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The 1D Discrete Module Beam (DMB) discretize hydro-elastic body into multiple independent rigid bodies (modules) connected by beam elements. The technique has proven accurate in representing structures with a large length-to-breadth ratio (L/B), such as floating bridge, barge, and ship structures. Only one horizontal bending axes are typically considered for such structures. Bending stiffness in the other horizontal axis is considered stiff and consequently treated as rigid. However, this assumption is not satisfied for structures with L/B closer to one. This study aims to extend the 1D DMB method into the 2D DMB method in the frequency domain to simulate hydro-elastic structures with two horizontal bending axes by arranging the beam elements into grid-like configurations in both horizontal axes (x and y). The method is then validated through comparisons with published experimental results and other numerical methods for various sea states. The method’s versatility is then demonstrated by incorporating linearized mooring stiffness in the frequency domain, which consequently changes the structures’ hydrodynamic response. It is found that for freely floating bodies, the centerline deformation of both the 1D and 2D DMB methods is the same. However, the 1D DMB method cannot properly capture the breadth-wise elastic-deformation in oblique waves and the moored case. In those cases, the 2D DMB formulation becomes particularly important. The presented 2D DMB method is projected to be a practical tool to simulate floating structures with unconventional configurations such as floating solar panels and other very large floating structures.

We develop a fully-coupled time-domain hydro-elasticity model for the Submerged Floating Tunnel (SFT) based on the Discrete-Module-Beam (DMB) method. Frequency-domain simulation based on 3D potential theory results in multibody’s hydrodynamic coefficients and excitation forces for tunnel sections. Subsequently, we build the time-domain model with the multibody Cummins equation and external stiffness matrix from the Euler-Bernoulli and Saint-Venant torsion theories. We establish the mooring line model with rod theory and couple components with translational springs at their respective connection locations. We then compare the dynamic motions, wave forces, and mooring tensions between the present and Morison-equation-based elastic models under regular wave excitations at different submergence depths. The present model is especially important for the shallowly submerged tunnel in which the Morison model shows exaggerated motions, especially at high-frequency range.

The aim of this paper is to evaluate the hydrodynamic performance of a heaving buoy type wave energy converter (WEC) and power take-off (PTO) system. To simulate the nonlinear behavior of the WEC with PTO system, a three-dimensional potential numerical wave tank (PNWT) was developed. The PNWT is a numerical analysis tool that can accurately reproduce experiments in physical wave tanks. The developed time-domain PNWT utilized the previously developed NWT technique and newly adopted the side wall damping area. The PNWT is based on boundary element method with constant panels. The mixed Eulerian-Lagrangian method (MEL) and acceleration potential approach were adopted to simulate the nonlinear behaviors of free-surface nodes associated with body motions. The PM spectrum as an irregular incident wave condition was applied to the input boundary. A floating or fixed type WEC structure was placed in the center of the computational domain. A hydraulic PTO system composed of a hydraulic cylinder, hydraulic motor and generator was modeled with approximate Coulomb damping force and applied to the WEC system. Using the integrated numerical model of the WEC with PTO system, nonlinear interaction of irregular waves, the WEC structure, and the PTO system were simulated in the time domain. The optimal hydraulic pressure of the PTO condition was predicted. The hydrodynamic performance of the WEC was evaluated by comparing the linear and nonlinear analytical results and highlighted the importance accounting for nonlinear free surfaces.

Ocean environmental conditions, such as waves, winds, and currents, are getting harsher due to climate change. This means that oil and gas production platforms in the ocean may experience unexpectedly large environmental loads bigger than previous design loads. Also, many platforms are reaching the end of their design lives. Ensuring riser integrity is one of the most important issues for platform safety and service-life extension. Currently, monitoring sensors are deployed on risers, and structural evaluation methods are utilized to examine riser integrity. However, there are some limitations to the structural evaluation methods. Furthermore, platform operators continue to seek for more direct and cost-effective riser monitoring method due to the low price of oil. In this study, the MultiSensor Fusion (MSF) system is proposed to surmount technical and economic obstacles in real-time riser-monitoring technology. The MSF system is validated for TLP (tension-leg platform) risers by using numerical sensors and numerical-simulation tools.

Coupled dynamic analyses of 900-m-long SFT (submerged floating tunnel) are conducted for various system parameters and random waves. The SFT dynamics between the clamped-clamped fixed-end conditions are caused by elastic modes, and thus hydro-elasticity simulation program has been modeled, from which various elastic modes and wet natural frequencies are identified. Parametric studies are conducted to investigate SFT horizontal and vertical motions and mooring tensions with varying SFT bending stiffness, mooring interval, mooring material, and 100-yr-storm-wave conditions. The BWR (buoyancy to weight ratio) and water depth are fixed to be 1.3 and 80 m. Various nonlinear phenomena including larger downward motion and the corresponding mooring snap loading are observed and discussed.

Active anti-rolling tank (ART) is sophisticated equipment on a floating vessel to reduce roll motion for the slender ship-shape vessel. Three-dimensional panel based diffraction and radiation linear potential program employed to obtain hydrodynamic coefficients of floating vessel. For the ship motion, a BEM (Boundary Element Method)-based ship motion program was used and inner sloshing effects were conducted by a particle-based CFD (Computational Fluid Dynamics) program which is the Moving Particle Semi-implicit (MPS). By using panel program, the hydrodynamic coefficients were obtained in frequency domain, and then were converted into time domain ship motion simulation program. In this procedure, time memory effect was considered by Volterra series expansion. The ship motion program and sloshing program was coupled dynamically; inner tank received displacement, velocity and acceleration data from ship motion program and use them for inner tank motion, while the ship motion program was waiting external forces due to sloshing impact loads and inertia forces/moments from sloshing simulation program. Thus, two programs run simultaneously and allowed real time coupling effects of inner sloshing on vessel motion. By comparing response amplitude operator (RAO) of the vessel without anti-rolling tank, it was shown both values have good agreement. And then comparing between vessels with and without anti-rolling tank, it is shown that the effects of ART changed and shift RAOs. Furthermore, by changing the location of ART, location effects of ART were also investigated.

A hybrid offshore renewable energy platform with multiple wind turbines and wave energy converters in a square-type host structure is proposed by Korea Research Institute of Ships and Ocean Engineering (KRISO). In this paper, we conduct a comparative study of wave run-up and slamming occurrence for the numerical simulations and physical 1:50 experiments. This hybrid renewable energy platform comprises a square frame structure with slender support structures as well as spread mooring system. Considering relative kinematics of the three-dimensional structure to waves especially for random seas, it may undergo not only vertical but also horizontal slamming impacts. Following the investigation of the wave run-up (airgap), the three-dimensional slamming occurrence is examined. Both regular waves and random waves are considered. The system identification of the numerical model is checked against the measured data from static-offset and free-decay tests. Following the systematic comparison between the experimental and numerical data, maximum wave run-up and the worst slamming occurrence in terms of the relative velocities are identified.

In this paper, a thrust allocation algorithm is proposed to minimize the fuel consumption and the gas emission of the offshore platform dynamic positioning system. The thrust allocation algorithm generates thruster commands that keep the position of offshore platform while physical limitation. Generally, the offshore platform control system is an over-actuated system. Thus, a thrust allocation problem of the offshore platform can be determined as an optimization problem. In this research, a thrust allocation problem is designed to minimize the fuel-consumption and the gas emission. Fuel-optimal thrust allocation was newly formulated and solved based on penalty-method based optimization. Developed thrust allocation method was evaluated by comparing to conventional pseudo-inverse based thrust allocation. The proposed thrust allocation method was validated with comparison with an offshore support vessel static allocation cases. A fully coupled dynamics of hull, mooring, riser, and dynamic positioning system were simulated in time domain. The proposed thrust allocation method that uses penalty-method achieved a 3% accumulated fuel consumption reduction compared to the conventional pseudo-inverse method based thrust allocation algorithm in GOM 1-yr storm condition.

To accurately describe the dynamic response of aquaculture cages exposed to current loading, it is necessary to consider the reduction of fluid velocity due to the presence of the net. In this study, three wake models, originally developed for riser interaction, are applied to a cylindrical net to evaluate drag force, volume and projected area reduction when exposed to steady currents. The results are compared to available experimental data to validate the methodology and evaluate the wake models’ accuracy. The net has been modeled using the commercial software OrcaFlex. This tool is a fully nonlinear finite element software, capable to compute forces and displacements of the net when exposed to current and wave loading. Calculations were performed under a series of current speed and three ballast weight conditions. The effect of the number of elements used to describe the net is analyzed as well. Results without including wake effect show good agreement for drag force up to moderate current speeds. Nevertheless a significant improvement on drag force results was achieved when wake effect is included. Yet, at high current speed and heavier ballast, drag force is over predicted. Minor differences were found when different wake models are used. In addition, it is concluded that an increase on the number of elements shows no significant influence on drag force results. On the other hand, calculations for volume and projected area reduction get closer to experimental values. The findings confirm the relevance of wake effect on nets and serve as validation of the present numerical procedure.

Numerical tools for a single floating offshore wind turbine (FOWT) have been developed by a number of researchers, while the investigation of multi-unit floating offshore wind turbines (MUFOWT) has rarely been performed. Recently, a numerical simulator was developed by TAMU to analyze the coupled dynamics of MUFOWT including multi-rotor-floater-mooring coupled effects. In the present study, the behavior of MUFOWT in time domain is described through the comparison of two load cases in maximum operational and survival conditions. A semi-submersible floater with four 2MW wind turbines, moored by eight mooring lines is selected as an example. The combination of irregular random waves, steady currents and dynamic turbulent winds are applied as environmental loads. As a result, the global motion and kinetic responses of the system are assessed in time domain. Kane’s dynamic theory is employed to formulate the global coupled dynamic equation of the whole system. The coupling terms are carefully considered to address the interactions among multiple turbines. This newly developed tool will be helpful in the future to evaluate the performance of MUFOWT under diverse environmental scenarios. In the present study, the aerodynamic interactions among multiple turbines including wake/array effect are not considered due to the complexity and uncertainty.

For oil/gas production/processing platforms, multiple liquid layers can exist and their respective sloshing motions can also affect platform performance. To numerically simulate those problems, a new multi-liquid MPS (Moving Particle Simulation) method is developed. In particular, to better simulate the relevant physics, robust self-buoyancy model, interface searching model, and surface-tension model are developed. The developed multi-liquid MPS method is validated by comparisons against Molin et al’s (2012) three-liquid-sloshing experiment and the corresponding linear potential theory. The verified multi-liquid MPS program is subsequently coupled with a vessel-motion program in time domain to investigate their dynamic-coupling effects. In case of multiple liquid layers, there exist more than one sloshing natural frequencies, so the relevant physics can be much more complicated compared with the single-liquid-tank case. The numerical simulations also show that liquid cargo can function as a beneficial anti-rolling device.

The exploration and extraction of offshore hydrocarbon is currently facing stricter requirements in environmental conditions, structural integrity, and dynamic performance. Vibration control may be a critical part of mitigating the excessive dynamic responses of the offshore floating structures. If the structural responses can be monitored and controlled, then smart-platform technology can greatly widen the applicability of current technology toward deeper waters and more severe environmental conditions. This paper is focusing on the numerical simulations and analyses of top-tension risers in a tension-leg platform (TLP), incorporated with a bang-bang controlled magneto-rheological (MR) damper and variable stiffness (VS) system. The specific characteristics of the innovative system in alternating the damping forces and system stiffness show great potential to interactively change the structural behaviors corresponding to various external loadings. This research is expected to provide a robust and cost-effective solution for greatly expanding the capability of future smart offshore-platform technology.Copyright © 2014 by ASME

Recent increasing demands for more ocean energy and space utilization require larger scale of offshore structures, and the large scale leads to needs for comprehensive hydroelastic analysis to accurately account for the interaction of the floating body’s deformation with waves or other environmental loads. In this study, as generalization of the previous hydroelastic analyses by the present authors [4, 5], the three-dimensional hydroelastic analysis including torsion is achieved. Ocean wave is assumed to be potential flow. The example large-scale floating body studied here is 480 m long and 400 m wide with 8 m draft. It is modeled by 7857 elastic plate elements which have 6 degrees of freedom at each element. Modal analysis by finite element method (FEM) for all free boundary conditions is conducted and provides mode shapes, modal inertia/stiffness, and dry natural frequencies. The mode shapes are verified against experimental results by Leissa [10]. Using the mode expanded boundary element method (BEM), hydroelastic dynamics is first solved in frequency domain. Subsequently, the large flexible platform is applied to irregular waves in time domain. To investigate the three-dimensional dependency of dynamics, a series of oblique waves are applied as well as head waves, and the dynamic responses of the elastic system are systematically analyzed. Considering remarkable effect of added mass due to large submerged volume, its effects on elastic modes and resonance phenomena are also investigated. To validate the accuracy, pertinent verifications are carried out for both frequency and time domains.Copyright © 2014 by ASME

In the present study, a numerical simulation tool has been applied for the time-domain turbine-floater-tether fully-coupled dynamic analysis of a FOWT. The fully coupled dynamic analysis includes aero-blade-tower dynamics and control, mooring dynamics, and platform motions. In particular, the effects of second-order sum-frequency wave excitations on the coupled dynamic analysis are investigated. The fully coupled simulations with full blade operation are compared with those with parked condition (without blade rotation). For this purpose, a mono-column TLP with 5MW turbine in 200m water depth is selected as an example. The time histories and spectra of the FOWT motions and accelerations as well as tether top-tensions are presented for the given random collinear wind-wave condition. The shift of original floater natural frequencies due to the inclusion of tower flexural modes is demonstrated. The increase of aero damping in the case of rotating blades is also explained. The second-order sum-frequency wave loading introduces high-frequency excitations near pitch-roll resonance frequencies or lowest tower flexural modes. Its effects are more clearly seen in the blade-parked condition than the blade-fully-operational condition. The increased high-frequency responses may significantly increase tower-top accelerations and accumulated fatigue.Copyright © 2013 by ASME

Increasing numbers of FOWTs (floating offshore wind turbines) are planned in the coming years due to their high potential in massive generation of clean energy from ocean-wind. In the present study, a numerical prediction tool has been developed for the fully coupled dynamic analysis of an FOWT system in time domain including aero-loading, blade-rotor dynamics and control, mooring dynamics, and platform motions so that the influence of rotor-control dynamics on the hull-mooring performance and vice versa can be assessed. Hywind spar design with 5MW turbine is selected as an example, and two different environmental conditions, maximum operational and survival conditions, are applied for this study. The maximum operational condition means the maximum environmental condition that wind turbine can work normally, and the survival condition represents the extreme situation without any blade-turbine operation. Through this study, it is seen that the design environments for different structural components of FOWT can be different. The developed technology and numerical tool are readily applicable to the design of any future FOWTs in any combinations of irregular waves, dynamic winds, and steady currents.

The hydrodynamic performance of Backward Bent Duct Buoy (BBDB), a floating-type wave energy converter, was evaluated in the time-domain simulation by using a two-dimensional fully-nonlinear numerical wave tank (NWT) technique. The developed NWT was based on potential theory, boundary element method with constant panels, and the mixed Eulerian-Lagrangian (MEL) approach to capture the nonlinear free-surfaces. The viscous damping at the chamber entrance due to oscillating water column and the shape of body causing generation of vortex shedding were modeled and applied to the free surface boundary condition inside the chamber. The calculated surface elevations inside the chamber with open chamber condition were compared with experimental data to select a proper viscous damping coefficient. Then, the surface elevations with a tuned viscous damping coefficient were calculated for various wave conditions. The results of linear and nonlinear time-domain simulation with two different corner-shaped BBDBs were compared to investigate the mean drift force of BBDB. Energy conservation in the computational domain was checked for all cases.