bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrodynamic Forces due to Oblique Wave and Current Loading on Untrenched Subsea Pipelines
Authors: Green Johnathan , Griffiths Terry , Craddock Chris
Published: September 2014

A number of oil and gas projects encounter significant costs to achieve subsea pipeline stabilization using present methods. The standard procedure to estimate pipeline stability is to consider the worst combination of amplitude and direction of the current and waves that the pipe will undergo during its operational lifetime. To calculate the hydrodynamic forces a common approach is to consider only the component of the fluid velocity perpendicular to the pipe axis according to the independence principle. The hydrodynamic coefficients are then taken from a case where the fluid flow is perpendicular to the pipe for similar flow characteristics.

A substantial amount of research has been carried out to assess the hydrodynamic forces on pipelines with the current and wave directions collinear and perpendicular to the pipe. However, only limited information is available on pipeline hydrodynamic forces for highly oblique current and wave flow. A Computational Fluid Dynamics (CFD) analysis was carried out to investigate the effect on pipeline hydrodynamic forces for highly oblique collinear and non-collinear current and wave directions. The work was carried out as part of the STABLEpipe JIP (1) (with participation by Woodside, Chevron, The University of Western Australia, and Wood Group Kenny) which aims to achieve a step-change improvement in the approaches to stability design, especially on mobile or erodible sea beds.
An Experimental Investigation of Hydrodynamic Impacts of Marine Growth on Mid-Water Arch System
Authors: Farmakis Michael , Jin Yuting , Chai Shuhong , Morand Henri , Izarn Cecile
Published: September 2014

The presence of marine growth modifies hydrodynamic effects to subsea structures and could lead to incorrect modelling if not properly accounted for. Widely-used design practice codes do not contain any specific guidelines or recommendations to account for the effects of marine fouling on complex subsea structures and due to the desired longevity of oil and gas constructs, considerable amounts of marine biofouling can accumulate. In the experimental investigation described in the paper, the impacts of different marine growth severities, current velocities and current directions on the hydrodynamic drag were carried out in the Flume Tank at the University of Tasmania. A 1:15 scale mid-water arch (MWA) was employed during this investigation. Several marine biofouling severities were tested as well as the structure without marine growth, representing scenarios based on realistic MWA operating conditions. Physical modelling was validated with numerical simulations using computational fluid dynamics. Experimental results gathered show a rise in drag forces when the artificial marine growth is attached. The highest force magnitudes were observed when the marine growth severity was at its maximum roughness. This has been complemented by numerical results, with input parameters coming from 3D scans of the artificial marine growth.
Assessment of Residual Ultimate Strength of VLCC According to Damage Extents and Average Compressive Strength of Stiffened Panel
Authors: Nam Ji-Myung , Choung Joonmo , Park Se-Yung , Yoon Sung-Won
Published: September 2014

This paper presents the prediction of residual ultimate strength of a very large crude oil carrier considering damage extents due to collision and grounding accidents. In order to determine extents of damage, two types of probabilistic approaches are employed: deterministic approach based on regulations based on ABS [1], DNV [2], and MARPOL [3] and probabilistic approach based on IMO probability density functions (PDFs) (IMO guidelines [4]). Hull girder ultimate strength is calculated using Smith method which is dependent on how much average compressive strength of stiffened panel is accurate. For this reason, this paper uses two different methods to predict average compressive strength of stiffened panel composing hull girder section: CSR formulas and nonlinear FEA. Calculated average compressive strength curves using CSR formulas (IACS [5, 6]) and nonlinear FEA are imported by an in-house software UMADS. Residual ultimate moment capacities are presented for various heeling angles from 0° (sagging) to 180° (hogging) by 15° increments considering possible flooding scenarios. Three regulations and IMO guidelines yield minimum of reduction ratios of hull girder moment capacity (minimum of damage indices) approximately at heeling angles 90° (angle of horizontal moment) and 180° (angle of hogging moment), respectively, because damage area is located farthest from neutral axis.
Numerical Simulation and Optimization of a Wave Energy Converter for the Izu Islands Sea in Japan
Authors: Kamio Takeshi , Iida Makoto , Arakawa Chuichi
Published: September 2014

The purpose of this study is the numerical simulation and control optimization of a wave energy converter to estimate the power at a test site in the Izu Islands. In Japan, ocean energy is once again being seriously considered; however, since there are many inherent problems due to severe conditions such as the strong swells and large waves, estimations are important when designing such devices. The numerical simulation method in this study combines the wave interaction analysis software WAMIT and an in-house time-domain simulation code using the Newmark-β method, and introduces approximate complex-conjugate control into the code. The optimized parameters were assessed for a regular sine wave and an irregular wave with a typical wave spectrum. With the optimized parameters, average and maximum output power were estimated for the observed wave data at the test site. The results show a more than 100 kW average power output and a several times larger maximum power output.
Assessment of the Integrity of the PVDF Barrier in Unbonded Flexible Pipes
Authors: Davidson Michelle , Fernando Upul S. , Hall John , O’Donnell Brendon , Latto James , Karabelas George
Published: September 2014

The polymer barrier is the most important component in unbonded flexible pipe, providing the leak-tight boundary for transporting hydrocarbon medium. Premature failure of the barrier during service can be costly and may lead to disastrous environmental consequences. Design of the barrier for 25 years’ service integrity is therefore a major requirement in the flexible pipe design process. However, the API design code does not give a specific procedure for the design of the barrier and is mainly concerned with the design of other layers in the pipe which are intended to provide integrity to the polymer barrier.

The selection of barrier material depends on many factors including the service temperature/pressure range and pipe bending requirements. Polyvinylidene fluoride (PVDF) is used as a barrier material in cases where high pressure and relatively high temperature applications are involved. However, a hard polymer such as PVDF can be susceptible to crazing and cracking under specific conditions and therefore the use of PVDF in flexible pipe barriers requires critical consideration of the above issues.

This paper discusses the general design requirements of a single layer barrier, and different barriers in relation to static and dynamic applications. The details of a qualification test program performed to establish service integrity of single layer Solef 60512 PVDF barriers is discussed. The unique testing facilities developed to test the integrity of the barrier are presented.
Analysis of End Fitting Barrier Seal Performance in High Pressure Unbonded Flexible Pipes
Authors: Fernando Upul S. , Karabelas George
Published: September 2014

End fitting is an essential component of flexible pipes enabling their connection between moving structures and pipes to create a complete pipe infrastructure. Designing the end fitting requires careful consideration of many factors including creating an effective barrier seal, termination of all the polymer and metal layers and anchoring armor layers so that the external loads of the pipe are transferred to the connecting structure without disturbing the seal. The most difficult challenge in designing end fittings for ultra-high pressure pipes is creating the necessary barrier sealing performance. In general, the seal needs to perform to approximately twice the design pressure of the pipe so that it can sustain the burst condition of the pipe. This requires the sealing components to be subjected to extreme loading where most of the sealing components are subjected to severe plastic deformation. Understanding seal deformations and quantifying and demonstrating sealing performance are important in the development of a reliable seal. This paper describes a method of evaluating the sealing performance of end fitting seals. Design requirements of end fitting seals are also described.

The barrier seals in end fittings are normally created by swaging a metal seal ring in between the polymer barrier and the body of the fitting. This creates two leak paths, one between the metal ring and the barrier and the other at the metal-to-metal contact interface between the seal ring and the body. Achieving sufficient compression on the polymer barrier and creating sufficient contact pressure over a reasonable distance at the metal-to-metal interface are the key requirements in developing a reliable high pressure barrier seal. During this study, finite element models have been used to evaluate the contact pressure at seal contacts. These results, with a specifically developed leak criteria, have been used to evaluate the sealing performance. A brief description of the models and some specific results are presented. The FE model predictions are compared with the actual contact pressure measurements of a sealing setup.
Ship Collision Simulations Using Different Fracture Criteria and Mesh Size
Authors: Kõrgesaar Mihkel , Tabri Kristjan , Naar Hendrik , Reinhold Edvin
Published: September 2014

There is a wide range of fracture criteria available in the literature to simulate the ductile fracture in large structures. Almost all criteria depend in some form on the mesh size and some criteria also account the effect of the stress state on the fracture ductility. Furthermore, a material model employed could considerably influence the analysis results. Therefore, in this study, four different fracture criteria, three different mesh densities and two different material models are used to simulate ship collision with a rigid bulb. Thereby, plastic dissipation energy, force-displacement curves and structural failure mechanism is compared between different fracture criteria. Advantages and disadvantages of each criterion are discussed.
A Model Experiment Investigation on Two Degrees of Freedom VIV of Cylinder
Authors: Zhai Yunhe , Song Ruxin , Kang Zh. , Sun Liping , Li Peng
Published: September 2014

An experimental investigation on vortex-induced vibration (VIV) response characteristics of a rigid cylinder was conducted at the Towing Tank Lab in Harbin Engineering University. The Reynolds Number based on proposed diameter ranged from 6×104 to 2.4×105, with the cross-flow mass ratio my* = 1.127 and the in-line mass ratio mx* = 1.363. In the experiment, the spring constants of the cross-flow and in-line flow directions were regulated to change the natural vibration frequency of the model system. One- and two-degree-of-freedom VIV experiment was respectively carried out to analyze the vibration characteristic and trajectory. It was found that the non-dimension in-line and cross-flow natural frequency ratio fx/fy, is an important parameter which not only affects cross-flow vibration peak but also affects the forms of vibration trajectory except the reduced velocity.
Influence of Wave Group Characteristics on the Motion of a Semisubmersible in Freak Waves
Authors: Deng Yanfei , Yang Jianmin , Xiao Longfei
Published: September 2014

In the last few decades, the hydrodynamic performance of offshore structures has been widely studied to ensure their safety as well as to achieve an economical design. However, an increasing number of reported accidents due to rough ocean waves call for in-depth investigations on the loads and motions of offshore structures, particularly the effect of freak waves.

The aim of this paper is to determine the sea conditions that may cause the maximum motion responses of offshore structures, which have a significant effect on the loads of mooring systems because of their tight relationship. As a preliminary step, the response amplitude operators (RAOs) of a semisubmersible platform of 500 meters operating depth are obtained with the frequency-domain analysis method. Subsequently, a series of predetermined extreme wave sequences with different wave group characteristics, such as the maximum crest amplitude and the time lag between successive high waves, are adopted to calculate the hydrodynamic performance of the semisubmersible with mooring systems in time-domain. The paper shows that the maximum motion responses not only depend on the largest wave crest amplitude but also the time lags between successive giant waves. This paper will provide an important reference for future designs which could consider the most dangerous wave environment.
Cyclic Behavior of Stiffened Plates Under In-Plane Loading by Numerical Analysis
Authors: Cui Hu-wei , Yang Ping
Published: September 2014

The aim of the present work is to analyze numerically the progressive collapse behavior of stiffened plates subjected to extreme and/or cyclic loads. In this paper, a series of elasto-plastic large deflection analyses is performed on stiffened plates subjected to cyclic uniaxial in-plane loading by non-linear Finite Element Method. The target stiffened plates with two different dimensions are analyzed by two kinds of models. In the analyses, forced displacements are applied, and all the cyclic loading starts from compression to clarify the effect of cyclic loading on the buckling/plastic collapse behavior of stiffened plates. There are totally 28 model cases under discussion to investigate the behavior of a stiffened plate under cyclic loadings. From the numerical simulation, some of the behavior of stiffened plates, such as axial rigidity, ultimate compressive strength and the hysteresis mean stress vs. mean strain curve can be found out and the primary conclusions are drawn.
Joint 4-D Variational Stereo Reconstruction and Camera Calibration Refinement for Oceanic Sea State Measurements
Authors: Shih Ping-Chang , Gallego Guillermo , Yezzi Anthony , Fedele Francesco
Published: September 2014

Validating modern oceanographic theories using models produced through stereo computer vision principles has recently emerged. Space-time (4-D) models of the ocean surface may be generated by stacking a series of 3-D reconstructions independently generated for each time instant or, in a more robust manner, by simultaneously processing several snapshots coherently in a true “4-D reconstruction.” However, the accuracy of these computer-vision-generated models is subject to the estimations of camera parameters, which may be corrupted under the influence of natural factors such as wind and vibrations. Therefore, removing the unpredictable errors of the camera parameters is necessary for an accurate reconstruction. In this paper, we propose a novel algorithm that can jointly perform a 4-D reconstruction as well as correct the camera parameter errors introduced by external factors. The technique is founded upon variational optimization methods to benefit from their numerous advantages: continuity of the estimated surface in space and time, robustness, and accuracy. The performance of the proposed algorithm is tested using synthetic data produced through computer graphics techniques, based on which the errors of the camera parameters arising from natural factors can be simulated.
Dynamic Response Control of Top-Tension Risers by a Variable Damping and Stiffness System With Magneto-Rheological Damper
Authors: Kang Hooi-Siang , Kim Moo-Hyun , Bhat Aramanadka Shankar S. , Kang Heon-Yong
Published: September 2014

The exploration and extraction of offshore hydrocarbon is currently facing stricter requirements in environmental conditions, structural integrity, and dynamic performance. Vibration control may be a critical part of mitigating the excessive dynamic responses of the offshore floating structures. If the structural responses can be monitored and controlled, then smart-platform technology can greatly widen the applicability of current technology toward deeper waters and more severe environmental conditions. This paper is focusing on the numerical simulations and analyses of top-tension risers in a tension-leg platform (TLP), incorporated with a bang-bang controlled magneto-rheological (MR) damper and variable stiffness (VS) system. The specific characteristics of the innovative system in alternating the damping forces and system stiffness show great potential to interactively change the structural behaviors corresponding to various external loadings. This research is expected to provide a robust and cost-effective solution for greatly expanding the capability of future smart offshore-platform technology.
Reduction of Pitch Motion of FPSO Vessels by Innovative OWC Passive Control
Authors: Seixas de Medeiros João , Bailly Guimarães Helio , Fernandes Antonio Carlos
Published: September 2014

The improvement of the seakeeping capabilities of Floating, Production, Storage and Offloading (FPSO) vessels increases safety and allows its operation on severe weather conditions. It also increases the fatigue life of the risers. Hence, any improvement on the FPSO motion is mostly welcome. Guimarães [1], following similar efforts by Silva [2], studied the reduction of pitch motions of FPSO vessels with the use of the OWCs (Oscillating Water Columns) passive system. However, both experimental and numerical results were inconclusive due to green water effects during experiments and panel issues with the panel code WAMIT [3], respectively. The objective of the present work is to report a series of new tests that prove the feasibility of an “L-shaped” moon pool concept and estimates and tests the ideal length of such concept that maximizes the restoring moment and minimizes pitch the most. The tests were conducted in the Laboratório de Ondas e Correntes (Laboratory of Waves and Currents) of the Federal University of Rio de Janeiro (COPPE/UFRJ).
An Experimental Investigation of Cylindrical Floater VIM in Current and Waves
Authors: Saito Masakatsu , Fujiwara Toshifumi , Maeda Katsuya
Published: September 2014

VIM (Vortex Induced Motion) is one of the important issues in the safety evaluation for cylindrical floating offshore structures. The VIM is basically placed as a phenomenon to occur in strong current, but that also appears in current and waves in the sea where offshore structures are installed.

The authors have recognized the phenomenon that the motion amplitude of a cylindrical floater in current and irregular waves together is larger than the sum of the motion amplitude in current and in irregular waves respectively in a VIM experiment. This VIM amplification phenomenon in the current and waves is remarkable when wave height is relatively low that has high occurrence frequency in the sea. It is, therefore, expected that the amplification phenomenon has large influence on the accumulative fatigue damages of the offshore structure’s mooring system.

In order to make clear this VIM amplification phenomenon, the authors have conducted detailed VIM experiment in waves using a circular cross sectional mono-column floater model. The results of the VIM experiment in current and waves are described in detail in this paper. The results of the experiment in irregular waves show different characteristics for VIM amplitude in current. The results in regular waves show the effect of wave height and wave period on VIM amplitude in waves. Using those results, the mechanisms of the VIM amplification in waves are investigated.
A New Method for Determining the Design Sloshing Loads for LNG FPSOs
Authors: Paik Jeom Kee , Lee Sang Eui , Kim Bong Ju , Seo Jung Kwan , Ha Yeon Chul , Matsumoto Toshiyuki , Kim Do Hyun
Published: September 2014

The aim of this study was to develop a new method for determining nominal values for sloshing loads in the design of storage tanks in LNG FPSO (liquid natural gas, floating production, storage and offloading units). Details of the procedure are presented in a flow chart showing the key sub-tasks. 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 concluded that the developed method is useful for determining the design sloshing loads of storage tanks in ship-shaped offshore LNG installations in combination with virtual metocean data and operational conditions.
Top-of-Line Spatial Corrosion Prediction in Gas Pipelines
Authors: Mustaffa Zahiraniza , Sadek Mohamad Mounes , Shitan Mahendran , Dawotola Alex W.
Published: September 2014

The top-of-line corrosion (TLC) is corrosion observed at the upper section of a pipeline, as measured from the circumference of the pipe. The magnitude of TLC is mostly determined using the mechanistic condensation models. The spatial prediction of TLC, however, has not been of the interest among researchers, so as to know at what o’clock orientations (with respect to pipeline cross section) corrosion may accumulate the most. TLC spatial prediction is directly related to the release of corrosion inhibitor (CI) in the pipeline. The probability of retaining CI at the upper part of the pipeline is always a challenge due to the inconsistency of the operational flow parameters coupled by acceleration due to gravity. Thus, there is the need to properly understand the development of TLC with regard to its space. This paper proposes the analysis on TLC spatial prediction to be carried out by means of statistical approaches called the Exploratory Data Analysis (EDA) due to the nature of corrosion that are random. EDA is a simple tool that is able to summarize the main characteristics of TLC data using visual methods. The TLC data was taken from a gas pipeline operating in Malaysian offshore region. A median polish model (of EDA) for the TLC was later generated. A prediction table was also developed to guide users on the estimate of TLC in gas pipelines with regards to the o’clock orientations of the pipe circumference.
Physical and Numerical Modeling of the Performance of Dynamically Installed Anchors in Clay
Authors: Liu X. D. , Sun J. , Yi J. T. , Lee F. H. , Gu H. , Chow Y. K.
Published: September 2014

Depletion of shallow-water hydrocarbons is increasingly forcing the oil and gas industry to explore in deeper water. Dynamically installed anchors (i.e. torpedo anchors and deep penetrating anchor) are increasingly used as a cost-effective solution for floating offshore structures in deep water environments because their installation cost is largely independent of water depth. In addition, dynamically installed anchors can be deployed accurately, and their performance is less dependent on accurate assessment of the soil shear strength since lower seabed strengths permit greater penetration depths. Despite of the economic advantages afforded by dynamically installed anchors, there remain significant uncertainties in the prediction of the embedment depth and verticality, which is likely to affect their long-term holding capacity. Currently, the holding capacity of the dynamically installed anchors is assessed using conventional pile capacity techniques, which neglect discrepancies in the rate of installation and failure mechanism between them.

This paper presents a series of model tests simulating dynamic installation and monotonic pull-out of dynamically installed anchors in normally consolidated clay. The model tests are carried out in a beam centrifuge at 100g, with varying penetration angles, extraction angles and model masses. A special designed apparatus allows model anchors to be penetrated and extracted with different penetration angles. The test results show that for models without fins, no matter by which angle the model penetrated the soil, the smallest value of holding capacity is obtained when the pullout and penetration directions are the same. Results also indicate that the penetration depth linearly increases with the anchor mass. This study also reported the results from finite element (FE) analyses. The Coupled Eulerian-Lagrangian (CEL) approach in the commercial FE package Abaqus/Explicit is carried out to simulate dynamic anchor installation.

The findings of this study points to a method of assessing the minimum holding capacity of the anchor and its depth of penetration. Further study is now on-going to study the behavior of finned anchors.
Integrity of Subsea Control Umbilical
Authors: Yasseri Ramin , Yasseri Sirous , Wang Bin
Published: September 2014

Controlling subsea oil and gas production systems requires fibre optics, power cables and tubes for the transportation of chemical for injection and fluid for hydraulic controls. These are arranged in helical bundles in successive layers. Shaped thermoplastic fillers are used to separate components and to fill out voids in the cross section. If necessary, armour is added to provide strength for the dynamic loads. This paper describes the application of 3D finite element method to assess the structural Integrity of subsea umbilicals, including the interaction between the constituent elements. It is suggested to uses a global analysis to determine the internal forces and local analyses (sub-modelling) for detailed stress evaluation. Abaqus is used for this purpose, due to its capability with large size problems under severe discontinuities due contact conditions. This paper also discusses the umbilicals interaction with seabed. The focus of this paper is on the fatigue life calculation.
Maneuvering in Waves Based on Potential Theory
Authors: Schoop-Zipfel Jochen , Abdel-Maksoud Moustafa
Published: September 2014

The forces acting on a maneuvering ship are determined with the in-house potential code panMARE. For slender ships with salient hull features, the forces and moments can be captured by properly treating the shed vorticity. For blunt ships it is not possible to directly determine the strength of the vorticity and the position where it leaves the hull. Therefore, it is easier and not less accurate to account for separation forces via semi-empirical formulae. These corrections are based on slender body theory or extensive RANS computations. The mass forces can be determined directly by potential theory. Forces and moments due to rudder and propeller are calculated using state-of-the-art procedures. Arbitrary maneuvers can be simulated by using the equations of motion. With the applied corrections a satisfactory agreement with model test results can be obtained. Wave excitation forces can be introduced to incorporate the influence of sea states. These forces are determined with strip theory. While the forces agree well with measured data, a deviation can be observed in the motions.
On the Relationship of Microstructure, Toughness, and Hardness Properties in a Submerged Arc Welded API-5L Grade X65 Pipeline Steel Section
Authors: Aucott Lee , Wen Shuwen , Dong Hongbiao
Published: September 2014

In order to further improve welded pipeline performance, a detailed knowledge of the key and interlinking relationships between the chemistry, microstructure and mechanical properties of the weld joint is needed. In this paper, the results of optical emission spectroscopy analysis on the as welded chemical properties of a submerged arc welded API-5L grade X65 linepipe are first presented. The microstructure of the various weld regions is then assessed against the results of the chemical analysis using a series of microscopy techniques. A fine grained ferrite-degenerate pearlite microstructure was observed within the base metal of the linepipe along with large (1.5 μm) cuboidal Ti (C, N) precipitates. Within the heat affected zone (HAZ) close to the molten weld joint, grain growth occurred with small volume fractions of induced upper bainite present within the microstructure. The fusion zone of the submerged arc weld joint consists of predominantly acicular ferrite with a small volume of grain boundary phases and a high number of large (0.8 μm) spherical Ti (C, N) precipitates. The results of Vickers hardness tests carried out at two length scales (macro + micro) show clear relations between the hardening effects of the cementite enriched degenerate pearlite and induced upper bainite phases within the base metal and HAZ respectively. Fractography analysis of Charpy impact test samples across the submerged arc welded joint found that the large Ti (C, N) precipitates within the fusion zone appear to be acting as microvoid initiation sites for the ductile fracture and as such contributing to the relatively low toughness properties within the fusion zone. Finally, the potential benefits of reducing the Ti content in both the welding wires and X65 base metal for further improvement of the mechanical properties of the linepipe weld joint are discussed in regards to reducing the size of the coarse Ti (C, N) precipitates within the base metal, HAZ and fusion zone.
Assessing the Importance of the Slow Drift Motion of Floating Wind Turbine Platforms
Authors: Lupton Richard C. , Langley Robin S.
Published: September 2014

As offshore wind turbines are installed in deeper water, interest is growing in floating wind turbines because, among other reasons, they may become cheaper than fixed-bottom turbines at greater depths. When analysing floating wind turbines, linear diffraction theory is commonly used to model the hydrodynamic loads on the platform. While it well known that slow drift motion due to second-order loads can be important for other floating offshore platforms, it has not yet been established how important such effects are for floating wind turbines. In this paper we aim to give a general result by developing approximate closed-form expressions to estimate the second-order slow drift motion of platforms of different sizes. The values are bench-marked against a typical calculation of the slow-drift response of a platform. The results show that floating wind turbines, which tend to have smaller dimensions than other floating structures, may be expected to show smaller slow-drift motions.
Influence of Added Mass Effect on Rotation of a Drifting Iceberg in Non-Stationary Current
Authors: Marchenko Aleksey
Published: September 2014

Observations of drifting icebergs by the ice trackers installed at their surface show their rotation relatively vertical axis. Iceberg rotation can influence drift characteristics and potentially dangerous for the iceberg towing. In the present paper 2D motion of floating icebergs in the presence of wind drag and unsteady water flow is considered. The water flow is assumed two dimensional and with uniform vertical profile. The direction and magnitude of the current speed are varying with the time. Temporal variability of the flow is associated with semidiurnal tidal. The inertial properties of an iceberg are characterised by three components of the added mass tensor. The iceberg motion is described by the Kirchoff equations written in the frame of reference sliding with the fluid. The rotation of cylindrical icebergs with elliptic horizontal cross-sections is investigated depending on the wind and fluid flow characteristics.
Full Scale Application of Thermal Ageing in Line Pipe Material Selection for Deepwater Pipelines
Authors: Kerstens Niels , Liu Ping , DeGeer Duane
Published: September 2014

Comprising 4 pipelines over 900 km in length, with 32-in diameter and traversing water depths over 2200 m, the South Stream project requires a step-out in technology application. Following several years of preparation, the project is now approaching its implementation.

In order to document the reliability of the collapse resistance for South Stream, an extensive material development program was initiated and executed, including small scale, medium scale, and full scale testing on over one hundred purposely manufactured pipe joints by world’s 5 leading mills.

Testing performed included plate tests, full scale collapse tests on various combinations of plate sources, steel grades, and thermal ageing condition, pressure-bend tests, and reverse bending tests. A large number of medium and small scale tests were performed to allow the development of a suitably reliable statistical database for the probabilistic wall thickness design.

In addition, programs were developed and executed for weldability tests, performing over one hundred trial welds, and for H2S resistance tests.

This material development program was built on INTECSEA’s extensive experience with deep water large diameter pipelines (i.e. Oman-India, Blue Stream, Medgaz, Mardi Gras, IGI, etc.). Due to its extent and rigorous approach the South Stream material development program was able to conclusively prove the feasibility of the selected technological approach at an industrial scale.

This paper provides an overview of the key design issues that were successfully addressed and the major technological advances that have been implemented as part of the linepipe material development process for deepwater pipelines in an H2S containing environment.

The practical significance of this program is to optimize the wall thickness to a level that is manufacturable by the industry and hence enables the South Stream Project to proceed with its unprecedented depth and diameter combination.
Towing Tank Experiments on the Vortex-Induced Vibrations of a Flexible Cylinder With Wake Interference
Authors: Huera-Huarte Francisco J. , Bangash Zafar A. , Gonzalez Leo M.
Published: September 2014

We describe recent results showing the dynamic response, excited by vortex shedding, of a long flexible cylinder subject to a stepped current immersed in the wake of another cylinder, placed upstream in tandem configuration. Experiments were conducted at the E.T.S.I. Navales towing tank of the Technical University of Madrid during March 2012. The tank is 80 m long with a cross-section of 4 × 2.5 m. A supporting structure was designed in order to provide support for a 3 m long cylinder with an external diameter of 16 mm. The cylinder was instrumented with strain gauges providing curvature measurements in the in-line and the cross-flow directions at 11 locations along its length. Tension and drag forces were also measured at both ends of the model. For these experiments, the upstream rigid cylinder was made stationary by fixing it at both ends, and it was located at different centre to centre distances.

More than 200 runs were conducted, with its lower 65% length under the water free surface, connected to the structure by means of universal joints. The supporting structure allowed to configure different top end conditions and to apply different top tensions. Tests were conducted with speeds up to 1.4 m/s. The cylinder had a low flexural stiffness of 6.04 Nm2 and low mass ratio of 2.7. Fundamental natural frequencies were in the range from about 2.3 to 6.2 Hz, and the cylinder responded in modes up to the third cross-flow.
Study on Slamming Pressure on the Bottom of Upper Hull of Semi-Submersible Type Floater
Authors: Cheon Jin Sol , Jang Beom Seon , Yim Ki Ho , Lee HoSeong Daniel , Koo Bon-Yong
Published: September 2014

Normally design bottom slamming pressure of semi-submersible type floater is determined in a simple way using a relative speed obtained from an air-gap analysis. However, there are few studies on a robust background on the estimation of the design pressure. In this study, to investigate the slamming pressure on the bottom of semi-submersible rig, a simplified deformable stiffened plate with 0° dead rise angle is simulated by using nonlinear FEM software, LS-DYNA which can take the influence of Fluid-Structure Interaction (FSI) into account. To simulate the slamming condition of semi-rig in a similar way, water hitting the structure with fixed boundary conditions is proposed. Various parametric studies are carried out to examine the effect of structural flexibility, coupling stiffness, mesh size, velocity, stiffener size, and air cushion effect. To further investigate the effect of horizontal velocity, the model is expanded and simulated with various horizontal water velocities. Pressure response on the plate by the coupling of fluid and structure is studied and the hydroelastic effect of each parameter is discussed.
Challenges for Using LNG Fueled Ships for Arctic Routes
Authors: Gaspar Henrique M. , Ehlers Sören , Æsøy Vilmar , Erceg Sandro , Balland Océane , Hildre Hans Petter
Published: September 2014

The utilization of the Northern Sea Route by commercial ships is an official fact. Since 2009 the number of international cargo vessels using the passage has been increasing, and a continuous raise in these numbers is expected if the route establishes itself as reliable. The route saves vessels approximately two weeks’ time in summer over a route via the Suez channel, but the increase brings concerns, regarding environmental impact, safety, and operability on the route.

This paper investigates the current challenges of using LNG fueled ships for arctic transport routes. A panorama of the recent conditions and predictions for the arctic environment regarding ice concentration and seasonal route availability is presented. The current development of LNG as a commercial fuel is discussed based on this arctic panorama, approaching key topics such as infrastructure, economic viability, propulsion requirements, and environmental impact. Special attention is given to the performance of LNG propulsion systems under arctic conditions, focusing on powering and air emissions. We conclude the paper by proposing the implementation of a lifecycle model to predict economical and environmental performance indicators when simulating a fleet of LNG fueled ships operating under many possible future ice conditions scenarios.
Flexible Riser Carcass Collapse Analyses: Sensitivity on Radial Gaps and Bending
Authors: Axelsson Gunnar , Skjerve Håvard
Published: September 2014

In the North Sea, carcass failures have over the last years been the largest failure category for flexible pipes, causing multiple riser replacements [1]. The carcass failures could be split into two groups: axial tear-off and collapse.

In order to determine the resistance to collapse, a 3D analysis model has been built using the explicit finite element (FE) program LS-Dyna.

In addition, the LS-Dyna results were compared with a simplified 2D FE approach, using the non-linear implicit solver MARC.

The 3D model allows for more complex sensitivities, such as curved pipe and carcass tension.

The FE analyses were based upon measured carcass profile geometries and material data from hardness measurements at several positions along the carcass profile, including effects of strain hardening during manufacturing.

Three different carcass profiles of three different risers were analysed. The study includes sensitivities on straight and curved pipe sections, axial preloaded carcass, carcass ovality, radial gap between carcass and pressure sheets and pressure increase velocity.

Collapse resistance of axial strained carcass and bent pipe is not well documented and this paper will highlight some of the observed effects.

The results from the FE analyses showed good correlation between the vendor data [2] both in shape of the buckling mode and capacity.
Optimized Mooring Line Simulation Using a Hybrid Method Time Domain Scheme
Authors: Christiansen Niels Hørbye , Voie Per Erlend Torbergsen , Høgsberg Jan , Sødahl Nils
Published: September 2014

Dynamic analyses of slender marine structures are computationally expensive. Recently it has been shown how a hybrid method which combines FEM models and artificial neural networks (ANN) can be used to reduce the computation time spend on the time domain simulations associated with fatigue analysis of mooring lines by two orders of magnitude. The present study shows how an ANN trained to perform nonlinear dynamic response simulation can be optimized using a method known as optimal brain damage (OBD) and thereby be used to rank the importance of all analysis input. Both the training and the optimization of the ANN are based on one short time domain simulation sequence generated by a FEM model of the structure. This means that it is possible to evaluate the importance of input parameters based on this single simulation only. The method is tested on a numerical model of mooring lines on a floating off-shore installation. It is shown that it is possible to estimate the cost of ignoring one or more input variables in an analysis.
Green Water Loads Determination for FPSO Exposed to Beam Sea Conditions
Authors: de Carvalho e Silva Daniel Fonseca , Rossi Ronaldo Rosa
Published: September 2014

Considering new offshore frontiers for oil exploration and production, specially the Santos Basin region, FPSOs will be exposed to more severe wave conditions. This scenario requires careful analysis with respect to the green water phenomenon. The complex physics involved in the water-on-deck flow implies on several uncertainties regarding green water loads analysis. Taking into account model tests, CFD simulations and analytical formulations, this paper aims to simplify the green water loads determination, proposing a methodology to estimate these loads considering the water elevation above deck measured from experiments or numerical tools. In order to accomplish this objective, CFD simulations with different solvers were run for a benchmark case, showing that it is a suitable approach for a global result in impact dam break cases. After that, a special boundary condition was calibrated to represent model test results of water propagation in a FPSO deck exposed to beam sea in terms of water elevation. Using this CFD model, the loads on exposed structures was determined and compared against the dam break analytical formulation, which was modified to take into account the gap between each structure and the deck. Finally some vane type protection structures were simulated and their efficiency in partially obstructing the water-on-deck flow was evaluated. As a global result from all these analysis, a more comprehensive strategy for green water loads determination is proposed.
The Influence of Spring Stiffness for a Passive Variable-Pitch Tidal Current Turbine
Authors: Sun Ke , Zhang Jian-hua , Zhang Liang , Shao Wei
Published: September 2014

In this present work, a kind of passive variable-pitch tidal current turbine named spring-control turbine is studied by computational fluid dynamics (CFD) numerical simulation. FLUENT is applied combined with its second developing custom interface-User Define Function (UDF), where torque balanced equation of blade are connected into the fluid conservation equation solver. The variable pitch angle of a single blade of a test model is used as the validation of the numerical model. Considering the variable rules of the torque and power with the pitch angle of a single blade, we find that the bigger spring stiffness is more benefit for energy extraction of a tidal current turbine.
The Effect of Different Drilling Fluids on Borehole Mechanical Friction
Authors: Taghipour Ali , Ytrehus Jan David , Lund Bjørnar , Saasen Arild
Published: September 2014

Mechanical friction is one of the most important aspects in highly inclined wellbores such as extended reach drilling (ERD) and through tubing extended reach drilling (TTERD). Friction caused by the contact between the drill string and the well casing or borehole is dependent to the drilling weight and fluid properties. Drilling fluids play an important role on mechanical friction and using oil based drilling fluids with higher lubricity can reduce torque and drag and minimize stick and slip concerns. Reducing mechanical friction will improve drilling efficiency in general, and will in particular enable longer reach for ERD wells.

This paper presents results from experimental laboratory tests where mechanical friction has been investigated in non-circular wellbore geometry. The experiments have been conducted as part of a research project in the tribology lab in Technical University of Luleå. The project was sponsored by the Research Council of Norway and four oil companies.

Friction behavior has been investigated for two different drilling fluids; water based and oil based drilling fluids both with and without solid particles. A pin on disc setup was used for these experiments where a spherical steel pin was sliding on a rotational disc made of granite. Friction force has been measured in constant sliding speed and in presence of particles in wet condition. The test results show that mechanical friction is smaller with oil based than water based drilling fluids in the presence of solid particles. In addition, the friction coefficient depends to the particle types and is higher when solid particles were added to the lubricants.

Such experiments in a tribology laboratory are important to identify the effect of drilling fluid on mechanical friction from a basic point of view isolated from the other wellbore parameters. Test results and the experimental approach could therefore be of value for any one working with drilling and well construction.
Assessment of Stabilized Phase-Field Lattice Boltzmann Method for Two-Phase Flows
Authors: Raman K. Ashoke , Lee T. S. , Jaiman Rajeev K. , Low H. T.
Published: September 2014

The simulations of many offshore flow problems are currently facing the physics of complex mixtures of moving fluids. Typical situations where two-phase flow should be taken into account are slamming, wave breaking, slug flows, water entry, LNG sloshing and green water loading.

In the present paper, a recent approach based on phase-field Lattice Boltzmann for high density ratios is examined. This scheme uses two distribution functions, one for pressure and momentum and the other distribution function for density with a compact isotropic differencing scheme to discretize the intermolecular forcing terms. Stress form of the intermolecular forcing terms is used to model pressure and momentum distribution function for its momentum conserving properties while potential form of the intermolecular forcing term is used for order parameter. We discuss the numerical method and implement it by solving various benchmark problems and compare with analytical solutions and results from previous literature. The method alleviates the issue of spurious currents which is investigated through the verification of convergence of Laplace law for a stationary bubble followed by oscillating droplet problem. Then we analyse a case of rising bubble under buoyancy for different Eotvos and Morton numbers and compare it with results from previous literature. To assess the capability of the method to solve complex problems the case of droplet impact on thin film has been performed. This problem studied for different Reynolds Number to examine the phenomenon of splashing and deposition of the droplet on thin films. Finally the capability of the method to simulate offshore problems is demonstrated by studying the ocean wave breaking problem with the periodicity in the flow direction.