bluuebunny/crossref_metadata_2025 huggingface dataset:
Background of ABS Buckling Strength Assessment Criteria for Cylindrical Shells in Offshore Structures
Authors: Sun Hai-Hong , Tan Pao-Lin
Published: October 2008

Cylindrical shells, unstiffened or stiffened with rings and/or stringers are commonly used in offshore structures as main loading-carrying members. Comprehensive theoretical work and experimental studies on the buckling behavior of cylindrical shells have been carried out in the past. The increasing offshore application of stiffened cylindrical shells has raised some new challenges that need to be addressed. This paper provides the fundamental principles and technical background of the ABS buckling strength assessment criteria for cylindrical shells applied in offshore structures. The accuracy of ABS buckling criteria for assessing the cylindrical shells is established by benchmarking the results against an extensive database of test results assembled by American Bureau of Shipping. The results are also compared against current recognized offshore standards, such as API Bulletins 2U and DnV CN30.1. It is demonstrated that the ABS criteria provide very effective and sufficiently accurate predictions for the cylindrical shell buckling calculations.
Reliability-Based Structural Optimization for Positioning of Marine Vessels
Authors: Leira Bernt J. , Berntsen Per I. B. , Aamo Ole Morten
Published: October 2008

The present paper is concerned with floating marine structures that are kept in position by means of mooring lines in addition to a thruster system. Different options relevant for construction of automatic control schemes for the thruster system are first investigated based on a simplified response model. In particular, the role of structural reliability criteria applied to the mooring system is investigated. Subsequently, a more refined control algorithm based on such reliability criteria is introduced. The performance of this control system is demonstrated by numerical simulations.
Investigation of Ship and Platform Accidents Due to Severe Weather Events: Results of the MAXWAVE Project
Authors: Lehner S. , Rosenthal W.
Published: October 2008

Heavy sea states and severe weather conditions have caused the loss of more than 200 super carriers within the last 20 years. In many cases single ‘rogue waves’ of abnormal height as well as groups of extreme waves have been reported by crew members of such vessels. The European Project MAXWAVE dealt with both theoretical aspects of extreme waves as well as new techniques to observe these waves using different remote sensing techniques. The final goal was to improve the understanding of the physical processes responsible for the generation of extreme waves and to identify geophysical conditions in which such waves are most likely to occur. This paper gives a summary of the results of the MAXWAVE projects with emphasis on the analysis of the marine and satellite radar data sets. Two dimensional sea surface elevation fields are derived from marine radar data and complex Spaceborne Synthetic Aperture Radar (SAR) images. Several ship and offshore platform accidents are analysed and new warning criteria are discussed.
The Preliminary Study on Comparison of Velocity Distribution Models Under the Same Roughness Length
Authors: Hao Jialing , Yan Yixin , Song Zhiyao , Wang Changnan
Published: October 2008

Previous studies pointed out that due to the acceleration or deceleration action of tide current, the flow structure deviates from traditional logarithmic law in estuary, coast or other near shore water. The tidal velocity distribution model was derived and compared with the traditional logarithmic model. It should be pointed out that the velocity data adopted have four layers within one meter above the bed, and the roughness length z0 is different in the two models even in the same velocity profile. Because the fluctuation of roughness length z0 is remarkable when determining by single velocity profile, some studies thought that the variation of roughness length was small between adjacent time when the change of topography was less obviously. Therefore, the measured data is divided into several sections by one day or a tidal cycle to fit the velocity profile of every section to obtain a roughness length z0, i.e., the roughness length z0 varies only after a day or a tidal cycle. The purpose of the paper is to expand the log-linear model to full depth by adding the surface boundary condition ∂u∂zz=D=0(Diswaterdepth) and to discuss the difference when 6 points (bottom layer, 0.2D, 0.4D, 0.6D, 0.8D, surface layer) velocity profile are fitted by logarithmic model, log-linear model, and extended log-linear model with the same roughness length z0 in different time section, respectively. The calculated friction velocity and friction coefficient and their correlation are discussed. The results show that the log-linear model and the log-linear extend model are closer to the measure velocity profile than that of the logarithmic model.
The Use of a Volume of Fluid (VOF) Method Coupled to a Time Domain Motion Simulation to Calculate the Motions of a Subsea Structure Lifted Through the Splash Zone
Authors: Bunnik Tim , Buchner Bas , Veldman Arthur
Published: October 2008

Existing simulation methods are not able to determine in detail the wave loads on and the motions of a complex subsea structure when it is passing through the splash zone. To determine these loads and motions, model tests are necessary. Otherwise only simplified formulations or empirical relations for added mass and damping can be used. The improved Volume Of Fluid (iVOF) method presented in this paper is capable of predicting the behaviour of a subsea structure in the splash zone. The simulated flow around and through the structure looks very realistic and shows a strong resemblance with observations from model tests. The quantitative comparison of the load variations in the hoist wire of the subsea structure shows that the total load levels and dynamics of the subsea structure are well predicted. This good comparison shows the potential of the improved Volume Of Fluid (iVOF) method for the simulation of the behaviour of subsea structures in the splash zone. However, significant further development is needed before long simulations in irregular waves can be carried out. At the moment, the method is limited to short runs (regular waves) because of the long simulation times required at the moment.
Application of Data Assimilation for a Spectral Wave Model on Unstructured Meshes
Authors: Hsu Tai-Wen , Ou Shan-Hwei , Liau Jian-Ming , Lin Jaw-Guei , Kao Chia-Chuen , Roland Aaron , Zanke Ulrich , Shin Chih-Young
Published: October 2008

The effect of the data assimilation of buoy data in the wind wave model (WWM) for wind wave simulations in both deep and shallow water regions developed by Hsu et al. [2005] is investigated. Following Lionello et al. [1992], the sequential method is implemented, where analyzed wave spectra and significant wave fields were assimilated by optimal interpolation (OI), then the analyzed values were used to reconstruct the wave spectrum. This paper examines the results of the assimilation of wave spectrum, significant wave height and significant wave period in a nearshore WWM model. The WWM model underestimates the wave period because it incorrectly applies past wave field data. The analysis has provided useful indications of the shortcomings of the WWM model. In summary, the OI approach is shown to be a reliable assimilation scheme in the WWM model.
Numerical Moonpool Modeling
Authors: Malta Edgard Borges , Cueva Marcos , Nishimoto Kazuo , Golc¸alves Rodolfo , Masetti Isai´as
Published: October 2008

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to protect drilling equipment from environmental forces, and its design aims the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies have been carried out to predict the water dynamics inside the moonpool. At most, analytical tools have been used with experimental results, to obtain a good evaluation of viscous effects. Another line of development uses the moonpools as a device to reduce motions of ships or oil platforms. In his context, the use of moonpools in monocolumn type platforms was studied during the development of the concept, through the partnership between PETROBRAS and University of Sa˜o Paulo–USP. An alternative that became viable in the last years is the use of numerical methods to evaluate potencial parameters, being only necessary simple experiments to obtains viscous data to complete the model. This work, that is a continuation of articles about the issue written before, intends to consolidate the calculation method of moonpool to monocolumn units.
Characteristics of Vortex Shedding Process Induced by a Solitary Wave Propagating Over a Submerged Obstacle
Authors: Lin Chang , Ho Tsung-Chun , Hsieh Shih-Chun , Chang Kuang-An
Published: October 2008

The generation and evolution of shedding vortices induced by a solitary wave propagating over a two-dimensional submerged obstacle, a rectangular dike or a vertical plate, was investigated experimentally. The vortex shedding process was observed qualitatively using laser induced fluorescence (LIF) technique and particle tracing technique. The velocity fields were measured quantitatively using particle image velocimetry (PIV). Base on the results of the flow visualization, the comparison of the vortex shedding processes between rectangular dike and vertical plate was qualitatively made under the same wave condition. It is noted that the process of either rectangular model or vertical plate can be divided into four common phases, but the characteristics of shedding vortices are different in the same phase. In addition, the velocity similarity profiles for the formation of separated shear layer and the formation of vertical jet were also quantitatively compared between rectangular dike and vertical plate.
On the Statistical Choice of Extreme Domains of Attraction in Long-Term Predictions of Significant Wave Height
Authors: Neves Cla´udia , Scotto Manuel G. , Guedes Soares C.
Published: October 2008

It has been recognized the potential of Extreme Value Theory in estimating and assessing statistical models for tail-related wave height measurements. Focusing solely on the tail of the underlying distribution through the consideration of both extreme and intermediate order statistics, three testing procedures are applied so as to determine the choice of the extreme model which more adequately describes the largest significant wave heights that occurred at Figueira da Foz, Portugal, from 1958 up to 2001. On the basis of the results yielded by these alternative tests of hypothesis, inferences are established about the tail behavior of the underlying distribution, namely about whether it decays exponentially fast and if it has finite endpoint or not. The statistical choice of extreme domains proved to be a fundamental step preceding the estimation of extreme characteristics since it provided, for instance, a way of distinguishing upon several estimation methods the most adequate to adopt within the present setting. The fact that the extreme value index γ of any distribution with at most moderately heavy tails (with γ ≤ 0) remains the same under integer-power transformations plays a prominent role in the analysis of the data.
A Statistical Approach to Extreme Ice Loads on Lighthouse Norstro¨msgrund
Authors: Fransson Lennart , Lundqvist Jan-Eric
Published: October 2008

Data from full-scale measurements of ice loads on lighthouse Norstro¨msgrund has been analyzed using basic statistical methods. Questions like scaling, duration of ice interaction and correlation of extreme ice loads on different segments of the structure are discussed. Typical ice conditions in the Baltic Sea are described in general and the region is divided into areas with similar ice and ice movements. Indications of strong influence of structure diameter on the effective ice pressure were confirmed by results obtained on other lighthouses in the area. The result can be used in simulations of ice load probabilities for fixed vertical structures with small diameters located in the Baltic Sea.
Integrity Assessment and Repair Techniques of Flexible Risers
Authors: Marinho Mauro G. , dos Santos Joilson M. , Carneval Ricardo de O.
Published: October 2008

Deep water oil and gas exploitation in Brazil remarkably increased the utilization of flexible pipes in conjunction with floating production systems. In Campos Basin nowadays oil and gas transfer, water and gas injection and well control and monitoring are carried out almost entirely by flexible pipes, including risers, flowlines and umbilicals. Periodic inspections have detected a considerable incidence of damage in the top section of risers, which may affect their structural integrity and eventually induce different failure mechanisms. These include mostly external sheath damage, corrosion and/or fatigue-induced damage to the tensile armours and torsional instability. These damages are generally originated during installation or, more frequently, during operation due to contact with another riser or the platform structure. In order to mitigate the progression of these damages, besides periodic inspections, repair techniques were developed for both emerged and submerged riser sections. Apart from the inspection program, surface monitoring procedures, such as nitrogen injection, pressure monitoring and flow measurements in the annular space are being implemented, for a continuous flexible riser integrity assessment. This paper describes and evaluates these techniques, as well as reports the results obtained from field experience.
Feedforward Neural Networks Applied to Problems in Ocean Engineering
Authors: Hess David E. , Faller William E. , Roddy Robert F. , Pence Anne M. , Fu Thomas C.
Published: October 2008

The Maneuvering and Control Division of the Naval Surface Warfare Center, Carderock Div. (NSWCCD) along with Applied Simulation Technologies have been developing and applying feedforward neural networks (FFNN) to problems of naval interest in Ocean Engineering. A selection of these will be discussed. Together, they show the power of the nonlinear method as well as its utility in diverse applications. Experimental data describing a subset of the B-Screw series of propellers operating in all four quadrants have been reported by MARIN in the Netherlands. The data contain varying pitch to diameter ratios, expanded area ratios, number of blades and advance angle. These four variables were used to train a FFNN to predict the four-quadrant thrust and torque characteristics for the entire B-screw series over a range of beta from 0 to 360 deg. The results show excellent agreement with the existing data and provide a means for estimating 4-quadrant performance for the entire series. For submarine simulation and design, knowledge of the total forces and moments acting on the hull as a function of angle-of-attack, sideslip angle and dimensionless turning rate across a large parameter space is required. This data is acquired experimentally and/or numerically and can be used to train a FFNN to act as a Virtual Tow Tank or Virtual CFD Code. The network not only recovers the training data but also serves as a very fast, nonlinear six degree-of-freedom look-up table of the forces and moments acting on the hull throughout the parameter space described by the vehicle dynamics. Example solutions demonstrating this approach will be presented. Wave impact loads pose continuing problems for vessels in high sea states, with damage to hatches and appendages, suggesting that these loads may be greater than current design guidelines. Such forcing is complex and often difficult to estimate numerically. Experimental data were acquired at NSWC to measure the hydrodynamic loads of regular, nonbreaking waves on a plate and a cylinder while varying incident wave height, wavelength, wave steepness, plate angle and immersion level of the plate/cylinder. Predictions of wave impact forces from a FFNN trained on the experimental data will be presented.
Hydrodynamic Properties of an Asymmetric Deepwater Riser Bundle
Authors: Zimmermann Charles , James Richard , Seguin Blaise , Lynch Mattias
Published: October 2008

The BP operated Greater Plutonio field development offshore Angola comprises a spread-moored FPSO in 1,300 m water depth, serving as a hub processing the fluids produced from or injected into the subsea wells. The selected riser system is a riser tower tensioned by a steel buoyancy tank at its top end and distributed foam buoyancy along a central structural tubular. The riser bundle is asymmetric in cross-section and this paper presents the work performed to determine the specific hydrodynamic characteristics of the design. Both basin tests and CFD analysis results are presented with discussion on some specific hydrodynamic issues: vortex-induced vibration (VIV) of the global riser tower system, VIV of individual risers, and the dynamic stability of the global system (i.e. galloping). Finally, guidelines for the assessment of the hydrodynamic behaviour of such system geometries are proposed. The results of this paper demonstrate that the Greater Plutonio riser bundle represents an effective solution in term of hydrodynamic behaviour and is not sensitive to VIV fatigue or galloping.
Probabilistic Fracture Mechanics Structural Reliability Analysis of Reeled Pipes
Authors: Ernst Hugo A. , Schifini Ricardo , Bravo Richard E. , Passarella Diego N. , Daguerre Federico , Tivelli Marco
Published: October 2008

Structural integrity analyses are used to guarantee the reliability of critical engineering components under certain conditions of interest. In general, the involved parameters have statistical distributions. Choosing a single set of values for the parameters of interest does not show the real statistical distribution of the output parameters. In particular, offshore pipes installation by reeling is a matter of concern due to the severe conditions of the process. Since it is necessary to guarantee the integrity of the pipes, a probabilistic fracture mechanics reliability analysis seems to be the most adequate approach. In this work, a probabilistic fracture mechanics assessment approach to perform the structural reliability analysis of tubes subjected to a reeling process was developed. This procedure takes into account the statistical distributions of the material properties and pipe geometry, using a fracture mechanics approach and the Monte Carlo method. Two-parameter Weibull distributions were used to model the variability of the input parameters. The assessment procedure was implemented as a self-contained executable program. The program outputs are: the statistical distribution of critical crack size, amount of crack extension, final crack size and the cumulative probability of failure for a given crack size. A particular case of interest was studied; a seamless tube - OD 323.9 × wt 14.3 mm, was analyzed. Tolerable defect size limits (defect depth vs. defect length curves) for different probability of failure levels were obtained. A sensitivity analysis was performed; the effect of material fracture toughness and misalignment was studied.
Synthetic Mooring Rope Jackets Submitted to Abrasive Actions: A Methodology Proposed to Determine Superficial Wearing Behavior
Authors: Rodriguez Natalie P. , Ribeiro Marcos F. , Chimisso Fulvio E. G.
Published: October 2008

Nowadays, the use of synthetic ropes in the anchorage of offshore platforms is becoming common. In spite of having a good knowledge of the mechanical behavior of this kind of rope, a study of abrasion wearing is presented here in this paper. Regarding the life of a synthetic rope during its use, in quantitative terms, little is known about wearing by abrasive contact with other materials and equipments. The main objective of this work is to establish procedures and criteria in order to determine the abrasion wearing of the jackets. It is intended to analyze behavior using abrasion tests in jackets of scale ropes made of different synthetic materials and to verify which would provide the best conditions of use. In order to do this, procedures were established for the preparation and testing of specimens as well as the criteria to analyze the results. The fundamental criterion adopted for determining abrasion wearing takes into consideration the mass loss of the specimen associated with the mechanical resistance loss of the yarns that constitute the jacket. Two materials were analyzed using these procedures thereby obtaining wearing curves according to time in different test conditions.
Experimental Analyses of Metal-Composite Pipes Under External Pressure
Authors: Oliveira S. C. , Pasqualino I. P. , Netto T. A.
Published: October 2008

Metal-composite pipes are alternatives for deepwater applications where thermal insulation and structural behavior requirements must be met. They are composed by an inner metal pipe and an outer structural composite layer that provides both strength and additional insulation. In this work, the behavior of such pipes under external pressure was studied through a combination of experiments. Twelve different specimens laminated through the vacuum-bag process using aluminum for the inner pipe were manufactured. Different mechanical tests were carried out so as to determine the material properties for future numerical analyses. The effect of the metal-composite adhesion strength on the collapse pressure was investigated by experimental simulation of different interface conditions. The orientation of the fibers in the laminate was also changed in order to assess better performance under external pressure.
Kinematics Under Extreme Waves
Authors: Stansberg Carl Trygve , Gudmestad Ove T. , Haver Sverre K.
Published: October 2008

Four different methods for prediction of wave-zone particle velocities under steep crests in random seas are compared. The study includes linear prediction, a second-order random wave model, Wheeler’s method, and a new method proposed by Grue et al. (2003). Comparison to laboratory data is also made. The purpose is to observe and evaluate differences in predictions for high and extreme waves, and how well they agree with measurements. The whole range from below still water level up to the free surface is considered. It is found that the second-order random wave model works best at all levels of the water column under a steep crest in deep water, and is therefore recommended. Grue’s method works reasonably well in many cases for z > 0, i.e. above the calm water level, but it overpredicts the velocities for z < 0. Wheeler’s method, when used with a measured or a second-order input elevation record, predicts fairly well the velocities at the free surface z = ηmax, but it underpredicts around z = 0 as well as at lower levels. The relative magnitude of this underprediction is slightly lower than the local steepness kA0 and can be quite significant in extreme waves. If Wheeler’s method is used with a linear input, the same error occurs also at the free surface.
Earthquake Resistance of Pipeline With Local Metal Loss Subjected to Cyclic Loading
Authors: Hashimoto Yoshikazu , Yatabe Hiroshi , Hagiwara Naoto , Oguchi Noritake
Published: October 2008

In this study, nine full-scale experiments were carried out to evaluate earthquake resistance of pipeline with local metal loss subjected to cyclic loading. The test pipes were API 5L X42 with a 762 mm diameter and 12.7 mm wall thickness. Each test pipe had an artificial local metal loss on its outer surface. Each test pipe was machined to have different size of the local metal loss. An internal pressure was hydraulically applied to 2.0 MPa in order to simulate an operating pipeline. A cyclic loading was applied in the axial direction. The strain amplitude of the cyclic loading was 0.5 % because pipelines are designed so that the maximum strain in a straight pipe should be less than 0.5 % during a severe earthquake in Japan. In all the experiments, buckling deformation was observed around the local metal loss after several cycles. The size of the local metal loss influenced the critical number of cycles for buckling. In the author’s previous investigation, the size of the local metal loss affected the peak load strain in pipelines subjected to axial compressive loading. The peak load strain of a pipeline with local metal loss was calculated using an FE analysis. The FE analyses were then conducted under an axial compressive loading using the same test pipe geometry as in the experiments. These experimental and FE analytical results indicated that the size of the local metal loss influenced both the critical number of cycles and the peak load strain. Furthermore, there is a relationship between the peak load strain under an axial compressive loading and the critical number of cycles. Consequently, the earthquake resistance was evaluated for a pipeline with local metal loss subjected to cyclic loading using the peak load strain under an axial compressive loading. Finally, we proposed the allowable size of a local metal loss for a pipeline to have a sufficient earthquake resistance.
Simulation of a Biomimetic Tidal Current Energy Conversion Device
Authors: Finnigan T.
Published: October 2008

Aquatic species have evolved highly efficient modes of swimming through natural selection. The most efficient in terms of forward thrust are Thunniform-mode swimming species (eg. tuna, shark). These species are propelled by lateral oscillation of a stiff lunate caudal fin with motion primarily limited to the fin and peduncle (aft tail) regions. In this paper we describe the application of biomimicry to the use of swimming hydromechanics in the development of a new tidal current energy conversion device. The development process is briefly described and some preliminary simulation results, based on computational fluid dynamics, are presented. Results indicate that potential exists for the development of low-cost efficient devices based on the biomimetic concept.
OCTOPLUS: A New Generation of FPSO for Deepwater
Authors: Geoffroy Guillaume , Pe´trie´ Franc¸ois , Vache´ Michel , Mitchell Karl , Guedes Soares C. , Pascoal R. , Cappoen Leo , Ival Nicolas
Published: October 2008

Today, FPSOs are usually converted tankers (VLCCs) or box-shaped new-built vessels. In either case, the hull provides oil storage capacity whereas process equipment, utilities and accommodation are spread over the deck area. The hull is built or converted following conventional shipyard practices before the topside are installed as small pre-assembled units built to offshore oil and gas standards. The OCTOPLUS is a new concept Floating Production Storage and Offloading (FPSO) vessel designed for deep and ultra-deep waters. This new generation FPSO vessel allows the hull and the topsides to be built and transported independently, and then mated at the field location. To provide confidence in the concept to potential end users, the integrity of the OCTOPLUS design has been evaluated over a three year EC development study, which included model testing. The cost, schedule, execution plan and operation of a development based on the OCTOPLUS were also established and compared to a standard FPSO. This paper presents the results and conclusions of this three year study and the current development status of the OCTOPLUS concept.
Global Structural Loads Induced by Abnormal Waves and Design Storms on a FPSO
Authors: Fonseca Nuno , Pascoal Ricardo , Guedes Soares Carlos
Published: October 2008

The structural loads on a FPSO induced by a large set of realistic (measured) abnormal waves are compared with the loads induced by “design storms” and also with the minimum values required by Classification Society rules. The design storms have a duration of 3 hours and correspond to the 100 years contour of the scatter diagram of the Northern North Sea. Time domain simulations are performed with a time domain seakeeping program that accounts for the most important nonlinear effects. The results are analyzed to obtain probability distributions of the sagging and hogging peaks of the vertical bending moment. Several theoretical distributions are fitted to the simulated realizations.
Response Spectra for Explosion Resistant Design and Assessment
Authors: Walker Steve , Corr Brian , Tam Vincent , Martland Roland , Shahsavar Rashid
Published: October 2008

This paper describes a project to examine the explosion test results obtained at Spadeadam in the 1990’s from the point of view of structural response. At an early project phase, when reliable simulations may not be available, nominal explosion overpressures and durations or past experience from similar situations may be used as inputs to the Response Spectrum approach. The results, however, will only be as accurate as the estimated loading. Structural engineers are often presented with complex pressure traces and are expected to design efficient structures to resist these loads without any further guidance. Response surfaces, representing peak deflection against scaled natural period and structural resistances, have been calculated for a large number of experimental and simulated pressure traces. Biggs response curves are a special case corresponding to triangular loading time histories. The Response Spectra are obtained from these response surfaces by taking horizontal sections of these surfaces at allowable ductility values. The required static resistance for a structure with a given natural period and pre-determined allowable ductility may be read from these curves. The design or assessment may then proceed with this required static resistance which may be reinterpreted as an equivalent static load. The project team has had access to FLACS simulated pressure traces corresponding to the Spadeadam tests. This has enabled a comparison to be made between experimental and simulated traces taking into account the target structures’ characteristics. A description of the method has been included in reference [1], in the Commentary of the new API RP on Fire and Blast which will be made available during 2006, and in a forthcoming OTO (HSE) report.
On the Structural Design of Bilge Keels
Authors: Mateus Anto´nio F. , Witz Joel A. , Silva Paulo P. , Pereira Carlos A.
Published: October 2008

The reduction of response motion experienced by monohull marine structures through energy dissipation performed by passive stabilisers such as bilge keels is an inexpensive system, when compared to the active motion reduction systems such as active stabilisers or active stabilising fluid tanks. This is the main reason why, despite the obvious advances in motion reduction and control, bilge keels are still designed, produced and fitted to the most recent ships and other marine structures produced. The design philosophy of passive roll stabilisers such as bilge keels has been, and still is, often based on empirical or design codes that define a standard structural arrangement, which is then sized in accordance with the specific characteristics of the vessel. This paper discusses the main internal structural arrangement configurations which may be adopted for bilge keel design, highlights the critical details which particular attention has to be paid to, presents the main steps involved in the sizing of the structural elements, and evaluates the more advantageous configurations with respect to performance vs. production cost. In what respects internal arrangement, the widest possible range of possibilities is considered, from the simple and fairly inefficient flat bar configuration, to the highly complex high performance diaphragm based designs. The advantages, disadvantages, estimated design and production efforts are presented and discussed. This work aims at providing a comprehensive review and a systematisation of the design principles applied for these structural components, envisaging to providing a simple and effective guide for the global and detailed design of their structures. Conclusions and further research work are drawn regarding the results obtained and the best solutions proposed.
Nonlinear Finite Element Simulations of Ice Sheet Forces on Conical Structures
Authors: Sand Bjo̸rnar , Fransson Lennart
Published: October 2008

The present paper deals with interaction between an ice sheet and fixed, conical structures. The ice sheet as well as the structure is discretizied by finite elements. The interaction between the ice sheet and the conical structure is simulated using a special contact algorithm which makes it possible to follow the gradually developing contact between the ice sheet and structure. As the configuration of the ice sheet changes during the interaction process, the buoyancy forces changes accordingly. This process is traced by introducing a continuous nonlinear foundation model to include the effects of buoyancy forces and specific weight of the ice. The mechanical behavior of ice is approximated using two different constitutive models. In the first material model the ice is treated as an isotropic, brittle material, while in the second model the ice is considered being a transversal isotropic, brittle material. When the state of stress at a material point in the ice reaches the failure surface, cracking or crushing is said to occur. After cracking or crushing, the post peak behavior of the ice is approximated as a rigid plastic material. The results obtained during the finite element simulations are compared with analytical methods for calculation of ice sheet forces on conical structures.
Strengthening Effect From Steel Pipe Cladding in Pure Bending
Authors: Ilstad Ha˚var , Nes Hroar A. , Endal Geir
Published: October 2008

Clad steel pipelines consist of a typically 3 mm thick internal corrosion resistant alloy with metallurgical bonding to the outer carbon steel pipe. The method of clad pipe manufacture gives intrinsic rise to disbonded areas (defects) in between the two materials, and there is concern that such disbondment defects may trigger local buckling of the cladding when the pipe is subject to severe bending, e.g. when installed by the reeling method. This paper addresses the possibilities for local buckling of the cladding material with the steel pipe in pure bending. Disbonded areas are studied numerically, and the critical defect size regarding local buckling of the cladding is established and compared with the allowable defect size as defined in the manufacture specification. It is shown that allowable disbondment defects of 500 mm2 are only 1/20 of the necessary area for local buckling during installation by the reeling method. In traditional pipeline design, the possible strengthening effect from the cladding on a steel pipe is not taken into account. In this paper, the strengthening effect from the cladding with the pipe in pure bending is studied. For the pipeline analysed it is shown that the deformation controlled local buckling resistance of the clad steel pipe is at least equivalent to a full thickness carbon steel cross-section. Hence, for a clad steel pipeline installed by the reeling method, the required wall thickness can be calculated by assuming the total wall thickness to be solid carbon steel.
Steep Transient Waves in Tanks: Experiments and Simulations
Authors: Shemer Lev , Goulitski Konstantin , Kit Eliezer
Published: October 2008

Very steep waves constitute an essentially nonlinear and complicated phenomenon. Inter-related experimental and theoretical efforts are thus required to gain a better understanding of their generation and propagation mechanisms. A nonlinear focusing process in which a single unidirectional steep wave emerges from an initially wide amplitude- and frequency-modulated wave group at a predicted position in the laboratory wave tank is studied both theoretically and experimentally. The spatial version of the Zakharov equation was applied in the numerical simulations. Experiments were carried out in the 330 m long Large Wave Channel (GWK) in Hannover, Germany and in the 18 m long Tel-Aviv University (TAU) wave tank. Quantitative comparison between the experimental and the corresponding numerical results is carried out. Good agreement is obtained between experiments and computations.
Fast and Reliable Underwater Communication: Successful Applications of Biologically Inspired Techniques
Authors: Bannasch Rudolf , Kebkal Konstantin , Yakovlev Sergey , Kebkal Alexej
Published: October 2008

The capability of the Sweep Spread Carrier (S2C) technology to overcome even the most crucial problems occurring in noisy ultra-shallow water channels could be shown in a wide range of technical applications. A milestone was the first real time transmission of side-scan sonar data while sea-floor mapping carried out by the AUV HUGIN 1000 running several hundred meters in front of the supply ship at a depth of 100–200 m (water depth ca. 400 m). Data speed was between 20 and 33 kBit/s in this trial. Based on this experience a new protocol providing two communication layers has been developed which allows to interpose high priority messages into an ongoing data stream when ever appropriate. So the S2C modems can be used for the transmission of large data files (measurements, digital images etc.) while serving as a fast and reliable control or command link, simultaneously. Current developments include integrated communication, tracking and positioning (combination with an USBL module) and enhanced networking capability. Various models ranging from high-speed S2C modems for short distance application (up to 72 kBit/s over several hundred metres) to efficient long-range modems became available. One of our latest models, the S2C M 7/17, is currently tested in the German Tsunami warning system in the Indian Ocean. The two directional acoustic link between the ocean bottom unit (OBU in 5000–6000 m depth) and the satellite buoy at the surface will be fast enough to transmit not only pressure data but also the records of 4 high-resolution seismic channels including the pre-history of a given Tsunami event. Via satellite scientists have permanent access to the OBU and can thus analyse any event of interest. This new technology opens many possibilities also for applications in Offshore Mechanics and Arctic engineering. Worthwhile to be mentioned, just the other day also the acoustic data transfer via the mud inside of a drill pipe was managed by using the S2C technique.
Vibration Control of Offshore Platforms Using a Wideband METMD System
Authors: Zhao Dong , Ma Rujian , Cai Dongmei
Published: October 2008

A wideband multiple extended tuned mass dampers (METMD) system has been developed for reducing the multiple resonant responses of the platforms to all kinds of loads, such as earthquake, typhoon, tsunami and big ice load. This system is composed of several subsystems, each of which consists of one set of extended tuned mass damper (ETMD) unit covering a specific frequency bandwidth, and its average frequency is tuned to one of the first resonant frequencies of the platform. The offshore platform is simplified to a single degree-of-freedom (DOF) system to which a METMD subsystem (composed of m ETMDs) is attached and constitutes m+1 DOFs system. The total mass ratio of the METMD subsystem to the platform is 14% and the frequency ratio of the exciting frequency to the platform’s natural frequency varies in [0.5, 1.5]. The theory analysis shows that: 1) the platform has the better vibration control effect when the non-dimensional frequency bandwidth Ω, which is defined as the ratio of the frequency range to the controlled (target) platforms natural frequency, is in [0.35, 0.6]; 2) the damping coefficient ξ of ETMD systems is in [0.05, 0.15] and 3) the number of the ETMDs is 5 when Ω = 0.45 and ξ = 0.1. The FEM simulation shows that the METMD has a better vibration control effect on the mega-platforms’ vibration control under the random ocean wave load.
Development of a Model for the Optimal Determination of the Deck Elevation of Offshore Platforms
Authors: Ortega Cesar , Campos Dante , Soriano Alberto , Alamilla Jorge , de la O. Ramirez Juan de Dios
Published: October 2008

In the present paper, a reliability-based cost-benefit model for the optimal determination of the deck elevation for new offshore platforms is proposed. The assessment of the benefit of reduced risks and the costs of increasing the deck elevation are weighed. If the deck elevation is increased, the risk associated with the wave forces on deck is reduced, because is less the probability of wave forces on deck; however the significant cost of increasing the deck elevation is magnified by the cost of mobilization of equipment in the life of platform, it cost is increased because boats with more capacity are required (in general, the rent is more expensive). For the propose of assessment the risk associated with the wave forces on deck a probabilistic model for the future maximum wave heights under the platform is studied taking in account the location of the platform. In addition, the expected cost of mobilization of equipment and the expected cost of maintenance of the structure and equipment after of the hurricanes futures are estimated, and then the expected life-cycle cost of the platform is determined. One example of application is proposed, where several elevations for the deck are studied and their corresponding life-cycle costs are estimated. The optimal deck elevation is chosen so that the expected life-cycle cost is minimized.
System Reliability-Based Assessment on Existing Jacket Platform
Authors: Jin Wei-Liang , He Yong , Gong Shun-Feng
Published: October 2008

A great deal of existing jacket platforms need to be assessed in order to determine whether they can be reused or not. In allusion to the differences between the existing service platforms and the drafting ones, a methodology of system reliability-based assessment of existing jacket platform is presented and is applied for the assessments of BZ28-1 three jacket platforms which have operated for sixteen years. In terms of nonlinear mechanical analysis, correcting the statistical parameters of the sea states and considering pile-soil-structure nonlinear interaction, the mechanics characteristics of three jacket platforms subjected to extreme loads were analyzed. Based on the FEM model, with Monte Carlo simulation, the reliability of the platforms was studied. From the practical application, the method can be used to estimate the service life of the existing platforms and ensure their secure operation.
Comparison Between Predicted and Measured Umbilical Bending Stresses During Manufacturing
Authors: Sævik Svein , Gjo̸steen Janne K. , Figenschou Arild
Published: October 2008

The present paper addresses comparison between predicted and measured bending stresses in one umbilical cross section during manufacturing. A factor with significant uncertainties has been which stresses that will occur during the manufacturing procedure when the umbilical is reeled on and off storage drums. The umbilical components may not only be exposed to pure elastic bending stresses due to the curvature radius but also second order axial stresses due to the combination of curvature gradients and finite length to the end fittings. In cooperation with Aker Kværner Subsea a.s., Marintek has developed a tool for stress analysis of Aker Kværner umbilical cross-section designs. The main motivation behind the development has been to reduce technical risk and costs associated with umbilical manufacturing, installation and operation. This is obtained by increasing the basic understanding of the umbilical cross-sections mechanical behaviour and the ability to quantify the stress response when exposed to various loads. In order to calibrate the mathematical model with regard to manufacturing stresses a 70 m long umbilical prototype was instrumented by strain gauges and reeled on an off a storage reel with radius 4.4 m. The paper presents the model applied to simulate this operation as well as the results from comparing predicted and measured stresses.
Risk Based Structural Integrity Management Using Bayesian Probabilistic Nets
Authors: Montes-Iturrizaga Roberto , Heredia-Zavoni Ernesto , Marcial-Marti´nez Enrique , Faber Michael Havbro , Straub Daniel , De la O. Ramirez Juan de Dios
Published: October 2008

The present paper presents a general framework for integrity management of offshore steel jacket structures allowing for the risk based planning of inspections and maintenance activities with a joint consideration of all relevant deterioration and damage processes. The basic idea behind the suggested approach is to relate the relevant deterioration and damage processes, i.e. the exposure events, to damage states which in turn may be related to the overall integrity of the jacket structural system as measured through the Reserve Strength Ratio (RSR). This facilitates that any state of degradation, irrespective of the cause, can be assessed in terms of their impact on the annual probability of failure for the structure. Taking basis in data as well as subjective information regarding the annual occurrence probabilities of the relevant deterioration and damage processes, together with a probabilistic modeling of the quality of condition control, it is possible to assess the effect of condition control of each type of deterioration and damage phenomena. This then facilitates the development of a general framework for risk based integrity management. In the present work such a framework is formulated using Bayesian Probabilistic Nets (BPN) for evaluating the time varying global structural reliability of jackets subject to progressive deterioration of its members due to the combined effect of different sources of damage.
Design of Pressure Hulls Using Nonlinear Finite Element Analysis
Authors: MacKay John R. , Smith Malcolm J. , Pegg Neil G.
Published: October 2008

Through the use of nonlinear finite element analysis (NLFEA), submarine pressure hull designs could potentially be based on calculated limit states that include the full geometric complexity of the structure, and real-world effects such as build imperfections. In addition, NLFEA could provide a rational means of assessing the effects of in-service damage on structural performance. Analysis of pressure hulls using 3D NLFEA is not currently supported in design codes, primarily because the uncertainty regarding the accuracy of the method has not been quantified. Defence Research and Development Canada (DRDC), the R&D branch of the Canadian Navy, is undertaking work to develop a partial safety factor for 3D NLFEA of pressure hulls, by comparison of numerically calculated collapse pressures to experimental results. Data from experiments previously conducted at various institutions will be augmented by a pressure hull testing program currently being undertaken by a joint project of DRDC and the Ministry of Defence of the Netherlands. The development of NLFEA modeling and analysis guidelines, as well as a revised design/analysis procedure, will be discussed, as well as a history of the DRDC submarine structures research program.
Suction Piles in Clay: Reliability Assessment of Deepwaters
Authors: Sa´nchez Moreno Jorge , Prates de Lima Edison Castro , Ellwanger Gilberto Bruno
Published: October 2008

This paper presents an illustrative example of the reliability assessment of suction piles foundation in clayey soils for deepwater applications. The results of two reliability approaches, using suction piles as the foundation system for tension leg platforms (TLPs) under deep waters environmental conditions in the Campeche Bay (Mexico), are compared. The first approach uses the first-order second moment (FORM) reliability method with a suction pile analytical model and the second one uses the Monte Carlo reliability method with a suction pile Finite Element model. The uncertainties of the loads, soils mechanical properties and analytical models were included in the reliability analysis. The results from this study point to important topics for future works.