Accepted Manuscripts

Debabrata Karmakar and Carlos Guedes Soares
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038500
The interaction of surface gravity waves with horizontal pitching plate for actively control waves is investigated based on the linearized theory of water waves. The two dimensional problem is formulated for the submerged plate pitching about its middle point and the other plate is considered to be floating above the submerged plate. The submerged plate's thickness is considered negligible in comparison with the water depth and wavelength of the incident wave. The study is carried out using the matched eigenfunction expansion method and the analytical solution is developed for the interaction of the surface gravity waves with horizontal submerged structure. The performance is analyzed for both impermeable and porous submerged pitching plate. The numerical results for the reflection coefficient, transmission coefficient and free surface deflection are computed and analyzed. The study is carried to find the optimal value of the length and depth of the submerged plate at which the dissipation of the incident wave energy is observed. The reduction the wave transformation due to the pitching of the plate with the change in angle of incidence is also analyzed. The present study will be helpful in the analysis of proper functioning of submerged pitching plate to control wave motion for the protection of offshore structures.
TOPICS: Wave motion, Plates (structures), Waves, Gravity (Force), Water waves, Energy dissipation, Eigenfunctions, Wavelength, Reflectance, Offshore structures, Deflection, Water, Wave energy, Submerged structures
M Liu and Colin Cross
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038502
A strain concentration factor is typically incorporated in the HPHT pipeline lateral buckling assessment to account for non-uniform stiffness or plastic bending moment. Increased strain concentration can compromise pipeline low cycle fatigue and lateral buckling capacity, leading to an early onset of local buckling failure. In this paper, the philosophy of local buckling mitigation using the strain concentration factor is examined. The local buckling behaviour is evaluated. Global strain reduction and evolution against buckling is analysed with respect to varying joint mismatch level. The concept of a strain reduction factor due to joint mismatch is developed based on the global strain capacity reduction with reference to the uniform configuration. It is demonstrated that the strain reduction factor is a unique characteristic parameter. As opposed to strain concentration it is an invariant insensitive to evaluation methods and design strain demand level, hence more representative as a limiting design metric to maintain the safety margin. The rationale for its introduction as an alternative to the strain concentration factor is outlined and its benefits are established. The method for obtaining the strain reduction factor and its application is developed. The discernible difference and scenarios for application of either factors are discussed, including low and high cycle fatigue, linearity and stress concentration, ECA and lateral buckling. Additional causal factors giving rise to mismatch such as pipe schedule transition and buckler arrestor are also discussed. Iterations of FE analyses are performed for a pipe-in-pipe configuration in a case study.
TOPICS: Underwater pipelines, Design, Buckling, Pipes, Pipelines, Safety, Evaluation methods, Failure, Low cycle fatigue, Stiffness, High cycle fatigue, Finite element analysis, Stress concentration
Qian Zhong and Ronald W. Yeung
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038503
Model-Predictive Control (MPC) has shown its strong potential in maximizing energy extraction for Wave-Energy Converters (WECs) while handling hard constraints. However, the computational demand is known to be a primary concern for ap- plying MPC in real time. In this work, we develop a cost function, in which a penalty term on the slew rate of the machinery force is introduced and used to ensure the convexity of the cost function. Constraints on states and the input are incorporated. Such a constrained optimization problem is cast into a Quadratic Programming (QP) form and efficiently solved by a standard QP solver. The current MPC is found to have good energy-capture capability in both regular and irregular wave conditions, and is able to broaden favorably the bandwidth for capturing wave energy compared to other controllers in literature. Reactive power required by the PTO system is presented. The effects of the additional penalty term are discussed.
TOPICS: Wave energy, Quadratic programming, Machinery, Control equipment, Waves, Optimization
Yiting Wang, Xuefeng Wang, Shengwen Xu and Lei Wang
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038501
Motion responses of moored very large floating structures (VLFSs) in coastal regions are remarkably influenced by shallow water, seabed topography, and mooring system, which were given particular focus in this paper. A three dimensional (3D) numerical model of a moored semisubmersible single module (SMOD) was described, time domain simulated and experimentally validated. A catenary-taut-hybrid mooring system was adopted considering coastal space limitations. Large scale catenary mooring lines were deployed on the deep water side, while taut chains were used on the shore side to decrease the anchor radius. Although the mooring system may induce a stiffness difference between the two sides, the effectiveness of the mooring system was demonstrated by time domain simulation and model tests. The moored semisubmersible SMOD in shallow water exhibits significant low frequency characteristics. Water depth, asymmetric stiffness and bottom topography effects were investigated by a series of sensitivity studies. The results show that these factors play an important role in motion responses of the moored SMOD, which can further conduce to better understandings on the hydrodynamic of the semisubmersible type VLFSs.
TOPICS: Semi-submersible offshore structures, Floating structures, Mooring, Seabed, Water, Shorelines, Stiffness, Chain, Computer simulation, Simulation
Sivabalan Ponnappan and Surendran Sankunny
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038396
A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes result in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin's (2001) theoretical and Fukuda's (1977) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross-sections. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and non-zero Froude number especially in the presence of waves.
TOPICS: Oscillations, Water, Drillships, Ships, Resonance, Waves, Shapes, Cross section (Physics), Downtime, Pistons, Drag (Fluid dynamics), Sloshing, Vessels
Suresh Rajendran, Nuno Fonseca and Carlos Guedes Soares
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038347
A time domain code based on strip theory is applied to calculate the probability distributions of relative motions and bending moments of a cruise ship in a set of extreme seas. The code includes two levels of complexity. The simpler one combines linear radiation and diffraction forces with nonlinear Froude-Krylov forces, hydrostatic forces and shipping of green water on the bow. Cummins formulation is used to represent the radiation forces. The second approach is a generalization of the first one and, although the formulation is based on the linear assumption (of the radiation forces), the effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces. The code is used to analyze the responses of a cruise ship in a set of extreme sea conditions. The short term nonlinear responses are represented by empirical probability distributions, obtained from the nonlinear time domain simulations, and the quality of the predictions is assessed by comparing with model tests experimental data. Finally, the long term value of the bending moment is calculated from the short term distribution of the nonlinear loads in a few extreme sea states selected based on coefficient of contribution method, and the results are compared with the IACS rule bending moment.
TOPICS: Stress, Waves, Statistical distributions, Vessels, Seas, Radiation (Physics), Diffraction, Ships, Information Analysis Centers, Water, Hydrostatics, Simulation, Strips, Engineering simulation
Decao Yin, Elizabeth Passano and Carl M. Larsen
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038350
Slender marine structures are subjected to ocean currents, which can cause vortex-induced vibrations (VIV). Accumulated damage due to VIV can shorten the fatigue life of marine structures, so it needs to be considered in the design and operation phase. VIV prediction tools are based on hydrodynamic coefficients, which are obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or cross-flow (CF) direction. Combined IL and CF forced motion experiments are also reported. However, measured motions from flexible pipe VIV tests contain higher order harmonic components, which have not yet been extensively studied. This paper presents results from conventional forced motion VIV experiments, but using measured motions taken from a flexible pipe undergoing VIV. The IL excitation coefficients were used by semi-empirical VIV prediction software VIVANA to perform combined IL and CF VIV calculation. The key IL results are compared with NDP flexible pipe model test results. By using present IL excitation coefficients, the prediction of IL responses for combined IL and CF VIV responses is improved.
TOPICS: Vortex-induced vibration, Cross-flow, Pipes, Marine structures, Excitation, Design, Damage, Ocean currents, Circular cylinders, Computer software, Fatigue life
Shengnan Liu, Muk Chen Ong, Charlotte Obhrai and Sopheak Seng
J. Offshore Mech. Arct. Eng   doi: 10.1115/1.4038349
Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semi-submerged circular cylinder. The 2D simulations are carried out by solving Navier-Stokes equations discretized by finite volume method (FVM). Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank. Validation studies have been performed by comparing the numerical results of free surface waves past the cylinder with the published experimental and numerical data. The present numerical results are in good agreement with both the experimental and the other numerical results in terms of hydrodynamic forces and free surface elevation. Subsequently, the effects of the wave height and the wave length on wave-structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semi-submerged cylinder at different wave heights and the wave lengths. The averaged and maximum vertical wave forces on the cylinder increase with the increasing wave height. The numerical results for the irregular waves are compared with those induced by the regular waves in terms of the maximum and averaged vertical wave forces. When the significant wave height and the spectral peak period of the irregular waves is equal to the wave height and the wave period of the regular waves, the maximum vertical wave force induced by the irregular waves is larger than that induced by the regular waves, meanwhile the average vertical wave forces have the contrary relationship.
TOPICS: Simulation, Waves, Computational fluid dynamics, Engineering simulation, Cylinders, Wave forces, Computer simulation, Fluids, Circular cylinders, Fluid-dynamic forces, Navier-Stokes equations, Significant wave heights, Finite volume methods, Surface waves (Fluid)

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