Hub Height 

100 m

Interface Level 

24 m

Water Depth 

25 m

0 100 200 300

Tower shear

Yaw Error [deg]

TowerBase

TowerTop

0

0.5

1

1.5

2

2.5

10 100 1000 10000

Load Ratio

Return Period (Years)

DLC 1.6a

DLC 1.6b

Required NSM = 1.94

IEC NSM = 1.6875

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

10 100 1000 10000

Load Ratio

Return Period (Years)

DLC 1.6a

DLC 1.6b

Required NSM = 1.63

IEC NSM = 1.6875

0

0.5

1

1.5

2

2.5

3

3.5

4

10 100 1000 10000

Load Ratio

Return Period (Years)

DLC 6.1a

DLC 6.1b

DLC 6.1c

Required NSM = 1.81

IEC NSM = 1.6875

Figure 1. Offshore wind turbine (Alstom Haliade 150) and support structure configuration considered in this study

Abstract

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

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Aeroelastic Modeling of Offshore Turbines and Support Structures in 

Hurricane-Prone Regions R. Damiani*, Y. Guo*, W. Musial*, and D. Arora**

*National Renewable Energy Laboratory; **Alstom Power, Inc

The information contained in this poster is subject to a government license.

AWEA WINDPOWER • Las Vegas, Nevada • May 5–8, 2014

NREL/PO-5000-61533

Conclusions 

This study revealed how CAE tools can be used to assess the reliability of OWT systems in hurricaneprone waters, and thus achieve a well-balanced design, in which reliability levels and costs are optimized.

• Tower drag is crucial when verifying the integrity of the substructure. Under parked DLCs, the tower and

substructure drag account for two-thirds or more of the tower-base shear and cannot be ignored. As a

result, the new FAST v8 Aerodyn module now includes tower aerodynamic drag.

• To assess the substructure, both operational and parked cases must be carefully investigated as they

both produce peak loads at the interface with the tower. Additionally, yaw-error-induced loading needs

to be assessed in hurricane ride-through strategies because tower-base shear can increase up to 4

times when the rotor is not aligned with the wind direction.

• Overload hazard in hurricane-prone waters may be significantly higher than in the North Sea. We

produced hazard curves that show how IEC basic conditions (met-ocean RP events and basic PSFs)

may not guarantee the ability to withstand a 500-yr RP event. Even the ABS approach may not suffice,

and PSF values need to be revised for each DLC, though specifically devised controls may mitigate this

requirement.

• A jacket that was designed with basic IEC requirements was shown to produce high utilization ratios

that are not compatible with a safe design in hurricane-prone regions where PSF and met-ocean RP

need to be increased.

Figure 4. Tower-base bending moment (in blue) and torsion (in 

red) maxima for ultimate limit state (ULS) load cases as 

calculated by FAST. Note: both parked and operational DLCs 

produce tower-base load maxima.

Figure 2. Tower-base bending moment as a function of wind 

speed and yaw error under parked conditions

Figure 3. Tower-top and tower-base shear forces as a 

function of yaw error; difference is a result of the tower 

aerodynamic drag now included in FAST v8

FAST v8 – Tower Drag Assessment

Figure 7. Hazard curves; DLC1.6 – normalized to 100 years 

(ABS); -15o wind-wave misalignment

PSF1.56

IEC NOT 

SUFFICIENT

PSF1.45

ABS/IEC NOT 

SUFFICIENT

Figure 8. Hazard curves; DLC6.1 – normalized to 100 

years (ABS); -15o wind-wave misalignment

PSF1.35

ABS OK

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

10 100 1000 10000

Load Ratio

Return Period (Years)

DLC 6.1a

DLC 6.1b

DLC 6.1c

Required NSM = 1.88

IEC NSM = 1.6875

Figure 9. Hazard curves; DLC6.1 – normalized to 100 

years (ABS); all wind-wave misalignments

PSF1.51

ABS/IEC NOT 

SUFFICIENT

Hazard Curves: 

ABS/IEC/API Design Approach Comparison

Figure 5. SACS adds substructure hydrodynamic loads and 

computes substructure overall overturning moment and 

stresses. 

References

Figure 6. Hazard curves; DLC1.6 – normalized to 50 years 

(IEC); -15o wind-wave misalignment 

[1] American Bureau of Shipping (ABS). (2013). Guide for Building and Classing Bottom-founded Offshore Wind Turbine Installations.

[2] American Petroleum Institute (API). (2000). RP 2A (WSD) - Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Working

Stress Design, 21st Edition, December, 2000, with Supplements in December, 2002, September, 2005, and October 2007.

[3] International Electrotechnical Commission (IEC). (2009). IEC 61400-3 Ed. 1.0: Design Requirements for Offshore Wind Turbines.

[4] American Wind Energy Association (AWEA). (2012). AWEA Offshore Compliance Recommended Practices.

[5] Damiani, R., Guo, Y., Platt, A., Meadows, R., Musial, W. (2013). Factors Affecting the Design and Reliability of Offshore Wind Turbines and Support Structures for the

VOWTAP Project. AWEA Offshore Windpower 2013, Providence, RI, October 22-23, 2012.

Tower-base shear dramatically increases 

when aerodynamic drag is included. Yawerror effects need to be accounted for in 

hurricane ride-through strategies. 

Figure 10. ULS analysis: member utilization 

according to API code checks (SACS output)

SACS model of the 

substructure

Structure designed per basic IEC 

requirements fails member APIcode verification.

FAST v8/SACS ULS Analysis of the Substructure

Tower-base loads calculated by FAST 

were applied at the substructure 

interface in SACS.

FAST v8 – SACS Coupling

Acknowledgments

U.S. offshore wind turbines (OWTs) will likely have to contend with hurricanes and the associated loading

conditions. Current industry standards do not account for these conditions in their design load cases

(DLCs), thus a new approach is required to guarantee that the OWTs achieve an appropriate level of

reliability. In this study, a sequentially coupled aero-hydro-servo-elastic modeling technique was used to

study two proposed design approaches: 1) The ABS approach [1], and 2) The hazard curve or API approach

[2]. The former employs [3] partial load factors (PSFs) and 100-yr return-period (RP) met-ocean events. The

latter allows setting PSFs and RP to a prescribed level of system reliability. The 500-yr RP robustness check

(appearing in [2] and [3] upcoming editions) is a good indicator of the target reliability for L2 structures [2,4].

Computer-aided engineering (CAE) tools such as the National Renewable Energy Laboratory’s (NREL’s)

FAST and Bentley’s SACS (offshore analysis and design software) can be efficiently coupled to simulate

system loads under hurricane DLCs [5] and were used in this project to examine the two design approaches.

For this task, we augmented the latest FAST version (v8) to include tower aerodynamic drag that cannot be

ignored in hurricane DLCs. In this project, Alstom provided the Haliade 150, 6-MW turbine model that was

simulated on a typical four-legged jacket for a mid-Atlantic site. FAST-calculated tower-base loads were fed

to SACS at the interface level (transition piece); SACS added hydrodynamic and wind loads on the exposed

substructure, and calculated mudline overturning moments, and member and joint utilization. Results show

that CAE tools can be effectively used to achieve a well-balanced, safe OWT design. The hazard curve yields a 500-yr, required 

nominal safety margin (NSM = Soil Safety 

Factor (0.8) * PSF), thus the required PSF.

Specifically devised controls 

may mitigate this requirement.

This research benefited from collaboration with the Virginia Offshore Wind Technology Advancement Project (VOWTAP) team, including Dominion Power, the KBR

team, and the Keystone Engineering team involved in the U.S. Department of Energy-funded, VOWTAP demonstration project, Award: DE-EE-0005985.