bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Strength Assessment of Pitted Stiffened Plates Under Biaxial Compression Loading
Authors: Amlashi Hadi K. K. , Moan Torgeir
Published: November 2008

Structural components of ships are subjected to corrosion, especially when exposed to sea environment. It is, therefore, essential to seek rational standards for the structural integrity of aging structures without economic penalties with respect to the repair and maintenance costs incurred over the life cycle of the structure. To make proper decisions about costs and safety, knowledge about the effect of corrosion and other degradation phenomena on the strength is of crucial importance. The main objective of the present study is to investigate the strength of pitted stiffened plates under biaxial compression loading. For this purpose, systematic numerical FE investigations of ultimate collapse and post-ultimate responses for the stiffened plates with pit corrosion wastage are undertaken, varying the degree of pit corrosion intensity (DOP), pit depth and the location of the densest pitted zone. The focus here is on local plate failure mode which is desired to be governing failure mode in designing of stiffened plates. Geometric imperfections and residual stresses were included in the models. It is found that the ultimate strength of a stiffened plate can be significantly decreased due to pitting corrosion. The ultimate strength of pitted stiffened plates is governed not only by the level of DOP, but also by the smallest sectional area and the location of densest pitted zone. This improved knowledge of the damage tolerance assessment of stiffened plates can be used to schedule repairs more efficiently, while risk or reliability assessment schemes are normally applied for that purpose.
Accurate Numerical Methods for Calculating the Response Statistics of Compliant Offshore Structures
Authors: Naess A. , Karlsen H. C. , Teigen P. S.
Published: November 2008

The state-of-the-art representation of the horizontal motions of e.g. a TLP in random seas is in terms of a second order stochastic Volterra series. Until recently, there has been no method available for accurately calculating the mean level upcrossing rate of such response processes. Since the mean upcrossing rate is a key parameter for estimating the large and extreme responses it is clearly of importance to develop methods for its calculation. The paper describes numerical methods for calculating the mean level upcrossing rate of a stochastic response process represented as a second order stochastic Volterra series. Since no approximations are made, the only source of inaccuracy is in the numerical calculation, which can be controlled.
Modeling Vortex Induced Motions of Spars in Uniform and Stratified Flows
Authors: Oakley Owen H. , Constantinides Yiannis , Navarro Claudia , Holmes Samuel
Published: November 2008

This paper examines the vortex induced motions (VIM) of a spar type floating production platform in uniform and sheared currents. The large draft of modern production platforms means that in some of the extreme current events the flow past the platform is highly non-uniform along the hull. We discuss the simulation of these stratified flows associated with hurricane events and loop currents and the implications for experiments and numerical simulations. Model testing options are reviewed along with the potential effects of buoyancy due to temperature and salinity variations in the current. Comparisons are made between experimental test results and numerical simulations of VIM at small scale and projections are made to full scale behavior using computational fluid dynamics (CFD) and detached eddy simulation (DES).
Epistemic Uncertainties in Decision Making
Authors: Faber Michael Havbro , Maes Marc A.
Published: November 2008

The present paper reviews and outlines the interpretation of uncertainties with a view to the various different categorizations introduced in the literature. A framework is then presented for risk based decision making taking basis in the Bayesian decision theory and recent methodical developments in risk assessment. It is emphasized that in principle all types of uncertainties should be included in formal decision analysis and that not doing so corresponds to informal decision analysis the quality of which may be difficult to judge. The controversial problem in engineering decision making of how to take into account uncertainties associated with the definition of the system being analyzed is outlined. For the typical situation where a discrete set of possible system representations is possible it is shown how a decision problem may be formulated for the identification of the optimal system to be considered as basis for decision making. The presented decision framework takes into account all prevailing uncertainties, epistemic as well as aleatory. Examples related to structural design and assessment problems relevant for offshore engineering are given illustrating how not to account for all types of uncertainties leads to sub-optimal decision making.
Active and Passive Control of Spar Vortex-Induced Motions
Authors: Korpus Richard , Liapis Stergios
Published: November 2008

Despite designer’s best efforts, Vortex-Induced Motion (VIM) of spars remains a significant problem. While strakes have proven highly effective at suppressing riser VIM, all three straked classical spars in the Gulf of Mexico continue to experience occasional VIM events. Since VIM can cause riser damage, mooring line fatigue, and even work stoppage, its suppression is a major priority. But designers will only successfully eliminate VIM once reliable predictions tools become available. VIM is both roughness and Reynolds number dependent, and spars lie outside the validated range of current computational tools in both these areas. This paper presents the development of Reynolds-Averaged Navier-Stokes (RANS) methods to predict real-world spar VIM behavior. It includes the ability to address rough surfaces and high super-critical Reynolds numbers. Validation is provided against existing experimental data where available. The resulting algorithms were used to assess effectiveness of two alternative control strategies for suppressing spar VIM. The first strategy consists of actively injecting fluid in a direction tangential to the spar. The second strategy is passive, and relies on adding an external sleeve to the spar. Active injection was found highly effective, and even capable of totally eliminating VIM. The passive alternative was also found to reduce VIM, but is not as effective as active injection.
Microalloy Precipitation Control in Development of TMCP
Authors: Suzuki Shuichi
Published: November 2008

This paper reviewed the development of microalloying technology, mainly took place in Japan. This is neither comprehensive survey nor scrutiny on a specific subject in the microalloying technology, but rather a summary of personal development experience in this field. Because too many papers have been already published on improvement of the base metal properties by microalloying and/or TMCP, I intentionally focused on the application side of microalloying to the HAZ and the weld metal. But every phase was surveyed by the common metallurgical effect of microalloying elements: that is, the kinetics of carbo-nitrides. Firstly the reason for the usefulness of microalloying is explained in relation with the TMCP. Secondly precipitation, growth, and dissolution kinetics of microalloying carbo-nitrides and their application to the HAZ property control are reviewed. Lastly, the influence of steel design on the weld metal property is introduced, by exemplifying the SAW material design of line pipes.
Seakeeping Tests With Gaussian Wave Packets
Authors: Matos Vini´cius , Sales Joel Sena , Sphaier S. H.
Published: November 2008

An important property used in the design stage of floating systems is the RAO (Response Amplitude Operator), the transfer function, for motions, forces and so on. This importance has motivated the development of several analytical, numerical and experimental tools to obtain the hydrodynamic behavior of platforms and ships. Experimental model tests in wave tanks are advisable for the accurate evaluation of the body movements. Three known techniques are used to obtain the RAO curves: tests with regular, irregular and transient waves. In the present work, special attention is given to the technique used to perform model testing with one type of transient wave: the Gaussian Wave Packet. The advantages of using such technique are discussed and results are also presented for a semi-submersible model during tests carried out at Laborato´rio de Tecnologia Oceaˆnica - LabOceano/COPPE/UFRJ, Brazil. Numerical calculations and tests with regular and irregular waves are used for validation and comparison.
On the Effect of Corrosion Defects in the Collapse Pressure of Pipelines
Authors: Netto T. A. , Ferraz U. S. , Botto A.
Published: November 2008

The loss of metal in a pipeline due to corrosion usually results in localized pits with various depths and irregular shapes on its external and internal surfaces. The effect of external corrosion defects was studied via a series of small-scale experiments and through a nonlinear numerical model based on the finite element method. After calibrated in view of the experimental results, the model was used to determine the collapse pressure as a function of material and geometric parameters of different pipes and defects.
Development of Piled Foundation Bias Factors in the Arabian Gulf
Authors: Zaghloul Hassan , Ronalds Beverley , Cole Geoff
Published: November 2008

Offshore sites in the Arabian Gulf are characterized by the presence of calcareous sediments. Research work on driven piles in calcareous sediments has been developing for over 40 years. Yet, international codes and standards do not provide, exploit or quantify guidance for driven piles in carbonated sediments. Lack of design methods is partly attributed to inability of conventional soil mechanics to predict appropriate engineering parameters in calcareous sediments. Further, the characteristics of the sediments vary between the geographical regions depending on the geological history forming that region. As a result, current industry practice follows a conservative and subjective approach at the mere mention of carbonated sediments. Consequently, reassessment of an existing platform may falsely indicate the need for expensive construction intervention. In this study, we reviewed current practice for assessment of piled foundation in the calcareous sediments of the Arabian Gulf, collated a database of actual pile driving records, developed and implemented a detailed back-analysis procedure and implemented to derive actual pile capacities. The statistics show that the use of a single capacity value, as implied by the deterministic method of codes and standards, is insufficient to describe the various conditions surrounding the as-installed driven piles in calcareous sediments.
Evaluating Fracture Toughness of Ductile Materials: A Critical Indentation Energy Model Based on Continuum Damage Mechanics
Authors: Lee Jung-Suk , Kim Eui-Hyun , Kwon Dongil
Published: November 2008

A new model for determining fracture toughness of ductile materials is presented that uses indentation and is based on continuum damage mechanics (CDM) and the concept of critical void volume fraction. Fracture toughness evaluated using the model showed good agreement with standard fracture toughness test results.
Large-Wave Simulation of Surface Tension Effect on Weak Spilling Breakers
Authors: Dimas Athanassios A.
Published: November 2008

The effect of surface tension on the evolution of weak spilling breakers is studied by performing large-wave simulations (LWS) of the free-surface flow developing by the interaction of a gravity free-surface wave and a surface shear-layer current. The flow models the evolution of gravity waves under the influence of wind shear. The surface tension modifies the dynamic free-surface condition and its effect depends on the dimensionless Weber number. The Euler equations are filtered according to the LWS formulation and solved numerically by a spectral method and a fractional-time-step scheme. The results indicate a stronger surface tension effect with decreasing Weber number values and increasing initial wave height. Specifically, decreasing the Weber number alters the size and shape of the characteristic bulge of spilling breakers and the toe position resulting in sharper slopes and angles of the free surface profile. The spiller wave height is reduced with decreasing Weber number.
Methods for Reducing a Large Number of Current Profiles to a Small Subset for Riser VIV Analysis
Authors: Lambrakos Kostas F. , Sidarta Djoni E. , Thompson Hugh M. , Steen Atle , Burke Roger W.
Published: November 2008

The paper presents two different approaches to construct subsets of current profiles from a large set of long term current profiles for the purpose of performing calculations for riser fatigue damage from vortex induced vibrations (VIV). The subsets are intended to reproduce the fatigue damage from the full set of current profiles. In the first approach, the full set of profiles is first sorted into bins based on current magnitude, direction and shear in the profile. The profiles within each bin are then reduced to a single constructed profile through one of many possible current averaging schemes. The present study includes two types of constructed profiles; one profile is generated by the average value of the currents for each bin and the other by the average value plus one standard deviation. The second approach is based on first performing a simplified and computationally efficient VIV analysis of the full set of profiles. The profiles are then sorted into bins by the dominant excitation mode, and then a single profile is chosen to represent all the profiles that excite the mode of interest. The chosen profile for the mode of interest has VIV power-in which is close to the average power-in for all the profiles that excite the mode. The number of profiles in the subset is equal to the number of modes that are excited by the full set of profiles. The VIV power-in in this paper is estimated through a simplified procedure that is consistent with the SHEAR7 methodology. Other available codes can also be used for the simplified VIV calculations.
Determination of Precise Contact Area During Spherical Indentation for Metallic Materials
Authors: Kim Sung-Hoon , Lee Kyung-Woo , Jeon Eun-Chae , Kwon Dongil
Published: November 2008

The continuous indentation technique, because it is fast, precise, and nondestructive, has been widely used to determine such mechanical properties as flow properties, residual stress, fracture properties, viscoelastic properties and hardness of materials and structural units. In particular, continuous indentation by a spherical indenter can provide hardness and flow properties such as yield strength, tensile strength, and work-hardening exponent, using the characteristic that strain from the loaded indenter changes with indentation depth. Since the stress and strain values on the flow curve are defined based on the contact area between the indenter and material in the loaded state, accurate determination of the contact area is essential. Determination of the contact area is closely connected with elastic deflection and plastic pile-up/sink-in behavior. In this study, the pile-up phenomenon is considered as two independent behaviors, elastic deflection and plastic pile-up/sink-in, which can each be described by a formula. The formulas can be obtained from FE simulation with conditions reflecting real indentation tests for materials used for various purposes and with a wide range of material properties. By analyzing indentation morphology from the FE simulation, the two phenomena were quantified as formulas. In particular, plastic pile-up/sink-in behavior was formulated in terms of work-hardening exponent and indentation ratio.
Dynamic Behaviour of Seismically Isolated Cylindrical Offshore Storage Tanks
Authors: Mastan-Zade Nijat , Yazici Gokhan
Published: November 2008

Cylindrical offshore storage tanks are mainly used to store critical liquids such as petroleum and LNG. Failure of these tanks after an earthquake not only results in high financial losses but can also create an environmental disaster as well. Seismic isolation technique has been utilized in this study to mitigate the damaging effects of lateral earthquake forces. In this study, seismic isolation is used in cylindrical offshore storage tanks. Seismic isolators are protected from the marine environment by a water-proof flexible membrane that allows the horizontal movement of the tank. The mathematical model takes the following into account: mass of the stored liquid, added mass of the water surrounding the tank, horizontal stiffness and damping of the isolation system. Internal forces due to the hydrostatic and hydrodynamic forces acting on the system during an earthquake have been calculated by using the shell-moment theory. The results of the analyses are compared for both the isolated and fixed-base tanks.
Spectral Analysis of Nonlinear Random Wave Loadings
Authors: Ma Rujian , Li Guixi , Zhao Dong
Published: November 2008

The spectral analysis of nonlinear random wave loadings on circular cylinders is performed in this paper by means of nonlinear spectral analysis. The study is carried out by expressing the wave profile and velocities of water particles as a nonlinear composition of the first order wave profile. Under the assumption of the first order wave profile being a zero-mean Gaussian process, the random wave spectra of finite amplitude waves are given. In order to solve the loading spectra of the finite amplitude random waves, the drag force is extended into power series of velocity. The loadings of the finite amplitude random waves are then expressed as nonlinear compositions of the first order wave profile and its derivatives. These techniques made it easier to compute the spectral densities of the finite amplitude random wave loadings.
A New Solution for the Return Period of a Sea Storm in Which the Largest Wave Height Exceeds a Fixed Threshold
Authors: Arena Felice , Pavone Diego
Published: November 2008

A new analytical solution for the return period of a sea storm in which the largest wave height exceeds a fixed threshold is obtained, by applying the Boccotti’s Equivalent Triangular Storm (ETS) model. An expression is then given for the probability that a wave, which is both higher than a fixed threshold and the highest of its own storm, will occur during a sea storm with a given intensity (the storm intensity being the maximum significant wave height during the storm). In the applications it is shown that the new solution has a simpler form than the Boccotti’s original one (two integrals to solve numerically, compared with the four integrals of the Boccotti’s solution), and that they give identical results. Finally it is achieved that the extreme waves of given height H, with large probability will occur in a sea storm with maximum significant wave height Hs max close to 0.5H.
Dynamic Behavior of a Vertical Riser and Service Life Reduction
Authors: Morooka Celso K. , Coelho Fa´bio M. , Ribeiro Elton J. B. , Ferrari Jose´ A. , Franciss Ricardo
Published: November 2008

In the last years, the most of offshore oil and gas reserves discoveries in Brazil are placed in ultra-deep water depths. Petroleum production from these offshore fields needs developments with novel solutions in terms of necessary technologies and economical viability. The use of vertical rigid risers such as top tensioned risers (TTR) and others like combined systems as self standing hybrid risers and steel catenary risers for ultra-deep waters have shown viable from both, technical and economical aspects. However, there are needs for detailed studies on their dynamic behavior in order to improve, particularly, the understanding of influence of the environment as wave and current, and floating platform oscillations at the riser top. The present work presents studies on vertical top tensioned riser dynamic behavior through time domain simulations of its displacements and respective, bending moments and stresses. Influences of the vortex induced vibrations (VIV) and waves on the riser service life reduction are analyzed. Maximum and minimum envelops for displacements and stresses along riser length are shown.
ECO-PLATFORM: A Solution for Solving Environmental Problems in Coastal Area
Authors: Hosoda Ryusuke , Murota Shigemochi , Yao Masahiko , Tamura Yuki
Published: November 2008

The authors proposed a concept of “ECO-PLATFORM” for solving environmental problems that people have been facing and will face in and around the coastal area, setting their sights on the contribution from the viewpoint of the sea [1]. The Eco-Platform is a complex on a floating platform and has functions for solving social and environmental problems. The functions to be installed are generation of heat energy and electric power from combustible waste without introducing fossil energy, cascade use of heat energy, application of marine biotechnology for processing the sewage and for purifying eutrophic seawater, reproduction/regeneration of material and energy resources from manmade waste and sewage. All the functions to be installed are integrated systematically and efficiently. To ensure the realization of the concept, it is necessary to study/investigate the feasibility of systematic combination of functions and facilities installed on the Eco-Platform. The authors have been studying the required functions and size of each facility, probable waste/sewage/energy/resource input and output relations. After the studies on the desirable combination and arrangement of installed facilities, they also studied the energy and material balance of the total system from the technical standpoints to implement the Eco-Platform system. They will show as the results of the study that the Eco-Platform is technically feasible introducing technologies mainly developed in the latter half of the 20th century and will show possible combination of functions/facilities supposing waste/sewage disposal in Sakai-city, Osaka, Japan.
Elasto-Plastic Behavior and Buckling Analysis of Steel Pipelines Exposed to Internal Pressure and Additional Loads
Authors: Schaumann Peter , Keindorf Christian , Bru¨ggemann Henning
Published: November 2008

The currently valid worldwide standards allow for taking into consideration plastic deformations in order to achieve a higher degree of utilization. The maximum plastic strains, which can be allowed for steel pipes subjected to internal pressure and additional loads, are particularly interesting. In this paper results of investigations on the elasto-plastic bearing behavior of steel pipelines subjected to internal pressure and bending are presented. Four-point bending tests on eight steel pipes were carried out in order to make the buckling analysis in the elasto-plastic range possible. Finite-element-models were checked by test results for the application on buried pipelines. Taking into account bedding conditions of the pipeline in the soil was made possible. Furthermore, an analytical method based on the differential equation for beams with longitudinal tensile force and variable bending stiffness was developed. It is suitable to determine the elasto-plastic bearing capacity for internal pressure and bending. The collapse due to plastic shell buckling is considered by a limit criterion based on critical strains.
Prediction of Safe Sea-State for Pile Driving
Authors: Bazargan Hamid , Bahai Hamid , Yasseri Sirous F. , Aryana Farzad
Published: November 2008

In this paper vertical piles have been studied with a view to identifying the range of sea-states suitable for the safe pile driving operation. Pile configuration, including the non-linear foundation and the gap between the pile and the pile sleeve shims have been modeled using the finite elements (FE) analysis facilities within ABAQUS. Dynamic analyses of the system for various sea-states characterized by significant wave heights and mean zero upcrossing periods and modeled as a combination of several wave components, have been performed. Repeating the above procedure can generate a table of safe and unsafe sea-states. If the prediction is repeated N times from which n times proved to be safe, then it can be said that the predicted sea-state is safe with a probability of 100(n/N)%. The significant wave height (Hs) and mean zero upcrossing period (Tz) of a future sea-state of a location in NE Pacific (near 46° N 131° W) were generated using the artificial neural networks (ANNs) already trained for this purpose — the location of US National Oceanographic Data Center (NODC) Buoy 46005 is used in this study. The Hs and Tz of some future sea-states were generated from their corresponding conditional 7-parameter probability density functions (pdf’s) given some information including a number of previously measured Hs’s and Tz’s. The parameters of the pdf’s have been estimated from the outputs of 2 different 7-network sets of trained ANNs. This gives a predicted sea-state for a specific time in future. The methodology explained in this paper can identify all control parameters and offer possible solution strategy. The finding reveals that how slight changes in the design configuration can be beneficially exploited to limit the pile response.
Design Drivers of Hybrid Riser Towers
Authors: Thie´baud Franc¸ois , Luteyn Ste´phane , Pe´rinet Dominique
Published: November 2008

DORIS Engineering and Stolt Offshore have jointly designed the three Girassol hybrid riser towers installed offshore Angola. They were the first riser towers designed to provide very efficient insulation properties in very deepwater conditions. Some towers are under development or construction for other fields but in similar design conditions. Based on the experience of Girassol, DORIS engineering and Stolt Offshore have developed the concept to adapt to much deeper waters (around 2500 m) and more severe environments such as the Gulf of Mexico loop current or hurricane waves. This paper will present the key drivers for design of riser towers in these environments addressing for example the impact on the bundle cross section, buoyancy requirements, top tank size, flexible jumper lengths. Limitations will be defined and explained both in terms of design, fabrication and installation. As FPSO units are likely to be part of the development of the Gulf of Mexico deepwater areas in the not-so-distant future, the benefit of riser towers will become obvious, given the existing facilities for fabrication along the coast and spreads for installation. Other areas such as the Mediterranean sea, or even the Northern North Sea are potential candidates for this type of riser arrangement which provides multiple benefits over steel catenary risers and flexible pipes.
A Large Deck Extendable Draft Platform Design for Ultra-Deepwater in the Gulf of Mexico
Authors: Murray John , Sablok Anil , Demerchant Todd , Xu Lixin , Finnigan Tim , Aramanadka Shankar S. Bhat
Published: November 2008

A new delivery scheme based on an Extendable Draft Platform (EDP), designed for large fields in ultra-deepwater, offers potential benefits that minimize start-up costs and enhance overall economics. This paper describes an EDP design intended for deployment in water depth of 8,500 ft in a Gulf of Mexico environment. This deck design is one of the largest to date, using the EDP delivery method. The paper discusses design philosophy in detail, explaining the unique design features of the topsides for dockside commissioning, the deck connection system, and the riser systems and their integration. The topsides has process capacity of 200,000 bopd and about 475 mmcfd of gas. The deck also supports a drill rig with a hook capacity of 1,500 kips. The 40,000-ton topsides are 316 ft by 316 ft, supported by four 76-ft diameter columns. In addition to supporting the equipment, the topsides design supports 12,500 tons of riser tension. The complete process and drilling systems are assembled and commissioned dockside in less than 40 ft water depth. The entire system is floated on the deck barge, with the columns lowered through the deck and locked into position on site. Then the columns are de-ballasted, and the platform is raised to the operating draft. The top tensioned risers (TTRs) are supported by hydraulic tensioners. The mooring system, a major component of the lowering mechanism that deploys the EDP, comprises 12 lines composed of chain and polyester rope. These lines are pre-set and connected to the hull prior to raising the deck.
Spar Technology as a Seabasing Enabler
Authors: Selfridge Mark , Kennell Colen
Published: November 2008

A concept was developed by the Seabasing Innovation Cell within the Center for Innovation in Ship Design (CISD) at the Naval Surface Warfare Center Carderock Division (NSWCCD). The study was undertaken during February–May 2003 with funding provided by the Office of Naval Research (ONR). The concept was developed in summer 2003 with further CISD funding and has been chosen for the 2004 Senior Year Design, Build and Test project by Florida Atlantic University’s (FAU) Ocean Engineering Department. The concept, known as the Deep Water Stable Craneship (DWSC) consists of two entities, a catamaran craneship and a detachable spar, which when connected form a self-deploying, open ocean capable trimaran. The spar can be rotated through 90 degrees, from horizontal to vertical, using seawater ballast. When vertical, partial de-ballasting ‘lifts’ the catamaran clear of the water surface allowing the system to operate as a spar and take advantage of the superior seakeeping afforded by the small waterplane area. The concept was inspired by the “ONR owned” and “Scripp’s Institute operated” FLIPSHIP, and was developed as a potential solution to the Seabasing goal of transferring containerized cargo at sea between large and small vessels in seastate four (significant wave-heights 1.25–2.5m). Current crane operations at-sea are limited to seastate two (significant wave-heights 0.1–0.5m), largely due to pendulation of the load. This paper presents the development of the concept, its performance and alternative uses. In addition, a 1:15 scale ‘demonstrator’ is discussed as is a proposal for a FLIPSHIP-II.
A 3-D Finite Volume Method for Green Water Calculations
Authors: Zhang Sheguang , Liut Daniel , Weems Kenneth , Lin Woei-Min
Published: November 2008

A 3-D Finite Volume method (FV3D) is developed and applied to green water problems. The Navier-Stokes (N-S) equations are discretized with the 3-D finite volume method on collocated Cartesian grids. The free surface motion is captured with the Volume of Fluid (VOF) method. The velocity and pressure fields are solved by the SIMPLER scheme with an alternating direction implicit solver. FV3D is validated against existing experimental and numerical results for tank sloshing and ship green-water-on-deck cases. This method is applicable to calculation of the green water effect on advanced wave-piercing hull forms.
Vibration Control of Offshore Platforms Using the Mega-Frame Theory
Authors: Zhao Dong , Ma Rujian , Wang Weiqiang , Cai Dongmei
Published: November 2008

The successful use of mega-frame theory for big buildings on the ground to control the vibration reminds us to investigate a new kind of platform using the theory. The concept platform is made up of major columns and major beams. The major columns stand vertical and horizontal load. The major beams endure the horizontal load and averagely transfer the weight of the platform to the columns. The static analysis, mode analysis and simulation of vibration control are performed for the concept platform and other three types of platforms for comparisons. It is concluded that the concept platform is feasible in functionality since the platform has high holistic intensity and can avoid the partial breakage because of the columns of the MFPs being more equal burden under static loadings as compared with other kinds of platforms. It can also avoid the big distortion under the severe external loadings because of the MFPs having the equal mass and stiffness distribution. And the displacement of MFPs is very small under the randomexternal loadings. Furthermore, the MFPs will have similar structures and suitable for blocking manufacturing.
Model Tests in Brash Ice Channels
Authors: Hellmann Jens-Holger , Rupp Karl-Heinz , Kuehnlein Walter L.
Published: November 2008

According to the present Finnish-Swedish Ice Class Rules (FSICR) the formulas for the required main engine power for tankers led to much bigger main engines than it is needed for the demanded open water speed. Therefore model tests may be performed in order to verify the vessel’s capability to sail with less required power in brash ice channels compared to the calculations. Several model test runs have been performed in order to study the performance of crude oil tankers sailing in brash ice. The tests were performed as towed propulsion tests and the brash ice channel was prepared according to the guidelines set up by the Finnish Maritime Administration (FMA). The channel width was 2 times the beam of the tanker. The model tests were carried out at a speed of 5 knots. For the tests a parental level ice sheet of adequate thickness is prepared according to HSVA’s standard model ice preparation procedure. After a predefined level ice thickness has been reached, the air temperature in the ice tank will be raised. An ice channel with straight edges will be cut into the ice sheet by means of two ice knives. The ice stripe between the two cuts will be manually broken up into relatively small ice pieces using a special ice chisel and if required the brash ice material will be compacted. Typically the brash ice thickness will be measured prior the tests at 9 positions across the channel and every two meter over the entire length of the brash ice channel with a special device, which consists of a measuring rule with a perforated plate mounted under a right angle at the lower end of the rule. As a result of the tests it could be demonstrated that tankers with a capacity of more than 50 000 tons require 50% and even less power compared to calculations using the present FSICR formulas.
Application of ERS and ENVISAT Synthetic Aperture Radar Images for Coastal Water Analysis
Authors: Nieto Borge Jose´ Carlos , Schneiderhan Tobias , Schulz-Stellenfleth Johannes , Niedermeier Andreas
Published: November 2008

The scientific developments carried out in the last years with spaceborne synthetic aperture radar (SAR), as well as the comparisons with in-situ sensors, have demonstrated that SAR is a reliable remote sensing tool to study wave fields on the open ocean. In their so-called SAR image mode, SAR systems on board satellites are able to scan ocean areas about 100 × 100 km2 with a spatial resolution about 20 × 20 m2. These SAR images are able to provide information about the spatial variability of wave fields, as well as other phenomena that occur on the ocean surface, such as the local wind field. This work investigates the capabilities of spaceborne SAR to extract sea state information for those areas close to coastal locations, where the incoming wave fields present high spatial inhomogeneities. For this purpose, ERS-2 SAR and ENVISAT Advanced SAR (ASAR) images acquired over the North Sea and the Bay of Biscay are used to study ocean waves in shallow waters under different climate and oceanographic conditions. In addition, the potential of ENVISAT ASAR capability to scan the ocean surface with dual polarizations (horizontal, HH, and vertical, VV) is analyzed in this work. The relevance of these investigations for different practical applications concerning the analysis of ocean waves is discussed.
Design Point Effect Due to Severe Storm Conditions on the Reliability Assessment of Offshore Structures: Case Study
Authors: Tawfik Ahmed Mostafa
Published: November 2008

Structural reliability is of an intense significance for the evaluation of failure probability and safety levels of offshore structures in their structural design stage, especially when the variables are eminently random. Hence, the environmental conditions are considered among the most important parameters in the design of offshore structures whose effect of great contribution to structural safety. The 100 year storm wave height and current velocity constitute the majority of uncertainty in the environmental conditions. Eventually, these values are used as stochastic values i.e. mean values in addition to standard deviation. This paper uses the stochastic values of 100 year storm wave height and current velocity to estimate the probability of failure. These simulations had been encountered in the structural analysis and design on practical case study representing an offshore structure located in the Mediterranean Sea to predict the reliability level. The outcome of these simulations uses the Response Surface Method with design point technique for both stress and displacement limit states. The results of this Reliability method lead to an acceptable level for reliability analysis and risk assessments.
Estimation of Damping Coefficients of Moonpools for Monocolumn Type Units
Authors: Cueva Marcos , Malta Edgard Borges , Nishimoto Kazuo , Costa Ana Paula
Published: November 2008

The use of moonpools in offshore technology are normally related to the hull opening in drilling units with the objective to have easy installation process and protect drilling equipment from environmental forces, and it is desirable the minimum motion of the water inside the moonpool, avoiding water impacts when lowering an equipment. Several studies, refs. /7,10,11,12,13/, have been carried out to predict the water dynamics inside the moonpool using analytical and experimental tools, this last due to the high viscous influence in the responses, and the study of coupled dynamics of floating units with moonpools have been presented in previous works (refs. /1,2,3,8,9,17/), with especial focus on FPSO’s and Monocolumn type units. In the present study, the objective is to verify the reliability of numerical approach in heave dynamic evaluation of monocolumns. With this approach the objective is to tune the natural period the floater and moonpool. Two monocolumn example units were numerically calculated using WAMIT software, making possible to verify the 6 degree of freedom of the unit and the 3 of the moonpool. As the software does not evaluate viscous forces, decay tests were performed in order to verify the external viscous damping to be considered in the numerical calculations. The calibrated numerical model transfer functions (RAO) were compared with regular and transient wave tests, where heave and moonpool heave amplitudes could be compared.
Parametric Evaluation of the Performance Characteristics of Tight Moored Wave Energy Converters
Authors: Mavrakos Spyridon A. , Katsaounis Georgios M.
Published: November 2008

The paper aims at presenting a numerical model to predict performance characteristics of tight moored vertical axisymmetric wave energy converters that are allowed to move in heave, pitch and sway modes of motion. The hydrodynamic characteristics (exciting wave forces, hydrodynamic parameters) of the floats are evaluated using a linearized diffraction–radiation method of analysis that is suited for the type of bodies under consideration. According to this method matched axisymmetric eigenfunction expansions of the velocity potentials in properly defined fluid regions around the body are introduced to solve the respective diffraction and radiation problems and to calculate the floats’ hydrodynamic characteristics in the frequency domain. Based on these characteristics, the retardation forcing terms are calculated, which account for the memory effects of the motion. In this procedure, the coupling terms between the different modes of motion are properly formulated and taken into account. The floating WEC is connected to an underwater piston that feeds a hydraulic system with pressurized fluid. Numerical results showing parametrically the performance characteristics in terms of the expected power production for several types of floats that are exposed to the wave climate conditions commonly encountered in the Mediterranean area are presented and discussed.
Numerical Modeling of Surface Wave Motion With a Bottom Turbulent Boundary Layer
Authors: Jamali Mirmosadegh
Published: November 2008

An effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the wave and boundary layer characteristics such as bed shear stress. A mixing-length theory is used for turbulence modeling. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems.
The Effects of Free Field Ground Motion Analysis on Seismic Response of Offshore Piles
Authors: Kimiaei Mehrdad , Shayandar Mohsen Ali , El Naggar M. Hesham , Aghakouchak Ali Akbar
Published: November 2008

Nonlinear response of piles is the most important source of potentially nonlinear behavior of pile-supported offshore platforms due to earthquake excitations. Earthquake design of offshore platforms is one of the main parts in offshore platforms design on which seismic soil pile structure interaction could be the main concern. In this paper, seismic response of offshore piles embedded in layered soil deposits is determined using a BNWF model (Beam on Nonlinear Winkler Foundation). Results of the free field ground motion analyses using equivalent-linear or nonlinear approaches are applied as input seismic excitations to the support nodes of BNWF model. Sensitivity of the results to input ground motions are discussed and addressed in this paper. Computed responses are also compared with centrifuge test results.
Strongly Coupled Fluid-Structure Interactions via a New Navier-Stokes Method for Prediction of Vortex-Induced Vibrations
Authors: Kallinderis Yannis , Ahn Hyung Taek
Published: November 2008

Numerical prediction of vortex-induced vibrations requires employment of the unsteady Navier-Stokes equations. Current Navier-Stokes solvers are quite expensive for three-dimensional flow-structure applications. Acceptance of Computational Fluid Dynamics as a design tool for the offshore industry requires improvements to current CFD methods in order to address the following important issues: (i) stability and computation cost of the numerical simulation process, (ii) restriction on the size of the allowable time-step due to the coupling of the flow and structure solution processes, (iii) excessive number of computational elements for 3-D applications, and (iv) accuracy and computational cost of turbulence models used for high Reynolds number flow. The above four problems are addressed via a new numerical method which employs strong coupling between the flow and the structure solutions. Special coupling is also employed between the Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model. An element-type independent spatial discretization scheme is also presented which can handle general hybrid meshes consisting of hexahedra, prisms, pyramids, and tetrahedral.
A New Concept in Joining Diagonal Bracings to Horizontal Ones
Authors: Khonsari S. V. , England G. L. , Joeafshan-Vishkaie B.
Published: November 2008

A new ‘universal’ structural joint with multiple applications was devised. Its application as a ‘beam-to-column joint’ was already investigated experimentally, and proven to be promising. The results were reported in previous OMAE conferences and elsewhere. Another fruitful application conceived for this joint is in the context of joining (diagonal) braces to the frame members of a structure. In particular, in chevron (inverted V) bracing systems, where the dominating exchanged forces between the combined diagonal braces and the horizontal brace (beam) are ‘shear,’ the use of the devised joint as a joining member can substantially improve the overall behaviour of the structure under dynamic loading, cyclic or else. This is due to the high ‘shear flexibility’ of this joint, emanating from 1) its high ‘shear deformation capacity,’ and 2) the ‘restored ductility’ of its material, both substantiated through experimental studies. However, numerical push-over study of a typical one-storey frame proved the effectiveness of using such notion in chevron-braced frames in every respect—the flexibility of the frame increased substantially. The elastic stiffness and the elastic limit decreased to more reasonable values, which guarantee the stable, non-violent failure of the frame if subjected to overload. Moreover, since this joint works in a ‘sacrificial capacity’ and is in the form of a ‘self-contained’ separate entity, connected to the joined-diagonals and the horizontal braces through bolts, it can be ‘replaced,’ if it receives excess damage, leaving the non-replaceable elements, braces, intact. Finally, since it is capable of joining structural elements of different materials to each other, it is expected to have a potential for being used in every structure, offshore or onshore, prone to be subjected to dynamic, cyclic or else, in its service life.
Nonlinear Wave Loads Acting on Cylinder Array Slowly Oscillating in Diffraction and Radiation Wave Fields
Authors: Bao Weiguang , Kinoshita Takeshi , Yoshida Motoki
Published: November 2008

The problem of a circular cylinder array slowly oscillating in both diffraction and radiation wave fields is considered in the present work. As a result of the interaction between the wave fields and the low-frequency motion, nonlinear wave loads may be separated into the so-called wave-drift added mass and damping. They are force components proportional to the square of the wave amplitude but in phase of the acceleration and velocity of the low-frequency motion respectively. The frequency of the slow oscillation is assumed to be much smaller than the wave frequency. Perturbation expansion based on two time scales and two small parameters is performed to the order to include the effects of the acceleration of the low-frequency motion. Solutions to these higher order potentials are suggested in the present work. Wave loads including the wave drift added mass and damping are evaluated by the integration of the hydrodynamic pressure over the instantaneous wetted body surface.
Conversions vs. Newbuildings: General Arrangement and Strength Issues in FPSO Design
Authors: Reyes Marta C. T. , Kaleff Peter , Fernandes Antonio C. , Ferreira Marcos D. A. S.
Published: November 2008

The conversion of ageing tankers is a tempting alternative to newbuildings when FPSO units are planned. The low cost of an already classified hull and an existing pumping facility compensate for the repair and updating costs a conversion may demand. Newbuildings on the other hand allow for improved production plant and storage tank lay-out. The trade offs between conversions and newbuildings are analyzed from the point of view of strength and general arrangement. Converted tankers carry a serious strength drawback since the emptied engine space enhances the high buoyancy region at the stern which significantly increases bending moment amidships. In general one or more cargo tanks must be kept at partial loading to compensate for the increased bending moment. A new building lay-out is analyzed in which the tanker engine space is completely eliminated. Utility systems are resized and placed on the main deck aft of the production plant. Accommodation spaces are also resized and rearranged in order to fit into the reduced deck space available. The gains in strength are quantified and the lay-out implications are discussed in detail. A comparative study is presented between a conversion of a hypothetical tanker and an equivalent new building.