bluuebunny/crossref_metadata_2025 huggingface dataset:
Predictions of Armour Wire Buckling for a Flexible Pipe Under Compression, Bending and External Pressure Loading
Authors: Sertã Otávio , Fumis Rafael , Connaire Adrian , Smyth John , Tanaka Rafael , Barbosa Teofilo , Godinho Carlos
Published: August 2013

During installation and operation a flexible pipe may be subjected to high compressive forces, high cyclic curvatures and external pressures leading to high reverse end-cap loads. Under such loading conditions, which occur particularly in the touchdown region for deep water applications, the limiting condition for the flexible pipe can be the compressive stability of the tensile armour wires. Two potential instability modes are possible: radial mode (birdcaging) and lateral mode (lateral wire disorganization). Previous work on the subject has established the key factors which influence the onset of each buckling mode [1],[2],[3] and [4].

In order to ensure the feasibility of flexible designs for applications with increasing water depth, it is important to improve the knowledge of the mechanisms which can lead to instability of armour wires and enhance the ability to predict with greater assurance, the particular conditions which increase the risk of wire instability.

The focus of this work is the comparison of finite element prediction of radial buckling (birdcaging) with physical testing results under loading states that lead a pipe to birdcaging failure.

The numerical model incorporates all tensile armor wires and their interactions with each other and adjacent layers. The outer sheath and reinforcing tape layers are explicitly represented, while the inner layers of the pipe (pressure armour and carcass sheath) are idealized using a homogeneous representation. The model also incorporates the effects of manufacturing pre-tension and hoop strength in the anti-birdcaging tape layers which are critical determinants for the onset of buckling.

A key aspect of the method presented is the means by which the loading is applied. Specifically, the modeling handles the simultaneous and controlled application of end rotations, axial compression and radial resistance of the tapes through to the point of tape failure, pipe ovalisation and subsequent radial displacement and buckling of individual wires.

In summary, in this paper a solid modeling approach is presented, which is compared with full a scale sample test data, that enables the simulation of a flexible pipe undergoing large combined compression, curvatures and pressure loading.
SCR Fatigue Feasibility Prediction for Deepwater FPSO Applications
Authors: Xiang Sherry , Cao Peimin
Published: August 2013

Many of the recent offshore oil and gas discoveries in the world have occurred in deep waters, at locations far away from the existing subsea infrastructure. Floating Production Storage and Offloading systems (FPSOs) are the preferred field development solutions for those locations with mild and moderate environments. The simplest fluid transfer system between the seabed and the FPSO is a simple catenary configuration, with Steel Catenary Risers (SCRs) as the preferred solution for deep water, high pressure and high temperature applications.

This paper introduces an effective engineering SCR fatigue screening methodology for FPSO applications. The focus of this study is to better understand SCR fatigue performance at Touch Down Point (TDP) and develop a simple but reasonable correlation between FPSO porch motion and the riser dynamic stress response at TDP. This methodology enables one to perform the porch motion statistics analysis and the riser dynamic response analysis independently. By using the wave screening approach, SCR fatigue feasible map can be effectively developed. The paper presents an example performing the fatigue screening evaluation. SCR feasibility design flow chart for FPSO application is also addressed.

Particular for FPSO application in a new development, the fatigue feasibility prediction approach along with the strength feasibility check can help the project make the informative decision on the overall FPSO and riser system selection at the early stage of the development.
A Study on the Extensional-Torsional Response of a Damaged Flexible Pipe
Authors: Merino Héctor E. M. , de Sousa José Renato M. , Magluta Carlos , Roitman Ney
Published: August 2013

Inspections in flexible risers have detected a considerable number of damages in their top section caused by the installation process or generated during their operation, due to the contact with another riser or components of the floating facility. Among the most common damages detected, the rupture of the tensile armor wires is critical to the structure integrity as its load capacity may be significantly reduced. The objective of this work is to present the main results involving pure tension and torsion with tension tests in a 4” unbonded flexible riser with five damaged wires in its outer tensile armor. These tests were carried out at the Structures Laboratory of Federal University of Rio de Janeiro (LabEst). Besides presenting full data concerning the internal structure of the riser, this paper describes the experimental procedures used to perform the tests and the main results obtained such as axial and torsional stiffness and the redistribution of forces in the wires. Comparisons between numerical simulations with those obtained experimentally are presented. A brief discussion about results and some simplifications assumed on the numerical model are presented at the end of the work.
Dynamic Behavior Analysis of a Deepwater Self Standing Hybrid Riser System
Authors: Morooka Celso K. , Shiguemoto Denis A.
Published: August 2013

Self Standing Hybrid Riser (SSHR) system is an attractive option for deepwater application. The system is composed by a vertical riser pipe coming from the sea bottom to a subsurface buoy which is placed in around one hundred meter water depth from the sea surface. It is connected to a floating production facility at the sea surface by a flexible jumper. It almost eliminates all the undesired dynamic effects from ocean waves, in addition to relief the total riser system weight from the floating platform. Recent discoveries of petroleum in ultra-deep water Pre-salt Offshore Brazil stimulated development of innovative and nontraditional riser system configurations.

The present work deals with the dynamic behavior of a SSHR system excited by sea current, waves and displacements induced by sea surface platform motions. In plane and out of plane displacements for the SSHR system is analyzed and subsurface buoy maximum motions amplitude have been observed. Effects from current drag and vortex induced forces are investigated by numerical simulations of the dynamic behavior of the SSHR system.

Analysis procedure is presented with details, and fundamentals of the semi-empirical approach for hydrodynamic drag and vortex induced forces in the riser length and subsurface buoy are described. Numerical simulation results are presented, and evaluations are conducted for deepwater condition. Maximum amplitude of vibration is observed near from to lock-in conditions, and effects of the subsurface buoy vortex induced motion (VIM) are discussed. Hydrodynamic forces previously obtained from reduced model tests are used for the numerical approach.
Methodology for Thermoplastic Umbilical Cross Section Analysis
Authors: Silva Danianderson , Balena Rosianita , Lisbôa Rafael
Published: August 2013

Umbilicals are composite cables responsible for communication, control and power supply of offshore systems for oil exploitation. They can be composed of elements such as power and signal cables, hoses, tubes and fiber optics. Due to their critical role of providing control and power, umbilicals properties and operational limits must be correctly calculated in order to prevent failures, which can be costly and harmful to the environment.

In order to address these needs, the main goal of this paper is to study the viability of using a finite element analysis tool for calculating the allowable bending radius (ABR) and the crush resistance of these cables. Using this tool, the engineer can have a sound basis for choosing the best cross section design at early stages of the project.

The ABR is particularly important at the point where the umbilical touches the seabed, named touchdown point (TDP). This parameter limits the possible operational configurations for the cable and has a significant influence in its fatigue lifespan. Currently, the methods used to predict the ABR of thermoplastic umbilicals are mostly empirical and/or based on samples, but, as we go to deeper areas — as the recent ultra-deep water field discoveries in Brazil, Angola and others — more accurate ways of estimating this parameter are needed.

Another effect of the higher water depths is the increase of tension on umbilicals during installation. Therefore, tensioners with higher load capacity are needed, requiring umbilicals with higher crush resistance. Sample testing during a qualification program is the main method to obtain the value of this parameter, but is costly.

Due to its reliability and relative low costs compared to sample testing, the use of a finite element tool is an attractive alternative for obtaining these limits. In this paper, crush resistance and ABR will be calculated for thermoplastic umbilicals — the most common type of umbilical used in Brazilian offshore oil exploitation — and the results will be compared with the ones achieved by traditional methods. This comparison shows the applicability of using finite element analysis for determining these parameters.
Conceptual Design of TMR-Less Manganese Nodule Mining System and the Economy Examination
Authors: Yamazaki Tetsuo , Yamada Amon , Arai Rei , Nakatani Naoki
Published: August 2013

Manganese nodules on deep ocean floors have been interested in as future metal resources these forty years. The Total Materials Requirement (TMR) of the conventional proposed mining method, however, is very high because of the much lifted water with the nodules from the seafloor to the sea surface and the longer transportation from the mining site to the smelting plant. An innovative conceptual design of the TMR-less mining system is presented. The economy is examined and compared with the one of the conventional method.
Extreme Values of Environment Parameters Under Super Typhoon in South China Sea
Authors: Li Chenfeng , Ren Huilong , Xu Donghao , Zeng Ji , Li Xiaoyu
Published: August 2013

Deteriorated environment leads to frequent occurrences of typhoon in South China Sea, especially exceeding 100-year super typhoon which is severely threatening the safety of offshore structures. In order to research its extreme wind resisting ability, it is necessary to analyze the exceeding 100-year wind with associated waves and currents. Based on probability theory and least square method, a parameter estimation method for three-parameter Weibull distribution is presented. According to the measured data in South China Sea, environment parameters’ distribution under super typhoon is obtained. Based on the distributions, wind speed with associated waves and currents at different return period in South China Sea is given, and its comparison with Gumbel distribution shows that it had high fitting precision and strong applicability. The wind with associated waves and currents parameters under different meteorological wind scale are given. At last, the structural response of one example Jack-up platforms are compared and analyzed in extreme conditions. The results indicate that the probability analysis of the environment parameters is essential to ensure the safety of structure, further analysis considering both probability estimation method and rules is necessary.
Design and Modelling of Accidental Ship Collisions With Ice Masses at Laboratory-Scale
Authors: Kim Ekaterina , Storheim Martin , von Bock und Polach Rüdiger , Amdahl Jørgen
Published: August 2013

Knowledge about the level of damage after a collision with an ice mass is necessary for designing ships and offshore structures operating in ice-infested waters. An understanding of the physical processes during such a collision is needed to prevent (or limit) accidents, causing loss of life, the loss of a ship or environmental pollution. This study was motivated by the lack of experimental data on ship collisions with ice masses where both the ship and the structure undergo deformations. Laboratory experiments of accidental collisions with ice masses (ACIM) are essential to verify current methods for integrated analysis of the crushing and deformation of the ice and the steel structure. ACIM tests are sensitive to the structural design, i.e., the design of a structure that is flexible enough in relation to the ice mass. Both the ice and the structure should be able to deform during the collision event. The paper addresses issues related to the planning of ACIM at laboratory scale with special emphasis on the choice of:

(i) process of ice manufacturing and ice mechanical properties;

(ii) flexibility of impacted structure;

(iii) scaling of the experiment.

Experimental setup of laboratory-scale ACIM for the Aalto Ice Tank is proposed. Non-linear finite element analysis is used as a tool to predict structural damage and to guide the planning of collision experiments. The predicted damage of the test specimens caused by collision is presented.
Reynolds Number Effects on the Vortex-Induced Vibration of Flexible Marine Risers
Authors: Resvanis Themistocles L. , Jhingran Vikas , Vandiver J. Kim , Liapis Stergios
Published: August 2013

This paper explores the Reynolds number dependence of the Vortex-Induced Vibration (VIV) of flexible marine risers. Emphasis is placed on revealing the trends that exist between the Strouhal number and the Reynolds number and between the dimensionless amplitude (A/D) and Reynolds number. Data is drawn from recent towing tank experiments which used flexible cylinders of three different diameters. The 38m long pipes were exposed to uniform and sheared currents. The Reynolds number range extended from approximately 5,000 to 220,000 — well into the critical regime — with the larger diameter pipes responding in up to the 13th mode and the smaller diameter pipe responding well above the 20th mode. The results and trends from this set of experiments are compared to previous results from laboratory and field experiments.
Nonlinear Instabilities of Spar Platforms in Waves
Authors: Rodríguez Claudio A. , Neves Marcelo A. S.
Published: August 2013

Spars are widely recognized as an excellent choice for deep water applications due to their hydrodynamic characteristics. However, some relatively recent works report the occurrence of large motions in the plane perpendicular to wave incidence direction, i.e., not directly excited by waves. These unstable motions have been attributed to Mathieu instabilities, caused by pure hydrostatic variations of the underwater hull geometry. Based on a new approach developed by the Authors for predicting parametric rolling in ships, this nonlinear phenomenon has been investigated for spar platforms. The proposed approach demonstrates analytical and numerically that unstable motions, in fact, may appear, but the mechanism that triggers parametric rolling is not related to variations in the hull hydrostatic characteristics. Nonlinear pressure variations induced by waves passing along the spar introduce parametric excitation. Different from typical ship forms, where this effect is negligible even in very long waves due to shorter draughts, in spars, this excitation can be significant, especially for very long waves. The present paper presents the analytical expressions for roll parametric excitation in spars and numerically explores the proposed approach applied to a typical spar under a wide range of wave heights and periods. Parametric Amplification Domains (PADs) were numerically computed, showing not only the boundaries of the instability regions but also the maximum roll amplitudes.
CFD Analysis of a LNGC Carrier in Waves
Authors: Guilmineau Emmanuel , Leroyer Alban , Visonneau Michel , Ory Emmanuel
Published: August 2013

Single Buoy Moorings (SBM) Offshore is a pioneer in the offshore and gas industry. Its product line is the supply of facilities and services for the development and production of offshore oil and gas fields as well as the systems relevant to the mooring technology at large. SBM is currently developing various concepts aimed at offloading LNG (Liquefied Natural Gas) carriers offshore. These concepts whether they assume tandem or side-by-side offloading have in common a building block: the LNG carrier. Owing to its unusual shape (shallow draft, non wall sided hull at the waterline with a flared bow and a transom stern and the presence of a bulb just below the sea surface) at least compared to standard VLCCs (Very Large Crude Carrier), difficulties arise when performing diffraction calculations and when comparing model test results in monochromatic, bi-chromatic and irregular waves with numerical time domain simulations. The main objective is to identify whether differences between model tests and standard numerical predictions based on linear potential theory can be bridged in increasingly complex wave fields by resorting to CFD simulations.

The CFD software used is ISIS-CFD, developed by the Numerical Modelling Group of the Fluid Mechanics Laboratory of Ecole Centrale de Nantes and distributed as commercial software by NUMECA International under the name FINE/Marine.

CFD simulations have been performed in monochromatic head wave condition with and without the 4-line mooring system to prevent the LNG carrier from drifting away. The CFD method is described and a comparison between model tests and simulations is presented. CFD shows that it is able to predict the motions measured in model tests. In addition, both the wave frequency and the natural frequency of the mass spring system are correctly linked with the frequencies of the predicted motions.
Bending Capacity of Tubes With Lüders Bands: Influence of Geometric and Material Parameters
Authors: Hallai Julian F. , Kyriakides Stelios
Published: August 2013

Previous studies involving a combination of experiment and analysis have shown that when bending a pipe with material that exhibits Lüders bands to strain levels of 2–3% the structure develops instabilities that can lead to premature collapse. It has been demonstrated that the associated events and limit states can be consistently simulated using 3D finite elements and an elastic-plastic constitutive model with an up-down-up material response. This paper presents the results of a parametric study of the problem in which a wide range of geometric and material parameters are considered. The parametric study quantifies the boundary between stable and unstable behavior and shows that the bounding values of Lüders strain depend on the D/t of the tube and the level of the Lüders stress.
Steel Jackets and Monotower Foundations for Offshore Wind Turbines
Authors: Wåsjø Kasper , Bjerkås Morten , Søreide Tore
Published: August 2013

Steel jacket support structures for offshore wind have an increasing popularity for water depths larger than 20 m. Traditionally, support structures such as monopiles and gravity based foundation have dominated at shallow water. Offshore wind industry deals gradually with larger water depths, and light weight foundations as steel jackets are believed to give a cost benefit. The present study shows that the quasi static overturning moment for monotower structures and steel jackets are of comparable magnitude. It is also shown that the most important benefit of steel jackets as support structures is to reduce the base shear. At last it is shown that foundation size is primarily driven by wave and current loads at water depth around 50–60 m.
Hydro-Mechanical Issues of Offloading Operations Between a Floating LNG Terminal and a LNG Carrier in Side-by-Side Configuration
Authors: Lécuyer Bruno , Ledoux Alain , Molin Bernard , Le Boulluec Marc , Heguiaphal Begnat
Published: August 2013

This paper presents a state-of-the-art methodology to build a numerical hydro-mechanical model of a floating LNG Terminal with a LNG carrier moored alongside, complying with the physical mechanical behavior and hydrodynamic phenomenology involved in such a complex system.

The required numerical functions are available for intensive engineering applications in the PRINCIPIA’s software DIODORE™ dedicated to sea-keeping and mooring analysis. The methodology has been validated by comparison with non-captive model tests in IFREMER basin in the frame of the SALT 2 JIP led by PRINCIPIA.

The paper describes first the academic side-by-side case that has been tested in basin, associated with the comparison between the experimental results and the numerical application using DIODORE™. In a second step, the main physical parameters of the simulation are analyzed and some gained insights are exposed.
Model Tests for a Floating Windturbine on Three Different Floaters
Authors: Koo Bonjun , Goupee Andrew J. , Lambrakos Kostas , Kimball Richard W.
Published: August 2013

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution, and therefore many countries build wind farms near shore. Up to now, most of offshore wind farms have been built in relatively shallow water (less than 30m) with fixed tower type wind turbines. Recently, several countries plan to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine.

The model tests described in this paper were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled.

The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for calibration and improvement of existing design analysis and performance numerical codes. An additional objective of the model tests was to establish advantages and disadvantages among the three floating platform concepts on the basis of test data.

The paper gives details of the scaled model wind turbine and floating platforms, the set-up configurations, and the instrumentation to measure motions, accelerations and loads as well as wind turbine rpm, torque and thrust for the three floating wind turbines. The data and data analysis results are the subject of another paper in this conference [1].
Comparison of Design and Operational Behaviour of an Offshore Pipeline With Controlled Lateral Buckling
Authors: Solano Rafael F. , Antunes Bruno R. , Hansen Alexandre S. , Bedrossian Arek , Roberts Graeme
Published: August 2013

It is imperative to adopt some conservative premises in the engineering calculations undertaken during the design stage of an offshore pipeline susceptible to lateral buckling, in order to achieve a design with adequate levels of robustness and integrity throughout the pipeline’s design life. The conservatism can be attached to many uncertainties such as the pipe-soil interaction — interpreted as-soil friction factors — the seabed stiffness and profile and even the as laid lateral out-ofstraightness. Once in operation, these effects will come into play and the pipeline may behave slightly differently to that anticipated in design, depending on the relative strength of the natural uncertainties compared to the design features such as engineered buckling triggers. The over-riding intention in design is, of course, to enable the pipeline to withstand, with sufficient safety margins, the maximum stresses and strains anticipated to occur by realistic predictions in the design stage.

In recent years, many kilometres of deepwater pipelines have been designed and installed along the Brazilian coast using the principle of controlled lateral buckling, in which engineered buckle triggers, such as sleepers and distributed buoyancy sections, are deployed at regular intervals along the pipeline. The purpose of these triggers it to initiate a sufficient number of benign buckles along the pipeline and thereby relax the compressive forces set up as a result of thermal expansion without violating safe limits on stress and strain in the pipelines. In addition to the engineered buckling sites, however, the natural seabed features and associated uncertainties will interact with the pipeline’s behaviour and create additional natural buckle sites. To anticipate these sites and discover their importance at the design stage is recognised as a real challenge, particularly as precise post-installed and in-operation surveys are not normally carried out with the intention of confirming such buckle sites and design assumptions.

The work reported in this paper is a detailed comparison between the initial design and observed operational behaviour of an offshore HP/HT pipeline, mainly in terms of the engineered and natural buckles actually formed along the pipeline, the severity of these buckles and some conclusions concerning the effects of initial imperfections and pipe-soil interaction characteristics considered in detailed design. It is hoped that this rare feedback from real operating conditions on installed pipelines, will be of great interest to pipeline designers and lead to more efficient and better understood design processes and encourage Operators to undertake more regular and sophisticated surveys of operating and installed pipelines for the benefit of future projects.
Ensemble Forecasts Used for Extreme Wave Warning and Platform Unmanning Strategies
Authors: Hagen Øistein , Riise Bjørn
Published: August 2013

Surface waves often represent the most critical environmental parameter for offshore activities, and reliable forecasts are in many cases essential for safe operation.

Currently, decisions on safety during extreme wave situations are generally taken based on deterministic wave forecasts, and uncertainty related to forecasts are acknowledged and accounted for via safety margins. However, unmanning criteria reflecting a consistent probabilistic model for the forecast uncertainty are generally lacking.

In 1998 the European Centre for Medium-Range Weather Forecasts (ECMWF) introduced a new method for ocean wave forecasting based on the Ensemble Forecasting technique. In this paper we address how the ensemble prediction system (EPS) can be used to establish a probabilistic model applicable for platform unmanning criteria and procedures.

Before ensemble forecasting techniques can be introduced operationally for this purpose it is important to investigate how the decision process can be improved with this method. By improvement we expect the probability forecasts to yield a higher hit rate and a lower false alarm rate.

Extreme wave warnings are simulated based on a data series established for a 10-year period and the hit and false-alarm rate from deterministic and probabilistic forecasts are compared with measured data. The ensemble forecasts are used to:

• establish a model for uncertainty of forecasts for bad weather

• update safety margins used at present such that target safety is obtained if deterministic forecasts are used

• discuss a revised scheme where the EPS forecasts are applied on an operational level
Hebridean Marine Energy Resources: Wave-Power Characterisation Using a Buoy Network
Authors: Vögler Arne , Venugopal Vengatesan
Published: August 2013

The Outer Hebrides of Scotland were identified as an area with a high wave power resource of 42.4kW/m. The Outer Hebrides of Scotland are currently targeted by a range of developers for demonstration and commercial developments of wave energy converters and current planning efforts are based on initial deployments by 2014. Technology providers with well advanced plans to develop the Hebridean wave resource include Aquamarine Power (Oyster) [1], Pelamis (P2) [2] and Voith Wavegen (OWC) [3]; all of these companies are partners in the Hebridean Marine Energy Futures project [4] to help move the industry into the commercialisation stage.

As part of the Hebridean Marine Energy Futures project, a three year programme aimed at developing a high resolution wave energy resource map to support the site selection process of marine energy developers, a network of three wave measuring buoys was deployed 15km offshore in a depth of 60m and at distances of 11km between buoys. Measured wind and wave data from this buoy network for autumn 2011 are analysed and presented in this paper along with estimated wave power for the same duration.
Vibration Decay Along Deepwater Moorings With Complex Geometry and Configuration, as a Tool for Truncation
Authors: Argyros Alex , Langley Robin S. , Ahilan R. V.
Published: August 2013

This paper is concerned with the difficulties in model testing deepwater structures at reasonable scales. An overview of recent research efforts to tackle this challenge is given first, introducing the concept of line truncation. Passive truncation has traditionally been the preferred method by industry; however, these techniques tend to suffer in capturing accurately line dynamic response and so reproducing peak tensions. In an attempt to improve credibility of model test data the proposed truncation procedure sets up the truncated model, based on line dynamic response rather than quasi-static system stiffness. Vibration decay of transverse elastic waves due to fluid drag forces is assessed and it is found that below a certain length criterion, the transverse vibrational characteristics for each line are inertia driven, hence with respect to these motions the truncated model can assume a linear damper whose coefficient depends on the local line properties and vibration frequency. Initially a simplified taut string model is assumed for which the line is submerged in still water, one end fixed at the bottom the other assumed to follow the vessel response, which can be harmonic or random. A dimensional analysis, supported by exact benchmark numerical solutions, has shown that it is possible to produce a general guideline for the truncation length criterion, which is suitable for any kind of line with any top motion. The focus of this paper is to extend this work to a more complex line configuration of a conventional deepwater mooring line and so enhance the generality of the truncation guideline. The paper will close with an example case study of a spread mooring system, applying this method to create an equivalent numerical model at a reduced depth that replicates exactly the static and dynamic characteristics of the full depth system.
SCR VIV Analysis Using Time Domain Code SimVIV
Authors: Sidarta Djoni E. , Rakshit Tuhin , Jaiswal Vivek
Published: August 2013

Vortex Induced Vibration (VIV) analysis of risers using time domain Finite Element Analysis (FEA) captures nonlinearity in the system. Such non-linearity may be in the form of boundary conditions or the imposed loadings. Riser VIV analysis using a frequency domain code has to linearize the system, such as imposing linearized boundary conditions.

Non-linear boundary conditions for steel catenary riser (SCR) could be in the SCR-soil interaction and in the hang-off system. An SCR is often supported by a flex joint that has non-linear rotational stiffness. The effect of these non-linear boundary conditions in SCR VIV response is investigated in this paper.

In addition, the effect of the directions of current profiles with respect to the riser on the SCR VIV response is also investigated. Comparison to standard industry practice, that is applying currents either in the plane or normal to the plane of the SCR, is presented.

Time domain VIV code SimVIV coupled with FEA code ABAQUS was used in the analysis.
Experimental Investigation Into the Behaviour of a New Replaceable Bracing System
Authors: Khonsari S. V. , England G. L. , Moradi M. , Farahani H. , Zarei D.
Published: August 2013

A new bracing system with new features somewhat different from those of the existing ones was devised and is presented. These features comprise some of the advantages of eccentric bracing systems including their ability to dissipate energy through bending of flexural elements while lacking some of the disadvantages of such systems including their inability of being replaced upon damage during events such as earthquakes, etc. While in traditional types of braces, including both concentric and eccentric ones, braces are under axial loading, in this new system they work in a bending capacity. As a result, the sort of flexibility which is introduced in the system in the presence of eccentric braces (definitely at the price of forcing the potential damage to occur and be concentrated at the girders/beams to which such braces are connected), is now provided by the braces and at the price of sacrificing them. Therefore, if the level of damage is such that the damaged element is no longer usable, the replacement of the braces is a more viable and economically justifiable than that of the girders. Moreover, since girders, as part of the deck/floor system, are normally engaged with other elements such as stringers/joists, their replacement may not be practically possible at all. Such bracing systems, called Broken Beam Bracing System (BBBS), have the potential of being used in offshore/onshore structures as originally-used elements or in a retrofit/repair capacity.
A Nonlinear Analytical Model for Flexible Pipe Crushing Analysis
Authors: Pesce Celso P. , Franzini Guilherme R. , Rabelo Marcos A. , Ramos Roberto , Gonçalves Rodolfo T. , Tanaka Rafael , Barbosa Teófilo , Godinho Carlos A.
Published: August 2013

This paper discusses a nonlinear analytical model for flexible pipe crushing analysis, improving analytical models previously published in OMAE2003, 2010 and 2011. Following that path, and still based on the concept of equivalent pipes and rings, the present model discusses a strategy to cope with an elastic-to-plastic analysis. Such a model considers the combined action of squeezing and concentrated loads applied as representing those due to the caterpillar shoes. Experimental results and finite element analysis are used to assess the pertinence of analytical modeling hypotheses.
Swell Attenuation due to Wave-Induced Turbulence
Authors: Babanin Alexander V.
Published: August 2013

In the paper, we will discuss two separate, but connected issues: wave-turbulence interactions, and estimates of swell attenuation due to such turbulence. Theoreticaly, potential waves cannot generate the vortex motion, but the scale considerations indicate that if the steepness of waves is not too small, the Reynolds number can exceed the critical values. This means that in presence of initial non-potential disturbances the orbital velocities can generate the vortex motion and turbulence. This problem was investigated by laboratory means, numerical simulations and field observations. As a sink of wave energy, such dissipation is small in presence of wave breaking, but is essential for swell. Swell prediction by spectral wave models is often poor, but is important for offshore and maritime industry. Based on the research of wave-induced turbulence, new swell-dissipation function is proposed. It agrees well with satellite observations of long-distance swell propagation and is being employed and tested in spectral wave models.
Determination of Infragravity Wave Conditions at a Nearshore Location With a Non-Uniform Coastline: Case Study of Baía de Todos os Santos, Brazil
Authors: van der Hout Arne J. , de Jong Martijn P. C. , Moerman Emiel , Timerman Gabriela J. , Ribeiro Eric Oliveira
Published: August 2013

The Petrobras energy company plans to build a new terminal in the Baía de Todos os Santos, Brazil. This paper describes a study in which an estimate has been provided for infragravity (or low-frequency, LF) wave conditions at this future LNG Regasification Terminal location. To aid the design of the terminal, numerical simulations of the propagation of these waves into the bay have been performed.

At the proposed terminal location, within a large coastal bay, LF waves can be related to long waves bound to the local primary sea and swell waves (bound LF waves) and to low-frequency waves that propagate into the bay after being made free near the bay entrance (free LF waves). While most of the primary waves will not fully penetrate into the bay, the long LF waves can. Due to the large area of the bay (approximately 40 km × 30 km), detailed modelling of all complex coastal processes related to infragravity waves was not efficient. Therefore, a practical approach has been followed in this study.

For a first evaluation, an estimate has been made of bound LF wave conditions at the terminal location inside the bay and at the mouth of the bay. This estimate is based on an equilibrium method using the local primary wave climate and assuming a flat bottom. In a second stage, free LF wave penetration into the bay has been simulated using the shallowwater model Delft3D-FLOW.

By applying the practical approach described in this paper, infragravity wave conditions (bound and free) have been efficiently estimated at the future terminal location for yearly-averaged and extreme primary wave conditions. The results show that the LF wave climate at the terminal will be very mild, corresponding to beneficial conditions for terminal operations. The outcome of this study will be taken into account in the further design of the terminal.
Station Keeping and Propulsive Performance of a DP Crane Barge Close to a FPSO: A Comprehensive Set of Experimental Tests
Authors: Tannuri Eduardo A. , Moratelli Lazaro , Vileti Vinícius L. , Silva João Luis B.
Published: August 2013

DP crane barges operate in close proximity to FPSO (Floating Production Storage and Offloading) during lifting operations. This operation is critical since small variations of the relative position may result in collision between the bodies or large motion of the lifting load. The hydrodynamic interactions between the units are an important effect that must be considered in the analysis of those operations. The interactions are an intricate problem that cannot be modeled analytically in an appropriate way. Hence, experimental and numerical tools are applied for the definition of the environmental conditions for a safe operation or the optimal relative positioning between the vessels.

A research project conducted by Petrobras in cooperation with Brazilian universities aims to develop and to validate numerical tools for aiding the design and analysis of the DP lifting operations. They are based on dynamical time-domain simulations of the operation and can be used for predicting several parameters of the operations. The experimental tests that will be used for validating this numerical model, concerning the DP performance when the crane barge is close to a FPSO, are presented in this paper. The tests were conducted in an ocean basin using the model of a moored FPSO and the DP crane barge. The station keeping ability (relative motion) and the performance of the propulsive subsystem of the DP crane barge (power and thrust utilization parameters) are measured. Several relative positions and wave conditions were considered.
A General Approach for DP Weathervane Control: Numerical Results
Authors: Miyazaki Michel R. , Tannuri Eduardo A.
Published: August 2013

With oil exploration occurring at increasing distances from the shore, the use of DP vessels becomes economically attractive for oil transfer and transportation. This operation has some peculiarities, such as the possibility to arbitrarily set the DP vessel heading.

Setting the correct yaw angle brings advantages to the vessel such as reducing the fuel consumption and minimizing the DP system output. However, it is not trivial to calculate the optimum heading, due to uncertainties at wave and current estimation. Moreover, these factors are not constant along time, and to keep the optimum position, the system must adapt with the changes.

A method to update the heading Set-Point based on the theory of Zero Power Control was developed, which has advantages over other weathervane control methods, such as an effective control of all horizontal movements and the possibility to control any reference-point, not only points located the vessel bow.

A fully non-linear dynamic simulator was then used to carry out a complete analysis of the controller.
An Improved Stochastic Modeling for Bayesian Wave Estimation
Authors: Iseki Toshio
Published: August 2013

A modified Bayesian modeling procedure for wave estimation is proposed. In this method, errors in the estimates of ship response functions can be taken into account. In order to discuss the relationship between the minimum ABIC and the accuracy of the estimated wave parameters, the ABIC surfaces and the optimum area of the wave estimation are shown with respect to the two hyperparameters. As a result, the modified Bayesian modeling makes the ABIC surface smoother and can provide stable wave estimation. This concludes that the modified Bayesian modeling is reliable within a certain accuracy to estimate the wave parameters.
Experimental Study of the WEPTOS Wave Energy Converter
Authors: Pecher Arthur , Kofoed Jens Peter , Larsen Tommy , Marchalot Tanguy
Published: August 2013

This paper presents the power performance results of the experimental study of the WEPTOS wave energy converter (WEC). This novel device combines an established and efficient wave energy absorbing mechanism with an adjustable structure that can regulate the amount of incoming wave energy and reduce loads in extreme wave conditions. This A-shaped floating structure absorbs the energy in the waves through a multitude of rotors, the shape of which is based on the renowned Salter’s Duck. These rotors pivot around a common axle, one for each leg of the structure, to which the rotors transfer the absorbed wave energy and which is connected to a common power take off system (one for each leg). The study investigates the performance of the device in a large range of wave states and estimates the performance in terms of mechanical power available to the power take off system of the WEPTOS WEC for two locations of interest. These are a generic offshore location in the Danish part of the North Sea (Point 3) and the location of the Danish wave energy centre (DanWEC) in front of Hanstholm harbour.
An Efficient 3-D FNPF Numerical Wave Tank for Virtual Large-Scale Wave Basin Experiment
Authors: Nimmala Seshu , Yim Solomon , Grilli Stephan
Published: August 2013

This paper presents an accurate and efficient three-dimensional computational model (3D numerical wave tank), based on fully nonlinear potential flow (FNPF) theory, and its extension to incorporate the motion of a laboratory snake piston wavemaker, to simulate experiments in a large-scale 3D wave basin (i.e. to conduct “virtual” or numerical experiments). The code is based on a higher-order boundary element method combined with a Fast Multipole Algorithm (FMA). Particular efforts were devoted to making the code efficient for large-scale simulations using high-performance computing platforms to complement experimental 3D wave basins. The numerical simulation capability can serve as an optimization tool at the experimental planning and detailed design stages. To date, waves that can be generated in the NWT include solitary, Cnoidal, and Airy waves. In this paper, we detail the model, mathematical formulation, and wave generation. Experimental or analytical comparisons with NWT results are provided for several cases to assess the accuracy and applicability of the numerical model to practical engineering problems.
Effect of Mud Shedding on Riser Anti-Recoil Control at Emergency Disconnect
Authors: Li Songcheng , Campbell Mike , Howells Hugh , Powell Simeon
Published: August 2013

With drilling capability extends to water depths up to 3000m, significantly increased is the risk associated with a failed riser recoil control in the event of an emergency riser disconnect due to loss of vessel station keeping. In deeper waters the tensioner system undertakes higher top tension due to the accumulation of riser length and mud weight. During emergency disconnect, the riser is disconnected between the blow-out preventer (BOP) and the lower marine riser package (LMRP), releasing the base tension and mud pressure. Impact between the top riser system and the diverter housing system should be avoided, and the clearance between the LMRP and BOP should be secured. Efforts have been continuously made in the industry to achieve a more accurate predict of the riser recoil response. The relevance of mud discharge to recoil control has been widely discussed but little quantitative data has been revealed in the literature.

In this paper, effect of mud shedding on riser recoil response is discussed. The Herschel-Bulkley rheology model is utilized for mud flow and is considered the latest advance in the drilling industry. Water hammer theories with column separation are modified to account for mud discharge in laminar, transitional and turbulent flow regimes.

As a case study herein, recoil response of a drilling riser attached to a dynamically positioned semi-submersible drilling vessel is assessed to present the mud discharge effect on riser anti-recoil control. At emergency events, the riser is disconnected above the BOP, which is located at a water depth of about 2150m for this study. Mud is generally preferred to be freely discharged during an emergency disconnect for ease of anti-recoil control and riser integrity. The density difference between drilling mud in the riser annulus and sea water outside the riser outer casing before disconnect induces a high pressure difference, which drives mud shedding at riser disconnect. Mud flow rate plays an important role in the speed control of the riser uplift. 3D finite element analysis is performed in time domain to simulate riser response before and after disconnect. 2HRECOIL software is integrated into ANSYS user programmable features to better model the riser response and mud discharge.
Experimental Investigation of Dynamic Response of a Flexible Cylinder in a Transverse Flow
Authors: Gu Jijun , Pinto Waldir , Coelho Jairo , Levi Carlos , Vitola Marcelo , Duan Menglan
Published: August 2013

Dynamic response of a vertical flexible cylinder vibrating at low mode numbers with combined x – y motion are investigated in this paper. The uniform flow was simulated by towing the flexible cylinder along the tank in still water, therefore the turbulence intensity of the free flow is negligible in order to obtain more reliable results. The dominant frequencies, maximum attainable amplitude, modal analysis and x – y trajectory in cross-flow and in-line directions have been reported and compared with literatures, some good agreements and justifiable discrepancies are obtained. These results could benefit the future experimental and numerical work in this area.