bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of Equivalent Dynamic Amplification Factor (EDAF) on a Jacket Structure
Authors: Baarholm Gro Sagli , Johansen Atle , Birknes Jørn , Haver Sverre
Published: November 2013

Non-linear time domain irregular wave simulations have been performed for the Kvitebjørn jacket platform located in the North Sea with the aim to quantify the dynamic amplification. The jacket is a slender structure installed in about 190m water depth.

For each of the selected extreme sea states both quasi-static and dynamic response simulations have been carried out for several wave realizations using different seeds. Based on the quasi-static response and dynamic response, equivalent dynamic amplification factors (EDAFs) were calculated for different response measures in the jacket. EDAF is the factor one has to multiply the q-probability quasi-static response with in order to obtain an adequate estimate of the q-probability dynamic response. The EDAFs are to be used in ultimate limit state (ULS) and accidental limit state (ALS) analyses of the platform.

Simulations were performed applying a Gaussian wave with Wheeler stretching and a second-order wave model.

This paper focuses on the selection of wave kinematics method and on the establishment of the EDAF analysis procedure.
Optimisation of Steel Catenary Risers
Authors: Voie Per Erlend , Sødahl Nils
Published: November 2013

The riser system is a vital component of deep water floating production installations. Field specific optimised solutions are generally required to obtain a technical solution fit for purpose and to secure an economic field development. In some cases, the riser system is a potential technology stopper preventing use of existing field proven floater/station keeping solutions for new applications. One such example is operation of large diameter steel catenary risers (SCR) from floating production units in harsh environmental conditions.

This study is focused on methodology for optimisation of riser systems by use of a mathematical optimisation scheme. An optimisation problem may formally be described as minimisation of a function in several variables (objective function) subjected to additional constraints. For the first time the general optimisation framework Optima is outlined allowing for use of standard riser analysis software for evaluation of objective function and constraints. Principles for selection of objective function and constraints are discussed for practical design applications with reference to design codes. Finally, an optimisation study of a steel riser for deep water and harsh environment is presented.
Sheared Current Generation in Flume Tank for Experimental Research
Authors: Islam Mohammed , Spencer Don , Herrington Paul , Walker Dan , Moideen Hisham , Park Y. C.
Published: November 2013

Current velocity, profile, direction, and duration may affect hydrodynamic loads and VIM of offshore structure. It is often recommended that physical experiments are carried out in sheared current, in multiple directions and for sufficiently long period of time to investigate the hydrodynamic characteristics of deep draft offshore structures to obtain better correlation to the field measurements. This necessitates generating sheared current with acceptable turbulence level. This paper presents a recent advancement in generating sheared current in a flume tank facility. In this process, the test specimen remains moored and the water flows past with its velocity varied with depth as long as necessary.

A combination of synthetic and wire meshes are used to provide the required amount of blockage onto the circulating channel flow of the flume tank to obtain specified current distribution across the cross-section and at the longitudinal center of the tank. The final set-up of the current screen provided a sheared flow distribution within 10% of the targets. Also, the measured turbulence level was below 10% in all the locations measured.

VIM studies of a model spar were successfully carried out in the generated sheared current in the flume tank facility. The ability to accurately model the sheared flow essentially improves the accuracy of the measured VIM type response measurements. The generated sheared current can also be applied for other hydrodynamic experiments where sheared current is relevant.
Hydrodynamic Modeling of Tension Leg Platform Wind Turbines
Authors: Bachynski Erin E. , Moan Torgeir
Published: November 2013

In order to compute the system response of tension leg platform wind turbines (TLPWTs), it is important to accurately capture the hydrodynamic loading not only at the wave frequency, but also in the low (difference) and high (sum) frequency ranges. The current work compares the dynamic response of several single column TLPWT designs in different wind and wave conditions using three hydrodynamic models: first order potential flow with viscous drag, first and second order potential flow with viscous drag, and a Morison’s equation model. Second order wave forces were found to have a relatively small effect on the structural load predictions: increased tendon tension variation of approximately 2–10% was observed in storm conditions, while negligible effects were observed in operational conditions. The Morison model, however, gave significantly larger pitch forcing near the natural period, leading to larger structural load predictions in all sea states.
Efficient Semi-Analytical Methodology for the Pre-Design Analysis of the Shock Response of Marine Structures
Authors: Iakovlev Serguei , Sigrist Jean-Francois , Leblond Cedric , Santos Hugo A. F. A. , Seaton Cris T. , Williston Kyle
Published: November 2013

We introduce a robust and computationally efficient methodology for numerical simulation of shock-structure interaction. The methodology is based on the use of some of the classical methods of mathematical physics, with the subsequent coupling between the fluid dynamics and structural parts using the finite-difference methodology. In order to demonstrate the versatility of the approach, we apply it to two rather different practically important problems of the interaction between shock waves and submerged cylindrical structures, aiming at providing insights that would be useful to engineers at the pre-design stage.

We first consider a submerged cylindrical shell subjected to two consecutive shock waves, and analyze the effect of such loading in the context of both hydrodynamic fields and the structural stresses it induces. The most important result of this analysis is the observation, for certain values of the distance between the wavefronts, of a very significant increase of the maximum stress observed in the structure.

Then, we consider a submerged cylindrical shell subjected to a single shock wave, but employ a more advanced shell theory than the one traditionally used, namely, the Reissner-Mindlin theory instead of the Kirchhoff-Love one. We demonstrate that such an advancement of the model not only leads to a very significant improvement of the accuracy of the respective simulations, but also allows for modeling relatively thick shells.
Influence of Particle Geometry on the Simulation of Sand Cutting Process
Authors: Chen Xiuhan , Miedema Sape A. , van Rhee Cees
Published: November 2013

Appropriate simulations of the sand cutting process are needed for dredging purposes. Software packages based on discrete element modeling (DEM) have offered useful tools to conduct the simulation and visualize the result. However, before moving to water saturated environment, simulations of the sand cutting process in atmospheric environment should first be treated. As known in reality the sand particles are not perfectly spherical, and their behavior is determined by their natural shape and material properties. Hence one of the key questions emerges to us, what particle geometry design is appropriate for a practical simulation.

To answer that question, a group of simulations with various particle geometry designs are set up to imitate the natural behavior of the sand particles in cutting process. The simulations are conducted in 3D by the software package EDEM™. The outcome shows that the macro behavior of the sand specimen could be different when different types of particles are used. And the cutting force records registered on the cutting blade are influenced by the particles’ ability to rotate, which is determined by the geometry of the particles, too. Besides, there is a risk that some of the particle shapes may create orderly placement in the sand pile.

Another topic discussed in this paper is the algorithm to calculate the porosity in the sand pile, which is useful for developing the simulation into water saturated environment. A numerical method based on a 3D grid system will be introduced in this part.

This paper will show the detailed methodology and results of the simulation and analysis.
Flow Assurance With Water Heated Pipe-in-Pipe in Fields With High Gas Oil Ratio and High Wax Appearance Temperature
Authors: Basilio Leandro P. , Drumond Fabiano G. , Mendes Pedro L. F. , França Alessandro , Prado Marcílio P. , Ribeiro Jonatas
Published: November 2013

The development of pre-salt fields in the Santos Basin has presented a plenty of technological challenges, which include the production of fluids with high values of GOR (Gas Oil Ratio) and high values of WAT (Wax Appearance Temperature).

To avoid the wax deposition, it is absolutely imperative handling the temperature by insulation and/or heating. If in one hand the production system thermal insulation minimizes the heat losses to the environment, on the other hand the cooling due the thermodynamic effect of gas expansion is inevitable, mainly in fields with high GOR values.

To mitigate the problem of temperature drop, mainly in the risers where the expansion effects are pronounced, various heating systems for subsea pipelines have been studied by PETROBRAS. In parallel to these studies, it was noted that the design philosophy of the offshore production units for the pre-salt fields have indicated the use of turbo–generators to supply electrical power to the production unit, leading to the opportunity of using the generated super heated gases, originally discharged into the atmosphere, in order to feed the riser heating systems based on heated fluids circulation.

Studies of heat exchange performed indicated a great potential for the risers heating by the flow of heated water through the annular of a Pipe-in-Pipe riser, by using a WHRU (Waste Heat Recovery Unit) to recover the power discharged into the atmosphere from the turbo–generators. It was identified the possibility of heating Pipe-in-Pipe risers by circulating heated fresh water in a closed loop with water return to the production unit, or by coupling the riser heating system with the water injection system, using all the facilities for treatment and pumping of water injection.

This paper aims to describe the methodology considered in the development of a conceptual design of a heating system for a Pipe-in-Pipe riser, by the flow of heated water in the annular, in a Free Standing Hybrid Riser configuration. The paper will also present the characteristics associated with the two possibilities of heated water circuit, as well as advantages and disadvantages of each system, and the aspects related to subsea production layout.
Stress Concentration Factors of Damaged FPSO Side Panels Under Cyclic Loads
Authors: Pasqualino Ilson , Pinheiro Bianca , Ferreira Carolina
Published: November 2013

FPSO (floating production, storage and offloading) units can be subjected to mechanical damage in their side panels caused by collision with supply vessels. Even if the ultimate strength of the panel is not significantly affected by small damage, the stress concentration in the collided region may lead to the initiation of fatigue cracks, considering the long period of operation undergone by these vessels. The aim of this work is to evaluate stress concentration factors (SCFs) in damaged FPSO side panels and estimate their effect on the fatigue life through a theoretical study. A finite element model is developed to reproduce a supply vessel collision and evaluate resulting SCFs under in-plane compression load. A parametric study is carried out considering different damage magnitudes and the results obtained are used to develop an analytical expression to provide SCFs as a function of dimensions of damage and panel. SCFs provided by this expression could be used in a theoretical fatigue life study that can estimate the residual fatigue life of collided FPSO side panels and help to forewarn a fatigue failure under the event of an accidental collision.
Wave Energy Estimation in Four Italian Nearshore Areas
Authors: Vannucchi Valentina , Cappietti Lorenzo
Published: November 2013

In this work, a set of analyses concerning the deep water wave power of the whole Mediterranean Sea has been carried out. These analyses cover the period from July 2009 to March 2012. Processes affecting waves as they propagate towards the coasts can modify the wave power, leading to reductions or, sometime, local enhancements due to focusing mechanisms. To quantify these processes, and thus to select the most energetic locations, numerical simulations were used to propagate the offshore time series into four selected near-shore areas. Monthly and yearly mean wave power maps are presented. Moreover some hot-spots, located at water depths in the range of 50 m to 15 m, are highlighted.
Micrositing Optimization of the Block Island Wind Farm, RI, USA
Authors: O’Reilly Christopher M. , Grilli Annette R. , Potty Gopu R.
Published: November 2013

The Rhode Island Ocean Special Area Management Plan (RIOSAMP) has been implemented in Rhode Island since 2008 to provide guidance to local regulators in the zoning of renewable energy, with a focus on the siting of offshore wind farms. The project culminated in the siting of the first North American offshore wind project, optimized using a spatial planning approach combining exclusionary and mitigating factors. The optimization of mitigating factors is based on a standard cost model approach and extended to include ecological and societal factors. This macro-siting optimization phase provided the framework to define a Renewable Energy Zone (REZ) for wind farm development and the present study seeks the siting optimization of the wind farm layout within this zone. The optimization considers the loss in power resulting from turbine wake interaction, a cable cost clustering algorithm, and the spatial variation of both foundation cost and the available wind resource within the REZ through a micrositing objective function. This initial objective function is extended to include ecological and social costs. The layout optimization is based on a Genetic Algorithm (GA) optimization scheme. The method is applied to the REZ area, demonstrating that a gain of approximately $10 million over 20 years could be obtained if an “optimal layout” would be selected over the initial layout chosen by the developers.
The Wind Tunnel Experiment Study of Wind Load on Jack-Up Drilling Unit
Authors: Hu An-kang , Lin Yi , Jiang Wei
Published: November 2013

Owing to the characteristics of the Jack-up drilling unit, the environment loads are important factors to the structure safety of jack-up. The wind load takes up a much larger proportion in structure analysis, and it has been now paid more and more attention in the field of offshore engineering. According to the MODU rule, the projected area method is used to calculate wind load. The height coefficient Ch and shape coefficient Cs are chosen by experience listed in MODU rule. However, the aerodynamic interference between platform components, such as wind shielding effects and acceleration flow effects are ignored. So the calculation results in accordance with MODU rule are conservative, and not good for structure optimization designing to some degree. Therefore, the wind tunnel experiment is very important and necessary. In this paper, a 400ft jack-up, which is developed by CIMC, is studied as an example. Considering the Reynolds number effects, the wind loads of jack-up both in normal drilling condition and severe storm condition are obtained in the experiment. Compared with the results, which are calculated according to MODU rule, it indicates that the experiment results are less, and the effect of aerodynamic interference and updraft can’t be ignored.
Study Advancement for Multiphase Flow and Heat and Mass Transfer Characteristics for Gas Hydrate Decomposition in South China Sea Offshore Sediment
Authors: Yu Xichong , Zhao Jiafei , Pang Weixin , Li Gang , Liu Yu
Published: November 2013

Gas hydrates are crystalline solids that consist of gas molecules, usually methane, surrounded by water molecules. According to the phase equilibrium characteristics of gas hydrate, there are three basic development methods, including heating, pressure decreasing and chemical injecting. The development process is actually the multi-phase flow process. Currently, there is no good commercial software used to simulate the multiphase flow, heat transmission and mass transfer in the gas hydrate decomposition process. The study is not mature, still in the development and trial stage. So in this paper, we will make a deeply study on the multi-phase flow simulation method of gas hydrate decomposition in the sediment. We try to make breakthrough in the theory and simulate method. According to the different scales, the simulation computation study of flow characteristics model has microcosmic, mesocosmic and macrocosmic scales. In this paper, mesocosmic scales is used to study for the multiphase flow, heat and mass transfer in the offshore gas hydrate decomposition process, and numerical simulation and experimental simulation are together used to study. Study advancements are shown as follows: firstly, conventional Lattice Boltzmann model is modified to new Lattice Boltzmann Model (LBM) based on sediment with gas hydrate and flow characteristics for gas hydrate decomposition, the interaction and density difference between the phases are considered, and Magnetic Resonance Imaging (MRI) visual technology is used to successfully verified to LBM methods. Secondly, contraction core reaction methods based on fractal theory is used to simulate heat and mass transfer in the offshore gas hydrate decomposition process and is successfully verified by experimental simulation for South China Sea offshore gas hydrate sediment.
Full-History Finite Element Modelling of Pipe-in-Pipe Flowline System: From Installation to Operation
Authors: Wang Facheng , Liu Zhigang , Liu Xinshuai
Published: November 2013

Developments of oil and gas reservoirs in South China Sea are presently accelerated, to cope with the significant increase in energy demand from the mainland. Pipe-in-Pipe (PIP) flowline systems have been widely employed in this region and are continuously being considered for further developments. This is due to its significant thermal insulation capacity to deal with the High Pressure and High Temperature (HPHT) issue. However, the methods in industry for design of PIP systems usually have two side extremes. Simplified analytical approach may lack of accuracy and detailed FE analysis always brings considerably sophisticated modelling and post-processing tasks. To overcome this situation, COTEC Offshore Solutions, together with its mother company, China Offshore Oil Engineering Company, have developed a cost-effective, beam elements based, 3D simulation model using ABAQUS, a general purpose finite element analysis (FEA) package. The mode allows complicated structures of PIP system to be represented in an effective way and adopts a representation of stinger for S-lay installation analysis. A full-history time-dominate analysis from installation to operation is performed in one model, rather than the commonly used ‘snapshot’ analysis.

In this study, a simplified modeling guidance of PIP components have been suggested. On the basis of the guidance, a novel 3D beam-elements based model has been produced to accurately represent complex PIP structural behaviors, but with minimum increase in modeling complexity. The analysis is carried out on the time-domain basis, which permits the full strain and stress history of the installation and operation to be observed and the most onerous time-point during the full installation and operation to be captured.
Experimental and Numerical Investigation of the Wave-Induced Loads on a Deep-V Catamaran in Regular Waves
Authors: Bashir Musa B. , Tao Longbin , Atlar Mehmet , Dow Robert S.
Published: November 2013

This paper presents the results of towing tank tests carried out to predict the wave loads in regular wave conditions on a Deep-V hull form catamaran model. The experiments were carried out at the Newcastle University towing tank using a segmented model of the university’s new research vessel, “The Princess Royal”. The vessel is a twin hull with a Deep-V shape cross-section. The model, divided into two parts at the cross-deck level, was fitted with a 5-axis load cell at the position of the vessel’s centre of gravity in order to measure the motions response and wave loads due to the encountered waves. The longitudinal, side and vertical forces, along with the prying and yaw splitting moments were measured. The results obtained were further compared with those from numerical predictions carried out using a 3D panel method code based on potential flow theory that uses Green’s Function with the forward speed correction in the frequency domain. The results highlight reasonable correlations between the measurements and the predictions as well as the need for a proper understanding of the response of the multihull vessels to the wave-induced loads due to the non-linearity that have been observed in the experimental measurements of wave loads.
Two-Dimensional Model for Wave-Induced Pore Pressure Accumulation in Marine Sediments
Authors: Zhao Hongyi , Jeng Dong-Sheng , Zhang Huijie , Zhang Jisheng
Published: November 2013

In this paper, a two-dimensional (2D) porous model is established to investigate the predication of the wave-induced pore pressure accumulations in marine sediments. In the new model, the VARANS equation is used as the governing equation for the wave motion, while the Biot’s consolidation theory is used for porous seabed. The present model is verified with the previous experimental data [1] and provides a better prediction of pore pressure accumulation than the previous solution [2]. With the new model, a 2D liquefied zone is formed at the beginning of the process, and then gradually move down. After a certain wave cycle (for example, 30 wave cycles in the numerical example), the liquefaction zone will become one-dimensional (1D) and continuously move down and eventually approaches to a constant. Numerical results also conclude the maximum liquefaction depth increases as wave height increases and in shallow water.
Prototype Measurement for Deep Water Floating Platforms Based on Monitoring Technology
Authors: Yu Du , Wu Wen-hua , Yue Qian-jin
Published: November 2013

Due to the complexity of ocean environmental conditions and non-linear structural responses, there are many uncertainties in establishing design of deep-water floating platforms. Traditional hydrodynamic analysis and model test, till now, still can’t provide the long-term safety and reliability for floating structure in harsh environments. Prototype test, as a full-scale experimental method, is carried out on in-service platforms to obtain the real response without simplification and hypothesis. The obtained data can be used not only to reduce uncertainties, but also to verify and further improve design specifications.

In this paper, the full-scale measurement technique for floating platforms is discussed. Four monitoring subsystems are built up related to different characteristics of data collection in-service platform. Individual Remote Monitoring System (IRMS) is established to perform the remote communication between platform and land. The full monitoring system is carried out on a FPS in South China Sea, which supported by a joint industry project (JIP). Some useful monitoring results are presented at last to demonstrate the reliability of the whole monitoring systems.
Uniaxial Bearing Capacity Factors and Failure Mechanisms for Skirted Spudcans
Authors: Vulpe Cristina , Gaudin Christophe
Published: November 2013

The undrained uniaxial bearing capacities of skirted footings for offshore structures, namely skirted spudcans, were investigated by means of finite element analysis (FEA). In order to increase the fixity and moment capacity of conventional spudcans, peripheral skirts are fitted to the shallow footings. Additional capacity is acknowledged by the offshore industry, but no formal recommendations have been established to quantify the capacity increase. The present paper assesses the behavior of skirted spudcans subjected to pure vertical, horizontal and moment loading on normally consolidated clay. The resulting bearing capacity factors and the failure mechanisms are presented and compared to both solid embedded footings and circular skirted footings solutions found in the literature.
Grade X65 and X70 Heavy Wall (up to 37mm in Thick) UOE Linepipe for Sour Service
Authors: Nakamichi Haruo , Ishikawa Nobuyuki , Endo Shigeru , Tanizawa Akihiko , Kawanaka Toru , Kakihara Shinichi , Uchitomi Noriaki
Published: November 2013

Based on the increasing demand for natural gas to be extracted under severe environmental conditions, requirements for thicker sour service pipe steels with high strength (X65 grades or more) are increasing. In order to achieve both a resistance to hydrogen induced cracking (HIC) and better mechanical properties, it is important to obtain a homogeneous microstructure. For this purpose, we manufactured API X65 and X70 heavy wall (up to 37mm in thick) UOE linepipe for sour service utilizing an advanced thermo mechanical control process (TMCP) employing the theoretical maximum cooling rate with water (the ‘ultimate cooling rate’) and homogeneous temperature distribution in accelerated cooling. For applications in deeper water, higher pipe thickness to diameter ratio (t/D) is required. However, during pipe forming, these high ratio pipes suffer higher plastic strain in the vicinity of the surface. This plastic strain causes increasing surface hardness and HIC resistance may deteriorate. It is important to evaluate the effect on strain systematically based on the difference of microstructure morphologies. Therefore, the effect of bending strain on HIC properties was investigated through a simulated laboratory bending test with a strain of up to 8.9%. It was found that homogeneous bainitic microstructure can prevent HIC even under a higher bending strain. On the other hand, crack area ratio (CAR) was almost doubled when 5% strain was applied near the surface area in the material in which inclusion morphologies were not optimized.
Statistical Methods for Prediction of Characteristic Loads for Free Fall Lifeboats Based on CFD Screening Results
Authors: Tregde Vidar , Nestegård Arne
Published: November 2013

Computational Fluid Dynamics (CFD) has been used in a screening process to calculate characteristic loads for a Free Fall Lifeboat (FFLB) during impact and submergence. The link between various input, e.g. environmental conditions and host specific data, resulting structural loads and motion of the lifeboat is explored. The screening can be used together with host specific environmental conditions to find structural design loads and motion restrictions.

Response based analysis have been developed for both short term and long term predictions. For the short term predictions a sea state given by (Hs, Tp) on the 100-year contour line is identified and a three hour irregular sea state is simulated. This time history of surface elevations is used for a large number of random lifeboat drops. From these random drops a distribution of wave height and corresponding wave steepness is derived which is then input to an interpolation in the database of CFD screening results. The resulting responses are fitted to a Weibull distribution and the 90% quantile in this short term load distribution is determined.

The long term response analysis is further developed from the short term analysis. The short term distributions for each (Hs, Tp) are combined with the probability of occurrence of the sea state, and long term distributions are derived for the responses similar to the short term analysis.

The screening results are used to identify critical load cases which are further investigated.
Development of 3MW Tidal Energy Platform
Authors: ter Brake Erik , Todman Mike , Armstrong John
Published: November 2013

The Triton-3 platform is a novel tidal energy harvester capable of producing 3MW from tidal flow. The platform is a floating structure moored to the seabed by a single-point fully articulated anchorage, and carries three power trains and a number of marine auxiliaries. The driver for the design as developed by TidalStream Ltd is to reduce the cost of energy production in order to compete with the current cost of offshore wind. Independently audited cost modelling shows that tidal stream energy can become competitive with offshore wind, achieving a generating cost as low as 10p/kWh at the best sites. This generating cost is estimated to be less than half that which could be achieved at a similar site from a single seabed-located turbine.

The driving aspects for the competitive cost are maximising the capacity per mooring point, reducing installation costs by float-out solutions and by providing easy access to the tidal equipment. Access is achieved by allowing the platform to come to the surface by means of de-ballasting. By doing so, there is no need for large workboats and/or diver activities to perform regular inspection and maintenance on the tidal equipment, reducing the cost significantly.

The technical aspects that arise when developing the tidal turbine platform for a typical offshore location are investigated by Houlder Ltd and discussed in this paper. A number of technical challenges have been addressed where the rotational stability in both roll and pitch are of interest. The roll of the platform is heavily affected by the performance of the turbines; sudden increase or reduction in thrust will induce significant rolling moments that must not impair the integrity of the platform. Pitching of the platform allows it to reach the surface when de-ballasted for maintenance and inspection. During normal operations, the platform remains aligned with the current and in doing so maximises the performance of the turbines.

The paper illustrates how these aspects have been achieved by means of passive solutions. By means of positioning and shaping the main body of the platform, a working configuration has been developed where the rotations of the platform remain within a limited window maximising the potential power production. The concept has been tested by TidalStream during a large-scale model testing campaign where the unit was subject to different current speeds and different turbine configurations and fault cases. This publication compares the results of the large scale model testing with numerical models developed in OrcaFlex and shows the effectiveness of the passive solutions.
A CIP-Based Numerical Simulation of Green Water on a 2-D Floating Body
Authors: Zhao Xizeng , Wang Fuqiang , Lu Feng , Wang Liyong , Zhang Yang
Published: November 2013

Numerical simulation of green water violent impact on a 2-D floating body is carried out using a Constrained Interpolation Profile (CIP)-based model and validated by a newly designed experiment, which is carried out in a two-dimensional wave flume in the Research Institute for Applied Mechanics, Kyushu University, Japan. In this work, focused wave are used for green water phenomena. Numerical simulations are performed by a CIP-based Cartesian grid method, which is described in the paper. Fluid-structure interaction is treated as a multi-phase flow problem. The CIP algorithm is adopted as the base scheme to obtain a robust flow solver for the multi-phase flow problem. The free surface/interface boundary is captured by THINC/SW (THINC: tangent of hyperbola for interface capturing; SW: Slope Weighting). A model of a box-shaped floating body with a small freeboard is adopted in order to easily obtain green water phenomena in the experiment. Main attentions are paid to the body responses, nonlinear phenomena, such as green water and violent impact forces. The highly nonlinear wave-body interactions, including significant body motion and water on deck, are modeled successfully in comparison with experimental measurement.
Investigations on Galloping of Non-Circular Cross Sections
Authors: Damblans Guillaume , Berhault Christian , Le Cunff Cédric , Molin Bernard , Wiet Paul , Cinello Alexandre , Deglaire Thomas , Legras Jean-Luc
Published: November 2013

Bundle arrangements are currently used in the design of underwater riser towers or oil export lines. A bundle is made of several parallel pipes linked together at intervals. Even if individual pipes are of circular section, the global external cross-section seen by the fluid is non-circular. When placed in a current, a bundle may be therefore prone to plunge instability [6], also know as “galloping”. When designing such bundle’s section, it is important to be able to predict its susceptibility to galloping and what are the implications on the whole structure.

Galloping is taking place in a low frequency range compared to VIV but with larger amplitude, up to several diameters. Instability can also occur in torsion through a coupling effect with transverse oscillations.

Riser Vortex Induced Vibrations have been studied for decades, and numerous experiments have been performed both in-situ and in model test facilities to understand and predict the instability of a slender cylindrical structure in current. The main motivation is the consequences of VIV on riser fatigue life.

If galloping and related instabilities are well known in aerodynamics [9], no large specific experiment or study exists for hydrodynamic flows [1], [7]. Therefore no guidelines exist to help prevent or predict galloping while designing cross-sections and pipe arrangements. Until recently, only the Blevins criteria [1] were available to predict the risk of instability. Based on recent examples of riser tower, experimental and numerical investigations are carried out within the “Gallopan” project in the frame of CITEPH (Concertation pour l’Innovation Technologique dans l’Exploration Production des Hydrocarbures) [11]. The main objective is to propose guidelines to avoid or reduce the risk of galloping in bundle cross section design.

Two cross section shapes are investigated, a square cross section for which results are available in the literature [1], [10], and a bundle cross section specifically designed to be unstable. Model tests are performed in two steps:

• Captive tests and transverse forced oscillation tests in steady current to derive hydrodynamic coefficients;

• Free oscillations tests in steady current to identify the range of reduced velocity where instability occurs as well as the response amplitudes. A specific experimental arrangement, based on a vertical pendulum system is used.

Numerical investigations are focused on the use of a standard riser analysis tool. Hydrodynamic coefficients issued from experiments are introduced. Model test set-up is reproduced for comparison purpose.
Risk Assessment on Subsea Gas Pipelines Located at Water Basin of Jetty Area due to Dredging and Operation After Dredging
Authors: Putranta Dinariyana Dwi , Artana Ketut Buda , Sambodho Kriyo , Ariana I Made
Published: November 2013

This study addresses risk assessment of 14″ gas pipelines buried closed to jetty area due to the plan to dredge the water basin at jetty area. The dredging plan is aimed to enable a more spacious maneuvering basin at the jetty area to enable larger vessel to be served and easier control of tug boat operation during berthing process. Before dredging, the gas pipeline is located 133 m from the slope. This condition provides enough protection to the gas pipeline since large vessel will not reach the pipeline due to vessel’s draft restriction. After dredging, however, gas pipeline will be located only 49 m away from the slope. The water area after dredging allows larger vessel will be in the vicinity of the pipeline and hence impose risk to the existence of the gas pipeline. Risk to the pipelines due to dredging activities (drop/drag anchor, drop clamshell/object, ship sinking, and ground instability) and due to various vessels operation after dredging (drop/drag anchor, ship sinking, and ship grounding) are assessed by implementing quantitative risk assessment. Hence, this study is focused on the assessment of risk to the 14″ gas pipeline due to dredging activity including risk assessment during operation of the jetty after dredging. Based on pipeline and environmental data, all possible hazards are identified. Some hazards are screened out using ‘Failure Modes and Effects Analysis’ (FMEA) to obtain the list of potential hazards. In order to evaluate the acceptance criteria of all potential risks, the risk profiles are composed according to DNV-RP-F107 “Risk Assessment of Pipeline Protection”. As part of the assessment, geotechnical assessments of submarine landslide due to dredging are also considered. The effective stress approach is implemented to the assessment and submarine slope stability is analyzed using Bishop’s and Janbu’s methods of analysis. The risk profiles for all potential hazards are reported, and simulation results for different slope ratios are given to illustrate the stability of slope configuration during dredging.
Numerical Modeling of Breaking Waves Over a Reef With a Level-Set Based Numerical Wave Tank
Authors: Chella Mayilvahanan Alagan , Bihs Hans , Kamath Arun , Muskulus Michael
Published: November 2013

Wave breaking is a highly unsteady, non-linear and extremely turbulent phenomenon. During the wave breaking process, the energy of the wave system is focused close to the crest of the wave and a spatial spread of wave energy occurs. Thus, the description of such a physical phenomenon is highly complex and it requires a deep insight into the breaking wave process. The accurate assessment of breaking wave kinematics is essential for an accurate prediction of hydrodynamic loads on structures. Besides, the understanding of the transformation of waves propagating over an artificial or natural reef is important concerning the coastal processes.

The numerical model used in this study is a two-phase model, which solves the flow problem for air and water simultaneously. The Navier-Stokes equations are solved on uniform Cartesian grids in two dimensions. The complex free surface is captured by the level set method. A staggered grid is used for the computation with the velocities defined at the cell edges and the pressure at the cell centres. This avoids unphysical pressure oscillations that can occur due to the coupling of pressure and velocity in the incompressible Navier-Stokes equations. The Ghost Cell Immersed Boundary Method is employed to handle the boundary conditions for complex boundaries. Turbulence modelling is carried out using the k-ω model. Discretization of the convective terms is performed using the 5th order Weighted Essentially Non-Oscillatory (WENO) scheme. In this study, a two-dimensional numerical wave tank is used to simulate waves propagating over steep slopes and wave dissipation. The main objective of the present study is to investigate the wave breaking process over a submerged reef. This is accomplished by examining the wave profile during wave breaking and the breaker indices. Also, the numerical results are compared to data from physical experiments and the numerical results exhibit reasonable agreement with experimental data.
Analysis and Offshore Support for the Float-Over of a 24,250mT Topsides on the North West Shelf
Authors: el Mouhandiz Abdel-Ali , Bokhorst Job
Published: November 2013

In April 2012 Heerema Marine Contractors (HMC) successfully performed the float-over of the North Rankin B topsides. With a weight of 24,250mT it is the heaviest deck installed in a single piece in the harsh offshore environment of Australia’s North West Shelf to date. During years of preparations extensive hydrodynamic analyses have been undertaken to achieve design loads, optimize the float-over equipment and ultimately determine operability limits for the installation.

As the float-over analysis contains various components with non-linear characteristics (e.g. fenders, leg mating units, mooring lines), a non-linear problem solving approach was used. The selected analysis tools use an efficient time step integration enabling a large number of simulations to be run. The strong non-linearity of the system requires a Monte Carlo simulation to obtain statistically reliable results. The numerical model included purpose built modules for the float-over equipment (such as LMUs) taking into account relevant details and project specific geometries. Available data from model tests was used in calibration and validation of the model.

Towards the offshore execution phase, the limiting combinations of wind sea and swell waves were identified. Forthese combinations the system response remained (just) within its ‘Not To Exceed’ values. During the offshore operation, prior to the actual float-over, the spectral wave conditions, wind and current were constantly monitored to validate the forecasts. Also the barge motions were measured and compared against results from the numerical models. This was used in support of the final decision to proceed with the float-over operation.
Efficient Estimation of Offshore Structural Response Based on Threshold Upcrossing Rates
Authors: Lambert Luke A. , Najafian G. , Cooper J. E. , Abu Husain M. , Mohd Zaki N. I.
Published: November 2013

Reliable estimation of offshore structural response due to random wave loading is essential for ensuring safe and economical designs. However, the conventional Monte Carlo time simulation method requires the simulation of an extremely large number of response records in order to derive extreme response probability distributions with acceptably low sampling variability. The Efficient Threshold Upcrossing (ETU) method, presented in this paper, enables rapid calculation of these probability distributions by using information about threshold upcrossing rates in conjunction with an Efficient Time Simulation (ETS) technique. Extreme response probability distributions from this novel technique are compared with those from the conventional and the ETS methods using a simple structural model exposed to Morison wave loading. It is shown that the method allows a very efficient calculation of response statistics.
Time Varying Forces on FPSO Bilge Keels
Authors: van 't Veer Riaan
Published: November 2013

One of the more challenging topics in offshore hydrodynamics remains the a-priori prediction of the roll response in irregular waves of a ship-shaped floating unit. It requires insight in how roll damping is generated by the bilge keels in particular. Such can be obtained through dedicated model tests measuring the bilge keel loads, and this paper presents several findings derived from FPSO sea keeping tests at model scale 1:70. Since the most practical and perhaps only feasible sea keeping prediction tool for engineering studies remains the well developed 3D potential flow, the global roll damping coefficients and the time varying bilge keel loads are studied based on fluid velocities calculated by the potential flow method. The findings of this paper enhance the understanding of roll damping and bilge keel loads.
Self-Submersible SPM Cage Simulation in Regular Waves With Oblique Currents
Authors: Shainee Mohamed , Decew Judson , Leira Bernt J. , Ellingsen Harald , Fredheim Arne
Published: November 2013

Meeting the gap between food demand and availability is of grave concern, and open ocean or offshore mariculture is identified as a promising solution for reducing this gap. In a series of studies performed by the authors, an offshore cage design concept (i.e., a self-submersible single point mooring (SPM) cage concept) was developed and analyzed in regular waves with a following current. The results of previous studies are promising, but further analysis is needed to determine whether the cage system can perform effectively in oblique waves and currents. Hence, the current study extended numerical simulations to include waves and currents running in oblique directions and also analyzed the cage system in a following and adverse current. The results of the simulations indicate that in oblique waves and currents, and even in adverse currents (relative to the direction of wave propagation), the self-submersible SPM cage system shows good submergence characteristics such that it can be effectively used in an offshore environment.
Collision of a Drifting Ship With Wind Turbines in a Wind Farm
Authors: Suzuki Hideyuki , Okayama Shinya , Fukumoto Yukinari
Published: November 2013

A multiple collisions caused by a drifting ship which lost control and entered into a wind farm (WF) may cause relatively large risk for a WF comprised of bottom mounted type offshore wind turbines. A bottom mounted type wind turbine will be installed relatively close to shore in Japan and sometimes close to dense marine traffic area. Consideration of the risk will be necessary in planning a WF.

This paper presented an estimation of a damage caused by collision with a drifting ship accidentally entered a wind farm. The WF is assumed comprised of bottom mounted type offshore wind turbines. The size of the drift ship considered in the analysis is 6788 GT. For smaller ships, damage to wind turbine considered to be small. Entry of ships from sides other than the one facing sea route was ignored because the number of ship entries from the sides was considered small. Under a number of limitations, risk of multiple collisions in WF by a drifting ship was formulated and quantitatively estimated.
On the Intrinsic Mechanism of Wakes Interference Past Two Cylinders Towed Beneath a Free Surface
Authors: Rajaona Dominique R. , Ramanakoto Toky
Published: November 2013

An experimental study of the flow past a set of two horizontal cylinders is presented. The cylinders are towed in a uniformly accelerated and decelerated motion in a visualization tank in order to enhance the vortex effects. The main cylinder (D = 0.04 m; L/D = 16) is placed in the flow past a front one (d = 0.002m; L/D = 16). They are towed beneath the free surface and the drag and lift forces are measured. The main cylinder wake pattern is visualized by an embarked CCD camera. The Reynolds number based on the maximum velocity is from 0 to 14000 and the Froude number based on the main cylinder immersion from 0.2 to 1.2 for an acceleration value of 0.15m.s-2. It is shown that the near wake is made of a combination of the main cylinder Von Karman vortices and those of the front cylinder. The interference phenomenon and the free surface effects are studied by varying the depth parameter and the two cylinders arrangements.