bluuebunny/crossref_metadata_2025 huggingface dataset:
Real-Time Downhole Weight on Bit (DWOB) Automation in Directional Drilling
Authors: Wu Andrew , Hareland Geir
Published: October 2015

This paper introduces the functionality of a new type of Autodriller software system, which can acquire downhole weight on bit (DWOB) based on surface rig measurement. Field tests are performed, including DWOB measured by downhole measuring tools and the hookload below the top drive using a TTS (Torque and Tension Sub). Three sets of drilling data from three horizontal wells in Western Canada were utilized to verify the models of this new Autodriller system. DWOB comparisons between the model and the measuring tools were carried out. The comparisons indicate a good agreement between the downhole measured DWOB and the new Autodriller predicted values. The difference between the new Autodriller prediction and downhole measured DWOB can be quantified using rooted mean square error (RMSE) or relative error (RE). This paper also analyzes the differences in some sections, and some measures are suggested to potentially reduce these differences. The new Autodriller is a closed loop control system which can automatically in real-time adjust surface weight on bit (SWOB) so that the DWOB is accurate, which will directly improve the performance of drill bits, and decrease the cost of drilling, especially in directional well drilling applications.
Lateral Pressure Effects on the Ultimate Strength of Plates
Authors: Jiang Lei , Zhang Shengming
Published: October 2015

During the operations of ships and offshore structures in the ocean environment, these structures are subjected to combined lateral pressure and in-plane stresses. However, in today’s ship design and analysis procedures, the effects of the lateral pressure on the ultimate strength of these structures are often ignored. Previous studies have indicated that the lateral pressure could have a noticeable influence on the ultimate load carrying capacity of stiffened panels when they are subjected to combined longitudinal and transverse stresses. The purpose of this paper is to present a systematic numerical study to quantify the lateral pressure effects on the ultimate strength of plates. The sensitivity of the plate’s ultimate strength to lateral pressure is characterized as a function of the plate geometry, the pressure magnitude and the ratio of the in-plane stress components. The present numerical study is performed by using LR’s in-house nonlinear finite element program VAST and the newly development LR procedure for nonlinear structural mechanics analysis was followed. The results and findings from this study are detailed in this paper.
Validation of Global Performance Numerical Design Tools Used for Design of Floating Offshore Wind Turbines
Authors: Allen Christopher K. , Goupee Andrew J. , Dagher Habib J. , Viselli Anthony M.
Published: October 2015

In an effort to harness the abundant offshore wind resource over deepwater, the development of numerical design tools for floating offshore wind turbines (FOWTs) has progressed steadily in recent years. However, at present, a validated model capable of completely coupling the full elastodynamic response between the mooring system, floating support structure, turbine tower and the wind turbine is not commercially available.

The University of Maine has developed a new FOWT design, VolturnUS, which utilizes a concrete semi-submersible hull. For the VolturnUS design effort a number of numerical models were developed to analyze the system’s global performance. This paper presents the results of a validation study conducted to quantify the accuracy and suitability of a subset of these models for use in the design of the VolturnUS FOWT. Validation was conducted via comparisons of numerical model results to test data obtained from a 1:50 scale model testing campaign conducted by the University of Maine at the Maritime Research Institute, Netherlands offshore basin.

The validation study evaluated the performance and capabilities of the numerical models over a range of design conditions. Emphasis was placed on design load cases (DLCs), which were found to govern the design of the FOWT. The DLCs follow the American Bureau of Shipping’s (ABS) Guide for Building and Classing Floating Offshore Wind Turbines.

Through this method of model validation this work sought to quantify the numerical models’ accuracy, highlight their limitations, justify design assumptions, and identify areas requiring further development in the field of FOWT numerical modeling.
Evolution of the Extreme Wave Region in the North Atlantic Using a 23 Year Hindcast
Authors: Ponce de León Sonia , Bettencourt João H. , Brennan Joseph , Dias Frederic
Published: October 2015

The IOWAGA data base for the North Atlantic region was used to identify the region where extreme values of significant wave height are more likely to occur. The IOWAGA database [1] was obtained from the WAVEWATCH III model [2] hindcast using the CFSR (Climate Forecast System Reanalysis) from NOAA [3,4]. The period of the study covers 1990 up to 2012 (23 years). The variability of the significant wave height was assessed by computing return periods for sea storms where the significant wave height exceeds a given threshold. The return periods of sea storms where the Hs exceeds extreme values for the north Atlantic region were computed allowing for the identification of the extreme wave regions which show that extreme waves are more likely to occur in the storm track regions of the tropical and extratropical north Atlantic cyclones.
Axisymmetric Mechanical Behavior of Flexible Risers
Authors: Junpeng Liu , Vaz Murilo A. , Duan Menglan , Li Xiaotian
Published: October 2015

Polymer properties, i.e. viscoelastic characteristics, may pose a non-negligible impact on the axisymmetric analysis, especially on the axial structural damping behavior. An available mechanical model taking viscoelastic into account is established for capturing such parameters as axial strains and forces on each layer, axial stiffness and so on. In this paper, the flexible riser is divided into helical and cylindrical elements to carry out the mechanical analysis due to the complexity of structure and material property. Prony Series is used to describe the creep and relaxation behavior of material viscoelasticity. A viscoelastic solution is obtained using the correspondence principle. Finally, a case study is performed and some interesting results are presented.
Numerical Simulation of Wake Effect in Nets Under Steady Current
Authors: Cifuentes Cristian , Kim M. H.
Published: October 2015

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.
Ultimate Limit State Based Ship Structural Design Using Multi-Objective Discrete Particle Swarm Optimization
Authors: Ma Ming , Hughes Owen , McNatt Tobin
Published: October 2015

Multi-objective optimization problems consist of several objectives that must be handled simultaneously. These objectives usually conflict with each other, and optimizing a particular solution with respect to a single objective can result in unacceptable results with respect to the other objectives. A reasonable solution to a multi-objective problem is to investigate a set of solutions, each of which satisfies the objectives at an acceptable level without being dominated by any other solution. Genetic or evolution algorithms have been demonstrated to be particularly effective to determine excellent solutions to these problems. Among many algorithms, the particle swarm optimization (PSO) has been found to be faster with less computational overhead. In this paper a multi-objective discrete particle swarm optimization is formulated and used to optimize a large and complex thin-wall structure on the basis of weight, safety and cost. The structure weight and cost are calculated using realistic finite element models. The design process has two stages: (1) the actual stresses are obtained by finite element analysis of the full ship, (2) for a midship segment of the ship (referred to as a “control cluster”) the structural safety is evaluated using the ALPS/ULSAP set of ultimate limit state criteria, and then the segment is optimized using any suitable optimization method (in this paper, the PSO method). Both stages involve iteration, but the process is arranged so as to keep the number of full ship finite element analyses to a minimum. The complete design process is illustrated for a 200,000 ton oil tanker. The numerical results show that the PSO method is very useful to perform ultimate strength based ship structural optimization with multi-objectives, namely minimization of the structural weight and cost and maximization of structural safety. The example also demonstrates that the proper definition of boundary conditions and design load cases is of paramount importance for design optimization.
Numerical Simulation of UOE Pipe Process and its Effect on Pipe Mechanical Behavior in Deep-Water Applications
Authors: Chatzopoulou Giannoula , Karamanos Spyros A. , Varelis George E.
Published: October 2015

Large-diameter thick-walled steel pipes during their installation in deep-water are subjected to a combination of loading in terms of external pressure, bending and axial tension, which may trigger structural instability due to excessive pipe ovalization with catastrophic effects. In the present study, the UOE pipe manufacturing process, commonly adopted for producing large-diameter pipes of significant thickness, is considered. The study examines the effect of UOE line pipe manufacturing process on the structural response and resistance of offshore pipes during the installation process using nonlinear finite element simulation tools.
Subsea Systems Functional Failure Consequence Classification: A Case Study From a Subsea Manifold
Authors: Ratnayake R. M. Chandima , Garten T. , Barre A.
Published: October 2015

Subsea systems’ functional failure consequence classification (FFCC) and initial screening has been a stringent requirement for subsea systems operating on the Norwegian Continental Shelf (NCS). Hence, it is of great importance to establish approach(es) for the subsea systems’ FFCC. The study performed in this manuscript focuses on adapting existing guidelines, which are available in NORSOK Z-008, to develop an approach for performing the FFCC for a subsea system. A case study has been carried out which is closely aligned with a real-time project to perform subsea manifold related FFCC. The approach has been developed with the help of existing internal documents, data, information, and the experiences of the subsea systems’ owner as well as requirements pertaining to regularity authorities’ related guidelines, other existing similar work and established standards. The manuscript also illustrates a framework of the work process and illustrative analysis results.
Hydrodynamic Responses of a Single Hinged and Double Hinged Articulated Towers
Authors: Bithin Ghorai , Selvam R. Panneer , Sundaravadivelu R.
Published: October 2015

Numerical modelling and hydrodynamic analysis of single-hinged and double-hinged articulated tower are carried out. A single-hinged articulated tower (SHAT) is designed and modelled in AQWA and response in the time domain and frequency domain for the selected degrees of freedom has been studied. For double-hinged articulated tower (DHAT) an intermediate hinge is introduced and the response of the same is analysed. A comparative study between SHAT and DHAT response is carried out through the work. Also the influence of buoyancy chamber size on SHAT response has been investigated in the work. The entire analysis is done assuming the body is behaving rigidly. From the analysis, it is concluded that the maximum tip displacement of double-hinged articulated tower (DHAT) is increased by 40% compared to single-hinged articulated tower (SHAT) in random waves. The maximum tip displacement of double-hinged articulated tower increases by 2.3 times if the position of the intermediate hinge is raised by 5% (11 m above from the primary position). The natural frequency of SHAT increases by 22% and maximum hinge rotation by 14% if the outer diameter of buoyancy chamber is increased by 20% from primary dimension although in terms of its numerical value these are fairly small.
Effect of Lay Angle of Anti-Buckling Tape on Lateral Buckling Behavior of Tensile Armors
Authors: Zhou Chongyao , Sævik Svein , Ye Naiquan , Ji Guomin
Published: October 2015

During deep water flexible pipe installation, the pipe is normally free hanging in empty condition from the installation vessel to the seabed. This will introduce large hydrostatic forces to the pipe causing true wall compression. In addition, the pipe will be exposed to cyclic bending caused by waves and vessel motion. The combination of true wall axial compression and cyclic bending may lead to tensile armor instability in both lateral and radial directions. If the anti-buckling tape is assumed to be strong enough, the inner tensile armor will lose its lateral stability first due to the gap that may occur between the inner tensile armor and the pipe core, hence restricting the available friction restraint forces. These may further be reduced by cyclic motions that act to create slip between the layers, hence introducing lateral buckling of the tensile armor, with associated severe global torsion deformation of the pipe, ultimately causing the pipe to lose its integrity.

The anti-buckling tape is designed to prevent the radial buckling behavior, however, its effect on lateral buckling has not yet been documented in available literature. In the present paper, the effect of the winding direction of the anti-buckling tape on the twist of the cross section is studied, including comparisons with available test data from literature.
Strain Capacity of X70 Pipeline Steel Subjected to Biaxial Loading Condition
Authors: Paredes Marcelo , Wierzbicki Tomasz
Published: October 2015

In this work the tensile capacity of circumferentially pressurized cracked pipes with varying crack parameters and pipe dimensions are numerically investigated. The biaxial loading mode includes internal pressure and tensile load, which are applied in sequence. The present physics-inspired fracture model based upon the original Mohr-Coulomb criterion enables not only the computation of global fracture response of pipe subjected to complex loading condition but also a thorough determination of the local evolving stress state around the growing cracks.
Approximation of Higher-Order Derivatives of the Frequency Domain Free Surface Green Function
Authors: Shi Yuyun , Li Hui , Li Zhifu , Ren Huilong
Published: October 2015

The higher-order derivatives of the free-surface Green Function are critically important in three-dimensional frequency-domain boundary element methods using mixed dipole-source distribution. To improve the accuracy and efficiency of numerical schemes, the computing domain is divided into five areas. Derivatives in four areas are calculated analytically since the Green function is defined analytically. The 5th area is divided into a number of sub-areas in which truncated Double Chebyshev series are used to approximate the Green function. Unlike the usual way in which the derivatives of Green function are obtained by differentiating the series, we re-approximate the derivatives by new Chebyshev series with new coefficients. Numerical results show that the new series are more accurate, in particular, second order derivatives.
Numerical Study on Fish Tail Shaped Rudder for Improved Ship Maneuvering
Authors: Babu Mannam Naga Praveen , Krishnankutty P.
Published: October 2015

Maneuvering is an important safety aspect in ship operations so as to avoid accident of ships in seaways and more critically in the restricted area of waterways. IMO stipulates many safety regulations on ship maneuverability in open sea conditions and the local authorities may have additional regulations in harbor, canal and other restricted waterways. The effectiveness of rudder has substantial influence on the maneuverability of a ship. It is often difficult to increase the size of the rudder, to get higher control force/moment, due to the geometrical restrictions of the aft aperture of the ship. A hydrodynamically efficient rudder section addresses this problem to some extent. Most of the fishes maneuver efficiently using their tail. The fish tail functions almost similar to that of a rudder for its movements and navigation. In general, ship with flap rudders and fish tail shaped rudders perform maneuverability better compare to a ship fitted with a conventional rudder having the same underwater surface area. In fishtail shaped rudders, the shape and movements promote good flow patterns in a wider range of rudder angles. In a fish tail, the trailing edge accelerates the flow and recovers lift over the aft section of the rudder. This results in the generation of a higher lift and thus helps in reducing the turning diameter of the vessel.

The studies carried out with two rudder types — conventional rudder and fish tail shaped rudder — are presented in this paper. Numerical simulations are performed on these two rudders, both having the same surface area, for different rudder angles in free stream condition. The lift force generated by the fish tail shaped rudder is found to be higher than the conventional rudder. The flow across and the hydrodynamic forces acting on the sections are determined using a commercial CFD code. The effectiveness of the fishtail rudder is also brought out from the numerically simulated turning maneuver of a chosen ship fitted with the same rudder.
A Finite Element Model for Umbilical Cable Crushing Analysis
Authors: Santos Caio C. P. , Pesce Celso P. , Salles Rafael , Franzini Guilherme R. , Tanaka Rafael L.
Published: October 2015

Umbilical cables are essential elements of offshore floating production systems. Due to their complexity, the offshore industry regularly counts on numerical tools to perform design assignments. One of these assignments is to evaluate strains and stresses states in all components due to distinct sets of external loads. The main purpose of this paper is to present a numerical model for prediction of the stress and strain fields in the umbilical cable components under crushing loads. Such loads, outcoming from the laying operation, comprise the caterpillar shoes load and the squeezing effects, associated not only to the tensile armours, but also to helical components under tension. The referred model comprises a joint analysis using a two-dimensional Finite Element Method (FEM) fed by an analytical model, which represent three-dimensional effects. A combined analytical-numerical approach is much easier to implement than a complete fully three-dimensional one and it is meant to obtain results efficiently, without the need of a large computational capacity. The paper presents and discuss modeling hypotheses and methodology, describing in which way three-dimensional effects and interactions among cable components were treated. Case studies with three umbilical cables are presented.
Risk Based Integrity Assessment and Control of Ageing Subsea Pipelines: An Expert System Based Approach
Authors: Ratnayake R. M. Chandima
Published: October 2015

Many of the subsea pipelines around the world are reaching the end of their design life, and some are already under operation beyond the design life. Inherently, it is expensive to replace or repair subsea pipelines. This is mainly due to the fact that cost of replacement or repair necessitates the cost of loss of production during the shutdown period. Hence, it is vital to maintain the integrity of pipelines at an anticipated level to optimize replacements and repairs. To conserve the integrity of pipelines, it is necessary to allocate a larger proportion of budgets for inspection, maintenance and subsequently for necessary replacements and repairs. In addition, in the light of a number of high profile incidents, political and regulatory pressure has grown on asset owners to certify that they maintain pipeline integrity at an anticipated level. Regulatory authorities shall check up on the integrity assessment and control (IA&C) process of pipeline systems to confirm that the risk of a potential failure is as low as reasonably possible (ALARP). Assessments of the risk of potential failures of ageing pipelines provide an opportunity to optimize expenditures on IA&C. In this context, setting economic inspection intervals to prevent failures, limit risks to ALARP, quantify the present integrity (using available design data and as-built data, inspection data, loading and environmental data), and predict the future integrity (forecast operating conditions using past and present operating conditions) play a significant role. There is a need to employ expert systems to undertake risk based IA&C to minimize the variability in the assessments and evaluations caused by frequent migration of experts from one organization to another and new recruits with lack of experience. This manuscript provides the state of the art of the IA&C of subsea systems and proposes an expert-systems-based approach to minimize the variability caused in the IA&C process.
A Forward-Speed Body-Exact Strip Theory
Authors: Bandyk Piotr J. , Hazen George S.
Published: October 2015

This paper develops an extension to the body-exact strip theory of Bandyk, Beck, and Zhang [1–8], focused on improved prediction of forward-speed effects. One of the known limitations of standard strip theory is the treatment of forward speed terms. The free surface boundary conditions completely neglect the forward speed, which is usually justified by the argument of high-frequency oscillations. The pressure equation on the body includes a speed-dependent term that must computed, most commonly using the Ogilvie-Tuck theorem or numerical approximations. The strip theory variation described here circumvents these deficiencies by applying the 2D+T approach. The model assumes that each two-dimensional frame, in which a boundary value problem (BVP) is solved, remains fixed relative to an earth-fixed frame. The numerical model is based on a time-domain Rankine source method, using the same body-exact approximation as described in earlier work [1]. A suitable acceleration potential BVP is derived. Added mass and damping coefficients are calculated for two Wigley hulls, using the the standard body-exact approach and forward-speed 2D + T variant, and compared to existing model test and numerical data.
Vibration Behavior of Marine Risers Conveying Gas-Liquid Two-Phase Flow
Authors: An Chen , Su Jian
Published: October 2015

Vortex-induced vibration (VIV) has been intensively studied both theoretically and experimentally due to its importance in the design of marine risers. In recently years, the effects of single phase, steady internal flow on the VIV of marine risers have received increasing attention. However, the effects of transient two-phase flow on the vibration behavior of marine risers have been seldom studied. In this work, a fluid-structural model for analyzing the dynamic behavior of riser vibration subjected to simultaneous internal gas-liquid two-phase flow and external marine current is proposed. Slug flow regime is considered as it causes most violent vibrations. An analytical model is adopted for the prediction of important flow characteristics of the gas-liquid slug flow. A wake oscillator is employed to model the vortex shedding behind the riser. The dynamic behavior of risers is analytically and numerically investigated by using the generalized integral transform technique (GITT), by which the transverse vibration equation is transformed into a coupled system of second order differential equations in the temporal variable. Parametric studies are performed to analyze the effects of the superficial velocities of liquid and gas on the dynamic behavior of risers.
Inherent Safety Aspects for Layout Design of a Floating LNG Facility
Authors: Xin Peiwei , Ahmed Salim , Khan Faisal
Published: October 2015

This article outlines the aspects of inherent safety for the topside layout design of a floating liquefied natural gas (FLNG) facility. An FLNG plant requires a compact design; meanwhile, it needs the safest layout to tackle multi-dimensional safety issues. Thus, the layout of the facility is a paramount factor for ensuring its safety in a cost effective way. Three layouts are proposed and evaluated from the inherent safety perspective. The layout of the process area is mainly focused due to its higher risks. Integrated inherent safety index, cost index and domino hazard index are used to evaluate the three layouts in quantitative terms. An optimal layout is finally chosen based on both inherent safety and cost performance.
High-Fidelity Fluid Structure Coupled Simulations for Underwater Propulsion Using Flexible Biomimetic Fins
Authors: Chung Howard J. , Kancharala Ashok K. , Philen Michael K. , Wang Kevin G.
Published: October 2015

The ability of fish to maneuver in tight places, perform stable high acceleration maneuvers, and hover efficiently has inspired the development of underwater robots propelled by flexible fins mimicking those of fish. In general, fin propulsion is a challenging fluid-structure interaction (FSI) problem characterized by large structural deformation and strong added-mass effect. It was recently reported that a simplified computational model using the vortex panel method for the fluid flow is not able to accurately predict thrust generation. In this work, a high-fidelity, fluid-structure coupled computational framework is applied to predict the propulsive performance of a series of biomimetic fins of various dimensions, shapes, and stiffness. This computational framework couples a three-dimensional finite-volume Navier-Stokes computational fluid dynamics (CFD) solver and a nonlinear, finite-element computational structural dynamics (CSD) solver in a partitioned procedure. The large motion and deformation of the fluid-structure interface is handled using a validated, state-of-the-art embedded boundary method. The notorious numerical added-mass effect, that is, a numerical instability issue commonly encountered in FSI simulations involving incompressible fluid flows and light (compared to fluid) structures, is suppressed by accounting for water compressibility in the CFD model and applying a low-Mach preconditioner in the CFD solver. Both one-way and two-way coupled simulations are performed for a series of flexible fins with different thickness. Satisfactory agreement between the simulation prediction and the corresponding experimental data is achieved.
An Assessment of Prediction Methods for Waves Inside the Moonpool of a Vessel (Comparisons of Numerical Solutions With Experiments)
Authors: Cao Yusong , Zhang Fuwei , Joung Tae-Hwan , Ostman Anders , Kristiansen Trygve
Published: October 2015

This paper presents a preliminary assessment of the computational accuracy and efficiency of three different prediction methods for the water motion inside the moonpool of a rectangular box with forced vertical motion in a water tank. The first method is a linear solution method based on the linear wave diffraction/radiation theory (WAMIT). The second one is a method based on a CFD simulation (STAR-CCM+), the third method is a hybrid method combining a potential flow solver and a viscous flow solver (PVC3D). The accuracy of each method is assessed by comparing the prediction with the physical test data. The computational efficiency (complexity of setting up the computation and the computation speed) of the methods is discussed.
Estimation of Most Probable Maximum From Short-Duration or Undersampled Time-Series Data
Authors: Agarwal Puneet , Walker William , Bhalla Kenneth
Published: October 2015

The most probable maximum (MPM) is the extreme value statistic commonly used in the offshore industry. The extreme value of vessel motions, structural response, and environment are often expressed using the MPM. For a Gaussian process, the MPM is a function of the root-mean square and the zero-crossing rate of the process. Accurate estimates of the MPM may be obtained in frequency domain from spectral moments of the known power spectral density. If the MPM is to be estimated from the time-series of a random process, either from measurements or from simulations, the time series data should be of long enough duration, sampled at an adequate rate, and have an ensemble of multiple realizations. This is not the case when measured data is recorded for an insufficient duration, or one wants to make decisions (requiring an estimate of the MPM) in real-time based on observing the data only for a short duration. Sometimes, the instrumentation system may not be properly designed to measure the dynamic vessel motions with a fine sampling rate, or it may be a legacy instrumentation system. The question then becomes whether the short-duration and/or the undersampled data is useful at all, or if some useful information (i.e., an estimate of MPM) can be extracted, and if yes, what is the accuracy and uncertainty of such estimates.

In this paper, a procedure for estimation of the MPM from the short-time maxima, i.e., the maximum value from a time series of short duration (say, 10 or 30 minutes), is presented. For this purpose pitch data is simulated from the vessel RAOs (response amplitude operators). Factors to convert the short-time maxima to the MPM are computed for various non-exceedance levels. It is shown that the factors estimated from simulation can also be obtained from the theory of extremes of a Gaussian process. Afterwards, estimation of the MPM from the short-time maxima is explored for an undersampled process; however, undersampled data must not be used and only the adequately sampled data should be utilized. It is found that the undersampled data can be somewhat useful and factors to convert the short-time maxima to the MPM can be derived for an associated non-exceedance level. However, compared to the adequately sampled data, the factors for the undersampled data are less useful since they depend on more variables and have more uncertainty. While the vessel pitch data was the focus of this paper, the results and conclusions are valid for any adequately sampled narrow-banded Gaussian process.
Effect of Compressibility on Peak Impact Pressure and Pressure Distribution During Water Entry of a Wedge
Authors: Zhang Yi , Mohtat Ali , Yim Solomon C.
Published: October 2015

Ship hull slamming and wave impact on wave energy converters (WEC) are an important problem in naval architecture and marine hydrokinetic for the survivability of vessels and WECs in adverse environmental conditions. An idealized canonical model for these problems can be traced back to the water entry of a wedge–shaped rigid object. Common asymptotic and empirical models are often based on the assumption that the fluid is incompressible and the initial wedge entry velocity to sound speed ratio (defined as the characterized Mach number) is low. In this paper, we present some initial results of a numerical study of the effect of fluid compressibility on the peak slamming impact pressure. The deadrise angle and entry velocity of the model are varied to examine the sensitivity of the peak and distribution of the pressure response with respect to these parameters. The fluid compressibility is controlled through the bulk modulus and manifested as various sound speeds. The normalization using this sound speed shows that the pressure coefficient changes consistently for a certain deadrise angle. The results indicates that despite a low Mach number, the fluid compressibility in the numerical model affects the peak pressure significantly, implying an overestimation in engineering application by the classical asymptotic incompressible theory.
Reinvestigation of Ice Speed Effect on Ice-Induced Vibration of Conical Structure
Authors: Liu Xiang , Chen Yingying , Wang Yihe , Zhang Dayong , Gu Hai
Published: October 2015

Offshore installations designed to withstand extreme ice actions in ice-covered waters have experienced ice-induced vibrations (IIVs). ISO 19906 states that the dynamic ice actions and the corresponding IIVs shall be considered in the design and characterized with respect to the fatigue limit state (FLS). Ice-breaking cones have been effectively adopted in several full-scale structures to reduce the magnitude of the ice loads. Full-scale experience from Bohai Bay has shown that a structure utilizing a conical waterline geometry reduces the magnitude of IIVs relative to the analogous vertical structure. The same data indicated that structures with a narrow cone at the waterline can still experience IIVs and the magnitude of acceleration varies by ice speed for a given ice thickness. Researchers have performed comprehensive model tests on sheet ice approaching flexible conical structures and found the same phenomenon. In this paper, both full-scale field data and model test data were reinvestigated in consideration of ice speed. Two dimensionless variables were constructed for this purpose, one dimensionless variable with respect to the ice velocity and the other with respect to the acceleration of the structure. It is found that these two dimensionless variables show a linear correlation. The proposed methodology can be used to investigate the ice-induced acceleration of the flexible conical structure subject to a moving ice sheet.
Residual Stress Evaluation and Buckling Analysis of Carcass Layers in Flexible Pipes Using 3D Finite Element Model
Authors: Tang Minggang , Yan Jun , Chen Jinlong , Yang Zhixun , Yue Qianjin
Published: October 2015

A carcass layer plays an important role in unbonded flexible pipes to resist the high level of external pressure without buckling collapse. The layer is made of an interlocked helically wound metal wire of profiled section that cold-deformed by a series of rollers. In typical design and analysis process of the carcass layer, the initial stress is always assumed to be zero. During the carcass layer manufacturing (a cold forming process), the metal strap is however subjected to varies of squeezing and bending and deformations to take “S” shape, and then the profiled wire to a cycling sequence of bending, squeezing and twisting deformations, which take it beyond its material elastic limit. The residual stress is therefore introduced and could has effect on the critical pressure of the carcass layer.

This paper presents a 3D finite element model to investigate the detailed residual stress distribution and variation during the forming process of the carcass layer. A study case using a 3D ring model is presented to systemically study the influence of imperfections, especially the residual stress, on the critical pressure of carcass layers.
Development of a Ship Performance Simulator in Actual Seas
Authors: Tsujimoto Masaru , Sogihara Naoto , Kuroda Mariko , Sakurada Akiko
Published: October 2015

Greenhouse gas shall be reduced from shipping sector. For that purpose the regulation of EEDI (energy efficiency design index for new ships) and SEEMP (ship energy efficiency management plan) were entry into force from 2013. In order to improve energy efficiency of ships in service it is necessary to predict the fuel consumption in actual seas.

In order to reduce GHG emission from ships, a Vessel Performance Simulator in Actual Seas has been developed. It simulates ship speed and fuel consumption at steady condition by using weather data and designated engine revolution. Physical models for hull, propeller, rudder and engine are used in the simulator. Especially steady wave forces, wind forces, drift forces, steering forces and engine/governor model are important factor for the estimation.

The fuel consumption should be evaluated combined the ship hydrodynamic performance with the engine/governor characteristics. Considering the external forces by winds and waves, the operation point of the main engine is important for the estimation, since the torque limit and the other limit of the engine/governor are affected to the ship hydrodynamic performance. To prevent the increase of fuel consumption in service, the engine control system by the Fuel Index has been applied to present ships. In rough weather condition the revolution of the main engine is reduced to lower revolution by the Fuel Index limit. It causes the large decrease of ship speed but reduces the fuel consumption due to reduction of engine revolution.

Using the simulator the navigation performance of a container ship, a RoRo vehicle carrier and a bulk carrier is simulated along the route.

In this paper following contents are discussed; 1) evaluation of the physical model; steady wave forces, wind forces, drift forces, steering forces and engine/governor model, 2) simulation and validation of the physical model by tank tests and on-board measurements and 3) effectiveness of the ship performance simulator for GHG reduction.
A Fundamental Research on the Effects of a Floating Berth on Reduction of Tsunami Damages in a Harbor: Reduction Effects of the Breaking of Mooring Lines and Running on a Quay
Authors: Murata Kazuki , Ohno Masato , Ikoma Tomoki , Masuda Mitsuhiro
Published: October 2015

Tsunami consecutively brings about several types of disasters, which is described as “disaster chain.” In the case that vessels are moored to the quay, the breaking of the mooring line possibly causes drift or collision with other vessels. There is also a case that a mooring tether of the vessel moored to the floating pier is not broken in the damage of tsunami, which was confirmed at Shiogama port in Miyagi when the East Japan Great Earthquake occurred on March 11, 2011.

The objectives of this paper are to clarify the reduction effect of mooring line force when a vessel is moored to a floating berth. The three-dimensional MPS method is used with advantages of floating berth considered. We simulated a vessel with the cargo carrier of 3,000t. Especially, by changing the wave elevation and the incident angle, the responses of the mooring tension which acts on the vessel moored to a floating berth and to the quay respectively are compared, and its reducing efficiency is examined. A conclusion is reached that, when tsunami incident wave angle is 90 degrees, mooring vessels to a floating berth can be expected as a solution to reduce the problems of vessel’s running onto a quay, in comparison with mooring them at a quay wall. Also, we propose suitable anchoring methods according to the size of the vessels.
Motion Response Analysis of FPSO’s CALM Buoy Offloading System
Authors: Sun Liping , Zhang Xu , Kang Youwei , Chai Shuhong
Published: October 2015

Catenary anchor legs mooring (CALM) buoy offloading system is widely used in offshore oil and gas exploitation engineering. Proper prediction of the motion response of CALM systems is very important to the fatigue analysis of offloading pipelines and the design of mooring lines. Numerical models of a CALM system in survival and operation conditions will be established in this paper, and motion characteristics of the buoy and its sensibility to environmental factors, as well as performance of slender bodies are derived. More importantly, the shuttle tanker’s stability in plane and the risk of its collision with other floating bodies are focused on in this paper. All of the conclusions will provide recommendation for designing in practical engineering.
Localizing Swimming Objects in Noisy Environments by Applying Nearfield Acoustic Holography and Minimum Energy Method
Authors: Ziemer Tim
Published: October 2015

A problem in localizing individual swimming objects acoustically is the high amount of strongly fluctuating ambient noise due to turbulent pressure fluctuations, thermal and seismic noise, motoring vessels, wind and marine mammals. In littoral and other shallow waters additionally the complex boundaries produce absorption of high-frequency components and strong reverberation in other frequency regions, refraction due to sudden changes in temperature and scattering from the rough sea surface and floor.

In this paper localization of a single swimming object in presence of disturbing sources and noise is simulated. Low-frequency nearfield acoustic holography (NAH) based on vector hydrophone array measurements is combined with minimum energy method (MEM) to increase detection certainty.

This combination of NAH and MEM appears to be a reliable and robust detection method suitable e.g. for sort-range navigation in littoral waters and for iceberg detection in open waters.
Long-Term Global Performance Analysis of a Vertical-Axis Wind Turbine Supported on a Semi-Submersible Floating Platform
Authors: Borg Michael , Manuel Lance , Collu Maurizio , Liu Jinsong
Published: October 2015

This study examines the long-term reliability analysis of a floating vertical axis wind turbine (VAWT) situated off the Portuguese coast in the Atlantic Ocean. The VAWT, which consists of a 5-MW 3-bladed H-type rotor developed as part of the EU-FP7 H2OCEAN project, is assumed to be mounted on the OC4 semi-submersible floating platform. Given metocean conditions characterizing the selected turbine site, a number of sea states are identified for which coupled dynamics simulations are carried out using the FloVAWT design tool. Short-term turbine load and platform motion statistics are established for individual sea states that are analysed. The long-term reliability yields estimates of 50-year loads and platform motions that takes into consideration response statistics from the simulations as well as the metocean (wind-wave) data and distributions. Results can be used to guide future floating VAWT designs.
The Research and Application of the Ice-Resistant Technology for the Offshore Structures in Bohai Sea
Authors: Du Xiaying , Cui Yujun , Liu Yuan
Published: October 2015

The sea ice of China’s Bohai Sea has strongly affected the safety of the offshore structures in every winter. The ice load has become the principle load to be controlled during the design of offshore structures. In order to minimize the damages caused by sea ice, the researchers in Bohai oil field have conducted thorough studies on the ice-resistant technology, which have been tested and improved through the application in the ice-resistant test. This paper presents summarization and review on the studies and applications of the ice-resistant technology in Bohai Sea.

Based on the observation data in the past several decades, this paper recommends design environment parameters for sea ice in Bohai Sea, including ice period, ice thickness, ice speed and ice flow direction. Fundamental research on the ice loads and ice-induced vibration are introduced in this paper. The methods and results of the experimental studies and field tests for the research are illustrated systematically. Moreover, this paper reviews the application of the ice-resistant technologies and structures in Bohai oil field and discussed their respective deficiency. Meanwhile, an ice-resistant emergency case is presented to illustrate the compositions and operation mode of the emergency management system of Bohai oil field.
Qualification of S-N Curve by Experiments: Estimation of Survival Probability for a Low Number of Experiments
Authors: Thibaux Philippe , Tonesi Cristina
Published: October 2015

Due to the cost of fatigue testing, qualification of new weld details or improved welding techniques is often performed by comparing the experimental results with a mean S-N curve. The experimental dataset is considered to qualify for a given S-N curve if the mean of the log of the experimental results is larger than the mean of the chosen S-N curve plus an interval depending on the chosen confidence level. The confidence level is generally chosen arbitrary high, typically 75% or 90%. The survival estimation does not take into account the risk incurred by the fact that the actual mean has a probability of 25% or 10% of being lower than the theoretical S-N curve. In the present paper, we propose a different methodology based on the design S-N curve. The probability of failure for a given load level is computed by combining the probability of failure considering the mean value of the S-N curve, and the probability of this mean value. Thanks to the present method it is possible to estimate the probability of failure without making an assumption on the confidence level, which reduces the arbitrary during the estimation of the test results, particularly when the number of experimental results is limited.
Hydrogen Induced Corrosion Cracking and Crevice Corrosion Behavior in Natural Sea Water of Super Duplex Seam Welded Tubes for Umbilicals Applications
Authors: Evin Harold N. , Peultier Jerome , Thevenet Gilles , Wagner Olivier , Romazzotti Henri , Roques Jean-Philippe
Published: October 2015

Umbilicals are important tubes assembly essentially used for subsea equipment controlling functions and fluid injection. Super duplex stainless steel 2507 (UNS 32750 / EN 1.4410) has been the historical grade used for its corrosion resistance and mechanical properties. The Oil and Gas industry is gradually working on the exploration and exploitation of offshore and deep offshore resources. These operating conditions lead to new philosophy in the design of subsea umbilicals, which are evolving to answer the needs of new functionalities. Recent years development supported by O&G operators, have seen innovative super duplex stainless seam welded tube solution being used for their high mechanical characteristics (YS0.2≥ 750 MPa; UTS≥ 900 MPa), tight dimension tolerances, and their ability to generate cost saving during installation. The specific metallurgical manufacturing route for the 2507 strip together with an efficient roll forming process and NDT control allows to obtained tubes for umbilicals which push the usual boundaries of utilization of this super duplex grades.

In this paper, Hydrogen Induced Stress Cracking (HISC) behavior, possible failure mode of the super duplex stainless steel tubes exposed to cathodic protection in the termination unit, was extensively studied. A stress load of 110% YS0.2 was applied using a dead weight bench and the tubes performance was characterized after 500h of exposure in synthetic sea water under cathodic protection. Crevice corrosion is also a reported failure mode for umbilical tubes. Corrosion resistance of the 2507 seam welded tubes was also assessed in natural sea water in crevice conditions at open circuit potential (OCP). No failure was observed during the HISC testing in synthetic sea water after 500hours of exposure and no micro-crack was detected on the tubes after characterization. The metallurgical microstructure of the laser weld is not preferential initiation zone for crevice corrosion.