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Contract No. DE-AC36-08GO28308

Verification of New Floating 

Capabilities in FAST v8

Preprint

F. Wendt, A. Robertson, J. Jonkman, 

and G. Hayman

National Renewable Energy Laboratory

To be presented at AIAA SciTech 2015

Kissimmee, Florida

January 5‒9, 2015

Conference Paper

NREL/CP-5000-63116

January 2015
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Verification of New Floating Capabilities in FAST v8 

Fabian Wendt1, Amy Robertson2, Jason Jonkman3, and Greg Hayman4

National Renewable Energy Laboratory, Golden, CO, 80401

FAST v8 is the latest release of the National Renewable Energy Laboratory’s wind 

turbine aero-hydro-servo-elastic simulation software, with several new capabilities and 

major changes from the previous version. FAST has been significantly altered to improve 

the simulator’s modularity and to include new functionalities in the form of modules in the

FAST v8 framework. This paper focuses on the improvements made for the modeling of 

floating offshore wind systems. The most significant change was to the hydrodynamic load

calculation algorithms, which are embedded in the HydroDyn module. HydroDyn is now 

capable of applying strip-theory (via an extension of Morison’s equation) at the member 

level for user-defined geometries. Users may now use a strip-theory-only approach for 

applying the hydrodynamic loads, as well as the previous potential-flow 

(radiation/diffraction) approach and a hybrid combination of both methods 

(radiation/diffraction and the drag component of Morison’s equation). Second-order 

hydrodynamic implementations in both the wave kinematics used by the strip-theory 

solution and the wave-excitation loads in the potential-flow solution were also added to 

HydroDyn. The new floating capabilities were verified through a direct code-to-code 

comparison. We conducted a series of simulations of the International Energy Agency Wind 

Task 30 Offshore Code Comparison Collaboration Continuation (OC4) floating 

semisubmersible model and compared the wind turbine response predicted by FAST v8, the

corresponding FAST v7 results, and results from other participants in the OC4 project. We 

found good agreement between FAST v7 and FAST v8 when using the linear 

radiation/diffraction modeling approach. The strip-theory-based approach inherently differs

from the radiation/diffraction approach used in FAST v7 and we identified and 

characterized the differences. Enabling the second-order effects significantly improved the 

agreement between FAST v8 and the other OC4 participants.

Nomenclature

A33(0) = zero-frequency limit of added mass in the heave direction (calculated via WAMIT)

CAA = added-mass coefficient in the heave direction of joint i

L = angular momentum

ρw = water density

Vnn ����⃗ = weighted normal reference volume vector of joint i

ω = angular velocity

I. Introduction

HE National Renewable Energy Laboratory’s (NREL’s) aero-hydro-servo-elastic wind turbine simulation 

software, FAST, is a comprehensive software tool for the simulation of land-based and offshore wind turbines

[1]. The latest release (FAST v8) introduced several major modifications and new capabilities. The architecture of 

FAST v8 is entirely different from FAST v7. FAST is now split into multiple modules coupled with a driver 

 

1

Engineer, National Wind Technology Center, 15013 Denver West Parkway, Golden, CO 80401 2

Senior Engineer, National Wind Technology Center, 15013 Denver West Parkway, Golden, CO 80401

3

Senior Engineer, National Wind Technology Center, 15013 Denver West Parkway, Golden, CO 80401, AIAA 

Professional Member

4

Senior Engineer, National Wind Technology Center, 15013 Denver West Parkway, Golden, CO 80401

T
2 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

program (glue code) [2]. The increased modularity aims towards simplifying and improving the development 

process of future FAST modules developed by NREL and the global user/developer community. Because of the 

significant program changes in FAST v8, we conducted a series of studies to verify the results generated by FAST 

v8 against those from FAST v7 and against the corresponding simulation results of other Offshore Code 

Comparison Collaboration Continuation (OC4) participants. The verification work covered in this paper focuses on 

the new features introduced to the HydroDyn module, in particular the ballasting feature and the implementation of 

strip-theory for multimember structures. The new quasi-static mooring line module (MAP) [3], the new secondorder hydrodynamics of HydroDyn, and the coupling approach that is used in the modular architecture of FAST v8

are also verified within the scope of this paper. The OC4-DeepCwind semisubmersible was used for the study, and 

the load cases simulated were those examined in the OC4 Phase II project [4] [5]. The simulated load cases covered 

a wide range of operating conditions and model complexity. All results in this paper are based on the latest release 

of FAST, v8.09.00a-bjj.

II. Major Changes Introduced in FAST v8 

In FAST v8, major changes have been introduced to the 

architectural concept of the software. FAST is now divided

into individual modules that are executed and coupled with 

the FAST driver (glue code). A comparison between FAST 

v7 and FAST v8 architectures is given in Figure 1. Because 

of its modular architecture, FAST v8 requires a coupling 

routine that connects different modules. The coupling 

approach used in FAST v8 features a mesh-to-mesh mapping 

scheme that allows the coupling of modules with highly 

different spatial discretization. Data between the modules are 

exchanged with an implicit predictor-corrector approach, 

which allows the transfer of platform accelerations from 

ElastoDyn to HydroDyn and the transfer of hydrodynamic 

loads, including acceleration-dependent added-mass loads,

from HydroDyn to ElastoDyn [6]. 

Several major changes have been introduced to the 

HydroDyn module. In FAST v7, HydroDyn calculates the 

hydrodynamic loads on floating substructures based on a 

potential-flow theory approach. This approach requires a set 

of hydrodynamic coefficients (restoring matrix and 

frequency-dependent damping matrix, added-mass matrix, 

and wave-excitation vector) that needs to be calculated 

separately, prior to running FAST. WAMIT [7] is commonly 

used to derive these coefficients for a user-specified platform geometry. FAST v7 also calculates the viscous drag on 

the central substructure member by dividing the member into strips and calculating drag based on the nonlinear 

viscous-drag term from Morison’s equation [8]. Although FAST v8 still offers the potential-flow-theory-based 

approach for the calculation of wave diffraction and radiation loads, the strip-theory functionality of HydroDyn has 

been significantly extended. The user can now specify an arbitrary multimember substructure geometry with varying 

drag, added mass, and dynamic pressure coefficients for each member in FAST’s HydroDyn input file. In addition, 

the other components of the relative form of Morison’s equation have been added to the code (fluid-inertia and

added-mass terms), enabling the user to model the hydrodynamic loads solely using strip-theory. Other attributes 

include extensions to Morison’s equation, including the ability to model axial hydrodynamic loads on tapered 

members and member end plates, buoyancy and ballasting of the members, and marine growth. A detailed 

description of the theory and equations behind the strip-theory-based modeling approach is given in the HydroDyn 

User’s Guide and Theory Manual [9]. Although FAST v8 now has the capability of modeling offshore fixed-bottom

multimember substructures with flexible members, structural flexibility is not currently a feature for floating 

platforms.

FAST v8 is now able to simulate second-order wave kinematics for the strip-theory-based modeling approach 

and second-order incident-wave excitation (diffraction) forces for the potential-flow-based modeling approach [9]. 

The implementation of second-order hydrodynamic theory enables more accurate modeling of sea states and the 

associated wave loads on fixed-bottom and floating wind systems. The magnitude and frequency content of secondFigure 1. Architectural comparison of 

FAST v7 and FAST v8 [2].
3 

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order hydrodynamic loads can excite structural natural frequencies, leading to greater ultimate and fatigue loads. A 

circumstantial verification of these new second-order modeling capabilities was not part of the scope of this paper, 

but selected examples of simulation results that illustrate the second-order effects are given in the results section. 

Further information on the verification of the second-order modeling capabilities implemented in FAST v8 can be 

found in [10]. 

 The new quasi-static mooring line module MAP was also part of this verification study. MAP replaces the 

original quasi-static mooring line model of FAST v7. In addition to quasi-static, single-segment mooring systems, 

MAP can also model multisegmented mooring lines [3]. Multisegemented mooring systems involving variable 

stiffness and mass properties and line-to-line interconnections have widespread, practical utility in realistic offshore 

anchoring designs. This new multisegment feature, however, is not part of the verification work covered within this 

paper. 

III. Model Description

The model used to verify the new HydroDyn capabilities follows the definition of the OC4-DeepCwind floating 

semisubmersible wind turbine [4]. The turbine used for this model was the NREL 5-MW reference turbine [11]. The 

floating semisubmersible platform was based on the DeepCwind project [12]. Because FAST v8 offers three 

different approaches for calculating hydrodynamic loads (potential-flow theory only, potential-flow theory 

combined with viscous drag from Morison’s equation, and strip-theory only), three different FAST v8 models were

created. All three models described below used HydroDyn’s new ballasting feature for modeling the water mass in 

the partially flooded platform columns [4]. The current implementation of the ballasting feature requires the user to 

hand-calculate the (4, 4), (5, 5), (4, 6), and (5, 6) entries of the 6 x 6 linear hydrostatic restoring matrix (AddCLin in 

the HydroDyn input file) to model the ballasting-induced restoring moments. A detailed description of these terms is 

given in Section 6.8.3 of the HydroDyn User’s Guide [9]. 

A. WAMIT-Only Model

From here on the potential-flow-only model is referred to as the 

WAMIT-only model. The approach used to calculate the first-order 

hydrodynamic loads for this model was the same as in FAST v7. 

Linear hydrostatic restoring, wave radiation, and diffraction forces 

were considered through a set of hydrodynamic coefficients that 

were calculated via WAMIT. In accordance with the OC4 model 

description [4], a quadratic drag matrix was added to the model to 

mimic viscous drag effects that otherwise would not be captured by 

the WAMIT-only approach. 

Second-order difference-frequency potential-flow terms can be 

included in FAST v8 via a Newman approximation, which is based 

on first-order WAMIT coefficients, or by considering the full 

difference-frequency quadratic transfer functions (QTFs), derived 

from a second-order WAMIT solution. Second-order sum-frequency 

potential-flow terms can also be included via a full sum-frequency 

QTF, calculated from a second-order WAMIT solution [9]. The 

second-order, potential-flow-based simulation results shown in this 

paper were computed with the full sum- and difference-frequency 

QTFs. 

B. WAMIT+Morison Model

The hybrid model that combines the viscous-drag term from 

Morison’s equation and the potential-flow theory approach will be 

referred to as the WAMIT+Morison model. In addition to the hydrodynamic load calculation features described for 

the WAMIT-only model, the WAMIT+Morison model calculates viscous drag forces for each member of the 

submerged platform geometry based on the local wave particle and structural velocities, rather than using an

additional quadratic drag matrix (as used for the WAMIT-only model). The ability to model viscous drag at the 

member-level for a complicated multimember substructure like the OC4-DeepCwind semisubmersible was not 

available in FAST v7 (in FAST v7, only viscous drag on a central column could be modeled). This new HydroDyn 

feature is a valuable tool for augmenting a potential-flow based model with viscous-drag loads induced by flow 

Figure 2. NREL 5-MW turbine 

installed on the OC4 platform [4].
4 

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separation. For a substructure that consists of members with different diameters, the viscous drag component can 

play an important role, especially for members that are relatively small in diameter.

The platform geometry and the corresponding drag coefficients were defined based on the OC4 model 

description [4], and on the information given in the following section. Only first-order wave kinematics were 

considered in this paper for this model (even for the model with second-order WAMIT). This WAMIT+Morison 

model with second-order WAMIT enabled is distributed with FAST v8.09.00a-bjj as test 25 in the certification test 

(CertTest directory).

C. Morison-Only Model

In the Morison-only model, hydrodynamic loads are calculated solely from a strip-theory approach. Compared to 

the potential-flow based approach, strip-theory hydrodynamic loads may be preferable for substructures with 

members that are relatively small in diameter, compared to the typical wave length. A Morison-only model does not 

require an additional preprocessor like WAMIT. Platform geometry and all required hydrodynamic coefficients are 

directly specified by the user in the HydroDyn input file. Because the present version of HydroDyn does not 

calculate the change in buoyancy with platform displacement when using a Morison-only approach, the user must 

manually include the hydrostatic restoring matrix through the definition of a linear stiffness matrix in the HydroDyn 

input file. The calculation of the required coefficients is described in [9]. In addition to the drag coefficients and 

platform geometry, the Morison-only model requires the definition of added mass and dynamic-pressure coefficients 

for each member and joint. The selection of the appropriate hydrodynamic coefficients was based on the information

provided in [4]. Several adjustments from the original OC4 model description [4] were introduced to the definition 

of added mass and drag coefficients for member joints in the heave direction. These adjustments were necessary 

because of the FAST v8-specific implementation of the hydrodynamic loads in the axial/heave direction. In [4], the 

base columns (Fig. 2) were considered as heave plates for the computation of drag forces in the axial direction. The 

axial drag coefficient of 4.8, as described in [4], was based on the assumption that the base column is a flat heave 

plate with an equal area on the top and bottom. Because FAST v8 considers the actual wetted surface of the 

members, and the top column is connected to another member, the top of the base column contributes a smaller 

surface area to the axial drag force calculation than the bottom. To create a FAST v8 model that behaves similar to

the one described in [4], the axial drag coefficient was doubled to 9.6 and applied only to the bottom of the base 

column.

The added mass coefficients in the heave direction were defined as 1.0 at the bottom of the central column (Fig. 

2) and the base columns. Based on the new implementation in HydroDyn, the relationship between the added-mass 

coefficient in the axial direction of each joint (CAA) and the zero-frequency limit added mass in the heave direction 

(calculated via WAMIT) for a structure with N joints (assuming negligible transverse added mass from the pontoons 

and braces), is as follows: 

33

1

33

(0) N T

Ai ni ni

T i w ni ni

C VV A

= V V ρ

 

  =    

∑

C C

C C (1)

with the added-mass coefficients at the bottom of the base and central column being defined as 1.0, and A33(0)/ρw

being calculated as 1.50E+04 m3 by WAMIT. The added mass coefficients for the top ends of the base columns can 

be calculated via Eq. (1). The weighted normal reference volume vector (Vnn ����⃗) is further specified in [9] and considers 

the size and orientation of all member endplates connected to joint i. This approach ensures that the nonwetted

contact area between the two connecting members on top of the base columns is not considered for the added mass 

force computation. Based on Eq. (1) and the definition of the normal volume vector given in [9], this eventually led 

to a CAA value of 0.428 for the top of the base columns. 

In addition to the added mass and drag forces, the FAST v8 implementation of the Morison-only approach also 

considered fluid-inertia forces on the member ends. Similar to the transverse fluid-inertia forces from Morison’s 

equation, the computation of the inertial forces at member ends is described in [9] and includes contributions from 

the local dynamic pressure (Froude-Kriloff) and scattering (fluid acceleration). The implementation in HydroDyn 

makes use of the added-mass coefficient discussed in the previous paragraph and requires the definition of dynamic 

pressure coefficients on the top and bottom of each member. The dynamic pressure coefficients at the bottom of the 

base column and central column were set to 1.0, and the dynamic pressure coefficient at the top end of the base 

columns were set to 0.5. The OC4 model description [4] did not address the scattering term or discuss dynamic 

pressure coefficients, and these values were found to create a reasonable system response for regular wave load 
5 

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cases when being compared to the corresponding WAMIT-only results. A detailed description of the hydrodynamic 

load calculation for the Morison-only modeling approach is given in the HydroDyn User’s Manual [9]. 

Second-order wave kinematics for the strip-theory-based approach, including second-order sum-frequency and 

difference-frequency terms, can be enabled by the user. These terms are computed from analytical full differenceand sum-frequency QTFs [9] and are included in some of the load cases. 

IV. Load Cases

The load cases included in the verification work are based on the OC4 project [5]: 

Table 1. Simulated Load Cases

Load Case Description Degrees of Freedom 

(DOFs)

Wind Waves/Current

LC 1.2 Static Equilibrium All No air Still water

LC 1.3a Free decay, surge Platform No air Still water

LC 1.3b Free decay, heave Platform No air Still water

LC 1.3c Free decay, pitch Platform No air Still water

LC 1.3d Free decay, yaw Platform No air Still water

LC 2.1 Regular waves Platform, Tower No air Regular Airy, H = 6 m, T = 10 s

LC 2.2 Irregular waves Platform, Tower No air Irregular Airy, Hs = 6 m, Tp = 10 s, 

JONSWAP

LC 2.3 Current only Platform, Tower No air 0.5-m/s surface current, 1/7th power 

law decrease with depth

LC 2.4 Current and regular 

waves

Platform, Tower No air 0.5-m/s surface current, 1/7th power 

law decrease, regular Airy, H = 6 m, T

= 10 s

LC 2.5 50-year extreme 

wave

Platform, Tower No air Irregular Airy, Hs = 15 m, Tp = 19.2 s, 

JONSWAP

LC 2.6 White noise waves Platform, Tower No air White noise waves, PSD = 1 m2/Hz 

for 0.05 Hz‒0.25 Hz

LC 3.1 Deterministic wind 

and waves

All Steady, 8 

m/s

Regular Airy, H = 6 m, T = 10 s

LC 3.2 Stochastic wind (at 

rated) and waves

All Turbulent,

11.4-m/s 

mean

Irregular Airy, Hs = 6 m, Tp = 10 s,

JONSWAP

LC 3.3 Stochastic wind

(above rated) and 

waves

All Turbulent,

18-m/s 

mean

Irregular Airy, Hs = 6 m, Tp = 10 s,

JONSWAP

LC 3.4 Wind/wave/current All Steady, 8 

m/s

0.5 m/s surface current, 1/7th power 

law, regular Airy, H = 6 m, T = 10 s

LC 3.5 50-year extreme 

wind/wave

All Turbulent,

47.5-m/s 

mean

Irregular Airy, Hs=15 m, Ts=19.2 s, 

JONSWAP

LC 3.6 Wind/wave 

misalignment

All Steady, 8 

m/s

Regular Airy, H = 6 m, T = 10 s,

direction = 30°

LC 3.7 White noise waves 

with steady wind

All Steady, 8 

m/s

White noise waves, PSD = 1 m2/Hz 

for 0.05Hz‒0.25 Hz

V. Verification Results

Key observations made during our FAST v8 verification work are discussed in the following sections. As 

described in [5], the large magnitudes of aerodynamic loads tend to mask the discrepancies between different 

hydrodynamic modeling approaches. Because this verification work is focused on the new hydrodynamic 

capabilities of FAST v8, the following sections do not cover load cases with aerodynamic loading. Even though the 

wind load cases (LC 3.x) are not further discussed within this paper, they were simulated as part of our FAST v8

verification work. Because no major changes were introduced to FAST’s aerodynamic modeling capabilities, no 
6 

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significant differences were found in a direct comparison of the LC 3.x simulation results generated by FAST v7 and

the FAST v8 WAMIT-only model. 

A. WAMIT Only

The WAMIT-only simulation results generated with FAST v8 (and second-order terms disabled) were virtually 

identical to FAST v7. Because FAST v7 did not have the capability of modeling partially flooded multimember 

substructure geometries directly in HydroDyn, the mass and inertia of any water ballast had to be included in the 

corresponding lumped mass and inertia values of the platform structural model. The new ballasting feature 

implemented in FAST v8 now enables the user to specify partially or fully flooded substructure members. The 

FAST v8 simulation results generated with this new ballasting approach showed small differences, compared to the 

corresponding FAST v8 simulations that included the water ballast as part of the lumped platform mass and platform 

inertia. 

Figure 3. LC 1.3b simulation results (FAST v8 with and without HydroDyn ballasting).

Figure 4. LC 1.3d simulation results (FAST v8 with and without HydroDyn ballasting).
7 

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The largest noteworthy differences between the FAST v8 simulations with and without HydroDyn ballasting 

occurred in the free-decay load cases: LC 1.3b and LC 1.3d. The system’s pitch response with HydroDyn ballasting 

was slightly different in amplitude compared to a model incorporating the ballasting in the structural properties. The 

system’s surge response, which was highly coupled to the pitch motion, also showed a small difference in 

amplitude. These differences can be explained as follows: the implementation of the ballasting feature in HydroDyn 

includes a small-angle assumption for the ballast-induced restoring moment in the pitch and roll degree of freedom 

(DOF) and it does not consider the nonlinear ω × L inertia term (with ω being the angular velocity and L the angular 

momentum) of Euler’s equation for rigid body rotations [13], which are both included when ballasting is part of 

FAST’s structural model. Also, the rotational inertia of the ballast mass within the cross section of a member is not 

considered by the HydoDyn ballasting feature. These simplifications of the HydroDyn ballasting implementation 

explain the small differences in the coupled pitch and surge response, illustrated in Fig. 3 and Fig. 4.

The lack of significant differences between the WAMIT-only results from FAST v7 and FAST v8 also served as 

a verification of the new mooring line module, MAP, and the coupling approach introduced as part of the FAST v8 

modularization framework. 

B. WAMIT+Morison

In this section, the WAMIT+Morison results are compared against the corresponding WAMIT-only results, as 

well as against simulation results of other OC4 participants that use a combined Morison/potential-flow theory 

approach. The results for the free-decay load cases (LC 1.3a – LC 1.3d) agreed with the WAMIT-only results. 

Differences were visible in the coupled DOFs of the main free-decay motion (Fig. 5). This outcome was likely 

related to missing off-diagonal terms in the drag matrix that were used in the WAMIT-only model to mimic viscousdrag effects, and did not indicate a problem with the WAMIT+Morison solution [5]. 

Figure 5. LC 1.3a simulation results (FAST v8 WAMIT+Morison, FAST v8 WAMIT only, WAMIT + 

Morison-only OC4 participants).

The regular wave response (LC 2.1) matched well with the corresponding WAMIT-only results. LC 2.3 and LC 

2.4 were not simulated with the WAMIT-only model because hydrodynamic loading from current required the 

computation of member-level viscous-drag forces. In comparison to the other OC4 participants, the general system 

response predicted by FAST v8 for both the WAMIT+Morison and Morison-only models appeared to be reasonable

(Fig. 6). The mean offset in surge due to the current induced viscous drag is adequately predicted by FAST v8.

Enabling FAST’s second-order wave potential-flow modeling capabilities for the WAMIT+Morison model in LC 

2.4 increased the mean value of the platform surge signal as a result of second-order mean wave-drift loads (orange

curve in Fig. 6). The results of several OC4 participants include corrections that induce higher order hydrodynamic 

effects (e.g. wave stretching or considering the instantaneous platform position). These higher order nonlinearities 

are not currently considered by FAST v8 and bring about an even larger increase of the mean platform surge.
8 

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Figure 6. LC 2.4 simulation results (FAST v8 WAMIT+Morison, FAST v8 Morison-only, WAMIT+Morison 

OC4 participants, Morison-only OC4 participants, FAST v8 WAMIT+Morison with second-order theory).

The largest noticeable differences between FAST v8 and the other OC4 participants were found in the irregular 

wave load cases LC 2.2 and LC 2.5 when second-order potential-flow effects were not included. Simulation results 

of irregular wave simulations were analyzed in the frequency domain by comparing the corresponding power 

spectral density functions (PSDs). In LC 2.2, pitch, surge, and heave responses were under-predicted by FAST v8 at 

low frequencies when compared to WAMIT+Morison results of other OC4 participants (Fig. 7). A similar and even 

larger under-prediction of the low-frequency pitch, surge, and heave response was also evident in the Morison-only 

FAST v8 model when compared to Morison-only results of other OC4 participants and is further discussed within 

the next section. Although the underprediction of the surge motion by the FAST v8 WAMIT+Morison model was 

still visible in the simulation results for LC 2.5 (Fig. 8), the pitch and surge response were in better agreement with 

the other OC4 participants that use a combined Morison/potential-flow modeling approach. Enabling the secondorder potential-flow terms in the WAMIT+Morison solution drastically increased the low-frequency response of the 

system in LC 2.2 and LC 2.5 (pink curves in Figs. 7 and 8) because of second-order slow- drift forces. This led to 

better agreement between FAST v8 and the other OC4 participants in the low-frequency range.

Figure 7. LC 2.2 simulation results (FAST v8 WAMIT+Morison, FAST v8 Morison-only, WAMIT+Morison

OC4 participants, Morison-only OC4 participants, FAST v8 WAMIT+Morison with second-order wave 

theory, FAST v8 Morison-only with second-order wave theory).
9 

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C. Morison-Only

Compared to the WAMIT+Morison and WAMIT-only models, the Morison-only model uses a significantly 

different approach for the hydrodynamic load calculation. Therefore, differences in the simulation results were 

expected and a simple direct comparison against the WAMIT-only and WAMIT+Morison simulation results was of 

limited value for verification purposes.

Figure 8. LC 2.5 simulation results (FAST v8 WAMIT+Morison, FAST v8 Morison-only, WAMIT+Morison 

OC4 participants, Morison-only OC4 participants, FAST v8 WAMIT+Morison with second-order wave 

theory, FAST v8 Morison-only with second-order wave theory).

For this reason, the FAST v8 Morison-only simulation results were mainly compared against the corresponding 

results of other OC4 participants that used a Morison-only modeling approach. The Morison-only FAST v8 results 

were similar to the other Morison-only OC4 participants for the free-decay (LC 1.3a – LC 1.3d) and regular wave 

(LC 2.1, LC 3.1, LC 2.4) load cases. The static-equilibrium position of the system with a constant surface current of 

0.5 m/s (LC 2.3) was accurately predicted as well. FAST v8 also computed a reasonable system response for regular 

waves in combination with a constant surface current of 0.5 m/s, as illustrated in Fig. 6. 

Differences between FAST v8 and the other Morison-only-based OC4 participants were found only in load cases 

with irregular waves (LC 2.2, LC 2.5). Between 0 Hz‒0.05 Hz in the LC 2.2 PSD plot (Fig. 7), FAST v8 with firstorder wave kinematics underpredicted the system response in surge, heave, and pitch. For LC 2.5, the FAST v8

Morison-only simulation results showed better agreement with the other Morison-only OC4 participants. As 

discussed for the WAMIT+Morison results, second-order hydrodynamics tended to increase the system response in 

the low-frequency band as a result of second-order slow-drift effects. This trend was also evident when comparing 

the blue (FAST v8 Morison-only) and the orange curves (FAST v8 Morison-only with second-order wave theory) in 

Figs. 7 and 8. The majority of the Morison-only OC4 participants did not consider second-order wave theory, but 

applied corrections by using the instantaneous position of the platform in the wave field or stretching the wave 

kinematics to the free surface, which induced higher-order hydrodynamic effects. As shown, the first-order FAST v8 

simulation results underpredicted the system response for low frequencies, but the inclusion of second-order wave 

theory tended to capture much of the nonlinear effect. For LC 2.2 (Fig. 7), the first-order wave-excitation-induced 

peak around 0.1 Hz was shifted towards lower frequencies in the Morison-only solution with second-order wave 

kinematics (orange curve). This frequency shift was not evident for LC 2.5 (Fig. 8) and is being investigated further. 
10

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

VI. Conclusions 

With the latest release of FAST (v8), the hydrodynamic modeling capabilities of the software have been 

significantly extended. The user can now choose between the potential-flow (radiation/diffraction) based modeling 

approach, a strip-theory based modeling approach, and a hybrid approach (radiation/diffraction combined with the 

quadratic drag term from Morison’s equation). The hybrid approach allows the user to augment a potential-flowbased model with additional viscous drag loads. The strip-theory-based approach may be preferable for 

substructures with members that are relatively small in diameter, compared to the typical wave length. Second-order 

hydrodynamic effects have also been implemented in HydroDyn, enabling the user to capture second-order

hydrodynamic excitation forces, which can play an important role in the assessment of fatigue and ultimate load 

conditions. 

Simulation results generated with FAST v8 were compared against the results of the previous FAST version (v7) 

and the results of other OC4 participants for a wide range of operating conditions. The load cases and model 

definition for this verification work was based on Phase II of the OC4 code comparison project. The new 

hydrodynamic modelling capabilities of FAST v8 were verified by comparing the simulation results against the 

results of OC4 participants with similar hydrodynamic modeling approaches. 

Good agreement was found between the FAST v7 and the FAST v8 WAMIT-only simulation results, which 

verified the accuracy and reliability of the new quasi-static mooring line module MAP and the coupling approach 

that was introduced as part of the modular FAST v8 architecture. 

The evaluation of the results generated with FAST v8 using a WAMIT+Morison and a Morison-only 

hydrodynamic modeling approach showed general agreement with the corresponding OC4 participants. The largest 

differences were found for load cases with irregular waves (LC 2.2 and LC 2.5) until second-order terms were 

included. The FAST v8 first-order results underpredicted the low-frequency system response compared to the 

majority of other OC4 participants. Enabling incident-wave excitation forces from second-order loads for the 

WAMIT+Morison potential-flow-based modeling approach significantly increased the low-frequency response of 

the system and improved the agreement between FAST v8 and the other OC4 participants. A similar trend was 

evident when enabling second-order wave kinematics for the FAST v8 Morison-only solution.

Acknowledgments

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the 

National Renewable Energy Laboratory. Funding for the work was provided by the DOE Office of Energy 

Efficiency and Renewable Energy, Wind and Water Power Technologies Office.

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