bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical Local Buckling Conditions for Deepwater Pipelines
Authors: Wolodko J. , DeGeer D.
Published: October 2008

In many deepwater pipeline applications, a clear understanding of pipe local buckling behaviour due to excessive bending and/or external pressure is required to adequately design subsea pipeline systems to an acceptable degree of risk. With ultra-deepwater pipelines being considered for water depths of nearly 3000 m, pipe collapse, in many instances, will govern design. For example, bending loads imposed on the pipeline near the seabed (sagbend region) during installation will reduce the external pressure resistance of the pipeline, and this design case will influence (and generally govern) the final selection of an appropriate pipeline wall thickness. To date, the deepest operating pipelines have been laid using the J-lay method, where the pipeline departs the lay vessel in a near-vertical orientation, and the only bending condition resulting from installation is near the touchdown point in the sagbend. More recently, however, the S-lay method is being considered for installation of pipelines to water depths of nearly 2800 m. During deepwater S-lay, the pipeline originates in a horizontal orientation, bends around a stinger located at the stern or bow of the vessel, and then departs the lay vessel in a near-vertical orientation. Thus, during S-lay, the installed pipe experiences bending around the stinger (overbend region), followed by combined bending and external pressure in the sagbend region. In view of these bending and external pressure loading conditions, analytical work was performed to better understand the local buckling behaviour of thick-walled linepipe due to bending, and the influence of bending on pipe collapse. Variables considered in the analytical evaluations include pipe material properties, geometric properties, pipe thermal treatment, the definition of critical strain, and imperfections such as ovality and girth weld offset. Presented is the finite element analysis (FEA) model developed to perform the assessments, validation of the model, the results of analyses and correlations to experimental data.
Towards Practical Implementation of Time Optimal Trajectories for Underwater Vehicles
Authors: Chyba M. , Haberkorn T. , Smith R. N. , Zhao S. , Choi S. K.
Published: October 2008

In this paper, we are concerned with the practical implementation of time optimal numerical techniques on underwater vehicles. We briefly introduce the model of underwater vehicle we consider and present the parameters for the test bed ODIN (Omni-Directional Intelligent Navigator). Then we explain the numerical method used to obtain time optimal trajectories with a structure suitable for the implementation. We follow this with a discussion on the modifications to be made considering the characteristics of ODIN. Finally, we illustrate our computations with some experimental results.
Detecting Internal Waves From Satellite Ocean Color Imagery
Authors: Ho Chung-Ru , Su Feng-Chun , Kuo Nan-Jung , Huang Shih-Jen , Chen Chun-Te , Zheng Quanan
Published: October 2008

Internal waves have been observed by lots of high resolution satellite images, such as Synthetic Aperture Radar (SAR) and optical images of SPOT and Landsat. These images are usually expensive. In this study, some free but lower spatial resolution satellite images are applied to observe the internal wave phenomena. The internal waves in the Sulu Sea are detected from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) onboard the Orbview-2 satellite. The SeaWiFS image has a spatial resolution of 1.1 km. It is acceptable to observe the internal wave phenomena while the soliton width is larger than the image resolution. The results show that the internal solitary in the Sulu Sea can be observed successfully with SeaWiFS chlorophyll images. The internal waves in the Sulu Sea have amplitudes of 10 to 90 m and wavelengths of 5 to 16 km. The large-amplitude internal solitary waves may significantly influence the near-surface chlorophyll concentration. The chlorophyll concentration would be lower when the depression internal waves passed through. A theoretic model is proposed and tested to estimate the amplitudes of internal waves from chlorophyll concentration images.
The Statistical Boundary Polygon of a Two Parameter Stochastic Process
Authors: Ottesen Torfinn , Aarstein Jon A.
Published: October 2008

The simultaneous values of tension and bending are of primary interest when checking the structural integrity of dynamic risers and umbilicals. Considering extreme checks, it is generally sufficient to check the simultaneous values along the convex hull of the extreme contour. A method for obtaining a boundary polygon that approximates the statistical extreme convex hull from time series data is described. The procedure for obtaining the extreme values from the most suitable extreme value distribution as determined by the Anderson-Darling Goodness-of-Fit test is outlined.
A Practical Model for Nonlinear Seismic Response Analysis of Jacket Type Fixed Offshore Platforms
Authors: Kimiaei Mehrdad , Aghakouchak Ali Akbar , Shayanfar Mohsen Ali , El Naggar M. Hesham
Published: October 2008

Offshore platforms in seismically active areas should be designed to survive severe earthquake excitations with no global structural failure. In seismic design of offshore platforms, it is often necessary to perform a dynamic analysis that accounts for nonlinear pile soil structures interaction effects. This paper summarizes an inexpensive and practical procedure compatible with readily available structural analysis software (ANSYS) for estimating the nonlinear lateral response of fixed offshore platforms resting on flexible piles subjected to seismic loading. In the proposed model, piles and jacket members are modeled using BNWF (Beam on Nonlinear Winkler Foundation) and FE (Finite Element) approaches respectively in an integrated model. In this paper, nonlinear seismic response analysis of an existing sample offshore platform has been performed and sensitivity of the results to the model main parameters is investigated.
Investigation Into the Effect of Initial and Secondary Ship Impact on the Integrity of a Typical Jack-Up Platform
Authors: Daraei J. , Gharabaghi A. R. M. , Chenaghlou M. R.
Published: October 2008

In this paper finite element analysis of a typical jack-up platform under different impact scenarios are performed using ANSYS software and the effect of various mechanisms in absorbing the energy of collision is determined. A 3D model of Iran-Khazar Jack-up operating in Caspian Sea is prepared. Dynamic nonlinear analyses, including geometric and material nonlinearity, are performed. The effects of preloading due to platform weight and environmental wave and current loading before collision are applied. In first collision between ship and studied jack-up, for impact to chords (leg-like vertical elements) as well as to braces (diagonal or horizontal elements), the main energy absorption mechanism is overall deformation of structure. For second impact it is assumed that the rebounded ship moves again toward the platform with relative wave induced velocity and worst impact scenario is considered. The results show that leg chords can resist to the second impact and the platform in both initial and secondary impacts is damaged locally such as indentation of the chord wall and the whole structure is safe.
Interaction of a Second-Order Solitary Wave With an Array of Four Vertical Cylinders
Authors: Basmat A. , Markiewicz M. , Petersen S.
Published: October 2008

In this paper the interaction of a plane second order solitary wave with an array of four vertical cylinders is investigated. The fluid is assumed to be incompressible and inviscid. The diffraction analysis assumes irrotationality, which allows for the use of Boussinesq equations. A simultaneous expansion in a small nonlinearity parameter (wave amplitude/depth) and small dispersion parameter (depth/horizontal scale) is performed. Boussinesq models, which describe weakly nonlinear and weakly dispersive long waves, are characterized by the assumption that the nonlinearity and dispersion are both small and of the same order. An incident plane second order solitary wave is the Laitone solution of Boussinesq equations. The representation of variables as the series of small nonlinearity parameters leads to the sequence of linear boundary value problems of increasing order. The first order approximation can be determined as a solution of homogeneous differential equations and the second order approximation follows as a solution of non-homogeneous differential equations, where the right hand sides may be computed from the first order solution. For the case of a single cylinder an analytical solution exists. However, when dealing with more complex cylinder configurations, one has to employ numerical techniques. In this contribution a finite element approach combined with an appropriate time stepping scheme is used to model the wave propagation around an array of four surface piercing vertical cylinders. The velocity potential, the free surface elevation and the subsequent evolution of the scattered field are computed. Furthermore, the total second order wave forces on each individual cylinder are determined. The effect of the incident wave angle is discussed.
A Numerical Model of Typhoon Surge Along the Coast of Taiwan
Authors: Li Hsien-Wen , Li Fang-Cheng , Tzang Shiaw-Yih , Tsai Cheng-Han , Lo Yao-Tsai , Huang Shih-Jen
Published: October 2008

The storm surge associated with a typhoon approaching Taiwan is one of the main causes of floods that frequently occur in the low land areas in certain exposed coastlines. Every year typhoons strike Taiwan, especially from June to October. In the past, typhoon surges have caused a lot of damages. A numerical prediction of typhoon surge in the seas around Taiwan is developed. The model is based on the hydrodynamical-numerical method. Air pressure gradient and wind stress are considered as the driving forces for the typhoon surge in this model. Here a circular storm is taken as the typhoon model. The wind stress and bottom friction stress are also computed by means of parameterization. In this model the bottom topography plays a great role. In the calculation the sea water is considered as a homogeneous medium. The Typhoon Tim in 1994, the Typhoon Doug in 1994 and the Typhoon Herb in 1996 are selected for case studies to verify this numerical model. These typhoons, which had different wind forces, passed Taiwan along different paths. Thus the effect of different typhoons can be studied.
Satellite Observation of the Mesoscale Features in the Taiwan Strait
Authors: Kuo Nan-Jung , Ho Chung-Ru , Huang Shih-Jen
Published: October 2008

In this paper, we discuss the kinematic properties of the mesoscale features in the Taiwan Strait (TS). Two kinds of mesoscale features can be seen in the TS. One is the shear wave; it is a lateral wavelike train with phase speed and wave period. The other one is the eddy-like cold-water wedge; it mainly rotates with a little translation. The shear wave mainly occurs in the boundary of the China cold water along the western TS in winter and spring. The eddy-like cold-water wedge can be seen in the mid-winter when the strong southward China coastal cold water covers up most of the northern TS and creates a very unstable front with the northward warm water. In this study, selected NOAA satellite AVHRR channel 4 infrared images during 2000–2003 are considered to detect the movement of the above mesoscale features through a feature-tracking method. It is found that the development of these features is highly related to the monsoon winds. Meanwhile, the local tidal pattern can also affect their movements.
Instantaneous Spectral Analysis of Non-Stationary Ship Motion Data
Authors: Iseki Toshio
Published: October 2008

The time varying coefficient vector autoregressive (TVVAR) modeling is applied to the cross-spectral analysis of non-stationary ship motion data. Introducing the instantaneous response, a vector autoregressive model can be reduced to simple time varying coefficient autoregressive (TVAR) models for each ship motion and the required CPU time is effectively reduced. The TVVAR model and stochastic perturbed difference equations are transformed into a state space model. The vector-valued unknown coefficients can be evaluated and the instantaneous cross spectra of ship motions can be calculated at every moment. The results showed good agreements with one of the TVAR modeling and also with the stationary autoregressive (SAR) modeling analysis under stationary conditions. Furthermore, the instantaneous relative noise contribution was also estimated using the TVVAR coefficients and illustrated how the structure of a spectrum changed according to the ship manoeuvres for the first time. Optimum order of the model and Akaike’s information criterion were also examined for several changes of parameters. Moreover, it is confirmed that the TVVAR modeling can estimate the instantaneous cross spectra and relative noise contribution of ship motions even under non-stationary conditions.
Effects of Heave Plate on the Hydrodynamic Behaviors of Cell Spar Platform
Authors: Zhang Fan , Yang Jianmin , Li Runpei , Hu Zhiqiang
Published: October 2008

This paper presents an experimental study on the effect of heave plates on the hydrodynamic performances of a cell spar platform. A variation of the cell spar concept, whose hard tank still consist of several cylinders (cells) with the same diameter and length, but the lower part is fitted with a truss section and several heave plates, is modeled and tested. As the effectiveness of heave plates is crucial to the heave performance, different types of heave plates are fitted to the truss structure, varying from the number and spacing to the form, such as perforated plates, to investigate various design aspects of the plates. Experimental results and numerical predictions for the responses of the spar to the wave loading, as well as loads and added mass and damping coefficients on the heave plates are presented.
Optimizing Ship Machinery Maintenance Scheduling Through Risk Analysis and Life Cycle Cost Analysis
Authors: Baliwangi Lahar , Ishida Kenji , Arima Hidetoshi , Artana Ketut Buda
Published: October 2008

Ship maintenance scheduling management integrated with risk evaluation and Life Cycle Cost (LCC) assessment approach is developed in this research. It improves upon existing practices in arranging an optimal maintenance schedule by modeling operational and economical risks. This paper researches maintenance scheduling algorithm with explicitly consider risks associated with some operation problems such as operating schedule, routes, ship position, resources availability, and achievement of reliability-availability-maintainability (RAM) of system. Modeling of components RAM with their failures consequences results risk evaluation. Time value of maintenance cost, replacement cost, earning rate, and penalty cost are also simulated. When the system reaches the lowest level of lower limit reliability, one or more components should be maintained or replaced. Since maintenance task may interrupt the operation, to minimize time-to-maintain all possible events of maintaining other components at the same time will be evaluated together with resources availability. By researching those possibilities, constraining the risk, and based on LCC calculation result, an optimal maintenance scheduling can be then well established.
Development of Environmental Assessment Technique for CO2 Ocean Sequestration
Authors: Shitashima Kiminori , Ohsumi Takashi
Published: October 2008

Increasing atmospheric concentrations of greenhouse gases are suspected of causing a gradual warming of the Earth’s surface and potentially disastrous changes to global climate. Because CO2 is a major greenhouse gas, CO2 ocean sequestration is being explored as one possible option to limit the accumulation of greenhouse gases in the atmosphere. In the case of CO2 ocean sequestration, an assessment of environmental impacts to the ocean is the most important issue. Sequestration research requires a development of new cost-effective observation techniques to monitor dilution and diffusion of the sequestrated CO2. We developed an in-situ pH/pCO2 sensor, a tracking neutral buoy system and a towing multi-layer monitoring system to observe the pH change, water movement and diffusion at a mid-depth of the ocean as an environmental assessment technique for CO2 ocean sequestration.
Weld Root Fatigue Assessment of Fillet-Welded Structures Based on Structural Stresses
Authors: Fricke Wolfgang
Published: October 2008

The problem of fatigue cracking is particularly significant for large welded plate structures being typical for ships and floating offshore structures. Here, fillet welding is applied to a large extent to join structural components. This leads to non-fused root faces, which can behave like initial cracks. In several cases the situation is even worse, when welding can be performed from one side only, resulting in a highly-stressed weld root on the other side. Although refined approaches exist for a fatigue assessment of such weld roots, for instance the crack propagation and the notch stress approaches, more practical approaches requiring less effort are demanded to handle typical problems occurring in practice. These problems are associated with locally increased load transfer, e.g. at crossing support structures and at fillet welded ends of attachments with additional stress concentrations. Also, pronounced bending can occur in fillet welds due to lateral loading of the attachment and/or the eccentricity of one-sided welds. In the paper, practical approaches for such problems are presented which have been developed in the recent past in different research projects and which are based on a structural stress or a local nominal stress in the weld. Their application is demonstrated by several examples taken from ship and offshore structures using relatively coarse finite element meshes for the stress analyses.
Pipe Response Under Concentrated Lateral Loads and External Pressure
Authors: Andreadakis Kyros P. , Karamanos Spyros A.
Published: October 2008

The paper focuses on the denting the response of pipeline segments subjected to lateral quasi-static wedge loading, in the presence of internal pressure. Nonlinear finite element models of pipeline segments, which are in good agreement with denting test data from internally pressurized pipes, are employed to obtain load-deflection curves for different levels of pressure, for various wedge shapes and for different types of boundary conditions. It is found that the presence of internal pressure increases significantly the denting resistance. The present study gives special emphasis on the development of a simplified three-dimensional analytical model, which yields closed-form expressions for the denting force and the corresponding denting length in terms of the corresponding denting displacement. The model, introduced elsewhere for non-pressurized tubes, is enhanced to include the pressure effects, accounting for different types of pipe end conditions. The analytical solution compares very well with the finite element results, and illustrates tube denting response in a clear and elegant manner.
Trimaran Concept for Offshore Operations in Northern Seas
Authors: Hovland Erlend , Gudmestad Ove Tobias
Published: October 2008

A trimaran concept for offshore operations has been developed. Vessel features include low response in waves, low resistance for efficient transfer over long distances, launching positions for intervention equipment and a large deck area. The vessel may, in different versions, represent an interesting design to be used for field development and for inspection, maintenance and repair in Northern Seas. Efficiency in operation has been focused on in the development of this concept. To build a vessel for the future one has to study where the industry is moving and where likely work will be coming up in the time to come. Advances in technology, both in ship design and in subsea intervention are hard to predict and the capacities and characteristics for the future are hard to foresee. Some characteristics are, however, likely to be desired in the future as well. These are characteristics such as low vessel motions, good fuel economy, a large deck area and high loading capacity, to name a few. These characteristics are, in turn, perhaps not easy to combine. One vessel concept that combines some of these features in an elegant way is the trimaran. Concept development and theoretical analysis were followed up with tank testing. A 1:50 scale model was built by Technical Services and tested in the 60 m towtank at Memorial University of Newfoundland. Resistance and Motion tests were performed and compared with computer analysis. Comparison of results show good consistency between analysis and tank tests.
GPS-Tracked Buoy for Water Level Measurements
Authors: Shih H. H. , Brennan R. , Cisternelli M.
Published: October 2008

With the advancement of differential, kinematic GPS techniques, water level measurements with the accuracy of centimeter- or decimeter-level is possible when buoys are placed close to a coastal base station or at distances of thousands of kilometers from shore, respectively. Applications of these techniques to observe tides and waves and to detect tsunamis have been demonstrated. This paper will first briefly review existing water level measurement methods, the needs for coastal and open ocean water level measurements, previous GPS buoy experiments, and GPS measurement uncertainties and precision positioning techniques. These will then be followed by a brief description of the application of GPS buoys in hydrographic surveying and the development of a real-time water level reporting GPS buoy to support US National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) nautical charting mission. The present measurement system consists of a portable spherical buoy and a shore base station. Utilizing Real-Time Kinematic (RTK) differential GPS technique, accurate water level data were collected and reported in real-time at six- minute intervals in complying with NOS water level measurement standards. The buoy’s motion sensors provide automated corrections for wave-induced buoy motions. Several field tests conducted near NOAA water level stations have shown that the root-mean-square (rms) of differences between the two measurement systems is on the order of 2 cm. The buoy also provides surface wave information. The portability and the accuracy of the system offer possibilities for other applications in coastal waters. Future enhancements include satellite data telemetry and monitoring, option for post processing, and solar power supplementation.
Sea Water Distillatory Using Rising Film on the Fluted Surface of Horizontal Tubes
Authors: Li Yan , Mei Ning , Sun Yesheng
Published: October 2008

The purpose of this study is to investigate the mechanism of the seawater distillatory using rising liquid thin film on the fluted surface of a horizontal tube. By analyzing the formation of the rising film, a process of the HRF evaporators was designed to analysis the efficiency of the system. The numerical solution of heat transfer model shows that the temperature difference of HRF in one effect is lower than that of HFF. The behaviors of the flow characteristics were discussed. The results show that the rising liquid thin film could be formed when the rate of roll equaled 15°. The results from theoretical analysis suggest that seawater distillatory using rising liquid thin film on the fluted surface of a horizontal tube was especially suitable for the wobble environment.
Study on CO2 Releasing Nozzle for CO2 Ocean Storage
Authors: Nakajima Yasuharu , Shirota Hideyuki , Kojima Ryuji , Yamane Kenji , Aya Izuo , Namie Sadahiro , Tamura Kenkichi
Published: October 2008

Ocean storage of CO2 is one of greenhouse gas control technologies, where CO2 captured from flue gas of fossil fuels is injected into deep sea below 3500m depth to be sequestered from the atmosphere. A CO2 sending method, COSMOS, was proposed as a method of ocean storage, which enables CO2 drops released in mid-depth water to descend to deep sea floor below the depth of 3500m. Then, the authors have worked for development and evaluation of COSMOS. In the first phase of the COSMOS project, the concept of COSMOS was demonstrated by in situ experiments of small-scale CO2 releasing at mid-depth water in Monterey Bay, U.S.A. Three models of CO2 releasing nozzle unit were developed for the experiments. The first model of nozzle unit released liquid CO2 as one mass; however, it was immediately broken into small droplets and soon turned to ascend. The second and third models were designed to have thermal insulator enough to keep low temperature so that both models successfully released liquid CO2 with dry ice, which continued descending for a few minutes. Based on these results, COSMOS was improved, where injection of a mixture of liquid CO2 and dry ice, CO2 slurry, is expected to enable small CO2 drops to descend to deep sea floor below 3500m depth. Then, in the second phase of the COSMOS project, the authors started an investigation on the effect of the aspects of releasing nozzle on the behavior of released slurry drops, and obtained a few results from lab experiment of CO2 slurry releasing from two types of nozzle head.
Risers Stability Under External Pressure, Axial Compression and Bending Moment Considering the Welded as Geometrical Imperfection
Authors: Sa´nchez Sa´nchez He´ctor A. , Corte´s Salas Carlos
Published: October 2008

In Mexico, PEMEX manage more than 60,000 Km of oil and gas land and marine pipelines. Therefore, their structural integrity must be carefully assessed. The current development of offshore structures requires the design of better risers capable of assuring an optimal level of security. For that, some factors must be taken into account, which in critical conditions might become risky. It is common to find situations, in the performance of fixed offshore platforms, in which risers might be found empty due to maintenance or repairing. This condition could represent an unfavorable situation that in many cases might become highly critical given the conditions they are found in. Several studies had been completed with the aim to define a design methodology for the local buckling of pipes. In this paper, the bending capacity of the empty steel marine pipelines (risers) with girth-welded, placed to the legs of the offshore platforms very near to the deep sea, is studied by numerical models. The objective is to study the behavior and local buckling of these pipes submitted to combine loads, bending, external pressure and axial force, considering the girth-welded as the geometrical imperfection and to estimate the bending capacity of the pipes. Critical pressure of buckling, and modal configurations are evaluated by theoretical methods and numerical approaches such as finite element method (FEM). The numerical results are compared with theoretical methods.
Coupled Dynamic Analysis of Vessel/Risers System Including VIV
Authors: Jung Jong-Jin , Korobkin Alexander A. , Shin Hyun-Soo , Lee Woo-Seob
Published: October 2008

The present paper describes general methodology and simulation results for risers coupled with a floating vessel. Equations of extensible riser and their finite element formulation are derived and presented in the form, which is suitable to accommodate the effects of contents flow and VIV. Nonlinear riser equations are solved by iterations in global variables. The analysis procedure of coupling between the several risers and the vessel dynamics is described in details. The low frequency coupled vessel motion and riser behavior are numerically simulated. The internal flow effects are investigated in uniform and pulsating cases. It is found that the internal content flow makes the riser stiffer and the riser curvature reduced. In the VIV analysis, the VIV exitation zone of the riser is obtained based on eigen frequencies and the maximum response is evaluated from the corresponding eigen-shapes in the VIV region.
A Structural Analytical Method of Underwater Observation Tower Due to Breaking Wave Forces
Authors: Saijo Osamu , Eto Hiroaki
Published: October 2008

A structural design of the underwater observation tower constructed at wave breaking zone was studied in this research. Though the under water observation tower seems to be a rotational shell structure from its shape, actually, the rigid frame of beams and ribs exits inside wall, in which the frames are covered with curved plate element firmly against seawater pressure. Assuming that the observation tower was a pseudo-cylindrical shell, two results through our conducted research were applied for a practical structural analysis of the underwater observation tower. One is the unique and simple formula on added mass depending on elastic vibration, the other is transformation of total breaking wave forces into the wave pressure distribution over the surface of the observation tower. Using those results, the structural characteristic concerning displacement of the under water observation tower was examined through our own results.
Behavior Characteristics of Hang Off Riser That Considers Oscillation of Floating Structure
Authors: Itoh Kazuaki , Tamura Kenkichi , Maeda Katsuya , Shintomi Kyoko
Published: October 2008

A riser system has been used widely as the progress of ocean developments. The vibration is one of the most important problems from the safety aspect of a riser system. In this paper, we conducted the model experiment of a hang-off riser at the Deep-Sea Basin in the National Maritime Research Institute of Japan. Translational and rotational motions of the upper floating structure are simulated by the forced-oscillation device. The motion of the riser was measured using 3D video tracking system. Numerical analysis program for simulating the behavior of riser was also developed. This program used the improved Morison’s equation as a method of evaluating hydrodynamic force. The validity of this calculation method was verified through the comparison with the experiment results.
Prediction of Oil Spill Contamination at Mirfa Power and Desalination Plant
Authors: Mohamed Khaled A. , Odeh M.
Published: October 2008

Mirfa Power and Desalination Plant is one of the main plants in Abu Dhabi Emirate. It is located at the Arabian Gulf coast. The plant is vulnerable to oil pollution because oil fields, offshore pipelines and refineries are located close to it. A risk analysis study was carried out to determine the trajectory and horizontal spread of oil slick released from different locations. The results give the potential risks of oil pollution at the seawater intake in terms of oil slick concentrations and properties. The study helps in the setup of effective oil protection measures at the plant intake channel.
A Study on Efficient Methods for Estimating Load Effects and Reliability and Their Application to Preliminary Structural Design of Rectangular-VLFS: 1st Report
Authors: Okada Hiroo , Tsubogo Takashi , Masaoka Koji , Taniguchi Yuichi
Published: October 2008

This paper is concerned with efficient methods for estimating load effects and reliability and their application to preliminary structural design of box-type VLFS supposed as a floating airport, which is to be put in the west area of the Inland Sea of Japan (“Setonaikai”). In this study, as the first report, structural responses in oblique waves are investigated by using the efficient method based on Weighted Residual Method combined with thin plate and shallow water approximation presented by Tsubogo (2004). Effects of wavelength and wave incident angles on characteristics of the structural response are clarified from results obtained by numerical calculation. These results can be used to evaluate the load effect and the reliability in next report.
Modes of Failure for Suction Caissons Under Vertical Pull-Out Loads
Authors: Zeinoddini Mostafa , Nabipour Mahmood , Abdi Mahmood R.
Published: October 2008

Suction caissons have a fair base in the offshore industry, however, prediction of their behaviour and capacities still need extra attentions. Understanding their modes of failure is crucial for any analytical solutions. In this paper, failure mechanisms noticed in a numerical study of the suction caissons behaviour under vertical pull-out loading are reported. The employed finite element models have been calibrated by and verified against different available experimental data. In principal four distinctive modes of failure have been recognized. The first mode corresponds to a shear sliding failure in the soil plug along the caisson’s wall interior. This mode of failure has been mainly found in drained but comparatively weak soils. The second mode of failure noticed under drained conditions in soils with enhanced strength characteristics is a local tension failure occurring in the bottom of the soil plug. In this case the detached soil plug accompanies the caisson in its movement upward. The third mode corresponds to a restricted shear failure outside the caisson. It develops to a local wedge initiating halfway through the caisson walls and extending out to the soil surface. This failure mode has been mostly noticed with clays under undrained conditions. The foruth mode has been found to be a general shear failure in the soil underneath and around the caisson. It occurs in a wider spread body of the soil and has been noticed with the undrained sands. The load-displacement curves present a virtually bi-linear behaviour.
The New ATLAS FANSWEEP 30 COASTAL: A Tool for Efficient and Reliable Hydrographic Survey
Authors: Ko¨nnecke Stefan
Published: October 2008

The main field of application of a multi-beam echosounder is hydrographic surveying, in other words acquisition of bottom topography information. This kind of bathymetric data is needed by a large number of users for harbour and coastal zone management, resource exploration, hydrographic charting for safety of navigation and many other tasks. Tools for highly efficient investigation of the seafloor, including rapid sediment classification, as well as reliable object detection are of high interest to offshore construction projects. Throughout the last couple of years, ATLAS Hydrographic has been concentrating on the development of the next generation of hydrographic survey echosounders. One of the results is the ATLAS FANSWEEP 30 family of multi-beam echosounders (MBES). This family of sonars offers an innovative new approach of multi-beam echosounder technology for hydrographic surveys. The ATLAS FANSWEEP 30 COASTAL, the first member of the ATLAS FANSWEEP 30 MBES family, is breaking new ground. Innovations, such as Simultaneous Multi-Ping, wide bandwidth transducer, multiple time and multi-view scanning of the seafloor for object search, address a single purpose: to improve survey efficiency while multiplying data reliability and resolution. With the FS30C the new Simultaneous Multi-Ping (SMP) is introduced. SMP utilises the complete frequency range between 100 kHz and 200 kHz to transmit and receive eight frequency separate pings at every ping interval. Four of these eight pings are forward, the other four backward looking mapping the seafloor from two independent directions. Thus, every bit of the mapped seafloor is looked at from two different directions. The 8x Simultaneous Multi-Ping enables the hydrographer to run the survey operation at high speeds. Still, the 8x SMP can ensure more than 300% bottom ensonification. That means every patch of seafloor is measured at least 3-times at one pass. The gained data redundancy is used to enhance not only data accuracy and reliability, but in particular the capability to recognise objects on the sea floor. Beginning at signal transmission, through reception beamforming to the point of real-time data gridding capabilities, the complete data flow is controlled by a built-in data evaluation solution. The new ATLAS FANSWEEP 30 COASTAL is a well suited tool to satisfy actual and upcoming demands in highly efficient and most reliable survey operations.
Non-Linear Effects of Roll Damping Tanks on Ship Motions
Authors: Schumann Carsten , Pereira Ricardo
Published: October 2008

This article describes the application of two numerical methods of computing the flow in u-tube and free surface roll damping tanks. These methods account for the most important non-linear effects in tank flows. i) The programs based on these methods are integrated in a non-linear time simulation strip program. ii) Response curves of tanks are computed with the mentioned tank programs and the results are incorporated in a linear strip program. iii) With both strip programs (linear and non-linear), sea keeping computations are carried out and the results are compared.
Investigation and Optimisation of Boat Deployment Systems at High Seas
Authors: Clauss Gu¨nther F. , Kauffeldt Andre´
Published: October 2008

There is a multitude of seagoing missions such as rescue missions, coast guard and pilot duties, whose success depend on ship-deployed boats. Launching these boats from a mother ship or recovering them by special deployment systems in a broad range of environmental conditions are key operations for a successive mission. In recent years, new boat deployment systems, promising better operational availability at high sea states, have evolved beyond the traditional side-davit system with dual falls. These new systems deploy their boats via stern ramps integrated into the transom of the mother ship, for example seen at numerous rescue cruisers around the world. This paper presents two different boat deployment systems. After a short discussion of the disadvantages of side-davit systems, a new type of a stern boat deployment device, the so called Janssen Docking System [1], is introduced. This system is equipped with an articulated ramp hinged to the stern. Whereas launching operations are less critical, the recovery of boats is quite hazardous. For this operation mode structural forces on critical areas as well as the relative motions between the ramp and the small boat are systematically investigated in model tests. Based on the results of these sea keeping tests the feasibility of the system has been analysed and improvements are recommended. As a second system a floodable dock integrated into a mother ship is presented. For launching and recovering small boats at high seas the swell inside the dock and the resulting relative motions between boat and dock ship are investigated. This leads to critical flow conditions inside the harbour in terms of sloshing waves with heights up to 3 meters. The analysis of local flow phenomena inside the dock dependent on the motion of the ship in a given sea state are the basis for the development of an optimized dock shape. Therefore an existing nonlinear numerical method for unsteady viscous computation based on Volume of Fluid (VOF) methods and Reynolds Averaged Navier Stokes Equations (RANSE) is used to simulate these phenomena. The time domain calculation allows to change local dock shapes systematically for further improvements. To validate the numerical solution the calculated results are compared to sea keeping tests at model scale. The paper concludes with a perspective for the further development of the dock shape.
Fatigue Under Combined High and Low Frequency Loads
Authors: Huang Wenbo , Moan Torgeir
Published: October 2008

Based on Gaussian load processes, a new formula suitable for evaluating the combined fatigue damage due to high and low frequency loads is derived. Then, by using of the Winterstein’s transformation, the developed formula is extended for the combination of non-Gaussian loads. The numerical simulation shows that the predicted damage by the derived formula is very simple to use and close to the rain-flow prediction.
Numerical Simulations of Sloshing in Rectangular Tanks
Authors: Ransau Samuel R. , Hansen Ernst W. M.
Published: October 2008

Simulations of two- and three-dimensional sloshing in rectangular tanks are performed using the commercial CFD code FLOW3D. Small amplitude freely oscillating sloshing and non-linear sloshing due to forced excitation were investigated. The results are compared to both experimental results and other numerical results; and tests are made with different grids. The purpose of this study was the validation of the new VOF algorithm under development at Flow Science and implemented in FLOW3D.
Effects of Pile-Soil Interaction on Push-Over Analysis of Jacket Type Offshore Platforms
Authors: Asgarian B. , Lesani M.
Published: October 2008

Pushover analysis is performed to determine the capacity of jacket type offshore structures subjected to lateral loads. Overall behavior of the platform in the nonlinear range of deformation can be observed from pushover analysis results. This analysis can be used for the platform capacity assessment. For performing pushover analysis, nonlinear behavior of structural elements and lateral load pattern should be obtained. Both displacement and load control type of the analysis can be performed. One of the important aspects of pushover analysis is the non-linear behavior of the underlying soil and its interaction with the pile. In this paper pushover analysis of sample jacket type offshore platforms considering pile soil interaction (PSI) is performed. In the pushover analysis performed, fiber elements are used for the modeling of the member and soil nonlinearities. Actual soil layer properties, which have been derived from geotechnical reports, are incorporated in the model using fiber elements of “DRAIN-3DX” software. Each soil layer is presented by an equivalent fiber element, having the same characteristics of the relevant soil. The pile structure from top elevation until its tip together with other jacket components including the legs and braces are also introduced by fiber elements. Push over analysis is performed for the platforms subjected to wave load pattern. In order to compare the Pile-Soil Interaction (PSI) effects, all of the analyses are also performed using fixed and pinned pile ends at mud line elevation and pile stubs. The significance of the Pile-Soil Interaction and soil non-linear behavior is pointed out from this comparison and analysis results.
Reliability-Based Design Criterion for TLP Tendons
Authors: Cicilia Federico Barranco , Prates de Lima Edison Castro , Sagrilo Lui´s Volnei Sudati
Published: October 2008

This paper presents a Load and Resistance Factor Design (LRFD) criterion applied to the design of Tension Leg Platform (TLP) tendons in their intact condition. The design criterion considers the Ultimate Limit State (ULS) of any tendon section along its whole length taking into account both dynamic interactions of load effects and the statistics of its associated extreme response. The partial safety factors are calibrated through a long-term reliability-based methodology for the storm environmental conditions, like hurricanes and winter storms, in deep waters of the Campeche Bay, Mexico. In the reliability analysis, the uncertainties in the definition of load effects and analytic limit state models for calculation of tendon strength and randomness of material properties are included. The results show that the partial safety factors reflect both uncertainty content and the importance of the random variables in structural reliability analysis. When tendons are designed according to the developed LRFD criterion, a less scattered variation of reliability indexes is obtained for different tendon sections across a single or various TLP designs.
A Genetic Algorithm Approach to Steel Riser Optimization
Authors: Tanaka Rafael Loureiro , de Arruda Martins Clo´vis
Published: October 2008

It is well known that the riser system represents an important part of a production unit’s total cost. To ensure economic feasibility it is important to find means to reduce this cost to its minimum. In order to achieve cost reduction and other design improvements, optimization algorithms have been used in several areas with great success. This paper presents the application of such an algorithm, known as Genetic Algorithm, to the optimization of a steel riser taking into account material and installation costs. From a given inner diameter and material for both pipe and floater, the optimizer is left free to vary the number of segments as well as the pipe thickness, length and floater diameter for each segment, inside user-defined ranges. Each obtained configuration is tested in a set of load cases with different currents and offsets. Restrictions such as maximum tension, minimum curvature radius and maximum stress are applied to verify the configurations’ feasibility. Only static analysis is performed.
Comparison of Numerical Results and Design Criteria for Buckle Propagation and Buckle Arrestor Design
Authors: Plonski Thomas , Stadie-Frohbo¨s Gundula , Jokisch Gordon
Published: October 2008

Buckle propagation is a relevant design criterion for deep-water offshore pipelines. Imposed by external impact or local bending, e.g. during laying, a local buckle can be initiated, thereby decreasing the collapse strength of the pipeline. As a result of the high ambient external pressure, the buckle can start to propagate. Several design criteria for buckle propagation exist. This paper compares different design criteria with numerical results to obtain an impression of the levels of conservatism applied in the various codes. In most design cases, it is not suitable to avoid buckle propagation by using an increased wall thickness over the entire pipeline. Therefore, buckle arrestors are installed to stop the propagation. The common technical solution is to install sections of thicker pipeline, which requires that the buckle arrestor wall thickness and the length of the buckle arrestor have to be determined during design. A distinction is drawn between short and long buckle arrestors. Both cases are considered here. The design of the buckle arrestor can be carried out by using well-known criteria, such as the criteria developed by Kyriakides, Langner or Torselletti et al. These criteria are compared with experimental data. Numerical calculations are carried out and the results are compared with the design criteria.
Riser Response Analysis by Modal Phase Reconstruction
Authors: Lucor Didier , Triantafyllou M. S.
Published: October 2008

A methodology is presented to identify the vortex induced vibrational modes of a riser, based on data from computational fluid dynamics coupled to a long beam under tension and placed in cross-flow; two inflow velocity profiles are tested: (a) uniform, and (b) exponential shear. The modes we estimate are in fact nonlinear equilibria between the flow-induced excitation forces and the structural dynamics and are characterized by varying amplitude and phase along the span; these are complex modes, mixtures of traveling and standing waves.
Low Cycle Fatigue Analysis of Marine Structures
Authors: Wang Xiaozhi , Kang Joong-Kyoo , Kim Yooil , Wirsching Paul H.
Published: October 2008

There are situations where a marine structure is subjected to stress cycles of such large magnitude that small, but significant, parts of the structural component in question experiences cyclic plasticity. Welded joints are particularly vulnerable because of high local stress concentrations. Fatigue caused by oscillating strain in the plastic range is called “low cycle fatigue”. Cycles to failure are typically below 104. Traditional welded joint S-N curves do not describe the fatigue strength in the low cycle region (< 104 number of cycles). Typical Class Society Rules do not directly address the low cycle fatigue problem. It is therefore the objective of this paper to present a credible fatigue damage prediction method of welded joints in the low cycle fatigue regime.