Showing papers in "International Journal of Offshore and Polar Engineering in 2017"

Microstructure and Mechanical Properties of Cryogenic High-manganese Steel Weld Metal

Abstract: Extant studies have focused on the development of high-manganese austenitic steel, which is a potential cost-effective alternative to commercial cryogenic materials such as 9% Ni steels, 304 stainless steels, and Al5083 alloys. The development of suitable welding consumables is of significant importance in the commercial application of this new material for cryogenic applications. Specifically, flux-cored arc welding consumables that allow all-positional welding for high-Mn steel are required to fabricate liquefied natural gas (LNG) tanks. Hence, a welding wire alloyed with austenite-stabilizing elements (e.g., C, Mn, and Ni) was developed for cryogenic toughness. The microstructure and mechanical properties were evaluated as a function of the alloy composition. This unique combination of strength and toughness demonstrated the potential of this newly developed high-Mn steel for cryogenic services.

Comparative Study of Water-Impact Problem for Ship Section and Wedge Drops

Abstract: A comparative study of a water-entry problem was conducted as a focused session of ISOPE-2016, Rhodes by the International Hydrodynamic Committee (IHC) of ISOPE. Thirteen institutions participated, and twenty different numerical results were investigated and compared with one another and with model test data. Some promising results were obtained even though there is still a long way to go to draw general conclusions. Four numerical cases were investigated, and several measures of comparison have been discussed.

On the Tidal Resonance of the Bristol Channel

Abstract: The Bristol Channel has one of the largest tidal ranges in the world. A key cause for this is the resonance with the dominant semidiurnal tides. In this paper we use numerical simulations to investigate this resonance. We first vary the frequency on the boundary of the model and examine at which frequency the model is excited. Second, we apply a disturbance to the model and analyse the frequency at which it resonates. We examine the sensitivity of these results, finding them sensitive to the bed friction used (with possible implications for energy extraction) but insensitive to small changes in the tidal amplitude on the boundary or the mean-water level.

Recent Advances in Anchor Design for Floating Structures

Abstract: This paper describes advances in offshore anchoring technology made possible through research undertaken recently at the Centre for Offshore Foundation Systems at the University of Western Australia. The paper focuses on the behavior and performance of statically and dynamically installed anchors for floating structures, addressing issues associated with anchor installation and anchor capacity under monotonic, sustained, and cyclic loading. The role of centrifuge modeling, numerical modeling using large deformation analysis, and analytical modeling based on plasticity analysis in advancing anchor technology is presented. This modeling has led to tools that can assist anchor design to reduce uncertainty and optimize performance.

Study of the Trajectory and Landing Points of Dropped Cylindrical Object with Different Longitudinal Center of Gravity

Abstract: Dropped objects are among the top ten causes of fatalities and serious injuries in the oil and gas industry (DORIS, 2016). Objects may accidentally fall down from platforms or vessels during lifting or any other offshore operation. The accurate prediction of the landing points of the dropped objects may protect underwater structures and equipment from being damaged. In this paper, the authors propose a three-dimensional (3D) theory to numerically simulate the dynamic motion of a dropped cylindrical object under the water and to investigate the influence of the longitudinal center of gravity (LCG) on the motion. A numerical tool called Dropped Objects Simulator (DROBS) has been further developed on the basis of this 3D theory. It is initially applied to a dropped cylinder with its center of gravity at the center of volume (cylinder #1, LCG = 0) falling from the surface through calm water. The calculated trajectories match very well with both the experimental and numerical results published in Aanesland (1987). Then DROBS is further utilized to simulate two dropped cylinders with positive LCG (cylinder #2) and negative LCG (cylinder #3), respectively. The simulated results from DROBS show better agreement with the measured data than the numerical results given in Chu et al. (2005). This comparison again validates and indicates the effectiveness of the DROBS program. Finally, the simulation is applied to investigate the effects of varying LCGs on the trajectory and landing points. The newly developed DROBS program can be used to simulate the distribution of the landing points of dropped cylindrical objects in order to predict risk-free zones for offshore operations.

Numerical investigation of water entry problems using immersed boundary method

Abstract: This paper presents a systematic numerical investigation of the water-entry problems associated with dropping triangular wedges or ship sections that uses an incompressible Immersed Boundary Method (IBM). In the IBM, the solid bodies are treated as an additional phase, and their motions are solved by a unified equation similar to those governing the air and water flows; a level-set technique is used to identify the air-water interface, and a projected Heaviside function is developed to identify the fluid-solid interface. For the purpose of comparison, a corresponding numerical simulation with or without consideration of the compressibility of the fluids is also carried out by using OpenFOAM. All results are compared with the experimental data provided by the comparative study of ISOPE 2016. The results suggest that the unified equation in the IBM can well predict the motion of the dropping bodies; the IBM can capture the entrapped air and produce an impact pressure and local and global forces that agree fairly well with the experimental data.

Prediction of Long-Term Deformations of Offshore Wind Power Plant Foundations Using HCA-Based Engineer-Oriented Models

Abstract: The paper presents three engineer-oriented models based on the high-cycle accumulation (HCA) model of Niemunis et al. (2005), dedicated to the prediction of long-term deformations of offshore wind power plant (OWPP) foundations caused by wind and wave action. A sublayering model for shallow foundations under vertical cyclic loading and two different approaches (a sublayering model and a stiffness-degradation model) for monopile foundations subjected to horizontal cyclic loading are presented. The results of these models are compared to the solution from 2-D or 3-D finite element simulations with the original HCA model. Furthermore, the prediction is confronted with the prognosis of other engineer-oriented models proposed for OWPP foundations in the literature. Finally, a simplified procedure for the determination of the HCA material constants is briefly explained.

Motion Response of a Moored Semi-Submersible-Type Single Module of a VLFS in Multi-Slope Shallow Water

Abstract: The motion response of a moored semi-submersible-type single module (SMOD) of a Very Large Floating Structure (VLFS) is significantly influenced by the seabed topography and shallow water effects. To investigate the motion response of a moored SMOD that is located near islands, both numerical and experimental studies have been conducted. The hydrodynamic parameters of the SMOD were acquired by the use of the panel method. A finite-element model was adopted to calculate the tension forces of the mooring lines. It was found that the moored SMOD exhibited low-frequency characteristics in shallow water. As the seabed became more inclined, the roll motion became larger, while the sway and heave motions hardly changed. As the water depth became shallower, the heave and roll motions were mitigated; however, the sway motion was aggravated.

Wave-Dissipating Performance of Twin-Plate Breakwater Under Oblique Random Waves

Abstract: The present study investigates experimentally the wave-dissipating performance of twin-plate breakwater under oblique random waves. Through extensive model tests, the effects of the relative plate width and relative wave height on the dissipating performance are investigated under various incident wave angles. The results show that the dissipating performance changes insignificantly within the measured range of 15∼60 incident angles. The transmission coefficient kt under oblique waves fluctuates around the value under normal waves. In addition, the transmission was directly affected by the relative plate width, i.e., the transmission was 001 < kt < 0015 for B/L < 005 and 0005 < kt < 0010 for B/L ≥ 005. In particular, under oblique waves the wave patterns after the twin-plate breakwater exhibited significant three-dimensional characteristics, which were different from those under normal incident waves. To validate the present experimental results, the measured transmission coefficients are compared to those of Neelamani and Gayathri (2006) under normal incident waves, and the comparison shows good agreement.

Case Study for Impact of D-String® on Levelized Cost of Energy for Offshore Wind Turbine Blades

Abstract: The rapid growth of the offshore wind industry has led to a considerable increase in the size of offshore wind turbine blades. Loading of these has become an important factor, particularly edgewise loads resulting from gravitational forces. These loads imply development of longitudinal cracking on the trailing edge, with a significant impact on maintenance cost. As a result, several devices are in development with the goal of stiffening the blades. This paper aims to analyze the potential impact of such a device on the levelized cost of energy using a discrete event simulator. A case study is made using the NORCOWE reference wind farm.

Holistic Modeling of the Global Propulsion Energy Index in Waves for Small Craft

Abstract: In the international regulation framework, the energy-efficient operation of ships is becoming standard. In this respect, restrictions on new construction appear to encourage improvement to existing vessels often equipped with outdated technologies. One of the relevant aspects of propulsion plant design and fleet management is the propulsion need to accomplish the design requirements in a wide set of sea states or in conflicting operative conditions (e.g., laden/ballast, sailing/trawling), requiring very different performances. A preliminary assessment of the energy efficiency of the ship system is then crucial for optimizing both the operating costs and the impact on the sea environment. A new efficiency assessment method that includes engine fuel consumption evaluated by ad hoc statistic regressions and ship resistance in calm water and in waves computed by a 3-D boundary element method is proposed. An application to a hard-chine 18 m trawler is proposed as part of a wider decision support system or weather routing algorithm.

Hydrodynamic Interactions of Multiple Bodies with Water Waves

Abstract: A hierarchical wave interaction theory is reviewed as an innovative idea for treating hydrodynamic interactions among a great number of bodies rigorously in the framework of linear potential theory. We also introduced experimental results that were obtained using a structure consisting of 64 truncated vertical circular cylinders arranged in a periodical array of 4 rows and 16 columns. From the observation of measured results and their comparison with computed results, the effects of multiple-body interactions on the wave elevation and local steady forces are noted with respect to the wave frequency and the spatial position inside the structure. It is observed that the characteristics of wave interactions clearly change depending on whether the wave frequency is below or above the near trapped-mode frequency. Overall agreement between measured and computed results is very good, although slight differences attributed to viscous effects are observed.

Performance of a Mono Bucket Foundation – A Case Study at Dogger Bank

Abstract: This paper presents the results from the measurements of a mono bucket foundation supporting a met mast at Dogger Bank, which is the location of a future wind farm. The foundation is installed at a water depth of 23 meters and in soil conditions with layers of dense sand and stiff clay. The mono bucket foundation has a diameter of 15 meters and a skirt length of 7.5 meters. The presented data are the water surface elevation, the inclination of the bucket lid, and the generation of excess pore pressure inside the bucket, which contain data from a half year of measurements. The measurements show that the structure has been exposed to more than one severe storm. This paper describes the behaviour of the mono bucket foundation in terms of rotation and excess pore pressure generation. The measurements are used to identify the general behaviour of the full-size mono bucket foundation and show evidence of the presence of suction pressures generated during impact loads, which creates undrained moment and uplift forces.

Modeling and Validation of Simulation Results of an Ice Beam in Four-Point Bending Using Smoothed Particle Hydrodynamics

Abstract: The goal of this article is to investigate the applicability of smoothed particle hydrodynamics (SPH) to simulate four-point bending failure of an ice beam. The possibility of improving the correlation between SPH and analytical results is explored by conducting a parametric study in both 2-D and 3-D, to determine the required formulation, number of particles, and appropriate values of the parameters affecting accuracy. This study was used to validate the method for simulating elastic bending and was extended to model bending failure of an ice beam. The results were subsequently compared with results from four-point bending experiments available in literature.

Numerical Studies of Coupling Effect of Sloshing on 3D Ship Motions

Abstract: The present work describes the development of an approach to study the problem of ship motions coupled with internal sloshing effects. A hybrid solution algorithm is developed that integrates a three-dimensional potential flow solver for ship motions into a viscous flow solver for sloshing loads. A three-dimensional time-domain forward-speed boundary element method (BEM), based on the transient Green’s function technique, is employed to capture the external ship hydrodynamics, while the open source finite volume method (FVM)-based solver, OpenFOAM, which incorporates an incompressible multiphase viscous flow model into an interface-capturing volume of fluid (VOF) technique, is employed to capture the internal sloshing hydrodynamics. After the individual solvers are validated against the experiments and other computations available in the literature, the solution from the coupled solver is discussed, and the influence of the coupling effects of sloshing loads on ship motions is brought out.

Stochastic Modeling of the Fate and Behaviors of an Oil Spill in the Salish Sea

Abstract: Expansion of oil pipelines in western Canada will significantly increase tanker traffic and the probability of oil spills in the Salish Sea. To study the potential environmental effects from an oil spill, a state-of-the-art three-dimensional oil spill model was forced by a newly developed, high-resolution, hydrodynamic and atmospheric model, to simulate the fate and transport of three selected oils in the Salish Sea. A stochastic approach under a wide range of environmental conditions indicated that there is a very high probability for contamination of the Haro Strait area and the majority of the oil would stay on the surface and accumulate on the shoreline, rather than disperse into the water column.

Modeling of Impact Waves in LNG Ship Tanks

Abstract: This paper focuses on large sloshing waves that may impact the side walls or the ceiling of prismatic liquefied natural gas (LNG) ship tanks. Large pressure peaks may occur locally, which may damage the containment system. The physics involved prior to such impacts is complex and includes the development of free-surface instabilities due to the shearing gas flow, the gas compression just before and after entrapment, and the emission of pressure waves into the liquid at the impact points. Furthermore, as the LNG is in a state close to saturation, phase change phenomena might play an important role during the impact process, e.g., during the phase of gas compression driving condensation. This clearly poses a challenge to the software codes. This paper focuses on some of these aspects, especially gas compressibility and the influence of phase change. This paper concludes with an investigation of the possibilities concerning nonisothermal experiments with liquid methane or LNG.

A Cost-Effective Method for Modelling Wave-OWSC Interaction

Abstract: Bottom-hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves. In our previous studies, wave and OWSC interaction has been investigated via computational fluid dynamics (CFD) models. However, these models were highly time-consuming, and significant re-reflection was observed. The present work couples a Boussinesq wave model with a CFD model in order to extend the scope of the applications of the previous models. This model takes advantage of the Boussinesq wave model, which simulates the wave propagation effectively, and the CFD model, which provides the local flow details comprehensively. The model is validated by a comparison of the present results with those obtained with the pure CFD model and the experimental tank testing. The final objective of the present work is to simulate some events experienced and recorded by the full-scale prototype (Oyster 800 developed by Aquamarine Power) incorporating the real bathymetry at the Oyster 800 site.

Global Performance of a KRISO Semisubmersible Multiunit Floating Offshore Wind Turbine: Numerical Simulation vs. Model Test

Abstract: The global performance of the KRISO square-type semisubmersible multiunit floating offshore wind turbine (MUFOWT) in irregular waves is numerically simulated by using a multiturbine floater-mooring coupled dynamic analysis program. The developed time-domain numerical-simulation tool is extended from the FAST–CHARM3D coupled dynamics program for a single turbine on a single floater. FAST has been developed by the National Renewable Energy Laboratory for years for the single unit. Recently, KRISO has designed and studied a square-type semisubmersible MUFOWT in which four 3 MW wind turbines are installed at each corner of a single floater. Additionally, 24 point-power-absorber-type linear-generator-based wave energy converters are set up, with six wave energy converters at each side of the platform. For verification, KRISO performed a series of model tests for this MUFOWT with 1:50 Froude scale. In this paper, the MUFOWT simulation program is used to reproduce KRISO’s model test results. In the fully-coupled multiturbine/hull/mooring dynamic simulations, the complete second-order difference-frequency wave forces are also included. The analysis results are systematically compared with the model test results, which shows reasonable correlation between them.

On the Applicability of Lattice Boltzmann Single-Phase Models for the Simulation of Wave Impact in LNG Tanks

Abstract: This contribution addresses the applicability of an efficient lattice Boltzmann-based single-phase free-surface model for the simulation of wave impact on the side walls of 2-D containers. The computational efficiency of the method is known to allow for very short turnaround times, but wave impact simulations have not been investigated in detail yet. Results for a selected wave impact case are discussed, the convergence behavior in space and time is analyzed, and limitations of the single-phase free-surface model are revealed. The results show that lattice Boltzmann method (LBM)-based single-phase free-surface models are a viable tool for predicting the impact wave behavior, but the quality of the pressure signal is limited, because of the absence of air in the simulations.

The Impact of Extreme Wave Events on a Fixed Multicolumn Offshore Platform

Abstract: This paper presents an experimental and numerical investigation into the magnitude and distribution of the hydrodynamic loads affecting a fixed multicolumn offshore platform (rigidly mounted tension leg platform) when subjected to extreme wave events All wave load components, including wave-in-deck slamming pressures, were predicted using a commercial computational fluid dynamics (CFD) code STAR-CCM+ and compared against experimental measurements Slamming pressures were calculated using both data obtained locally at discrete points and globally averaged over the whole exposed area of the deck In all simulated cases, the deck area exposed to a wave-slamming event was found to be in contact with a water–air mixture with a significant proportion of air phase It was concluded that the slamming pressure data for the exposed area provided better insights into the pressure changes due to air compressibility and its content

Numerical Simulation of Highly Nonlinear Sloshing in a Tank Due to Forced Motion

Abstract: FSID stands for free-surface identification. This is the name of a computational code that simulates highly nonlinear 2-D free-surface flow in potential theory. This theoretical framework is shown to be still valid to describe the interaction between two nonmiscible fluids (gas/liquid) in a closed tank, whatever the density ratio. That is why GTT has used FSID for years. This code quickly generates flow conditions before impact, hence providing more sophisticated computational fluid dynamics (CFD) codes with initial flow conditions. The last developments of the code concern the forced motion in three degrees of freedom and two-phase flow modeling. In the present paper the governing equations of the model are presented along with illustrative results.

Experimental Observation of the Effects of Liquid Temperature and Bubbles on Impact Pressure Inside Gas Pocket

Abstract: In this study, a series of drop tests were carried out to observe the effects of the impact pressure inside a gas pocket of water with different temperatures and/or a lot of bubbles. Cylindrical gas pockets were generated by dropping the body into the water with different temperatures and/or different bubble sizes. Dynamic pressures in gas pockets were measured by various types of pressure sensors installed on the disc that was part of the body. The results of this study are expected to be very useful in understanding the physical phenomena of sloshing inside real liquefied natural gas (LNG) cargo.

Numerical modeling of the configuration of a long-distance free-spanning submarine pipeline on an uneven seabed

Abstract: In this paper, a procedure is developed to analyze long-distance free-spanning pipelines on an uneven seabed through the incorporation of the Vector Form Intrinsic Finite Element (VFIFE or V-5) method with the bubble model, which is also called the UWAPIPE model. A dynamic contacting scheme is proposed to model variations of the contact situation between a seabed and a deforming pipeline on it. Specifically, a high-efficiency Message Passing Interface (MPI) parallel scheme is adopted to reduce the computing time of the procedure. Through the use of the procedure, the configuration of a 10 km long-distance pipeline lying on a real irregular seabed in the South China Sea is simulated, and the results are compared with a Remotely Operated Underwater Vehicle (ROV) survey and Det Norske Veritas (DNV) recommendations. The effects of the submerged weight and internal pressure of the pipeline are evaluated. Subsequently, the unevenness ratio is defined, and its relation to the pipeline bending moment is presented. The effects of the seabed undulation and adjacent spans are proved to be significant for the examined case.

Simplified Evaluation of Brittle Crack Arrest Toughness in Heavy-Thick Plate by Combined Small-Scale Tests

Abstract: To develop a simplified evaluation method for brittle crack arrest toughness in a heavy-thick plate, the correlation between large-scale tests (such as the ESSO test) and small-scale tests (such as the Naval Research Laboratory (NRL) drop-weight test and the Charpy V-notch impact test) was investigated. It was found that the Nil-Ductility Transition Temperature (NDTT) obtained by the NRL drop-weight test at the surface layer and the fracture appearance transition temperature (vTrs) obtained by the Charpy V-notch impact test at the quarter thickness position have a high correlation with brittle crack arrest toughness. A simplified evaluation equation for arrest toughness was suggested by the combination of the results of the small-scale tests. It was confirmed that the developed equation can be applied to various steels independent of the chemical composition and the manufacturing process.

Study of Water-Entry Impact of Wedge and Ship-Like Section Using Potential Theories and CFD

Abstract: This paper presents the results of potential-based methods and computational fluid dynamics (CFD) for the water-entry impact of a wedge and ship-like section. In the potential-based computation, a Generalized Wagner Model (GWM) and a Modified Logvinovich Model (MLM) were used. In the CFD computations, a constrained interpolation profile (CIP)-based method and commercial software were used for the prediction of fully nonlinear slamming phenomena. The grid convergence index for the peak pressure was analyzed for both CFD computations. The computational results were also compared with the experimental results obtained by the Korea Research Institute of Ships and Ocean Engineering (KRISO) as part of the Wave Induced Loads on Ships Joint Industry Project-III (WILS JIP-III). In the experiment, free drop tests were performed for 2-D wedges and ship sections of a containership. Accuracy was investigated in terms of the peak pressure, pressure distribution, local hydrodynamic force, and free-surface shape.

Onboard Monitoring System of Overbend Pipe During Deepwater S-Lay Installation

Abstract: In S-lay operations, the pipeline passes over the stinger and is laid on the seabed after welding and nondestructive tests. The high combined loadings of axial tension, bending moment, roller reaction force, and the pipelay vessel motion may result in plastic deformation in the pipeline, which is difficult to accurately quantify. A real-time onboard monitoring system is recommended to guarantee the safety of the pipelaying process in challenging projects that include very large strain in the pipeline. However, most of the current onboard monitoring systems focus on the submerged section and sagbend section of the pipeline. The overbend section is ignored because of the uncertainty resulting from the combined loadings and the dynamic process during the pipe passing over the stinger. The present paper proposes a novel online monitoring method based on the roller reaction force measurement, which can monitor the dynamic overbend pipe strain fluctuations in real time. Relevant analytical equations in the model are first derived, then a large-scale hybrid model test and numerical simulation are carried out to verify the proposed monitoring system.

Hull Form Multi-Objective Optimization for a Container Ship with Neumann-Michell Theory and Approximation Model

Abstract: With the continuous development of the shipbuilding industry and shipping business, hydrodynamic optimization of hull forms has drawn the attention of both academia and industry. This paper reports the details of an efficient, numerical, design optimization tool for hull form for container ships. This tool is composed of three functional modules: hull form deformation, hydrodynamic performance prediction, and optimization. The free-form deformation (FFD) and radial basis function (RBF) methods are employed to modify the ship hull globally and locally, respectively. To reduce the cost of the numerical optimization, which is always a challenging problem, a new potential theory, the Neumann–Michell (NM) theory, and the approximation model are adopted. In addition, the analysis of variance (ANOVA) method is used to represent the influence of each design variable on the objective functions. The high efficiency is illustrated by the optimization for a container ship. Wave resistance coefficients at three design speeds are minimized, and a Pareto front of solutions is obtained. The optimal hulls are verified and analyzed by the NM theory and a Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics (CFD) solver. Numerical results confirm the availability and reliability of the optimization tool described.