Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2017;140(3):031101-031101-10. 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 three-dimensional (3D) 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.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(3):031102-031102-11. 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, and 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.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;140(3):031103-031103-11. doi:10.1115/1.4038586.

Two methods are investigated to simultaneously obtain both three-dimensional (3D) velocity field and free surface elevations (FSEs) measurements near a surface piercing foil, while limiting the equipment. The combined velocity field and FSE measurements are obtained specifically for the validation of numerical methods requiring simultaneous field data and free surface measurements for a slender body shape. Both methods use stereo particle image velocimetry (SPIV) to measure three component velocities in the flow field and both methods use an off the shelf digital camera with a laser intersection line to measure FSEs. The first method is performed using a vertical laser sheet oriented parallel to the foil chord line. Through repetition of experiments with repositioning of the laser, a statistical representation of the three-dimensional flow field and surface elevations is obtained. The second method orients the vertical laser sheet such that the foil chord line is orthogonal to the laser sheet. A single experiment is performed with this method to measure the three-dimensional three component (3D3C) flow field and free surface, assuming steady flow conditions, such that the time dimension is used to expand the flow field in 3D space. The two methods are compared using dynamic mode decomposition and found to be comparable in the primary mode. Utilizing these methods produces results that are acceptable for use in numerical methods verification, at a fraction of the capital and computing cost associated with two plane or tomographic particle image velocimetry (PIV).

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;140(3):031104-031104-10. doi:10.1115/1.4038677.

The paper presents an experimental study in a laboratory flume to investigate the combined wave–current flow over a pair of hemispherical obstacles placed at a relative spacing L/d = 4, where L is a center to center distance between the obstacles and d is the obstacle height. Detailed velocity data were collected using three-dimensional micro-acoustic Doppler velocimeter from upstream to downstream of the pair of obstacles along the centerline of the flume. This study examines the mean flows, eddy viscosity, mixing length, turbulence kinetic energy (TKE) flux under the influence of two hemispherical obstacles in combined wave–current flow conditions. The analysis reveals that higher level of turbulence including maximum eddy viscosity and TKE flux is observed near the top of the obstacles. Further, the power spectral density (PSD) for velocity fluctuation is also analyzed to study the internal structure of turbulence due to combined wave–current flow over hemispherical obstacles.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2017;140(3):031301-031301-8. 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 results 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, “On the Piston and Sloshing Modes in Moonpools,” J. Fluid Mech., 430, pp. 27–50.) theoretical and Fukuda's (1977, “Behavior of Water in Vertical Well With Bottom Opening of Ship and Its Effects on Ship-Motion,” J. Soc. Nav. Archit. Jpn., 1977(141), pp. 107–122.) 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 section. 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 nonzero Froude number especially in the presence of waves.

Commentary by Dr. Valentin Fuster

Research Papers: Materials Technology

J. Offshore Mech. Arct. Eng. 2018;140(3):031401-031401-9. doi:10.1115/1.4038582.

Residual stress produced by cold bending and welding processes contributes to the collapse pressure reduction of submarine hulls. Usually, the residual stress profiles used to quantify this reduction are obtained from analytical or numerical models. However, such models have limitations to take into account cold bending and welding in the same time. Hence, experimental analyses are necessary to better quantify the residual stress. Based on that, this paper presents residual stress experimental results obtained at six points on a pressure hull prototype using X-ray portable system. Based on these results, the residual stress profiles through the material thickness were estimated for each region on the frame by using a polynomial approximation. These profiles were introduced in a nonlinear finite element numerical model to study the collapse pressure reduction. Experimental results available on the literature were also used. Material and geometric nonlinearities were considered in the analysis. The results show that the residual stress reduces the collapse pressure as part of the frame web has stress level higher than the material yield. The preload introduced by the residual stress plays a less important role for the collapse pressure reduction at higher out-of-roundness and out-of-straightness defect amplitudes.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;140(3):031402-031402-8. doi:10.1115/1.4038502.

A strain concentration factor is typically incorporated in the higher-pressure and high-temperature (HPHT) pipeline lateral buckling assessment to account for nonuniform 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 behavior is evaluated. Global strain reduction and evolution against buckling are analyzed with respect to varying joint mismatch level. The concept of a strain reduction factor (SNRF) due to joint mismatch is developed based on the global strain capacity reduction with reference to the uniform configuration. It is demonstrated that the SNRF in terms of strain capacity reduction 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 SNRF and its application is developed. The discernible difference and scenarios for application of either factor are discussed, including low and high cycle fatigue, linearity and stress concentration, engineering criticality assessment (ECA), and lateral buckling. Additional causal factors giving rise to mismatch such as pipe schedule transition and buckler arrestor are also discussed. Iterations of finite element (FE) analyses are performed for a pipe-in-pipe (PIP) configuration in a case study.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2018;140(3):031601-031601-11. doi:10.1115/1.4038581.

This paper analyses and evaluates the variability of seismic demand and capacity of a case study jacket offshore platform considering different sources of uncertainty. The aleatoric uncertainty due to variability of near-fault ground motions, as well as uncertain properties of the damping ratio, material elastic modulus, material yield strength, mass, and gravity load have been investigated. The main aim of this study is to pursue which sources of the uncertainty considerably affect the seismic response of the platform. To this end, the sensitivity analysis was conducted not only in the particular earthquake level, but also in all other ranges of intensity using incremental dynamic analysis (IDA) with a special focus on the seismic collapse fragility. In order to reduce the number of simulations, the Latin hypercube sampling (LHS) scheme has been utilized as an efficient sampling procedure to combine the effects of modeling random variables. The collapse fragility curves are derived for each of the model realizations created with LHS technique. Thereafter, the summarized random fragility curve is compared with the deterministic mean parameter model fragility curve. It is found that the uncertainty in the mass and gravity load on the platform are the most influential variables, which can notably alter the IDA and collapse fragility curves. Furthermore, the random combination of the considered sources of uncertainty shifts the median of collapse fragility away from the mean parameter model collapse fragility. Overall, the effects of the uncertainties do not lead to notable changes in the summarized collapse fragility curve.

Commentary by Dr. Valentin Fuster

Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2018;140(3):031701-031701-9. doi:10.1115/1.4038583.

This paper addresses the statistical uncertainty in long-term fatigue damage in offshore structures due to the short-term simulation length used in time domain analysis of stresses. The paper focuses on steel risers applications. A new simulation-based estimator for the variance of the short-term fatigue damage is presented. The proposed estimator is based on a variation of the original nonparametric bootstrap. It works with blocks of data instead of discrete values, in order to better account for the autocorrelation of the stress cycles in the stress time series. This versatile estimator can be applied in time-domain fatigue analyses to assess the variance of the fatigue damage using a single stress time series and does not require any previous assumptions on the stochastic stress process.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2018;140(3):031702-031702-12. doi:10.1115/1.4038580.

Umbilicals, which link top floaters and subsea devices, provide control functions through electrical cables and hydraulic remote transmission. These cables are considered the “lifeline” of subsea production systems for offshore oil and gas exploitation. Umbilicals should undertake self-weight and periodic loading during operation because of the severe conditions of the ocean environment. Heat is released to the umbilical body during power transmission in electric cables, which influences the mechanical properties and optical transmission in the cable. However, several sectional arrangements can be applied to a number of umbilical components. Thus, sectional layout design with multiple components should be treated as a multidisciplinary optimization problem. From the mechanical point of view, the umbilical structure should be designed with compact and symmetric layout to obtain an even probability of resistance to loads and reduce structural stress, thereby improving fatigue performance. In terms of thermal effect, these electric cables should be arranged to dissipate heat easily and avoid influence on functional and structural components. This study quantifies compactness, symmetry, and temperature distribution by introducing corresponding indices. A multidisciplinary optimization framework is then established. Particle swarm optimization (PSO) intelligent algorithm is adopted to perform optimization and obtain the optimal solution, which is superior to the initial design. The optimization design strategy is proven effective and efficient by a case study, which provides a reference for umbilical design.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2017;140(3):031801-031801-11. doi:10.1115/1.4038349.

Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semisubmerged circular cylinder. The 2D simulations are carried out by solving Navier–Stokes equations discretized by finite volume method. Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank (NWT). 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 wavelength on wave–structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semisubmerged cylinder at different wave heights and the wavelengths. 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 are 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: Waves , Simulation , Cylinders
Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2017;140(3):031802-031802-8. 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. Semi-empirical VIV prediction tools are based on hydrodynamic coefficients. The hydrodynamic coefficients can either be calculated from experiments on flexible beams by using inverse analysis or theoretical methods, or obtained from forced motion experiments on a circular cylinder. Most of the forced motion experiments apply harmonic motions in either in-line (IL) or crossflow (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 Norwegian Deepwater Programme (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.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2017;140(3):031901-031901-10. 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 applying 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 the literature. Reactive power required by the power-take-off (PTO) system is presented. The effects of the additional penalty term are discussed.

Commentary by Dr. Valentin Fuster

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