bluuebunny/crossref_metadata_2025 huggingface dataset:
A Three-Dimensional Curved Beam Element for Helical Components Modeling
Authors: Provasi Rodrigo , Martins Clo´vis de Arruda
Published: November 2011

The offshore industry is in constant evolution due to the need of reaching new oil fields in increasingly water depths. In this scenario, not only new types of platforms are being designed, but also new types of flexible pipes and new umbilical cable configurations. The greatest difficulty to generate a new concept for a riser is to determine if it is viable or not. Flexible pipes and umbilical cables are complicated to model, due to the interactions between their layers and the large number of possible arrangements. To predict their behavior, adequate models are necessary. One can rely on finite element models, which show a great difficulty in mesh generation and convergence (especially due to the contact pairs). One can also rely on analytical models, which have many limitations due to simplifications (even though necessary ones). Another possible approach is to define macro elements, which represent a component, instead of classical finite elements (such as tetrahedral elements). Related to that approach, a numeric method using macro-elements is proposed. It consists in creating elements which has the desired characteristics of the problem in its formulation, leading to robust custom-made elements and to coarse meshes (since the complexity of the problem is within the element). Some elements are proposed in this model: a concentric one for cylindrical layer modeling; a three-dimensional curved beam for helices; a bridge element for node connection; and a contact element, for gap and friction treatment. The first two of them are already concluded and the later ones are being designed. This paper presents the three-dimensional curved beam element, which takes into account the effects of curvature and tortuosity. This is accomplished by using a strong coupling between displacements and assuming that the twist and shear strains varies linearly within the element. Using such hypothesis, the shear lock phenomenon is also avoided. This formulation is implemented and their results compared to those obtained by a classical finite element modeling tool, with good agreement.
On the Simplified Analysis of Square Plates With Holes
Authors: Schleyer Graham K. , Underwood Nicholas J. , Do Hyung Min , Paik Jeom Kee , Kim Bong Ju
Published: November 2011

This paper reports the development of a simplified energy solution to a complex problem involving large inelastic deformation in thin clamped square ductile plates with a square hole under the action of transverse pressure loading. The work is part of a project to study blast loading of steel plates with penetrations as used for deck plating or bulkheads that may be required to resist loading far in excess of their design limit due to the effects of an accidental explosion. It is important to develop criteria for the ultimate load-carrying capacity of such structures and guidance for the industry. In seeking this goal, the project will utilize experimental methods for conducting 1/8 scaled pulse pressure tests on 0.5 m square plates with a central aperture. The data will be used to help develop both finite element numerical simulations and simplified analytical models. Initial studies show, based on comparisons with previous test data, that acceptable engineering solutions can be achieved with simplified methods provided the user is aware of the limitations of the method. Furthermore, preliminary results suggest no significant reduction in resistance due to the central hole, the load being reduced in proportion to the hole area. Detailed analysis of the local stresses and strains around the hole will be carried out in the next stage of the project.
Analysis of a Two-Body Floating Wave Energy Converter With Particular Focus on the Effects of Power Take Off and Mooring Systems on Energy Capture
Authors: Muliawan Made Jaya , Gao Zhen , Moan Torgeir , Babarit Aurelien
Published: November 2011

The present paper summarizes analyses of a two-body floating wave energy converter (WEC) including the mooring system. An axi-symmetric Wavebob type WEC is chosen as the object of investigation here. However, the PTO system is modeled in a simplified manner as ideal linear damping and spring terms that couples the body 1 and the body 2 motions. The analysis is done using SIMO, a time domain simulation tool which accommodates simulation of multi-body systems with hydrodynamic interactions. In SIMO, docking cone features have been introduced between the two bodies to let them move as per actual operation and fenders are applied to represent end stops. Six alternative mooring configurations are applied to investigate the effect of mooring on power capture. In this paper, the software HydroD using WAMIT for hydrodynamic is used to determine hydrodynamic loads. The analysis is carried out for several regular and irregular wave conditions as representative of operational conditions. Simulations are performed with the purpose to study the effects of power take off (PTO) system, end stops setting and several mooring configurations on power captured by the WEC.
Wind Turbine Installation Vessel of a New Generation
Authors: Bereznitski Alexei
Published: November 2011

A new type of the wind turbine installation vessel is developed. The concept utilizes the Small Waterplane Area Twin Hull (SWATH) vessel. High transit speed, excellent seakeeping, and quick installation sequence allow placing a large number of wind turbines within short time minimizing the time of offshore construction works. The paper deals with the design development of this concept. Such major subjects as seakeeping, model testing in seakeeping tank, wind turbine landing sequence, and the workability are covered in the paper. Special attention will be given to the design of the active motion compensation system applied in the hoisting system. Dynamic behavior of this system is studied. The motions of the vessel are also compensated by dedicated active anti roll and anti-pitch systems. These systems were also tested in a seakeeping tank.
A BEM-Isogeometric Method With Application to the Wavemaking Resistance Problem of Ships at Constant Speed
Authors: Belibassakis K. A. , Gerostathis Th. P. , Kostas K. V. , Politis C. G. , Kaklis P. D. , Ginnis A. I. , Feurer C.
Published: November 2011

In the present work IsoGeometric Analysis (IGA), initially proposed by Hughes et al (2005), is applied to the solution of the boundary integral equation associated with the Neumann-Kelvin (NK) problem and the calculation of the wave resistance of ships, following the formulation by Brard (1972) and Baar & Price (1988). As opposed to low-order panel methods, where the body is represented by a large number of quadrilateral panels and the velocity potential is assumed to be piecewise constant (or approximated by low degree polynomials) on each panel, the isogeometric concept is based on exploiting the NURBS basis, which is used for representing exactly the body geometry and adopts the very same basis functions for approximating the singularity distribution (or in general the dependent physical quantities). In order to examine the accuracy of the present method, in a previous paper Belibassakis et al (2009), numerical results obtained in the case of submerged bodies are compared against analytical and benchmark solutions and low-order panel method predictions, illustrating the superior efficiency of the isogeometric approach. In the present paper we extent previous analysis to the case of wavemaking resistance problem of surface piercing bodies. The present approach, although focusing on the linear NK problem which is more appropriate for thin ship hulls, it carries the IGA novelty of integrating CAD systems for ship-hull design with computational hydrodynamics solvers.
Large Marine Lifts in Shallow Water
Authors: Crowle Alan P.
Published: November 2011

Large energy developments are taking place near shore, in locations around the world, including LNG jetties, oil production in shallow water and renewable energy projects. Crane vessels of all sizes are required to install the component parts for these projects. This paper explains current techniques and the design requirements to carry out the lifting of large units required for shallow water installations. Recent developments have seen the introduction of new vessels for offshore wind farm installation and their features are discussed.
A Study on Akaike’s Bayesian Information Criterion in Wave Estimation
Authors: Iseki Toshio
Published: November 2011

A feasibility study of Bayesian wave estimation was carried out to investigate the relationship between the minimum Akaike’s Bayesian information criterion (ABIC) and the estimated wave parameters. The ship response functions, which were used for the Bayesian wave estimation together with the ship motion cross spectra, were simply modified and compared with the normal response functions in connection with the accuracy of estimated wave parameters. Moreover, the concept of the ABIC surfaces was introduced to investigate the optimum estimates from the stochastic viewpoint and the physical viewpoint. As the result, it was revealed that the minimum ABIC did not always provide the best estimates from the viewpoint of wave estimation and the simply modified response functions could reduce the estimating errors in some cases. The reasons were considered that the estimating error at the sharp peak of response amplitude operators was closely related to existence of the local minima of the ABIC surface and the simply modified response functions had some effects to make the ABIC surface smoother. It is pointed out as the conclusion of this report that any estimating errors of the ship response functions were not considered in the Bayesian modeling.
On the Derivation of Design S-N Curves Based on Limited Fatigue Test Data
Authors: Lotsberg Inge , Ronold Knut O.
Published: November 2011

Qualification of new characteristic S-N curves for fatigue life assessment of structures is considered to be a significant engineering challenge. First, representative fatigue test data for the actual structural connections have to be derived. Then these test data have to be transferred into characteristic S-N curves that represent a predefined probability of survival. Characteristic S-N curves are also often denoted design S-N curves as these curves are often used directly for fatigue life assessment of structures without application of a material factor. A few large scale tests can add significant confidence to a design S-N curve dependent on the type of structural detail to be designed. The reason for this is that a prototype test specimen can be fabricated in a similar way as the actual connection and it is similar in geometry, material characteristics, residual stress, and fabrication tolerances. In addition it can likely be subjected to a more relevant loading and boundary conditions as compared with that of small scale test specimens. When a limited number of test data are available, it is questioned how a characteristic S-N curve can be derived with a well defined probability of survival. The mentioned issues are further considered in this paper together with some recommendations on how to derive design S-N curves based on limited data.
Seismic Performance Evaluation of Fixed Steel Jacket Platforms Retrofitted With Buckling Restrained Braces
Authors: El-Din Mohamed Nour , Kim JinKoo
Published: November 2011

In recent years there have been significant developments in seismic retrofit of building structures. For steel off-shore jacket structures, however, researches on the seismic performance evaluation and retrofit are relatively rare. In this study, the advantages of using Buckling-Restrained Braces (BRBs) in offshore jacket structures and its effect on the seismic performance of the platform are highlighted. The potential failure mechanisms of the structures using both normal braces and BRBs under seismic loads were investigated first. Pushover analyses of fixed jacket offshore platforms comprising the most common types of the platforms in practice were conducted. In those model structures, two different schemes of BRB retrofitting were utilized and compared with the normal bracing case. After that, an example for the seismic assessment of an existing 8 legged service platform located in the Gulf of Thailand was investigated. The model was designed based on loads according to the environmental conditions in Gulf of Thailand. Different retrofitting schemes using normal braces were investigated to improve the seismic performance of the structure and the results were compared with the results of the seismic retrofit using BRBs. The analysis results showed that the seismic performances of the jacket structures were greatly improved when bracing members were replaced with BRBs compared to the case with using normal bracing. Numerical results revealed that the plastic hinges were more uniformly distributed over the structures in case of retrofit using BRB, while they were concentrated at the top of the structures when normal bracings were used.
Structural Integrity Assessment of a Pipeline Subjected to an Underwater Explosion
Authors: Monti Paolo , Molinari Caterina , Bocciarelli Massimiliano , Corigliano Alberto , Mariani Stefano
Published: November 2011

Several trunklines cross either areas which in the recent past were war theatre or dumping areas used for burying weapons after the last war. The presence of unexploded mines, bombs or torpedoes on the seabed constitutes a potential hazard for the structural integrity of submarine pipelines. Before laying, it is therefore necessary to remove the unexploded charges within a corridor including the route. Risk still remains during the pipeline life, since annual surveys can show evidences of unexploded mines or torpedoes dragged by fishing gears till the protruding pipeline. Consequently, the structural integrity assessment of a submarine pipeline subjected to underwater explosions is of the utmost importance. The aforementioned assessment involves several aspects: the characterization of shock wave and gas bubble pulsation in water; the definition of loading conditions on the pipeline; the characterisation of the strain-rate properties of the steel; the local and global structural analysis; the pipe verification criteria. Aim of this study is to describe how the aforementioned aspects can be managed. Analytical and numerical approaches concerning the assessment of the structural response of the pipeline are presented, and criteria for Serviceability and Accidental Limit States are proposed.
Design Methodology for Axial Force Response of Pipe-in-Pipe: A Probabilistic Approach
Authors: Fyrileiv Olav , Marley Mark , Pettersen Sune
Published: November 2011

As the easy oil is more or less gone, the typical offshore development faces several challenges in the future. These may be related to ultra deep water or difficult operational conditions like high pressure and temperatures. In addition there are often challenges related to flow, for example wax or hydrates during shut-downs or in tail production. Prevention of wax and hydrates is often solved by injection of chemicals or alternatively by some sort of heating, e.g. direct electrical heating. It may also to some degree be solved by superior thermal insulation or a combination of the methods mentioned. A thick insulation coating may give additional challenges with respect to submerged weight. Pipe-in-pipe (PIP) designs, where the flowline is insulated and covered by an outer pipe, solve this challenge and are becoming more and more popular. However, the pipe-in-pipe concepts also provide some specific challenges. DNV has recently been involved in a PIP project with quite challenging operational conditions. The combination of high temperature and high pressure (HTHP) and a corrosive well fluid with a buried pipe-in-pipe without any release of axial force leads to a very conservative design using conventional design approach. This challenge can be solved by applying a stochastic design approach avoiding conservative assumptions on top of each other. A probabilistic analysis targeting an acceptable probability of failure according to DNV-OS-F101 [1] resulted in an optimised design with a balanced selection of input parameters and avoiding ultra-conservative, worst case input combinations.
Accelerating the Deployment of Offshore Renewable Energy Technologies (ADORET): Presentation, Findings and Recommendations
Authors: Dudziak Gregory , Kolliatsas Christos , Schaefer Julia , Myers Noah
Published: November 2011

The outcome of the ADORET project, commissioned by the International Energy Agency Implementing Agreement for Renewable Energy Technology Deployment (IEA-RETD), and executed by Mott MacDonald, will be presented. Globally, offshore renewable energy is a burgeoning industry which has the potential to grow rapidly in a number of countries. While tidal and wave energy is at an earlier stage of development with technology demonstrations being pursued in many locations worldwide, the offshore wind industry is already considered mature. The overall objective of the ADORET project is to assist policy makers and project developers by providing them a better understanding of the specifics of offshore renewable energy (ORE) technologies and to give them practical guidelines in how to foster their deployment. The project has an international perspective, focusing on the 11 IEA-RETD members as well as 7 additional countries. In an introductory phase, the project has delivered a brief of the current status of advancement of ORE technologies. It then highlighted the available resources, and gave an overview of policies in place and deployment to date. In a second phase, the economics and financing of ORE projects have been examined. The respective maturity levels of technologies have been discussed, as well as capital and operational costs structures. An analysis of commercial and technical risks, and of their impact on the financing costs, has been performed. Mitigation measures can reduce these to an acceptable level. The respective merits and constraints associated to the main financing options available to ORE projects will be presented. An in-depth review of technical and non-technical barriers and challenges to the deployment of ORE technologies has been performed and will be presented. Technical barriers covered included technology and design optimisation, reliability, installation and decommissioning, operation and maintenance, grid connection and integration. Non-technical barriers investigated included environmental, safety, regulatory and licensing, competing uses, skills, supply chain and infrastructures, and financial issues. Examples of country and technology specific barriers are highlighted. In the final part the project, a detailed review of policies in place to promote ORE technologies deployment and of their effectiveness has been performed. Mitigation measures to barriers previously identified are also presented. Based on this analysis, a generic model policy framework is proposed, to be used as a possible template by policy makers in interested countries. Guidelines for project design and development are presented. The full version of the ADORET report and its appendices are shortly to be made available to download at www.iea-retd.org.
Forces on Pairs of Closely Spaced Unequal Circular Cylinders in an External Flow
Authors: Eatock Taylor R.
Published: November 2011

The paper examines how interference effects between a pair of closely spaced cylinders influence the fluid forces acting on them in a low Keulegan Carpenter number regime. The forces are characterised by inertia coefficients, and the analysis is based on potential flow theory using a mapping based on bipolar coordinates. The analysis corresponds to the long wave limit. The dependence of the force coefficients on cylinder spacing and radius ratio is illustrated by contour plots, which might be used as design charts to determine the fluid forces on arbitrary configurations of the two cylinders. Results are also given for some representative geometries indicating the effects of wave diffraction on closely spaced vertical cylinders.
Subsea Hydrocarbon Processing and Treatment: Twister Subsea
Authors: Balk Wouter F. , Tjeenk Willink Kees
Published: November 2011

Twister™ is an innovative gas conditioning technology which is now generally accepted for natural gas applications. Condensation and separation at supersonic velocity provides some unique benefits. The extremely short residence time (∼ 2ms) prevents hydrate problems, and eliminates the requirement for chemicals and associated regeneration systems. The simplicity and reliability of a static device, with no rotating parts, operating without chemicals, ensures a simple, environmentally friendly facility, with high availability. Previous studies have confirmed that Twister is a key enabler for subsea gas conditioning, enabling direct sales gas export, without the need for topside facilities. The objective of this paper is to present the results of a recent study which evaluates the technical feasibility of using Twister technology for subsea dehydration and hydrocarbon dew-pointing. Four Twister subsea gas production scenarios were studied. Different gas compositions were used to simulate both lean and rich gas reservoirs. Four different process schemes were simulated using UniSIM© and evaluated for each subsea scenario. The process schemes include a Chemical-free, MEG inhibited, a Hybrid and a Liquid Re-injection scheme. The Chemical-free and Hybrid schemes use electricity to prevent hydrate blockage. The MEG inhibited scheme utilizes MEG (Mono-Ethylene Glycol) for hydrate inhibition. This scheme requires some 85% less MEG than wet-gas evacuation under the same production conditions. The Hybrid scheme combines these two processes, and injects MEG after bulk water removal. The Hybrid system can operate using significantly less MEG than that required for the MEG inhibited process, and some 95% less than wet-gas evacuation. Liquid Re-injection produced on-spec gas, but wastes part of the value in the gas stream, as well as MEG. It is concluded that the Hybrid and Chemical-free schemes require some additional subsea technology, which is not yet available. The MEG inhibited scheme, however, uses available subsea technology and can be applied to create on-specification gas ready for direct subsea pipeline export, using available technology. Subsea compression will be required somewhat earlier when compared to wet-gas evacuation. This apparent disadvantage is however overcome by the elimination of platform required for production, the elimination of long multiphase gas lines with inherent pressure losses and overall flow assurance improvement.
Model-Scale and Full-Scale CFD Calculations for Current Loads on Semi-Submersible
Authors: Koop Arjen , Bereznitski Alexei
Published: November 2011

In this paper results of CFD calculations with the MARIN in-house code ReFRESCO are presented for the JBF-14000 Semi-Submersible designed by Huisman Equipment BV. The objective of the CFD calculations is to investigate the applicability, the costs and the accuracy of CFD to obtain the current coefficients of a semi-submersible for all headings. Furthermore, full scale CFD calculations are carried out to investigate possible scale effects on the current coefficients. An extensive verification study has been carried for the model-scale current loads on a semi-submersible using 10 different grids of different grid type for 3 different headings, i.e. 180, 150 and 90 degrees. These headings represent the main different flow regions around the semi-submersible. The CFD results are compared with the results from wind tunnel experiments and tests in the Offshore Basin for a range of current headings. The results for the force coefficients are not very dependent on grid resolution and grid type. The largest differences found are less than 10% and these are obtained for CX results for 180 degrees. For the results obtained on the same grid type the results change less than 4% when the grid is refined. These verification results give good confidence in the CFD results. For the angles with larger forces, i.e. the range [180:130] for CX and the range [150:90] for CY the CFD results are within 12% or better from the experiments. Full-scale force coefficients are calculated using 5 subsequently refined grids for three different headings, i.e. 180, 150 and 90 degrees. Scale effects should only be determined when the effect of grid refining is investigated. The trend of the force coefficients when refining the grid, can be different for model-scale and full-scale. The use of coarse grids can lead to misleading conclusions. On average the full-scale values are approximately 15–20% lower than for model-scale. However, larger differences for a number of angles do exist.
Effects of Twist on Chain Strength and Fatigue Performance: Small Scale Test Results
Authors: Ridge Isabel , Smedley Philip , Hobbs Roger
Published: November 2011

Mooring chain can twist in response to torque generated in the adjacent elements of a mooring, or even be inadvertently installed in a twisted condition. This paper explores the effect of initial twist on the static strength and fatigue life of studless chain. It reports test results for 16mm studless chain (with similar proportions as the much larger chains now employed in hydrocarbon production vessels) with known levels of initial twist subject to axial tensile loading. Twist levels of up to 24° per link were examined. The results were generally better than had been expected. There was little scatter in the results, giving confidence in the test procedures. The effect of initial twist on static strength was modest, with a maximum strength loss of about 4% at 18° twist per link. The effects of initial twist on fatigue life were also small. Indeed, some twist seemed to improve fatigue performance, perhaps because the contact between links was displaced from the region of high residual (compressive and tensile) stress resulting from proof loading during manufacture. While the results of these small scale tests suggest that twist levels up to 24° per link were not detrimental, other practical reasons remain for continuing to minimize twist in mooring chain. Furthermore, caution should be applied in extrapolating these results to predict the effect of twist in full scale offshore mooring chains.
A Simplified Approach to Estimate Wake Oscillations in Riser Arrays
Authors: Le Cunff Ce´dric , Toumit Ste´phane , Heurtier Jean-Michel
Published: November 2011

In the offshore industry, modeling pipe vibrations due to current is important to predict structural fatigue life. In the case of Wake Induced Oscillations (WIO), clashing is also an issue during the design phase to be able to define enough clearance to prevent clashing. If not possible, it is then needed to estimate contact energy between pipes and ensure that clashing is acceptable. Wake induced oscillations are difficult to predict, involving Vortex-Induced Vibrations (VIV) at relatively high frequency and small displacement together with larger motion at lower frequency leading to potential contact. A hydrodynamic model is proposed to predict WIO of a flexible pipe in the wake of an upstream pipe. The structural displacement of the two pipes is computed with a classical finite element model. The pipes are linked thru two dimensional strips where the hydrodynamic loads are computed based on the pipe distance in the strip. Since both pipes are flexible, the upstream pipe is subjected to VIV while the downstream pipe is subjected to the mean wake created by the pipe, to VIV as well as WIO. Each effect is represented by a simplified model. A Blevins model is used to represent the quasi-static drag and lift forces on the downstream riser. The VIV on the upstream riser is computed with a Van der Pol oscillator model, and a similar model is used for the downstream riser with an added term to account for the upstream riser presence. Experimental results on two tandem jumpers are used to validate the approach in steady current and in current plus wave. The database has a large number of model tests with different initial gaps between the two jumpers. Dynamic response of the two pipes is measured thru accelerometers and tension sensors. Some of the configurations exhibit clashing and/or overlapping of the jumpers. Amplitude and spectra of vibrations are compared to the proposed model; the general characteristics of the interaction (contact, overlapping) are also addressed.
Comparative Study of Hydroelastic Impact for One Free-Drop Wedge With Stiffened Panels by Experimental and Explicit Finite Element Methods
Authors: Luo Hanbing , Wang Hui , Guedes Soares C.
Published: November 2011

This paper studies the hydroelastic responses of a complex structure for one water entry problem. One steel wedge with complex stiffened panels was designed with a deadrise angle of 22 degrees and a series free-drop model tests were carried out. An explicit finite element code is adopted to simulate this coupled hydroelastic impact problem. The motion, stress responses on longitudinal stiffeners and transverse frames are obtained. Comparisons of the numerical and the experimental results are carried out. Good agreement is achieved. The hydroelastic effect is discussed.
The Effect of Pressure Solution in SPH Simulations of Sloshing Flow
Authors: Rafiee Ashkan , Cummins Sharen , Rudman Murray , Thiagarajan Krish
Published: November 2011

In modelling incompressible flows using the Smoothed Particle Hydrodynamics method (SPH), an equation of state with a large sound speed is typically used. This weakly compressible approach (WCSPH), results in a stiff set of equations with a noisy pressure field and stability issues at high Reynolds number. As a remedy, an incompressible SPH technique was introduced [1] (ISPH), which uses a pressure projection technique to model incompressibility. In this paper, the incompressible and weakly compressible forms of the SPH method are employed to study sloshing flow. Both methods are compared with experimental data. The results show the incompressible SPH method provides more accurate pressure fields and free-surface profiles when compared to experiment.
Time Domain Analysis of Springing and Whipping Responses Acting on a Large Container Ship
Authors: Lee Yongwon , Wang Zhenhong , White Nigel , Hirdaris Spyros E.
Published: November 2011

As part of WILS II (Wave Induced Loads on Ships) Joint Industry Project organised by MOERI (Maritime and Ocean Engineering Research Institute, Korea), Lloyd’s Register has undertaken time domain springing and whipping analyses for a 10,000 TEU class container ship using computational tools developed in the Co-operative Research Ships (CRS) JIP [1]. For idealising the ship and handling the flexible modes of the structure, a boundary element method and a finite element method are employed for coupling fluid and structure domain problems respectively. The hydrodynamic module takes into account nonlinear effects of Froude-Krylov and restoring forces. This Fluid Structure Interaction (FSI) model is also coupled with slamming loads to predict wave loads due to whipping effects. Vibration modes and natural frequencies of the ship hull girder are calculated by idealising the ship structure as a Timoshenko beam. The results from springing and whipping analyses are compared with the results from linear and nonlinear time domain calculations for the rigid body. The results from the computational analyses in regular waves have been correlated with those from model tests undertaken by MOERI. Further the global effects of springing and whipping acting on large container ships are summarised and discussed.
Comparative Study of Seakeeping Analysis Results From Various Methods
Authors: Dhavalikar Sharad S.
Published: November 2011

Over the years many methods are evolved for computing ship motions and loads, i.e. seakeeping analysis. All of these methods are known as potential flow methods where fluid is assumed to be irrotational. In these methods velocity vector is represented as a gradient of scalar potential phi (φ) known as velocity potential. One of the oldest methods is strip theory method where ship is descretized into number of 2D strips. Other methods are panel methods where entire hull is descretized into number of panels. These are also known as Green’s function methods. Various formulations of Green’s function do exist. In a recent development Green’s function methods are extended to Rankine Panel method (RPM) where free surface in the vicinity of vessel is panelized for computations. In RPM Green’s function computations are made easy. Again these methods are divided into frequency domain and time domain. Time domain methods take into account various nonlinearities which generally cannot be handled by frequency domain methods. For zero and non-zero forward speed of the vessel different formulations exist. All these methods have their own advantages and disadvantages. Hence, it is very important in the initial design stage to decide on method of seakeeping analysis. Here an attempt is made to compare the results of seakeeping analysis using various tools based on various seakeeping methods.
Mooring Line Components With Semi-Brittle Behavior: Verification of Fitness for Purpose
Authors: Lassen Tom
Published: November 2011

Low impact energy for Charpy V Notch (CVN) specimens and associated low Crack Tip Opening Displacement (CTOD) values have occurred on several occasions in high strength steel offshore mooring components. In the present work an Engineering Critical Assessment (ECA) for shackles has been carried out to demonstrate fitness for purpose. Typical values for CVN and CTOD are 17 Joules and 0.01 mm respectively at design temperature. The purpose of the present work is to demonstrate that even in the case where normal quality requirements are not met, the shackles may still have enough structural integrity and fatigue durability to withstand the load spectrum in the field during the planned target service life of typically 20 years. The ECA is based on applied fracture mechanics as outlined in the BS7910 document. Shackles made of QR4 high strength steel with different geometries and loading modes are analyzed. Extreme load cases and fatigue load spectra are treated and fracture mechanics modelling is discussed. A CTOD value as low as 0.01 mm may give critical surface crack depth close to 1% of the diameter in an ultimate limit state condition. For normal ductile steel behaviour the critical crack depth is usually close to 15% of the diameter. However, under the assumption that the pre-existing crack depth is 0.25 mm, the predicted fatigue life based on fracture mechanics analysis is still over 100 years. For a target service life of 20 years this corresponds to a Design Factor Fatigue (DFF) of 5. This is close to the requirement given by DNV for mooring chains. Based on the present analysis it can be concluded that the shackles are fit for purpose as manufactured even under unfavourable and unlikely assumptions.
Constructing the Shields Curve
Authors: Miedema Sape A.
Published: November 2011

Prediction of the entrainment of particles is an essential issue for the study of erosion phenomena in many applications. The original Shields curve describes the entrainment of many particles at many locations and is thought critical to general transport. The mechanisms involved in general are sliding, rolling and lifting, new models of which have been developed. I will introduce concepts for the determination of the effective velocity and the acting point of the drag force, based on integration of the drag force over the cross section of the exposed particle (where most earlier models were based on integration of the velocity), the behavior of turbulence intensity very close to the virtual bed level and the factor of simultaneous occurrence of the small turbulent eddies. The resulting values of the Shields parameter, based on practical and reasonable properties, are compared with data, resulting in the best correlation for the sliding mechanism with the data of many researchers. The Shields parameter found for rolling and lifting overestimates the measurements from literature. Sliding seems to be the mechanism moving the top layer of the particles, while rolling and lifting are much more mechanisms of individual particles. In the new model it is considered that in the laminar region entrainment is dominated by drag and the influence of small turbulent eddies reducing the thickness of the viscous sub-layer, while in the turbulent region this is dominated by drag and lift. The transition region is modeled based on sophisticated interpolation. The model also describes the influence of exposure and protrusion levels and is compared with data of laminar main flow. The model correlates very well with the original data of Shields and data of others and also matches empirical relations from literature. The model is suitable for incorporating exposure and protrusion levels and laminar main flow.
An Assessment of the Potential of Tidal Power From Minas Passage, Bay of Fundy, Using Three-Dimensional Models
Authors: Karsten Richard
Published: November 2011

Large tidal currents exist in Minas Passage, which connects Minas Basin to the Bay of Fundy off the northwestern coast of Nova Scotia. The strong currents through this deep, narrow channel make it a promising location for the generation of electrical power using instream turbines. These strong currents are clearly illustrated in the results of a high-resolution, three-dimensional model of the flow through Minas Passage presented here. The simulations also clearly indicate the asymmetry of the flood and ebb tides and the 3D structure of the flow. A previous study has indicated that as much as 7000 MW could be extracted from the tidal currents through Minas Passage. However, this estimate was based on a complete fence of turbines across the passage, in essence a tidal barrage. In this paper, the power potential of partial turbine fences is examined. In order to estimate the power potential of turbine arrays, the theory of partial turbine fences is adapted to the particular dynamics of Minas Passage. The theory estimates the potential power of the fence and the change in flow that would result. The results are presented in terms of the portion of the cross-sectional area that the turbines occupy and the drag coefficient of the turbines. When the turbine fence occupies a large portion of the passage, the potential power of the fence rises significantly, to a value much larger than estimates based on the kinetic energy flux. The increase in power comes from the increased tidal head that a large turbine fence creates and the resulting increase in the turbine drag. We also present the efficiency of the turbine fence — given as the ratio of the power associated with the turbine drag over the total power extracted from the flow — and the impact of the turbines on the tidal flow. The results of the theory are compared to numerical simulations of the flow through the passage with turbines represented as regions of increased drag. The numerical simulations give power values that are three to six time as high as the theory suggests is possible. This discrepancy is examined by plotting the changes in tidal currents caused by the turbine fence.
Potential Cost Savings in Wall Thickness Design of Ultra Deepwater Rigid Pipelines
Authors: Oazen Eduardo , Escudero Carlos , Azevedo Fa´bio , Basi´lio Leandro
Published: November 2011

The wall thickness is always a cost-related issue for pipeline projects. In some extreme cases thicker pipelines may implicate not only in high CAPEX, but also may threat the whole project viability. For ultra deep water projects, the pipeline wall thickness issues are naturally amplified and sometimes may cause a huge impact in the predicted costs and in the feasibility of the project. Due to future scenarios up to 2500 m (8200 ft) for rigid pipeline installation, several improvement possibilities are discussed in order to allow the reduction of the pipeline wall thickness. The article intends to present the evaluated possibilities to optimize the wall thickness design. Strategies for design and fabrication requirements are discussed in relation to most utilized standards, such as DNV-OS-F101.
Experimental Study of a Negatively Buoyant Horizontal Jet in Wave Environment
Authors: Lin Jian-Feng , Hsiao Shih-Chun , Hsu Tai-Wen , Chang Kuang-An
Published: November 2011

The present study is to investigate the mechanism of turbulent fluctuation between regular progressive wave and non-buoyant and buoyant jet experimentally. A particle image velocimetry (PIV) technique was employed to measure the instantaneous velocity field and provide quantitative results about mean and turbulent properties via phase-averaged method of all instantaneous velocity maps located at that spatial location. Two kinds of initial effluent were used to examine the effects of buoyancy on the interactions of a turbulent jet under water waves. The experimental results demonstrate that the diffusion of salt water jet changes to a wider profile caused by the wave oscillatory motion. As expected, the width of turbulence intensity of salt water jet increased when it under waves. Furthermore, this confirmed that width and magnitude of the mean flow and turbulence intensity were significantly increased and shifted when the salt water jet was under the wave environments.
Performance Evaluations of Taut-Wire Mooring Systems for Deepwater Semi-Submersible Platform
Authors: Chen P. , Chai S. , Ma J.
Published: November 2011

In order to investigate the effect of taut-wire mooring system on the motion performance of semi-submersible platforms, parametric studies of coupled motion responses are conducted using a time domain analysis in this study. The nonlinear dynamic characteristics of mooring lines and the interactions of platform and mooring lines are investigated. The parametric studies consist of investigating the effects of the hydrodynamic coefficients CA and CD of mooring line, tension dip angle, mooring line pretension, different taut-mooring arrangements and total number of mooring lines on the motion performance of a semi-submersible platform in water depth of 1500 meters, which is subjected to a 100 year return significant wave height of 13.3 meters, a peak period of 15.5 seconds, a current speed of 1.97 meters per second and wind speed of 55 meters per second. The wind and current both act in the same direction as the ocean waves in this study in order to estimate the maximum mooring line loads. The environmental load direction is varied from 0° to 90° at the interval of 15 degrees. Seven directions are calculated in total. The research results show that the different parameters, such as the hydrodynamic coefficients of the mooring line, tension dip angle, pre-tension, arrangement angle of mooring lines and total number of mooring lines, have different effects on the coupled motion responses. In particular, the arrangement angles of mooring lines have significant effect on motion responses and dynamic loads of mooring lines. The motion performance of semi-submersible platform and mooring line dynamic loads can be controlled effectively when these parameters are selected reasonably throughout parametric studies carefully designed and conducted.
Strategy of the Kara Sea Oil and Gas Field Development and Evaluation of Economy Uncertainties
Authors: Bulakh Maria , Zolotukhin Anatoly B. , Gudmestad Ove T.
Published: November 2011

Huge reserves of hydrocarbons on the shelf of the Kara Sea, adjacent to the west coast of the Yamal Peninsula are to be developed in the XXI century. This large oil and gas area in northern West Siberia has prospective resources of 56 trillion barrels of oil equivalents (boe). The Russia Federation has no other regions with similar concentrations of undeveloped hydrocarbons. Creating this unique Kara Sea gas production centre could provide in the years 2015–2030 a production of at least 800 billion Sm3 of gas per year providing both domestic and external demand with this fuel, and supply it to the external market, (http://oilgasindustry.ru).
Laboratory Observations on the Evolution of Deep-Water Wave Packets
Authors: Ma Yuxiang , Dong Guohai , Ma Xiaozhou
Published: November 2011

New experimental data for the evolution of deep-water wave packets has been presented. The present experimental data shows that the local maximum steepness for extreme waves is significantly above the criterion of the limiting Stokes waves. The wavelet spectra of the wave groups around the breaking locations indicate that the energy of higher harmonics can be generated quickly before wave breaking and mainly concentrate at the part of the wave fronts. After wave breaking, however, these higher harmonics energy is dissipated immediately. Furthermore, the variations of local peak frequency have also been examined. It is found that frequency downshift increases with the increase of initial steepness and wave packet size.
Research on the Time-Frequency Energy Structure of Freak Wave Generation and Evolution
Authors: Cui Cheng , Zhang Ning Chuan
Published: November 2011

In this paper, simulations of freak wave generation and evolution are carried out in a 2-D Numerical Wave Tank (NWT), which is based on the improved Volume of fluid method (VOF). The freak wave can be generated by the individual superposition model or the dual superposition model (a transient wave is embedded into a random wave train). Wavelet analysis method has been adopted to calculate the time-frequency energy spectrums of the wave trains collected during freak wave generation and evolution. In the generation course of the freak wave, the energy is focused gradually and the energy is gradually diffused in the course of evolution. When the most appropriate energy concentration occurs, the freak wave results. The moderately large wave, which occurs during the period around the freak wave and does not satisfy all the qualifications of the adopted definition, has a similar time-frequency energy structure with the freak wave. In its wavelet spectrum, strong energy density surges instantaneously and seemingly carried over to the high frequency components at the instant the large wave occurs. The external characteristic parameter α1 (the ratio of the maximum wave height to the significant wave height) and the internal characteristic parameter αE (energy concentration) appear to correspond.
Design and Research on Some Key Technologies of Combined Hydrocyclone
Authors: Zhang Yong
Published: November 2011

Almost all hydrocyclones used for the separation of produced water are static equipments. Just as its name implies, the static hydrocyclone has no internal moving components or external power supply. Special structure and pressure difference between its inlet and outlets can lead multi-phase liquid to flow under helical track. Thus, insoluble multi-phase media will be separated under the centrifugal force by helical motion. Static hydrocyclone has been widely used especially on offshore platforms because it need minor space and few supporting facilities. But every hydrocyclone should be customized because the given structural parameters will bring corresponding separating effect. This costs longer time and more money during the stage of design and performance test. Building upon previous work on static hydrocyclone and some efforts about dynamic hydrocyclone, we designed the combined hydrocyclone which combines the power section of dynamic hydrocyclone and separating section of static hydrocyclone, but has independent transformation principle and control mode. It can adapt to wider flow range by preparing reasonable power supply and processing capacity of the separating section. Dynamic matching of velocity and liquid flow, optimum design of the key components, stability of the flow field of the separating section and some performance tests is discussed in this paper.