bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature Effects on FPSO Ultimate Strength: A Reliability Based Approach
Authors: Moatsos Ioannis , Das Purnendu K.
Published: January 2009

Structural Reliability analysis is based on probability theory and a significant amount of literature exists on the subject and continues to be a subject of ongoing research. When the structural safety of a ship’s hull is considered, the ultimate hull girder strength should be evaluated. Existing literature on the subject often neglects the effect of temperature. This paper aims to determine the effects of temperature in ship structural reliability and to propose a procedure for analysing structures by taking into account temperature effects. The ultimate strength of the hull girder was calculated using a component approach, where the behaviour of the hull is evaluated based on the behaviour of the single structural components. A sample analysis for Tanker/FPSO structures is provided where the reliability analysis was carried out using a First and Second Order Reliability Method (FORM and SORM) analysis. The loading component was handled using extreme wave statistics and the Ferry Borges-Castanheta load combination method. Annual reliability indices and probabilities of failure were calculated for hogging and sagging conditions showing the effects of temperature along with Partial Safety Factors for all variables taken into account.
Viscous Free-Surface Flow Past Twin Vertical Cylinders
Authors: Chen Tong , Chwang Allen T.
Published: January 2009

The laminar flow behaviors around two vertical circular cylinders (in a tandem arrangement) that pierce a free surface are investigated by the finite element method in this paper. The computational results exhibit two major free-surface effects: the presence of a free surface allows the occurrence of small-scale Kelvin-Helmholtz instabilities, but suppresses the onset of large-scale vortex alternating behavior. It is also found that the vorticity will expand in a necklace shape adjacent to the free surface. The second cylinder may experience a persisting suction force due to “trapped” vortices in the gap between the two cylinders, which may not happen in the absence of a free surface.
Bearing Capacity of a Skirted Foundation Under VMH Loading
Authors: Gourvenec Susan , Randolph Mark
Published: January 2009

This paper presents results from a series of three-dimensional finite element analyses investigating the ultimate limit state of a circular skirted shallow foundation over a comprehensive range of combined vertical, moment and horizontal loading. Failure loci in V:M:H load space derived from the finite element analyses are compared with limit state predictions from the offshore industry design guidelines [1]. The comparison highlights considerable conservatism of the current design method largely due to poor representation of the response to fully combined loading and neglect of the tensile capacity achieved with foundation skirts. These shortcomings are particularly significant with respect to foundations for offshore conditions and result in an oversight of considerable potential load capacity in design.
Field Tests on Pullout Loading Capacity of Suction Piles in Clay
Authors: Cho Y. , Lee T. H. , Chung E. S. , Bang S.
Published: January 2009

A series of field suction pile pullout tests were conducted inside the Okpo harbor located along the southern coast of Korea. Suction piles having inside diameters of 0.5, 1.0 and 1.5 meters and the length of five meters were tested. The seafloor condition at the Okpo harbor is predominantly clay. The piles were first installed by applying well-controlled suction pressure inside the pile. During the pullout tests, the load vs. pile displacement relationship and the water pressure variation inside the pile were carefully measured. The measurements were then used to validate an analytical solution that was developed previously to estimate the suction pile pullout loading capacity.
On Second-Order Roll Motions of Ships
Authors: Liu Yonghui
Published: January 2009

More and more vessels for offshore engineering applications have a roll period beyond 20 seconds in order to avoid the wave frequency excitations on the roll motion. That is particularly true for the latest drilling vessels. However, this will lead to an unexpected second-order roll motion of the ships. This paper will present a new methodology to evaluate the second-order roll motions of the ships in random seas. The higher-order boundary element method (HOBEM) has been utilized to generate the second order difference-frequency roll excitations on the ships and the second-order ship roll motions were predicted by a frequency-domain method. A series of model tests have been conducted and the test results have a good agreement with the numerical predictions. Therefore, this new method has been verified and validated.
Deformation Measurements on Full-Scale Fracture Tests of Pre-Cracked Tubular Joints
Authors: Talei-Faz Bijan , Brennan Feargal P. , Robson Stuart
Published: January 2009

A series of six static strength destructive tests were performed on full-scale pre-cracked tubular welded T-joints manufactured from a high strength weldable steel used in the construction of offshore Jack-Up platforms. All specimens had at least one through-thickness fatigue crack at the weld toe, from a previous fatigue-testing programme. The tests were aimed at analysing the residual static strength of the cracked members. As destructive tests are costly to perform, every effort was made to maximise the data collected. This included the use of a novel photogrammetric technique to provide three-dimensional measurement in real time of the deformation in the vicinity of the brace-chord intersection. The technique has been used for large-scale structural testing in a number of civil and aerospace applications, but to the author’s knowledge this is the first time that it has been employed for the full-scale mechanical testing of large steel structures. This paper describes the details of the photogrammetric technique applied to the large steel specimens which were loaded to failure, resulting in the total separation of the intersecting members. It is hoped that the technique can be used to generate information which can be used in conjunction with finite element or other numerical analyses to increase the accuracy and reliability of modelling cracked tubular joints.
Hydroelastic Responses of a Very Large Floating Structure Supported by Footing-Columns in Waves
Authors: Murai Motohiko , Inoue Yoshiyuki , Kibe Tomoyuki
Published: January 2009

A Very Large Floating Structure (VLFS) is expected as a new utilization of ocean space and coastal region. In Japan, there are many feasibility studies about the hydroelastic responses induced by wave forces for the reason of the relative small rigidity of VLFS. The VLFS supported by columns generally shows smaller elastic responses than those of the pontoon type VLFS. In this paper we discuss the hydroelastic responses of the VLFS supported by large number of columns which have one or two wave-free frequencies for heave. We investigate the hydroelastic behaviors of the VLFS in regular and irregular waves. Furthermore we compare the hydroelastic responses of following three types VLFS in irregular waves. First one is supported by cylindrical columns without wave-free frequency, second one is supported by columns with one wave-free frequency and third one is supported by columns with two wave-free frequencies.
Stochastic Response and Stability Analysis of Single Leg Articulated Tower
Authors: Banik A. K. , Datta T. K.
Published: January 2009

The stationary response and asymptotic stability in probability of an articulated tower under random wave excitation are investigated. The articulated tower is modelled as a SDOF system having stiffness nonlinearity, damping nonlinearity and parametric excitation. Using a stochastic averaging procedure and Fokker-Plank-Kolomogorov equation (FPK), the probability density function of the stationary solution is obtained for random sea state represented by a P-M sea spectrum. The method involves a Van-Der-Pol transformation of the nonlinear equation of motion to convert it to the Ito’s stochastic differential equation with averaged drift and diffusion coefficients. The asymptotic stability in probability of the system is investigated by obtaining the averaged Ito’s equation for the Hamiltonian of the system. The asymptotic stability is examined approximately by investigating the asymptotic behaviour of the diffusion process Y(t) at its two boundaries Y = 0 and ∞. As an illustrative example, an articulated tower in a sea depth of 150 m is considered. The tower consists of hollow cylinder of varying diameter along the height, providing the required buoyancy of the system. Wave forces on the structure are calculated using Morrison’s equation. The stochastic response and the stability conditions are obtained for a sea state represented by P-M spectrum with 16m significant wave height. The results of the study indicate that the probability density of the stationary response obtained by the stochastic averaging procedure is in very good agreement with that obtained from digital simulation. Further, the articulated tower is found to be asymptotically stable under the parametric excitation arising due to hydrodynamic damping.
Analysis of the Response of Flexible Risers to the Loads Imposed by Hydraulic Collars
Authors: de Sousa Jose´ Renato M. , Magluta Carlos , Roitman Ney , Ellwanger Gilberto B. , Lima Edison C. P. , Papaleo Arnaldo
Published: January 2009

Despite being largely used to install flexible risers, neither the loads imposed by hydraulic collars nor the response of the flexible riser to these loads have been previously studied. Hence, this paper aims at presenting a finite element (FE) model to calculate not only the loads imposed by a typical hydraulic collar to a flexible riser, but also the response of this structure to the imposed loads. A 9,5” flexible riser is analyzed and the obtained results are compared to the ones from experimental tests carried out by PETROBRAS and COPPE/UFRJ.
Reinforced Thermoplastic Pipe: Innovative Technology for Onshore Field Developments
Authors: Avery Andy , Martin Sally
Published: January 2009

Sovereign Pipe (formally Halliflow™), a Reinforced Thermoplastic Pipe has been developed by Halliburton Wellstream as an alternative pipeline technology for onshore infield flowlines. This technology builds on Wellstream’s strengths in the provision of unbonded flexible pipe solutions for the offshore oil and gas industry since 1989. A significant proportion of global production originates from onshore developments, and this sector has become increasingly concerned with optimizing production output and maximizing the productive life of fields. This requires the flexibility to manage fields and associated wells efficiently and life cycle costs effectively. Reinforced Thermoplastic Pipe technology is ideally suited to meet this need, providing a robust product that can operate efficiently in extremes of temperature, for extreme services that have corrosion implications for traditional carbon steel pipes and yet provides the flexibility for re-deployment as field conditions change over the product life. This paper details the Sovereign Pipe design methodology, service criteria, Arctic and tundra applications and associated thermal dynamics which make this product suitable for such life of field applications. Factors specifically considered here include ease of manufacturing and installation, and technical characteristics such as thermal dynamics, chemical resistance to internal fluids and resistance to ultra violet degradation.
Modelling the Heave Response of Tension Leg Platforms Using the Volterra Series
Authors: Taylor Scott , Haritos Nicholas , Thiagarajan Krish
Published: January 2009

Tension Leg Platforms (TLPs) are predominately used for deep water oil and gas production. The use of tendons creates a small amplitude, high cyclic response in the vertical plane (heave, roll and pitch). Under these conditions fatigue cracking becomes an important consideration. The amplitude of the vertical motion is minimised by ensuring the natural frequency of the TLP lies above the energetic part of the wave spectrum. However, due to non-linear wave loading effects, it is possible for waves to create an output at their sum-frequency, which may consequently equal the natural frequency of the platform. This phenomenon is more commonly known as ‘springing’. The Volterra method [1] is a technique used to model the behaviour of TLPs under these conditions. This approach quantifies the linear and non-linear (quadratic, cubic, etc) responses separately using transfer functions, which are determined from the input and output of the system. In this paper an orthogonalised Volterra series for use with both Gaussian and non-Gaussian input data is presented. The data used in the Volterra modelling was collected from tests conducted on a model TLP. The wave height and platform motion were measured at wave frequencies around one, a half and a third of the model’s heave natural frequencies. Both regular and irregular wave tests were performed to varying wave heights and frequencies. Using the Volterra method, the transfer functions were calculated up to the third order. Difficulties encountered due to the use of discrete data were identified and where possible their effects minimized. The results demonstrate clear evidence of springing, with dynamic amplification present at sum-frequencies close to the natural frequency of the platform for the non-linear responses.
Generating Power From Ocean Waves Using a Float With Excessive Buoyancy: Theory and Dynamic Model Results
Authors: MacCready T. , Zambrano T. , Hibbs B. D.
Published: January 2009

We are exploring a new approach to ocean energy extraction through a device that we refer to as the NAF (an acronym for Non-Archimedean Float). The NAF is a fully submerged body with excess buoyancy; i.e., the mass of the body is far less than the mass of the water it displaces. When such a float is tethered beneath the ocean surface the buoyancy yields a large force vector in the direction perpendicular to the isobaric surfaces that parallel the water/air interface. The constant shifting of the wave troughs provides the opportunity for energy extraction using turbines affixed to the float. We are exploring the NAF concept because its simplicity results in many inherent benefits. The device has few moving parts, gathers energy from waves coming in any direction, and exists as a non-obtrusive, completely submerged installation. A numerical model of the NAF has been created to determine the dynamic behavior and power output for various configurations and under various wave conditions. The numerical model is set up to calculate the various forces experienced by the NAF float, and from these it calculates the velocity and position of the float through time series steps. The model effectively demonstrates which variables are important and how power output relates to NAF dimensions. One early finding from the model result relates to tuning the natural frequency of the NAF to match the natural frequency of the waves. The NAF moves like an inverted pendulum, and its natural frequency is primarily dependent on the length of the pendulum. Regardless of the actual float buoyancy, the 6 to 12 second periods that typify average wave conditions dictate that the NAF tether should be between 30-m and 60-m long. Also, a scale version of this novel energy device consisting of a float tethered beneath the ocean surface was deployed off the coast of southern California. The deployment yielded rich data sequences that are sufficient for comparison with a dynamic numerical model.
Recent Development in High Strength Linepipe for Sour Environment
Authors: Ishikawa Nobuyuki , Shinmiya Toyohisa , Endo Shigeru , Wada Tsunemi , Kondo Joe
Published: January 2009

This paper firstly summarizes the design concepts for controlling crack resistant property and mechanical properties of high strength linepipe steels for sour gas service. Optimum conditions of controlled rolling and accelerated cooling that balances crack resistant property and toughness were investigated. It was demonstrated that higher cooling rate in accelerated cooling process brings tremendous advantages for balancing toughness and strength by fine bainitic microstructure even for heavy wall thick pipes. Production results of high strength sour resistant linepipes were introduced. In order to increase strength grade of sour linepipes, further investigation was made using the steels with different microstructures. It was found that precipitation hardened ferrite-bainite steels have extremely high resistance against HIC even for Grade X80. Mechanical properties and microstructural characteristics of this newly developed steel were introduced in this paper.
Assisted Dynamic Positioning of a Moored FPSO: Robustness Aspects Regarding Current Forces Modeling
Authors: Tannuri Eduardo A. , Simos Alexandre N. , Pesce Celso P. , Donha De´cio C.
Published: January 2009

This work treats the problem of assisted dynamic positioning (DP) of moored FPSOs, focusing on robustness issues regarding the modeling of hydrodynamic current forces and moment. Modern DP systems usually employ Extended Kalman Filters associated to LQG or Adaptive controllers to estimate current forces and velocity, performing a direct compensation of these environmental loads. Such a procedure guarantees better performance of the controller under a wide range of environmental conditions, which is known as a “large environmental window”. However, it demands a large number of control parameters, usually requiring a time consuming tuning process. More recently, Sliding Mode Controllers (SMC) with an alternative compensation technique [1] have also been investigated and proved to be an appropriate choice. Robustness properties of SMC allow the use of a simple hydrodynamic current force modeling and rough measurements of current velocity and direction to perform a real time estimation of current loads. Robust performance and stability are guaranteed even in the presence of current measuring and modeling errors. This procedure preserves the advantages of current compensation and avoids the parameter tuning process. In the present paper the so-called ‘Short Wing-Cross Flow Current Model’ [2] is used. This current force model has the advantage of being semi-explicit, in the sense that it depends on only three experimental coefficients, obtained from simple rotating-arm experiments in a towing tank. Nevertheless, it is a common practice in Ocean Engineering to use the well-known ‘Hydrodynamic Derivative Models’, for low maneuvering speeds as, e.g., the ‘Takashina model’. Those models require a large number of towing tank tests, and are usually believed to represent current loads with very small errors. The present work shows simulations of the SMC technique (with the simpler Short Wing Current Model), controlling a tanker that is otherwise modeled with a ‘Hydrodynamic Derivative Model’. The results confirm that, for control purposes, the ‘Short Wing-Cross Flow Current Model’ current model can be used, since performance and stability requirements are preserved.
Dynamic Positioning Systems: Comparison Between Wave Filtering Algorithms and Their Influence on Performance
Authors: Tannuri Eduardo A. , Bravin Tiago T. , Pesce Celso P.
Published: January 2009

Wave filtering is an essential function of a Dynamic Positioning System, being responsible for the separation between high-frequency wave induced motions and low-frequency ones, which must be controlled. Low attenuation of the first components may cause oscillatory control action, high fuel consumption and can damage propeller systems. On the other hand, depending on filtering design, high levels of attenuation may be associated with non-admissible delay-times, which may cause instability in the closed-loop system. Traditionally, low-pass or notch-type filters have been used since the first DP systems, due to the simplicity, acceptable performance and possibility of being implemented using analog circuits. Alternatively, observers based on Kalman Filtering Theory have also been used, based on simplified ship models, separating low and high frequency motions. Two wave filters, representing each of those categories, were implemented in a complete DP simulator. All DP components and algorithms are considered and modeled in the simulator, including propellers, thrust allocation, wind-feedforward and different control strategies. Environmental loads are evaluated using fully validated models, including wind, current and wave actions. A pipe-laying barge under typical Campos Basin environmental conditions has been considered as example. Several aspects of the filtering algorithms were analyzed and compared, involving the influence of each filter in the overall DP performance, relationship of design parameters with physical system, ease of commissioning and tuning. The trade off between low frequency tracking (which is associated with the delay time) and wave response suppression is analyzed and discussed for both categories of filters.
Lazy-Wave Steel Rigid Risers for FSO With Spread Mooring Anchoring System
Authors: Lima Torres Ana Lu´cia F. , Gonzalez Enrique Casaprima , da S. Ferreira Marcos Donato Auler , de Siqueira Marcos Queija , Mourelle Marcio Martins , Correia da Silva Renato Marques
Published: January 2009

Petrobras developed projects with European companies and Brazilian universities in order to study different configurations of steel risers using flexibilization elements. For the bow turret-moored FPSOs the lazy-wave configuration was considered the most adequate due to its structural behaviour and costs when compared to other configurations. A detailed study was performed by the Petrobras R&D Center to verify the structural integrity of a lazy-wave SCR (SLWR) attached to a turret-moored FPSO at a water depth of 1290 m. The results for the installed riser showed its feasibility. Petrobras continued the studies of the SLWR to verify its behaviour when connected to a FSO with a spread-mooring anchoring. This paper presents the approach and methodology adopted in Petrobras to verify the structural integrity of a SLWR attached to a FSO with spread-mooring anchoring at a water depth of 1800 m. The riser analysis was performed using the Petrobras’s in-house computer codes ANFLEX and POSFAL developed and implemented as part of projects from CENPES with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”. For VIV (Vortex Induced Vibration) fatigue damage calculation SHEAR7 was used. Maximum stresses were verified through a deterministic non-linear time domain-analysis. The time-domain random nonlinear analysis was considered to be the most appropriate to be used for fatigue damage calculation due to the possibility of representing the existing non-linearities of the model and random characteristic of the environmental loading. For the fatigue damage analysis, a set of load cases that considers the bimodal / bi-directional characteristics of sea-states, probability of occurrence and energy content, was used.
Predicting the Failure Pressure of Pipelines Containing Nonuniform Depth Corrosion Defects Using the Finite Element Method
Authors: Benjamin Adilson Carvalho , de Andrade Edmundo Queiroz
Published: January 2009

PETROBRAS is conducting a research project with the purpose of investigating the behavior of pipelines containing long nonuniform depth corrosion defects. In the first phase of this project, burst tests of two tubular specimens were carried out. Each of the two specimens had one external nonuniform depth corrosion defect, machined using spark erosion. This defect consists of two short and deep defects within a long and shallow corrosion patch, longitudinally oriented. The second phase of the project aims at appraising the performance of two different finite element models: a shell model and a solid model. This paper describes the application of these models in the analysis of the two tubular specimens containing a long nonuniform depth defect that were tested in the first phase of this project. The failure pressures predicted by the two types of FE models are compared with the burst pressures measured in the laboratory tests. Also a comparison between the results obtained by these models is presented. It is concluded that the solid model is more accurate than the shell model, but both models proved to be capable of simulating the failure behavior of defects constituted by a long and shallow corrosion patch with deep defects over it.
The Prince TLP Steel Catenary Risers: Design and Fatigue Challenges
Authors: Mekha Basim B. , Heijermans Bart
Published: January 2009

The Prince field in Ewing Bank Block 1003 in the Gulf of Mexico was developed using a “Moses” Tension Leg Platform in a water depth of 455m (1492 ft). Two 12-inch Export Steel Catenary Risers (SCRs) connected to the TLP were designed for oil and gas transportation. The Prince field water depth is the shallowest so far for SCRs connected to a floating structure. Having the SCRs in such relatively shallow water provided many challenges to the SCRs designers, as well as the TLP hull designers. The SCR design required large departure angles of 24 degrees to mitigate the touchdown area stresses and to accommodate the TLP surge (i.e. near and far) motions. The large SCR departure angles resulted in long suspended sections of the SCRs and consequently long sections were required on the seabed. Also, the SCR configuration caused high static and dynamic horizontal loads that were directed to the TLP hull. The motions of the TLP in this water depth affect the entire SCR length and thus, the fatigue and the corresponding fracture mechanics assessment became a very significant aspect of the SCR design. This paper gives a brief introduction to the Prince TLP and its two export SCRs. It also provides an insight into the challenges faced and overcome during their design. The fatigue approach and results with data from the fatigue test programs are also presented. A summary of the fracture mechanics assessment and the critical flaw sizes derived based on BS 7910 (1999) are given.
Full Stochastic Fatigue Analysis for Kizomba ‘A’ FPSO-Hull Interface Design
Authors: Na Jeong Heon , Lee In Hye , Sim Woo Seung , Shin Hyun Soo
Published: January 2009

As the offshore oilfields shift into the deeper ocean of less infra, FPSOs become a popular solution. Many FPSOs operate in fields where they are expected to stay for decades without drydocking for maintenance. Currently one of the most active area of the FPSO installation is the west Africa offshore where the sea state is benign and the swell from the south is dominant and the water depth is deep. In the case of long operation in the benign site, the fatigue is rather important in the structural design point of view. This paper describes the first extensive practical application of the full stochastic fatigue analysis method to the design of the FPSO-hull interface design like topside supports, flare tower foundation, riser porches, caissons, crane pedestals, mooring foundations, helideck connections, deck penetrations and etc. Especially, topside support is highlighted in detail because of its potential inertia loading due to heavy weight and importance in fabrication stage. The full length of Kizomba ‘A’ FPSO being built in Hyundai Heavy Industries is modeled for the global analysis and the boundary conditions from the global model analysis are transferred for the local model analysis. Some discussions of screening analysis and hot spot stress calculation and roll damping evaluation are also presented.
Development of an Analytical Tool for the Design of Deepwater Riser/Flowline Thermal Insulation Systems
Authors: Williams Dara , Harte Annette , Grealish Frank
Published: January 2009

The offshore oil and gas industry is predicting the discovery of more and more deepwater reservoirs. Increased water depths create a requirement for reliable pipelines to economically recover these deepwater fields and also to minimise flow assurance problems. Increased flow assurance problems in deeper waters increase the need for thermally insulated pipelines. This paper presents an overview of the key issues in the analysis and design of thermal insulation systems, identifies and discusses how these are addressed by the design tools developed within the DeFRIS [1] project and presents results used to validate the algorithms incorporated into the design tool.
Ultradeepwater Monobuoys
Authors: Oliveira Mauro C.
Published: January 2009

This work deals with the analysis and design of monobuoys for deep waters. The monobuoy performance evaluation is carried out using a time domain computer program due to the non linearities present in this problem. This program is used to simulate the behavior of the monobuoy under the action of waves, wind and current. A coupled analysis between the floater and the mooring lines, considering its inertia, is also employed. Initially a validation study is conducted comparing the numerical simulations with model test results for a 400 meters water depth CALM buoy. The test comprises an operational condition with a tanker connected to the buoy under the action of wave, current and wind loads. From these results a calibration procedure of the numerical simulation is proposed and different hull forms are assessed in order to verify its global behavior. The main objective is to check if there are improvements in comparison with more conventional shapes relatively to the riser forces in the connection point.
An Experimental Rig for Verification of the Mechanical Properties of Welds Produced at In-Service Welding
Authors: Lindstro¨m Per R. M. , Ulfvarson Anders
Published: January 2009

The strength of a weld joint is determined by its geometry and its metallurgic structure, which is dependent on the cooling rate, its chemical composition and the original grain size of the base material. During in-service welding of structures affected by a forced flow of fluid on its reversed side the cooling rate depends on the fluid’s boundary layer, the material’s thickness and the heat input of the welding process. Currently, the calculation of the cooling rate during in-service welding is made by means of numerical methods such as the Finite Element Method, FEM. Through the introduction of an apparent thermal conductivity, kPL, it possible to determine the cooling rate for specific welding parameters by means of Rosenthal’s equation. This can be done with a standard pocket calculator. An experimental rig for measurement of the heat transfer during the in-service welding of structures affected by a forced flow of fluid on its reversed side has been designed and built. The physical principles of welding on plates affected by a forced flow of fluid on their reverse side are the same as for welding on the circumference of a pipe containing a forced flow of fluid. In the rig, the required boundary layer is built up in a pipe system by means of a pump. As the flow and the temperature of the fluid can be controlled to simulate the specific heat transfer, it is now possible to verify the values of the apparent thermal conductivity, kPL, that were calculated by means of FEM. A quantitative database will be filled with values of the apparent thermal conductivity, kPL, for various configurations. For the purpose of evaluation and qualification of in-service Welding Procedures Specifications, WPS, the sponsors of the research project use the experimental rig.
A Fast Calculation Procedure for Fatigue Reliability Estimates of Offshore Structures
Authors: Karadeniz H.
Published: January 2009

This paper presents formulations and procedure of a fast and efficient computation of fatigue reliability estimates of offshore structures, which eliminates repetitive execution of spectral analysis procedure so that it is performed only once for all reliability iterations. This is archived by a suitable uncertainty modelling and spectral formulation of the stress process. For this purpose, a new uncertainty variable is defined to represent all uncertainties in the stress spectrum, except those in the damping and inertia force coefficients, thicknesses of marine growths and structural members, which are represented by their own uncertainty variables. Apart from uncertainties in the stress spectrum, a detailed modelling of the fatigue-related uncertainties is presented. Uncertainties in SCF, damage model (S-N line), analytical modelling of the probability distribution of non-narrow banded stress process, long-term probability distribution of sea states and in the reference damage at which failure occurs, are all considered in the group of fatigue-related uncertainties. Formulation of the stress spectrum and stress spectral moments is presented explicitly in the idealized uncertainty space. Then, the failure function of the reliability analysis is expressed in terms of uncertainty variables as being independent of the spectral analysis. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty origins. The procedure presented in the paper is demonstrated by an example jacket type structure and the results are compared with previously calculated results using more sophisticated uncertainty modelling of the stress spectrum.
Obstacle Avoidance Considerations for the REMUS Autonomous Underwater Vehicle
Authors: Fodrea Lynn R. , Healey Anthony J.
Published: January 2009

Future Naval operations necessitate the incorporation of autonomous underwater vehicles into a collaborative network. In future complex missions, a forward look capability will also be required to map and avoid obstacles such as sunken ships. This work examines obstacle avoidance behaviors using a hypothetical forward-looking sonar for the autonomous underwater vehicle REMUS. Hydrodynamic coefficients are used to develop steering equations that model REMUS through a track of specified points similar to a real-world mission track. A two-dimensional forward-looking sonar model with a 120° horizontal scan and a 110 meter radial range is modeled for obstacle detection. Sonar mappings from geographic range-bearing coordinates are developed for implementation in MATLAB simulations. The product of bearing and range weighting functions form the gain factor for a dynamic obstacle avoidance behavior. The overall vehicle heading error incorporates this obstacle avoidance term to develop a path around detected objects. REMUS is a highly responsive vehicle in the model and is capable of avoiding multiple objects in proximity along its track path.
Bending Fatigue Analysis of Sea Floor Observatory Moorings
Authors: Han Seon , Grosenbaugh Mark
Published: January 2009

The purpose of this paper is to provide design criteria for seafloor observatory moorings where the principle mooring line element is an electro-mechanical (EM) cable, and the primary cause for failure is the bending fatigue at terminations. A single point mooring system that consists of a discus surface buoy, an S-tether, a subsurface buoy, and an anchor is considered for this analysis. The waves are assumed random and modeled using the Pierson-Moskowitz spectrum. Raoof’s contact stress-slip parameter at the connection between the surface buoy and the EM cable is used to evaluate a particular design and to provide general guidelines to arrive at optimal design.
Water Collection and Discharge From an Opening in Ships
Authors: Akıntu¨rk Ayhan , C¸alıs¸al Sander M. , Sireli E. Mete
Published: January 2009

In this study, experimental results on water collection and discharge from an opening on a floating body at sea are reported. The objectives of the tests were to establish relationships between the geometrical characteristics of an open deck or a damaged hull — its beam, height of the discharge gap (e.g. freeing port) — and wave parameters such as wavelength and wave height. Two separate series of experiments were designed and models constructed for water discharge and collection. A two-dimensional model for water discharge experiments was built to visualize the free surface and to measure the flow discharge from a tank. Additionally, for the study of water collection experiments, a similar plexi-glass model was constructed. This model was used to measure time dependent volume of water accumulation on deck. The amount of water collected inside the plexi-glass tank was measured by four load-cells, at different tank opening heights and wave frequencies and amplitudes.
Collapse of Imperfect Corroded Steel Tubes
Authors: Mateus Anto´nio F. , Witz Joel A.
Published: January 2009

This paper presents the results of an investigation into the structural mechanics of corroded steel tubes and quantifies the discrepancies resulting from two approaches to modelling the effects of corrosion: uniform thickness reduction and quasi-random thickness. It is shown how the geometrical configuration of the tube’s corroded surface influences its strength under external pressure and how it may lead to collapse localisation phenomena.
Assessment of VIV Induced Fatigue in Long Free Spanning Pipelines
Authors: Mo̸rk Kim , Fyrileiv Olav , Chezhian Muthu , Nielsen Finn G. , So̸reide Tore
Published: January 2009

Current design practice for free spanning pipelines is to allow free spans as long as the integrity with respect to potential failure modes are checked and found acceptable. The case study for Ormen Lange (OL) pipelines planned in the deep waters of the Norwegian Sea is associated with a large number of very long free spans, which requires significant intervention work if based on the state-of-practice acceptance criteria. The design philosophy of the state-of-the-art design code DNV-RP-F105 “Free Spanning Pipelines” is applied in combination with the experience gained from dedicated OL model tests. Updated project specific design guidelines with multi-mode behavior, typical for OL long free spans, is taken into account and an updated Cross-Flow (CF) response model has been developed. An approach to select the In-Line (IL) mode excited by CF response is suggested. Methods for combining stresses from multiple active modes have been proposed and tested, for both IL and CF Vortex Induced Vibrations (VIV). Fatigue analysis has also been performed on the stress series measured in the model tests and this has been successfully used to verify and validate the presented computational procedure. Uncertainty in the model test based fatigue estimates has been assessed and sensitivity studies have been carried out. Reasons for deviations and potential problem areas for long free spanning pipelines have been identified.
Nonlinear, Second-Order Response Statistics of Compliant Offshore Structures
Authors: Naess A. , Karlsen H. C.
Published: January 2009

The paper continues previous work to develop a new method for calculating the mean level crossing rate of a stochastic process represented as a second order stochastic Volterra series. The derivation of this procedure consists of three stages. First the expression for the mean crossing rate is rewritten in terms of a joint characteristic function. Secondly, it is noted that a closed form expression for this joint characteristic function can be derived. Thirdly, it is then demonstrated how the method of steepest descent can be applied to the numerical calculation of the mean crossing rate. It is shown by an example that the numerical accuracy of this method is apparently very high.
Mobile Marine Riser Inspection and Management Program
Authors: Disher Chris , Harthorn Larry
Published: January 2009

The new R.A.D.A.R. (Riser Active Data Acquisition Recorder) program allows for full marine drilling riser inspection while the joints are stacked on the deck of the rig with no disassembly. With the utilization of ultrasonics, robotic tools have been developed to pass through the inside diameter of the main tube and auxiliary lines to obtain potentially thousands of pipe wall readings and to record the weld integrity of each weld. The tools are also capable of video recording the pipe interior. Trained inspectors complete the riser survey with dimensional, visual, and magnetic particle inspections on the end connections of the main tube and auxiliary lines. Inspection data is compiled and stored in a database and is added to previous inspections and will be utilized for trend analysis. Using statistical methods the inspection history can predict trends of riser life and the potential for future repairs. This will assist operators to predict riser repairs, out of service time, cost of repairs, and most importantly eliminate extremely costly rig down time. The RADAR program has successfully inspected over 110,00 feet of pipe since its inception in September 2001.
The Effect of Mooring System and Sheared Currents on Vortex Induced Motions of Truss Spars
Authors: van Dijk Radboud R. T. , Voogt Arjan , Fourchy Paul , Mirza Saadat
Published: January 2009

Since 1996 Spars have been used as production platform in the Gulf of Mexico. Spar Vortex Induced Motions (VIM) in strong currents like the hurricane and loop currents are an important consideration for the design of the mooring system and risers. This is important for the extreme offsets as well as fatigue in risers and the mooring system. This paper compares the VIM behavior of a truss Spar in sheared currents, like the Hurricane current in the Gulf of Mexico, with tow test results. Experiments have been carried out on a scaled model in both a complete mooring system and in a towing set-up with a simplified horizontal mooring. The Spar model consists of a hard tank with removable helical strakes, a truss section and a square soft tank. The results of this model test program show that both the choice of the mooring system and current profile have a significant influence on the VIM response of the Spar. The paper discusses the results of this research and also addresses important issues and considerations for VIM model tests.
Dynamic Behavior of a Vertical Production Riser by Quasi 3D Calculations
Authors: Morooka C. K. , Kubota H. Y. , Nishimoto K. , Ferrari J. A. , Ribeiro E. J. B.
Published: January 2009

A study on dynamics of a vertical production riser under environmental loads is carried out. Current and wave loads along the riser, and surface vessel motions due to the environment at the riser top are taken into account. Based on descriptions of fundamentals of the Quasi 3D model previously proposed by Ferrari&Bearman (1999), numerical simulations for riser displacements have been performed. In line and transverse hydrodynamic current and wave loads on the riser are estimated and riser displacements thought Quasi 3D fashion are estimated in the time domain. Comparisons with literature results and experimental ones are carried out.
Behaviors of a Very Large Floating Structure Supported With Dolphins Under Earthquake Loading
Authors: Watanabe Yoshiyasu
Published: January 2009

In recent years, tremendous research efforts have been directed toward developing very large floating structures (VLFSs) for the purposes of airports, terminals etc. on the sea to utilize ocean space. There is a VLFS which has a pontoon type structure supported with many dolphins and it is important to investigate sufficiently the behaviors of such VLFSs under earthquake loading, because large reaction forces will be exerted on the dolphins and fenders that connect the dolphins and the floating structure. This paper reports the results of the time history response analyses of a VLFS supported with dolphins with varying the period and velocity of the applied sinusoidal waves as an earthquake loading when gaps between fenders and dolphins are equal to zero. In the analyses, the floating structure supported with dolphins is modeled as a horizontally elastic plate and a rigid plate both supported with springs and dashpots through gap elements.
A Concept Design and Hydrodynamic Behavior of a Spar Platform
Authors: Grove Marcio de Abreu , Levi da Conceic¸a˜o Carlos Antonio , Schachter Richard David
Published: January 2009

This paper presents a study of a Production and Storage Spar Platform concept design for operations in deep water, under the environmental conditions of the Campos Basin, Brazil. This study is the result of a research sponsored by Ageˆncia Nacional do Petro´leo – ANP. The theme was chosen due the good results obtained by this kind of platform in the North Sea and the Gulf of Mexico. Since the exploration of hydrocarbonets is moving to deeper waters, where the Spar concept seems to be one of the best options for the task, due to its simplicity in construction and operation. Although it looks simple, this work had required perceptive studies, and the best way chosen to carry it out, was dividing it in two directions: one dealing with the design method and naval architecture aspects and other with its hydrodynamic behavior. The first line took into consideration the hull generation, compartmentation, weight distribution and stability. A design methodology is presented and the interaction of these factors are showed, taking into account the hydrodynamic results. The second line has favored the evaluation and the control of the vertical motion, excited by dominant waves from the spectra of energy of the sea, and the horizontal motions due the low frequency excitations, and its interaction with the mooring system.
Response of the Mega-Float Equipped With Novel Wave Energy Absorber
Authors: Masanobu Sotaro , Kato Shunji , Maeda Katsuya , Namba Yasuhiro
Published: January 2009

The hydroelastic response is significant from the viewpoint of the structural design of a Mega-Float. Equipping a Mega-Float with some additional structures, such as vertical plates, is one of the ways to reduce the hydroelastic response easily. However, in general, wave drift force acting on the Mega-Float may be increased, when the Mega-Float is equipped with the additional structures to reduce the response. In the present study, we developed a novel additional structure that was effective in the reduction of both hydroelastic response and wave drift force. Furthermore, we estimated the response of Mega-Float equipped with the additional structures, and compared the result with the value measured in at-sea experiments. As a result, we confirmed both the effectiveness of the novel additional structure and the validity of the estimation method.
Fuzzy Reliability Assessment for Offshore Structures Considering Inspection and Maintenance
Authors: Xu Liangbin , Chen Guoming
Published: January 2009

It has been shown that probabilistic model is an effective tool in assessing structural reliability incorporating the information from non-destructive inspection (NDT) of offshore structures. However, the preceding theorems fail to adequately deal with the uncertainties of fuzzy character that are associated with NDT for offshore structures. This paper is aimed at an initial investigation on reliability theory of structure involved with fuzziness considering inspection and maintenance. Based on the basic principles of fuzzy set theory, a fuzzy Bayesian procedure is proposed to quantify the fuzzy uncertainty, including the NDT uncertainty in the flaw size determination and the flaw detection, the modeling uncertainty in model selection and the uncertainty in distribution parameter. This paper presents an algorithm to compute the posterior probability of model weights and the posterior distribution of distribution parameter employing the fuzzy Bayesian theorem. The prior probabilities of model weights may be determined based on fuzzy synthetic evaluation. A method for reliability assessment upon inspection and maintenance considering fuzzy uncertainties is put forward in this paper. The example is investigated to illustrate the validation of the proposed method. It is found that the effect of fuzzy uncertainties could be very significant in assessing structural reliability.