bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient Finite Element Modeling of Marine Riser Systems
Authors: Li Yousun
Published: January 2009

During the time domain simulation of a marine riser under stochastic wave loads, the small time increment and long time duration require efficiency in the finite element modeling. It is expected that the riser system can be modeled by larger elements, or fewer degrees of freedom. In this presentation, higher-order beam elements are introduced, with their tensions treated as Lagrangian multipliers. Each element can have zero, one, or two mid-nodes with varying geometry. Relatively few high-order elements can produce the same accuracy as a large number of conventional beam elements. Normally, the constraints have to be imposed to the element nodes. When a large number of constraints are involved, such as in the multitube analyses, the higher-order elements alone cannot reduce the number of elements. In the present methodology, this difficulty is also circumvented. A constraint is allowed to be applied to any location within an element. Even it is possible to model a lateral constraint sliding along a riser. The cross-sectional forces and moments can be output at any location along the riser. They are continuous within and across the elements by a special interpolation technique. These techniques help to enhance computational efficiency in the riser analyses, especially for the long-duration time simulation of the riser motions.
Numerical Simulation of Flow Around a Circular Cylinder Using a Fourier-Chebyshev Collocation Spectral Method
Authors: Marti´nez R. Johnny J. , Esperanc¸a Paulo T. T.
Published: January 2009

Vortex-induced vibrations (VIV) of slender structural elements (marine cables, pipes and risers) are very important aspects to be considered in the design stage of many offshore structures, because the fatigue life of risers installed in deep water are often dominated by VIV effects. Despite the relatively fundamental nature of the problem, a small amount is known about the nature of the fluid-structure interaction. The purpose of this paper is to develop a Fourier-Chebyshev collocation spectral method for computing unsteady two-dimensional viscous incompressible flow past a circular cylinder for moderate Reynolds numbers. The incompressible Navier-Stokes equations (INSE) are formulated in terms of the primitive variables, velocity and pressure. The incompressible Navier-Stokes equations in curvilinear coordinates are spectrally discretized and time integrated by a second-order mixed explicit/implicit time integration scheme. This scheme is a combination of the Crank-Nicolson scheme operating on the diffusive term and Adams-Bashforth scheme acting on the convective term. The projection method is used to split the solution of the INSE to the solution of two decoupled problems: the diffusion-convection equation (Burgers equation) to predict an intermediate velocity field and the Poisson equation for the pressure, it is used to correct the velocity field and satisfy the continuity equation. Finally, the numerical results obtained for the drag and lift coefficients around the circular cylinder are compared with results previously published.
Laboratory Measurements of the Dynamic Properties of Intact and Remolded Offshore Clays From Campeche Bay
Authors: Valle Celestino , Stokoe Kenneth H.
Published: January 2009

Comparisons of the dynamic properties of intact and remolded offshore clay specimens has been carried out. The clay specimens were obtained from Campeche Bay, offshore Mexico. Combined resonant column and torsional shear (RCTS) equipment at the University of Texas at Austin was used to determine the dynamic soil properties. Each soil specimen was tested twice, first in the intact condition and second as remolded material. Remolding was done by kneading the intact material and then reforming the specimen by compacting in a mold. The effects on the dynamic properties, expressed by shear modulus and material damping ratio, between intact and remolded conditions are discussed. As expected, shear modulus and material damping at small and large strains are affected by remolding. Interestingly, the normalized modulus degradation curves were changed very little by remolding up to strains between 0.06 and 0.1%. The results offer insight into the effects of sampling disturbance on linear and nonlinear dynamic soil properties.
Probabilistic Estimation in Soil Dynamic Properties to Obtain Response Spectra at Campeche Bay
Authors: Garcia Vargas Joel A. , Ruvalcaba Gonza´lez Antonio
Published: January 2009

In the present study, response spectra for seismic design in the Bay of Campeche for offshore platforms, using probabilistic models in soil dynamic properties (shear modulus “G”, normalized shear modulus “G/Gmax” and damping ratio “D”), to evaluate their influence in the response of the site of interest, are presented. In this study the probabilistic model of the soil, take into account the uncertainty in soil dynamic properties, due to the fact that the soil dynamic properties play an important role on the interpretation of free-field motion. In this work an explicit form of the seismic wave characteristics were supposed and their likely source-site propagation characteristics were performed with two computer programs. In order to obtain seismic design parameters, three types of seismic event records were associated to the soil properties at the site of interest, taking into account local effect conditions. Geothechnical, seimologycal, tectonical and geological information, similar to the area of interest, were used. The maximum acceleration on the surface, the soil dominant period of vibration, the duration and the frequency content due to a dynamic excitation applied in the rigid base of the soil column were considered. We assumed that each one of the three types of earthquakes are representative of strong motions that might be potentially affecting the Bay of Campeche. In the study thirty-one sampling in several sites located at the Bay of Campeche were used as well as field and laboratory tests. The dynamic properties were determined in each test type by different ranges according to their shearing strain. From these results test, we know the G and D variation law in the range of interest. In the same way, the uncertainty level influence of the soil dynamic properties was modeled with the expected value of the soil dynamic properties. Six different sets including the soil modulus plus/minus a standard deviation, coefficient from the dynamic properties-time variant evaluations, for each one of the shearing strain, which are contained between 10−4 a 10, were considered. Expected site response spectra for different recurrence periods, may be used to complement available information in order to update the present seismic design spectra for offshore facilities located in this area.
Effects of Aircushion Division to Hydroelastic Responses of an Aircushion Type Very Large Floating Structure
Authors: Ikoma Tomoki , Masuda Koichi , Maeda Hisaaki , Rheem Chang-Kyu
Published: January 2009

A target offshore structure in this study is an aircushion supported very large floating structure. The aircushion type VLFSs behave elastically in water waves. Corresponding aircushions are very large or relatively small size. The VLFSs considered in this study are supported by a large aircushion, two aircushions, or several module aircushions. The zero-draft theory is applied to the prediction of the hydrodynamic forces. The zero-draft theory is based on the pressure distribution method. The elastic deflection predicted by the zero-draft method is compared with that by another three-dimensional method in order to confirm the validity of it. In addition, the steady wave drifting forces on VLFSs with the aircushion are shown and their characteristics are examined. Then, the momentum theory is applied to the prediction. In the final section, effects of aircushion division to the elastic deflection and the wave drifting force are investigated. From the results, it is confirmed that the elastic deflection is can be reduced in the specification relation between the wavelength and the length of a module aircushion. In addition, it is possible to ajust the aircushion setting in order to simultaneously reduce the elastic deflection and the steady wave drifting force of the aircushion type VLFS on the case.
Effect of Volcanic Fines and Gradation on the Deformation and Strength Properties of Detrital Marine Sediments in Hawaii
Authors: Brandes Horst G. , Johnson Tonya
Published: January 2009

The compressibility and triaxial response of mixed sediments, consisting of volcanic and marine calcareous fractions, are investigated for a series of samples taken in the nearshore shallow waters of Kaneohe Bay in Hawaii. The results suggest that the presence of coarse detrital calcareous sands dominates the overall behavior if present in sufficiently large quantities, resulting in low compressibilities similar to that of silica sands and friction angles larger than that for most terrigenous sands at comparable densities. Larger than expected frictional resistance is attributed to the volume change tendencies of the non-uniform grains. On the other hand, when significant fines are present, whether of a volcanic nature or consisting of silty carbonate residue, compressibility tends to increase and frictional strength tends to decrease.
3D High Resolution Survey of Natural Seeps in the Campeche Sound
Authors: Nu´n˜ez Farfa´n Jaime , Cruz Roque Diego , Barrera Nabor Pro´coro , Koh Cambranis Wilbert
Published: January 2009

In order to define the zones at risk by the shallow gas and natural seeps of hydrocarbons in the Campeche Sound, a 3D geophysical study of 4 areas has been carried out in the region of the Cantarell oil field [1]. The results of this high resolution survey were correlated with previous explorations and it was possible to define the migration route and evolution of the shallow gas accumulations. The volume increase of the accumulations detected can not be explained by the lack of precision of the instruments and is more likely connected to the exploitation of the reservoir. The faults that transport the hydrocarbons from the reservoir to shallow strata define an area that contains several important platforms likely to be affected by the gas migration process. This geophysical survey was also used to define the location of three geotechnical borings for studying the degradation on the properties of the soils due to the presence of hydrocarbons that support the platforms.
Advances in Instrumentation to Monitor and Assess the Structural Integrity of Steel Jacket Platforms
Authors: Huxtable John , Stevenson Paul R. , Dover W. D. , Brennan F. P.
Published: January 2009

Structural Integrity Monitoring (SIM) is a growing practice, offering alternatives to periodic inspection, that can remove uncertainties and allow service work to continue without interruption. As computing power has increased, the abilities of SIM systems have similarly changed. Demands of the local environment and structural design have also resulted in changes to the capabilities of SIM. Changes in technology are examined with specific examples of enhancements particularly in monitoring of offshore structures. A UK-based organisation aiming to harmonise SIM methods to improve safety by removing common errors and reduce SIM development costs is also discussed.
Hydroelastic Modeling of a Compliant Offshore Tower: Formulation
Authors: Xia Jinzhu , Miao Quanming , Haritos Nicholas , Ronalds Beverley
Published: January 2009

Offshore oil and gas can be produced using a variety of platform types. One option, the compliant offshore tower, has proven to be an economic solution in moderately deep water (300–600m). In this paper, the wave-induced global dynamic responses of a compliant tower in wind, current and waves are studied in the context of fluid-structure interaction. A beam undergoing transverse and axial motion models the vertical member of the tower. The beam is supported by a linear-elastic torsional spring at the bottom end and a point mass and a buoyant chamber is located at the top free end. The fluid forces on the beam are modeled using the Morison equation while the hydrodynamic forces on the chamber are obtained based on the three-dimensional diffraction-radiation theory. By applying Hamilton’s variation principle, the equations of motion are derived for the coupled fluid-structure interaction system. The non-linear coupled system equations that emanate from this new approach can then be solved numerically in the time domain.
Parametric Study of Mooring Lines on a Small Buoy
Authors: Miao Quanming , Xia Jinzhu
Published: January 2009

A shallow-draft cylindrical buoy and mooring lines comprise an integrated dynamic system responding to environmental loading due to wind, current and waves in a complex way. In this paper, a time-domain decoupled buoy motion analysis method will be applied to study numerically the effect of the length, pre-tension and coordinates of attached points of the mooring lines on the motions of the buoy and the loads of the mooring lines. The wind and the current speeds are assumed to be constant and the wind and current forces are estimated from empirical formulations. The hydrodynamic coefficients, wave exciting forces and slow drift forces of the buoy are obtained from the 3D diffraction-radiation theory. The numerical results show how the length of the mooring line influences the maximum mooring loads at severe sea. The results also quantitatively show how the pre-tension and the coordinates of attached point of the mooring lines affect the motions and loads of the moored system.
High Heat Input Welding of 12Cr-6Ni-2.5Mo Supermartensitic Stainless Steel
Authors: Aune Ragnhild , Fostervoll Hans , Akselsen Odd Magne
Published: January 2009

In conventional welding of 13% Cr supermartensitic stainless steels, the normal microstructure that forms on cooling is martensite. Although high heat input tends to give a certain coarsening of the final microstructure, the eventual accompanying loss in toughness is not known. The present study was initiated to examine the effect of heat input on weld metal and heat affected zone mechanical properties of a 12Cr-6Ni-2.5Mo grade. The results obtained showed that the notch toughness is low (25 J) and independent of heat input for the weld metal, while it is reduced with increasing heat input for fusion line and the heat affected zone locations. Subsequent post weld heat treatment gave a substantial increase in toughness for all notch locations. Based on these results, indications are that a specified maximum heat input is not applicable in welding of supermartensitic stainless steels, allowing more production efficient techniques to be used, both in longitudinal seam and girth welding.
A Comparison of Dirichlet and Neumann Wavemakers for the Numerical Generation and Propagation of Nonlinear Long-Crested Surface Waves
Authors: Baddour R. E. , Parsons W.
Published: January 2009

We are studying numerically the problem of generation and propagation of gravity long-crested waves in a tank containing an incompressible inviscid homogeneous fluid initially at rest with a horizontal free surface of finite extent and of infinite depth. A non-orthogonal curvilinear coordinate system, which follows the free surface is constructed and the full nonlinear kinematic and dynamic free surface boundary conditions are utilized in the algorithm. “Wavemakers” are modeled using both the Dirichlet and Neumann lateral boundary conditions and a full comparison is given.
Hydrogen Assisted Cracking in Welding of 13% Cr Supermartensitic Stainless Steels
Authors: Aune Ragnhild , Fostervoll Hans , Akselsen Odd Magne
Published: January 2009

Supermartensitic stainless steels are known to be prone to hydrogen induced cold cracking. Therefore, the objective of this work was to assess the susceptibility to hydrogen cracking in Gas Metal Arc Welding (GMAW) with use of matching base and filler materials (supermartensitic stainless steel), using the Instrumented Restraint Cracking (IRC) test. Root welding in the IRC test did not result in hydrogen induced cracking, neither for low nor high weld metal hydrogen content. Because of the martensitic transformation, the global residual stresses are very low after welding (below 100 MPa). Since the yield strength (Rp0.2%) of the material is about 600–720 MPa, it implies that the IRC test method is not very suitable for supermartensitic stainless steels. However, by performing IRC test multi-layer welding, micro-cracks were found in the last pass. An increase in the weld metal hydrogen content resulted in reduced fracture stress and ductility, as observed in tensile testing of IRC test specimens directly after welding. Investigation of the fracture surfaces of the specimens with high hydrogen contents showed fish eyes, which are strong indications of hydrogen embrittlement. By performing heat treatment (225°C for 24 hours) of specimens with high hydrogen contents and subsequent tensile testing, the fracture stress and ductility were restored to the initial base metal level. Slow Strain Rate Testing (SSRT) with and without Cathodic Protection (CP) was performed on test specimens sampled transverse to the welding direction. CP has detrimental effect on the fracture stress and ductility due to the high weld metal hydrogen pick up.
The Ocean Thermal Gradient Hydraulic Power Plant and Its Scope
Authors: Beck Earl J.
Published: January 2009

Heretofore, the concept of developing power from the tropical oceans, (Ocean Thermal Energy Conversion, or OTEC) has assumed the mooring of large platforms holding the plants in deep water to secure the coldest possible condensing water. As the Ocean Thermal Gradient Hydraulic Power Plant (OTGHPP) does not depend, on the expansion of a working fluid, other than forming a foam of steam bubbles. It does not need extremely cold water as would be dictated by Carnot’s concept of efficiency and the 2nd Law of Thermodynamics. Plants may be based on or near-shore on selected tropical islands, where cool but not extremely cold water may be available at moderate depths. This paper discusses the above possibilities and two possible plant locations, as well as projected power outputs. The location and utilization of large of amounts of power on isolated islands, where cabling of power to major population centers would not be feasible are discussed. Two that come to mind are the reduction of bauxite to produce aluminum and the of current interest is the electrolyzing of water to produce gaseous hydrogen fuel to be used in fuel cells, with oxygen as a by-product.
A Study for the Ecological Development Policy of the Coastal City on Osaka Bay Coastal Zone
Authors: Sawada Yumiko , Fujii Yoshiyuki , Murata Buichiro , Kondo Takeo
Published: January 2009

After World War, the national land planning has taken the highest priority to the economic recovery and the regional development in our country. Especially the enclosed sea area has been utilized for the base of the exportation and importation and the high industrial growth. However, in that process, the environmental policy that was considered all aspects of coastal sea and marine organisms wasn’t enough carried out. In result, since the 1960’s the industrial accumulation and the coastal population explosion caused the pollution problems and the coastal environment problems in the whole country. Particularly overdevelopments of the coastal sea area and coastal line have deteriorated the amenity for human being. This paper makes clear the concept of the ecological development policy for the model of the coastal city, and studies for the ecological development policy of the coastal city on Osaka Bay coastal zone. On the occasion, we have to recognize the aspects of the coastal zone such as the property of the space, which gives new value from exchange and the fusion at the coastal zone, and the importance of desirable ecosystem involved human being.
Study of Porosity Location in Fresh Water Wet Welds
Authors: Pessoa Ezequiel C. P. , Bracarense Alexandre Q. , Liu Stephen , Pe´rez-Guerrero Faustino
Published: January 2009

The objective of this work was to determine for particular water depth, the optimum current ranges in terms of best porosity location. Pores located at the top part of each weld bead were generally eliminated through re-melting by the next weld bead. This finding was consistent in fresh water wet welds on ASTM A36 steel using three commercial electrodes: E6013, E7018 and E7024 grades. Two depth levels were studied, 50 and 100 meters. A gravity feed welding system was used in a hyperbaric chamber of 200 m pressure capability. Through visual analysis and with help of a computer software, the optimal weld parameters for each electrode and depth regarding pore location were determined. Additionally, it was observed that porosity increases with increasing current and depth. Similar to welding in seawater, it was also verified that the E6013 electrodes presented greater arc stability and produced the best beads, while the E7018 electrodes did not produce good results. The E7024 electrode exhibited easy arc opening and reopening but did not present good stability. Finally, it was observed that the optimal current range reduced as depth increases.
WORLDWAVES: High Quality Coastal and Offshore Wave Data Within Minutes for Any Global Site
Authors: Barstow Stephen F. , Mo̸rk Gunnar , Lo̸nseth Lasse , Schjo̸lberg Peter , Machado Ulla , Athanassoulis Gerassimos , Belibassakis Kostas , Gerostathis Theodore , Stefanakos Christos , Spaan Gerard
Published: January 2009

There has been a growing demand for reliable information on the wave conditions, in particular at coastal sites, as a result of increased utilisation of the coastal zone to a multitude of activities including various shoreline developments related to transportation, tourism, fish farming and recently wind and wave energy industries. This trend is likely to continue. Reliable data is also needed with respect to the management and protection of these often fragile environments. Many of those concerned with these wave-impacted environments still use antiquated data sources, usually from offshore waters as, in the absence of long term wave data collected at the site of interest, the calculation of reliable wave statistics at a coastal site is a complicated, time consuming and expensive business, requiring various data sets to be assembled. WORLDWAVES simplifies and speeds up the modelling of wave conditions in coastal waters by integrating the following under a single Matlab toolbox: High quality long-term wave data offshore all global coasts; worldwide bathymetric and coastline data; SWAN and backward raytracing wave models; sophisticated offshore and nearshore wave statistics toolboxes with tabular and graphical presentations, including a facility to export ASCII time series data at offshore or inshore locations; a geographic module with easy zooming to any area worldwide; tools to set up model grids and display and edit bathymetry and coastline; a facility for the import of user offshore data and export of inshore time series data. In this paper we describe the design and implementation of WorldWaves including the fusion of satellite, model and buoy wave and wind data in the global offshore database and the new raytracing model.
A Finite Element Method for a Nonlinear Sloshing Problem
Authors: Kyoung J. H. , Kim J. W. , Bai K. J.
Published: January 2009

A nonlinear sloshing problem in LNG tanker is numerically simulated. During excessive sloshing, the sloshing-induced impact load can cause a critical damage on the tank structure. Recently, this problem became one of important issues in FPSO design. A three-dimensional free surface flow in a tank is formulated in the scope of potential flow theory. The exact nonlinear free surface condition is satisfied numerically. A finite-element method based on Hamilton’s principle is employed as a numerical scheme. The problem is treated as an initial-value problem. The computations are made through an iterative method at each time step. The hydrodynamic loading on the pillar in the tank is computed and compared with other results.
The Analysis of Practical Usage Experience of the Ship Structures Fatigue Strength Criterion
Authors: Egorov Gennadiy V. , Kozlyakov Vitaliy V. , Sivoborodchenko Nickolay N.
Published: January 2009

It is shown in the report, that the trial-and-error requirements of the Rules of classification societies to the minimum section modulus of transport ships, including the FPSO-type ships, are based, mainly, on the maintenance of their strength, the lack of which in riveted ships exhibits in the form of weakening of rivet joints, and in welded ships in the form of cracks. The analysis of proposals for analytical formulation of the fatigue strength criterion (F.S.C.) during 150 years of development of steel shipbuilding confirms this thesis. It is shown, that modern recommendations of IACS (1999) do not provide stable reserves of fatigue resistance for ships of different standard-sizes, especially those constructed from high-tensile steels. In the report the ways of perfecting of the requirements to fatigue strength are offered.
The Experimental Study and Theoretical Analysis About the Ice-Wave Interaction
Authors: Du Heqing
Published: January 2009

Experiments were performed to examine ice-wave interaction that the waves attenuate under continuation ice sheet and at ice floes. The analysis of ice-wave interaction and numerical calculation of the wave attenuation in floe zone are still important problems. It was understood that the reasons of the attenuation are creep, viscous, reflection and collision, and the viscosity has been calculated by using hydrodynamics method in the water boundary layer. Proportion of the every reason and the quantitative analysis has been done.
Nonlinear Free-Surface and Viscous-Internal Sloshing in 2-D Numerical Wave Tanks
Authors: Valentine Daniel T. , Frandsen Jannette B.
Published: January 2009

This paper examines free-surface and internal-pycnocline sloshing motions in 2-D numerical wave tanks subjected to horizontal base excitation. In all of the cases studied, the rectangular tank of liquid has a width-to-depth ratio of 2. The first set of results are based on an inviscid, fully nonlinear finite difference free-surface model. The model equations are mapped from the physical domain onto a rectangular domain. Case studies at and off resonance are presented illustrating when linear theory is inadequate. The next set of results are concerned with analyzing internal waves induced by sloshing a density-stratified liquid. Nonlinear, viscous flow equations are solved. The influence of the side-wall boundary layers on sloshing motions as well as the onset of internal breaking of the primary sloshing mode are discussed. The frequencies that characterize the motion of internal waves are also reported.
Promotion of Symbiotic Relationship Between Human and Ecosystems in a Small Island
Authors: Hosoda Ryusuke , Hanano Koichi , Fukutani Yoichi
Published: January 2009

People have become aware that maintenance of symbiotic relations between people and their surrounding environment is important not only for people’s happiness but also for their own existence on the earth system. Facing up to reality, however, there have arisen matters of regret caused by the human lives and activities. One of big problems we have to consider is scatter of waste and debris on natural beaches, especially in the coastal area near the dense populated area. We have to remember that the scatter is the results of human activities mainly on the coastal land area. The problem had become serious on a small island located near the mouth of Osaka Bay, where rubbishes such as tins, bottles and plastic bottles, bags and sheets have been piled and remained on the beach for more than half a century. People living in a small community, who were worrying about the decline of the ecosystem of the small island, started the activity of beach cleaning more than ten years ago. Since then, the quality of the ecosystem on the island has been maintained in a permissible condition. They also started the field researches on the fauna and flora in and around the island to make an inventory of fauna and flora. Their activities were appreciated and they were honored with testimonials commending their contribution on the maintenance of symbiosis. In the present paper, people’s activities are introduced with the importance of promoting the symbiosis with ecosystem in surrounding natural environments.
System Identification of Nonlinear Coupled Ship/Offshore Platform Dynamics in Beam Seas
Authors: Cheng Jun , Falzarano Jeffrey M.
Published: January 2009

The Mobile Offshore Base (MOB) is designed to transit to anywhere in the world in the required time frame. This means that the MOB must be able to transit in severe environmental conditions. In these extreme sea conditions, a primary cause for concern is the large accelerations that the vessel motions might experience due to the high static stability of the MOB at Transit Draft. Furthermore, since the vessel has minimum freeboard in this condition, it is exposed to green water over the pontoon tops. The submergence of the pontoon deck causes a considerable loss in the vessel’s restoring moment. These concerns have warranted a study by the Office of Naval Research into the Transit Draft Dynamics of the MOB. During the research of MOB dynamical properties, a nonlinear system modeling and optimization tool utilizing Reverse MI/SO (Multiple-Input / Single Output) techniques was developed and applied to different aspects of MOB dynamics analysis. MISO is based on statistical signal processing of the recorded time histories of the excitation and response of the non-linear multi-degree-of-freedom system. This method of analysis is functional and reliable in identifying an optimum representation of the linear and non-linear parameters of the system under consideration. In this paper, we analyze the model testing data in beam seas using the Reverse MISO technique. We expected to identify significant nonlinear roll damping for the nonlinear integro-differential equation as is the case with ships. Instead, a significant nonlinear heave damping related with the nonlinear relative heave velocity has been found during the analysis. This reminds us again that due to the strong nonlinearity of MOB motions in the severe sea ways, nonlinear analysis methods such as Reverse MISO are important and need to be applyed in order to fully identify the dynamics of the MOB motion.
Force-Feedback Control in VIV Experiments
Authors: Smogeli O̸yvind N. , Hover Franz S. , Triantafyllou Michael S.
Published: January 2009

A force-feedback control system for VIV experiments is designed and evaluated for the purpose of achieving high accuracy free vibration tests. Through an organized approach, this work details specific methods for minimizing the combined effect of mass and damping using control system fundamentals. The dynamics of the closed-loop system are analyzed, a numerical model constructed and a control scheme is chosen and implemented in real-time. The control system performance is evaluated by performing frequency response tests in air. Free vibrations of a smooth aluminum cylinder are performed at Reynolds number 19000. Test series with damping ratios of one, two and five percent are performed, all with nondimensional mass four. A peak amplitude ratio of 1.15 is observed for the case of lowest damping. Forced vibration tests with the same setup are performed and compared to the free vibration results, giving consistent results.
Reliability of Pipeline Affected by Corrosion and Third Parts
Authors: Alamilla Jorge L. , Flores-Maci´as Oscar , Vargas Alejandro
Published: January 2009

An approach to evaluate the structural safety of pipelines affected by corrosion and third parts is presented. This formulation is applied when the information about the corrosion state of the system is available. This information includes: the total number of detected corrosion defects, depth and length of each measured defect. The probabilistic model considers the detected and undetected corrosion, and generation of new corrosion defects over time. In other words, the model takes into account the corrosion state of the system and the quality of the inspection tool. The failure is defined by a safety margin that is evaluated in terms of pressure stress. The undetected defects and the generation of new defects are modeled by a Poisson process. The contribution of failure by third parts to the failure probability is taken into account in a simplified way. For this case the probability function is obtained from the reported failures. The proposed model is applied to a pipeline segment in which reliability indexes over time are represented in terms of reliability functions that characterize the probability distribution between failures.
Hybrid Event Tree Analysis of Ship Grounding Probability
Authors: Li Dianqing , Tang Wenyong , Zhang Shengkun
Published: January 2009

The probability of occurrence of human-error-dominated event can not be effectively handled by using conventional event tree method. Furthermore, it is impossible to consider the sensitivity and uncertainty of basic events’ probabilities in conventional event tree method. To overcome these disadvantages, the concepts of the fuzzy error rate and the fuzzy error possibility were introduced to represent the failure probability of human-error-dominated event, while the fuzzy probability was used to represent the failure probability of non-human-error-dominated event. Based on them, the fuzzy probability of ship grounding with piloting failure was calculated by hybrid event tree method. Moreover, the fuzzy sensitivity index and the fuzzy uncertainty index were defined. Some grounding probability reducing measures were verified by sensitivity and uncertainty analysis. The results indicate that the proposed approach is very useful in analyzing the probability of occurrence of human-error-dominated event.
Design of Mono-Column TLPs for Ultra-Deepwater Applications
Authors: Davies Kent , Leverette Steve , Williams Neil
Published: January 2009

Previous industry discussion has notionally limited the TLP concept to water depths less than 1,500m (∼4500 ft). This paper presents the results of analysis performed to clarify the limits for mono-column TLP’s and explores means to extend the applicable range of TLP’s to ultra-deep water. Detailed coupled motion and fatigue analyses were carried out for a realistic production case over a wide range of water depths (1,000 to 3,000m) for moderate (Gulf of Mexico) and mild (West Africa) conditions. The existing Matterhorn SeaStar TLP design was used as the test platform for the study. The topsides payload and surface-well count were held constant while adapting the hull and tendons for increasing water depth. Consideration was given to both conventional and nonconventional, but near-term, tendon solutions. Following the engineering analysis, design alternatives are compared on an economic basis.
A Feasibility Study on Generation of Electricity by the Tidal Current in Kanmon Strait
Authors: Fujiwara Takamichi , Hamada Takaharu , Kyozuka Yusaku
Published: January 2009

Promotion of the utilization of natural energy is urgent as a countermeasure to global warming. There are several kinds of energy resources in the ocean: waves, thermal differences, tidal levels and tidal currents. A feasibility study on tidal current power generation in Kanmon Strait, Japan is introduced in this report. A variable mesh version of the Marine Environmental Committee ocean model is used to reproduce the tidal current and to learn the distribution of tidal kinetic energy. Based on the simulation results, the optimal location for installation of a power plant is determined. The electric power created by a propeller type generator with a 10m diameter is also calculated as the average annual consumption of a household and availability of the power generation by the tidal current in Kanmon Strait is discussed.
A Numerical Simulation of Physical Environments in the Ariake Sea Including the Effects of the Tidal Flat
Authors: Yamaguchi Soichi , Kyozuka Yusaku
Published: January 2009

The Ariake Sea surrounded by Nagasaki, Kumamoto, Saga and Fukuoka prefecture, Japan has a huge tidal flat area and very high productivity. However, human activities such as reclamation of this tidal flat and loading from the rivers have polluted its environments. As a result, the number of red tides and amount of oxygen deficient water in the Ariake Sea and the area where those phenomena occur has increased from 1980s. The red tide by Rhizosolenia imbricata that occurred November, 2000 caused particularly severe damage to the laver cultivation. The real cause for these phenomena has not clearly been identified, But resolution of these problems requires a physical and biochemical study of the environments. As the first step in such an approach, the authors developed a method to reproduce the physical environments with high accuracy by using numerical simulation which considers the effects on tidal flat and compares them with the observed physical environments. The simulation shows good agreement with the observations made in the Ariake Sea itself.
Benefit of Partly Displacement Controlled Condition in Sagbend
Authors: Collberg Leif , Aronsen Kristoffer , Palmer Andrew , Hahn Guillermo
Published: January 2009

Some codes like BS8010 and DNV-OS-F101 distinguish between load controlled condition or displacement controlled (or strain based) condition. Even though the principles are clear, it is often hard to determine the difference in practice. A displacement controlled condition allows a higher utilization and is therefore beneficial, and can be applied provided that the condition can be classified as a displacement controlled condition. Many projects have had intense discussions on this matter during the last 15 years, however, without much progress. The objective of this paper is two-fold. First it will show that a discussion on load controlled versus displacement controlled is of limited value. The discussion should rather be on how to take benefit of a partially displacement controlled condition. Second, the paper gives a suggestion on how to allow for a partially displacement controlled condition, also determining the degree of displacement control. The suggestion is supported by specific FE-calculations.
Model Testing of Ice Barriers Used for Reduction of Design Ice Loads
Authors: Jochmann Peter , Evers Karl-Ulrich , Kuehnlein Walter L.
Published: January 2009

The drilling rig Sunkar, owned and operated by Parker Drilling under contract to Agip KCO, was the first drilling rig in the North Caspian Sea. In this area sea ice may occur between November and April. The original ice protection concept of the drilling rig was based on the fact that ice loads were partly taken and/or reduced by two rows of heavy piles. As an alternative concept ice model tests for Sunkar with installed ice barriers (Ice Rubble Generators) around the rig were carried out in the Large Ice Model Basin of HSVA in order to establish the design ice loads and to prove that the design forces can be reduced significantly by using these ice barriers. The test series were carried out in 1.3 m thick level ice with a bending strength of 770 kPa. Ice drift angle, ice drift speed, spacing between the ice barriers, as well as the angle of the ice barriers were varied. The design water level simulated in the model tests was about 4 m. As maximum measured ice load values are a result of coincidental ice failure occurrences these values are much more scattered than the mean values of ice model tests. Even if an individual model test could be repeated exactly, i.e. exactly within measurable limits, the maximum load would be different. Therefore the design load needs to be obtained by using a sophisticated statistical approach. To establish the design load an extreme value distribution, a Gumbel-Probability-Distribution (GPD) for each individual model run has been applied. The ice model tests have shown that a significant ice force reduction can be achieved if the drilling rig Sunkar is protected with ice barriers. The reduction of the maximum horizontal global ice load amounts to approx. 63% when Sunkar is protected by ice barriers. The ice barriers initiate ice rubble and areas of rafted ice as well as ice accumulation between the barriers, which lead to ice bridging with a spacing of 60 to 80 m between the ice barriers. As a final result it was found that the stability of Sunkar will be sufficient under any angle of drifting ice if ice barriers are installed.
Tests on Dynamic Ice-Structure Interaction
Authors: Ka¨rna¨ Tuomo , Kolari Kari , Jochmann Peter , Evers Karl-Ulrich , Bi Xiangjun , Ma¨a¨tta¨nen Mauri , Martonen Petter
Published: January 2009

This paper addresses the problem of ice induced vibration of offshore structures. Compliant structures having vertical ice walls may suffer from very severe vibrations. Several theories have been proposed to predict these vibrations. However, physical details of this phenomenon are not fully understood. Conical structures are deemed to be less sensitive to vibrations. Recent full-scale measurements made in the Bohai Bay indicate that also these structures my experience excessive vibrations caused by ice. Indentation tests were done at the ARCTEC laboratory at the Hamburg Ship Model Basin, Hamburg to clarify details of ice induced vibration. Sheets of columnar grained saline ice was used in the tests. The parameters that were varied included the structure’s compliance and damping, indentation velocity and the structural geometry at the water line. The paper provides tests results that were obtained using vertical and conical model structures.
PROCAP-3000: New Challenges for 3000m Water Depth
Authors: Salie´s Jacques Braile
Published: January 2009

This paper presents the main drives that PETROBRAS has to overcome the challenge of producing in water depth up to 3000m. It presents also the PROCAP-3000, the in-house Technological Program on Ultra -Deepwater Exploitation System, and its main projects developed to be able to achieve the goals. The paper presents in adition the major challenges in the following areas: flow lines, risers, completion riser, boosting, and subsea equipment. The main projects of each area will be discussed, the main results presented and the challenges pointed. Petrobras technical staff believes that the above technical areas and the chosen projects, to be presented, play a key-hole in ultra-deepwater production, not only in Brazil, but also all over the word in deepwater developments.
Inspection Reliability for Offshore Structures
Authors: Dover W. D. , Brennan F. P. , Kare´ R. F. , Stacey A.
Published: January 2009

The assessment of fatigue cracks in offshore structures involves stress analysis, fatigue fracture mechanics analysis and knowledge of the likely loading on the damaged member. Before embarking on the analysis it is necessary to assess the input information on the presence, location and size of any defect. This requires detailed information on the reliability of the nondestructive testing technique used for the inspection. Inspection reliability data is also extremely important for inspection scheduling especially in situations where reliability based inspection scheduling is to be used. To produce this type of data is expensive and time consuming as it involves blind trials, and a large sample containing realistic defects. The ICON project [1], a major EU Industry funded project, addressed this problem and conducted inspection reliability trials for the offshore industry. ICON and the subsequent ICON User Group produced a large database on fatigue crack detection and sizing techniques. This data is now available in the ICON User group Software. The paper has used this software to produce POD data for ACFM and MPI. The POD data has been assessed in terms of crack categories (including categorisation based on BS7910), the influence of spurious indications, and in terms of the influence it has on fatigue crack growth predictions.
A Feasibility Study for Steel Catenary Risers Connected to a Spread-Moored FPU Monohull in a Field Offshore Brazil
Authors: Pollack Jack , Davies Kent B. , Riggs David C.
Published: January 2009

Until recently, the feasibility of connecting Steel Catenary Risers (SCRs) to monohull FPU / FPSOs (Floating Production Units / Floating Production Storage Offloading) had not been proven for deepwaters offshore Brazil. Extreme deepwater applications have required the use of steel risers to avoid hydrostatic collapse. Since SCRs are usually limited by fatigue, due to seabed interaction in the Touch Down Zone (TDZ), the effect of monohull vessel motions on SCR fatigue life had been an issue to be resolved. In the past, only semi-submersibles had been considered for these Brazil applications. Today it can be shown that spread-moored ship-shapes are sufficiently stable for the connection of several large diameter SCRs.
Coupled Analysis of a Spar Structure: Monte Carlo and Statistical Linearization Solutions
Authors: Spanos Pol D. , Ghosh Rupak , Finn Lyle D. , Halkyard John
Published: January 2009

The response of a combined dynamic system consisting of a Spar, a mooring line system, and top tension risers is presented. The top tension of a riser is provided by a buoyancy can that is laterally supported by a preloaded compliant guide. The overall system is nonlinear due to the mooring line restoring force, the friction between the buoyancy cans and the compliant guides, and the friction between the risers and the guides at the Spar keel. The friction between the Spar and the riser system is captured by a Coulomb model, whereas the stiffness of the mooring line system is accounted for by using a regression analysis of pertinent load versus displacement data. The combined model is numerically integrated using input time histories of force and moment that are compatible with a random representation (Jonswap spectrum) of a typical extreme environmental loading in the Gulf of Mexico. The input time histories of the force and the moment for the combined model are obtained at the center of gravity of the Spar by using a standard motion analysis program (MLTSIM). The numerical results of the time domain analysis of the combined model, in conjunction with ergodicity, are used for an associated Monte Carlo study. The Monte Carlo study provides useful information regarding the probability densities and the power spectra of various response components of the combined system. These results serve as a benchmark for additional analytical studies that use the technique of statistical linearization to derive equivalent linear stiffness and damping coefficients for the combined system.
Multi-Tube Model Application in Inner Riser Centralizer Analysis
Authors: Chang Roger , Yu Jim
Published: January 2009

For top-tensioned riser analyses, the multi-tube modeling scheme is increasingly being used due to the growing application of the concentric dual casing riser systems. One of the major advantages of using the multi-tube modeling scheme over the conventional composite modeling scheme is the multi-tube model’s capability to determine the size and locations of the inner riser centralizers, which are used to prevent the inner riser from buckling when the riser is in production, protect the inner riser and its couplings from rubbing the outer riser, and minimize the contact load between the outer and inner risers. This paper explains the methodology used to perform this unique analysis and introduces the concept of the stress increase factor that should be included in the riser strength check and fatigue life analyses. This paper also demonstrates that misplacing centralizers or improper spacing will increase the contact load at the two most critical regions of the riser system, the stress joint and keel joint, rather than decrease the contact load. Based on the analysis results, guidelines for designing and analyzing centralizers are developed.