Second-order Wave Forces Research Articles

FPSO ROLL MOTIONS DRIVEN BY SECOND-ORDER FORCES AND THE IMPACTS ON SLWR RISER DESIGN

Abstract The dynamic behavior of risers is highly influenced by the FPU motions, associated with environmental loads, caused by waves, current and wind. Particularly, as well known, under potential flow modelling, wave loads may be represented through power series expansions, with respect to a small parameter ε. First-order wave forces pulsate in the frequency range of the incident wave spectra, whereas second-order wave forces are related to sum or difference frequencies of the same spectra. The so-called second-order difference frequencies forces may then be responsible by resonating motions of the vessel in that range. Significant low-frequency motions due to second order forces are commonly observed in the horizontal plane, resonating low natural frequencies of moored floating systems characterized by low mooring stiffnesses. Recently, however, this phenomenon was also observed for roll motions in several new FPSOs that will operate in the Pre-Salt of Santos Basin. The phenomenon had already been detected in a deep draft semi-submersible in Campos Basin, but only more recently in FPSOs. The new floating units are characterized by high roll natural periods, above the typical wave period range in the pre-salt region, and the roll response to the second-order difference frequencies forces can be significant. This phenomenon was indeed detected in model tests and confirmed by numerical studies. In fact, this additional FPSO roll behavior occurs in typical periods slightly above 20 s, imposing displacements and accelerations that can directly affect the dynamics of the riser top region. The present paper addresses this phenomenon and its possible impact on steel lazy-wave riser (SLWR) designs.

Effects of the second-order hydrodynamics on the dynamic behavior of the platform among the wind-wave hybrid systems

This paper presents three different wind-wave hybrid systems (HSs) with multiple point-absorption wave energy converters (WECs) integrated onto a floating semi-submersible wind turbine (WT). The focus of the present study is to explore the effects of the second-order wave forces on the dynamic behavior of the platform in the HS, including the platform motion and the mooring line tension, under both operational and extreme sea states. ANSYS/AQWA based on 3D diffraction/radiation potential theory is used to conduct numerical investigation on the second-order hydrodynamics of the HS. To ensure the reliability of the investigation, the validations of the semi-submersible platform and point-absorption WEC are done based on the available experimental data. Additionally, two models, "1st-order" and "1st&2nd-order", are set up for comparative analysis, to highlight the significance of the second-order forces. The simulation results show that the second-order hydrodynamics have a significant influence on the surge and pitch responses of the platform, but almost no effects on the heave response. In addition to this, ignoring the second-order hydrodynamics will significantly underestimate the mooring line tension response. Overall, this study suggests that the second-order hydrodynamics should be considered in the design of wind-wave HSs, in order to ensure their performance and safety, especially under the severe sea states.

Large Scale Experimental Study on Waves and Submerged Horizontal Cylinders

A large-scale physical model experiment was conducted to study the interaction between regular waves and submerged horizontal cylinders, breaking through the bottleneck of a low Reynolds number in a traditional small water tank. The surface pressure distribution and overall force of submerged horizontal cylinders at different submerged depths in waves with different heights and periods under a high Reynolds number were analyzed, and the variation in first- and second-order wave forces with wave height periods was analyzed. Subsequently, the influence of the cylinder on the wave field was analyzed, and the reflection coefficient was obtained through the three-point separation method. According to the analysis of the experimental results, the horizontal and vertical forces acting on the submerged cylinder per linear meter are similar, with the horizontal force slightly greater than the vertical force; at the same time, the submerged cylinder has a certain reflection effect on long-period waves. The summary, analysis, and display of relevant experimental results provide validation support for the current high-precision mathematical model as much as possible.

A Novel Method for Fatigue Damage Assessment in Bimodal Processes Considering High- and Low-Frequency Reduction Effects

Due to inherent nonlinearities within floating systems and the second-order wave forces affecting them, the dynamic responses of floating systems manifest as bimodal Gaussian processes. Consequently, the classical spectral fatigue assessment method grounded in the Rayleigh distribution cannot be applied. This paper introduces the double frequency coupled (DFC) method as a spectral fatigue assessment approach, providing an accurate estimation of fatigue damage originating from bimodal Gaussian processes. Within the DFC method, the bimodal Gaussian process is partitioned into two components: low-frequency (LF) and high-frequency (HF) processes. A Gaussian distribution is employed to describe the probability distribution function (PDF) of the amplitude reduction induced by the interaction between LF and HF processes. The PDF of small-cycle fatigue can be computed by convoluting the PDF of HF amplitudes and the reduction amplitude between LF and HF. Similarly, the PDF of large-cycle fatigue can be determined through convolution, which involves the PDF of LF amplitudes and small-cycle fatigue. The overall fatigue damage arising from the bimodal Gaussian process is obtained by directly summing the contributions of small-cycle and large-cycle fatigue. Numerical investigations of the DFC method’s effectiveness are presented through a series of parametric studies, demonstrating its robustness, efficiency, and accuracy within engineering expectations. Furthermore, the DFC method is found to be applicable to both single-slope and two-slope S-N curves.

Investigation of the dynamic response of a floating wind-aquaculture platform under the combined actions of wind, waves and current

Floating wind-aquaculture platforms are drawing increasing attention from the academic and engineering communities due to their potential to fully exploit and utilize marine space and its resources. However, these platforms integrate both the hydrodynamics and aerodynamics of floating wind turbines and aquaculture cages, making their mechanical properties more complex. This study aims to evaluate the effects of three different hydrodynamic and aerodynamic damping components on and the contribution of the stochastic environmental loads to the dynamic response of a floating wind-aquaculture platform. A coupled hydro-aero-servo method is established. Decay and forced oscillation tests of the platform in still water are firstly numerically performed, followed by simulation of the dynamic behavior under different combinations of environmental loads, including the fluctuating wind load of the blades, stochastic wave excitation forces on the floating body and viscous force of the aquaculture cage system. The aerodynamic damping of the wind turbine and the hydrodynamic damping of the floating body are dominant in low- and wave-frequency range, respectively. Regarding the environmental load components, the second-order wave force and the turbulent wind load are dominant in the surge direction in the low-frequency range. The dynamic response of the platform in the wave-frequency range is mainly induced by the first-order wave force. Fish net can suppress the low-frequency motion but has almost no influence on the wave-frequency motion.

Hydroelastic analysis of a floating bridge under spatially inhomogeneous waves, with emphasis on the effect of drift force modeling

The pontoon-supported floating bridge is an alternative concept for crossing a fjord with a width of up to 5 km. Due to the huge span, the first two natural periods of the bridge are more than 60 s, indicating that the corresponding modes can be excited by slowly-varying drift forces on the pontoons. A simplified approach for determining the drift forces is to assume that the pontoons are independent of the floating bridge, i.e., fixed or freely floating. However, the first-order motions of the pontoons, which are restricted by the deformation of the floating bridge, have a significant effect on the drift forces. To evaluate the uncertainties implied by the simplification of drift forces, in this study, a second-order beam-connected-discrete-modules (BCDM) hydroelastic method is adopted to assess the effect of different drift force models on the floating bridge. In this method, the first-order response amplitude operators (RAOs) of bridge-restricted pontoons are first solved in the frequency-domain. Based on the RAOs, the mean drift forces for each pontoon are then determined by using potential theory. Considering the spatial inhomogeneity of the wave field, the time series of the first- and second-order wave forces are generated by using the linear transfer functions and Newman’s approximation, respectively. Then, the method is applied to investigate the hydroelastic responses of a straight side-anchored floating bridge for the crossing site of Bjørnafjord. The results show that the horizontal displacement is very sensitive to the different force models implied by various pontoon boundary conditions, i.e., free, fixed, and bridge-restricted. The drift forces on the bridge-restricted pontoons are approximately 10%–20% larger than those for the fixed pontoons. Moreover, wave inhomogeneity results in increased vertical displacements and weak axis bending moments.

Numerical study of second-order wave forces using a Rankine panel method

Numerical analysis of the wave forces acting on ships can be complex primarily because of the interactions between incident waves and structures. In this paper, the three-dimensional frequency domain Rankine panel method is used to study the seakeeping and second-order wave drift forces of a ship in waves. The model considers second-order wave forces acting on the ship hull and therefore accounts for slow drift ship motions and the added resistance. The second-order wave forces are evaluated using the pressure integral method, also known as the near-field approach. The unsteady velocity potential is divided into three parts namely (a) incident, (b) diffraction and (c) radiation to satisfy the Laplace equation. To avoid wave reflection the Rayleigh’s artificial friction is used in way of the free surface boundary. This method is equivalent to introducing a numerical damping beach for energy dissipation. The numerical results are compared with those of experiments for Wigley-III hull in head waves and the SCb-84 hull in oblique waves. Good agreement is obtained.

Investigation of second-order low-frequency wave forces approximations for moored floating structures

Floating structures are widely used in both shallow and deep waters in industries such as offshore oil, gas, and wind. However, the safety of floating structures subject to long-crested irregular waves is threatened by slowly varying wave drift forces. In this study, the mesh-convergence laws for a panel model, which is used to calculate second-order potentials using a near-field approach, are investigated using a semi-submersible platform as an example. The effect of the relative-wave-height component of low-frequency (LF) wave forces is sensitive to the mesh near the waterline; therefore, it should be fully refined. The key difference-frequency band of the quadratic transfer function (QTF) matrix is investigated using time-domain-coupled dynamic analysis with a truncated QTF matrix. The results show that a difference-frequency limit of 0.4 rad/s is sufficient to calculate the slow drift and mooring tension. Based on prior approximations of the second-order LF wave force, a new hybrid approximation method is proposed. The discrepancies between the approximations and the full-QTF method are investigated in the time domain. This shows that the approximations are greatly affected by the water depth and the spectral peak period. This is because shallow-water mooring systems are more rigid than deep-water mooring systems; thus, the structural response is strongly dependent on a wider difference frequency and the off-diagonal elements in the QTF matrix (with a bigger difference frequency), which contain more errors. In approximations, the mooring-tension error is more sensitive to the water depth than the slow drift owing to the effects of heave and pitch. If the maximum values of the drift offset and mooring tension are considered, the approximation without the free-surface integral and the proposed “WN + Newman” hybrid approximation are recommended for use in shallow water.

A finite element-scaled boundary finite element coupled method for corner singularities in 2D second-order radiation problems

In potential flow analyses, singularities at the corners of bodies lead to erroneous pressure integrations and unsolved high-order velocity potentials. While much research has been devoted to addressing the former difficulty, fewer solutions have been proposed for the latter. In non-linear problems, higher derivatives are required on the boundaries, but usually, they are inaccurate and even singular at the corners. This paper introduces a coupling method that combines the scaled boundary finite element method (SBFEM) and the finite element method (FEM) to overcome this challenge. The SBFEM is semi-analytic and can accurately describe the singularities; the FEM is enhanced with Adini elements to achieve higher continuity. By integrating them, the proposed method allows for evaluating second derivatives on the body surface. The singular boundary conditions at the corners are automatically satisfied by the SBFEM, enabling the well-posed solution of boundary value problems (BVPs). The implementation of the proposed method is demonstrated through the solution of the second-order radiation problem of a rectangular box, with fine convergence observed for the first- and second-order velocity potentials and their gradients. The method also provides accurate and efficient direct pressure integrations for second-order wave forces, with results in good agreement with established indirect methods.

Hydrodynamic and environmental modelling influence on numerical analysis of an innovative installation method for floating wind

This paper presents a numerical modelling methodology for an innovative installation method of floating wind turbines. The study analyses the influence of hydrodynamic and environmental modelling factors on the dynamics of a multibody system consisting of a catamaran with three wind turbines and a spar foundation with a mooring system. The research focuses on features such as viscous drag on the spar, hydrodynamic interaction between floating bodies, long- and short-crested waves, first- and second-order wave loads, wave directions, and wave peak periods. The study found that hydrodynamic modelling factors had a significant effect on the horizontal motion of the installation system but a negligible effect on vertical motion and mooring system tension. Therefore, it is recommended to consider viscous drag forces, hydrodynamic interaction, and first- and second-order wave forces when developing mechanical damping and control systems to reduce horizontal motion. The study also suggests using both long- and short-crested waves for numerical analysis when estimating the weather window for installation operability analysis. Overall, this study contributes to the development of an efficient and practical installation method for floating wind turbines by analysing the impact of hydrodynamic and environmental factors on the dynamics of the multibody system.

Research on coupled dynamic characteristics of Floating Wind Turbine under complex environment

The fully coupled aerodynamic-hydrodynamic-mooring numerical model of floating offshore wind turbine(FOWT) has been established. The complex conditions such as second-order wave force, full-field turbulent wind, and mooring break have been taking into account. Using OrcaFlex and FAST to calculate the dynamic response of FOWT under the combined action of wind, wave and current. The time-domain response of floating foundation, mooring tension, blade root contact force, aerodynamic power, and wind rotor thrust are obtained. And the effect of the turbulent wind and mooring break at different positions on the FOWT are studied. The results show that compared with the platform motion and mooring tension, the aerodynamic power and rotor thrust are more sensitive to the turbulent wind, and there are fluctuations in amplitude and multiple peaks in the frequency spectrum. Compared with the leeward side, the mooring break on the windward side is a more dangerous situation, and it is easy to cause the collapse of platform and the break of the remaining mooring. The mooring break on the leeward side is relatively safe, and it will produce a drift in the sway direction, and have little impact on the safety of the wind turbine.

On-site wave-wind observation and spectral investigation of dynamic behaviors for sea-crossing bridge during tropical cyclone

The dynamic behaviors of a sea-crossing bridge during a tropical cyclone event which passed through the bridge site in September 2015 are simulated using in-situ observation wave and wind data. The hourly observation data during the cyclone event at the bridge site including averaged wind speed, turbulence intensity, direction, significant wave height, mean period, spectral variations are recorded. The wave and wind spectra are analyzed from raw data and are fed to a frequency-domain framework for response modeling. The stochastic wave actions considering foundation geometry, diffraction effect, wave coherence and second-order quadratic wave force transfer functions are considered using boundary element methods and the wind actions including buffeting loads, aerodynamic stiffness and damping are modeled based on stationary buffeting theory. Results reveal that the maximal structural response occurs near the cyclone eye wall while the storm center is still 80–100 km away but experiences sudden drop when the cyclone eye reaches the bridge site. Deck responses are much more sensitive to wind, but wave dominates the tower and base force responses. Second-order wave load is found to be critical to responses because the sum-frequency wave force could undesirably coincide with the bridge eigenfrequencies.

Wave force identification for a large-scale quasi-elliptical cylinder from monitored wave elevation

Based on the potential theory, the wave force on a large-scale quasi-elliptical cylinder was identified using monitored wave elevation data. Expressions with underdetermined coefficients of quadratic transfer functions for a noncircular cylinder are presented. The linear wave forces, second-order wave forces, and hydrostatic forces on the cylinder are identified using recursive least squares in real time. A comparison between the experimental and identified results of wave forces demonstrates the effectiveness of the identification method for a noncircular cylinder. The identified wave force consisted of several force components that correspond to the Fourier coefficients of the radius function. Generally, at a low wave number, the force components corresponding to the Fourier coefficient of the radius function decrease as the order of the harmonic term increases. With increasing wave number, the force components at the high-order harmonic term may significantly increase and become the dominant components of the wave force. The phase difference between the force components was studied, in which the phase difference of the first two main components was zero in the short-side direction and is π in the long-side direction. Phase jumps occur when the force components are zero.

Motion Analysis of A Wind-Wave Energy TLP Platform Considering Second-order Wave Forces

Offshore wind energy has become a major energy source, and various studies are underway to increase the economic feasibility of floating offshore wind turbines (FOWT). In this study, the characteristics of wave-induced motion of a combined wind-wave energy platform were analyzed to reduce the variability of energy extraction. A user subroutine was developed, and numerical analysis was performed in connection with the ANSYS-AQWA hydrodynamic program in the time domain. A platform combining the TLP-type FOWT and the Wavestar-type wave energy converter (WEC) was proposed. Each motion response of the platform on the second-order wave load, the effect of WEC attachment and Power take-off (PTO) force were analyzed. The mooring line tension according to the installation location was also analyzed. The vertical motion of a single FOWT was increased approximately three times due to the second-order sum-frequency wave load. The PTO force of the WEC played as a vertical motion damper for the combined platform. The tension of the mooring lines in front of the incident wave direction was dominantly affected by the pitch of the platform, and the mooring lines located at the side of the platform were mainly affected by the heave of the platform.

The Effect of the Second-order Difference-frequency Wave Force on the Motions of an Underwater Vehicle Near the Surface

_ When an underwater vehicle performs tasks, such as power recharge, air exchange, deployment, and retrieval near the surface, the low-frequency suction force and pitch moment will interfere with its motion. This paper presents a study on the effect of second-order difference-frequency wave force on the motions of the DARPA SUBOFF model at various submergence depths and environmental conditions. Ignoring the second-order wave force component induced by the second-order difference-frequency force may underestimate the second-order effect in irregular waves in the time domain. Comparisons were made between Newman’s approximation, which sets the off-diagonal difference-frequency quadratic transfer function (QTF) value to the average over the corresponding diagonal values, and the full QTFs based on the Pinkster approximation in the frequency and time domains. The QTFs-based prediction is significantly larger values than Newman’s approximation. Time-domain simulations were conducted, to investigate the effect of the second-order difference-frequency wave loads on motion responses at different nondimensional submergence depths and various sea conditions. The results of the computation indicate that ignoring the second-order difference-frequency wave force may underestimate the motion and dynamic responses of underwater vehicles; in addition, the pitch and heave motion responses are dominated by the low-frequency response. Introduction When an underwater vehicle travels close to the water surface, it experiences an upward force and pitch moment, known as surface suction. This can significantly affect the underwater vehicle’s behavior when power recharge, air exchange, deployment, and retrieval are incorporated into the underwater vehicle simulations. Veillon et al. (1996) noted that for a 10,000-ton submarine at a depth of 50 m, the surface suction due to waves will require around 20–30 tons of compensation to stop the submarine from surfacing. In addition, Hirom (1974) indicated that in waves, the fluctuating forces on a submarine can be on the order of 1000 kN, with a steady component of approximately 10 kN. In calm water, suction occurs owing to a higher flow velocity and a lower pressure above an underwater vehicle when sailing near the surface, and the wave pattern generated by the underwater vehicle complicates the surface effect. The longitudinal position of the center of pressure is after the amidships, which induces the pitch moment. In the presence of waves, the first-order wave force, which is the oscillatory force at the wave frequency, acts on the underwater vehicle. In addition, waves and model motions induce low-frequency suction force and pitch moment. Using any internal compensation and control surfaces to offset the induced suction force and pitch moment requires a good and early understanding of the magnitude of these forces, for designing sufficient-capacity compensation tanks (Crossland 2013). Therefore, the surface suction and pitch moment effects are essential components of the hydrodynamic characteristics of underwater vehicles sailing near the water surface.

Discrete-module-beam-based hydro-elasticity simulations for moored submerged floating tunnel under regular and random wave excitations

This study investigates the hydro-elastic behaviors of a moored submerged floating tunnel (SFT) using a fully-coupled time-domain hydro-elastic model based on the discrete-module-beam (DMB) method. In frequency domain, multibody hydrodynamic coefficients and wave excitation forces are obtained from 3D potential theory. In the time-domain DMB-based hydro-elastic model, the multibody Cummins equation and coupled stiffness matrix according to the theory of Euler-Bernoulli beam and Saint-Venant torsion are used. Mooring lines are modeled with rod theory and high-order finite-element formulations. The tunnel is then coupled with mooring lines by linear springs at respective connection locations. The present DMB model is validated through comparisons with two independent commercial programs, i.e., quasi-static computation based on the 3D solid model by ABAQUS and dynamic simulation using the semi-empirical Morison model by OrcaFlex. Then, the effects of various design parameters, such as submergence depths, buoyancy-weight ratios (BWRs), and wave conditions, on dynamic responses and mooring tensions are evaluated. The effects of nonlinear snap loadings and first- and second-order wave forces on SFT dynamics in regular and random waves are also investigated. Wet natural frequencies and mode shapes, hydro-elastic lateral and vertical displacements, mooring tensions, and bending moments for various cases are presented and analyzed.

Study on dynamic responses of hydraulic pneumatic tensioner for a TLP under hydraulic component failure

This paper takes the hydraulic pneumatic tensioner as the research object. In order to study the dynamic response of hydraulic pneumatic tensioner, a coupled model of tension leg platform was established. Three kinds of sea conditions including first-order irregular wave, second-order wave and freak wave are simulated. The dynamic response of the hydraulic pneumatic tensioner is analyzed under the action of different pressure accumulators. It is found that the greater the pressure of accumulator, the smaller the displacement of hydraulic cylinder and the greater the tension under the action of the second-order wave force and the freak wave force. In the second-order waves, the average drift force has great influence on the amplitude of displacement and tension of hydraulic cylinder, while the second-order sum frequency force has great influence on the frequency of displacement and tension of hydraulic cylinder. In addition, the dynamic responses of the tensioner under second-order waves are also studied when one cylinder fails. When one cylinder fails or is in semi-failure state, the hydraulic pneumatic tensioner can still provide a nearly constant tension to the riser. The remaining normal piston cylinders’ tensions will increase, but the total tension will decrease.

Dynamic analysis of bridge structures under combined earthquakes and wave loadings based on a simplified nonlinear Morison equation considering limit wave steepness

Utilizing the traditional Morison equation to calculate wave forces in cylindrical structures is cumbersome, and there are currently few studies regarding the high-order nonlinear Morison equation. In this study, the traditional Morison equation is simplified, and the simplified second-order nonlinear Morison equation based on the Stokes second-order wave is derived. The correctness of the simplified Morison equation is verified, and the use of the stability point method to solve the second-order nonlinear wave force is proposed. Based on the simplified second-order nonlinear Morison equation, the nonlinear dynamics of bridge structures under the combined loadings of earthquakes and second-order nonlinear waves are investigated. The results indicate that the displacement and stress responses of the second-order nonlinear wave to the bridge structure are significantly greater than those of the linear wave. When under second-order nonlinear waves and earthquake loadings combined within bridge structures, earthquakes will significantly affect these bridge structures. The formula for the breaking wave force based on second-order nonlinear waves is proposed. The breaking wave force based on second-order nonlinear waves has a large load on the bridge structures, and it will induce a weak high-frequency effect. When under combined the second-order nonlinear breaking wave and earthquake loadings on the bridge structure, earthquakes will still play a major role.

Coupled dynamic response analysis of multi-column floating offshore wind turbine with low center of gravity

To realize the application of the floating offshore wind turbine (FOWT) from deep to relatively shallow waters, a new concept of multi-column floating wind turbine platform with low center of gravity (CG) is designed and validated. The multi-column low CG platform is designed to support a 6MW wind turbine class and operated at a water depth of 50m in the South China Sea. The frequency domain software WADAM and time domain software NREL-FAST are used to simulate coupled dynamic responses of the floating wind turbine system with second-order wave loads considering. The dynamic behaviors of multi-column low CG FOWT system under normal operation and parked conditions are presented. The influence of second-order wave force on the motion responses of the multi-column platform, fore-aft force and moment of the tower base and mooring force are researched respectively. The results demonstrate that the coupled dynamic responses at rated operating condition and extreme condition meet the normal operating requirements and extreme survival requirements of FOWT system in the shallow water (50m) of South China Sea. In addition, it is found that, the wave frequency response gradually replaces the second-order low frequency response as the main influencing factor of the coupled dynamic response of the FOWT system with the increasing severity of the sea states. However, in general, the magnitude of second-order low frequency response increases with the increasing severity of the design load case. Thus, in the subsequent design of the shallow water FOWT system, the second-order effects should be paid enough attention.

Insights from detailed numerical investigation of 15 MW offshore semi-submersible wind turbine using aero-hydro-servo-elastic code

Offshore wind turbines are getting larger in terms of dimensions to maximize power generation. Large wind turbine offshore structures can be installed in deep water locations, where they have the advantage to generate higher power due to high wind speeds. To achieve this, wind turbine manufacturers are introducing larger and scaled-up designs to meet the global energy demands. However, these models suffer from large structural loads and deformations when subjected to varying environmental loadings. Therefore, it is necessary to fully comprehend the multibody coupled dynamics of such large offshore structures before proceeding towards their installation in open oceans. This paper carries out an extensive investigation on the ultimate load analysis and fully coupled dynamic response of the 15 MW reference turbine mounted over a UMaine VolturnUS-S semi-submersible floating platform. We used OpenFAST to carry out the stability and full-system linearization studies followed by a detailed time-domain analysis for the power production, parked and fault conditions. We began with a brief background theory on linearization and eigenanalysis for identifying the natural frequencies followed by discussion of the Campbell diagram showing the full-system natural frequencies. It was observed that the multi-blade coordinate code does not correctly pick the natural frequencies. Additional free decay tests have been carried out to verify the full-system natural frequencies. The variation of mean, minimum and maximum values of various characteristic loads and platform motions with respect to different environmental conditions are also presented. We also discussed the change in dynamic response due to the yaw misalignment, wind–wave misalignment, and second-order wave forces. The parked and fault conditions were found to be the critical design load cases (DLCs) for the design of IEA 15 MW wind turbine mounted on Volturn US-S semi-submersible. Furthermore, this paper gives an insight of the important DLCs to be considered while computing the design loads for similar large wind turbine structures.