bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Joint Behavior on the Reassessment of Fixed Offshore Platforms in the Bay of Campeche, Mexico
Authors: Chakrabarti Partha , Mukkamala Adinarayana , Abu-Odeh Ibrahim , de la O. Rami´rez Juan de Dios
Published: November 2008

Pemex Exploration y Produccio´n owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of these fixed offshore platforms were built during the 70s and 80s and have already exceeded their design service life. To meet the growing demand for oil and natural gas it was found economic to extend the service life for these platforms by at least another 15 to 30 years. To meet this extended service life, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness and possible locations of fatigue problems. To extend the fatigue life of the welded joints, inspections are required to be performed according to a risk based inspection planning procedure. As a part of the reassessment study non-linear pushover and spectral fatigue analyses are conducted. The effect of joint behavior, viz. the local joint flexibility and strength, on the structural ultimate capacity and fatigue life is discussed in this paper. In conventional analysis the tubular joints are assumed to be rigid and the flexibility effects due to shell deformations are ignored. In this present paper, the effect of the joint behavior is included in the analysis and its implications on the results are discussed. For the ultimate strength analysis both API and MSL formulations for the load-deformation behavior of the joint are studied and compared. For the fatigue analyses, local joint flexibility modeling using Buitrago’s formulation is used. Results including and excluding these effects are compared. Effect of grouting of the joint is also studied. Comprehensive results of the study for a number of platforms, which cover the categories of Drilling, Production, Gathering and Habitation, are presented. The effects of local joint flexibility and joint strength on structural behavior have been recognized to be important in the recent publications of the recommended practices and the codes such as the API RP 2A. However, comprehensive discussions and the results of application of these aspects are rare in the published literature. This paper addresses these issues and presents the results of a large number of platforms of different configurations, indicates some noticeable trends and suggests some general conclusions.
Analysis of Fatigue Crack Propagation in Typical Welded Joints of FPSOs
Authors: Zhang Bin , Moan Torgeir
Published: November 2008

The purpose of this paper is to predict crack propagation and especially the remaining life after through-the-thickness crack at typical joints in deck and bottom structures of FPSOs. This information can be used in planning inspection, repair and maintenance. The growth of fatigue cracks is studied in typical welded joints through the use of analytical and numerical methods. The simplified analytical model is based on the British Standard 7910 [3] and Dexter’s analytical model [1–2]. Numerical analysis is performed with the finite element method, considering the effect of residual stresses, and using the J-integral approach to determine the stress intensity factor at the crack tip during different stages of crack growth. The first stage is referred the surface crack stage, in which the crack grows from an initial surface crack of a size of about 0.1 mm in depth and 0.2mm-1.0mm in length to the through-thickness crack. The second stage is named the long crack stage, in which the crack grows from an approximately 40–100mm long through-thickness crack to the final critical long crack. The computed stress intensity factors, along with the Paris law, are used to predict the crack propagation at each stage with reasonable accuracy. The effect of welding residual stresses on fatigue behaviour is considered by introducing an effective SIF concept. It is concluded that stable crack propagation behaviour can be conservatively predicted by using relatively simple approaches. These techniques can be used in making rational decisions regarding scheduling of inspections, repairs, and allow a better prediction of the structural reliability in view of fatigue cracks.
Fatigue Loads on Offshore Wind Turbines Due to Weakly Non-Linear Waves
Authors: Trumars Jenny M. V. , Tarp-Johansen Niels Jacob , Krogh Thomas
Published: November 2008

Due to the pronounced dynamic behavior of wind turbines, fatigue load effects may be quite sensitive to the precise modeling of the frequency content of the wave loading. As the offshore wind turbine technology progresses, larger and larger turbines will be placed at still deeper waters, causing the resonant frequency of the first eigen mode of a traditional bottom-fixed support structure to be typically in the range from 0.25 Hz to 0.35 Hz. The deeper the water and the larger the turbine, the lower the frequency will be. As an example, wave measurements from the offshore wind farm Bockstigen show a second peak at approximately 0.3 Hz in the wave spectrum. Thus this peak, or similar peaks realized at shallow water sites, may very well be dynamically amplified in the response. This second peak cannot be modelled with a linear wave model, and a wave model taking non-linearities into account has to be used. In the presented work, both a linear and a non-linear wave model are used to study the fatigue in an offshore wind power plant and the difference is compared. Time series of irregular linear and non-linear waves are calculated, and structural calculations of an offshore wind turbine with a slender support structure are used in the analysis of the fatigue loads. The forces on the structure are calculated using Morison’s equation, integrating along the structure and lumping the loads in nodes for the structural calculations. The difference between the wave models is significant and the non-linear model yields higher fatigue damage than the linear one.
Evolution of Pipe Properties During Reel-Lay Process: Experimental Characterisation and Finite Element Modelling
Authors: Martinez Michae¨l , Brown George
Published: November 2008

The development of finite element analysis, in terms of simulation power and theoretical model accuracy, enables one to understand and simulate industrial processes more precisely, especially those involving non linear behaviour and analysis. Reeled pipe technology is one of these, and has a lot to gain from this increasing efficiency. In the reel-lay process the pipe is first reeled onto a drum on a vessel for transportation. During offshore installation the pipe is unreeled, straightened and deployed into the sea. During the process, the pipe is fully and cyclically plastified. Plastification modifies the pipe properties, which is not by itself detrimental but should be understood by the designer. Pipe properties are affected in three ways: geometrical shape – reeling and straightening induce some residual ovalisation; mechanical properties – yield stress, hardening slope, isotropy are modified; and fatigue properties. Technip and IFP have studied these property evolutions for many years, both from an experimental and a numerical point of view. The present paper discusses the first two points. A wide experimental programme has been performed. Full scale pipes were reeled and straightened on a bending rig device especially built for that purpose. Pipe ovalisation was monitored through the whole process. Pipe mechanical properties were also fully characterised in the pipe axial, hoop and thickness directions, both in tension and compression, before and after reeling process. Extruded and UOE pipes were tested and characterised. Pipe initial properties are dependent on the manufacturing process but they are modified by the reeling process. Reeling induces some anisotropy that cannot be properly accounted for by usual plasticity models. Finite element simulations with Abaqus software, using the material behaviour of unreeled pipe, underestimate stiffness evolution in the hoop direction and overestimate ovalisation induced by the reeling process. Anisotropy has indeed a great effect on ovalisation that results from an interaction between axial and hoop loading. Hardening is also a key parameter. A new plasticity model has been written in an Abaqus User Material Model, known as UMAT. The new model is based on an anisotropic Hill criterion and special attention is paid to the hardening. This new model reduces by more than two the error on ovality estimation, and gives a realistic prediction of material anisotropy evolution through the process. Although, the tuning of the model coefficients is more complex than for usual models, its use is quite straightforward and does not increase computation time.
A Coupled-Mode, Phase-Resolving Model for the Transformation of Wave Spectrum Over Steep 3D Topography: A Parallel-Architecture Implementation
Authors: Gerosthathis Th. P. , Belibassakis K. A. , Athanassoulis G. A.
Published: November 2008

The problem of transformation of the directional spectrum of an incident wave system over a region of strongly varying three-dimensional bottom topography is studied, in the context of linear theory. The Consistent Coupled-Mode Model (Athanassoulis and Belibassakis 1999, Belibassakis et al 2001) is exploited for the calculation of the linear transfer function, connecting the incident wave with the wave conditions at each point in the field. This model takes fully into account reflection, refraction and diffraction phenomena. The present approach permits the consistent transformation of any incident directional wave spectrum over a variable bathymetry region and the calculation of the spatial evolution of point spectra of all interesting wave quantities (free surface elevation, velocity, pressure), at every point in the domain. This approach can be extended to treat weakly non-linear waves.
Residual Stresses Evaluation on Radial Friction Welded Joints
Authors: Pinto Rosa Irene Terra , Strohaecker Telmo Roberto
Published: November 2008

The Radial Friction Welding (RFW) is a solid-state welding process in which two long elements of several metallic alloys can be joined, without the occurrence of common problems to the conventional welding processes that include fusion. During friction welding the temperature evolution is directly related with the deformation gradient, and these fields govern the joint properties. In this work, the finite element method was used to solve the full coupled termomechanical problem in order to determine the deformation and the stress fields and the variation of the temperature during RFW process. The simulation of the RFW process permitted to establish the influence of the welding parameters, like rotation and approximation speed, on the joint quality. Furthermore, the knowledge of the temperature gradient and cooling rates allowed the prediction of the resulting microestruture and determination of the level of residual stresses of the joint. To verify the analytical results the determination of the residual stresses was accomplished by the hole drilling method in several points along the perimeter of two welded workpieces.
A Comparative Assessment of the Use of Spudcans and Caissons as the Foundations of Jack-Up Structures
Authors: Vlahos G. , Cassidy M. J. , Knowles B.
Published: November 2008

More than ever before, operators are advancing mobile jack-up units into deeper waters and harsher environments. Constraining their use is the capacity of the shallow foundations (typically inverted conical spudcans) to withstand the larger wave, wind and current loads of these environments. The use of foundation skirts (or caissons) is often touted as an alternative, though today only a small percentage of jack-up units use caisson foundations. One restraining feature is limited understanding of their behaviour when compared to conventional spudcan. This paper addresses this by comparing the results of push-over experiments of a three-legged jack-up with similarly sized caisson and spudcan foundations. The tests were conducted on a 250:1 scale jack-up on an overconsolidated clay sample. With improved understanding of the overall response of a jack-up platform with caisson foundations, operators should have greater confidence to use these footings.
Tendency of Damping Ratio of Floating Artificial Base by Experiment and Measurement
Authors: Maruyoshi Koichi , Saijo Osamu
Published: November 2008

It is important to estimate both added mass and radiation damping of floating artificial base, which is floating structure, in fluid-structure interaction problem. When a floating artificial base shows elastic behavior, its damping contains both structural damping and radiation damping. Therefore it is difficult to estimate the damping. We aimed to grasp tendencies of damping, when a floating artificial base shows elastic behavior, by both experiment and field measurement. The experimental modal analysis using elastic plate models was carried out and tendencies of damping ratio of experimental models were grasped. Besides, damping ratio of an existent floating artificial base was calculated from measured vibration data by the Random Decrement Technique (RDT). In our experimental results, the damping ratio became larger value in lower vibration mode, and then it decreased in higher vibration mode. In higher vibration mode, we could not confirm radiation damping. It is thought that structural damping accounts for nearly all of obtained damping ratio. Consequently, it is appropriate that damping ratio of the floating artificial base is evaluated as Rayleigh type damping.
On the Non-Linear Decay Motion of an Oscillatory Plate
Authors: Abdolmaleki K. , Thiagarajan K. P. , Monaghan J. J.
Published: November 2008

We study the non-linear decay motion of a 2D plate experimentally and analytically. The plate was hinged to the bottom of a wave flume and was positioned at a certain initial angle. The restoring force on the plate was derived from two horizontal pre-tensioned springs. To maintain the system characteristics linear, the springs were selected to allow a maximum 18 degrees of rotation for the plate. The position, velocity and the acceleration of the plate were retrieved from the load cells attached to the springs. The plate was released from its initial position at t = 0 and allowed to oscillate. The free-surface elevation was captured using a high frame per second (200 fps) digital camera. In addition, two wave probes on either side of the plate were installed. It was observed that the high stiffness of the springs produced a mild impact to the water that caused a relatively large water run-up and water jet. This event, consequently, made the decay motion very non-linear. A formulation based on the linear theory was developed to help with the understanding and interpreting the physics of the problem. The presented experiment aims to benchmark various numerical techniques such as Smoothed Particle Hydrodynamics (SPH) that intend to simulate free-surface and water impact problems. Although the setup did not model a green water incident, most of the features in the problem, like initial water impact, run up and water jet resemble the physics of green water. In the designed experiment, not only body 3D effects were minimum, but also the system characteristics were linear. Moreover, in contrast to the dam break experiments, perfect initial conditions were achieved. Therefore, the effects of the flow nonlinearities such as the plate impact to the water, water run up-down and water jet were studied without interference of the body nonlinearities. The impact of these effects on the damping and the added mass were highlighted.
Probabilistic Analysis of Characteristic Pressure for LNG Tanks
Authors: Graczyk Mateusz , Moan Torgeir , Rognebakke Olav
Published: November 2008

The paper addresses the problem of determining characteristic extreme values of sloshing pressures for structural design. This involves estimating ship motion in a long-term period, fluid motion in the tank, excited pressures and relevant structural responses. Ship motion analysis is based on linear strip theory. In order to investigate the dependence of the sloshing response on sea conditions, a multimodal approach for an initial prediction of pressures in the tank is utilized. However, an appropriate theoretical / numerical approach which can be used for realistic prediction of the most extreme pressure has not yet been developed. Thus, experiments are utilized for the most severe sea states for a chosen tank filling level. In the analysis of experimental short term pressure distribution the choice and fit of probability distribution models is addressed, with due account of different physical mechanisms causing impacts. The most critical tank areas for sloshing loads are briefly discussed. Appropriate dynamic response of the tank structure needs to be investigated by accounting for temporal and spatial distribution of sloshing loads. These two factors are also addressed in the paper.
Application for Estimating Motion Behaviors of FPSO/FSRU and Shuttle Tanker in Alongside and Tandem Mooring Conditions
Authors: Yokozawa Hitoshi , Ito Akio
Published: November 2008

With increasing attention to global warming, expectations for Floating Production Storage and Offloading (FPSO) System to develop a gas field of which reserve is smaller, or Floating Storage and Regasification Unit (FSRU) as LNG terminal are growing. Offloading transfer of LNG requires rigid loading arm instead of flexible hose because of low temperature of LNG. It involves the relative motion estimation between LNG FPSO/FSRU and a shuttle tanker to design loading arm and to estimate operability of offloading work. The operability in environmental conditions of the sites affects on the decision of the storage capacity and the necessary number of LNG carriers for transportation. For this task of motion study, the motion estimation programs for alongside and tandem alignment have been developed. This paper presents outline of the programs and some validation results. An example of application of the calculation results to design FSRU is also shown.
Space Shuttle MODUS: Key System for the Installation of Networks of Benthic Stations
Authors: Gerber Hans W. , Clauss Gu¨nther F.
Published: November 2008

In the last decade within several European and bi-national (German-Italian) projects a set of equipment and technology for benthic stations, and their deployment and recovery has been developed. Starting with a single prototype station which operated autonomously for several months in the Mediterranean Sea (GEOSTAR2 – Geophysical and Oceanographic Station for Abyssal Research – Deep Sea Mission, 1999–2001, /1/) with a near-real-time-communication-system including a acoustic link from station to buoy and a shore radio link. This concept has been extended to realize a sub-sea network of several benthic stations (ORION- Ocean Research by Integrated Observation Networks, 2002–2005, /1/). This includes an extended version of the GEOSTAR acoustic underwater communication system between the stations and the buoy unit for direct shore communication. For the deployment and recovery of these benthic stations the deep-sea shuttle MODUS (Mobile Docker for Underwater Sciences) has been developed and used (operation depth 4000 m). All operations up to now were successfully conducted. The ORION-network is under operation at a water depth of 3700 m North of Sicily/Italy since December 2003. In addition to this MODUS has been used to install the first long-term benthic station directly cable connected to shore. Thus, the first European long term benthic station has been established, with the aim to demonstrate a technology for the ongoing plans for a European deep-sea shore network (ESONET – European Seafloor Observatory Network, /1/). The deployment took place east of Sicily using MODUS and a ROV for the realization of the cable connection between the station and the cable termination in January 2005.
Thermal-Hydraulic Analysis of Heavy Oil Transportation in Heated Sandwich Pipelines
Authors: Su Jian , Estefen Segen F.
Published: November 2008

Heavy oil production in deepwater is a major challenge to pipeline technology due to high viscosity and associated high pressure drop. In this paper, we propose to apply active electrical heating in multilayered composite pipelines for heavy oil production in deepwater. A thermal-hydraulic analysis is carried out to evaluate the pressure drop reduction and power input requirement. We concluded by comparative numerical studies that the heated sandwich pipeline provides a significant pressure drop reduction and thus a boost of mass flow rate of the oil production for a given available pressure drop between the wellhead and the separator.
Applying Partial Safety Factors in Mooring System Design
Authors: Okkenhaug Siril , Sogstad Bjo̸rn , Mathisen Jan
Published: November 2008

The DNV offshore standard for position mooring, DNV-OS-E301 [1], was issued June 2001 based on the result from a joint industry project. A new revision was issued in October 2004. The consequences for mobile units when applying the new standard, compared to the old class rules, have been a major concern for operators of mobile units. A comparison study has therefore been initiated, where four relevant units are considered. We have applied the mooring design for existing mobile units that operate in Norwegian waters. Two different water depths are covered. The new standard, DNV-OS-E301, applies a partial safety factor format. However, the main difference when applying DNV-OS-E301 compared to the old POSMOOR [2] rules is that low frequency (LF) motions will have to be taken into account when calculating the line tensions. The results for the four mobile units are compared also to other relevant codes, i.e. the Draft International Standard ISO 19901-7 [4] and the present Norwegian regulations for offshore structures, NMD [5 & 6]. It should be noted that the present NMD regulations still do not require that LF motions are taken into account. Due to the partial safety factor format in DNV-OS-E301, more or less all of the units fulfill the requirements even though LF motion is accounted for. However, when comparing the results to the NMD regulations, the introduction of LF motion is crucial for almost all of the mobile units studied, as they have problems in fulfilling the requirements when this response is accounted for. Simply including LF motion in design would tend to increase the required strength of the resulting mooring line designs, and thereby raise the safety level if nothing else is done with the NMD regulations. Thus, provided that the present safety level for mobile units is sound, the present NMD safety factors could either be reduced or the partial factor format in DNV-OS-E301 could be adopted in order to maintain the safety level for mobile units when LF motion is taken into account.
Interaction of a Solitary Wave With a Vertical Permeable Elliptical Breakwater
Authors: Basmat A.
Published: November 2008

In this paper the diffraction of a plane first-order solitary wave by a vertical permeable breakwater with calculation of the wave loading is investigated. The interaction between long non-linear water waves and dissipative/absorbing coastal structures that are commonly used in coastal engineering is investigated using both analytical and numerical technique. The breakwater consists of a vertical permeable surface-piercing elliptical cylinder fixed in the ground. The analytical model herein is based on the application of Boussinesq equations to describe the diffraction of the first-order solitary wave by an elliptical breakwater. The method of solution is based on perturbation theory, using a perturbation parameter defined in terms of surface geometry of the cylinder. The analysis includes terms up to the first-order in this parameter, where the zeroth-order solution corresponds to a circular cylinder. The velocity potentials at the zeroth and first orders are expressed as eigenfunction expansion involving unknown coefficients that are determined through the boundary conditions on unperturbed cylinder. The flow through the porous breakwater is assumed to obey Darcy’s law. The total force onto the elliptical cylinder is obtained by integration of the pressure over permeable cylindrical wall. The analytical solution is obtained by means of a Fourier transformation technique. The effects of porosity, relative wave length and the incident wave angle are discussed. Numerical results compare well with previous predictions for the limiting cases of a permeable cylindrical breakwater.
Dynamic Interaction Between Spans in a Multi Span Pipeline Subjected to Vortex Induced Vibrations
Authors: Soni Prashant K. , Larsen Carl M.
Published: November 2008

Pipelines are the most convenient and safe medium of oil and gas transportation from offshore fields. Such pipelines will often have free spans when passing through areas with uneven sea floor. These free spans may experience vibrations due to vortex shedding when subjected to ocean current, which may lead to unacceptable accumulation of fatigue damage. The bottom topography may result in multi-span pipelines with short contact area between adjacent spans. Such neighbor spans may interact dynamically, meaning that the response of one span can not be found unless its neighbors are taken into account. The purpose of the present paper is to illustrate possible consequences of interaction and present a consistent method for classification of interaction. Two aspects of interaction are studied, namely the possible energy transfer due to hydrodynamic forces from one span to its neighbor of different length, and how the stiffness properties and length of the contact area between spans influence the interaction. The first aspect is studied by using an empirical model for the analysis of vortex induced vibrations (VIV) of multi-span pipelines. This model applies a frequency domain solution procedure that must describe the interaction between the pipe and the seafloor by linear theory. It was observed from the results that the spans interact dynamically, and the distribution of lift forces along the length of pipe shows that energy is transferred from the span with smallest amplitude to the span with largest amplitude. The second aspect is studied by use of an analysis model where the contact between pipe and seafloor has a more realistic representation. This model is based on a non-linear finite element method, and the dynamic analysis is carried out in time domain. By using this model it is possible to characterize the mechanical coupling between the spans. A systematic study illustrates how seafloor stiffness and local profile of the seafloor will influence the interaction.
Analysis of Duplex Yoke Mooring System for Tandem Offloading of LNG
Authors: Liu Yonghui (Allen) , Boatman L. Terry
Published: November 2008

The technology for transferring LNG between two floating vessels offshore is a crucial component in floating LNG facilities now in the planning stage. A duplex yoke mooring system and conventional boom-to-tanker LNG offloading arms were recently developed to carry out tandem offloading of LNG in open sea areas with significant wave heights up to 5.5 meters. This paper will present a methodology to evaluate the operability of the LNG offloading in severe environments. The relative motions between the FPSO and the carrier were predicted by a frequency-domain method. A series of model tests have been carried out. A good agreement between the predictions and measurements has verified and validated the analysis method and procedures. Berthing operations in seastates with a significant wave height of 3.5 meters were also conducted successfully. Therefore, it can be concluded that the technology of the duplex yoke mooring and boom-to-tanker loading arms is ready to be implemented for a safe and reliable transfer of LNG in harsh open sea environments for LNG facilities.
Modelling and Assessment of an Offshore Rescue Vessel
Authors: Crossland Paul , Furnell Alistair , Mills Matthew
Published: November 2008

BP Exploration has developed a new concept in providing safety cover for personnel working on offshore installations. The concept involves the use of a new design of rigid inflatable boat, which can be deployed in emergencies to recover casualties from the water, sustain their life and then return them to a surgical facility for primary care. This paper describes the innovative trials run by QinetiQ in order to simulate elements of the vessel and to build a more complete picture of its usability and operational capability. The team has taken a total systems multi-disciplinary approach to the solution, encompassing offshore engineering, surgical expertise and ergonomics. The trials encompass the retrieval of casualties from a wave tank onto an afterdeck mock-up, their treatment under conditions of motion in a large displacement motion simulator and the flow of casualties within a mock-up of the vessel. The paper describes how the data gathered from these trials was then integrated to allow an appreciation of likely casualty handling capability of the vessel.
A Numerical Method to Solve the Three-Dimensional Static Problem of a Riser With Bending Stiffness
Authors: Yamada da Silveira Lauro Massao , de Arruda Martins Clo´vis
Published: November 2008

Numerical integration of the governing differential equations of three-dimensional riser statics may experiment large stability problems due to the inclusion of bending stiffness, as the leading order term becomes usually very small. These numerical problems may be avoided by working with a perfectly flexible cable model. However, such a model cannot deal with all the boundary conditions, as for an ideal cable there is no continuity of curvature at the touchdown point, at the top and at the points where there is change in the submerged weight. At the touchdown region, for instance, the perfectly flexible cable model overestimates the maximum curvature. To overcome these difficulties, one approach that can be adopted is to use firstly a perfectly flexible cable model and correct later the results with analytical expressions obtained from a boundary layer method. This approach is based on the fact that bending stiffness is relevant only at small boundary layers around the points where the perfectly flexible cable model cannot represent the curvature continuity. For a two-dimensional formulation it was already shown that this approach is very good. For a three-dimensional formulation, however, the analytical expressions become too cumbersome, the authors could not so far find an exact analytical solution and, therefore, the problem must be solved numerically. Another possible approach is to use finite elements method, as many full nonlinear commercial softwares do. However, it is not difficult to face convergence problems. This work presents a numerical method to solve the set of differential equations of the three-dimensional riser statics, including the bending stiffness. The results obtained are compared to a full nonlinear well-known commercial computer code.
An Adaptive, Robust, Model-Referenced Control Strategy for the Dynamic Positioning of a Shuttle Tanker During Offloading Operations
Authors: Tannuri Eduardo A. , Kubota Leonardo K. , Pesce Celso P.
Published: November 2008

In deep water oil production, Dynamic Positioning Systems (DPS) strategy has shown to be an effective alternative to tugboats, in order to control the position of the shuttle tanker during offloading operations from a FPSO (Floating Production, Storage and Offloading System). DPS reduces time, cost and risks. Commercial DPS systems are usually based on control algorithms which associate Kalman filtering techniques with Proportional-Derivative (PD) or Optimal Linear Quadratic (LQ) controllers. Since those algorithms are, in general, based on constant gain controllers, performance degradation may be encountered in some situations, as those related to mass variation during the loading operation of the shuttle tanker. The positioning performance of the shuttle changes significantly, as the displacement of the vessel increases by a factor of three. The control parameters are adjusted for one specific draught, making the controller performance to vary. In order to avoid such variability, a human-based periodic adjustment procedure might be cogitated. Instead and much safer, the present work addresses the problem of designing an invariant-performance control algorithm through the use of a robust model-reference adaptive scheme, cascaded with a Kalman filter. Such a strategy has the advantage of preserving the simple structure of the usual PD and LQ controllers, the adaptive algorithm itself being responsible for the on-line correction of the controller gains, thus insuring a steady performance during the whole operation. As the standard formulation of adaptive controllers does not guarantee robustness regarding modeling errors, an extra term was included in the controller to cope with strong environmental disturbances that could affect the overall performance. The controller was developed and tested in a complete mathematical simulator, considering a shuttle tanker operating in Brazilian waters subjected to waves, wind and current. The proposed strategy is shown to be rather practical and effective, compared with the performance of constant gain controllers.
Component Approach for Confident Predications of Deepwater CALM Buoy Coupled Motions: Part 2 — Analytical Implementation
Authors: Santala M. J. , Huang Z. J. , Wang H. , Yung T. W. , Kan W. , Sandstrom R. E.
Published: November 2008

This paper describes an analytical implementation of the component approach for motion predictions of a deepwater CALM buoy as described in the companion paper “Component Approach for Confident Predictions of Deepwater CALM Buoy Coupled Motions — Part 1: Philosophy”. The implementation of the approach starts with a “model-of-the-model” validation of the analytical tool. Emphasis is given to making an accurate analytical characterization of the model as tested. To capture the strong coupling between the buoy motions and line dynamics the analyses described herein were carried out in the time-domain. This allows a rigorous treatment of the hydrodynamic forces on the buoy as well as the non-linear mooring loads when analyzing the buoy responses in waves. Since the validation analysis is a model-of-the-model practice at model scale, the proper application of the validated tool to the full-scale system is discussed. This involves modeling of the exact full-scale system and the proper selection of the hydrodynamic coefficients for the buoy and lines. In this paper we will present the numerical modeling procedures and the results from validation work to confirm that the analytical tool is validated correctly. Detailed results from validation analysis versus model test data will be shown for system components including buoy hydrodynamics from the forced oscillation test, line tension from line oscillation test, and the motions and tensions of integrated buoy/mooring/riser system. We point out that the hydrodynamic coefficients at model scale can not be directly applied to the full-scale system analysis even though they are from model test measurements. We will present the difference between the results of the model-scale system using model scale hydrodynamic coefficients and those based on a proper range of the coefficients at full-scale. This will highlight the need to design component tests to determine appropriate full scale coefficients in order to improve the accuracy of full-scale design predictions. These results will show the advantages of adopting a component approach over the common industry practices in the areas of correct use of model test data, validation analysis and the analysis of the coupled CALM buoy system responses in waves.
VIV Analysis of a Riser Subjected to Step and Multi-Directional Currents
Authors: Schulz Karl W. , Meling Trond S.
Published: November 2008

A multi-strip numerical method combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a finite-element structural dynamics response has been developed previously to analyze the flow-structure interaction of a flexible riser subjected to fixed and non-uniform, two-dimensional shear currents. In this paper, we expand on the previous work using the tool to numerically compute the VIV loads and motions of a vertically tensioned riser in a stepped current. The flow conditions for this stepped current configuration were chosen to match a set of laboratory experiments carried out in the Delta Flume at Delft Hydraulics. In addition to the stepped current, the multi-strip method was extended to accommodate a three-dimensional skew current exposed to a vertically tensioned riser. Note that in this case, the skew current is non-uniform in both direction and magnitude, and the flow conditions and riser configuration were chosen to match a set of rotating-rig experiments made by Marintek. For both configurations (stepped and skewed currents), comparisons of in-line and transverse VIV displacements are presented between numerical and experimental results.
Riser Stability Under External Pressure and Axial Compression Observing Geometrical Imperfections
Authors: Sa´nchez Sa´nchez He´ctor A. , Corte´s Salas Carlos
Published: November 2008

Steel pipes are studied considering external pressure, axial compression and bending actions to empty condition, taking into account the initial geometrical imperfections. The main objective is to study the behavior and structural stability of these pipes submitted to combine loads, considering the influence of the geometrical imperfections and to estimate the critical external pressure. Critical pressure of buckling, and modal configurations are evaluated by theoretical methods and numerical approaches such as finite element method (FEM). The numerical results are compared with theoretical results.
Development of High Strength Linepipe With Excellent Deformability
Authors: Okatsu Mitsuhiro , Shinmiya Toyohisa , Ishikawa Nobuyuki , Endo Shigeru , Kondo Joe
Published: November 2008

Extensive studies to develop high deformability linepipe have been conducted. In case of linepipes laid at seismic region, higher resistance to buckling against large strain induced by earthquake related ground movements are required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, base on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine M-A constituents particles was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure were evaluated by axial compression test, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Tensile properties after thermal coating of developed high deformability pipe was also investigate. It was shown that increase in yield strength by thermal strain aging was minimized and round-house type stress-strain curve was maintained for the linepipe manufactured by new conceptional TMCP process.
Design Optimization of a Ship Hull Block Based on Finite Element Analysis
Authors: Jang Chang Doo , Song Ha Cheol , Hwang In Seop , Shin Sang Hoon , Lee Jong Hwan , Choe Ick Hung , Park Jinsoo
Published: November 2008

Optimization techniques for shipbuilding have been developed to design large-scaled commercial vessels based on classification rules and analytical data, but most of the target model in the past were restricted to the mid-ship section design in the preliminary design stage. By contrast, preceding researches about the design optimization of detailed structures are relatively rare since narrow information has been provided as the design guide for such structures. In this study, the minimum weight design of a ship hull block was carried out based on the finite element analysis. The target model for design optimization is a stiffened deck block of LNG carrier for upholding a main boiler. The weight of a ship hull block and the main dimensions of structural members are chosen as an objective function and design variables, respectively. An optimization algorithm was adopted to escape the local minima and reduce CPU time in analysis procedure, and finite element analysis was performed to determine the constraint parameters at each iteration step in optimization loop. Optimization results were compared with an existing ship, and the effects of optimum design were examined from various points of view.
Jacket Type Offshore Structure Pile-Soil Interaction Modeling Using Fiber Elements
Authors: Asgarian B. , Haghshenas S. A. , Soltani R. H.
Published: November 2008

In this paper a nonlinear fiber elements is used for modeling of pile soil intraction. In the model, both of steel pile and surronding soil nonlinear behavior is considered using fiber element. In this paper the model is developed using DRAIN-3DX software. The method used in this paper, however, allows pile and surronding soil inelastic behavior to be modeled accurately using a single elements. The model is used to simulate nonlinear behavior of pile -soil system and the results are compared with the other analytical and available experimental results. The lateral capacities of offshore piles can be calculated using methods presented in this paper. The analysis results using method presented in this paper in terms of pile head load deformation, pile lateral capacity and pile internal forces are in a good agreement with the other available analytical or experimental results. The model can be used for the pile soil structure interaction analysis of jacket type offshore structures.
Wave Forcing and Wave Scattering From a Vertical Surface-Piercing Cylinder
Authors: Swan Chris , Masterton Stephen , Sheikh Rizwan , Cavalletti Alessandra
Published: November 2008

This paper concerns the nonlinear, higher-harmonic, wave-forces acting on a vertical surface-piercing cylinder. New laboratory data is presented which confirms that in the case a vertical cylinder, the diameter of which is large but not sufficiently large that the body lies within the linear diffraction regime, the second- and third-harmonic forces are not well described by existing models. This is particularly apparent when the incident waves are steep and have a relatively small wave period. Indeed, under these conditions the second-, third- and fourth-harmonic forces are shown to be comparable in size. This is clearly at odds with the results of a traditional perturbation expansion. An explanation for this lies in the nature of the scattered wave field, particularly the high-frequency waves identified by Sheikh & Swan [1]. The phasing of these scattered waves are, at least in part, dependent upon the motion of the fluid around the circumference of the cylinder and will not therefore be captured by a series solution based solely on the harmonics of the incident wave motion. The paper considers several test cases, fully exploring the correlation between the nonlinear forcing and the high-frequency scattering. The practical implications of these results are also addressed.
Studies in the Stability of a Numerical Wave Tank Model for Generation and Propagation of Steep Nonlinear Long-Crested Surface Waves
Authors: Parsons W. , Baddour R. E.
Published: November 2008

We are studying numerically the problem of generation and propagation of gravity long-crested waves in a tank containing an incompressible inviscid homogeneous fluid initially at rest with a horizontal free surface of finite extent and of infinite depth. A non-orthogonal curvilinear coordinate system, which follows the free surface is constructed which gives a realistic “continuity condition”, since it tracks the entire fluid domain at all times. A depth profile of the potential is assumed, and employed to perform a waveform relaxation algorithm to decouple the discrete Laplacian along dimensional lines, thereby reducing it’s computation over this total fluid domain. In addition, the full nonlinear kinematic and dynamic free surface boundary conditions are utilized in the algorithm, and a suitably tuned numerical beach is used to avoid reflections. It is well known that instability, in the form of generated spurious “sawtooth waves”, plagues this problem, leading to numerical overflow. This makes it very difficult to generate steep waves for sufficiently long simulation times. The authors have struggled with this problem for some time, with significant success, by employing an “aliasing filter”. This paper outlines our ongoing study of the stability of the model, including an analysis of the possible nature of the underlying causes including compatibility conditions. We conclude by giving a simple practical technique for greatly improving the stability.
Reliability of TLP Tendons Under Storm Sea States
Authors: Cicilia Federico Barranco , Prates de Lima Edison Castro , Sudati Sagrilo Lui´s Volnei
Published: November 2008

This paper presents a methodology for reliability analysis of Tension Leg Platform (TLP) tendons subjected to extraordinary sea state conditions like hurricanes or winter storms. A coupled approach in time domain is used to carry out TLP random nonlinear dynamic analysis including wind, current and first and second order wave forces. The tendons Ultimate Limit State (ULS) condition is evaluated by an Interaction Ratio (IR) taking into account dynamic combination among tension, bending and hydrostatic pressure. Expected long-term extreme IR is obtained through the integration of cumulative probability functions (CPFs) fitted to response maxima associated to individual short term sea states. The reliability analysis is performed using a time-integrated scheme including uncertainties in loads, tendon strength, and analytical models. Failure probabilities for the most loaded tendon of a TLP in Campeche Bay, Mexico, considering a 100-yr design sea state and the 100-yr extreme response generated by long-term observed storms are compared.
The Long-Term Wind and Wave Spectra of the Mediterranean Sea
Authors: Stefanakos Christos N.
Published: November 2008

It is a well-known fact that long-term time series of wind and wave data are modelled as nonstationary stochastic processes with yearly periodic mean value and standard deviation (periodically correlated or cyclostationary stochastic processes). Using this model, the initial nonstationary series are decomposed to a seasonal (periodic) mean value m(t) and a residual time series W(t) multiplied by a seasonal (periodic) standard deviation s(t), of the form Y(t) = m(t) + s(t)W(t). The periodic components m(t) and s(t) are estimated using mean monthly values, and the residual time series W(t) is examined for stationarity. For this purpose, spectral densities of W(t) are obtained from different seasonal segments, calculated and compared with each other. It is shown that W(t) can indeed be considered stationary, and thus Y(t) can be considered periodically correlated. This analysis has been applied to model wind and wave data from several locations in the Mediterranean Sea. It turns out that the spectrum of W(t) is very weakly dependent on the site, a fact that might be useful for the geographic parameterization of wind and wave climate.