 

force was calculated independently following Eq. (2.3.2-8) in API RP-2A (see also Eq. 2 below), 

using a logarithmic vertical wind profile and mean wind speed at hub-height. This static force 

was then superimposed with the aerodynamic loads obtained by FAST. 

S ACuF 2 = ρ )2( (2) 

where 

u is wind velocity 

CS is shape coefficient (0.5 for cylindrical sections) 

 A is the projected area of the tower facing the incoming wind. 

Note that both API and IEC suggest modeling all physical loads; the API formulation was used 

only as a matter of convenience.

4.2 Equivalent Monopile Properties for FAST Analyses

The FAST program is limited to the modeling of fixed monopile support structure 

configurations. For monopiles, the program represents the supporting structure both in terms of 

applied wind and wave load, and more importantly, also in terms of the structural vibration 

characteristics that affect the resulting turbine wind loads. Any support structure other than a 

fixed monopile requires an equivalent representation to capture at least the first mode of 

vibration, wave drag, and inertial loading. An equivalent fixed-base monopile was “tailored” to 

match the desired structural period for each case study by changing the depth of fixity and also 

calibrating the system damping. The target structural period was selected to avoid resonance

caused by the turbine rotor rotation and the aerodynamic effect of the blade passing in close 

 

8 This limitation will be more important for the smaller waves that contribute to fatigue.
23 

proximity to the support tower. This concept is explained in detail in Section 3.1.3, with the use 

of a Campbell diagram. The equivalent representation of the monopile is illustrated in Figure 13. 

Figure 13. Tower and monopile model properties. (Illustration from MMI Engineering) 

4.3 Coupled Wave and Wind Load Analyses with FAST

The numerical results for the load analysis are presented in Sections 5.1 and 6.2. This section 

provides a general description of the analysis method. 

A range of wave and wind conditions were specified for the site. Ten simulations were 

performed for each case using a combination of wind and wave conditions. For each simulation, 

a different turbulent field was specified for the mean wind speed. It was determined through an 

additional set of parametric analysis that 10 simulations provide a stable representation of the 

wind force, specifically the mean value of the maximum wind force. 

The FAST program can model both the operating (rotating blades) and parked (stationary blades) 

conditions. During extreme wind conditions (for mean hub-height, 10-minute average wind 

Mt

Vt

Mb

Vb

77.6 m 10

m

Water Depth

Depth of Fixity

Tower Base

Mean Water Level

Mudline
24 

speeds above 25 m/s) the turbine is positioned as parked. The value of 25 m/s is the cut-out wind 

speed specified in NREL’s 5-MW baseline wind turbine. 

Four internal force and moment components were obtained for each analysis (or case) in the 

form of a time-history; namely, tower top shear and over-turning moment (Vt and Mt in Figure 

13), and monopile base shear and over-turning moment (Vb and Mb). The maximum values of 

these components were recorded for each simulation. The mean of these maxima were calculated 

over the 10 simulations. The mean of these maximum force components were then used for the 

reliability assessment of each structure.

4.4 Breaking Wave Forces

The breaking wave limit identifies the combination of wave height, wave period, water depth, 

and seafloor slope that causes instability in the wave form that leads to the breaking wave 

condition. For this study, a flat sea bed is assumed. A zero seafloor slope produces a spilling 

breaking wave condition. Impact forces were calculated in cases where wave instability was 

indicated. If the wave was numerically unstable prior to wave breaking, additional drag-inertia 

forces were also calculated using Stream function particle kinematic theory. To obtain the total 

base shear and OTM, these forces were added to what was calculated by the FAST analysis and 

with the wind forces on the tower. 

Figure 14 shows the wave conditions for the site along with the breaking wave limit. It also 

shows the numerical limit to the range of applicability for Stream function. 
25 

Hmax (m) Tmax

(sec)

Physically 

Stable9

Numerically 

Stable10

Return 

Period 

(years)

3.92 6.56 Yes Yes <1

7.17 8.37 Yes Yes 1 

8.54 8.98 Yes Yes 5 

9.91 9.54 Yes Yes 10

11.24 10.03 Yes Yes 25

13.85 10.92 Yes Yes 50

15.27 11.35 Yes Yes 100

19.72 12.58 Yes No 1,000

21.01 12.91 No No 2,000

23.97 13.61 No No 10,000

Figure 14. Breaking waves at the site. (Tapp is the apparent wave period.) 

Below is the procedure to calculate impact forces caused by wave breaking. A conceptual 

illustration of the problem is provided in Figure 15. The breaking wave impact force was 

calculated based on the guidelines provided in IEC [3] using Equation 3. 

 9 Physically stable implies whether wave breaking has occurred or not

10 Numerically stable implies whether Stream Function Wave theory is numerically well behaved and provides results or not

0

0.005

0.01

0.015

0.02

0 0.01 0.02 0.03 0.04

Hmax/(g*Tapp^2)

d/(g*Tapp^2)

physical breaking limit

Waves Considered

numerical limit for stream
26 

Figure 15. Impact load from a breaking wave. (Illustration from MMI Engineering)

where 

C: wave celerity

Hb: wave height at the breaking location 

ηb: maximum elevation of the free water surface above the still water level

Rmp: radius of the cylinder (also referred to as R here)

λ: curling factor (approximately 0.5) 

ρ: water density

SWL: Still water level

The impact force is given by

( )

 8C

R 0for 

 422 Arctan 1 2

≤≤















 = − −

t

R

Ct

R

Ct tF λη b RC πρ

(3a)

( )

 

C

R

32

1 ' here 

C

R

32

12 ' 323 for 

 '6

4

' Arctan 1

3

'8

'6

4 2

≤≤ −=















 = − −

ttwt

C

R

R

Ct

R

Ct

R

Ct

Ct

R tF λη b RC πρ

(3b) 

The first term in Equation 3a is the maximum value; the second term is the decay of force with 

time. The maximum value is twice that given using API guidelines. 

A representative impact force time-history is provided in Figure 16 for a maximum wave height 

of 20.07 m (17.37 m after breaking). 

λ.ηb

ηb Hb

SWL

Rmp

C

area of 

impact
27 

Figure 16. Impulse force on structure from a breaking wave of height 17.4 m 

A CAP analysis was performed to calculate the dynamic response of the monopile to this 

impulse loading. The mudline OTM time-history is provided from this analysis in Figure 17. 

This response shows significant transient behavior in the monopile. This behavior is caused by

the instantaneous nature of the loading. The peak at 208 MN-m is the OTM caused by impulse 

loading from the breaking wave that is added to other load components to calculate the overall 

mudline OTM. 

Figure 17. Response of the monopile to the impulse force from a breaking wave of height 17.4 m 

Based on Equation 3, the amplitude and the effective duration of the impact force is a function of 

the structural member diameter (or radius R). Figure 18 shows that for jacket members, which 

are smaller than the monopile in diameter, both the amplitude and the duration of the impact 

force is less compared to those for the monopile. As a result, the input energy, which is the area 

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 0.02 0.04 0.06 0.08 0.1

Impact Force (kN)

Time (sec)

-200,000

-150,000

-100,000

-50,000

0

50,000

100,000

150,000

200,000

250,000

0 2 4 6 8 10

OTM (kN-m)

Time (sec)
28 

under the curves, is significantly less for the jacket structure. This means that the response of the 

jacket structure to the impulse loading is significantly less, compared to the response of the 

monopile. Therefore, jacket structures, being more transparent, are less sensitive to the impact 

loading from breaking waves. 

Figure 18. Comparison of impact forces from breaking waves on monopile and jacket structures 

(member diameters shown in parenthesis)

4.5 Additional Drag and Inertia 

For wave heights that exceed the numerical applicability of Stream Function wave theory (see 

Figure 14), drag and inertia forces from Stream theory can only be calculated up to the indicated 

limit. Above this limit, drag and inertia forces still exist and were calculated using the approach 

discussed in this section.

Figure 19 shows the concept of how this force is treated. We assume that CX/HX= CY/HY (i.e., 

the wave steepness remains the same above the Stream limit) and the velocity profile stays 

constant above CX. The kinematic wave profile is calculated for CX using Stream theory, and the 

kinematics above CX (up to CY) are assumed to be constant. This is a reasonable approach to 

incorporate this small correction effect.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 0.02 0.04 0.06 0.08

Impact Force (kN)

Time (sec)

monopile (6.5 m)

jacket leg (0.9 m)

jacket diagonal (0.4 m)
29 

Figure 19. Additional drag and inertia forces for modeling missing effects in wave theory near a 

breaking wave. (Illustration from MMI Engineering)

The formulation is provided for a maximum wave height of 17.22 m and the corresponding surge 

of 1.50 m. Based on stream function, one calculates CX as 13.07 m, maximum horizontal 

velocity as 13.04 m/sec, and maximum horizontal acceleration as 5.51m/sec2

. The equivalent 

drag and inertia coefficients are 1.03 and 1.30. The maximum stable wave height HY is 74% of 

the effective water depth. The effective water depth of 26.5 m results from the actual water depth 

of 25 m plus surge of 1.5 m during the analyzed storm. 

m96.1422.17/07.1372.19

m72.1950.2674.0

×=⇒ =

=×=

Y

Y

C

H

The base shear caused by additional drag and inertia is: 

MNm

kN

kN

g

CF

kN

g

CF

t

u V

g

CF

Au u

g

CF

FF

mI

DD

mI

DD

ID

62) 2

)07.1396.14( Additional OTM 57.39(1526

 Additional BaseShear 526,1

45051.5)07.1396.14(25.3

076,104.13)07.1396.14(5.6 2

|| 2

 Additional BaseShear

2

2

= − +×=

⇒ =

=×−×××=

=×−××=

=

=

+=

π ρ

ρ

δ

δρ

ρ

CX HX

HY CY

Y: Physical limit for breaking

X: Numerical limit for stream

C: Crest Height

H: Wave Height
30 

5 Monopile Substructure Analysis

A series of wave and wind load analyses were performed at the start of the development of the 

monopile concept to assess the variation in load as a function of projected area of the monopile, 

its stiffness, and the corresponding first mode of vibration. These analyses indicated clearly that 

the basic properties of a monopile (i.e., base diameter and thickness) at its critical section (i.e., 

the location of maximum bending moment immediately below mudline) would be controlled by 

the allowable range in the first mode period that was established to avoid accelerated fatigue. 

This assessment was applicable for both the IEC and API conditions, because it was found that 

the required minimum section properties to provide adequate strength for the 50- or 100-year 

design conditions were significantly less than those required to achieve a maximum first mode 

vibration period of 4 seconds. 

There are a number of combinations of monopile diameter and wall thicknesses that can be used 

to achieve the required maximum 4-second vibration period. Generally speaking, in terms of 

stiffness, greater overall efficiency is achieved with larger diameter and smaller wall thickness. 

Other factors that must be considered include the feasibility and cost of driving large-diameter 

piles, the larger drag and inertia wave forces, local buckling, soil-pile interaction, and foundation 

stiffness. A monopile diameter of 6.5 meters with a mudline wall thickness of 65 mm was found 

to provide a reasonable balance of all design parameters. Additional detail on this design is 

provided in Figure 5. This configuration was used for the remainder of the calculations. 

5.1 Wind and Wave Load Analyses with FAST

FAST is configured to model monopile support structures; therefore, most of the properties of 

the structure were represented directly. FAST does not model soil-pile interaction and the 

additional flexibility that occurs from lateral soil strain. This soil-pile interaction was represented 

in the model by changing the depth of fixity. FAST is not capable of formulating variable drag 

and inertia coefficients that would be appropriate for conditions where marine fouling would 

occur underwater. An equivalent set of drag and inertia coefficients were determined through a 

separate set of wave load time-history analyses using the Capacity Analysis Program (CAP) 

software, which does address variable drag and inertia coefficients (Appendix B). A few 

parameters, such as assumption of fixity depth, overall damping of the system, modulus of 

elasticity, and coefficients for drag and inertia coefficients, formed part of the “tuning” exercise 

so as to represent in FAST the physical effects available in CAP. 

To verify the structural modeling in two different programs, i.e., CAP and FAST, frequency 

analyses were performed on the response values. Three different response quantities were 

considered: displacement, base shear, and OTM. The wave frequencies were also needed to 

understand the “forcing” frequencies. Therefore, first the frequency analyses of the input waves 

were performed. Figure 20 shows the first and the higher harmonics of the input wave. One can 

observe the first structural frequency and the wave harmonics from the frequency analyses of the 

nacelle displacement in the figure. The same peaks are visible on the frequency response 

analyses of the base shear and the OTM. Some of the important observations based on Figure 20

are listed below.
31 

 Wave Input Nacelle Displacement

 Base Shear Overturning Moment 

Figure 20. Comparison of CAP and FAST results in the frequency domain for the monopile

• The first structural mode frequency obtained from the CAP and FAST models match

closely. 

• There is no higher mode contribution observed beyond the second mode. Therefore, 

modeling the turbine by the first two modes suffices for the purposes of this study. 

• The first structural mode is away from the first wave harmonic (first modal frequency is 

different from first wave harmonic frequency); however it is closer to the second wave 

harmonic. Although the energy from the second wave harmonic is less than from the first 

wave harmonic, resonance at the first structural mode may still occur from the second 

wave harmonic. 

• If the frequency responses of the displacement and the base shear are compared, one can 

observe that structural response (energy) is higher in displacement response, whereas 

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0

1

2

3

4

5

6

7

8

Frequency (Hz)

PSD

First Wave Harmonic

Second Wave Harmonic

Third Wave Harmonic

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0

0.005

0.01

0.015

0.02

0.025

Frequency (Hz)

PSD

CAP

FAST

First Structural Mode 

Second Wave

Harmonic

First Wave 

Harmonic

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0

1

2

3

4

5

6

7

8

9 x 105

Frequency (Hz)

PSD

CAP

FAST

Second Wave Harmonic 

First Wave Harmonic 

First Structural Mode 

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0

0.5

1

1.5

2

2.5

3

3.5

4 x 108

Frequency (Hz)

PSD

CAP

FAST

First Structural Mode

Second Wave 

Harmonic

First Wave 

Harmonic
32 

wave energy is higher for base shear. This means that displacement response is 

dominated mostly by structural characteristics, whereas base shear is mostly dominated 

by wave loading characteristics. One can see that these two effects (i.e., contribution of 

the loading itself and the response of the structure) are comparable to each other in the 

frequency response of the OTM. This is because overturning moment is a function of 

both the wave loading and the response of the nacelle mass. Response of the nacelle mass 

is influential for the base overturning moment because of the long moment arm (height of 

the turbine above the mudline). 

Structural damping, soil damping, aerodynamic damping, and hydrodynamic damping form 

components of overall damping that reduce the response of a structure once the imposed load is 

removed. In the CAP analysis, these components are modeled implicitly or explicitly, and an 

attempt was made to capture the overall damping via a single parameter within FAST to mimic 

the physical effects in CAP. Figure 21 shows the effect of damping on both nacelle displacement 

and OTM for the monopile structure. Note that this is an overall “damping” parameter to permit 

FAST to mimic physical effects in CAP and is not to be confused with the physical structural 

damping in the system. The structural damping is usually on the order of a percent or so. The 

target nacelle displacement and the overturning moment time-histories were obtained by explicit 

CAP analysis. A parametric study for the damping values of the tower was performed in the 

FAST analysis to replicate the target time histories from CAP. The priority was given to match 

the maximum or minimum values (i.e., the extremes) obtained from two different analyses, the 

CAP and FAST analyses, because the maximum OTM is the main parameter of interest for 

structure design and reliability analysis in this study. One can conclude that 10% is a good 

representation of the damping in the system for all the ranges of the heights, i.e., 1 year-, 50 

year-, and 100-year waves. 
33 

Operating Case (1-Year Wave) 

50-Year Wave

100-Year Wave

Figure 21. Effect of damping on monopile structure (“target” is from CAP; numbers in legend 

denote FAST analyses with different overall damping values)

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

30.00 35.00 40.00 45.00 50.00 55.00

Disp (m)

Time (sec)

target

5%

10%

15%

-100,000

-80,000

-60,000

-40,000

-20,000

0

20,000

40,000

60,000

80,000

100,000

30.00 35.00 40.00 45.00 50.00 55.00

OTM (kN-m)

Time (sec)

target

5%

10%

15%

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

30.00 35.00 40.00 45.00 50.00 55.00

Disp (m)

Time (sec)

target

5%

10%

15%

-100,000

-80,000

-60,000

-40,000

-20,000

0

20,000

40,000

60,000

80,000

100,000

30.00 35.00 40.00 45.00 50.00 55.00

OTM (kN-m)

Time (sec)

target

5%

10%

15%

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0.80

30.00 35.00 40.00 45.00 50.00 55.00

Disp (m)

Time (sec)

target

5%

10%

15%

-150,000

-100,000

-50,000

0

50,000

100,000

150,000

30.00 35.00 40.00 45.00 50.00 55.00

OTM (kN-m)

Time (sec)

target

5%

10%

15%
34 

The resulting equivalent monopile properties that were used for the FAST analyses are 

summarized in Table 5. 

Table 5. FAST input data for monopile 

Monopile thickness (m) 0.065 

Tower base outer diameter (m) 6.5

Tower base wall thickness (m) 0.03

Tower top outer diameter (m) 3.87

Tower top wall thickness (m) 0.02

Steel density (effective) (kg/m3) 8,500.00

Steel Young's modules (E) (MPa) 2.100E+5

Steel shear modulus (G) (MPa) 8.080E+4

Water depth, d (m) 25 

Height monopile extends above MSL (m) 10

Length of tower + monopile (m) 112.6

Cd 1.03

Cm 1.30 

Damping 10%* 

Structural fore-aft period (s) 4.08 

*This is a “tuning” number and includes all sources of damping so that the response of the 

structures match from two different analyses programs: CAP and FAST. In this sense it is an 

overall “tuned” damping number to permit FAST software to mimic the physical effects missing 

in FAST that are captured explicitly in CAP (e.g., soil structure interaction). 

5.2 Total Wind and Wave Load Demand

Figure 22 presents the results of coupled wind and wave analyses with FAST in terms of base 

shear and OTM. One can observe that for a constant wave height, base shear and the overturning 

moment first increase and then decrease. Prior to the cut-out wind speed, the range of wind speed 

values are those at which the turbine operates and generates power. After the cut-out wind speed, 

the response values (base shear and the overturning moment) increase because of the increase in 

the wind speed itself. Another observation is that for larger waves and under operating wind 

speeds the response values get magnified due to the interaction between the wave and wind 

loading. Although the response surfaces for base shear and OTM are shown for all of the wave 

heights and wind speeds are presented in the figure, one should focus on the response values that 

lie on the diagonal, i.e., wind speed and wave height with the same return periods. These 

response values are the most probable ones to happen in reality because the wave height and the 

wind speed are correlated. Table 6 provides the results for the metocean loads used in the design 

checks. These results are nominal loads and do not include any load factors. 
35 

Figure 22. Coupled wind and wave loads for monopile for blade loads and hydrodynamic loads. 

(Illustration from MMI Engineering)

2.00

3.99

5.79

8.50

9.50

12.74

13.70

15.94

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9.20 12.65 15.55 21.64 35.15 39.02 51.61 55.37 64.12

Hs (m)

Base Shear (kN)

Ws (m/sec)

(1hr-10 m)

2.00

3.99

5.79

8.50

9.50

12.74

13.70

15.94

0

50,000

100,000

150,000

200,000

250,000

300,000

9.20 12.65 15.55 21.64 35.15 39.02 51.61 55.37 64.12

Hs (m)

Over Turning Moment (kN-m)

Ws (m/sec)

(1hr-10 m)
36 

Table 6. Monopile design loads at mudline section

Base Shear (kN)

Storm Type Coupled 

Wind-Wave

Tower 

Wind Total* 

Operating Storm 2,542 7.26 2,549

50-year Storm (Wind Driven) 3,455 117 3,572

50-year Storm (Wave Driven) 4,046 90 4,136

100-year Storm (Wind Driven) 4,035 145 4,180

100-year Storm (Wave Driven) 5,152 112 5,264

Mudline Overturning Moment (MN-m)

Storm Type Coupled 

Wind-Wave

Tower 

Wind Total* 

Operating Storm 130 0.51 130

50-year Storm (Wind Driven) 90 8.66 99 

50-year Storm (Wave Driven) 84 6.64 91

100-year Storm (Wind Driven) 101 10.8 112

100-year Storm (Wave Driven) 104 8.30 112 

 * Note: There are no breaking wave effects in these storms

Superimposing the different components of load (aerodynamic and hydrodynamic loads from 

FAST, tower wind, as well as slam, and stream correction, if present) is conservative. The intent 

is to estimate the annual maximum load in a storm; more specifically, the maximum combined 

wind and maximum wave load in a storm with a given frequency (return period). The 

conservatism comes from algebraically adding peak value of each component, which assumes 

these peaks occur at the same time. In reality, there is a lag between the different component

peaks, depending on whether a component is drag dominated or inertia dominated. The drag 

components are driven by wind or wave velocity, while the inertia components are dominated by 

wind or wave acceleration. The velocity and accelerations achieve their maximum values at 

different times. The FAST analysis includes this effect for the wind and wave force components 

that are modeled explicitly (i.e., rotor wind and normal substructure wave), but the maximum 

values for remaining components are simply added; the time phase lag between the components 

is ignored. It is reasonable to make this conservative assumption for the objectives of this study 

because it has a consistent effect for both the API and IEC analyses. 

5.3 Strength Checks

Structural response was assessed for each load condition (i.e., unfactored mudline base shear and 

OTM) using the CAP model. CAP includes a pile-soil interaction capability (Figure 23) and thus 

properly represents the effect of soil bearing on both pile fixity and bending moment. 

The maximum demand in the monopile generally occurs below the mudline and Table 7 lists the 

unfactored design loads used in the CAP analyses at the most critical cross-section within the 

monopile. API RP2A utilizes these unfactored loads, whereas IEC/ISO requires the application 

of load factors (1.1 for gravity loads, and 1.35 for extreme environmental loads) prior to the 
37 

structural response analysis. These loads are compared to the monopile loads for the 15-m water 

depth case (from Phase I) in Section 7.1. A material strength of 250 MPa (36 ksi) was assumed 

for the initial utilization ratio check for the monopile. The analyses indicate that higher-strength 

steel is not needed for this case.

Figure 23. Monopile model in CAP

Table 7. Monopile Design Loads at Critical Section (Highest Bending Stress) Below Mudline

Load Cases

Unfactored Loads

Design Loads per API (no 

load factors)

Design Loads per IEC/ISO

Gravity 

Analysis

Wind/Wave 

Analysis

(1.1 Gravity + 1.35 

Environmental)

Axial 

Load 

(kN)

Shear 

(kN)

Moment 

(MN-m)

Axial 

Load 

(kN)

Shear 

(kN)

Moment 

(MN-m)

Axial 

Load 

(kN)

Shear 

(kN)

Moment 

(MN-m)

Power 

Production 10,020 4,770 141 10,020 4,770 141 11,020 6,430 191

Parked (50-yr 

Metocean 

Criteria / Wavedriven) 10,020 4,150 113 11,020 5,600 152

Parked (50-yr 

Metocean 

Criteria / Winddriven) 10,020 3,910 116 11,020 5,280 156

Parked (100-yr 

Metocean 

Criteria/ Wavedriven)* 10,020 5,280 141 10,020 5,280 141

*The wind-driven 100-year storm generates similar or smaller loads for both shear and overturning, so only the 100-year wavedriven storm was used as the design storm load. 
38 

The monopile cross-section was evaluated using both the API and IEC design formulations. The 

maximum utilization ratios (i.e., ratio of stress demand by external loads to allowable stress 

according to API; ratio of the stress demand by factored external loads to factored strength 

according to IEC/ISO) for the monopile are summarized in Table 8. The utilization ratios for 

shear do not govern design of the section. 

Table 8. Member Utilization Ratios for Monopile 

 

Based on API RP2A Based on IEC / ISO

Power 

Production a Parked/Idling Power 

Production b Parked/Idling

Combined Axial

Load and Bending 0.529 0.395 0.402 0.343

Shear 0.073 0.06 0.071 0.062

a

 Without one-third allowable stresses increase factor b

 With the same load and material factors from parked/idling condition

The results shown above indicate that the design is well within the acceptable limits for both API 

and IEC. There were no changes. 

These results confirm that the design of the monopile is predominantly controlled by the 

maximum first mode period of vibration (i.e., resonance avoidance for rotor and blade-passing 

frequency). 

A perfectly tuned design would achieve utilization ratios of 1.0 for all components of the 

structure using factored loads and factored strength. Such a design would achieve the reliability 

implicit in a given design code. The utilization ratios for this design are significantly less than 

1.0 (the largest utilization is 0.53). Therefore, this structure will generate a very high level of 

reliability (or very high beta values, presented later). 

These results indicate very clearly that the basic properties of the monopile (i.e., base diameter 

and thickness) at its critical section (i.e., the location of maximum bending moment immediately 

below mudline) are controlled by the resonance avoidance criteria. The minimum section 

properties required to provide adequate strength for the 50- or 100-year design conditions were 

significantly less than those required to achieve a maximum first mode vibration period of 4 

seconds. Thus, this assessment was applicable for both the IEC and API conditions. 

5.4 Capacity Analysis

A series of structural capacity analyses were performed using CAP software to determine the 

total capacity of the monopile design. The capacities were determined for each pair (each design 

storm) of base shear and base OTM separately. These capacities were then input to the reliability 

analysis. 
39 

The monopile was modeled in CAP to establish the effect of the soil-pile response and to 

determine the location of maximum pile bending below mudline. CAP includes an explicit 

representation of both material and geometric nonlinear behavior. 

In the capacity analysis, the lateral load on the monopile is increased incrementally until the

structure reaches its ultimate point. Figure 24 shows the results from the capacity analysis with 

the API extreme load case as an example. As the lateral load is increased, the elements below 

mudline are forced beyond their initial yielding level. The lateral capacity was assumed to be 

reached when the first full plastic hinge developed in the monopile below mudline. Table 9

presents the monopile capacity for each load case. 

Figure 24. Results of a typical capacity analysis. 

(Left: Load-displacement curve; Right: deflected shape with nonlinear events.) 

Table 9. Lateral Load and Overturning Capacity for the Monopile

Load Case

Capacity Capacity/Demand Ratio

Base 

Shear (kN)

Base OTM 

(MN-m)

Base 

Shear Base OTM

Power Production 11,500 586 4.5 4.5

Parked/Idling (50-yr Extreme / 

Wave-driven ) 23,100 508 5.58 5.58

Parked/Idling (50-yr Extreme/ 

Wind-driven) 19,600 537 5.48 5.48

Parked/Idling (100-yr 

Extreme) 23,500 505 4.47 4.47

The ratios of the capacity-to-design-load, which are far greater than 1, provide a clear indication 

of the reliability implicit in the design. If the ratio of the capacity to design load is greater than 

the “net” safety factor, then the design would result in a higher safety factor than suggested by 

the design guide. The ratio of capacity to design load is about 4.5 for the operating case, about 

5.5 for the 50-year cases, and about 4.5 for the 100-year case. 

0

5000

10000

15000

20000

25000

30000

0 1 2 3 4 5

Lateral Displacement at Yaw Bearing (m)

Lateral Force (kN)

First plastic hinging at 23,500 kN

Mean Water Level

First plastic hinge formation 

below mudline
40 

5.5 Monopile Reliability Analysis

The reliability formulation utilized here is adopted from Phase I and is repeated below for 

convenience. 

To calculate the reliability of the monopile, the following simple limit state function was used: 

 G() = R/L – 1 (4) 

When G() is less than zero, failure is assumed to happen in the ultimate strength. The resistance 

or capacity is modeled as a lognormal random variable with a CoV of 15%, and a 5-percentile 

value equal to C (=505 MNm)

The load L is modeled as L=LBW XBW + Ltower XTower + LSlam XSlam + LCorr XCorr

where 

• LBW is the aerodynamic blade load and the hydrodynamic wave load, which is calculated 

using FAST and is a function of wind speed and wave height. 

• XBW is the model uncertainty and variability for the aerodynamic load on the blade and 

the hydrodynamic load from drag and inertia from wave particle kinematics acting on the 

foundation. This factor is assumed to have a lognormal probability distribution with a 

mean value of 1 and CoV of 7% (7% CoV was obtained from the FAST analysis of LBW). 

• Ltower is the wind load on the tower, and is a function of wind speed. 

• XTower is the model uncertainty on the tower wind load and is assumed to be Lognormally 

distributed with a mean of 1 and CoV of 5%. 

• LSlam is the wave slam load, if any, on the structure's portion below water, and is a 

function of wave height and total water depth (i.e., surge height plus water depth). 

• XSlam is the model uncertainty for slam loads and is assumed to be lognormally 

distributed with a mean of 1 and CoV of 20%. Note this CoV is relatively large to 

represent the uncertainty associated with the breaking wave forces on large-diameter 

structures.

• LCorr is the additional load associated with the Stream function correction, and is a 

function of wave height and total water depth. 

• XCorr is the model uncertainty for the load correction LCorr. This factor is assumed to be 

lognormally distributed with a mean of 1 and CoV of 5% 

In this formulation, the metocean parameters that drive the load calculations are the wind speed 

and significant wave height for the annual maximum storm condition. The significant wave 

height implicitly defines the zero-crossing wave period, current velocity, and the surge height 

experienced for different storm severities (return periods). The wind speed and significant wave 

height are in turn correlated for the site. The metocean data at the site indicate that the wind 

speed (Ws) and significant wave height (Hs) have a correlation coefficient of 84%. In the 

reliability analysis, Ws and Hs are each modeled using the Gumbel distribution for tropical 

storms as found from the site-specific data for this site, and the correlation between Ws and Hs is 
41 

modeled as 84%. The alternate approach would be to model either Ws or Hs as an independent 

distribution and the second parameter as conditional on the first distribution of the wave. This 

approach was not used because of limitations on the probability distribution of the conditioned 

parameter. Also, generally for fixed offshore platforms, Hs is selected as the independent 

variable and Ws is chosen as the conditioned variable. However, for OWTs, Ws may be far more 

critical and thus should be used as the independent variable, while Hs may be critical when slam 

loads come into effect for extreme storms. Therefore, the correlation coefficient approach was 

adopted to give equal weight to Ws and Hs in the reliability calculation.

The variation of OTM with Ws and Hs is shown in the figures below. In the analysis performed,

the aerodynamic load on the blade and the hydrodynamic load on the monopile are analyzed in 

FAST using simulated wind time-histories and a Stream nonlinear wave-elevation profile. The 

data provide average values of the maximum load (mudline OTM) for each simulated case. The 

load that is most relevant to the reliability analysis is the annual mean maximum OTM at 

mudline. 

For wind speeds less than about 19 m/s at a 10-m height (i.e., cut-out wind speed of 25 m/s at a 

hub height of 90 m) the turbine is operating, so the aerodynamic loads are high. As the wind 

speed becomes greater than 19 m/s, the turbine is assumed to be in parked mode, where the 

blades are “feathered” into the wind to reduce wind forces on blades. The wind load drops 

considerably as wind speed goes from an operating range (< 25 m/s at a 90-m height) to a parked 

 0

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 160000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Significant Wave Height, Hs = 2m

L_Blade_Wave

L_Tower

L_Total

Figure 23. Mudline overturning moment (OTM) versus wind speed for the 

smallest storm analyzed.

(There is no breaking wave phenomenon for this small storm.)
42 

range (> 25 m/s at a 90-m height). Beyond wind speeds of 25 m/s, the load gradually increases 

with increasing wind speed. 

Figure 26. Mudline OTM vs. wind speed for the largest storm analyzed. 

(Includes breaking wave effect.) 

The preceding figure shows the contribution from different effects on the total load imposed on 

the structure. Note that the slamming load is in a similar order of magnitude as the wind and 

wave load on the structure.

 0

 100000

 200000

 300000

 400000

 500000

 600000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Significant Wave Height, Hs = 15.94m

L_Blade_Wave

L_Tower

L_Slam

L_Corr

L_Total
Offshore MA (blade load and drag-inertia wave load) 

 300000

 250000

 200000

 150000

 100000

 50000

 0

Wind Speed (m/s) (1 hr. avg. at 10m height) 

Figure 27. Loads on blade and wave loads only as a function of Ws, shown for all Hs values 

analyzed 

In Figure 27, the uppermost three cases for Hs 12.74 m (1000-year return period) and greater 

show much higher loads compared to smaller Hs values. These cases show prominent shallowwater wave kinematics for the larger storms. Note that the breaking wave effect is not yet 

included in these loads (these are included later). Mudline OTM (kNm) 

Hs = 2m 

Hs = 3.99m 

Hs = 5.79m 

Hs = 8.50m 

Hs = 9.50m 

Hs = 12.74m 

Hs = 13.70m 

Hs = 15.94m 

0 10 20 30 40 50 60 70
 

43 
Offshore MA (blade load and drag-inertia wave load) 

0

 50000

 100000

 150000

 200000

 250000

 300000

Mudline OTM (kNm) 

Ws= 9.2 m/s 

Ws=12.65m/s 

Ws=15.55m/s 

Ws=21.64m/s 

Ws=35.15m/s 

Ws=39.02m/s 

Ws=51.61m/s 

Ws=55.37m/s 

Ws=64.12m/s 

0 2 4 6 8 10 12 14

Significant wave height, Hs (m) 

Figure 28. Loads on blade and wave load as a function of Hs 

Figure 28 is similar to the previous figure, except the metocean parameters are shown differently 

now. The variation of the load can be seen as a function of Hs for the different wind speeds 

analyzed. 

44 

 16 
Offshore MA (Total OTM, incl all effects) 

 600000

 500000

 400000

 300000

 200000

 100000

 0

Wind Speed (m/s) (1 hr. avg. at 10m height) 

Figure 29. Total loads (including all applicable components) vs. wind speed. 

(1-hour average, at 10-m reference height.) 

In Figure 29, the upper set of Hs values result in much higher loads than the smaller Hs set, which 

is primarily due to the occurrence of breaking waves for storms with Hs ≥ 13.70 m. Now the 

slam load contribution from the breaking wave is included in the large storms, in addition to the 

wave kinematics from shallow-water depth effects. Mudline OTM (kNm) 

Hs = 2m 

Hs = 3.99m 

Hs = 5.79m 

Hs = 8.50m 

Hs = 9.50m 

Hs = 12.74m 

Hs = 13.70m 

Hs = 15.94m 

0 10 20 30 40 50 60 70 

45 
Offshore MA (Total OTM, incl all effects) 

Mudline OTM (kNm) 

 600000

 500000

 400000

 300000

 200000

 100000

 0

 0 2 4 6 8 10 12 14 16 

Ws= 9.2 m/s 

Ws=12.65m/s 

Ws=15.55m/s 

Ws=21.64m/s 

Ws=35.15m/s 

Ws=39.02m/s 

Ws=51.61m/s 

Ws=55.37m/s 

Ws=64.12m/s 

Significant wave height, Hs (m) 

Figure 30. Mudline overturning moment (including all load components) of monopile as a function 

of Hs 

Once the breaking wave height (Hs ≥ 13.7 m) has been reached, the loads do not increase 

significantly because of the similarity of slam loads (Figure 30). 

Figure 31 and Figure 32 present the capacity and total load, in terms of mudline overturning 

moment. The first figure shows how these values vary with an annual maximum 1-hour average 

wind speed. The second figure provides variation with an annual maximum significant wave 

height. The capacity of the system is different for the operating and parked conditions because of 

the different proportions of wave and wind load. This difference causes the plastic hinge to form 

(see Figure 24) at different elevations along the pile. The capacity is substantially greater than 

the loads at the 1,000-year level. The loads are higher than some of the likely Hs and wind speed 

combinations at the 10,000-year return period as well. Additionally, as noted earlier, the 

capacity-to-demand ratio indicates a very high reliability index for this design. This reliability is 

expected, given the small utilization ratios and large reserve-strength ratios for the monopile. 

The 19 m/s value of the 1-hour average at 10 m translates to the 25 m/s cut-out speed for 10-min 

average at 90m (hub height). Employing the reliability formulation with the loads and capacities 

shown in Figure 32, along with their uncertainty values, the reliability index comes to 5.83, 

which corresponds to an annual failure probability of 2.79×10-9. The ultimate strength-limit state 

does not govern the design of the monopile. The monopile design is controlled by the resonance 

avoidance requirement; therefore, the capacity far exceeds extreme loads for this monopile.

46 
Offshore MA (Total OTM, incl all effects) 

0

 100000

 200000

 300000

 400000

 500000

 600000

Mudline OTM (kNm) 

Failure Mechanism 1 

Failure Mechanism 2 

Hs = 2m 

Hs = 3.99m 

Hs = 5.79m 

Hs = 8.50m 

Hs = 9.50m 

Hs = 12.74m 

Hs = 13.70m 

Hs = 15.94m 

Capacity

 0 10 20 30 40 50 60 70 

Wind Speed (m/s) (1 hr. avg. at 10m height) 

Figure 31. Total load and capacity for monopile as a function of wind speed. 

(Failure mechanism is different for operating vs. parked modes.) 

Offshore MA (Total OTM, incl all effects) 

0

 100000

 200000

 300000

 400000

 500000

 600000

Mudline OTM (kNm) 

Failure Mechanism 1 

Failure Mechanism 2 

Ws= 9.2 m/s 

Ws=12.65m/s 

Ws=15.55m/s 

Ws=21.64m/s 

Ws=35.15m/s 

Ws=39.02m/s 

Ws=51.61m/s 

Ws=55.37m/s 

Ws=64.12m/s 

Capacity for Ws<19m/s 

Capacity for Ws>19m/s 

0 2 4 6 8 10 12 14

Significant wave height, Hs (m) 

Figure 32. Total load and capacity for monopile as a function of significant wave height

47 

 16 
48 

6 Jacket Substructure Analysis

6.1 Design Requirements for Resonance Avoidance

As was the case with the monopile, the initial configuration of the jacket was developed to 

achieve the maximum 4-second period of vibration. The resonance avoidance requirement was 

initially assumed to control the design of the jacket-type substructure, both in terms of jacket 

elements and piles. Note that later, it was found that some jacket members were overstressed

(utilization ratios were greater than 1.0) due to ultimate strength requirements and these members 

had to be redesigned. The conclusion appears to be that the jacket design was controlled by 

ultimate strength requirements in addition to resonance avoidance. 

6.2 Wind and Wave Load Analyses with FAST

A representation of the jacket was developed as an equivalent monopile for input into the FAST 

program for wind and wave load analysis. In the case of the jacket, the equivalent monopile 

properties that were needed for the FAST analysis required additional effort. An equivalent 

monopile model was developed using the CAP program to properly represent the effect of the 

foundation, additional framing, and the variation of drag and inertia coefficients above and 

below water. The modulus of elasticity of the equivalent monopile model (including the tower) 

was scaled to set the period at 4 seconds. A series of analyses were performed to assess the best 

overall matching properties for an equivalent monopile in FAST in terms of total wave load and 

nacelle displacement. 

Figure 33 shows the effect of damping and the drag/inertia coefficients in the overturning 

moment and the nacelle displacement. The target nacelle displacement and the overturning 

moment time-histories were obtained by explicit CAP analysis. Parametric study for the damping 

values of the tower was performed by FAST analysis to replicate the target time histories. 

Priority was given to match the maximum (or minimum) values obtained from FAST and CAP

analyses. In contrast to the monopile case, one damping value was not sufficient to obtain similar 

response values from CAP and FAST analyses for different wave heights. It was concluded that 

a 20% damping value up to a 100-year wave, a 30% damping value for a 100-year wave, and a 

40% damping value beyond the 100-year wave were good representations of the damping in the 

system. Note again that damping is an overall tuning parameter to permit FAST to mimic the 

dynamic physical effects missing in FAST but available and modeled in CAP. As a result of 

parametric studies mentioned previously, equivalent monopile properties were used in the FAST 

analyses (Table 10). 
49 

Operating Wave

50-Year Wave

100-Year Wave

Figure 33. Effect of damping on jacket structure

-6000

-4000

-2000

0

2000

4000

6000

8000

30 35 40 45 50 55

Base OTM (kN-m)

Time (s)

FAST

Jacket Model (CAP)

-0.04

-0.03

-0.03

-0.02

-0.02

-0.01

-0.01

0.00

30 35 40 45 50 55

Nacelle Displacement (m)

Time (s)

FAST

Jacket Model (CAP)

-20000

-10000

0

10000

20000

30000

40000

50000

30 35 40 45 50 55

Base OTM (kN-m)

Time (s)

FAST

Jacket Model (CAP)

-0.15

-0.10

-0.05

0.00

0.05

0.10

30 35 40 45 50 55

Nacelle Displacement (m)

Time (s)

FAST

Jacket Model (CAP)

-30000

-20000

-10000

0

10000

20000

30000

40000

50000

60000

70000

Base OTM (kN

30 35 40 45 50 55

-m)

Time (s)

FAST

Jacket Model (CAP)

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

30 35 40 45 50 55

Nacelle Displacement (m)

Time (s)

FAST

Jacket Model (CAP)
50 

Table 10. FAST Input Data for Jacket Structure 

Monopile thickness (m) 0.067 

Tower base outer diameter (m) 6 

Tower base wall thickness (m) 0.03

Tower top outer diameter (m) 3.87

Tower top wall thickness (m) 0.02

Steel density (effective) (kg/m3) 8500

Steel Young's modules (E) (MPa) 1.995×105

Steel shear modulus (G) (MPa) 7.676×104

Water Depth, d (m) 25 

Height monopile extends above MSL (m) 10

Length of Tower+Monopile (m) 112.6

Cd

0.55 (<100 year)

0.60(=100 year) 

0.70(>100 year)

Cm 0.2 

Damping

20%(<100 year)

30%(=100 year) 

40%(>100 year)

Structural fore-aft period (s) 3.84

6.3 Total Wind and Wave Load Demand

Figure 34 presents the results of coupled wind and wave analyses with FAST in terms of base 

shear and overturning moment. A detailed discussion on the coupled wave and wind loading is 

provided for the monopile case after Figure 22. The main observation regarding the difference 

between the monopile and jacket responses is that both base shear and over turning moment 

decrease significantly in the case of the jacket-type structure. Base shear decreases almost by 

half because the projected area of the jacket structure is almost half that of the monopile. The 

overturning moment does not decrease by half because there are two components affecting the 

overturning moment: wave loading and wind loading times the height of the structure above the 

mudline. Although the overturning moment caused by wave loading decreases by half for the 

jacket structure compared to monopile, the effect of the wind loading and the height of the 

structure stays constant for both types of structures. Therefore, the decrease in the overturning 

moment is not by half for the jacket-type structure compared to the monopile. Table 11 provides 

the results for the metocean loads used in the design checks. These are nominal loads and do not 

include any load factors.
51 

Figure 34. Coupled wind and wave loads (top-base shear, bottom-overturning moment) for a 

jacket-type structure for blade and hydrodynamic loads

2.00

3.99

5.79

8.50

9.50

12.74

13.70

15.94

0

1,000

2,000

3,000

4,000

9.20 12.65 15.55 21.64 35.15 39.02 51.61 55.37 64.12

Hs (m)

Base Shear (kN)

Ws (m/sec)

(1hr-10 m)

2.00

3.99

5.79

8.50

9.50

12.74

13.70

15.94

0

50,000

100,000

150,000

200,000

9.20 12.65 15.55 21.64 35.15 39.02 51.61 55.37 64.12

Hs (m)

Over Turning Moment (kN-m)

Ws (m/sec)

(1hr-10 min)
52 

Table 11. Jacket Design Loads at Mudline Section

Base Shear (kN)

Storm Type

Coupled 

WindWave

Tower 

Wind Total*

Operating Storm 1,266 7.26 1,273

50-year Storm (Wind Driven) 1,632 117 1,749

50-year Storm (Wave Driven) 2,206 90 2,296

100-year Storm (Wind Driven) 2,043 145 2,188

100-year Storm (Wave Driven) 2,536 112 2,648

Mudline Overturning Moment (MN-m)

Storm Type

Coupled 

WindWave

Tower 

Wind Totala 

Operating Storm 107 0.51 107

50-year Storm (Wind Driven) 53 8.66 61

50-year Storm (Wave Driven) 61 6.64 68

100-year Storm (Wind Driven) 67 10.8 78

100-year Storm (Wave Driven) 75 8.3 83

 * Note: Breaking waves occur in storms larger than a 100-year storm

6.4 Strength Checks

Each unfactored base shear and overturning moment pair (at mudline) from Table 11 was

converted into a shear-moment at the top of the jacket and applied to the jacket model in CAP 

(Figure 35). The analyses were repeated for three wave heading directions (0o, 90o, and 180o

clockwise from north). In each analysis, the maxima of the axial load, shear, and bending 

moment demand on the main member types (central column, piles, legs, and braces) were 

obtained. The design checks showed that the preliminary design dimensions were sufficient 

according to both IEC and API. 

Table 12 and Table 13 show the final member dimensions and the factored loads according to 

IEC and API, respectively. The maximum member utilization ratios are summarized for the API 

and IEC jacket designs in Table 14. In all of the cases, the pile elements were the ones to be most 

utilized. 
53 

 

Figure 35. Jacket model in CAP. (Illustration from MMI Engineering)

Table 12. Jacket Design Loads per IEC/ISO

Jacket Design 

Checks per IEC/ISO

Diameter 

(m)

Thickness 

(m)

Operating Load Case (Factored Loads) 

Axial 

Load 

(kN)

Bending 

Moment 

(kNm)

Shear, 

V (kN)

Utilization 

Ratio 

(N+M) 

Utilization 

Ratio (V)

Vertical Brace 0.4 0.02 254 131 29 0.241 0.018

Horizontal Brace 0.3 0.01 134 18 6 0.243 0.01

Leg (below El.-16m) 1 0.05 11,939 4,677 920 0.813 0.09

Leg (above El. -16m) 0.9 0.045 12,485 2,244 430 0.774 0.052

Pile (below El.-31m) 1.2 0.04 9,514 871 173 0.376 0.017

Pile (above El.-31m) 1.2 0.045 11,397 5,450 1,298 0.711 0.116

Tower

Jacket

Piles and 

Soil Springs

V

M’

V

M

M’-V pair at top-of-jacket 

elevation 

(applied to CAP model)

M-V pair at mudline

(coupled analysis 

results after correction)
54 

Jacket Design 

Checks per IEC/ISO

Diameter 

(m)

Thickness 

(m)

50-Year Extreme Environmental Load Case (Factored 

Loads) 

Axial 

Load 

(kN)

Bending 

Moment 

(kNm)

Shear 

(kN)

Utilization 

Ratio 

(N+M) 

Utilization 

Ratio (V)

Vertical Brace 0.4 0.02 612 82 15 0.247 0.009

Horizontal Brace 0.3 0.01 190 19 8 0.318 0.012

Leg (below El.-16m) 1 0.05 8,273 2,037 391 0.451 0.038

Leg (above El. -16m) 0.9 0.045 5,241 1,071 204 0.341 0.025

Pile (below El.-31m) 1.2 0.04 11,397 5,450 1,298 0.363 0.01

Pile (above El.-31m) 1.2 0.045 8,676 6,530 1,816 0.707 0.162

Table 13. Jacket Design Loads per API

Jacket Design 

Checks per API 

Diameter 

(m)

Thickness 

(m)

Operating Load Case

Axial 

Load 

(kN)

Bending 

Moment 

(kNm)

Shear 

(kN)

Utilization 

Ratio 

(N+M) 

Utilization 

Ratio (V)

Vertical Brace 0.4 0.02 170 103 22 0.308 0.018

Horizontal Brace 0.3 0.01 110 16 7 0.327 0.015

Leg (below El.-16m) 1 0.05 9,272 3,936 715 0.989 0.096

Leg (above El. -16m) 0.9 0.045 9,607 1,754 336 0.91 0.056

Pile (below El.-31m) 1.2 0.04 7,425 662 138 0.426 0.019

Pile (above El.-31m) 1.2 0.045 8,887 4,262 1,006 0.863 0.123

Jacket Design 

Checks per API 

Diameter 

(m)

Thickness 

(m)

50-Year Extreme Environmental Load Case

Axial 

Load 

(kN)

Bending 

Moment 

(kNm)

Shear 

(kN)

Utilization 

Ratio 

(N+M) 

Utilization 

Ratio (V)

Vertical Brace 0.4 0.02 552 71 13 0.269 0.008

Horizontal Brace 0.3 0.01 171 17 7 0.346 0.012

Leg (below El.-16m) 1 0.05 7,474 1,854 356 0.471 0.036

Leg (above El.-16m) 0.9 0.045 4,560 942 178 0.343 0.022

Pile (below El.-31m) 1.2 0.04 6,566 1,483 88 0.371 0.009

Pile (above El.-31m) 1.2 0.045 7,853 5,897 1,639 0.761 0.151
55 

Table 14. Utilization Ratio Summary for Jacket

Section

Dimensions

API IEC/ISO

Power 

Production a Parked/ Idling

Power 

Production b Parked/ Idling

D (m) t (m) N+M V N+M V N+M V N+M V

Vertical Brace 0.4 0.02 0.308 0.018 0.269 0.008 0.241 0.018 0.247 0.009

Horizontal Brace 0.3 0.01 0.327 0.015 0.346 0.012 0.243 0.01 0.318 0.012

Leg (Lower) 1 0.05 0.989 0.096 0.471 0.036 0.813 0.09 0.451 0.038

Leg (Upper) 0.9 0.045 0.91 0.056 0.343 0.022 0.774 0.052 0.341 0.025

Pile (Lower) 1.2 0.04 0.426 0.019 0.371 0.009 0.376 0.017 0.363 0.01

Pile (Upper) 1.2 0.045 0.863 0.123 0.761 0.151 0.711 0.116 0.707 0.162 a

 Without one-third allowable stresses increase factor b

 With the same load and material factors from parked/idling condition

All utilization ratios are below 1.0, and most ratios are far below 1.0. This implies that factored 

strength is generally much greater than the factored loads in each design code. The largest ratio, 

0.989, is for a leg member in power production condition. When weather severity pushes the 

structure beyond its elastic limit, the legs are mostly likely to be the first members to fail, given 

that the legs undergo the greatest utilization. The reliability indices for both API and IEC can be 

expected to come close to the safety levels implicit in these codes, especially for the legs that

have utilization ratios close to 1. 

6.5 Capacity Analysis

A nonlinear model of a jacket structure was developed using the CAP software and was used for 

capacity analyses. The analyses were repeated for each load case (i.e., operating, 50-year storm, 

100-year storm). The model included soil-pile interaction explicitly. 

Figure 36 summarizes the results of the analysis with the operating load case. As the lateral load 

on the structure is increased, initial yielding was observed at two locations on the compression 

leg and one location on the compression pile (Step 6). The lower spot on the compression leg 

became fully plastic (also referred to as a “plastic hinge”) at Step 12. Note that neither of these 

events caused a significant softening in the load-displacement curve. The structure softens 

because there are other locations along the compression leg reaching the initial yield level. The 

structure was assumed to reach the ultimate point when the compression pile plunged at Step 

198. The lateral load on the structure at this step is the capacity used in the reliability analysis. 

Note that this is a conservative assumption because the structure could have been pushed further 

until the compression leg buckles at Step 238. 

The response of the structure under storm loads (both 50-year and 100-year storms) is slightly 

different. The results of the capacity analysis with the 50-year storm load case are presented as 

an example in Figure 37 (results for the 100-year storm are similar and not shown in this report). 

In these analyses, the nonlinear events were concentrated mostly on the piles. The first plastic 

hinging was observed in the compression pile. This was followed by buckling of one of the 

horizontal braces at the mudline elevation. The structure was assumed to reach its capacity when 

the compression pile buckles. If the analysis is carried out further, the next significant event 
56 

would be the double hinging of the piles. Note that there was no significant increase in the lateral 

resistance after the pile plunging, as shown in the load-displacement plot. 

The capacities based on these analyses are listed in Table 15. 

 

Step 6: First initial Step 12: First Plastic Step 198: Pile Plunging Step 238: Leg Buckling 

yielding Hinging (Leg)

Figure 36. Results of a typical capacity analysis under operating storms

(Bottom: load-displacement curve; Top: deflected shape with nonlinear events at important 

steps.) 

Q-z Spring Response (Under Compression)

0

200

400

600

800

1000

1200

1400

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

Pile Tip Displacement (m)

Pile Tip Resistance (kN)

0

1000

2000

3000

4000

5000

6000

0 0.5 1 1.5 2 2.5 3 3.5 4

Nacelle Displacement (m)

Base Shear (kN)

First initial 

yielding

First plastic 

hinging (leg)

Pile Plunging Leg 

Buckling

Capacity: 4,830 kN

Loading 

Direction
57 

Step 6: First Step 9: First Plastic Step 14: Horizontal Step 32: Pile Plunging Step 37: Pile Double 

Initial yielding Hinging (Pile) Member Buckling Hinging

Figure 37. Results of a typical capacity analysis for a 50-year wave

(Bottom: load-displacement curve; Top: deflected shape with nonlinear events at important 

steps.)

Q-z Spring Response (Under Compression)

0

200

400

600

800

1000

1200

1400

0.00 0.10 0.20 0.30 0.40 0.50

Pile Tip Displacement (m)

Pile Tip Resistance (kN)

0

2000

4000

6000

8000

10000

12000

14000

0 0.5 1 1.5 2 2.5 3 3.5 4

Nacelle Displacement (m)

Base Shear (kN)

First initial yielding

First plastic 

hinging (pile)

Horizontal Capacity: 12,600 kN

member buckling

Double plastic 

hinging (pile)

Pile 

plunging
58 

Table 15. Capacity Analysis Results for Jacket

Load Case

Demand Capacity

Capacity/Demand 

Ratio

Base 

Shear 

(kN)

Base 

OTM 

(MN-m)

Base 

Shear 

(kN)

Base 

OTM 

(MN-m)

Base 

Shear

Base 

OTM

Power Production 1,273 107 4,833 419 3.8 3.92

Parked (50-Year 

Metocean Criteria) 2,296 68 12,644 400 5.51 5.88

Parked (100-Year 

Metocean Criteria) 2,648 83 12,611 400 4.76 4.82

The ratio of the capacity to the design loads provides an initial indication of reliability. Larger 

ratios imply greater levels of reliability for ultimate strength failure. The ratio for operating cases 

is 3.92. The IEC capacity-to-50-year-load ratio is 5.88 and the API capacity-to-100-year load is 

4.82. The ratio of capacity to a reference load is commonly referred to as a reserve strength ratio

(RSR). The API ratio is lower than the IEC ratio for the parked case because of the larger 100-

year load compared to the 50-year load, and the capacity is the same for both designs. The ratio 

of the 100-year load to the 50-year load itself is about 1.15, while the capacities of the two 

designs are essentially the same. This implies that both IEC and API designs could be expected 

to result in levels of reliability that are higher than what is implicit in each code, given the 

relatively large reserve strength ratios (a high reserve strength ratio is expected to imply a high 

reliability, assuming the physics of the load and capacity do not change materially after the 100-

year storm condition). A better indication of the reliability levels achieved in each design can be 

made by comparing the capacities to the same reference load.

6.6 Jacket Reliability Analysis

Figures 38 through 43 show the variation of the total mudline overturning moment with the 

annual maximum wind speed and with the annual maximum significant wave height. A 

comparison of the total load and the resulting capacity for the jacket is provided in Figure 44 and 

Figure 45. 
59 

Figure 38. Mudline overturning moment (OTM) versus wind speed for the smallest storm 

analyzed.

(There is no breaking wave phenomenon for this small storm.)

Figure 39. Mudline OTM vs. wind speed for the largest storm analyzed. 

(Includes breaking wave effect.)

 0

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Significant Wave Height, Hs = 2m

L_Blade_Wave

L_Tower

L_Total

 0

 50000

 100000

 150000

 200000

 250000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Significant Wave Height, Hs = 15.94m

L_Blade_Wave

L_Tower

L_Slam

L_Corr

L_Total
60 

Figure 40. Mudline overturning moment due to aerodynamic wind load and hydrodynamic wave 

loads. 

(No breaking wave effect included yet.) 

 

Figure 41. Loads from aerodynamic and hydrodynamic effects (excluding breaking wave effects) 

vs. Hs

 0

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 160000

 180000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Offshore MA (blade load and drag-inertia wave load)

Hs = 2m

Hs = 3.99m

Hs = 5.79m

Hs = 8.50m

Hs = 9.50m

Hs = 12.74m

Hs = 13.70m

Hs = 15.94m

 0

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 160000

 180000

 0 2 4 6 8 10 12 14 16

Mudline OTM (kNm)

Significant wave height, Hs (m)

Offshore MA (blade load and drag-inertia wave load)

Ws= 9.2 m/s

Ws=12.65m/s

Ws=15.55m/s

Ws=21.64m/s

Ws=35.15m/s

Ws=39.02m/s

Ws=51.61m/s

Ws=55.37m/s

Ws=64.12m/s
61 

Figure 42. Total load (including all effects) vs wind speed for different Hs values 

Figure 43. Total load (including all effects) vs Hs for different wind speed values 

 0

 50000

 100000

 150000

 200000

 250000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Offshore MA (Total OTM, incl all effects)

Hs = 2m

Hs = 3.99m

Hs = 5.79m

Hs = 8.50m

Hs = 9.50m

Hs = 12.74m

Hs = 13.70m

Hs = 15.94m

 0

 50000

 100000

 150000

 200000

 250000

 0 2 4 6 8 10 12 14 16

Mudline OTM (kNm)

Significant wave height, Hs (m)

Offshore MA (Total OTM, incl all effects)

Ws= 9.2 m/s

Ws=12.65m/s

Ws=15.55m/s

Ws=21.64m/s

Ws=35.15m/s

Ws=39.02m/s

Ws=51.61m/s

Ws=55.37m/s

Ws=64.12m/s
62 

Figure 44. Total load and capacity for jacket as a function of wind speed 

These figures are meant to show the variation of loads with different combinations of wind 

speeds and significant wave height. This does not address the likelihood of each Ws and Hs

combination. The design codes use the 50-year and the 100-year storms as the basis for design 

development and these loads are much less than the capacity. The reliability index calculation11

includes a range of Hs up to the 10,000-year value of 15.94 m. Note that the ultimate strength 

capacity of the structure is much larger than the loads, either in the operating or parked 

conditions for the blades. 

 

11 As additional insight, the pair that corresponds to the 1000-year Ws (60m/s) and the 1000-year Hs (13.13m) is 

likely to occur in a storm with a return period larger than 1000 years. This is due to the correlation between Ws and 

Hs which is less than 1. A correlation of 1 (100%) would result in the 1000-year storm having the 1000-year Ws 

value (independent of Hs) and the 1000-year Hs value (independent of Ws). A correlation less than 1 would result in 

a 1000-year storm having either Ws less than the 1000-year Ws or Hs less than the 1000-year Hs. 

 0

 50000

 100000

 150000

 200000

 250000

 300000

 350000

 400000

 450000

 0 10 20 30 40 50 60 70

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Offshore MA (Total OTM, incl all effects)

Failure Mechanism 1

Failure Mechanism 2

Hs = 2m

Hs = 3.99m

Hs = 5.79m

Hs = 8.50m

Hs = 9.50m

Hs = 12.74m

Hs = 13.70m

Hs = 15.94m

Capacity
63 

Figure 45. Total load and capacity of jacket as a function of Hs

The reliability index is 5.45, which corresponds to an annual failure probability of 2.51×10-8. The 

ultimate strength limit state does not govern the design of the jacket. The jacket design is 

controlled by the resonance avoidance requirement; and the capacity far exceeds extreme loads 

for this jacket. 

 

 0

 50000

 100000

 150000

 200000

 250000

 300000

 350000

 400000

 450000

 0 2 4 6 8 10 12 14 16

Mudline OTM (kNm)

Significant wave height, Hs (m)

Offshore MA (Total OTM, incl all effects)

Failure Mechanism 1

Failure Mechanism 2

Ws= 9.2 m/s

Ws=12.65m/s

Ws=15.55m/s

Ws=21.64m/s

Ws=35.15m/s

Ws=39.02m/s

Ws=51.61m/s

Ws=55.37m/s

Ws=64.12m/s

Capacity for Ws<19m/s

Capacity for Ws>19m/s
64 

7 Comparison of Monopile and Jacket 

This section presents some key comparisons of monopile to jacket results for the site analyzed at 

a water depth of 25 m, and a comparison of the monopile at the 15-m water depth from Phase I. 

7.1 Monopile Results, 25-m versus 15-m water depth 

Figure 46 compares the monopile total mudline overturning moments for the 25-m water depth 

case with the 15-m water depth case analyzed in Phase I. The 25-m results are shown for wind 

speed and moment ranges in the exhibit (a) to match those in exhibit (b) to permit direct 

comparison of the loads across similar Hs values. The following observations can be made from 

this figure: 

• For a 2-m Hs storm, there are no breaking waves in either storm and the 25-m case results 

in slightly larger loads than the 15-m case due largely to the increased lever arm for the 

same aerodynamic loads on the blades in the two cases. 

• Notice that for the 25-m depth case, the loads increase substantially in going from Hs 

9.5 m to 12.74 m, the onset of the breaking condition for the 25-m depth case. On the 

other hand, the onset of breaking happens earlier for the 15-m depth case (shallower 

depth causes breaking for smaller waves), as seen in the increase of loads going from Hs

of 6.69 m to 9.5 m. 

• For the same Hs value of 9.5 m, focus is on the loads around 52 m/s wind speed. The 

15-m depth case indicates a larger OTM of 190 kN-m, compared to an OTM of 

140 kN-m for the 25-m depth case. This larger OTM indicates the dominant effect of a 

breaking wave on the total overturning moment. (The breaking wave is present for the 

15-m case, and absent for the 25-m case). 

 
65 

(a) Monopile at 25-m depth site (range shown corresponding to plot below, for convenience) 

(b) Monopile at 15-m depth site 

Figure 46. Monopile loads for 15-m and 25-m depth as a function of wind speed and significant 

wave height 

Table 16 and Table 17 compare the specific design loads and the utilization ratio for the design 

loads for ultimate strength for the two water depth cases. Again, note that the design loads are 

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 160000

 180000

 200000

 220000

 5 10 15 20 25 30 35 40 45 50 55

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Offshore MA (Total OTM, incl all effects)

Hs = 2m

Hs = 3.99m

Hs = 5.79m

Hs = 8.50m

Hs = 9.50m

Hs = 12.74m

Hs = 13.70m

Hs = 15.94m

 20000

 40000

 60000

 80000

 100000

 120000

 140000

 160000

 180000

 200000

 220000

 5 10 15 20 25 30 35 40 45 50 55

Mudline OTM (kNm)

Wind Speed (m/s) (1 hr. avg. at 10m height)

Offshore MA (Total OTM, incl all effects)

Hs = 2m

Hs = 3.99m

Hs = 4.88m

Hs = 5.79m

Hs = 6.69m

Hs = 9.5m

Hs = 12.74m
66 

lower for the 25-m depth case for the 50-year and 100-year storms because of the absence of 

breaking loads for the 25-m case. The operating loads (about a 1-year storm) for the 25-m depth 

case are larger due to the larger level arm of the deeper case; note that the base shear is similar in 

magnitude because the hub height remains unchanged and the minor difference can be attributed 

to nominally larger hydrodynamic loads for the deeper water case. The utilization ratios are 

similar in the two depth cases, and this confirms the observation that the monopile design is 

largely driven by resonance avoidance and results in higher-than-needed ultimate strength per 

design loads. 

Table 16. Design Load Comparison for Monopile in 25-m Depth to 15-m Depth (from Phase I) 

Storm Type 

Monopile 15-m water depth Monopile 25-m water depth 

Base Shear (kN) 

Mudline 

Overturning 

Moment (MN-m) 

Base Shear (kN) 

Mudline 

Overturning 

Moment (MN-m) 

Operating 2,450 113 2,549 130 

50-year 6,740 142 4,136 99 

100-year 6,910 153 5,264 112 

Table 17. Utilization Ratio Comparison for Monopile in 25-m Depth to 15-m Depth (from Phase I) 

Based on API RP2A Based on IEC / ISO 

Power 

Productiona Parked/Idling Power 

Productionb Parked / Idling 

Monopile at 15-m 

water depth 0.570 0.605 0.433 0.595 

Monopile at 25-m 

water depth 0.529 0.395 0.402 0.343 

a

 Without one-third allowable stress increase factor 

b

 With the same load and material factors from parked/idling condition 

7.2 Jacket versus Monopile at 25-m water depth site 

The next two figures, 47 and 48, show the mudline overturning moments for jackets (denoted 

“Jkt”) compared to monopile (denoted “Mp”) for a select set of return periods. The following 

observations can be made from these exhibits: 

• The jacket loads are substantially smaller than the monopile loads for each return period. 

• The 100-year jacket load is similar to the 1-year monopile loads, meaning again that 

jacket loads are much smaller than the monopile loads. 

• The effect of shallow-water breaking waves appears much more pronounced for the 

monopile compared to the jacket. (Compare the large increase in the monopile load from 

Hs of 9.5 m to 13.7 m versus that of the jacket.) 
67 

Figure 47. Jacket vs. monopile loads for 25-m water depth for key return periods as a function of 

wind speed 

 0

 100000

 200000

 300000

 400000

 500000

 600000

 0 10 20 30 40 50 60 70

Wind Speed (m/s) (1 hr. avg. at 10m height)

Jacket vs. Monopile: Offshore MA (Total OTM, incl all effects)

Jkt: Hs = 3.99m

Jkt: Hs = 8.50m

Jkt: Hs = 9.50m

Jkt: Hs = 13.70m

Mp: Hs = 3.99m

Mp: Hs = 8.50m

Mp: Hs = 9.50m

Mp: Hs = 13.70m

1yr

50yr

100yr

2000yr
68 

Figure 48. Jacket vs. monopile loads for 25-m water depth for key return periods as a function of 

significant wave height 

7.3 Overall Comparison 

Table 18. Comparison of reliability index of monopile to jacket 

Monopile Jacket

Controlling Condition Fatigue Fatigue and strength 

Utilization Ratio 0.5 (operating case/storm) 

0.4 (parked case, extreme storm) 

1.0 (operating case/storm) 

0.8 (parked case, extreme storm) 

Reliability Index, β 5.8 5.4 

Conclusion The monopile design likely cannot

be optimized much due to fatigue 

resonance constraint 

The jacket design can likely be 

optimized further to achieve β=4 (or 

an appropriate value) and still avoid 

resonance 

 

 0

 50000

 100000

 150000

 200000

 250000

 300000

 350000

 400000

 450000

 500000

 550000

 0 2 4 6 8 10 12 14 16

Significant wave height, Hs (m)

Jacket vs. Monopile: Offshore MA (Total OTM, incl all effects)

Jkt: Ws=21.64m/s

Jkt: Ws=35.15m/s

Jkt: Ws=39.02m/s

Jkt: Ws=55.37m/s

MP: Ws=21.64m/s

MP: Ws=35.15m/s

MP: Ws=39.02m/s

MP: Ws=55.37m/s

1yr

50yr

100yr

2000yr
69 

8 Summary and Conclusions This study has included a comparison of API RP-2A (the standard used by BSEE for the 

regulation of offshore structures in OCS waters) and the IEC 61400-3 (a new standard developed 

by an international committee specifically to address the design requirements for OWT support 

structures). This review has included a direct comparison of the standards in order to provide an 

assessment of their applicability to the design of typical wind turbine support structures in U.S. 

OCS waters. 

8.1 Key Findings

8.1.1 Effect of Breaking Waves

The shallow-water wave effects related to wave breaking occur in a much more severe storm for 

25-m water depth compared to the 15-m depth. For the 25-m depth, wave breaking effects appear 

in a 1000-year storm versus wave breaking seen for only 25-year storms for the 15-m depth (in 

Phase I). At the 15-m depth site, wave breaking occurs in smaller storms compared to the 25-m 

site; owing to this effect, slam loads occur in smaller storms at the 15-m depth than at the 25-m 

depth. In these smaller storms, then, the total loads end up being larger at the 15-m depth than at 

the 25-m depth, implying the criticality of shallow-water effects on load calculation and the 

effect of water depth on the onset of wave breaking. 

8.1.2 Effect of Change in Water Depth on Monopiles 

For the 25-m depth site, a longer pile penetration is generally needed to achieve resonance 

avoidance for the soil profile assumed (60 m for a 25-m water depth, versus 50 m for a 15-m 

depth in Phase I12). Additionally, a larger-diameter pile, was adopted for resonance avoidance

(6.5 m for the 25-m depth, versus 6 m in Phase I). For smaller storms, the loads are similar for 

the two water depths, ignoring the difference in diameters for the monopiles used in the two 

phases. The design storm loads (i.e., for operating storms and for 50- and 100-year storms) have 

been reduced for the monopile because of the increase in water depth and the delay in the onset 

of breaking waves to storms with a return period 1000 years or greater. As compared to the phase 

I results, the 25-m monopile design was somewhat larger in diameter (6.5 m versus 6 m) to 

provide the stiffness required for resonance avoidance. The storm and operation wave load 

demands have been reduced from that of the more shallow-water case study because of the 

elimination of the breaking wave condition for the design event and because of some reduction 

in wave particle velocity for deeper water.

8.1.3 Effect of Structure Type on Loads, Design, and Reliability 

The design and extreme loads on the jacket structure are much less than on the monopile because 

of the jacket's more transparent profile. The primary difference between the jacket and monopile 

is in the load transfer mechanism between the structure and foundation. The jacket obtains its 

foundation fixity through compression and tension; the monopile achieves fixity through bending 

and soil bearing. In the storms with breaking waves, the jacket sees much smaller slam loads, 

again owing to its more transparent profile. The monopile design is predominantly constrained 

by resonance avoidance and cannot be optimized much for ultimate strength considerations (i.e., 

 

12 Note that these pile penetration lengths are indicative of the design selected, and do not necessarily represent 

optimized designs in which the penetration length is optimized for minimal cost, for example.
70 

the monopile ends up with high ultimate strength versus design loads). The jacket, however, can 

be optimized for both resonance avoidance and for ultimate strength to achieve a more 

“balanced” design for both of these design criteria. 

8.2 Recommendations

The results of this study, complemented with the experience from the oil and gas industry, lead 

to the following recommendations for further study. 

• Develop a comprehensive tool for use by the offshore wind industry to perform a coupled 

aerodynamic and hydrodynamic load analysis, including the effects of soil structure 

interaction and breaking wave effects (strength and fatigue). 

• Develop recommendations for OWTs using a large-diameter monopile foundation for 

appropriate development of drag and inertia forces from hydrodynamic loads, for 

estimation of loads from breaking waves, and for appropriately characterizing the soilstructure interaction to include large-diameter effects.

• Quantify uncertainties associated with large pile/column effects (load and soil-pile 

interaction) for inclusion in an overall safety-level assessment. 

• Study fatigue reliability for OWTs, especially from loads resulting from breaking waves 

for sites with shallow waters. 

• Include fatigue reliability over the design life of the structure and ultimate strength 

reliability as part of a systems reliability assessment of OWTs. 

• Develop an understanding of the implications of structural frequencies approaching blade 

frequency and other nacelle-related frequencies to be avoided. Assess the implications of 

uncertainty in installed OWT frequencies on structure dynamic behavior and the resulting 

change in safety levels.

• Study the effect of variation of turbine size for design loads and fatigue and ultimate 

strength to understand consequent effects on system reliability.

• Develop an appropriate safety level (reliability index) that would be acceptable by 

regulatory agencies and the offshore wind industry for OWTs in U.S. waters. This can be 

done via specification of safety factors in the design guidelines that would apply to U.S. 

waters.

These recommendations are intended to be used to develop a comprehensive view of all of the 

engineering issues involved in OWT design, and then to develop a robust design guideline 

applicable to U.S. waters.
71 

References

1. Jonkman, J., Dynamics Modeling and Loads Analysis of an Offshore Floating Turbine, 

National Renewable Energy Laboratory, Technical Report NREL/TP-500-41958, November 

2007. 

2. Argyriadis, K., Gill, L. and Schwartz, S., “Loads for Offshore Wind Turbines; the 2nd edition 

of the GL Wind Guideline”, 2004. 

3. International Electrotechnical Commission (IEC), IEC 61400-3 Ed.1, Wind Turbines – Part 

3: Design requirements of offshore wind turbines, IEC TC 88 WG3 Committee Draft, 

December 2005. 

4. American Petroleum Institute (API), Recommended Practice for Planning, Designing and 

Constructing Fixed Offshore Platforms-Working Stress Design, API RP-2A-WSD, 21st

Edition 2000. 

5. Comparative Study of OWTG Standards (prepared for JIP sponsorship), MMI Engineering, 

March 2009. 

6. A.K. Jha, W. Musial, C. Smith, D. Dolan. “On Hurricane Risk to Offshore Wind Turbines in 

US Waters”. Paper No. 20811. Offshore Technology Conference, Houston, Texas, 3-6 May, 

2010. 
72 

Appendix A: Soil Springs in the Analyses

The shear strength profile used in this phase of the study is shown below. The profile belongs to 

a clay site where the shear strength linearly increases from 12 kPa to 120 kPa in the top 4-m 

portion and then remains constant below that. This profile was adopted from an earlier MMI 

project. Note that this is the same as the profile used in the earlier analyses. 

The monopile and the jacket models developed for this study include soil springs to properly 

capture the soil-structure interaction. There are two types of soil springs in the models: (1) lateral 

springs, commonly known as p-y springs; (2) vertical springs. These springs were distributed 

along the pile as shown below. The vertical springs along the pile are called t-z springs, whereas 

the spring at the bottom tip is called a q-z spring. 

0

10

20

30

40

50

60

70

80

0 50 100 150

Undrained Shear Strength (kPa)

Depth Below Mudline (m)
73 

The properties of the soil springs were obtained using the equations provided in API RP2A 21st

ed. The force-displacement curves of the top three lateral (p-y) and vertical (t-z) springs are 

provided below. Given the soil profile is uniform below 4 m under mudline, the spring properties 

also remain constant in that section. 

MWL

Mudline

Monopile

Tower

Lumped mass for 

nacelle, hub & blades

El. 0m

El. -25m

El. -85m

Soil 

Springs

El. -25m

El. -85m

0

200

400

600

800

1000

1200

0 0.2 0.4 0.6 0.8

y/D

p (kPa)

Mudline

2m from mudline

4m from mudline and below

0

20

40

60

80

100

120

140

0 0.01 0.02 0.03 0.04 0.05 0.06

z/D

t (kPa)

Mudline

2m from mudline

4m from mudline and below
74 

The tip spring (q-z) is shown next. Note that the vertical axis is expressed in stress units in the t-z 

plots, whereas it is expressed in force units in the q-z plot. 

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

0 0.1 0.2 0.3 0.4 0.5 0.6

z/D

q (kN)

Pile tip
75 

Appendix B: Drag and Inertia Coefficients for Large 

Diameter Turbine Components

The calculations below are performed using the approach outlined in API RP2A Section 

C2.3.1.b7. 

Monopile diameter= 6.5 m 

Assumptions: 

• 25-m water depth

• 1.1-m surge 

• 15-m wave height with 11 s period 

• 0.4-m/s uniform current 

• Above high-tide level (smooth), average peak-to-valley height of hard growth, 

k = 0.05 mm; relative surface roughness, e = k/D = 0.05E-3 m/ 6.5 m = 8E-6 

Maximum water particle velocity normal to the cylinder axis, Um = 10.2 m/s

Wave period, T= 11s 

Keulegan-Carpenter Number, K= UmT/D = 10.2 x 11 / 6.5 = 17.3

Below high-tide level (rough surface drag and inertia):

Cds = 1.0 

K/Cds = 17.3 / 1.0 = 17.3  Cd/Cds = 1.4 (Figure C2.3.1-5) 

Cd = 1.4 x 1.0 = 1.4 

K/Cds = 17.3  Cm = 1.2 (Figure C2.3.1-8, “rough” curve) 

Above high-tide level (smooth):

e=8E-6  Cds = 0.64 (Figure C2.3.1-4) 

K/Cds = 17.3/ 0.64 = 27.0  Cd/Cds = 1.2 (Figure C2.3.1-5) 

Cd = 1.2 x 0.64 = 0.77 

K/Cds = 27.0  Cm = 1.6 (Figure C2.3.1-8, “smooth” curve) 

Summary: 

Rough Smooth

Cd 1.4 0.77

Cm 1.2 1.6
76 

Equivalent Monopile Properties for FAST Analyses: 

A wave analysis (with 10-m wave with 10-s period) was performed in CAP using a model of a 

6-m monopile with the actual Cd/Cm values calculated, shown previously, per API RP2A. The 

total base shear and the overturning moment histories are shown in the figures below as a target. 

Then the wave analysis was repeated for the cases of “Cd=1.0; Cm= 0” and “Cd= 0; Cm=1.0” . 

The results of these analyses are shown in the plots with the blue and the pink curves. The 

equivalent monopile with uniform properties was estimated to match the base shear and 

overturning moment histories. The unit Cd and the unit Cm analyses were added to each other in 

various proportions until the total overturning moment matched the target curve. The best result 

was obtained for the 1.03Cd + 1.30Cm case (shown in the plots below). The maximum 

overturning moment was matched 100%. The maximum base shear is 5.6% below the target.

As a result, the drag and inertia coefficients that are applicable to the equivalent monopile 

analyses with FAST analyses are as follows: 

Cd= 1.03 

Cm = 1.30 
77 

Appendix C: Study on Sensitivity of Load Results to 

the Number of Simulations

The coupled wave and wind load analysis was performed for a number of simulations because of 

the stochastic nature of the turbulent wind load. For each simulation, the maximum shear force 

and overturning moment were obtained throughout the time-history. The mean of these 

maximum values was used for the reliability assessment of the structure. The figures below show 

that, for all the operating wind speeds and all wave heights, enough accuracy is achieved after 10 

simulations. 

One can observe that after two simulations the variation is 4%, 5%, and 15% for the Ws=9.2, 

12.7, and 15.6 m/s respectively. The variation drops to 2%, 3%, and 5% after five simulations. 

This level of variation was considered to be significant. Therefore, 10 simulations were 

performed to achieve an acceptable level of accuracy. 

0.95

0.96

0.97

0.98

0.99

1

1.01

0 2 4 6 8 10 12

NormalizedMean

number of seeds

Ws=9.2 m/s

2.0

4.0

5.8

8.5

9.5

12.7

13.7

15.9

Hs (m)
78 

0.94

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

0 2 4 6 8 10 12

Normalized Mean

number of seeds

Ws=12.7 m/s

2.0

4.0

5.8

8.5

9.5

12.7

13.7

15.9

Hs (m)

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

0 2 4 6 8 10 12

Normalized Mean

number of seeds

Ws=15.6 m/s

2.0

4.0

5.8

8.5

9.5

12.7

13.7

15.9

Hs (m)