bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Full- and Small-Scale Reeling Simulation on Mechanical Properties of Weldable 13Cr Seamless Line Pipe
Authors: Shitamoto Hidenori , Hisamune Nobuyuki
Published: November 2013

There are several methods currently being used to install offshore oil and gas pipelines. The reel-lay process is fast and one of the most effective offshore pipeline installation methods for seamless, ERW, and UOE line pipes with outside diameters of 18 inches or less. In the case of the reel-laying method, line pipes are subjected to plastic deformation multiplication during reel-laying. It is thus important to understand the change of the mechanical properties of line pipes before and after reel-laying. Therefore, full-scale reeling (FSR) simulations and small-scale reeling (SSR) simulations are applied as evaluation tests for reel-laying. In this study, FSR simulations were performed to investigate the effect of cyclic deformation on the mechanical properties of weldable 13Cr seamless line pipes. Furthermore, SSR simulations were performed to compare the results obtained by FSR simulations.
An Experimental Study of the Hydrodynamic Effects of Marine Growth on Wave Energy Converters
Authors: Tiron Roxana , Gallagher Sarah , Doherty Kenneth , Reynaud Emmanuel G. , Dias Frédéric , Mallon Fionn , Whittaker Trevor
Published: November 2013

Even though the outstanding energy resource provided by ocean surface waves has long been recognized, the extraction of wave power is still in its infancy. Meanwhile, the increased interest in sustainable energy alternatives could lead to large-scale deployments of wave energy convertors (WECs) worldwide in the near future. In this context, the interaction of WECs with the marine environment is an issue that has come under increased scrutiny. In particular, the accumulation of biological deposits on the device (commonly referred to as biofouling) could lead to a modification in the behaviour and performance of the device design.

For coastal devices in the North-Eastern Atlantic region, the main contributors to biofouling are likely to be the brown algae from the genus Laminaria. In the experimental study described in this paper, we have investigated the effects of algal growth on a scale model of the Oyster 800 WEC, a technology developed by Aquamarine Power. The experiments were carried out in the wave tank at Queens University Belfast. The algal growth on the device has been emulated with plastic stripes attached on the surface of the device. Several configurations with various placements and stripe dimensions were tested, in sea states typical to the targeted deployment sites. Our experiments were designed as a worst-case scenario and provide first insights into the potential effects of biofouling on the performance of a WEC. The experiments indicate that the effects of biofouling could be significant and suggest the need for further investigation.
An Experimental Investigation of Scale Effects on Offshore Wind Turbine Blades
Authors: Muthanna Chittiappa , Visscher Jan Hendrik , Egeberg Tale Fjell
Published: November 2013

In hydrodynamic model testing, the choice of the scaling factor is often a compromise between the size of the facility and close matching of the characteristic numbers such as Reynolds and Froude. When combining the two aspects into model testing of offshore floating wind turbines, limitations on the size of the model test facilities apply and it is not possible to use both Froude and Reynolds number scaling effects at the same time. To investigate Reynolds number similarity, two wing geometries, a NACA 0012 and a NREL S825 were manufactured in several sizes and tested at identical Reynolds numbers. The wakes behind the airfoils were measured using Particle Image Velocimetry. One would expect to obtain similar results, but differences in the wake profiles were observed. Understanding the mechanisms involved are important as it then enables model building techniques to be improved, and thus make model testing more effective.
Empirical Orthogonal Function Analysis of 2D Current Transects in the Fehmarn Belt
Authors: Golestani Maziar , Sørensen Jacob Tornfeldt
Published: November 2013

Describing spatial coherence of hydrodynamic conditions typically includes analysis of long time series of model results and site specific bathymetric and hydrodynamic features. This complex task often involves a time-consuming qualitative analysis to identify the critical physical processes for normal and extreme conditions. A methodology for skillful reduction of the system dimensions and determination of the most important current patterns can provide a more quantitative analysis of the coherence and variability of complex spatial time series. The objective of this study is to decompose transects of velocity in the hydrodynamically complex Fehmarn Belt area into Empirical Orthogonal Functions (EOF) and determine their relative contribution to the total variance. This will help marine engineers and contractors to gain a more quantitative and accessible picture of the changes in the current transects and to obtain an overview of current shear pattern while performing complex and exquisite operations.

18 years of hindcast data from a three-dimensional flow model are used for performing the EOF analysis. After performing the EOF analysis, the most important and dominant current patterns are extracted. The analysis reveals that the first eigenmode explains about 89 % of the variance and resembles the barotrpic flow at the cross-section while other EOF modes represent various modes of the baroclinic flow. The results are compared to EOF analysis of two ADCP measurements installed on the seabed and comparisons with similar analysis of model output are performed.

It is shown that the whole time series can be reconstructed with much fewer degrees of freedom and almost no data loss by using only the first five EOF modes.
Roll Damping Decay of a FPSO With Bilge Keel
Authors: Avalos Gustavo O. Guarniz , Wanderley Juan B. V. , Fernandes Antonio C.
Published: November 2013

The roll damping decay is investigated for a Floating Production Storage and Offloanding (FPSO). For this purpose, a roll decay test of FPSO is simulated by means of the numerical solution of the incompressible two-dimensional Navier-Stokes equations. The numerical results are compared with experimental data for validating the numerical scheme implemented. The simulations indicated the strong influence of the bilge radius in the damping coefficient of the FPSO section. Interesting results were obtained regarding the time series of the displacement of the body and vortex shedding around the bilge keel.
Framework for Risk-Based Inspection of an Offshore Wind Farm
Authors: Guédé Z. , Bigourdan B. , Rouhan A. , Goyet J. , Renard P.
Published: November 2013

In the present paper, a framework for Risk-Based Inspection planning is set up for a park of wind turbines taking into account the fact that only a sample of wind turbines is inspected at the scheduled inspection dates. A risk-based strategy is proposed to select the wind turbines to inspect, which significantly reduces the computational effort required by a crude application of Risk-Based Inspection analysis. The proposed framework is illustrated on a simple example of a wind farm where the failure of a wind turbine is driven by the fatigue crack of one of its critical structural detail.
Modeling of Ship Manoeuvring Motion in 4 Degrees of Freedom Based on Support Vector Machines
Authors: Wang Xuegang , Zou Zaojian , Xu Feng
Published: November 2013

Ship manoeuvring motion in waves is usually accompanied by nonlinear roll motion of large amplitude. In this paper, by taking account of the influence of the roll motion, a 4 degrees of freedom mathematical model of ship manoeuvring motion is derived. A method based on least squares support vector machines (LS-SVM) is proposed for identifying the hydrodynamic derivatives in the mathematical model by analyzing the data of surge speed, sway speed, yaw rate, roll rate, roll angle and rudder angle. To verify the identification method, 10°/10° zigzag tests are simulated for a container ship by using the hydrodynamic derivatives obtained from the roll planar motion mechanism (RPMM) test; the simulation data are used to identify the hydrodynamic derivatives, and the identification results are compared with those of RPMM test. The 10°/10° zigzag manoeuvring motion is predicted with the identified hydrodynamic derivatives. Besides, the identified model is used to predict the 20°/20° zigzag and 35° turning circle manoeuvres, and the predicted results are compared with those of simulation tests to demonstrate the generalization performance of the identified mathematical model.
A Study for Worst Periods and Load Cases Selection in Dynamic Analysis of Flexible Riser
Authors: Zhang Yanqiu , Tan Zhimin , Hou Yucheng , Yuan Jiabei , Dong Qian
Published: November 2013

A study was conducted to examine the effectiveness of the methodology which is generally used in screening for the worst wave period and load-case for dynamic analysis of flexible riser. For the purpose of conservatism, it is often required to assume a large load-case matrix by considering all probable loading conditions with different wave and current directions, different wave heights and periods and different vessel orientations with respect to the riser system. The screening for the worst load-cases is performed to reduce the total number of load-cases to be simulated given a limited time-frame and in order to focus attention on the most critical loading conditions. A popular method for the screening is to calculate the vertical motion of the riser hang-off for each load-case with the RAO data of the vessel on which the riser is to be hung. The idea is in that the larger the vertical motion, the worse the dynamic behaviors of the riser. With this method, three criteria should be used during the selection, looking for 1) the largest vertical displacement, 2) the largest vertical velocity and 3) the largest vertical acceleration of the riser hang-off. In this studyconclusions are drawn about the effectiveness of the method for some extreme loading conditions (e.g. the maximum tension and compression), and assessments are made of the relative strengths of influence that the three criteria have on the outcome of the analysis.
Structural Impact in FPSO Life Extension: A Class Perspective
Authors: Machado Christiane L. , Chand Sudheer
Published: November 2013

The Offshore Oil and Gas Industry has converted a large number of units from trading tankers and carriers into Floating Production, Storage and Offloading units (FPSOs). Several of these have been moored offshore Brazil during the last 15 years. Following the discovery of offshore pre-salt fields some years ago, demand for FPSOs has increased, and the forecasts for productive field lives have grown. The result of these developments is the need to extend the service lives of existing FPSOs.

The main aim of this study is to investigate FPSO structural response to environmental conditions and functional loads, considering the actual available tools for numerical simulations and Rule requirements, which currently are basic requirements for design review for Classification.

The procedure was developed from one selected FPSO converted from a trading Very Large Crude Carrier (VLCC) tanker approximately 15 years ago and includes investigation of the impact on hull behavior comparing the motion analyses of the production unit under environmental data and software capabilities available at the period of conversion and actual performance: variances in the environmental (sea scatter diagrams) datasets; updates to Classification requirements for defining offloading conditions, environmental loads, acceptance criteria and remaining fatigue life (RFL); and incorporating the most recent gauged thickness for primary structure.

The selected FPSO was evaluated according to prescriptive Rule requirements and also using finite element analysis, taking into account the previous conditions of Classification approval as well as the actual requirements and available data. Structural analysis included one global model and some local refined models to address strength, buckling and fatigue capacity of the typical portions/connections of the hull.

The comparisons performed from the results of these analyses are a crucial step toward understanding the structural capacity of the FPSO at the conversion stage, its performance during the last 15 years, and its remaining service life.

Differences were tabulated and evaluated so that a more precise level of uncertainty could be achieved for predicting the estimated remaining service life, and consequently, a new and dedicated approach to investigate the existing FPSO fleet is being generated.
Computation of Stress Concentration Factors for Tubular Joints
Authors: Thibaux Philippe , Cooreman Steven
Published: November 2013

Stress concentration factors for tubular joints were computed using solid quadratic elements. The results of the computations are compared with experiments reported in the literature and with expressions reported in the literature and in design codes. An influence of element size and element type was observed, which leads to recommendations regarding element size of four quadratic elements in thickness, which is finer than in different published recommendations. A parametric study was performed, showing that stress concentration factors from the literature are not always conservative, particularly at the crown toe of the chord, while they tend to be overconservative at the chord saddle. The stress concentration factor for the inside of the member was also computed; it is found that it can be close to the stress concentration factor at the weld toe for both the in plane or out of plane bending modes.
Tandem Riser Hydrodynamic Tests at Prototype Reynolds Number
Authors: Constantinides Yiannis , Raghavan Kamaldev , Karayaka Metin , Spencer Don
Published: November 2013

Deepwater riser interference is an area of significant technical complexity and uncertainty in the design cycle due to the intricacies of wake hydrodynamics. Existing models, found in industry guidelines, are based on approximate theoretical models of bare cylinder wake and nominally checked against small scale tests at low Reynolds numbers. In actual conditions the Reynolds number is sufficiently higher and the risers are fitted with vortex-induced vibration (VIV) suppression devices. This raises questions on the applicability of the standard models and hydrodynamic coefficients used, especially if the geometry is different than a circular cylinder. A series of full scale tests, at supercritical Reynolds numbers, were conducted to address these uncertainties and obtain hydrodynamic coefficients for interference design. The tests were carried out utilizing two full scale cylinders fitted with actual VIV suppression devices and towed either in fixed or spring supported configurations. The paper discusses the experimental methodology and findings from the testing program, showing deviations from the standard models found in industry codes.
FEA-Based Stability Analysis of Mudmats: Coupled Soil-Structure-Flowline Interaction Model
Authors: Abdalla Basel , Wang F. Steven , Hossain M. Kabir
Published: November 2013

The traditional method of foundation stability assessment for subsea structures is to calculate the bearing capacity factor of safety using classical approach given in the API-RP-2A/2GEO. This classical approach can be overly conservative for foundations under complex loading conditions (e.g., multiple interacting loads). A typical example is pipeline end manifold or flowline sled, which can be subject to self-weight, structure-soil interaction, and multiple interface loads from flowline and jumpers under operational condition.

A more rigorous 3D-FEA based assessment approach is developed in this paper to achieve more accurate bearing capacity estimates for a flowline sled supported by mudmat. This fully combined global model comprises the structure (with sliding mechanism), soil foundation, jumpers, and flowline as realistically as possible so as to capture the more accurate interactions among the different parts of whole sled-soil system. The use of such advanced numerical modeling has proven to improve the mudmat bearing capacity factor of safety.
The Ship-Routing Optimization Based on the Three-Dimensional Modified Isochrone Method
Authors: Lin Yu-Hsien , Fang Ming-Chung
Published: November 2013

In this paper, the authors proposed a ship weather-routing algorithm based on the composite influence of dynamic forces, i.e. wind, wave and current forces, for determining the optimized transoceanic voyages. Our developed routing algorithm, three-dimensional modified isochrones (3DMI) method, utilizes the recursive forward technique and floating grid system for both the east- and west-bound ship routes in the North Pacific Ocean. In order to achieve the goals of minimized fuel-consumption or the maximized-safety routes for the transoceanic voyages, two sailing methods are applied as the prerequisite routes in the earth coordinate systems. The illustrative analysis of ship routes has been presented and discussed based on the realistic constraints, such as the presence of land boundaries, non-navigable sea, external forces, parametric roll responses as well as ship speed loss. As a result, the proposed calculation is verified to be effective for the optimized sailings by adjusting the weighting parameters in the objective functions.
A Numerical Simulation Procedure for Vortex-Induced Vibration of Pipelines With Free Span
Authors: Tsukada Raphael I. , Morooka Celso K.
Published: November 2013

Long subsea pipelines are generally used to connect an offshore petroleum production facility and a petroleum terminal at the coast, to export produced oil or gas. In the pipeline way though the sea bottom, free pipeline spans happen due to the uneven and irregularities of ocean ground. In this portion of the pipeline length, the sea current can cause forces in the pipe, as the drag and vortex-induced vibration forces, respectively. These forces can have great influence in the structural stress and fatigue damage. Therefore, they must be carefully analyzed and considered in the pipeline design. The present work aims to introduce a numerical simulation procedure based on a semi-empirical VIV model to predict dynamic response of a pipeline with free span. Computations were carried out in time domain using finite element method. Beam elements considering large displacements and rotation were used to represent the pipe behavior. The VIV forces are calculated based on hydrodynamic coefficients, like added mass, lift and drag coefficients. In order to verify the accuracy of the VIV estimation procedure, comparisons with experimental results are presented.
A Comparison of Coupled and Uncoupled Dynamic Analysis for the Flexible Riser in Shallow Water
Authors: Jo Chul-hee , Kim Do-youb , Rho Yu-ho
Published: November 2013

Flexible risers have been used extensively in recent years for floating and early production systems. Such risers offer the advantage of having inherent heave compliance in their catenary thereby greatly reducing the complexity of the riser-to-rig and riser-to subsea interfaces. Another advantage with flexible risers is their greater reliability. Concerns about fatigue life, gas permeation and pigging of lines have been overcome by extensive experience with these risers in production applications. In this paper, flexible riser analysis results were compared through coupled and uncoupled dynamic analyses methods. A time domain coupled analysis capability has been developed to model the dynamic responses of an integrated floating system incorporating the interactions between vessel, moorings and risers in a marine environment. For this study, SPM (Single Point Mooring) system for an FSU in shallow water was considered. This optimization model was integrated with a time-domain global motion analysis to assess both stability and design constraints of the flexible riser system.
A Feasibility Study on Hybrid Use of Ocean Renewable Energy Resources Around Japan
Authors: Taniguchi Tomoki , Ishida Shigesuke , Minami Yoshimasa
Published: November 2013

This paper addressed assessing feasibility of hybrid use of ocean renewable energy, such as wave and wind energy around Japanese coast. At first, wave and wind energy theoretical potentials were calculated and, in the second step, correlation coefficient between wave and wind energy was computed around Japanese coast. Sea area suitable for hybrid use of ocean renewable energy resources is supposed to have high potential for some types of energy resources. Furthermore, correlation of power generation between wave and wind energy resources should be low because one energy resource needs to complement another one for stabilizing power generation.

Based on the assumptions, feasibility of wind and wave energy was evaluated on some sea areas where R&D project are ongoing.
Weather Window Determination of Top-Tensioned Riser Deployment With Guidewires
Authors: Fan Bin , Thomas William J.
Published: November 2013

To replace 20 of the 42 top-tensioned production risers for the Statoil Snorre A platform, the planned riser deployment will use a guidewire and guide frame system. A suitable weather window needed to be determined for the deployment to avoid clashing the deploying risers and adjacent connected risers. Such clashing could cause severe consequences such as connector leakage and equipment damage.

Riser collision during operation and production was studied extensively [1, 2]. However, little investigation has been conducted on clashing during deployment. Due to the importance of the disturbed wave elevation and wave kinematic effects on the riser response by earlier study [3], RIFLEX was chosen for the simulation. RIFLEX simulates the disturbed wave kinematics. However, RIFLEX has limited capabilities to model the deployment guidance system and calculate the minimum riser clearance.

This paper focuses on the simulation of the riser deployed with guidewires and guide frames and calculation of the clearance between the deploying riser and the connected risers.

First, the guide frame and 4-guidewire system (see Fig. 2 and 3) was simplified as a rigid beam and a sliding linear spring (see Fig. 4 and 5). A derivation of the relationship between the guidewire tension and deployment depth was developed and implemented into a RIFLEX model.

Second, due to different responses caused by the disturbed wave kinematics, different deploying and installed riser pairs in the well bay were selected to find the smallest clearance during a three-hour random sea state. The smallest clearance was determined from five different deployment depths and weather conditions in the initial plane of the riser pairs.

Last, a clashing criterion is discussed and used for the deployment weather window determination. This paper also points out that a slightly wider weather window could be possible, depending on the deployment depth.

Recent successful deployment of the new risers confirmed the riser installation analysis.
Overcoming Installation Challenges to Wellhead and Conductor Fatigue
Authors: Lim Tze King , Koska Ryan , Tellier Elizabeth
Published: November 2013

Subsea wells are typically drilled and completed using marine risers with subsea BOP stacks deployed from mobile drilling units. Wave and current induced riser motions are transferred to the wellhead, conductor and casing system which can then cause fatigue issues at the critical connectors and welds. In recent years, the risk of fatigue failure has increased due to the use of 5th and 6th generation rigs with larger BOP stacks, and longer well drilling and completion durations. During the design stage, the fatigue performance of the wellhead, conductor and surface casing is often overlooked.

During well construction, variations from design can occur in terms of wellhead stickup, wellhead lockdown and cement levels, both around the conductor and in the annulus between the conductor and surface casing. This paper describes analysis results showing the effect of these variations on the fatigue performance of the wellhead, conductor and surface casing. The recommended analysis load case matrix to account for these uncertainties is also presented, together with the need for as-installed assessments.

Recommendations on fatigue resistant designs with equipment configuration and specification options capable of meeting these challenges are presented. Wellhead monitoring and fatigue tracking strategies that can be implemented during drilling and workover operations to address these potential concerns and manage the risk are also discussed.
Numerical and Experimental Study on a Floating Platform for Offshore Renewable Energy
Authors: Hu Changhong , Sueyoshi Makoto , Liu Chen , Kyozuka Yusaku , Ohya Yuji
Published: November 2013

This paper presents recent experimental and numerical work on dynamic analysis and load prediction of a floating platform. A new offshore renewable energy platform is designed for the second stage on-sea experiment of Kyushu University. An experiment is carried out in the towing tank with a 1/50 scale model, to verify the hydrodynamic performance of the platform and to prepare a benchmark database for validation of the numerical simulation method. The in-house CFD code, RIAM-CMEN, is extended for numerical simulation of the platform in harsh sea conditions. Numerical simulation is carried out and validated against the experiment.
Floating Axis Wind and Water Turbine for High Utilization of Sea Surface Area: Design of Sub-Megawatt Prototype Turbine
Authors: Nakamura Takuju , Mizumukai Kentaro , Akimoto Hiromichi , Hara Yutaka , Kawamura Takafumi
Published: November 2013

Paradoxically, sea surface area for offshore wind farms is limited in some countries like Japan. While the distance from the shore to a wind farm is limited due to the cost of electric transmission, the near shore area has considerable traffic of ships and is restricted by complex rights of local fisheries. One approach to the problem is inviting the local fishermen to the management of the wind farm. It requires higher economic performance of the wind farm than the original fishing activity in the same sea surface area.

The proposed concept is the combination of a floating wind turbine and counter-rotating water current turbine. The water current turbine partially cancels the reaction torque of electric generator and provides additional energy production from water current. The rotating axis of the turbine is not fixed in the upright position to reduce the size of supporting structure. Regular maintenance work is only in low altitude on the float. These features lead to the reduction of O&M costs and higher utilization of sea surface area.

This paper describes the prototype sub-megawatt turbine under construction and its expected economic performance.
Synthesis and Optimization of Submarine Pipeline Routes Considering VIV-Induced Fatigue on Free Spans
Authors: Baioco Juliana Souza , Blanco Juan Pablo Inza , Jacovazzo Bruno Martins , Albrecht Carl Horst , de Lima Beatriz Souza Leite Pires , Jacob Breno Pinheiro , Cardoso Carlos de Oliveira , Rocha Djalene Maria
Published: November 2013

Researchers from Petrobras and LAMCSO/COPPE have been involved in the development and implementation of a computational tool, based on Evolutionary Algorithms, for the synthesis and optimization of submarine pipeline routes. In this tool, randomly generated candidate routes are evaluated in terms of several criteria, incorporated in an objective (or fitness) function to take into account the relevant aspects that should be considered in the design of a route.

Previous works described the initial steps taken towards the development of such tool, including the geometrical representation of a route, and some of the terms of the objective function associated with a preliminary, global step of the optimization process (such as total pipeline length, and geographical-topographical issues associated with the route geometry and to the seabed bathymetry and obstacles). Special attention was dedicated to the implementation of On-Bottom Stability (OBS) criteria such as the proposed in the DNV-RP-F109 code.

This work is focused on another aspect related to the structural behavior of the pipe under hydrostatic and environmental loadings; more specifically, fatigue induced by vortex induced vibrations (VIV) on free spans along the candidate routes. Special attention is dedicated to the implementation of the screening criteria proposed in the DNV-RP-F105 code. Case studies are presented to assess the influence of the VIV criteria on the results of the optimization tool.
Design Requirements for Floating Offshore Wind Turbines
Authors: Chen Xiaohong , Yu Qing
Published: November 2013

This paper presents the research in support of the development of design requirements for floating offshore wind turbines (FOWTs).

An overview of technical challenges in the design of FOWTs is discussed, followed by a summary of the case studies using representative FOWT concepts. Three design concepts, including a Spar-type, a TLP-type and a Semisubmersible-type floating support structure carrying a 5-MW offshore wind turbine, are selected for the case studies. Both operational and extreme storm conditions on the US Outer Continental Shelf (OCS) are considered. A state-of-the-art simulation technique is employed to perform fully coupled aero-hydro-servo-elastic analysis using the integrated FOWT model. This technique can take into account dynamic interactions among the turbine Rotor-Nacelle Assembly (RNA), turbine control system, floating support structure and stationkeeping system. The relative importance of various design parameters and their impact on the development of design criteria are evaluated through parametric analyses.

The paper also introduces the design requirements put forward in the recently published ABS Guide for Building and Classing Floating Offshore Wind Turbine Installations (ABS, 2013).
Power Output Performance and Smoothing Ability of an Oscillating Water Column Array Wave Energy Converter
Authors: O’Sullivan Keith , Murphy Jimmy , O’Sullivan Dara
Published: November 2013

This paper presents the physical model testing results of a floating oscillating water column (OWC) array wave energy converter (WEC) and the power smoothing ability inherent in the OWC chamber arrangement in the structure. The device can be categorised as a very large floating structure (VLFS) with structure dynamics which may make it a suitable device on which to mount wind turbines. It incorporates 32 individual OWC chambers in a “V” shaped arrangement such that there is a phase-lag between successive wave crests in the OWC chambers as an individual wave passes the structure. This OWC array was tested in both monochromatic and panchromatic unidirectional wave fields and the motion response amplitude operators (RAO) have been calculated. The time series of absorbed power from panchromatic waves was then used as input to a simple Well’s turbine power take-off (PTO) Simulink model to estimate the electrical power produced by each chamber and the additive power produced by the 32 OWC’s. A simple control law of optimum speed of the generator was used for these simulations. The time series of total electrical power from the 32 chambers was compared to the time series of an individual chamber and the standard deviation of the signals were also compared. The OWC array achieved a much smoother power output signal than a device with one chamber. Further smoothing of the output signal is possible by increasing the inertia of the turbine however, this may have implications for the mean efficiency of the power train. A preliminary design of the Well’s turbine is included, both in terms of mechanical parts and generator rating.

This paper focusses on the power absorption and motion performance of the device and discusses the potential for the addition of wind turbines.
Estimating Characteristic Load Effects on Floating Structures in Ice
Authors: Walter Erik Løkken , Støle-Hentschel Susanne , Moslet Per Olav
Published: November 2013

The present paper presents research results from the IceStruct Joint Industry Project (Det Norske Veritas, 2012), in which a simplified methodology for determining characteristic ice load effects on floating and fixed structures was developed. The simplified methodology was calibrated to the results of a probabilistic model and the work presented herein describes how the characteristic mooring line tension for a spar located in an environment with drifting level ice and ice ridges was determined. The probabilistic model includes a description of several stochastic variables (ice ridge keel depth, consolidated layer thickness, parameters describing the strength of the keel etc.) where some variables are dependent on each other. A computational response model of a spar structure was used to determine the maximum mooring line tension during each ice ridge interaction event. Estimating the characteristic mooring line tension involves estimation of low probability levels of a multivariate stochastic system. Direct Monte Carlo simulation is usually not applicable to estimate such low probability levels as it would require a lot of simulation time. When solving similar problems for the design of floating structures in waves, response surfaces are often applied, and this methodology was also selected for the current work. In this example, six governing stochastic variables were identified through sensitivity studies, resulting in a response surface that was embedded in a seven-dimensional space. This response surface was approximated and represented by a Beziér surface, which was obtained by evaluating the computational response model for a number of points on a predefined grid in the variable space. The Beziér surface is well suited for representing response surfaces as it is fast to evaluate and does not introduce artificial fluctuations on the surface. The characteristic mooring line tension (corresponding to a given annual probability of being exceeded) was then estimated by a direct Monte Carlo simulation on the Beziér surface.
Effective Shear Area in One Dimensional Ship Hull Finite Element Models to Predict Natural Frequencies of Vibration
Authors: de Souza e Silva Osvaldo Pinheiro , da Silva Neto Severino Fonseca , Pasqualino Ilson Paranhos , Troyman Antonio Carlos Ramos
Published: November 2013

This work discusses procedures used to determine effective shear area of ship sections. Five types of ships have been studied. Initially, the vertical natural frequencies of an acrylic scale model 3m in length in a laboratory at university are obtained from experimental tests and from a three dimensional numerical model, and are compared to those calculated from a one dimensional model which the effective shear area was calculated by a practical computational method based on thin-walled section Shear Flow Theory. The second studied ship was a ship employed in midshipmen training. Two models were made to complement some studies and vibration measurements made for those ships in the end of 1980 decade when some vibration problems in them were solved as a result of that effort. Comparisons were made between natural frequencies obtained experimentally, numerically from a three dimensional finite element model and from a one dimensional model in which effective shear area is considered. The third and fourth were, respectively, a tanker ship and an AHTS (Anchor Handling Tug Supply) boat, both with comparison between three and one dimensional models results out of water. Experimental tests had been performed in these two ships and their results were used in other comparison made after the inclusion of another important effect that acts simultaneously: the added mass. Finally, natural frequencies experimental and numerical results of a barge are presented. The natural frequencies numerical results of vertical hull vibration obtained from these approximations of effective shear areas for the five ships are finally discussed.
Wave Interaction With Piled Structures: Application With IH-FOAM
Authors: Lara Javier L. , Higuera Pablo , Guanche Raul , Losada Inigo J.
Published: November 2013

This paper presents a numerical analysis of the interaction of waves with piles. A model called IH-FOAM, based on OpenFOAM®, is used. IH-FOAM is able to simulate and to absorb waves in three-dimensional domains, reducing the computational cost and extending the range of applicability of the CFD modelling to the study of offshore and coastal structures. In this work, a detailed analysis of mono and multi-piled structures is carried out. Several piles layouts are studied Wave run-up and forces have been studied for the multi-piled structures. Those magnitudes have been compared with the single piled structure pointing out the difference in the wave induced hydrodynamics and the non-linear interaction between the waves and the structures. The work contained in this paper presents a first step which will be extended in the future to analyse more complex layouts and the effects of broken waves.
Fluid Dynamical and Modeling Issues of Chemical Flooding for Enhanced Oil Recovery
Authors: Daripa Prabir
Published: November 2013

This paper evaluates the relevance of Hele-Shaw (HS) model based linear stability results to fully developed flows in enhanced oil recovery (EOR). In a recent exhaustive study [Transport in Porous Media, 93, 675–703 (2012)] of the linear stability characteristics of unstable immiscible three-layer “Hele-Shaw” flows involving regions of varying viscosity, an optimal injection policy corresponding to the smallest value of the highest rate of growth of instabilities was identified among several injection policies. Relevance of this HS model based result to EOR is established by performing direct numerical simulations of fully developed tertiary displacement in porous media. Results of direct numerical simulation are succinctly summarized including characterization of the optimal flooding scheme that leads to maximum oil recovery. These results have been compared with the HS model based linear stability results. The scope for potential application of the HS model based results to the development of fast methods for optimization of various chemical flooding schemes is discussed. Numerical experiments with more complex flooding schemes in both homogeneous and heterogeneous reservoir are also performed and results analyzed to test the universality of the generic optimal viscous families in a broader setting.
A New Slender-Ship Theory Valid for all Oscillatory Frequencies and Forward Speeds
Authors: Kashiwagi Masashi , Wang Xin
Published: November 2013

A new theory is presented for the radiation problem of heave and pitch of a slender ship advancing at arbitrary forward speed. The theory has no restriction on the order of forward speed and oscillation frequency. The general inner solution is constructed by the source distribution with Green function over the ship’s hull surface plus a line distribution along the ship’s centerline on the free surface with the radiation-wave related residue part of the Green function. By matching the inner solution with the outer solution, the source strengths on both hull surface and line distribution can be obtained. Numerical results of the added-mass and damping coefficients based on the present theory are shown for two modified Wigley models and compared with the unified theory and experiment results.
Flow-Induced Vibrations of High Gas Rate Well Jumpers: Tees vs. Bends
Authors: Pontaza Juan P. , Menon Raghu G. , Okeremi Akinyemi , Spritzer Jared , Widjaja Shington
Published: November 2013

Subsea well jumpers are steel pipe sections that connect the flow path between the tree and the manifold. They are designed with bends to accommodate limited expansion due to variations in temperature and pressure. The unsteady change of flow direction in these bends induces fluctuating forces with broadband frequency content. If the fluctuating forces are of significant amplitude and/or induced in the right frequency range, they may lead to flow-induced vibrations (FIV) of the well jumper. FIV poses structural integrity concerns for subsea piping in terms of cyclic stressing and, over time, a threat of fatigue failure. A comprehensive FIV screening approach, based on computational fluid dynamics (CFD) and structural finite element analysis (FEA), and associated fluid-structure interaction (FSI) is used to estimate well jumper fatigue life due to FIV — for high gas rate production conditions. Although the FIV screening approach itself is novel, the main contribution of this work is in comparing FIV response and fatigue life of a well jumper outfitted with the traditional impact tees Vs. one outfitted with short radius bends. We find that the resulting flow-induced stress cycling is similar in magnitude in both geometries, but with different frequency spectra — giving the well jumper with short radius bends a 2x increase in fatigue life, relative to the well jumper with impact tees. We also perform a sand erosion analysis comparison between the two well jumpers, and find that the well jumper with short radius bends has a 3x diminished erosion rate, relative to the well jumper with impact tees.
Propped Fracture Conductivity in Shales
Authors: Zhang Junjing , Kamenov Anton , Zhu Ding , Hill A. Daniel
Published: November 2013

The successful development of the major shale gas plays in North America hinges upon the creation of complicated fracture networks by pumping low viscosity fracturing fluid with low proppant concentrations at high flow rate. Direct laboratory measurement of hydraulic fracture conductivity created in the networks is needed for reliable well performance analysis and fracture design optimization.

A series of experiments were conducted under realistic hydraulic fracturing conditions to measure the conductivity using a modified API conductivity cell. Natural fractures were preserved and fracture infill was kept for initial conductivity measurement. Fractures were also induced along the natural bedding planes to obtain fracture surface asperities. Proppants of various sizes were placed between rough fracture surfaces at realistic concentrations. The two sides of the rough fractures were either aligned or displaced with a 0.1 inch offset.

Results show that the hydraulic fracture conductivity of shale samples with rough surfaces can be accurately measured in a laboratory with appropriate experimental procedures and good control on experimental errors. The unpropped offset fracture can create conductivity as much as poorly cemented natural fracture, while the conductivity of unpropped matched fracture is minor. The presence of proppants can elevate the fracture conductivity by 2 to 3 orders of magnitude. Propped fracture conductivity increases with larger proppant size and higher proppant concentration. This study also indicates that within 20 hours propped fracture conductivity can be reduced by as much as 24% as shown in the longer term fracture conductivity measurements.
Predicting Well Performance in Naturally Fractured Reservoir
Authors: Hwang Yunsuk , Lin Jiajing , Schechter David , Zhu Ding
Published: November 2013

Multiple hydraulic fracture treatments in reservoirs with natural fractures create complex fracture networks. Predicting well performance in such a complex fracture network system is an extreme challenge. The statistical nature of natural fracture networks changes the flow characteristics from that of a single linear fracture. Simply using single linear fracture models for individual fractures, and then summing the flow from each fracture as the total flow rate for the network could introduce significant error. In this paper we present a semi-analytical model by a source method to estimate well performance in a complex fracture network system. The method simulates complex fracture systems in a more reasonable approach. The natural fracture system we used is fractal discrete fracture network model. We then added multiple dominating hydraulic fractures to the natural fracture system. Each of the hydraulic fractures is connected to the horizontal wellbore, and some of the natural fractures are connected to the hydraulic fractures through the network description. Each fracture, natural or hydraulically induced, is treated as a series of slab sources. The analytical solution of superposed slab sources provides the base of the approach, and the overall flow from each fracture and the effect between the fractures are modeled by applying the superposition principle to all of the fractures. The fluid inside the natural fractures flows into the hydraulic fractures, and the fluid of the hydraulic fracture from both the reservoir and the natural fractures flows to the wellbore. This paper also shows that non-Darcy flow effects have an impact on the performance of fractured horizontal wells. In hydraulic fracture calculation, non-Darcy flow can be treated as the reduction of permeability in the fracture to a considerably smaller effective permeability. The reduction is about 2% to 20%, due to non-Darcy flow that can result in a low rate.

The semi-analytical solution presented can be used to efficiently calculate the flow rate of multistage-fractured wells. Examples are used to illustrate the application of the model to evaluate well performance in reservoirs that contain complex fracture networks.
SPH Multiphase Simulation of Bubbly Flows: Towards Oil and Water Separation
Authors: Grenier N. , Le Touzé D. , Colagrossi A. , Colicchio G. , Antuono M.
Published: November 2013

The multi-fluid SPH formulation by [1] is studied in the context of engineering flows encountered in the offshore industry where bubbly flows are of importance in some production processes. These particular flows being dominated by viscous and surface tension effects, the considered formulation includes models of these physical effects. This model is then used to simulate viscous incompressible bubbly flows of increasing complexity. These flows include the merging of two bubbles, the separation process in a bubbly flow in a closed tank and then in a simplified separator. Results are compared to numerical solutions when available. The influence of the Bond number on these interfacial flow evolutions is investigated in detail.
Numerical Simulation of Ship Collision With Gravity Base Foundations of Offshore Wind Turbines
Authors: Hamann Thorben , Pichler Torben , Grabe Jürgen
Published: November 2013

For the installation of offshore foundations several countries (e.g. Germany) require a proof of averting environmental disasters in case of ship collision. The aim is to prevent possible discharge of supplies or even loss of the vessel. Especially for gravity base foundations this load case is problematic due to their larger stiffness and mass compared to monopiles, tripods or jacket foundations. The finite element method provides a powerful tool to predict the collision behaviour in a realistic way taking into account the complex interaction between vessel, foundation and soil. The collision between a fully loaded single hull tanker and a gravity base foundation is subject of numerical analysis. The calculated contact forces between vessel and foundation are compared to a simplified calculation approach. For evaluation of the foundation deformations and areas of failure of the vessel are investigated. The influence of the water depth, the diameter of the foundation and an embedment in the seabed are determined in a parametric study. It can be shown that the finite element method is a suitable approach for investigation of the collision behaviour of offshore structures. The design of gravity base foundations can be optimized with respect to ship collision in a fast and cost-effective manner using this method.