The motivation and objective of the present study are to propose a hybrid control system for offshore jacket platforms to mitigate the vibrations induced by multiple hazards, namely, the earthquakes and regular and irregular waves. State-of-the-art indicates that not much work is reported on hybrid control of offshore jacket platforms for multiple hazards using a control algorithm, which is robust against uncertainties. A decentralized sliding mode control algorithm using magneto-rheological (MR) dampers is employed for the semi-active controller because of its robustness against parametric uncertainties and reliability. Passive shape memory alloy rubber bearings (SMARBs) are selected as passive isolators because of their high damping capacities, high fatigue resistance, and super elastic behavior, which are highly desirable for offshore applications. The scope of the present study is to demonstrate the efficiency of the proposed controller and investigate the effects of different influencing parameters. A jacket platform, reported in the literature, is taken as an illustrative example. A significant reduction in the top deck displacement is observed. The position and number of MR dampers affect the performance of the controller significantly. Limitations of the controller imposed due to the greater weightage or penalty imposed on displacements by the semi-active control algorithm as well as due to the magnetic saturation of MR dampers are overcome by the high energy dissipation of the passive SMARBs, thus making the hybrid controller highly efficient. The effectiveness of the controller is more for the earthquakes and random waves than for the regular waves.

In this paper a detailed procedure to study the mooring line strength reliability of a semi-submersible platform is presented. A fully coupled analysis is conducted to calculate the mooring line tension of a deep water semi-submersible floating system operated in the 100-year wave condition in South China Sea. The Average Conditional Exceedance Rate (ACER) method is applied to predict the 3-hour extreme mooring line tension based on 10 and 20 minutes fully coupled dynamic simulation and the results are validated by the global maximum method. A Kriging metamodel is trained to predict the 3-hour mooring line extreme tension taking into account the effect of random hydrodynamic drag coefficients. The hydrodynamic sampling points are generated by Latin Hypercube Sampling technique. A reliability analysis is carried out by Monte Carlo simulation considering the random hydrodynamic drag coefficients and mooring line breaking strength.

Wave interaction with multiple bottom-standing rectangular porous structures of different structural parameters is numerically modeled using the multidomain boundary element method and the matched eigenfunction expansion method, while assuming linear water wave theory. The sensitivity of wave reflection and transmission to the wave and structural parameters is analyzed with the objective to maximize wave energy attenuation. Different configurations of multiple structures are tested for maximizing the efficiency of dissipation. Furthermore, wave trapping by multiple porous structures near a sloping rigid wall is studied. Bragg resonance is observed in the case of wave scattering by multiple structures irrespective of wave and structural parameters and is found as proportional to the number of structures deployed. In addition, the study reveals that in the presence of multiple structures, due to more wave energy dissipation, wave transmission in the lee side of the structures is reduced significantly when compared with that observed for a single structure.

Dynamic analysis of floating wind turbines often considers the hull as a rigid body. This paper explores the consequences of modeling the pontoons of a tension leg platform (TLP) wind turbine as flexible beams. The analysis is based on numerical simulations of free decays, structural response to wave excitation and short-term fatigue damage accumulation at tower base and tendons. In addition, the importance of hydroelastic effects due to the pontoons' vertical deformations is evaluated. Pontoon flexibility changed the platform natural periods and motion amplitude significantly, and the adoption of flexible pontoons reduced the predicted fatigue damage in the tower base and tendons. On the other hand, hydroelasticity had negligible consequences for motion and load responses considered here.

The derivation of a discrete mooring model for floating structures is presented in this paper. The method predicts the steady-state solution for the shape of an elastic cable and the tension forces under consideration of static loads. It is based on a discretization of the cable in mass points connected with straight but elastic bars. The successive approximation is applied to the resulting system of equations which leads to a significant reduction of the matrix size in comparison to the matrix of a Newton–Raphson method. The mooring model is implemented in the open-source computational fluid dynamics (CFD) model REEF3D. The solver has been used to study various problems in the field of wave hydrodynamics and fluid–structure interaction. It includes floating structures through a level set function and captures its motion using Newton and Euler equations in six degrees-of-freedom (6DOF). The fluid–structure interaction is solved explicitly using an immersed boundary method based on the ghost cell method. The applications show the accuracy of the solver and the effects of mooring on the motion of floating structures.

The dynamic response of a tension leg platform (TLP) floating offshore wind turbine (FOWT) was analyzed with considering the aero-hydro characteristic of the whole floating wind turbine system including the wind turbine, TLP platform, and tethers. The “aero-hydro” coupled dynamic analysis was conducted in ansys-aqwa with a dynamic link library (DLL) calculating the aerodynamics loading at every steptime based on the blade element momentum theory. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and regular wave show that the wind loads influence mainly the low-frequency response of the TLP FOWT. The wind loads have a large impact on the offsets of the TLP away from the initial position while the wave loads influence mainly the fluctuation amplitude of the TLP FOWT. The average TLP pitch response under the wind load is significantly larger due to the large wind-induced heeling moment on the wind turbine. In addition, the tension of tethers at the upwind end is greater than that at the downwind end. The wind loads could reduce effectively the average tension of the tethers, and the tension of tethers is significantly affected by the pitch motion. Results from the coupled dynamic analysis of TLP FOWT under the condition of turbulent wind and irregular wave show that the surge and pitch of TLP result in an obvious increase of thrust of the turbine and the amplitude of torque fluctuation, more attention should be paid to the pitch and surge motion of TLP FOWT.

Floating offshore wind platforms may be subjected to severe sea states, which include both steep and long waves. The hydrodynamic models used in the offshore industry are typically based on potential-flow theory and/or Morison’s equation. These methods are computationally efficient and can be applied in global dynamic analysis considering wind loads and mooring system dynamics. However, they may not capture important nonlinearities in extreme situations. The present work compares a fully nonlinear numerical wave tank (NWT), based on the viscous Navier–Stokes equations, and a second-order potential-flow model for such situations. A comparison of the NWT performance with the experimental data is first completed for a moored vertical floating cylinder. The OC5-semisubmersible floating platform is then modeled numerically both in this nonlinear NWT and using a second-order potential-flow based solver. To test both models, they are subjected to nonsteep waves and the response in heave and pitch is compared with the experimental data. More extreme conditions are examined with both models. Their comparison shows that if the structure is excited at its heave natural frequency, the dependence of the response in heave on the wave height and the viscous effects cannot be captured by the adjusted potential-flow based model. However, closer to the inertia dominated region, the two models yield similar responses in pitch and heave.

The simulation of wind and wave fields for the evaluation of jacket-supported offshore wind turbines during Hurricane Sandy of 2012 is the focus of this study. For realistic load assessment of offshore wind turbines, it is important that coupled wind, wave, and current fields with appropriate spatial resolution are provided throughout the evolution of the hurricane. A numerical model describing the hurricane track and intensity variation with time is used to generate the coupled wind, wave, and current fields. Time series of turbulent wind fields and coupled wave kinematics are generated using the hurricane simulation output, and a detailed procedure is presented. These coupled wind, wave, and current fields form the inputs for the analysis of a 5-MW offshore wind turbine supported on a jacket platform sited in 50 m of water and assumed located in the path of Hurricane Sandy. The turbine’s and support structure’s performance under the simulated loading conditions is the subject of a separate study.

In the present study, the wave interaction with the very large floating structures (VLFSs) is analyzed considering the small amplitude wave theory. The VLFS is modeled as a 2D floating elastic plate with infinite width based on Timoshenko–Mindlin plate theory. The eigenfunction expansion method along with mode-coupling relation is used to analyze the hydroelastic behavior of VLFSs in finite water depth. The contour plots for the plate covered dispersion relation are presented to illustrate the complexity in the roots of the dispersion relation. The wave scattering behavior in the form of reflection and transmission coefficients are studied in detail. The hydroelastic performance of the elastic plate interacting with the ocean wave is analyzed for deflection, strain, bending moment, and shear force along the elastic plate. Further, the study is extended for shallow water approximation, and the results are compared for both Timoshenko–Mindlin plate theory and Kirchhoff’s plate theory. The significance and importance of rotary inertia and shear deformation in analyzing the hydroelastic characteristics of VLFSs are presented. The study will be helpful for scientists and engineers in the design and analysis of the VLFSs.

The main purpose of the study is to investigate the breaking wave interaction with a group of four circular cylinders. The physical process of wave breaking involves many parameters, and an accurate numerical modeling of breaking waves and the interaction with a structure remain a challenge. In the present study, the open-source computational fluid dynamics (CFD) model REEF3D is used to simulate the breaking wave interaction with multiple cylinders. The numerical model is based on the incompressible Reynolds-averaged Navier–Stokes (RANS) equations, the level set method for the free surface, and the k–ω model for turbulence. The numerical model is validated with experimental data of large-scale experiments for the free surface elevation and the breaking wave force on a single cylinder. A good agreement is obtained between the numerical results and experimental data. Two different configurations with four cylinders are examined: in-line square configuration and diamond square configuration. For both configurations, three different tank widths and four different spacings between the cylinders are investigated. The breaking wave forces on each cylinder in the group are computed for each case for the two configurations, and the results are compared with the breaking wave force on a single isolated cylinder. Furthermore, the study investigates the water surface elevations and the free surface flow features around the cylinders. For the closely spaced cylinders in a relatively narrower tank, the cylinders in both configurations experience the maximum forces lower than the maximum force on a single cylinder. But for the widely spaced cylinder in a relatively wider tank, the forces are higher and lower for the upstream cylinders and downstream cylinders, respectively, than the maximum force on a single isolated cylinder. The results of the present study show that the interference effects from the neighboring cylinders in a group strongly influence the kinematics around and the breaking wave forces on them.

This paper explores the performance of a 10 MW offshore wind turbine (OWT) supported either on a large diameter monopile or a 4-legged jacket emphasizing on the nonlinear response of its belowseabed foundation. The seabed foundation alternatives, a monopile and a multipod foundation, are compared under monotonic, cyclic, and seismic loading. For all nonseismic scenarios considered, the monopile is more flexible than the jacket and transmits higher rotations at the OWT base. The differences between the two alternatives are amplified in the case of nonsymmetric cyclic loading; the monopile not only deforms more than the jacket but tends to accumulate irrecoverable rotation with increasing loading cycles. The seismic performance of the alternative support structures is assessed for a comprehensive set of earthquake motions. It is concluded that both systems are seismically robust especially when subjected to pure earthquake loading, neglecting the simultaneous action of wind and waves. Alarming issues for OWT performance may arise when a nonzero steady wind force is superimposed to the kinematically induced stressing of the seabed foundation due to the seismic wave action. Jacket legs settle unevenly, while monopiles are building up rotations at increasing rates. Assuming a design-level earthquake and a wind thrust of the order 60% of the NC wind loading amplitude, this seismically induced residual rotation for the monopile may often exceed the deformation tolerance criterion. For the same loading combination, the corresponding rotation of the Jacket installation remains safely within the prescribed limits.

To improve our current understanding of tsunami-like solitary waves interacting with sandy beach, a nonlinear three-dimensional numerical model based on the computational fluid dynamics (CFD) tool OpenFOAM ® is first self-developed to better describe the wave propagation, sediment transport, and the morphological responses of seabed during wave runup and drawdown. The finite volume method (FVM) is employed to discretize the governing equations of Navier–Stokes equations, combining with an improved volume of fluid (VOF) method to track the free surface and a k–ε model to resolve the turbulence. The computational capability of the hydrodynamics and the sediment transport module is well calibrated by laboratory data from different published references. The results verify that the present numerical model can satisfactorily reproduce the flow characteristics, and sediment transport processes under a tsunami-like solitary wave. The water-sediment transport module is then applied to investigate the effects of prominent factors, such as wave height, water depth, and beach slope, in affecting the beach profile change. Finally, a dimensionless empirical equation is proposed to describe the transport volume of onshore sediment based on simulation results, and some proper parameters are recommended through the regression. The results can be significantly helpful to evaluate the process of transported sediment by a tsunami event.

A numerical model is developed based on multidomain boundary element method (MBEM) to investigate the scattering of surface gravity waves by a pair of floating porous boxes. This study reveals that there is a significant reduction in wave forces acting on the boxes with the introduction of structural porosity. Moderate porosity is highly effective for shallow and intermediate water depth for dissipating wave energy, while high porosity is more suitable in case of deep water. A major part of the wave energy is trapped by the porous boxes when the widths of the boxes are close to half of the wavelength of the incident waves. Apart from wave reflection and energy dissipation, a pair of floating porous boxes traps a part of wave energy, which helps in the wave attenuation on the lee side of the porous box. Horizontal and vertical forces of the seaside porous box are 180 deg out of phase, while they are in-phase in case of the box located on the lee side. Optima in the horizontal and vertical forces are attained when the distance between the boxes are at periodic intervals of half the wavelength. This study will be of immense help in the design of box-type porous structures as effective wave barriers for the creation of tranquility zone in the marine environment.

This study suggests three different designs of the moonpool to reduce the internal flow of the moonpool. The experimental and numerical studies were performed to improve the dillship's operability by the flow reduction in the moonpool. Based on the previous result of model tests in two-dimensional (2D) wave flume, three different moonpool designs were proposed: design-1: combination of forward recess deck, vertical bulkhead and beach-type wave absorber; design-2: a step-type forward recess deck; and design-3: a forward recess deck with beach-type wave absorber. Experimental study was carried out in the Offshore Engineering Basin for 87 K compact drillship which was 87,000 Mt as design gross tonnage. To verify the experimental results and clearly understand the internal flow of the drillship's moonpool, numerical study was carried out using Reynolds-averaged Navier–Stokes based computational fluid dynamics (CFD) code. From these studies, it was found that all the proposed designs showed significantly reduced internal flow at resonance frequency compared to the original moonpool design of the target drillship, although the characteristic of internal flow reduction was different for each moonpool design. Additional studies were carried out to obtain the improved moonpool design that is expected to further reduce the internal flow of moonpool through CFD. Based on the results of three moonpool designs, the combined one with the step-type recess deck and the wave absorber presents best performance in the viewpoint of flow reduction in the moonpool.

The performance assessment of a wave energy converter (WEC) is a key task. Depending on the layout of the WEC system and type of power take-off (PTO) mechanism, the determination of the absorbed power at model scale involves several challenges, particularly when the measurement of PTO forces is not available. In irregular waves, the task is even more difficult due to the random character of forces and motions. Recent studies carried out with kinetic energy harvesters (KEH) have proposed expressions for the estimation of the power based only on the measured motions. Assuming that the WEC behaves as a KEH at model scale, the expressions for power estimation of KEHs have been heuristically adapted to WECs. CECO, a floating-point absorber, has been used as case study. Experimental data from model tests in irregular waves are presented and analyzed. Spectral analyses have been applied to investigate the WEC responses in the frequency domain and to derive expressions to estimate the absorbed power in irregular waves. The experimental transfer functions of the WEC motions demonstrated that the PTO damping is significantly affected by the incident waves. Based on KEH approach's results, absorbed power and PTO damping coefficients have been estimated. A linear numerical potential model to compute transfer functions has been also implemented and calibrated based on the experimental results. The numerical results allowed the estimation of combined viscous and losses effects and showed that although the KEH approach underestimated the absorbed power, qualitatively reproduced the WEC performance in waves.

A three-dimensional (3D) numerical two-phase flow model based on solving unsteady Reynolds-averaged Navier–Stokes (URANS) equations has been used to simulate breaking waves and steep waves past a vertical cylinder on a 1:10 slope. The volume of fluid (VOF) method is employed to capture the free surface and the k– ω shear–stress transport (k– ω SST) turbulence model is used to simulate the turbulence effects. Mesh and time-step refinement studies have been conducted. The numerical results of wave forces on the structure are compared with the experimental data (Irschik et al., 2004, “Breaking Wave Loads on a Slender Pile in Shallow Water,” Coastal Engineering, Vol. 4, World Scientific, Singapore, pp. 568–581) to validate the numerical model, and the numerical results are in good agreement with the measured data. The wave forces on the structure at different Keulegan–Carpenter (KC) numbers are discussed in terms of the slamming force. The secondary load cycles are observed after the wave front past the structure. The dynamic pressure and velocity distribution, as well as the characteristics of the vortices around the structure at four important time instants, are studied.

For the estimation of wave loads on offshore structures, relevant extreme wave events need to be identified. In order to achieve this, long-term wave simulations of relatively large scales need to be performed. Computational fluid dynamics (CFD) based numerical wave tanks with an interface capturing two-phase flow approach typically require too large computational resources. In this paper, a three-dimensional (3D) nonhydrostatic wave model is presented, which solves the Navier–Stokes equations and employs an interface tracking method based on the continuity of the horizontal velocities along the vertical water column. With this approach, relatively fewer cells are needed in the vicinity of the air–water interface compared to CFD-based numerical wave tanks. The numerical model solves the governing equations on a rectilinear grid, which allows for the employment of high-order finite differences. The capabilities of the new wave model are presented by comparing the wave propagation in the tank with the CFD approach in a two-dimensional (2D) simulation. Further, a 3D simulation is carried out to determine the wave forces on a vertical cylinder. The calculated wave forces using the new approach are compared to those obtained using the CFD approach and experimental data. It is seen that the new approach provides a similar accuracy to that from the CFD approach while providing a large reduction in the time taken for the simulation. The gain is calculated to be about 4.5 for the 2D simulation and about 7.1 for the 3D simulation.

Jacket structures have been widely used in oil and gas industry and are increasingly becoming competitive as a support structure of wind turbines at different water depths. These types of structures usually fix in transition or shallow waters where numerous field observations and experiments have shown that water particles tend to exhibit non-Gaussian characteristics. However, current engineering practice ignores the wave nonlinearity for the analysis and design of these structures. The application of linear irregular models might result in considerable uncertainties in the obtained wave loads and consequently the dynamic response and thus it is highly questionable. Therefore, it is crucial to calculate the dynamic response of jacket structures under both linear and nonlinear wave models to investigate the validity of linear wave models in different sea states. In this paper, the finite element (FE) model of a jacket structure located in Persian Gulf (SP17 jacket) is setup and applied to perform a comparative study of the dynamic response to both linear and weakly nonlinear random waves. The fatigue life of the jacket structure is then calculated under both wave models. This paper will substantially improve the understanding of the dynamic response of jacket structures under fatigue damage.

Accurate prediction of long-term extreme loads is essential for the design of wave energy converters (WECs), but it is also computationally demanding due to the low probabilities associated with their occurrence. Although a full long-term probabilistic analysis using integration over all sea states or Monte Carlo simulation (MCS) may be used, these methods can be prohibitively expensive when individual response simulations are complex and time-consuming. The application of polynomial chaos expansion (PCE) schemes to allow the propagation of uncertainty from the environment through the stochastic sea surface elevation process and ultimately to WEC extreme load response prediction is the focus in this study. A novel approach that recognizes the role of long-term ocean climate uncertainty (in sea state variables such as significant wave height and spectral peak period) as well as short-term response uncertainty arising from the unique random phasing in irregular wave trains is presented and applied to a single-body point-absorber WEC device model. Stochastic simulation results in time series realizations of various response processes for the case-study WEC. We employ environmental data from a possible deployment site in Northern California (NDBC 46022) to assess long-term loads. MCS computations are also performed and represent the “truth” system against which the efficiency and accuracy of the PCE surrogate model are assessed. Results suggest that the PCE approach requires significantly less effort to obtain comparable estimates to MCS.

In this paper, detailed experimental results are reported to study the effect of the surface wave of different frequencies on unidirectional current over the bed-mounted train of rib roughness. The model roughness used in this study is transverse square ribs that lengthened across the entire width of the recirculating wave channel. The center-to-center rib pitch (P) was constant during the experiments, thus generating a broad range of near-bed flow patterns for each of the three different surface wave frequencies studied here. The relative submergence associated with the roughness height (k) was 8, which fall in the category of large roughness. Velocity measurements were conducted using acoustic Doppler velocimeter (ADV), and a surface wave of different frequencies was generated using the plunger-type wavemaker. The measured velocity data were analyzed to determine the relative importance of mean flow over the train of rib roughness. Mean velocity profiles illustrate the well-known downward shift from the flat surface data of the semi-logarithmic portion of the law of the wall. The width of the turbulent boundary layer increases with the superposition of surface wave compared to that of the current-only flow. The results also show that the mean reattachment length decreases due to the superposition of surface wave on unidirectional current.