bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Analysis of Crack Interactions Using Weight Function Method
Authors: Rangasamy V. , Wang X. , Bell R.
Published: March 2009

A method for the analysis of interaction effect on stress intensity factors of two coplanar cracks using weight function method is presented. It provides more insight and understanding of the interaction behavior of cracks. It is demonstrated that the existence of the second crack will affect the first crack on both weight function and uncracked stress distribution. Application of the weight function method to analyze two coplanar through cracks and two coplanar semi-circular surface cracks are conducted. It is shown that for a wide range of distances between neighboring cracks, most of the interaction effect comes from the contribution of the uncracked stress distribution. Therefore, for engineering applications, the interaction effect can be accounted for from the uncracked stress distribution accurately. This provides a simplified method for the crack interaction analysis.
An Analysis of Practical Use Experience of Ultimate Hull Girder Strength Criterion and Some Features of Hulls Ductile Collapses
Authors: Egorov G. V. , Kozlyakov V. V. , Stankov B. N.
Published: March 2009

Follow-up to the survey report [1], containing in-depth study of this problem, basically, since 1965, in the present report the almost 100-year’s experience of usage of the ultimate plastic strength criterion (U.S.C) in the Russian and Ukrainian practice of design of hulls of ship of different destination is reviewed. On the basis of comparison of results of computational and experimental estimations of an ultimate bending moments (Mult) and characteristics of their variability the expediency of usage of the (Mult) lower boundary by Boobnov-Papkovich-Shimansky method with a more precise estimation of carrying capacity of longitudinals by the generalized method of C. Smith is substantiated. These recommendations may be used also for estimation of the hull ultimate strength of FPSO (Floating Production Storage and Off-loading Units), the structure of which is similar to the hulls of modern tankers with the double bottom and double sides.
Dimensioning of a Dynamic Positioning System for a Shuttle Tanker
Authors: Defilippo Rocha Stefania , Portella Ricardo B.
Published: March 2009

The increase of the offshore deep water production activities in Brazil based on the growth of the FPSO/FSO fleet, became the offloading operations a critical issue related to the possibilities of platform downtime. In order to improve the actual operational behavior, a new generation of shuttle tankers equipped with dynamic positioning systems is now under construction. The intent of this paper is to present the work performed for the dimensioning of the first generation of the DP Shuttle Tankers specifically projected to operate on Brazilian deep waters [1]. The system is intended to be installed in existing ships converted to operate as DP shuttle tankers.
Experimental Studies of Surface Wave Reflection by Longshore Bars and a Sloping Seawall
Authors: Dong P. , Wang Y.
Published: March 2009

Laboratory experiments have been performed to investigate the effects of a seawall on resonant reflection by a fixed periodic bar system on the seabed. The experiments were conducted in a wave flume. A series of five fixed bars was placed in front of a seawall with 1:1 slope. Regular waves with wave height 0.04m and wave periods from 0.8 to 1.20 seconds were used. The tests were performed for the cases with and without the seawall. The wave surface elevations were measured at seven locations in front of the seaward and in either side the bar patch. Based on the preliminary analysis of measured wave time series, it was found that the presence of the seawall has considerable influence on the reflection characteristics of the waves in front of the bar field.
Critical Compressive Strain of Linepipes Related to Workhardening Parameters
Authors: Suzuki Nobuhisa , Toyoda Masao
Published: March 2009

An approximate solution to predict a critical compressive strain at the peak load (the peak load strain hereinafter) of linepipes subjected to axial compression is proposed in this paper. The approximate solution is derived from the deformation theory applying the stress-strain relationship with non–linear hardening properties, which have not been taken into account in a number of current equations applied for pipeline design. The approximate solution proposed in this paper is a closed form equation, which is useful and effective for the practical application. The parameters in the Ramberg-Osgood representation expressing workhardenability of the linepipes are successfully introduced for the approximate solution. The effectiveness and accuracy of the approximate solution are verified comparing the critical compressive strains of several API 5L grade linepipes obtained by finite element analyses.
A Numerical Treatment of the Boundary Conditions for Stable Assessment of Hydrodynamic Impact Pressure
Authors: Cheng Liang-Yee , Arai Makoto
Published: March 2009

One of the critical points in the numerical assessment of hydrodynamic impact loads is accurate and stable descriptions of the pressures. Due to discrete approximations adopted by available numerical methods, unnatural violently-oscillating time histories of the impulsive pressure, caused by unstable or even non-converging numerical computation, are generally obtained. The aim of this paper is to investigate the unstable nature of the impulsive pressure computation. Also, a new numerical treatment of the boundary condition for accurate and stable assessment of the sloshing impact pressure based on rectangular grid system is proposed. The comparisons of the computed results with the experimental ones confirmed the accuracy of the proposed technique. The improvements achieved over the existing approaches are also shown herein.
Model Tests for the Validation of Extreme Roll Motion Predictions
Authors: Kuehnlein Walter L. , Brink K.-E.
Published: March 2009

At present common stability criteria are based on practical knowledge gained from the operation of ships. Therewith the assessment of ship safety against capsizing is partly determined by long-term statistics of accidents. Regulations like the IMO-Resolution A 167 do not rate the typical seakeeping characteristics of different hull form geometries. Therefore strictly speaking, these criteria are just applicable for ships of similar types as included in statistics. Rapid development in ship design calls for the determination of ship and cargo safety in regard of extreme roll motions or capsizing during early design stage. Within the ROLL-S project, which was founded by the German Federal Ministry of Education and Research, dynamic stability tests with a box shaped Container Ship and a RO-RO vessel have been performed. The performance of model tests, which are intended to serve for the validation of numerical simulation methods, put high demands on test and data acquisition techniques. The data of the waves encountered, course and position, as well as the response of the model had to be determined by model tests in order to use these data for the validation of numerical ship motion simulations. During the tests extreme roll motions of the two considered vessels could be observed in head seas and in following seas. Besides critical motion characteristics in following seas, like broaching, parametric induced roll motion effects were investigated in head sea condition. Remark: This paper should be read in conjunction with paper OMAE 2002-28297 which describes generation and transformation of the used waves.
Model Test and Analysis on Oil Skimmer at Rough Sea
Authors: Ono Shirou , Omori Hideyuki , Iwasaki Toru
Published: March 2009

In stormy weather on January 2, 1997, the Russian tanker NAKHODKA broke up at midship into two parts in the Sea of Japan just off the Japanese coast. The recovery operation was hampered by high waves, and the most serious problem was the lack of equipment capable of recovering the spilled oil in stormy weather. In 1998, in response to this accident, the Association for Structural Improvement of the Shipbuilding Industry (ASIS) initiated a three-year research and development project on large-scale recovery systems for collecting highly viscous oil with a minimum kinematic viscosity of 100,000 centi-stokes under severe sea conditions. This paper focuses on the design and evaluation of a newly developed oil skimmer system suitable for the recovery of highly viscous spilled oil emulsified under rough sea states. The oil skimmer system is composed of a float, pump, hose, oil/water separator, etc. The basic design of the oil skimming and treatment system has been completed incorporating the wave-response technology, highly viscous oil suction technology and highly viscous oil collection technology. Furthermore, synthesized model experiment was carried out in a water tank and oil basin using a large-scale model of a developed oil skimming system to acquire the performance data on the constituent technologies and to confirm synthetic function as an oil skimming and treatment system.
Structural Health Monitoring in Marine Structures
Authors: Burton Belinda , Verijenko Viktor
Published: March 2009

The reliability or safety of structures may be determined in a number of ways, but one of the most accurate must surely be by direct measurement of the stresses or strain incurred by the infrastructure. Structural Health Monitoring (SHM) as it has become known, is usually accomplished by means of sensor placement at strategic locations within the given structure. While some may argue that this becomes expensive, a new method of monitoring structural health may make use of cost effective smart materials, which possess not only monitoring capabilities, but also significant load-bearing capabilities. Research is being conducted on various metastable alloys, which exhibit an austenitic phase at room temperature, but will transform to a martensitic phase structure upon application of strain. The martensitic phase has not only a greater strength than the austenitic phase, but also displays significant ferromagnetism, while the austenite is paramagnetic. Since the martensite forms in direct proportion to the strain experienced by the material, the magnetic signature increases in direct proportion to the strain. A measurement of the magnetism therefore yields a strain reading. Because some of these metastable alloys can be produced in forms, which possess significant strength, various key structural members may then be replaced with the smart material generating self-monitoring components. Only the peak strain is “recorded” within the material though, since the transformation is irreversible. This peak strain is however usually the limiting variable in determining the remaining life of the structure. The varying degrees of strength, corrosion resistance and sensitivity to strain, which can be engineered into the material, mean that a structural element correctly designed using either a metastable alloy or TRIP steel, forms the perfect passive peak strain sensor. This becomes particularly useful in inaccessible situations where non-destructive assessment is required but difficult to carry out in-situ.
Modelling and Scaling of the Impulse Turbine for Wave Power Applications
Authors: Thakker Ajit , Hourigan Fergal
Published: March 2009

This paper addresses the dimensional analysis of experimental data for the Impulse Turbine and the use of that data to create a model to predict the performance characteristics of an arbitrarily sized turbine under arbitrary operating conditions. The model assumes that the performance of the turbine is a function of flow coefficient only. The model is used to compare the performance of different turbines at the scaled-up level and under varying conditions of axial velocity and angular velocity. Also, the model is used to identify the optimum turbine rotational speed, for maximum output power, at practical sizes over a range of input power levels. This paper clarifies issues relating to the sizing and optimum operating point of the Impulse Turbine over variable sea conditions which oblige the turbine to operate over a design range rather than at a single design point and shows how this optimum operating point may be obtained.
Derivation of Sea Ice Parameters Using ENVISAT ASAR Data
Authors: Ko¨nig Thomas , Lehner Susanne , Schulz-Stellenfleth Johannes
Published: March 2009

This study presents methods for sea ice analysis which make use of SAR wave mode data products, so-called imagettes, to be provided by the European satellite ENVISAT every 100 km along the track, thus forming a particulary dense coverage of the polar regions. Prior to ENVISAT a reprocessed ERS-2 wave mode data set is used to derive sea ice variables like deformation energy. The results are compared to sea ice model runs and SSM/I data. Furthermore, ENVISAT data are of particular interest for observation of the damping of ocean waves by sea ice as pairs of high resolution images with different polarizations will be available. It is e.g. shown that ENVISAT data allow a better distinction between effects due to ocean wave motion and mechanisms. The basic principles of extracting parameters describing sea ice damping characteristics are demonstrated using ERS-2 SAR data.
Interaction of In-Line and Cross-Flow Vortex Induced Vibrations in Risers
Authors: Hansen Erik Asp , Bryndum Mads , Mayer Stefan
Published: March 2009

VIV in pipeline and risers has been studied through numerous experimental investigations using simplified model set-up, consisting of spring mounted rigid cylinders. Models have been constructed to allow in-line, cross-flow, or both in-line and cross-flow motions. Comparison of the model results shows overall agreement, although distinct differences exist between the individual model test series. Different explanation models have been established to try to improve the consistency, however, seldom definitive conclusions have been reached. The present paper presents the use of CFD to document the importance of the interaction of in-line and cross-flow motions on VIV response. 2D numerical tests have been performed using NS3 (DHI-CFD code) for a model undergoing in-line, cross-flow, combined in-line and cross-flow, and cross-flow in combination with forced in-line motions. The paper compares the results with some recent model tests and quantifies the significance of interaction.
Challenges in the Design of an Offshore Wind Turbine Foundation for Arctic Conditions
Authors: Gill Alistair E. , Fraser Ramsay
Published: March 2009

NEG Micon’s Yttre Stengrund Offshore Wind Turbines in the Kalmarsund, Sweden, are supported on unique foundations designed by AMEC. The chosen foundation comprised a steel monopile secured into a rock socket drilled out beneath a deep layer of overlying soil. Sea-ice loading and fatigue governed the design. To minimise the sea-ice loads the foundation was fitted with an ice protection shield. In order to achieve the required fatigue life, time domain simulations were conducted to determine the response of the turbine to combined wind and wave action. Details of the fatigue analyses and methods used to calculate the sea-ice loading are presented herein.
Three-Dimensional Wakes of Curved Pipes
Authors: Miliou Anthi , Sherwin Spencer J. , Graham J. Michael R.
Published: March 2009

A numerical investigation into the flow past curved pipes has been performed. The first group of computations includes curved pipes for which the incoming flow vector is normal to the plane of curvature. For the second group of computations the free-stream flow vector is parallel to the plane of curvature. Different flow conditions have been applied and the corresponding results compared. The vortex shedding patterns and the wake topology are studied and the sectional forces at different locations of the span are also presented. The fully three-dimensional simulations were computed with a spectral/hp element method. The computational results were compared with visualisation experiments undertaken in the towing tank facility of the Department of Aeronautics of Imperial College.
A Comparison of Self-Rectifying Turbines for OWC Based Wave Energy Power Extracting Device Using Numerical Simulation
Authors: Ansari A. R. , Khaleeq H. B. , Thakker A.
Published: March 2009

This paper presents a comparison of self-rectifying turbines for the Oscillating Water Column (OWC) based Wave Energy power extracting device using numerical simulation. The two most commonly used turbines for OWC based devices, the Impulse and the Wells turbines were evaluated under real sea simulated conditions. Assuming the quasi-steady condition, experimental data for both 0.6m turbines with 0.6 hub to tip ratio was used to predict their behavior under real sea conditions. The real sea water surface elevation time history data was used to simulate the flow conditions using standard numerical simulation techniques. A simple geometry of the OWC was considered for the simulation. The results show that the overall mean performance of an Impulse turbine is better than the Wells turbine under unsteady, irregular real sea conditions. The Impulse turbine was observed to be more stable over a wide range of flow conditions. This paper reports the comparison of performance characteristics of both these turbines under simulated real sea conditions.
Numerical Representation of the Collision Interaction Mass for Dual Riser Systems
Authors: Neto Euclydes Trovato , Ruggieri Claudio , Ferrari Jose´ Alfredo
Published: March 2009

This work presents a numerical procedure to represent interaction masses for the collision of adjacent risers. The approach is based on the concept that cyclic interaction between two mechanical systems can be described by representative masses which are associated with the energy dissipated during collision. Development of the numerical formulation employs key concepts connected to group theory for mechanical systems to define an expression for the collision interaction mass incorporating the dissipative behavior of each riser system. An application then follows to predict the contact (collision) regions and collision participating masses for a pair of adjacent risers subjected to different load and environment conditions.
A New Formulation for Equivalent Hydrodynamic Modeling of the Jack-Up Legs
Authors: Daghigh M.
Published: March 2009

Regulations of offshore structures suggest the application of Morison type equation for the estimation of forces induced by wave and current on the slender bodies of Jacket and Jack-up structures. However, common values of hydrodynamic coefficients are rarely defined in two different regulations. Estimation of global responses of Jack-up structure, the simplified geometrical model is used, therefore we will try to modify the DNV formulations in order to estimate the hydrodynamic forces on equivalent pile. Finally, the forces on the real structure and the equivalent pile model are compared and it has been shown that the approximation of the inertia forces has more accuracy comparing to the drag force, due to the nonlinear effect in drag term.
Steel Plate Serviceability in Marine Structures
Authors: Mateus Anto´nio F. , Witz Joel A.
Published: March 2009

Plates are structural components that are able to provide support beyond compressive collapse, i.e. in the post-buckling regime. To date the criterion used to define plate failure under compressive loading is the plate maximum load carrying capacity which is identified as the peak of the associated load versus plate end-shortening curve. Recent advances in structural analysis techniques, such as nonlinear finite element analysis, have enabled the reduction of uncertainty that is embedded into the behaviour and strength prediction of new and in-service structures. Consequently, a new criterion for ultimate plate compressive strength is suggested and justified. This criterion is based on the unloaded edge resultant stress reaching yield. It is considered to be a suitable threshold of serviceability based on the relation between drop of strength versus the gain in plate end-shortening. In practical terms this means that a plate will be able to work in an extended range of end-shortenings.
A Numerical Study of the Initial Mixing Zone Downstream of a Submerged Fish Cage
Authors: Helsley Charles E. , Kim J. W.
Published: March 2009

A fish cage of bi-conical shape (OceanSpar SeaStation 3000) has been placed in the waters of the coast of Ewa, Hawaii. As part of the HOARP Phase II research program, the cage was stocked with 133,000 moi (Pacific threadfin). One of the goals of the work is to examine the nature of the waste product stream of the fully-submerged fish cage. Federal regulations (40 CFR 122.24) require aquaculture systems to obtain a NPDES permit if they annually produce more than 45,454 harvest weight kilograms (about 100,000 pounds) of warm water fish (CWA sections 318 & 402). Should the fish cage in Ewa meet this criterion, it would need to meet the NPDES regulation criteria. This requires that a ‘mixing zone’ be established downstream of the cage. A CFD simulation was made to find the intensity of turbulence downstream of the fish cage. This turbulence increases the diffusion of the pollutant and the size of the mixing zone as a result. Parametric studies have been made to examine the effect of current speed on the initial mixing thereby enabling an estimation of the total mixing downstream of the cage. This estimate of the effective diffusion is essential for the establishment of the boundaries of the mixing zone.
Non-Linear Response Analysis of Moored Structures
Authors: Shashikala A. P.
Published: March 2009

The paper presents the application of spectral method to non-linear random vibration problems in offshore engineering. In the case of structures like single point mooring systems, the non-linearity comes from the non-linear restoring coefficient of mooring lines. The response spectrum for the non-linear dynamic response of the structure is studied by introducing the Hermite polynomial. The method provides an efficient tool for analysing moored structures.
Flexible, High Temperature Thermal Insulation Materials for Subsea Wellhead and Production Equipment
Authors: Janoff Dwight
Published: March 2009

As oil and gas wells are being drilled in deepwater, hydrate formation in the well has become a major concern. During a production shut down, gas hydrates can form and plug the bore of a subsea tree, tree piping, jumper, manifold and flow lines. During shut downs, hot produced fluids become stagnant and are cooled by the surrounding cold water, resulting in hydrate formation [1]. Thermal insulation is necessary to slow down this cooling process to prevent hydrate formation until the well production or hydrate inhibitor injection can be restored [2]. Currently available insulation materials for subsea applications are rated for internal temperatures of up to 121 °C (250 °F). These materials include NovoTherm, urethanes, and epoxy/syntactic foams, and vulcanized rubbers [3]. These materials may thermally age at above 121 °C (250 °F) such that the insulation bond line will become weakened or softened over time. Recently, insulation that will withstand 177 °C (350 °F) internal temperatures for high temperature, high pressure (HTHP) projects are being required by operators. Two new insulation materials, proposed for use on subsea wellhead and production systems at temperatures up to 177 °C (350 °F) will be discussed. The first material is based on addition cured silicone elastomer. The second material is based on a flexible Novolac epoxy. Both materials are cast in place into molds, have high tensile elongation, and possess the thermal properties necessary to meet cool down requirements for subsea production equipment. These materials are being proposed for use on manifolds, jumpers, production trees, and other subsea equipment.
Conditional Short-Crested Waves in Shallow Water and With Superimposed Current
Authors: Jensen Jo̸rgen Juncher
Published: March 2009

For bottom-supported offshore structures like oil drilling rigs and oil production platforms, a deterministic design wave approach is often applied using a regular non-linear Stokes’ wave. Thereby, the procedure accounts for non-linear effects in the wave loading but the randomness of the ocean waves is poorly represented, as the shape of the wave spectrum does not enter the wave kinematics. To overcome this problem and still keep the simplicity of a deterministic approach, Tromans, Anaturk and Hagemeijer (1991) suggested the use of a deterministic wave, defined as the expected linear Airy wave, given the value of the wave crest at a specific point in time or space. In the present paper a derivation of the expected linear short-crested wave riding on a uniform current is given. The analysis is based on the conventional shallow water Airy wave theory and the direction of the main wind direction can make any direction with the current. A consistent derivation of the wave spectrum taking into account current and finite water depth is used. The numerical results show a significant effect of the water depth, the directional spreading and the current on the conditional mean wave profile. Extensions to higher order waves are finally discussed.
Probabilistic Study of the Effects of Leaving Fatigue Cracks Unrepaired in a Ship
Authors: Kent J. S. , Das P. K. , Barltrop N. D. P. , Sumpter J. D. G.
Published: March 2009

A methodology to quantify the risk of leaving fatigue cracks unrepaired while a ship is in service is described. The basic variables used and the approaches to quantify their variability are described. Three different methods have been used to characterize the variation in toughness values between plates. Two of these keep the sampled toughness value constant as the crack grows while the third, a variable toughness model, tries to account for the variation in the toughness values as the crack grows. Preliminary results are also presented and the reasons for the results are discussed.
Review of Risk Assessment for FPSO Structural Design
Authors: Fyfe Alexander J. , Frieze Paul A. , Wang Xiaozhi , Spencer Jack
Published: March 2009

This paper presents a review of the state-of-the-art of risk assessment as it applies to the hull structures of FPSOs. Risk definitions are examined critically to determine how they should be applied in this case. The roles of human and organisational errors and their proper identification during design of intact and damage scenarios are discussed. It has long been accepted that FPSOs and trading tankers are subject to different environmental loading conditions. Even more substantial differences become evident when operating practices are examined and it is realised that to manage risk properly these differences need to be reflected in design and the preparation of operation procedures. Against a background of code development, design experience and detailed knowledge of many things that have gone wrong, the review examines: risk evaluation for hull structures; specific risks relating to FPSO hull-structure configurations; environmental issues for a moored FPSO; operational issues and risks arising from interaction between the hull and the mooring system.
Fatigue of Alumina-Chrome Carbide Composite Coating
Authors: Kireitseu Maksim , Nemerenco Ion , Yerakhavets L.
Published: March 2009

The fracture and fatigue characteristics of alumina coating strengthen with chrome carbide layer, is presented and discussed. The essential aspects of the methodology include acoustic emission monitoring of crack activity and its propagation under cyclic loads. Structural analysis of the alumina revealed an effect of α, γ phases of alumina and CrC thickness on fatigue life of the coating. The fatigue life of the coatings is found to be no longer than 103 cycles. None of the coatings resist above 103 cycles. It is found to be cracks initiation and propagation in the coating interface that result in the catastrophic failure of the composite. An ultimate load resulted in intensive crack growth in CrC-Al2O3-Al is found to be about 2 GPa that is higher than that in alumina coating.
Inelastic Analysis of Jacket Type Offshore Structures Using Fiber Elements
Authors: Aghakouchak Ali Akbar , Asgarian Behrouz
Published: March 2009

The fiber beam-column post-buckling element has been formulated and implemented in the non-linear program DRAIN-3DX to predict Buckling, Post-buckling and hysteresis behaviour of tubular Struts and Portals. In this element both material and geometric non-linearities are considered. The element is applied to simulate post buckling and hysteric response of tubular members subjected to cyclic loading. Also Psuedo-Static analysis of X-braced frame subjected to cyclic load has been perfomed. The predicted overall response matched well with the available experimental and other analytical results.
Safety and Environment Requirements and Systems for Offshore Installations Operating in Arctic Environments
Authors: Medonos Sava , Brewerton Robert W. , Jouravel Valentin I.
Published: March 2009

Recent offshore developments in the Arctic in Russia, the US, Canada and other locations have brought additional requirements on critical systems for safety and environmental protection. The design of the systems broadly follows the design principles for temperate climate. However, a substantial number of key additional requirements exist that are paramount for operations of the systems at down to −45 °C and in ice and snow. Equipment reliability reduces with reducing sub-zero temperature. Production, emergency and ancillary equipment have to be heated to prevent the equipment malfunctioning and to prevent the development of upset conditions into hazardous situations. This is valid for both preventative and mitigating measures. The heating has to have a high reliability, should it be HVAC systems or trace heating. A specific focus should be given to material selection, and this not only for operating conditions, but also for transportation and storage where equipment may be exposed to low temperature extremes, which may permanently change the material crystalline structure and its properties. The extent to which optimum solutions to these problems can be found is often dependent upon the nature of the overall platform layout, especially where designing against major hazards is concerned. Whole-year protected tunnels are preferred as means of escape, with heating for winter conditions to prevent blockage by ice and snow and ensure high availability whatever weather. They have to be ventilated by highly reliable ventilation systems. A number of fields in the Arctic have a very high hydrogen sulfide content, which together with sea and ice conditions sets requirements for special amphibious evacuation craft. Evacuation by means of icebreakers is often not possible for all conditions as icebreaker engines cannot be operated in air with hydrogen sulfide content. A possible solution may be an amphibious survival craft ARKTOS that have been used to date on some installations. A greater emphasis has in recent years been put on the protection of the environment against operational emissions and discharges, and accidental releases and spills. Whilst the incipient causes of accidental releases and spills are similar to those leading to fire and explosion hazards, recovery from a hydrocarbon spill situation requires specialised equipment with satisfactory performance specific for the Arctic. Based on the authors’ experience from several recent projects, the Paper presents additional requirements for safety and environmental protection systems for Arctic conditions, addresses incipient causes of equipment and structural failures and it outlines technical solutions for risk reduction, many of which have been used in practice.
Interaction of Water Gravity Waves With Elastic Ice Floes
Authors: Lavrenov I. V. , Novakov A. V.
Published: March 2009

The self-consistence motion of a fluid and elastically oscillating plates partially covering fluid is simulated numerically. The problem is reduced to simulation solution of the Laplace equation for the fluid and equation of elastic plate oscillations for ice motion. The numerical and analytical solutions, obtained using an integral equation containing the Green function, are compared. In order to solve the problem numerically the boundary elements method for the Laplace equation and the finite element method for the equation describing the elastic plate motion are proposed. The coefficients of surface gravity wave transmission and reflection from the floating plate are estimated. It is shown that the solution can be presented as quasi-periodic one with characteristics determined by the initial value of the wave and ice foe parameters. The ice floe exert a filtering effect on the surface wave spectrum, essentially reducing its most deflectable components.
Measurement of Organisational Maturity in Designing Safe Offshore Installations
Authors: Sharp J. V. , Strutt J. E. , Busby J. , Terry E.
Published: March 2009

The measurement of health and safety performance is an important requirement but most performance metrics are lagging indicators, measuring lost time incidents, dangerous occurrences etc. The challenge is to develop metrics that can be applied at the design stage. It is widely recognised that most accidents are influenced by the design stage, and many can be directly attributable to deficiencies in design. This paper is concerned with a design capability maturity model’, which is complementary to the design safety performance indicator model developed to apply to the design process itself. It has been developed to measure the capability of an organisation to design a safe installation, and is based on five maturity levels, ranging from level 1 (initial or learner approach) to optimised or best practice at level 5. This maturity model was originally developed for the software industry and has now been applied to offshore safety. A similar maturity model for quality assurance is now incorporated in the latest version of ISO 9004. Eleven characteristics associated with safety have been identified, in three main groups representing formal safety demonstration, safety implementation and longer term investment in safety. A maturity level is assigned to each of these characteristics and the profile produced reflects the organisation’s overall maturity in design for safety. An important aspect of the model is that it enables an organisation to establish its current level of maturity for each of the characteristics and to identify what steps are necessary to enable the organisation to progress to a higher level. The model can be used as a self assessment tool or applied through an external independent body to the different organisations involved in design (contractor’s design team, duty holder’s team etc).
Investigating Novel Foundations for Offshore Windpower Generation
Authors: Byrne B. W. , Houlsby G. T.
Published: March 2009

In recent years there has been a worldwide increase in the pressure to develop sources of renewable energy. The UK government is committed to ensuring that ten percent of UK energy consumption will be supplied by renewables by the year 2010. Central to this commitment is the need to develop wind farms particularly in the offshore environment. Moving offshore will allow very large wind turbines capable of supplying 2 MW (first generation) to 5 MW (second generation) of power to be installed in large farms consisting of up to fifty or more turbines. In contrast to typical oil and gas structures the foundation may account for up to forty percent of the projected installed cost. The weight of each structure is very low, so the applied vertical load on the foundation will be small compared to the moment load derived from the wind and waves. Further, it will be necessary to have a single design that can be mass-produced over each site rather than have each foundation individually engineered. In combination these points lead to a very interesting engineering problem where the design of the foundation becomes crucial to the economics of the project. One solution is to use conventional piling. However, at some sites it may prove more economical to use shallow foundations, and, in particular suction installed skirted foundations [1]. It will be necessary to develop an adequate design framework for these no vel foundations under the relevant combinations of load so that the optimum structural configuration can be achieved. At Oxford University a program of research on skirted foundations has been underway for the last five years, and much progress has been made on the understanding of this type of foundation under combined loading. This progress has been in both experimental and theoretical areas. This paper explores various structural options that might be used for the wind turbine application. These different options lead to different loading conditions on the foundations. Experiments investigating these different loading conditions are explored. A theoretical approach that describes the experimental results in a way that can be implemented in typical structural analyses programs is outlined. Finally details of a major research program into developing the necessary design guidelines for foundations for offshore wind turbines is described.
System Effects in Generic Risk Based Inspection Planning
Authors: Straub Daniel , Faber Michael Havbro
Published: March 2009

Reliability and risk based inspection planning (RBI) has been developed in the past for single details subject to fatigue based on structural reliability analysis. This paper extends the methodology to systems with a stochastic dependency between the individual hot spots. It addresses the general decision theoretic problems that arise when dealing with RBI of systems. The differences to inspection planning for individual hot spots are listed and discussed. Using a generic format for single hot spots, a consistent approach to the problem is proposed, based on the concept of Value of Information. Possible application of the methodology is outlined.
Titanium Riser Configurations
Authors: Leira Bernt J. , Bjo̸rset Arve , Berge Stig
Published: March 2009

The present paper addresses mechanical design aspects for titanium riser configurations. Three main types of riser systems are considered: • Vertical top-tensioned risers suspended from a TLP; • Catenary risers suspended from a TLP; • Pliant-wave risers suspended from a Semi-submersible. For each configuration, characteristic response properties related to bending moment and axial force diagrams are presented. Critical sections for each riser system are identified. For the vertical top-tensioned riser, design of a bending stiffener located at the seabed is briefly addressed. Capacity formulations and the relative importance of the following mechanical limit states are addressed: • Yielding; • Local buckling; • Fatigue. Differences between parameters influencing the various limit states for titanium respectively steel are highlighted. The local buckling limit state is subsequently focused upon in some more detail in relation to loads due to combined external overpressure and bending.
Estimation of Extreme Response and Fatigue Damage for Colliding Risers
Authors: Leira Bernt J. , Holma˚s Tore , Herfjord Kjell
Published: March 2009

For design of deep-water riser arrays, consideration must be made of the possibility for mechanical contact between the different riser pipes. Both the anticipated frequency of collision and the resulting stresses in the pipes need to be estimated. Such an assessment need to cover a certain range of conditions regarding environmental loading and surface floater motions. The present paper outlines a procedure which admits the most “critical” conditions to be identified based on an iterative approach. For each “load case”, which corresponds to a certain combination of environmental actions and surface floater motion, the corresponding contact stresses are computed. Furthermore, the accumulated damage for each load case (referred to a certain duration) is estimated. The numerical procedure for external load calculation is based on Computational Fluid Dynamics (CFD). For a given riser spacing, interpolation is performed during the response simulation in a pre-established data base. Contact between pipes is checked at each time step by looping through the nodal coordinates of the Finite Element Mesh which represents the pipe geometry. By assembling the response and accumulated damage which correspond to all the different load cases, the long-term probability distributions and weighted damage can be calculated. The procedure is applied to a particular example riser configuration. The effect of varying current direction (relative to the symmetry axis of the floater and riser configuration) on fatigue damage is focused upon in particular.
So Much for Safety
Authors: Skjong Rolf , Ronold Knut O.
Published: March 2009

The paper demonstrates that a simple method exists, by which easily accessible information aggregated in national statistics can be used to derive acceptance criteria for use in cost effectiveness evaluations. Cost effectiveness assessment is normally used for risks that should be made as low as reasonably practicable. These are risks, which are neither intolerably high nor negligible. Examples of users of such criteria are the national and international regulators that implement safety related regulations and industrial companies that operate in an industrial self-regulation regime and therefore define and implement their own risk control strategies. The criteria are derived by combining societal indicators published by the United Nations development program and national statistics. It is observed that in an unregulated market, individuals invest in their own safety or in the safety of their own family. In the same way as the societal indicators indicate how much the regulator should use on safety, the data on how much individuals spend on safety when the decision is up to them indicate when the regulator should not regulate. The idea is then that when individuals make better use of the available resources there is no reason to regulate. As a last point, when it comes to cost effectiveness, the paper demonstrates that situations may well occur for which a wealthy country should invest in safety and a poor country should not.
Design Study of a Deepwater Compliant Vertical Access Riser for the Gulf of Mexico
Authors: Brinkmann Carl R. , Whooley Kevin T.
Published: March 2009

This paper presents the results of a design study of a compliant vertical-access riser (CVAR) for the deepwater Gulf of Mexico area. The CVAR concept modifies traditional, tensioned risers by providing vertical compliancy through its geometric shape that is achieved by elongating the riser and providing buoyancy along the lower section. A major benefit of this geometric compliancy is that surface trees can be fixed relative to the floating vessel. Also, there is a potential for significant cost savings as compared to alternate, top-tensioned riser systems. A key design feature is the tapering of the buoyancy transition to create a nearly neutrally buoyant region. This tapering reduces bending stress between the weighted and buoyant regions of the CVAR. The design basis for this study is a production riser system for a deep draft caisson vessel (DDCV) in 4900 feet of water. The design study begins by establishing a base configuration for operational conditions. This configuration is then checked for extreme loading and fatigue due to DDCV motions and riser VIV. The potential for riser interference is also checked. The study also includes a brief assessment of fabrication and installation procedures. Finally, a cost estimate is made for comparison with alternate production riser systems.
The Development of Estimators for the Peaks-Over-Threshold Method
Authors: Naess A. , Clausen P. H.
Published: March 2009

The paper discusses the accuracy and efficiency of some of the standard estimators used in conjunction with the Peaks-Over-Threshold (POT) method. A comparison is made between some commonly adopted estimators and two types of estimators proposed by the authors. The comparison is based on an extensive set of synthetic data simulated from a range of different statistical distribution functions that have been assumed to describe wind speed processes.
Random Waves and Capsize Probability Based on Large Amplitude Motion Analysis
Authors: de Kat Jan O. , Pinkster Dirk-Jan , McTaggart Kevin A.
Published: March 2009

The objective of this paper is to apply a methodology aimed at the probabilistic capsize assessment of two naval ships: a frigate and a corvette. Use is made of combined knowledge of the wave and wind climate a ship will be exposed to during its lifetime and of the physical behavior of that ship in the various sea states it is likely to encounter. This includes the behavior in extreme wave conditions that have a small probability of occurrence, but which may be critical to the safe operation of a ship. Time domain simulations provide the basis for deriving short-term and long-term statistics for extreme roll angles. The numerical model is capable of predicting the 6 DOF behavior of a steered vessel in wind and waves, including conditions that may lead to broaching and capsizing.
Analysis and Applications of Second Order Models for Maximum Crest Height
Authors: Krogstad Harald E. , Barstow Stephen F.
Published: March 2009

Expressions for the maximum crest height are reviewed and tested on data from five different sensors in the WACSIS data set. The overall agreement is good and the analysis supports that second order models give accurate expressions for the distribution of the maximum crest height for varying water depth and wave steepness. In the second part of the paper, the expressions are combined with the existing extreme crest and wave height framework and applied to sets of time series and long term wave data. It is concluded that the 2nd order models represent a definite improvement over earlier empirical parametrizations.
Prediction of Green Water Loads for a RoRo Passenger Ferry
Authors: Sames Pierre C.
Published: March 2009

Selected results of the European research project DEXTREMEL are presented with emphasis on green water loads on the forward bulkhead of the superstructure of a RoRo passenger ferry. The finite volume code COMET was used to predict these loads. Model tests were carried out in a seakeeping basin. Experimental measurements comprised wave heights along the deck contour and local pressures at the forward bulkhead. In addition, a laser-based measurement technique visualized green water contours on the deck. Effects of boundary conditions and initial conditions were studied for simulation of flow over the deck. Predicted wave heights and impact pressures were compared with measurements. It is shown that some features of the complex flow over the deck can be predicted and that resulting impact pressures are in fair agreement with measurements.
Modelling of an Internal Wave Gravity Current Using Eulers Equations
Authors: Wood Deborah J.
Published: March 2009

In nature where thermoclines exist an internal wave may form, and if a sloping bottom is also present then a gravity current may occur. In this study we use a Navier-Stokes solver to solve Eulers equations to simulate the generation and evolution of such a wave. The thermoclines used in this study are similar to those seen in nature except scaled down to the laboratory scale used by some ongoing experiments. We find that the Navier-Stokes solver generates and evolves a wave similar to experimental observations. The head of the gravity current is dominated by medium density fluid with the thermocline thickness growing and becoming thickest at the centre of the head. Maximum velocities of approximately 0.5 of the linear wave speed are found which are similar to experimental and field observations.