bluuebunny/crossref_metadata_2025 huggingface dataset:
Deformation Capacity of Induction Bends
Authors: Bruschi Roberto , Barbas Serghios , Mandke Jay , Bartolini Lorenzo , Vitali Luigino
Published: October 2008

In the last two decades, reliability methods have been extensively used to calibrate rationally-based Load and Resistance Factored Design (LRFD) equations for the design of offshore and onshore pipelines. Several experimental and theoretical studies have been carried out with the aim to assess the strength and deformation capacity of induction bends, when buried and subject to severe and extreme loading conditions such as the ones caused by an earthquake crisis. Design criteria for induction bends do not reflect the R & D efforts of the recent years and working stress design in generally accepted by the Industry. The scope of this paper is: • Review the relevant literature on bends discussing analytical solutions, numerical analyses and experimental tests carried out aiming to predict the limit loads/deformations of induction bends and to define design criteria for induction bends. • Discuss the ABAQUS Finite Element Model (FEM) developed; • Present the FEM development, calibration and validation; • Show the parametric study results considering relevant parameters including bend geometry (wall thickness and bend angle), fabrication tolerances (thickness variation, bend cross section etc.), steel material characteristics (hardening and shape), and loading conditions (inner pressure, steel axial force, bending moment under closing and opening mode).
Qualitative and Quantitative Validation of a Numerical Code for the Realistic Simulation of Various Ship Motion Scenarios
Authors: Hennig Janou , Billerbeck Heike , Clauss Gu¨nther F. , Testa Daniel , Brink Kay-Enno , Ku¨hnlein Walter L.
Published: October 2008

There is an ongoing discussion on safety guidelines to be considering more recent developments in ship design. Numerical simulations of ship motions are considered as powerful tool for the safety evaluation of a given design. However, the consequent use of numerical codes calls for their thorough validation which has to be performed both qualitatively and quantitatively. This paper focuses on a code used and further developed by the Flensburg Shipyard. For its validation, the capsizing scenario in steep wave sequences is realized in the wave tank first. The dedicated computer controlled experimental technique ensures the exact phase correlation of wave excitation and resultant ship motions. Thus, the registered wave and the track of the ship model in the model test serve as input to the numerical simulation which results in the specific motion time traces. These are now directly compared to the motion registrations from the model tests. First results of the validation by direct comparison of time series have been presented in earlier publications, still with the restriction that only a few cases have been investigated. In this paper, the promising method is applied to another scenario in a long-crested sea state including steep wave combinations. Different aspects are discussed which results in the conclusion that the method is feasible for free running ships in stern and stern quartering seas.
Frequency Proximity for Multi-Mode VIV Interaction
Authors: Ilstad Trygve , So̸reide Tore , Nielsen Finn Gunnar
Published: October 2008

Uneven seabed and seabed intervention may introduce free-spans in pipelines where interaction between VIV responses of several modes occurs, including longer single spans and moderate length multiple spans. A series of model tests were performed within the Ormen Lange project to study the phenomenon of multi-mode behaviour. While for a beam controlled single span the modes are well separated, the multi-span modes are closer in frequency, and a clear reduction of response amplitude is observed. For single mode response the maximum cross flow amplitude may be 1.3 times the pipeline diameter, whereas in the case of mode interaction the amplitude is restricted to below 0.8 of diameter. To benefit of the test results in detail design a general procedure is implemented into fatigue analysis where frequency proximity is implemented as a criterion for deciding cross flow amplitude. In the case of cross flow modes occurring with small frequency distance, as for multiple spans and for longer single spans, the response amplitude in cross flow direction is reduced from the maximum 1.3 of diameter. The use of frequency as a parameter instead of mode shape evaluation procedure has proven simple to implement into existing fatigue analysis schemes. The paper demonstrates the applicability of the updated analysis scheme and comparisons are made with the test cases where predicted curvatures are compared with measured data for cross flow responses. Similar analyses are also performed by procedures based on existing guideline procedures, and the effect on allowable free spans from the present update is illustrated.
Collision Risk Analysis for Offshore Structures and Offshore Wind Farms
Authors: Povel Daniel
Published: October 2008

For the approval process of offshore platforms and offshore wind farms, risk analyses that consider collisions between ships and stationary offshore structures are helpful and sometimes necessary. Ship traffic, meteorological and hydrological data are important input information for the investigated sea area. Possible collision scenarios with powered ships sailing into the structures and disabled ships drifting into the structures have to be investigated to determine the collision probabilities and the consequences. Furthermore, risk control measures have to be examined and assessed in such risk analyses. In recent years, Germanischer Lloyd has developed and applied suitable methods and procedures to evaluate the collision risk, including the influence of risk control measures. For these analyses, Germanischer Lloyd developed an analysis software that uses Monte Carlo simulations. For the evaluation of different risk control measures, Bayesian nets are used to determine the reduction of the collision probabilities for powered and drifting vessels and the corresponding risk. In this paper, the methods and processes that were developed are presented together with results of a sample collision risk analysis. Moreover, experience in the practical application of collision risk analyses is discussed, as part of the approval process for offshore wind farms.
Numerical Reproduction of Extreme Wave Loads on a Gravity Wind Turbine Foundation
Authors: Bredmose H. , Skourup J. , Hansen E. A. , Christensen E. D. , Pedersen L. M. , Mitzlaff A.
Published: October 2008

A fully nonlinear 3D Navier Stokes solver with VOF (Volume of Fluid) treatment of the free surface is used to reproduce two extreme laboratory wave impacts on a gravity wind turbine foundation. The wave climate is irregular waves with a current. Numerical results for inline force, overturning moment and run-up are compared to measurements. The extreme wave loads for the two events are associated with slamming onto the under side of a horizontal platform placed 9.1m above the still water level. For such impacts, the computed wave loads are strongly sensitive to the shape of the incoming waves. A comparison with a Morison-type estimation of the wave loads shows that this much simpler approach can reproduce the overall trend of the wave load history, but not the extreme moment.
PE-RT: A New Class of Polyethylene for Industrial Pipes
Authors: Schramm Detlef
Published: October 2008

The development of a new family of PE materials with significantly improved processability and long term strength at high temperatures is discussed. These polymers form the basis for a new ISO class of polyethylene materials: PE-RT (Polyethylene of Raised Temperature resistance) for hot and cold water as well as industrial pipe applications. These materials have a unique molecular structure and crystalline microstructure, which provides excellent Long Term Hydrostatic Strength at high temperature without crosslinking the material. PE-RT type materials are successfully used in domestic hot and cold water piping systems for most applications. The easy processing and outstanding material properties made these resins also attractive for use in many industrial applications, where larger dimensions are required and regular Polyethylene cannot be used or has temperature limitations. They also compete against high end engineering plastics, offering significant cost savings. These materials provide significant process advantages to the converters, allowing high line speed pipe production and providing excellent flexibility and ease of installation for the application. A recently developed PE-RT type material offers still higher long-term strength at high temperature and further improved processability. This combination makes this resin particularly suitable for high temperature applications. An example of the latter is in larger diameter cooling water pipes in power plants. Pipes based on these materials can be connected via heat welding or by the use of mechanical fittings. Furthermore this material can be used in industrial applications, were traditional Polyethylene is limited by the temperature resistance and metallic materials suffer from corrosion. The excellent weldability of these materials provides various opportunities to connect also larger dimensions in industrial applications. Another example of this is the use in multi-layer structured oil pipelines on-shore and off-shore. This paper presents the material science and product design concepts that govern the high long-term hydrostatic strength at high temperatures. By controlling the molecular structure, the melt rheology and solid state properties can be influenced. This results in a unique balance of processability and hydrostatic strength. Further discussed are the product features and benefits of PE-RT materials. The paper shows examples of the application range for this type of products, using applications in the domestic pipes market as a reference study.
Collisions of Ships and Offshore Wind Turbines: Calculation and Risk Evaluation
Authors: Biehl Florian , Lehmann Eike
Published: October 2008

In Germany and other European countries, a large number of offshore wind parks will be installed within the next decade. Even though this development is promoted by governmental agencies in order to achieve positive effects such as installing a sustained energy and resource management, risks such as pollution due to collisions of tanker vessels with offshore wind energy conversion systems may cause the acceptance of wind energy to decrease. After a collision oil or other harmful substances may be spilled into the sea. It might even be possible for a ship to break apart and sink in a worst case scenario that includes failure of the bearing of the installed nacelle and subsequent impact onto the ship. Numerical crash tests were carried out in order to determine worst cases and to give hints to improve different offshore wind turbine support structures. In combination with statistical data and determination of probabilities of the occurrence of different scenarios, safety assessment analysis can be carried out by certification agencies and administrative authorities.
Generation of Rogue Waves With Predefined Steepness
Authors: Clauss Gu¨nther F. , Schmittner Christian E. , Klein Marco
Published: October 2008

For the deterministic investigation of cause-reaction relationships and for analyzing structure responses due to special wave scenarios methods for the precise generation of tailored wave sequences are required. In this paper we present an optimization approach for the experimental generation of wave sequences with defined characteristics, in particular with predefined wave steepness. Firstly, target parameters, such as wave crest front steepness, wave height and wave period at a particular time are defined for a wave group. Starting with a random phase distribution in frequency domain the phases are optimized in order to satisfy the target parameter in time domain. From this optimized target wave sequence a control signal based on linear wave theory for the wave generator is derived and the wave sequence is registered in a physical wave tank. As nonlinear effects like wave-wave interaction and wave breaking are insufficiently covered by the generation process the registered wave train may differ from the target parameters defined. To improve the accuracy of the measured wave sequence at target location the control signal is iteratively improved by a fully automated optimization process, controlling the wave generator, measuring and analyzing the created waves and modifying the control signal. As a result of the optimization process, a control signal for a wave train, satisfying all target parameters is obtained. The method is applied to generate a breaking rogue wave with defined wave crest front steepness superimposed to irregular seas.
Fully Nonlinear Energy Flux Calculations for Long-Crested Surface Waves
Authors: Parsons W. , Baddour R. E.
Published: October 2008

We employ a fully nonlinear numerical model to generate and propagate long-crested gravity waves in a tank containing an incompressible invicid homogeneous fluid, initially at rest, with a horizontal free surface of finite extent and of finite depth. A non-orthogonal curvilinear coordinate system is constructed which follows the free surface and is “fitted” to the bottom topography of the tank and therefore tracks the entire fluid domain at all times. A waveform relaxation algorithm provides an efficient iterative method to solve the resulting discrete Laplace equation, and the full nonlinear kinematic and dynamic free surface boundary conditions are employed to propagate the solution. In addition, a monochromatic deterministic theoretical wave-maker, employing a Dirichlet type boundary condition, and a suitably tuned numerical beach are utilized in the numerical model. We first show that the model generates the appropriate dispersion relation for both the deep water and shallow water conditions. Subsequently, we calculate the energy and energy flux for small steepness waves and show that the model produces the expected linear results. We complete the paper by considering steeper waves where the linear results are suspect.
Service Life Reduction of a Top Tensioned Vertical Riser and VIV Effect
Authors: da Costa Matt Cyntia G. , Franciss Ricardo , Morooka Celso K. , Coelho Fabio M.
Published: October 2008

In the present paper, a brief introduction of the fundamentals of TTR dynamic behavior is provided along with service life estimation utilizing numerical simulations in time domain. The simulation of riser motions due to environmental loads, ocean waves and marine current, was carried out along with bending moment and stresses. In-line and transverse riser behavior will also be considered. Vortex induced vibration (VIV) in risers due to waves and current is modeled. Riser service life calculations are shown with and without the influence of VIV. Lateral motions and offset at the top of the riser due to the surface floating production unit also are considered in the simulation. Time series from numerical simulations are presented for different environmental conditions with explanation regarding the impact on riser service life.
Efficient Analysis of a Catenary Riser
Authors: Passano Elizabeth , Larsen Carl M.
Published: October 2008

The paper deals with the challenge of predicting the extreme response of catenary risers, a topic of both industry and academic interest. Large heave motions introduced at the upper end of a catenary riser can lead to compression and large bending moments in the region immediately above the touch down area. In the worst case, dynamic beam buckling may occur. The focus of the paper will be on understanding the riser behaviour in extreme, low-tension response and in establishing suitable analysis strategies to predict the extreme response. Results from long nonlinear stochastic simulations of many sea states with varying environmental and operating conditions may be combined to describe the long-term response of a nonlinear structure such as a catenary riser. However, this theoretically straight-forward approach is very demanding computationally and ways to limit the extent of nonlinear stochastic simulations are therefore sought. The usefulness of simpler methods such as regular wave analysis to improve understanding of the physical behaviour and to aid in concentrating the nonlinear simulations to where they are most useful, will be demonstrated.
An Investigation of the Fatigue Performance of Riser Girth Welds
Authors: Maddox Stephen J. , Speck Julian B. , Razmjoo G. Reza
Published: October 2008

Increasing deep-water oil and gas recovery has highlighted the need for high integrity, high fatigue performance girth welds in steel catenary riser systems. Such systems include girth welds made from one side. However, the widely used fatigue design classification, UK Class F2, for such welds is not well founded, but probably over-conservative for pipeline welds. In an attempt to justify upgrading current fatigue design classifications and providing a better basis for design, fatigue tests were performed on a range of girth-welded pipes produced by pipeline welding contractors. This paper presents the results of those tests and their evaluation in terms of the factors that influence the fatigue performance of girth welds, including welding process, welding position, backing system, joint alignment, weld quality, specimen type and fatigue loading conditions. Conclusions are drawn regarding the scope for adopting higher design classifications and the conditions that must be met to justify them.
Squall: Nightmare for Designers of Deepwater West African Mooring Systems
Authors: Legerstee Franck , Franc¸ois Michel , Morandini Ce´dric , Le-Guennec Ste´phane
Published: October 2008

Squalls have been present in the environmental specifications for floating units in West Africa for the last couple of years. However it appears that such phenomena tend to be the designing factor for mooring systems of deepwater FPSO’s (in spread or turret configuration) and offloading buoys. At that stage, due to the lack of proper modelling/characterisation, squalls tend to be represented for design purposes by on-site recorded time series of varying wind velocity and associated relative headings applied from any direction. This leads to rapid changes in offsets and loads in the mooring lines induced by the transient response of the vessel to sudden load increase generated by such squall signal. Through diverse exemplary simplified calculations, this paper illustrates the influence of the consideration of squalls in the design process, together with the present shortcomings in the modelling process, either in terms of extreme conditions, or in terms of operating conditions, knowing that such events are difficult to forecast. In addition the effect of tugs, and associated operating limitations are also discussed. Areas needing further investigation are finally identified.
Vortex-Induced Vibration Tests of Two Faired Cylinders in Tandem at Prototype Reynolds Numbers
Authors: Lee Li , Allen Don W. , Henning Dean L.
Published: October 2008

This paper presents and discusses vortex-induced vibration (VIV) test results for two faired flexible cylinders in tandem at three spacings (5, 10, and 20 cylinder diameters), which were subjected to uniform flows. It starts with a description of the test facility, test setup, data acquisition, and data processing. It then presents and discusses the VIV responses and motion of both cylinders. Conclusions on the fairing performance are drawn at the end of this paper.
High Reynolds Number Flow Tests of Flexible Cylinders With Helical Strakes
Authors: Allen Don W. , Henning Dean L. , Lee Li
Published: October 2008

Tow tests have been performed on flexible circular cylinders, with and without short helical strakes, towed in a basin at critical and supercritical Reynolds numbers. The tests were conducted at the Naval Surface Warfare Center’s David Taylor Model Basin in Carderock, Maryland. Measurements were made of both the drag and acceleration (due to vortex-induced vibration) of the cylinder. A 3-1/2 inch diameter ABS pipe was used to achieve Reynolds numbers ranging from about 2×105 to 5×105, and a 5-9/16-inch diameter PVC pipe was used to achieve Reynolds numbers ranging from about 7×105 to 1.5×106. Tests were also conducted with aluminum inserts (strong-backs), made to fit just inside the test cylinders, in order to obtain stationary (rigid) cylinder drag measurements for comparison purposes. The test results for cylinders fitted with triple-start helical strakes are presented in this paper.
Steady-State and Transient Dynamical Systems Analysis of Uncoupled and Coupled Very Large Amplitude Roll Response of a Vessel in Regular and Random Waves
Authors: Falzarano Jeffrey M. , Vishnubhotla Srinivas , Juckett Sarah E.
Published: October 2008

This paper contains the most recent results of our analysis of the DTMB 5415 model hull. This hull is an early stage version of the US Navy’s DDG-51 and approximately represents the last traditional destroyer hull form designed, built and operated by the US Navy. Moreover, the US Navy has recently proposed an alternative simulation based approach to traditional static stability analysis for advanced hull forms. Static stability analysis has been in use by the US Navy for at least 50 years. This paper proposes an alternative analysis technique which uses modern techniques of dynamical systems to analyze the large amplitude (nonlinear) roll response of a vessel in waves. The technique considers the steady state (local) roll response, the transient (global) roll response in both regular and random waves. Moreover, the effects of coupling, damping and reduced GM are all investigated.
A Reliability-Based Comparative Study for Mooring Lines Design Criteria
Authors: Vazquez-Hernandez Alberto Omar , Ellwanger Gilberto Bruno , Sagrilo Lui´s Volnei Sudati
Published: October 2008

The characteristic load effect for the design of mooring systems can be defined by means of three procedures: 1) an extreme sea state with a given return period, 2) a set of sea states on a contour line associated to a return period or 3) extreme response (tension) statistics for a long-term period. This work presents the result of a reliability-based partial safety factor calibration study for a LRFD mooring line design criteria considering the three approaches mentioned above. The calibration exercise is applied to three FPSOs considering North Sea environmental conditions and different water depths: 200m, 800m and 3000m. The mooring systems investigated take into account lines made up of chains and polyester ropes. It is shown that the design procedure based on the long-term response, among all water depths investigated, is the one that presents less scattered reliability indices around the target level.
Optimization of Sensor Placement to Capture Riser VIV Response
Authors: Natarajan Shreenaath , Howells Hugh , Deka Dhyanjyoti , Walters David
Published: October 2008

Riser VIV response due to ocean current loading is a complex phenomenon governed by both the hydrodynamic and structural properties. In order to obtain better understanding of the global riser VIV response and assist in the improvement of riser VIV design, riser monitoring is being increasingly used. An optimization technique to identify the number of sensors required and the sensor locations for monitoring riser VIV fatigue is presented. The optimization technique has been developed using modal decomposition and linear regression. The paper explains why monitoring at selected locations with limited instrumentation is sufficient to capture global riser response. The principles and methods of using multiple measurement quantities in the optimization technique are also presented along with the adopted methodology, limitations and key conclusions.
Development of a Weather Adaptive Navigation System Considering Ship Performance in Actual Seas
Authors: Tsujimoto Masaru , Tanizawa Katsuji
Published: October 2008

Aiming to reduce fuel consumption and CO2 emission a new navigation system called WAN has been developed. The system provides optimum route and engine revolution with constraints of schedule and seakeeping criteria. In this paper, simulations of a container liner on transpacific route are demonstrated using WAN. Weather data used here are composed of 8 items; significant height, period and peak direction of wind wave, significant height, period and peak direction of swell, and mean speed and direction of wind. Ship responses; i.e. ship speed, fuel consumption and vertical acceleration at F.P., are calculated based upon the enhanced unified theory. To optimize route and engine revolution the augmented Lagrange multiplier method is applied. The objective function of minimization is fuel consumption with constraints of the schedule and the service limit. From the simulations, the effectiveness of WAN resulted very high and it is shown that the reduction of the fuel becomes 26.1% on average. Concerning the schedule keeping, accuracy of weather forecast must be examined. The influence on the system is evaluated using two kinds of weather data; one is a forecast received at departure and the other is the dataset extracted from the sequential forecasts of every 24 hours. From the simulations it is found that the fluctuation of fuel consumption due to updating the weather forecast is much smaller than the reduction of the fuel by WAN. However, from the viewpoint of ship safety, it is necessary to execute the system again whenever the weather forecast updated.
Submarine Pipeline Installation JIP: Strength and Deformation Capacity of Pipes Passing Over the S-Lay Vessel Stinger
Authors: Torselletti Enrico , Vitali Luigino , Levold Erik , Mo̸rk Kim J.
Published: October 2008

The development of deep water gas fields using trunklines to carry the gas to the markets is sometime limited by the feasibility/economics of the construction phase. In particular there is a market for using S-lay vessels in water depth larger than 1000m. The S-lay feasibility depends on the applicable tension at the tensioner which is a function of water depth, stinger length and stinger curvature (for given stinger length by its curvature). This means that, without major vessel up-grading and to avoid too long stingers that are prone to damages caused by environmental loads, the application of larger stinger curvatures than presently allowed by current regulations/state of the art is needed. The work presented in this paper is a result of the project “Development of a Design Guideline for Submarine Pipeline Installation” sponsored by STATOIL and HYDRO. The technical activities are performed in co-operation by DNV, STATOIL and SNAMPROGETTI. The scope of the project is to produce a LRFD (Load Resistant Factor Design) design guideline to be used in the definition and application of design criteria for the laying phase e.g. to S and J-lay methods/equipment. The guideline covers D/t from 15 to 45 and applied strains over the overbend in excess of 0.5%. This paper addresses the failure modes relevant for combined high curvatures/strains, axial, external pressure and local forces due to roller over the stinger of an S-lay vessel and to sea bottom contacts, particularly: • Residual pipe ovality after laying, • Maximum strain and bending moment capacity. Analytical equations are proposed in accordance with DNV OS F101 philosophy and design format.
Equilibrium Profile of Current-Induced Scour Around Submarine Pipelines
Authors: Mousavi Mir Emad , Bakhtiary Abbas Yeganeh , Enshaei Nastaran
Published: October 2008

Submarine pipelines installed on seabed disturb the local flow field and bring imbalance in local sediment transport that leads to scouring. Local scouring can leave the pipeline unsupported in significant distances. The free spans of the pipeline are susceptible to damages arising from environmental forces or human activities. This paper presents results of experimental investigations on the equilibrium profile of the local scouring around the pipe due to unidirectional current. New equations are proposed for the prediction of the scour depth and width before installation of the pipe. It is evident from the experimental results that the equilibrium depth of the current-induced scour is a function of pipe diameter and Shields number. Also scour width at upstream and downstream of the pipe and the total width of the scour are investigated. The results are simplified in the form of diagrams for prediction of the scour depth/width in practice based on flow velocity, bed roughness condition and size of the granular materials. The results can be used either for the design purposes or for the calibration of numerical models. One of the results of this work is that the scour width at downstream of the pipe is about 2.5 times of the width at upstream.
Safety Policy as a Tool in New Technology Offering: A Flow Assurance Case
Authors: Munier Bertrand , Besse Pierre , Lassagne Marc
Published: October 2008

Flow Assurance technologies are currently experiencing steady progress. Most new technologies are in many ways riskier than older ones, but they are also more efficient. It would be optimal for offshore operators ready to buy some novel technology to select the right offer, given their respective safety policies, from all existing possibilities. But operators hardly know of all these possibilities; the contractors who develop new technology do. But how could the latter optimally select the design option which should be offered to a given operator, to best serve its needs? The present paper deals with this question by connecting technical possibilities with company strategies in an integrated and quantified model: the output data of a physical model are entered as input into a psycho-management risk model. As a result, one can compute optimal characteristics for each operator of a new technology. This helps the contractor sell and monitor its offer under uncertainty. It also induces the operator to buy the innovative technology. We use some specific software developed at GRID (Group of research on Risk, Information and Decision, affiliated to ENSAM) to assess safety policies of operators. A real case is analyzed and presented. We argue that our model produces insight, by revealing knowledge which officers of the contractor company in long lasting contact with a given operator have internally accumulated, but which they are not able to express under any operational format. Such knowledge is ignored by industrial engineering, but often turns out to be richer than what can be extracted from any available data. One difficulty is to manage the software’s session in such a way that officers will answer the questions. The other difficulty is to convince engineering and risk managers that the “Human factor” is much richer than it is given credit for in textbooks and even more accurate than most existing data. Thus, we substantially improve existing industrial management techniques as currently applied to offshore technology, and more generally existing practice in the industry.
Wave-Induced Fatigue Damage of Mooring Chain Under Combined Non-Gaussian Low and Wave Frequency Loads
Authors: Gao Zhen , Moan Torgeir
Published: October 2008

This paper deals with the fatigue damage of catenary mooring chain under combined low frequency (LF) and wave frequency (WF) loads which are both non-Gaussian. The distribution of quasi-static LF mooring line tension is obtained by representing the slowly-varying wave force acting on the vessel as a second-order Volterra series and by taking into account the nonlinear relationship between the vessel motion and the resulting line tension. The dynamic WF tension distribution is estimated when both the drag and inertia forces on the mooring line are considered. Finally, with the narrow-band assumption for both the LF and WF response spectra, the total fatigue damage is approximated by the sum of the fatigue damages due to the WF tension process and the sum process of the WF tension envelope and the LF tension. The procedure is illustrated by considering of the mooring system of a semi-submersible. Short-term distributions of LF and WF line tensions are given and compared with the results from the linearized analyses with Gaussian tension. Relative LF and WF long-term contributions to the total fatigue damage from different sea states are also provided.
A Proposal for the Control of Teams of Autonomous Underwater Vehicles (AUVs) in Hierarchical and Peripheral Modes Using the Concept of Adaptive Autonomy
Authors: Glotzbach Thomas , Wernstedt Ju¨rgen
Published: October 2008

In this paper we discuss the further development of approaches for the control of vehicles with Adaptive Autonomy that have already been successfully used in single Autonomous Underwater Vehicles (AUVs) for cooperating teams of AUVs. A hierarchical control approach for a single AUV based on the Rational Behaviour Model (RBM) is presented. After the explanation of the concept of Adaptive Autonomy, this concept will be used to transfer the RBM- approach for single AUVs into another which can be used for the control of teams of AUVs. This new concept will contain a software task called ‘team instance’ that is responsible for the realisation of the cooperation between the vehicles. Finally, two concepts for the realisation of the ‘team instance’ are discussed and compared with each other, referring to possible real missions with teams of AUVs.
CFD Simulation of Truss Spar Vortex-Induced Motion
Authors: Atluri Sampath , Halkyard John , Sirnivas Senu
Published: October 2008

Helical strakes are used to suppress the Vortex-Induced Motion of Truss Spars. Model experiments have demonstrated the efficiency of strakes in the Truss Spar design but also indicate that the VIM response is sensitive to the details of strake design and placement of appurtenances around the Spar hull. It is desirable to study these hydrodynamic effects using CFD. The following paper is a continuation of some of the earlier CFD simulations on this subject (see, J. Halkyard, et al., “Benchmarking of Truss Spar Vortex-Induced Motions Derived from CFD with Experiments”, Proceedings of OMAE’05). This paper in particular deals with the effect of holes in the strakes and appurtenances and their placement. All the simulations were done at model scale (1:40 scale model of an actual Truss Spar design) to compare the motions with experimental results. Mesh sensitivity and turbulence modeling issues are also discussed. Calculations were done using general purpose CFD code Acusolve™.
Whipping Response of Vessels With Large Amplitude Motions
Authors: Fonseca Nuno , Antunes Eduardo , Guedes Soares Carlos
Published: October 2008

The paper presents a time domain method to calculate the ship responses in heavy weather, including the global structural loads due to whipping. Since large amplitude waves induce nonlinear ship responses, and in particular highly nonlinear vertical structural loads, the equations of motions and structural loads are solved in the time domain. The “partially nonlinear” time domain seakeeping program accounts for the most important nonlinear effects. Slamming forces are given by the contribution of two components: an initial impact due to bottom slamming and flare slamming due to the variation of momentum of the added mass. The hull vibratory response is calculated applying the modal analysis together with direct integration of the differential equations in the time domain. The structural dynamic characteristics of the hull are modeled by a finite element representation of a Timoshenko beam accounting for the shear deformation and rotary inertia. The calculation procedure is applied to a frigate advancing in regular waves. The contribution of whipping loads to the vertical bending moments on the ship structure is assessed by comparing this response with and without the hull vibration.
Buckle Propagation in Pipelines Under Non-Uniform Pressure
Authors: Talebpour R. , Abedi K. , Gharebaghi A. R. M.
Published: October 2008

Preventing the occurrence of local buckling due to external pressure is one of the main concerns in design of offshore pipelines. However, when a pipeline is designed for deepwater, prevention of the propagation of local buckling along the pipeline has more importance. Therefore, the study of buckle propagation phenomenon and its prevention is a subject of many researches in the past 25 years. Great amount of these researches have focused on appropriate estimation of buckle propagation pressure. In this paper, details of 3-D finite element modeling for buckle propagation simulation are outlined. In order to verify the accuracy and validity of the finite element modeling, the numerical results, obtained from nonlinear finite element analysis have been compared with the results of the experimental study on full-scale models, undertaken by C-FER Technologies, Canada, which have been published by Toscano et al. (2002). Comparison shows that the finite element results have very close agreement with the experimental behaviour. Also, in the present paper, numerical results of Toscano et al. are discussed; and the study shows that the proposed method, outlined in this paper, gives more appropriate results than the proposed method by Toscano et al. In addition, the buckle propagation of pipeline under the uniform external pressure as well as non-uniform pressure, due to the presence of the internal flow (when the pipe is not full of fluid) is studied. The change of buckle propagation pressure is determined by the proposed method.
Ultimate Strength of Stiffened Panels Assesment Taking Into Account Model Uncertainty
Authors: Toderan C. , Richir T. , Caprace J. D. , Rigo Ph.
Published: October 2008

This paper presents a methodology to take into account model uncertainties related to the load-end shortening curve of stiffened panels. This method is a part of a research activity carried out at University of Liege. His goal is to propose a reliability based model to assess hull girder ultimate strength using a progressive collapse algorithm. The numerical results presented here concern the load-shortening model chosen for this research, which is based on Bureau Veritas rules. Model uncertainty is quantified as a parameter considered as a random variable. Four statistical moments (mean, standard deviation, skewness and kurtosis) of this parameter are calculated and analyzed using a data-base of stiffened panels test results published in recent years. In order to increase the number of “actual” values for the statistical assessment, the data-base is completed with non-linear finite element analysis results. The paper contains also some recommendation for the implementation of the proposed method in ultimate strength reliability based analysis.
A Methodology for In-Line VIV Analysis of Risers in Sheared Currents
Authors: Sidarta Djoni E. , Lambrakos Kostas F. , Thompson Hugh M. , Burke Roger W.
Published: October 2008

As exploration and production move to even deeper water and more severe environment, the need to have a methodology for analyzing risers for in-line VIV fatigue damage without undue conservatism increases. The methodology presented in this paper reduces the conservatism in available methods by accounting for (1) the power-in region, (2) the power-out region (hydrodynamic damping), (3) competing modal excitation in the case of multiple mode excitation, and (4) the multiple constraints, if available, in the riser that result in irregular modal shapes. This methodology requires the use of a cross-flow VIV code with sheared flow capability such as SHEAR7, VIVA, or VIVANA. In this methodology the riser over the current profile is split into sections of cross-flow excitation and sections that have potential for in-line VIV excitation only. The cross-flow VIV code defines the sections for cross-flow excitation. All sections are analyzed for in-line VIV with the cross-flow VIV code using the appropriate in-line VIV force coefficients and Strouhal numbers. The assumptions implicit in the cross-flow VIV code regarding power-in, power-out, etc., are assumed valid for the in-line VIV analysis. The in-line VIV coefficients used in the analysis reported in this paper have been obtained from laboratory data, and are functions of both the VIV response amplitude and reduced velocity. The coefficients have been modified to give in-line VIV response amplitudes with the methodology presented that are consistent with DNV-RP-F105. The fatigue damage along the riser represents the sum of the damages produced by in-line VIV excitation for each of the riser sections.
Methodology for Application of Model Test Data in TLP Global Design
Authors: Phadke Amal C. , Mansour Alaa M. , Huang Edward W.
Published: October 2008

Sophisticated frequency and time domain software tools are available for the global performance analysis of Tension Leg Platforms (TLP). Time-domain tools allow realistic simulation of the response of TLP systems compared to frequency-domain tools, but they are generally computationally intensive. Rapid advances in computer technology have made it possible to employ sophisticated time-domain techniques as primary tools for the global performance analysis of TLP systems. However, response characteristics such as higher-order tendon response, wave-runup, airgap etc. cannot still be accurately predicted using the available numerical tools. Wave basin model tests, therefore, are indispensable to designers for estimating responses that cannot be reliably predicted. At the same time, using model tests alone as an analysis tool is not practical due to large number of design cases typically defined in global performance analysis. It is necessary to verify and calibrate numerical tools using model test data prior to their application in global performance analysis. This paper describes a methodology for calibrating and correlating predicted response from time-domain software tools against wave basin model tests. The application of correlation data in conjunction with predicted response to obtain various design quantities of interest has been investigated. Discussion for determination of model test correlated design maximum/minimum tendon tension, higher-order tendon tension response, and incorporation of vortex induced motion is presented.
Design and Construction of the Planned Research Station Neumayer III in the Antarctic
Authors: Nitschke Andreas
Published: October 2008

The Federal Republic of Germany operates research stations on the Ekstro¨m Ice Shelf west of the Atka Bay in the Antarctic since 1981. Due to the annual accumulation of snow and thereby increasing pressures, sub-surface stations have a limited service life time of about 15 years. The planned new Neumayer Station III ought to allow for the increased demands of scientific research and for the grown environmental awareness. The design concept considers an elevated station body and a service garage underneath. A hydraulic mechanism allows an annual lifting of the station to compensate for snow accumulation and settlements. Thus, the projected service life time for the new station is about 30 years, twice as long as for the existing station. The weather conditions in the Antarctic call for high standards of safety and functionality of the applied techniques and are the determining parameters for design and manufacturing of the station.
Physical Modeling of Snow Drift and Wind Pressure Distribution at the Proposed German Antarctic Station Neumayer III
Authors: Leitl Bernd , Schatzmann Michael , Baur Tillmann , Koenig-Langlo Gert
Published: October 2008

Snow drift performance and wind pressure distribution was studied at scaled wind tunnel models of the new Antarctic research station Neumayer III. One objective of the project was to identify possible problems due to wind driven erosion and accumulation of snow around the station body to be constructed on the Antarctic shelf ice. Based on systematic wind tunnel testing including flow visualization experiments, wall shear stress visualization, flow measurements and physical modeling of wind erosion and snow drift, a comprehensive insight into the complex flow phenomena around the station was gained. In a second set of wind tunnel tests, wind pressure distributions were measured for the final station design in order to assist the structural design process. Both, snow drift modeling as well as the wind flow and wind pressure measurements at the station delivered relevant information integrated into the design process and the operational advice of the station.
Verification of a 3rd Generation FEM Spectral Wave Model for Shallow and Deep Water Applications
Authors: Roland Aron , Zanke Ulrich , Hsu Tai-Wen , Ou Shan-Hwei , Liau Jian-Ming , Wang Swun-Kwang
Published: October 2008

This paper shows some results of the work currently carried out to improve the wave forecasting and hindcasting in oceanic and coastal regions. A new spectral wave model with a flexible numerical scheme using triangular elements to describe the model domain was developed by Hsu et al. (2002). This new spectral wave model called WWM (Wind Wave Model) is feasible for the spectral wave modeling of irregular coastlines and complicated bathymetries because of its numerical scheme. The Wave Action Equation is solved with the aid of the Fractional Step Method (Yanenko, 1971). The Integration in the spatial space is carried out with the Taylor-Galerkin Method and the terms describing depth and current induced refraction are integrated with the aid of Leonard’s (1979) TVD Ultimate Quickest scheme, which was already introduced in the WWIII (H. Tolman, 1991) for the same purpose. In three applications the wave model was verified against in-situ spectral measurements of directional and non-directional wave buoys. The results show that the new spectral wave model is capable of hindcasting the wave climate with a comparable accuracy like the SWAN model (Ris et al., 1998), though with a better efficiency since fewer nodes are necessary to resolve the model domain and the boundary conditions adequately.
RiserView: An Open Source, Multiplatform Post-Processing Tool for the Visualization and Analysis of Riser Dynamics
Authors: Bernardes Joa˜o L. , Silveira Lauro M. Y. , Martins Clo´vis A.
Published: October 2008

Offshore oil exploitation, given its importance, is the target of intense research throughout the world. Much of this research generates large amounts of raw data of difficult interpretation. This paper presents RiserView, a free, open-source and multiplatform post-processing tool that merges virtual reality and scientific visualization techniques to allow a three-dimensional interactive visualization of these data for the specific domain of riser dynamics. This tool, its use, the results of performance tests and how the tool may aid in the analysis of riser dynamics, in view of the visualization tools in commercial riser analysis software, are discussed.
Non-Linear Random Wave Groups With a Superimposed Current
Authors: Nava Vincenzo , Arena Felice , Romolo Alessandra
Published: October 2008

In this paper a new solution for non-linear random wave groups in the presence of a uniform current is obtained, by extending to the second-order the Boccotti’s ‘Quasi-Determinism’ (QD) theory. The second formulation of the QD theory gives the mechanics of linear random wave groups when a large crest-to-trough wave height occurs. Here the linear QD theory is firstly applied to the wave-current interaction. Therefore the nonlinear expressions both of free surface displacement and velocity potential are obtained, to the second-order in a Stokes’ expansion. Finally some numerical applications are presented in order to analyze both the wave profile and the wave kinematics.