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Research Papers: Piper and Riser Technology

J. Offshore Mech. Arct. Eng. 2015;137(2):021701-021701-11. doi:10.1115/1.4028904.

Reinforced thermoplastic pipe (RTP) is a composite thermoplastic pipe, which is increasingly being used in oil and gas industry. In practical applications, RTPs inevitably experience bending during reeling process and offshore installation. The ovalization instability of RTP under pure bending was investigated. Several fundamental assumptions of RTP were proposed from the engineering application point of view. Then, based on nonlinear ring theory initially proposed by Kyriakides et al., the effect of transverse deformation through the thickness was introduced, and the ovalization growth of cross section during bending was studied according to nonlinear kinematics. The formulation was based on the principle of virtual work and was solved by a numerical solution. Inelastic material behavior of high density polyethylene (HDPE) was included, and a simplified method was proposed to simulate the behavior of fiber reinforced layer. A detailed Abaqus model was established using solid and truss elements to simulate the HDPE layer and reinforced fiber, respectively. The results obtained from the theoretical method were compared with Abaqus simulation results and test data of verification bending experiment and the results show excellent agreement. The proposed methods are helpful for RTP's engineering applications.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021702-021702-9. doi:10.1115/1.4029357.

Closed-form analytical expressions are derived for the displacement field and corresponding stress state in two-layer cylinders subjected to pressure and thermal loading. Solutions are developed both for cylinders that are fully restrained axially (plane strain) and for cylinders that are axially loaded and spring-mounted. In the latter case, it is assumed that the combined two-layer cross section remains plane after deformation (generalized plane strain). The analytical solutions are verified by means of detailed three-dimensional finite element (FE) analyses, and they are easily implemented in, and suitable for, engineering applications. The chosen axial boundary conditions are demonstrated to be particularly relevant for pipeline and piping applications. By applying the exact solutions derived in the present study to typical offshore lined or clad pipelines, it is demonstrated that thermal expansion of the liner or clad layer may cause higher tensile hoop stresses in the pipe steel wall than accounted for in current engineering practice. It is shown that repeated cycles of start-up and shut-down phases for lined or clad pipelines cause significant plastic stress cycles in liners or claddings, which may pose a risk to the integrity of such pipelines.

Commentary by Dr. Valentin Fuster

Research Papers: Offshore Technology

J. Offshore Mech. Arct. Eng. 2015;137(2):021301-021301-10. doi:10.1115/1.4029211.

Oil spills can cause severe environmental damage. In situ burning or chemical dispersant methods can be used in many situations; however, these methods can be highly toxic and fail in slightly rough seas. Oil recovery techniques have also been developed to recover oil using skimmer equipment installed in ships. The challenges arise when a vessel is operated in heavy sea and current conditions. An oil skimmer has recently been developed by Extreme Spill Technology (EST) Inc. for automated oil recovery using a vacuum device installed in a vessel. Initial tests have shown that the prototype vessel is efficient in oil recovery. This paper presents the numerical and experimental studies of the hydrodynamic performance of the vacuum tower installed in the oil skimmer developed by EST. While the principle of the vacuum mechanism for oil skimming is simple, the hydrodynamic aspects of the recovery process is very complicated since it involves multiphase and multiscale moving interfaces, including oil, water, atmospheric air, and attenuate compressible air on the top part of the vacuum tower, and moving interface of oil slick, oil droplets, and air bubbles of different scales. The recovery process was simplified into a three-phase flow problem involving oil, water, and air and was simulated by using a computational fluid dynamics (CFD) method. The volume of fluid (VOF) method was employed to capture the moving surfaces between the fluid phases. Model tests were carried out to simulate the oil recovery process and for validation studies. Numerical results were compared with the experimental data. Studies were also extended to optimize the geometry of the tower for maximum oil recovery.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021302-021302-10. doi:10.1115/1.4029634.

A Rankine source method with a continuous desingularized free surface source panel distribution is developed to solve numerically a wave–body interaction problem with nonlinear boundary conditions. A body undergoes forced oscillatory motion in a free water surface and the variation of wetted body surface is captured by a regridding process. Free surface sources are placed in continuous panels, rather than points in isolation, over the calm water surface, with free surface collocation points placed on the calm water surface. Nonlinear kinematic and dynamic free surface boundary conditions along the collocation points on the calm water surface are solved in a time domain simulation based on a Lagrange time dependent formulation. Compared with isolated desingularized source points distribution methods, a significantly reduced number of free surface collocation points with sparse distribution are utilized in the present numerical computation. The numerical scheme of study is shown to be computationally efficient and the accuracy of numerical solutions is compared with traditional numerical methods as well as measurements.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Renewable Energy

J. Offshore Mech. Arct. Eng. 2015;137(2):021901-021901-9. doi:10.1115/1.4029212.

The accuracy of predicted loads on offshore wind turbines depends on the mathematical models employed to describe the combined action of the wind and waves. Using a global simulation framework that employs a domain-decomposition strategy for computational efficiency, this study investigates the effects of nonlinear waves on computed loads on the support structure (monopile) and the rotor–nacelle assembly of a bottom-supported offshore wind turbine. The fully nonlinear (FNL) numerical wave solver is invoked only on subdomains where nonlinearities are detected; thus, only locally in space and time, a linear solution (and associated Morison hydrodynamics) is replaced by the FNL one. An efficient carefully tuned linear–nonlinear transition scheme makes it possible to run long simulations such that effects from weakly nonlinear up to FNL events, such as imminent breaking waves, can be accounted for. The unsteady nonlinear free-surface problem governing the propagation of gravity waves is formulated using potential theory and a higher-order boundary element method (HOBEM) is used to discretize Laplace’s equation. The FNL solver is employed and associated hydrodynamic loads are simulated in conjunction with aerodynamic loads on the rotor of a 5-MW wind turbine using the NREL open-source software, fast. We assess load statistics associated with a single severe sea state. Such load statistics are needed in evaluating relevant load cases specified in offshore wind turbine design guidelines; in this context, the influence of nonlinear wave modeling and its selection over alternative linear or linearized wave modeling is compared. Ultimately, a study such as this one will seek to evaluate long-term loads using the FNL solver in computations directed toward reliability-based design of offshore wind turbines where a range of sea states will need to be evaluated.

Commentary by Dr. Valentin Fuster

Research Papers: Structures and Safety Reliability

J. Offshore Mech. Arct. Eng. 2015;137(2):021601-021601-10. doi:10.1115/1.4029316.

A Rankine source method is extended and applied to ship–ship interaction problems. The method covers the nonlinear steady flow and linear seakeeping in the frequency domain. The nonlinear steady flow solution accounts for the nonlinear free-surface conditions, ship wave, and dynamic trim and sinkage. Periodic flow due to waves is linearized with respect to the wave amplitude, taking into account interactions with the nonlinear steady flow following Hachmann approach, which considers the steady perturbation potential as constant in the body-fixed reference frame. This is advantageous for the prediction of ship motions at moderate to high Froude numbers. In this context, a new formulation of the boundary condition for the multibody case is derived. Two examples are considered, overtaking in calm water and replenishment at sea. For a feeder vessel overtaken by a container ship, horizontal forces and yaw moment are computed and compared with reference data. As an example of replenishment operation, interaction between a frigate and a supply vessel is studied. Ship motions are computed for two relative positions and three forward speeds and compared with model test data for the largest forward speed. The Rankine source method proves as more accurate compared with a zero-speed free-surface Green function method.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021602-021602-7. doi:10.1115/1.4029370.

In the reliability engineering and design of offshore structures, probabilistic approaches are frequently adopted. They require the estimation of extreme quantiles of oceanographic data based on the statistical information. Due to strong correlation between such random variables as, e.g., wave heights and wind speeds (WS), application of the multivariate, or bivariate in the simplest case, extreme value theory is sometimes necessary. The paper focuses on the extension of the average conditional exceedance rate (ACER) method for prediction of extreme value statistics to the case of bivariate time series. Using the ACER method, it is possible to provide an accurate estimate of the extreme value distribution of a univariate time series. This is obtained by introducing a cascade of conditioning approximations to the true extreme value distribution. When it has been ascertained that this cascade has converged, an estimate of the extreme value distribution has been obtained. In this paper, it will be shown how the univariate ACER method can be extended in a natural way to also cover the case of bivariate data. Application of the bivariate ACER method will be demonstrated for measured coupled WS and wave height data.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021603-021603-10. doi:10.1115/1.4029483.

Horizontal sectional loads (horizontal shear force and horizontal bending moment) and torsional moment are more difficult to predict with potential flow methods than vertical loads, especially in stern-quartering waves. Accurate computation of torsional moment is especially important for large modern container ships. The three-dimensional (3D) seakeeping code GL Rankine has been applied previously to the computation of vertical loads in head, following and oblique waves; this paper addresses horizontal loads and torsional moment in oblique waves at various forward speeds for a modern container ship. The results obtained with the Rankine source-patch method are compared with the computations using zero-speed free-surface Green functions and with model experiments.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021604-021604-7. doi:10.1115/1.4029536.

The results of a four points bending test on a box girder are presented. The experiment is part of series of tests with similar configuration but with different thickness and span between frames. The present work refers to the slenderest plate box girder with a plate's thickness of 2 mm but with a short span between frames. The experiment includes initial loading cycles allowing for partial relief of residual stresses. The moment curvature relationship is established for a large range of curvature. The ultimate bending moment (UM) of the box is evaluated and compared with the first yield moment and the plastic moment allowing the evaluation of the efficiency of the structure. The postbuckling behavior and collapse mode are characterized. Comparison of the experiment with a progressive collapse analysis method is made taking into consideration the effect of residual stresses on envelop of the moment curvature curve of the structure.

Commentary by Dr. Valentin Fuster

Research Papers: Ocean Engineering

J. Offshore Mech. Arct. Eng. 2015;137(2):021101-021101-9. doi:10.1115/1.4029484.

Extreme waves have led to many accidents and losses of ships at sea. In this paper, a two-dimensional (2D) hydroelastoplasticity method is proposed as a means of studying the nonlinear dynamic response of a container ship when traversing extreme waves, while considering the ultimate strength of the ship. On one hand, traditional ultimate strength evaluations are undertaken by making a quasi-static assumption and the dynamic wave effect is not considered. On the other hand, the dynamic response of a ship as induced by a wave is studied on the basis of the hydroelasticity theory so that the nonlinear structural response of the ship cannot be obtained for large waves. Therefore, a 2D hydroelastoplasticity method, which takes the coupling between time-domain waves and the nonlinear ship beam into account, is proposed. This method is based on an hydroelasticity method and a simplified progressive collapse method that combines the wave load and the structural nonlinearity. A simplified progressive collapse method, which considers the plastic nonlinearity and buckling effect of stiffened, is used to calculate the ultimate strength and nonlinear relationship between the bending moment and curvature, so that the nonlinear relationship between the rigidity and curvature is also obtained. A dynamic reduction in rigidity related to deformation could influence the strength and curvature of a ship's beam; therefore, it is input into a dynamic hydrodynamic formula rather than being regarded as a constant structural rigidity in a hydroelastic equation. A number of numerical extreme wave models are selected for computing the hydroelastoplasticity, such that large deformations occur and nonlinear dynamic vertical bending moment (VBM) is generated when the ship traverses these extreme waves. As the height and Froude number of these extreme waves are increased, a number of hydroelastoplasticity results including VBM and deformational curvature are computed and compared with results obtained with the hydroelasticity method, and then, some differences are observed and conclusions are drawn.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021102-021102-15. doi:10.1115/1.4029639.

Specification of realistic environmental design conditions for marine structures is of fundamental importance to their reliability over time. Design conditions for extreme waves and storm severities are typically estimated by extreme value analysis of time series of measured or hindcast significant wave height, HS. This analysis is complicated by two effects. First, HS exhibits temporal dependence. Second, the characteristics of HSsp are nonstationary with respect to multiple covariates, particularly wave direction, and season. We develop directional–seasonal design values for storm peak significant wave height (HSsp) by estimation of, and simulation under a nonstationary extreme value model for HSsp. Design values for significant wave height (HS) are estimated by simulating storm trajectories of HS consistent with the simulated storm peak events. Design distributions for individual maximum wave height (Hmax) are estimated by marginalization using the known conditional distribution for Hmax given HS. Particular attention is paid to the assessment of model bias and quantification of model parameter and design value uncertainty using bootstrap resampling. We also outline existing work on extension to estimation of maximum crest elevation and total extreme water level.

Commentary by Dr. Valentin Fuster
J. Offshore Mech. Arct. Eng. 2015;137(2):021103-021103-10. doi:10.1115/1.4029252.

A boundary element method (BEM) designed for solving the symmetric generalized Wagner formulation is presented. The flow field is parameterized with analytical functions and can describe the kinematics at any free surface or body location using a small set of parameters obtained from a collocation scheme. The method is fast and robust for all deadrise angles, even for flat plate impacts where classical BEMs usually fail. The method is easy to implement and is easy to apply. Given a smooth body contour the only additional input is the requested accuracy. There is no mesh involved. When solving the temporal problem, we exploit the analytical distribution of free surface velocities and apply an integral equation formalism consistent with the Wagner formulation. The output of the spatial and temporal scheme is a set of functions and parameters suitable for fast computation of the complete kinematics for any impact trajectory given the position of the keel and the body velocity. The method is developed to be combined with seakeeping programs for statistical impact and whipping assessment.

Commentary by Dr. Valentin Fuster

Research Papers: CFD and VIV

J. Offshore Mech. Arct. Eng. 2015;137(2):021801-021801-17. doi:10.1115/1.4029666.

The aim of this study is to develop a new probabilistic approach to determine nominal values for tank sloshing loads in structural design of LNG FPSO (liquefied natural gas, floating production, storage, and offloading units). Details of the proposed procedure are presented in a flow chart showing the key subtasks. The applicability of the method is demonstrated using an example of a hypothetical LNG FPSO operating in a natural gas site off a hypothetical oceanic region. It is noted that the proposed method is still under development for determining reliable estimates of extreme sloshing induced impact loads. It is concluded that the developed method is useful for determining the sloshing design loads in ship-shaped offshore LNG installations in combination with virtual metocean data and operational conditions.

Commentary by Dr. Valentin Fuster

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