bluuebunny/crossref_metadata_2025 huggingface dataset:
A Monte Carlo Approach for Estimating Extreme Currents in the Singapore Straits
Authors: Jones Oliver , Ewans Kevin , Chuah Stanley
Published: November 2013

Utilizing the independency of tide, through-flow, surge and high-frequency currents in the Singapore Straits, a Monte Carlo simulation method of combining the different components is proposed, expanding the horizon of available measured and modelled data and facilitating the definition of design current speeds. The statistical model proceeds by, first, making N number of random picks from the non-exceedence probability distributions of the surge, through-flow and high-frequency components. The number of random picks made in a given year for each component, N, is defined by assuming its occurrence rate is Poisson-distributed around a known annual mean value. N number of random start times are then chosen from each year and the maximum value of tidal current predicted over an ensuring 3-day window is combined with the randomly sampled component (either surge, through-flow or high-frequency current). Assuming an intended design life of 50 years, this process is repeated N number of times in each of the 50 years and for each current component, yielding 50 annual maximum values. For random 3-day windows that overlap, the model takes the vector sum of the maximum tidal current and the 2 (or 3) concurrent components. The process is repeated 1000 times, producing 1000 * 50 values of annual maxima which are then assigned non-exceedence probabilities. Return Period levels are obtained directly from the non-exceedence probabilities. The method provides a reduction in design current when compared to values derived by multiplying the exceedence probabilities of the varying independent contributions directly.
Preliminary Analysis on Integrated Transportation Technique for Offshore Wind Turbines
Authors: Zhang Puyang , Ding Hongyan , Le Conghuan , Zhang Siyu , Huang Xu
Published: November 2013

With current construction technology of offshore wind turbines, there is a need for a major marine spread to install the foundation, tower and turbine. There is a clear benefit that offshore installation can be integrated into one operation. The large-scale composite bucket foundation is a basis for the one-step integrated transportation and installation technique with a special vessel. The proposed transportation and installation technique will minimize offshore spread and maximize the proportion of work carried out onshore with consequent benefits in terms of cost, quality and safety. The composite bucket foundation with self-floating property would be towing into a semicircle groove of the vessel and connected with wireropes of the fixed crane. Afterwards, tower and turbine are attached onto the foundation at shore and the whole unit of foundation, tower and turbine is loaded out from the quayside, transported to site and set down on the seabed. During transportation, half of the unit weight is taken by the hoisting system of the vessel and other weight is supported by air cushions inside the composite bucket foundation. A detailed case is studied to determine the motions of the vessel with two units to confirm the viability and feasibility of such a method of the integrated transportation. The transportation and installation methodology are developed to reduce the time spent on offshore works in order to fit the installation work within the time windows with great economic benefits.
Fatigue Performance of Submarine Pipeline Girth Welds Installed by S-Lay and Reeling
Authors: Hval Morten , Lamvik Trond
Published: November 2013

Submarine pipelines installed on very uneven sea bottom usually results in many long free spans. If the free spans are above a certain length the pipeline will be subjected to vortex induced vibration (VIV) caused by sea currents or wave motions transverse to the pipeline direction. The onset and level of VIV will depend on several factors such as pipe dimensions and submerged weight, current velocity, seabed soil properties and span boundary condition. For uneven sea bottom, seabed intervention may be very costly if there are many high free spans that are longer than what is acceptable with respect to fatigue. There is obviously a possible economical benefit if longer free spans can be accepted due to improved fatigue performance, which may result in less intervention work by either excavation, trenching or rock/mechanical supports.

This paper will summarize results from a comprehensive full scale fatigue test program that has been performed in order to verify potential enhanced fatigue design parameters of high quality pipeline welds compared to standard fatigue design parameters according to DNV-RP-C203. The testing included pipe dimensions from 6 to 16 inch OD, and tests were performed on both as welded pipes and pipes subjected to simulated reeling strains. In addition to normal welds, also welds with deliberately introduced weld defects/weld indications, maximum acceptable axial misalignment and repaired welds were tested. The test results were compared with similar recommended design parameters for pipeline girth welds. Furthermore, the effect of reeling, weld residual stresses, weld defect and repair will be presented and discussed.
Minimizing Interference Between Top Tension Risers for Tension Leg Platforms
Authors: Koska Ryan , Kaculi Jim , Campbell Mike , Mills Darren
Published: November 2013

Interference between TTRs is a key design challenge. For spars, this is typically mitigated by having a large spacing between wellheads, thus increasing the distance between risers along the water depth. However, due to lower TTR tensioner stroke limits combined with larger vessel offsets, the feasibility of spacing out the wellheads for TTRs on a TLP is limited. The result is TTRs that are closer together through depth and therefore more likely to contact each other in extreme current conditions.

This paper presents the approach used to minimize the interference between the top tension risers on TLPs. Specific topics include considering current profiles with varying directions through the water depth, adjusting the top tension of the risers, and utilizing low-drag vortex-induced vibration (VIV) suppression strakes. The relative effect of each of these topics is also discussed.
The Development of a Fibre-Optic Based Breach Detection System for Flexible Pipe
Authors: Nott Phil , McNab John , Fernando Upul , Sheldrake Terry
Published: November 2013

In service, flexible pipes are exposed to harsh environments and challenging conditions that subject the structure to demanding physical rigour that is largely unknown or unnoticed throughout the service life of the asset. Resistance to extremes and large fluctuations in strain, temperature and pressure are built into the pipe design, however the pipes are often breached due to accidental damage to the outer shield caused by external objects. Early knowledge of this damage is vital to limit the degradation of the metallic wires due to contact with seawater, and with the early knowledge of breach prompt remedial actions can be triggered to prevent further deterioration of the pipe that may lead to expensive repair or replacement. Using fibre optics as a distributed temperature sensor, the entire length of the asset can be continuously monitored, and the incidence of a breach in the outer shield of the pipe can be efficiently captured.

This paper describes a fibre optic-based breach detection system developed to monitor offshore flexible pipes. A brief outline of the development programme is presented. The work involved selection of fibre/monitoring equipment, physical encasement of the ‘harsh environment’ fibre optics in the pipe structure, subsequent termination in the pipe end-fitting, and the procedures for interrogation and interpretation of the Raman-based optical signals. A prototype system has been implemented on mid-scale pipe samples and the transient and steady state thermal behaviour of the pipe has been investigated under a simulated and wide range of controlled breach test conditions. A summary of the test results is presented.

Numerical models of the pipe geometry have been developed to predict the transient thermal behaviour of a shield breach. Good correlation of experimental results with the predictions shows the sound adaptation of fibre optic-based sensing technology in offshore flexible pipes.
Port Logistics Policy of Japanese Government for Strengthening Global Competitiveness of Industry in Case of Ocean Space Utilization
Authors: Takahashi Koji , Urabe Shinichi , Umeno Shuichi , Kozawa Keiji , Fukuda Isao , Kondo Takeo
Published: November 2013

A port logistics is classified into two kinds, a container logistics and a bulk logistics.

In the field of a container logistics, a terminal operator pursues large-scale management of its container terminals by global M&A corresponding to the global growth of the container handling number, which resulted in container terminal operators’ oligopolization.

In the field of a bulk freight logistics, the private enterprises, which invest in and improve port equipment by themselves, are changing in the direction to reduce number of handling ports and to invest in port equipment intensively corresponding to the vigorous resource demand of the world.

The economic activities of the private enterprises are progressing on the basis of the management strategy which differs between “oligopolization” of a container logistics and “selection and concentration” of a bulk freight logistics.

On the other hand, since management of ports is the basis supporting a national logistics, each country of the world carries out various public participations, such as a legal support and a financial support.

For example, in Japan, although ‘a port authority system’ was brought to Japan almost 60 years ago by U.S.A. and the management right of main ports were transferred to local governments from national government, Japanese government has been taking the lead in implementation of many policies to strength global competitiveness of industry reflecting the flow of global privatization and concession.

Recently, many countries have been changing their public participation policies. A country has established the system which increases private investment and reduces public financial support since capital investment remaining power has decreased according to aggravation of the finances of national government and local governments. The other country has established the system which backs up logistics activity with public support.

This paper, to utilize ocean space, will show a port logistics policy of Japanese government, particularly the historical flow and the prospective view of the public participation to a port logistics based on the logistics trend of the world.
Consideration of a Rotary Power Take-Off Mechanism as a Wave-Energy Converter on the WindFloat Platform
Authors: Kanner Samuel , Yeung Ronald W.
Published: November 2013

The possibility of incorporating a wave-energy extractor into a current design of the WindFloat platform is examined. First, to absorb wave energy, a rolling cam shape, with rotary power take-off, is attached to a tubular truss member of the WindFloat located above the calm-waterline. Based on the assumption that the extractor is operating in beam seas, numerical predictions for the coupled 3-DOF system (surge, heave and pitch motions) were completed for an ideal-fluid situation. The degradation of the performance of the wave energy extractor because of viscous effects was discussed in [1]. Second, a design of a versatile bi-directional rotary system, named the UC Berkeley Double-Ratchet Mechanism (UCB-DRM) was made. This mechanism can produce a unidirectional rotational motion, thus facilitating the power take off by a generator. A physical unit was constructed. The efficiency and performance of this mechanical system is assessed by introducing a known, bi-directional torque input and measuring the torque output.
Neutron Diffraction Measurement of Weld Residual Stresses in an UOE Linepipe Subjected to Mechanical Expansion
Authors: Wen Shuwen , Dong Hongbiao , Zhang Shu Yan , Bannister Adam , Connelly Martin
Published: November 2013

Weld residual stresses in double submerged arc welded (DSAW) UOE linepipes can have significant implications on the integrity and in-service performance of the pipeline from which it is normally constructed, often for deep and ultra-deep water applications. Therefore they have to be considered as a crucial data input for accurate Engineering Critical Assessments (ECAs) of offshore pipelines. This has so far been based on the assumption that the residual stresses in the UOE pipe seam weld are equal to the yield strength of the parent material: an assumption that is likely to cause over-conservatism in conventional pipeline design.

In the current study neutron diffraction technique was used to quantify the weld residual stresses of an UOE linepipe (18″ OD × 25.4mm WT, grade X65) in post-expansion condition. The UOE linepipe manufacturing process is briefly introduced first, which involves forming steel plate into pipe, followed by DSAW seam welding and then pipe mechanical expansion. The sample preparation for neutron diffraction measurement of lattice parameters is described next in detail. Neutron diffraction was used to measure the residual elastic strain within the pipe through the precise characterisation of the interplanar crystal lattice spacing and the atomic lattice itself was used as a strain gauge. The measurements were mainly taken in and near the seam weld area of the pipe along the pre-defined lines in through-thickness and cross-weld orientations. The weld residual stresses in three primary directions, namely axial, hoop and radial, were calculated from the measured lattice parameters in as-supplied (welded and expanded) and stress-free conditions using Hooke’s law of linear elasticity, and are systematically presented in the paper. It was found that weld residual stresses of the UOE linepipe in post-expansion condition were significantly lower than the yield strength of the parent material, and in most cases these were less than 50% of the actual yield strength of the parent material. The mechanism of weld residual stress formation and the influence of post-weld expansion on the reduction and through-thickness normalisation of weld residual stresses were studied through finite element (FE) modelling analysis.

Finally an example ECA analysis was carried out for the current linepipe under an assumed service loading condition. The degree of over-conservatism that could result by assuming weld residual stresses to be equal to the yield strength of the parent material was demonstrated, as compared to the situation where the values of the required residual stresses were taken from the neutron diffraction measurement. It was therefore shown by the current study that the combined approach of neutron diffraction measurement and FE modelling could greatly enhance the understanding of performance of seam-welded UOE pipes.
Conceptual Design of a Single-Point-Moored FOWT and Tank Test for Its Motion Characteristics
Authors: Iijima K. , Kawai M. , Nihei Y. , Murai M. , Ikoma T.
Published: November 2013

A new design concept of a semi-submersible type floating offshore wind turbine (FOWT) moored by a single-point mooring is proposed. The FOWT model adopting 5MW class wind turbine is designed. The motion characteristics of the FOWT are evaluated by a series of tank tests. To this end, a scaled model with a scale ratio 1/100 is fabricated. The scaled mode tests are performed under winds, waves, and combined winds and waves to check its fundamental feasibility. It is observed that the motion characteristics under wind and waves are acceptable in general, and the combination of the single point mooring and the down-wind type rotor is effective in terms of weathervane. It is also shown that the difference between the two transfer functions to wave loads, one with and the other without wind loads, is small except pitch response at low frequencies.
Comparison of Simulation and Tank Test Results of a Semi-Submersible Floating Wind Turbine Under Wind and Wave Loads
Authors: Philippe Maxime , Courbois Adrien , Babarit Aurélien , Bonnefoy Félicien , Rousset Jean-Marc , Ferrant Pierre
Published: November 2013

A model of the Dutch Tri-floater semi-submersible platform equipped with the NREL 5MW wind turbine has been tested in the hydrodynamic and ocean engineering tank of École Centrale Nantes under wind and wave loads. This paper aims at comparing the results obtained with numerical simulations with these experimental results. The numerical model is based on the FAST design code from NREL and a user defined platform load model for calculating hydrodynamic and mooring loads. This hydrodynamic model includes non linear hydrostatic and Froude-Krylov forces, diffraction/radiation forces obtained from linear potential theory and Morison forces to take into account viscous effects on the braces and heave plates.

First the hydrodynamic model is calibrated against the results of free decay tests without wind. A good agreement is achieved by calibrating mooring properties and heave plates properties of the numerical model. Then a comparison of regular wave cases without wind is realised, and a fair agreement is observed for surge, heave and pitch motions of the floating system. Finally comparisons are realised for regular wave cases with a constant wind speed. A good agreement is observed for the steady state surge and pitch offset. Surge and heave motions also shown a good agreement, these degrees of freedom are not being strongly influenced by wind loading. For pitch motion, numerical simulations show differences around 0.4 rad/s wave frequency, for which model tests have shown a significant influence of wind loading on system motion.
Study of Pollution Transport Based on a Dynamical System Theory in Xiangshan Bay, China
Authors: Han Songlin , Liang Shuxiu , Sun Zhaochen
Published: November 2013

Understanding the characteristic of pollution transport is essential for protecting the water quality in coastal areas, or bays. Lagrangian Coherent Structure (LCS), a concept from dynamical system theory, is applied to study pollution transport in Xiangshan Bay located in the middle coast of Zhejiang Province in China with strong tidal dynamics. The LCSs of flow field are defined as the attracting or repelling material curves in Finite-time Lyapunov exponent (FTLE) fields. The FTLE fields are computed based on the velocity data of the three-dimensional, high resolution hydrodynamic model developed for Xiangshan Bay. The LCSs can reveal the accumulation or dispersion characteristics for different waters in Xiangshan Bay. Further, some particle tracking experiments were conducted to study how these curves affect pollution transport through analyzing the Lagrangian trajectories. The main transport patterns in Xiangshan Bay are given finally. The results can help optimizing the sewage draining outlets location to make the pollution leave the bay more efficiently and managing the coastal pollution.
Ocean Turbulent Mixing in Northern Bohai Strait, China
Authors: Liang Shu-xiu , Sun Zhao-chen , Han Song-lin , Yin Hong-qiang , Bai Bo
Published: November 2013

The measurements of ocean microstructure through which ocean internal mixing mechanism is revealed are taken more often recently. Free-falling turbulence microstructure profiler TurboMAP-9 is used to take a field observation on the area of northern Bohai Strait. 13 stations distributed in Bohai Sea, Yellow Sea and the “division line” between them are measured. Turbulent mixing characteristics of northern Bohai Strait for different seasons are described by analyzing the observation data of ocean turbulence microstructure profile. The results show that the northern Bohai Strait is a strong mixing area during non-stratification period. Turbulent energy dissipation rate ε of winter is bigger than that of autumn and it is strongest near the bottom layers which is in the order of 10−5W/kg. Heat dissipation rate χθ is in the same order of 10−6–10−5°C2/s in autumn as ε and 2–3 orders smaller than ε in winter. Thermal diffusivity coefficient kθ is a little bigger than turbulent mixing rate kρ in autumn and 1–2 orders smaller than kρ in winter. Both the kρ and kθ along the “division line” of Bohai Sea and Yellow Sea are bigger than that of the Bohai Sea and Yellow Sea. Base on the measured data and the analysis, heat dissipation rate and thermal dispersion coefficient can change 2–3 orders in non-stratification seasons which should be paid much attention to, especially for ocean model parameterization and pollutant discharge modeling.
Methodology for Performance Assessment of a Two-Body Heave Wave Energy Converter
Authors: de Andrés Adrian , Guanche Raúl , Armesto José A. , del Jésus Fernando , Vidal César , Losada Íñigo J.
Published: November 2013

A wave energy farm composed by several two-body heaving wave energy converters is being developed by IH Cantabria. This study presents a methodology to obtain the power performance of an isolated two-body heaving wave energy converter, previously presented and analyzed by [1]. The methodology relies on a numerical model which represents the motion of the two bodies in the time domain. This time domain model has been built substituting the entire Cummins equation system with a state-space system, thereby avoiding the convolution integral of the radiation force term with a state-space subsystem, previously used in [2] and [3].

The performance of the device along its life cycle has been estimated based on a proposed new methodology. The new method is proposed in order to obtain the long term power production of a device with the same computational effort than the classical method based on the power matrix. The proposed method is able to estimate long term power production time series. This long time series is obtained using the MaxDiss selection technique from [4] in order to compute only the power of the most representative sea states and the Radial Basis Function interpolation technique (RBF) to obtain the complete power series.
Experimental Study of Hydraulics and Cuttings Transport in Circular and Non-Circular Wellbores
Authors: Taghipour Ali , Lund Bjørnar , Ytrehus Jan David , Skalle Pål
Published: November 2013

Cuttings transport is one of the most important aspects to control during drilling operations, but the effect of wellbore geometry on hole cleaning is not fully understood. This paper presents results from experimental laboratory tests where hydraulics and hole cleaning have been investigated for two different wellbore geometries; circular and a non-circular, where spiral grooves have been deliberately added to the wellbore wall in order to improve cuttings transport. Improving hole cleaning will improve drilling efficiency in general, and will in particular enable longer reach for ERD wells.

The experiments have been conducted as part of a research project where friction and hydraulics in non-circular wellbores for more efficient drilling and well construction is the aim. The experiments have been performed under realistic conditions. The flow loop includes a 12 meter long test section with 2″ diameter freely rotating drillstring inside a 4″ diameter wellbore made of concrete. Sand particles were injected while circulating the drilling fluid through the test section in horizontal and 30° inclined positions. The test results show that borehole hydraulics and cuttings transport can be significantly improved in a non-circular wellbore relative to a circular wellbore.

Investigating the cutting transport in non-circular wellbores with available models is even more complex than for circular wellbores. Most drilling models assume circular wellbores, but in reality the situation is often different. Also, it may be possible to create non-circular wellbores on purpose, as in the present study. Such a comparative, experimental study of hole cleaning in different wellbore geometries has to our knowledge previously never been performed, and the results were obtained in a custom-made and unique experimental flow loop. The results and the experimental approach could therefore be of value for any one working with drilling.
Crescent Waves on Finite Water Depth
Authors: Zou Zhili , Zhou Yalong , Yan Kai
Published: November 2013

A laboratory experiment on generation and evolution of L2-type crescent waves was performed with focus on the effects of finite water depth on crescent waves. The new results include the critical wave steepness for triggering crescent waves, the characteristics of the wave surface pattern and amplitude spectrum, and the parameters of surface elevation.
Geology-Based Reservoir Model Building for Carbonate Reservoirs
Authors: Lagkaditi Lydia , Srivastava Ashok , Gupta Anuj
Published: November 2013

Environment in which carbonate reservoir rocks are deposited was studied by visiting and collecting rock samples from a carbonate reservoir analog located at Jabal Fuwairit in the Northeastern coast of Qatar. The primary objective of this study is to develop methods to characterize carbonate reservoirs.

The experimental procedure included collecting samples based on geology, mapping the location and altitudes of the samples using a high-resolution Global Positioning System (GPS) with an altimeter, permeability measurements at selected locations, conducting petrophysical measurements on the samples, Conventional and Micro-X-ray Computed Tomography Scanning (CT Scanning), Scanning Electron Microscopy (SEM), X-ray Energy Dispersive Spectroscopy (EDS) and Atomic Force Microscopy (AFM) imaging.

It is observed that the permeability measurements show an alternating pattern as a function of height above sea level. The cyclicity is probably representative of repeating sequences of sea level changes over geological time. CT Scanner and Micro-CT Scanner measurements were employed to obtain vital information about the flow pathways in the rock, thus assisting in calculating the porosity to compare with values measured in the lab. SEM results gave direct visualization of the pore network and information about grain size (500 microns) and intergranular porosity, mineralogy, compared with the EDS results, and lithotype. EDS results showed that reservoir rocks directly exposed to environment had significant alteration due to weathering. However, it is encouraging to learn that samples even few inches away from the surface had minimal environmental influence.
Evaluation of Loads During a Free-Fall Lifeboat Drop
Authors: Califano Andrea , Brinchmann Kristoffer
Published: November 2013

A new Offshore Standard (DNV-OS-E406) [1] has been developed for the design of Free Fall Lifeboats (FFLB) used for emergency evacuation from offshore structures. This standard aims at ensuring safe evacuation, focusing on the design at an early stage. Load (CFD) and structural (FE) analyses are used to assist in determining whether or not the free fall lifeboats comply with the Target Safety requirements as set forth by the Standard.

The present paper describes the methodology used when assessing the structural capacity of FFLB. The structural integrity is established throughout every phase of a lifeboat drop, linking computational fluid dynamics (CFD) and finite element analysis (FEA). Loads computed from CFD on a finite volume mesh are interpolated to a finite element model where the structural analysis can be performed in order to obtain stresses, strains and deflections, and finally assess the structural capacity of the lifeboat.

Emphasis in this work is given to the hydrodynamic model and loads, while structural analyses are discussed more in depth by Brinchmann et al. [2].
Wave Modelling for Potential Wave Energy Sites Around the Outer Hebrides
Authors: Greenwood Charles E. , Venugopal Vengatesan , Christie David , Morrison James , Vögler Arne
Published: November 2013

This paper presents the results of a numerical wave modelling study carried out to assess the near shore wave energy resource around potential wave energy sites at the Outer Hebrides in the United Kingdom. This study uses Danish Hydraulic Institute’s MIKE 21 Spectral Wave model suite. Input boundary conditions are obtained from a Datawell directional wave buoy located approximately 16 km off the coast of Lewis in 60 metre depth. Additional data collected from a submerged Acoustic Wave and Current profiler (AWAC) located at 13 metre depth offshore at one of the wave energy development sites was used to calibrate and validate the wave model for separate time periods. The calibration process allows the manipulation of white capping, bottom friction and wave breaking parameters to alter the energy dissipation across the model domain. The altered parameters gave a significantly better agreement between modelled and measured results than the model defaults. While the average wave conditions provided a relatively straightforward calibration process the more extreme storm events significantly under predicted the wave height. After several trials in altering model coefficients a good agreement was reached between the model results and the AWAC data. These new sets of calibration parameters enable the simulation of wave heights within 13% for the AWAC data and marginally more for wave periods for the first 6 months of 2012.
Numerical Analysis of Contact Stresses Between Mooring Chain Links and Potential Consequences for Fatigue Damage
Authors: Bastid Philippe , Smith Simon D.
Published: November 2013

Design codes for offshore mooring systems recommend proof loading chain links to around 70% of the specified breaking load of the chain (API RP 2FP1, Lloyd’s Register). This is primarily to check that the chain will safely resist the service loads and will not excessively elongate. It is assumed that the proof load also generates compressive residual stresses at the interlink contact region and also at the point of the intrados (KT point) where a high stress concentration occurs during tensile loading. Tests have shown that proof loading improves the fatigue performance of chain under cyclic axial loads.

Elastic-plastic finite element analyses of the proof loading have been performed. These analyses have shown that the proof loading also generates very high tensile residual stresses in the region surrounding the interlink contact zone. This region also experiences significant in-service cyclic stresses under cyclic tension or out-of-plane bending. The combination of the cyclic stresses and high tensile residual stress is of concern and it is proposed that the periphery of the interlink contact zone should be carefully reviewed. It is understood that chain link fatigue at present is only based on the risk of fatigue damage at the KT point.

This paper presents and discusses results of finite element stress analyses of studless chains of different sizes and grades, and show the relative fatigue sensitivity of the KT and contact regions. The chain grade, dimensions and loading regime are shown to be important.
Procedure to Obtain Added Mass in Ships From Natural Frequencies Measured in Hull Vertical Vibration Full Scale Tests
Authors: de Mesquita Ortiz Luiza , da Silva Neto Severino Fonseca , Sphaier Sergio Hamilton
Published: November 2013

This work shows an experimental-numerical procedure used to determine added mass of ships. A numerical Finite Element ship hull model is constructed according to the weight distribution and the drafts over its length during the full scale vibration measurement. The rate breadth/draft for each frame of the hull is considered as the independent variable of a quadratic function representing the added mass distribution, which coefficients are determined in order to minimize the sum of squared differences between the natural frequencies obtained numerically in respect with those obtained in the correspondent full scale measurement. The added mass coefficients obtained from the experimental data of a first ship are then used to predict numerically the natural frequencies of a second ship of the same type.
Material Testing for the Prediction of the Effect of Deformation and Aging Thermal Treatments on Collapse Resistance of UOE Pipes
Authors: Serebrinsky Santiago , Mantovano Luciano , Arroyo Fábio , Valdez Martín , Ernst Hugo , Silva Ronaldo
Published: November 2013

Oil & Gas offshore exploration and production increase continuously in deep waters. This trend requires pipes with increasing collapse pressure (pc), which is the primary design variable. The prediction of pc, based on an appropriate set of input variables, allows for the appropriate design of deep and ultra-deep water projects.

Elastic and plastic material behavior is one of the main factors affecting pc. International application codes (e.g., DNV OS-F101) incorporate the yield strength into their formulas for pc. In the present work, an assessment of the requirements on material characterization tests for a reliable prediction of pc has been performed. The most appropriate testing direction is the transverse compression. Moreover, since for large diameter pipes the plastic strain levels attained at collapse are often below 0.2%, the sample should allow for an accurate determination of compression behavior in this very low deformation range. This is particularly relevant for cold-formed pipes, as with the UOE process.

Based on these guidelines, a testing procedure has been designed. This analysis has been applied to the prediction of the effect of thermal cycles on pc. Calculated values show a very good agreement with experimental pc values determined for a series of UOE processed pipes that had been thermally treated and collapsed.
Wave Drift Forces and Responses in Current
Authors: Stansberg Carl Trygve , Hoff Jan Roger , Hermundstad Elin Marita , Baarholm Rolf
Published: November 2013

The influence from a current on wave drift forces and resulting slowly varying vessel responses can be quite significant. In this paper the effect is reviewed and further investigated. Several works have been published on this complex topic during the last 20–25 years, while it is only to a little extent taken consistently into account in standard industry tools. Simplified methods are often used, if any, and /or empirical correction from model test data. Thus there is a need to improve standard tools in this respect.

The effects on slowly varying vessel motions and resulting extreme mooring line loads are demonstrated through time series sequences from selected, previous model tests with FPSO’s and semisubmersibles in steep irregular waves. Wave-current interaction effects that can be larger than the effects from current and wind alone are identified. It is also confirmed from these examples that extreme mooring forces usually occur due to extreme slow-drift motions.

An overview description is given of a new, general numerical potential theory code for industry use, MULDIF-2, where wave-current-structure interaction is consistently included as a basic element in the formulation. Main items in the approach are addressed and referred to previous works in the literature. Results from an initial comparison against previous results on drift forces on a vertical column are given, and a good agreement is found. Further verification and validation work is in progress.
Wave-in-Deck Loads for an Intricate Pile-Supported Pier and Variation With Deck Clearance
Authors: Cornett Andrew , Anglin David , Elliott Trevor
Published: November 2013

Many deck structures are located at elevations low enough to be impacted by large waves. However, due to the highly complex and impulsive nature of the interactions between wave crests and intricate deck structures, establishing reliable estimates of extreme pressures and forces for use in design remains challenging. In this paper, results from an extensive set of three-dimensional scale model tests conducted to support the design of a large pile-supported pier (or jetty) are presented and discussed. Relationships between maximum wave-in-deck loads and the deck clearance (air gap) are presented and discussed. Results from numerical simulations of the wave-structure interaction process obtained using the three-dimensional CFD software FLOW-3D® are also presented and discussed. Finally, some initial comparisons between the numerical and physical modelling are also included. This paper provides new insights concerning the character and magnitude of the hydrodynamic pressures and loads exerted on intricate pile-supported deck structures due to impact by non-linear shallow-water waves, and the relationships between the hydrodynamic forcing and the deck clearance or air gap.
Seakeeping Characteristics of a Wave-Adaptive Modular Unmanned Surface Vehicle
Authors: Dhanak Manhar R. , Ananthakrishnan P. , Frankenfield John , von Ellenrieder Karl
Published: November 2013

Sea-keeping characteristics of a wave-adaptive modular catamaran, developed by Marine Advanced Research Inc., is studied using at-sea experiments and dynamic modeling. The vehicle is based on the Proteus design and is supported by two inflatable pontoons with each pontoon attached to a center payload tray using an independent suspension system connected in such a way that the demi-hulls perform somewhat independent motions through a wave field. The suspension system is designed to isolate the center payload tray from the motions induced by waves incident on the demi-hulls. The vehicle is propelled by two water jets and is capable of achieving speeds of 8–15 knots. Accelerometers have been mounted on the pontoons and the center payload tray of the vehicle and field tests in the open ocean have been carried out to study sea-keeping properties of the vehicle. Heave and surge motions for the cases of following seas, beam seas and head seas have been recorded. In each case the center payload tray is found to experience attenuated heave accelerations compared with the two pontoon hulls. The reductions in surge and sway motion are, however, not evident. The associated spectra suggest that heaving motion of the center payload tray over a range of frequencies is attenuated by the suspension system. No attenuation is apparent in the surge and sway motions. The WAM-V dynamics is modeled as a three-body problem with the pontoons corresponding to bodies 1 and 2, and the central tray to body 3. Results of the analysis are compared with the sea trials.
Prediction of VIV for Risers With Effective Strakes
Authors: Campbell Brad , Slocum Scott
Published: November 2013

Accurate prediction of straked riser response can be important for determining the minimum strake coverage needed to ensure adequate riser fatigue life in demanding high-speed current environments. Previously published results from ExxonMobil’s 2003 vortex-induced vibration (VIV) model tests on a long, flexible, straked cylinder showed that strakes can greatly reduce, but may not entirely eliminate flow-induced vibration. The data also showed that the low-amplitude vibrations that persist when the cylinders are straked are different in character from VIV on bare cylinders, suggesting that the strakes are effective in disrupting the alternating vortex shedding mechanism that excites bare-cylinder VIV and that other flow mechanisms may drive the straked-cylinder vibration. This further implies that classic VIV models that have been developed to predict bare-pipe response due to alternating vortex shedding may not be accurate for straked cylinders even if the excitation model (“lift curve”) is adjusted to represent lower excitation levels on strakes.

Classic VIV prediction methods for bare risers rely on special-purpose theoretical models to account for the strongly nonlinear interactions between bare cylinder motion and hydrodynamic excitation. However, for the low amplitudes of vibration observed in tests with straked cylinders, the influence of the cylinder motion on the flow may be less important. In an effort to more accurately predict the performance of strakes, ExxonMobil has recently explored a new approach for predicting flow-induced vibrations for straked risers that neglects any influence of riser motion on excitation, and instead assumes that hydrodynamic excitation can be approximated to useful accuracy by excitation measured on a fixed straked cylinder. New experiments show that this excitation is stochastic, with broad band-width, and that it conforms to relatively simple scaling laws.

The measured stochastic excitation data has been used with a conventional linear random vibration model to predict the response data acquired from the 2003 model tests on the long, flexible, straked cylinder. Comparison shows a dramatic improvement in prediction scatter compared to predictions using a classic bare-pipe VIV formulation with a “lift curve” model of the strake hydrodynamic excitation.
Analytical Models for Seabed Interaction Effects on Steel Catenary Riser in Touchdown Zone
Authors: Rezazadeh Kosar , Bai Yong , Tang Jiwei , Zhang Liang
Published: November 2013

The complex nature of seabed interaction with steel catenary risers (SCR) in touch down zone (TDZ) of SCRs makes serious difficulties for engineering design industry. Design must ensure that the curvature remains well within elastic limits, and that fatigue damage remains acceptable during the life. Analytical methods, despite it is limited in the accuracy because of idealizations of the system response, it offers a first step in assessing the system performance. The paper compares the results of different seabed interaction models with those from finite element analysis to evaluate the accuracy and consistency of solutions for initial design assumptions and fatigue assessment.
Changes in Pipeline Embedment due to Sediment Mobility: Observations and Implications for Design
Authors: Rodriguez Antonio Borges , Bransby M. Fraser , Finnie Ian M. S. , Low Han Eng , White David J.
Published: November 2013

This paper describes temporal variations in embedment of several existing pipelines on the North-West Shelf (NWS) of Australia, and the sediment mobility processes that cause them. Distinct and explainable patterns in the extent, distribution and rate of the development of pipeline embedment have been revealed through systematic detailed examination of repeated annual integrity surveys by ROV. This represents a unique data-set that has been used to optimize the reliability of a newly designed pipeline. This paper explains why these clear findings should not be overlooked in both the buckling and stability design of initially unburied pipelines, which is in contrast to currently established industry practice. This new information supports the presumption that conventional approaches for calculating the hydrodynamic stability of unburied pipelines may be more conservative than necessary. Conversely, and arguably more importantly, it is shown that conventionally accepted methods for calculating pipe-seabed resistance forces when planning buckling schemes should be considered unsafe if embedment due to sediment mobility is possible. Consequently, this paper proposes an innovative calculation methodology that statistically captures these sediment mobility effects, and which facilitates a more justifiable geotechnical input to pipeline engineering than what is conventionally adopted. This methodology is currently being used by the authors as a state-of-the-art design practice for unburied offshore pipelines in regions of sediment mobility.
Performance Comparison Between Linear and Nonlinear Controllers for a Dynamic Positioning System
Authors: Morishita Helio Mitio , Moratelli Lázaro , Martinez Rodrigo Santos
Published: November 2013

This paper addresses some preliminary discussion concerning the performance of different dynamic observer-controller-based positioning system for a shuttle tanker by considering her operational condition changes. The observer-controller models are nonlinear passivity based observer and backstepping controller, Extended Kalman Filter and Proportional-Derivative like controller, and notch filter and Proportional-Integral-Derivative controller. The variation of the operational condition of the vessel is obtained by changing the vessel draft and the environmental condition, such as relative incidence of the current, waves and wind, and the peak period of the sea wave spectrum. The performance of the dynamics of the vessel is investigated through numerical simulation in which a saturation model for the actuators is included. Preliminary results have indicated that the nonlinear observer-backstepping controller is the easiest approach to select the observer and control parameter in the case of variation of the draft.
A Possible Failure Mode for Leeward Mooring Lines on a Floating Storage Unit
Authors: Aksnes Vegard , Nybø Terje , Lie Halvor
Published: November 2013

The floating storage unit Navion Saga at the Volve field in the North Sea suffered from two mooring line breaks in steel wire ropes in 2011. Investigations of the broken ropes indicated that a possible failure mechanism could be high stresses near the wire socket induced by large bending moments in leeward mooring lines. The scope of the current study has been to make a numerical model capable of capturing such behaviour of the steel wire rope and to check if the minimum bending radius could be as low as the rope’s specified minimum bending radius.

The numerical model has revealed a possible failure mechanism. The connecting link plate between the upper chain segment and the upper wire segment lies initially on the seabed. When lifted off the seabed, the link plate and the wire socket will fall to the seabed at a higher speed than the upper wire segment. A transverse wave in mooring line plane propagating towards fair-lead is generated when the wire socket hits the seabed. The wave leads to large curvature in the wire near the socket. Sensitivity studies of the governing parameters have been performed to assess the uncertainties of the numerical model. A modified system is presented and it is shown that the phenomenon which is likely to have caused failure in the original system will not occur for the modified one.
Increasing Water Depth Window for Moored Subsea Structures and Operations With Low Weight Hybrid Ropes
Authors: Amils Xavier , Durmus Beste , Smeets Paul , Boesten Jorn , Davies Peter
Published: November 2013

This paper introduces new hybrid wire rope concepts by combining steel wire and UHMWPE fibers. These novel High Modulus Hybrid Ropes (HMHR) result in enhanced strength and significant reduction of linear weight compared to steel wire rope (SWR) of equivalent constructions. Compared to fiber ropes (FR) this new family of ropes shows increased protection against crushing, fretting and wear.

In-house and independent laboratory tests showed increased breaking load / weight advantages compared to SWR, while also revealing improved bending fatigue resistance. Additionally, test results show similar axial stiffness and torque performance to SWR, which can be modeled by using analytical algorithms. Furthermore creep rate of these ropes is comparable to that of SWR.

Finally, the authors discuss the expected performance of these ropes working in deep waters and under high load conditions in offshore and subsea systems. Additional work required to validate and introduce these ropes in the industry is addressed as well.