bluuebunny/crossref_metadata_2025 huggingface dataset:
Reliability Analyses of Suction Caissons for FPSO Systems
Authors: Valle-Molina Celestino , Heredia-Zavoni Ernesto , Silva-Gonza´lez Francisco L.
Published: July 2009

The reliability formulation for the analyses of suction caissons subjected to environmental loadings from FPSO systems is presented in this paper. The capacity model for the suction caisson assumes normally consolidated clays with a linear variation on the undrained shear strength. The limit equilibrium method was used to assess the inclined capacity of suction caissons. The probabilistic characterization of the environmental loading is associated to deep water sites in the Bay of Campeche in the Gulf of Mexico. The reliability of the suction caissons was performed by means of Monte Carlo simulations and calibration of partial safety factors was carried out for the ultimate limit state using the design equation proposed by DNV [1].
Numerical Study of the Ship Motion in Waves Using the Three-Dimensional Time-Domain Forward-Speed Free-Surface Green Function and a Second-Order Boundary Element Method
Authors: Hong D. C. , Hong S. Y. , Sung H. G.
Published: July 2009

The radiation and diffraction potentials of a ship advancing in waves are calculated in the time-domain using the three-dimensional time-domain forward-speed free-surface Green function and the Green integral equation on the basis of the Neumann-Kelvin linear wave hypothesis. The Green function approximated by Newman for large time is used together with the Green function by Lamb for small time. The time-domain diffraction problem is solved for the time derivative of the potential by using the time derivative of the impulsive incident wave potential represented by using the complementary complex error function. The integral equation for the potential is discretized according to a second-order boundary element method where the collocation points are located inside the panel. It makes it possible to take account of the line integral along the waterline in a rigorous manner. The six-degree-of-freedom motion and memory functions as well as the diffraction impulse response functions of a hemisphere and the Wigley seakeeping model are presented for various Froude numbers. Comparisons of the wave damping and exciting force and moment coefficients for zero forward speed, calculated by using the Fourier transforms of the time-domain results and the frequency-domain coefficients calculated by using the improved Green integral equation which is free of the irregular frequencies, have been shown to be satisfactory. The wave damping coefficients for non-zero forward speed, calculated by using Fourier transforming of the present time-domain results have also been compared to the experimental results and agreement between them has been shown to be good. A simulation of coupled heave-pitch motion of the Wigley seakeeping model advancing in regular head waves of unit amplitude has been carried out.
Spectral Fatigue and Stress Based Reliability Assessment of Offshore Steel Structures
Authors: Karadeniz H.
Published: July 2009

Having summarized briefly uncertainties in spectral fatigue damages of offshore structures, this paper presents the formulation and procedure of an efficient computation of reliability estimates on basis of fatigue damages and stresses. Most of uncertainties are embedded in response characteristics of the stress process and the damage-model used. Uncertainties in stress statistical characteristics are associated with the modeling of structures, random wave environment, wave loading and the analysis used. In the fatigue damage, additional uncertainties arise from the modeling of damage-mechanism. These uncertainties are due to experimental fatigue data and structural joint configurations. All these uncertainties can be classified into the categories as a) those naturally inherent (aleatory) and b) those due to lack of knowledge (epistemic). The second part of the paper is devoted to a fast and efficient computation of the fatigue reliability. This algorithm prevents repetitive execution of spectral analysis procedure during the reliability iteration. In this technique, a suitable formulation of the stress spectrum is used with a model uncertainty parameter representing most of uncertainties in the stress spectrum. The failure function of the reliability analysis is expressed independently of the spectral analysis. For the stress based reliability calculation the mean stress-amplitude of the stochastic stress variation is used to define a limit state function. The related uncertainties are the same as those aforementioned. The advanced FORM reliability method is used to calculate the reliability index and to identify important uncertainty parameters. The procedure is demonstrated by an example jacket structure. The third part of the paper explains the inverse reliability method to determine some parameters, which may be deterministic or probabilistic, under required reliability constraints.
A Physical Model of Wind Waves in the Coastal Zone
Authors: Saprykina Yana , Andreeva Natalia , Kuznetsov Sergey , Cherneva Zhivelina , Guedes Soares C.
Published: July 2009

The variability of the amplitude-frequency structure of wind waves in space and time during their transformation in the coastal zone are considered. Wave time series, measured synchronously in 15 points along the wave propagation, obtained at field and laboratory experiments, were used for the analysis. Free surface elevation time series were represented as a sum of first and second harmonics with amplitudes slowly varying in time (or envelopes of the waves of corresponding frequency bands). Relative changes of these amplitudes in space and time were studied also. It was revealed, that at the initial stage of the wave transformation, the changes of amplitudes of the first and the second harmonics are similar and amplitudes of the second harmonics are proportional to the squared amplitudes of the first harmonics. At this stage the variability of parameters of individual irregular waves can be explained by Stokes theory. Nearer to the coast the instantaneous values of the amplitudes of the first and the second harmonics varies in time chaotically and is not possible to construct a simple model of the variability of the parameters of individual irregular waves. The main reason for this effect is the backward energy transfer from the second to the first harmonics of the waves during nearly resonant non-linear triad interactions.
Wave Fields Diffracted by an Array of Truncated Circular Cylinders
Authors: Zhao Fenfang , Kinoshita Takeshi , Bao Weiguang
Published: July 2009

Along with the continual expanding of sea cage culturing recently, the floating breakwaters that protect the seawater fish cages from the incoming waves are more and more popularly utilized. In addition to the advantage of low cost and movable, it has fewer blockages to the normal seawater exchange compared to the fixed ones. As a first step for the purpose of designing more effective floating breakwaters, the diffraction of waves by the arrays of bottom-mounted and truncated circular cylinders, based on the linear wave theory, is considered in detail in this paper. The transmission of waves propagating through the arrays is also evaluated. The phenomenon of near-trapping mode at certain wavenumber is investigated, where it has peak forces and low transmission rate. The arrays with unevenly spaced arrangement of circular cylinders are studied, which has an expected efficiency on reducing the peak forces.
Hull-Girder Reliability of an Aged Oil Tanker
Authors: Parunov Josˇko , Mage Paulo , Guedes Soares C.
Published: July 2009

The aim of the paper is to assess the hull-girder reliability of an existing single-hull oil tanker after 25 years of service using a reliability method and assumptions very similar to those proposed recently by International Maritime Organization (IMO). The evaluation of the wave-induced load effects that occur during long-term operation of the ship in the seaway is carried out in accordance with the International Association of Classification Societies (IACS)-recommended procedure, while transfer functions are calculated using the linear strip theory. The still-water loads are defined on the basis of statistical analysis of data recorded on single-hull oil tankers. The ultimate collapse bending moment of the midship cross section, which is used as the basis for the reliability formulation, is evaluated by progressive collapse analysis and by a single-step procedure according to Common Structural Rules (CSR). The reliability analysis is performed for three states of the hull: as built ship with scantlings of the new ship, CSR corroded state with net thicknesses of the structural members according to the corrosion deduction thickness proposed by CSR and survey state with scantlings according to the thickness measurements after 25 years of service. The yearly probability of structural failure is calculated using the first-order reliability method, while sensitivity analysis and a parametric study are performed to investigate the variability of the results with changes of the random variables within their plausible ranges. Results of the analysis are compared with recommended target reliability indices proposed by IMO and also with the results of the reliability assessment of the new double-hull tanker. Such comparison is possible since similar reliability methods and uncertainty models are employed. Results of the analysis may have several useful applications related to the decision-making aspects of the future of an aged oil tanker.
A Prediction Method of Hydroelastic Motion of Aircushion Type Floating Structures Considering With Draft Effect Into Hydrodynamic Forces
Authors: Ikoma Tomoki , Kobayashi Masato , Masuda Koichi , Rheem Chang-Kyu , Maeda Hisaaki
Published: July 2009

An aircushion type floating structure can prevent to enlarge the wave drifting force restraining the hydroelastic response of it in water waves. The floating structure should be large scale to incident waves in order to make the best use of such advantages, i.e. it is a very large floating structure. The linear potential theory is useful to easily handle the wave force etc. on the aircushion type floating structure theoretically because it is predicted that its theory can give good results of behaviors of water elevation within aircushions and pressure and of wave loads on the structure qualitatively. The authors have confirmed from our past model experiments that non-linear effect does not always increase but for some exceptions. A prediction method of hydroelastic responses for the aircushion type very large floating structure by using the three-dimensional linear potential theory is shown in this paper. The validity of the method is proven and the application of the method is investigated by comparing the theoretical results with the results of the past model experiments.
Hierarchical Bayes Analysis of Rare Events Using High-Dispersion Poisson Mixtures
Authors: Maes Marc A. , Dann Markus , Faber Michael Havbro
Published: July 2009

Modeling the occurrence of rare events such as multiyear ice or iceberg encounters, ship collisions, and several types of accidental events is often challenging because considerable dispersion is found to be associated with discrete count data. This may be due to fluctuations in the processes generating the events, or they may arise because of a complicated mixture of causal events or there may be other unexplained discontinuities. In such cases, the traditional use of the Poisson distribution is inadequate, especially when the event frequency is subsequently used to formulate design criteria based on extreme values. In this paper, the use of discrete Poisson mixtures is suggested as opposed to the simple Poisson process and continuous Poisson mixtures. One objective is to ensure that the uncertainty regarding event occurrence is well represented in both the central and tail parts of count data. The analysis of discrete Poisson mixtures involves the estimation of the number k of mixture components, the k Poisson occurrence rates, and the k weights of the mixture. Until recently such an analysis was considered daunting at best. However, the analysis can be re-cast as an equivalent Hierarchical Bayes (HB) net using an auxiliary variable vector Z of variable dimension. A Markov Chain Monte Carlo analysis can then be used to obtain the posterior distributions of the dimensionality of the mixture, the mixture weights and the occurrence rates themselves. Also, posterior distributions can be found for iceberg collision risks and iceberg scour rates. The approach is illustrated for an iceberg risk estimation.
Orbital Velocity in Spatial Ocean Wave Elevation Measurement: Nonlinear Computation and Approximation
Authors: Grue John , Romero Leonel , Kleiss Jessica , Melville W. Kendall
Published: July 2009

A data base of the recent Gulf of Tehuantepec experiment, of measured elevation of ocean surface waves in strong wind conditions, over swaths that are 6 km long by 200 m wide, is used as input for computation of the orbital velocity of the waves. Linear, quadratic and cubic approximations are compared to computations using a fully nonlinear-dispersive method in three dimensions, where the effect of a current is accounted for. A cancellation among the leading order quadratic terms is observed, and explains why a linear approximation when corrected by evaluation at the actual free surface provides a good estimate of the orbital velocity. Highly accurate results are obtained by including terms up to the cubic nonlinearity. The waves examined here have an estimated nonlinearity Kˆηm up to 0.4, Kˆ local wavenumber, ηm = 5 m maximal elevation, while historically high waves like those in Hurricane Camille and the Draupner wave have Kˆηm = 0.49 and ηm = 18.5 m (Draupner wave).
Numerical Prediction of VIV and Comparison With Field Experiments
Authors: Constantinides Yiannis , Oakley Owen H.
Published: July 2009

Computational fluid dynamics are employed to simulate vortex-induced vibrations of risers. The current method uses a three dimensional model with an optimized mesh to analyze high aspect ratio risers. A riser with an aspect ratio in excess of 4,000 is modeled and compared with field experiments. Results show good agreement with experiments and the ability of the method to predict observed harmonic content. With the computational resources that can now be brought to bare, simulations of deepwater risers are possible for design validation and guidance of empirically based tools that currently do not model high harmonics.
On Modelling Kinematics of Steep Irregular Seaway and Freak Waves
Authors: Clauss Gu¨nther F. , Stempinski Florian , Stu¨ck Robert
Published: July 2009

The realistic modelling of velocity and pressure fields in steep, irregular seaways is still a challenging task, especially when extreme events such as freak waves are under investigation. Conventional wave theories provide fast and reliable results while CFD-codes based on RANSE or potential theory are gaining more acceptance for simulating water waves even though they are still considerably time consuming. This paper presents an approach to approximate irregular wave trains with known surface elevations by interacting Stokes waves of up to third order. This is a fast method to determine the wave potential of wind generated waves for long lasting wave registrations with arbitrary origin. The technique is applied to a steep breaking wave package as well as to a realization of a wave train in a wave tank (scale 1:120) which contain a measured extreme wave sequence. Here, special attention is paid to the distinction between the kinematics of the wave crests in extremely high waves and their surrounding irregular wave field. The predicted wave kinematics are validated by experiments employing particle velocity measurements (by Laser Doppler Velocimetry) as well as by pressure recordings. Kinematics of breaking waves are not covered by concurrent analytical wave theories. To address this deficiency a coupling mechanism between a conventionally determined velocity field with a RANSE/VoF-method is applied.
Fatigue Testing and S-N Data for Fatigue Analysis of Piles
Authors: Lotsberg Inge , Wa¨stberg Stig , Ulle Hugo , Haagensen Per , Hall Michael E.
Published: July 2009

Design S-N curves in design codes are based on fatigue test data where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. During 2006 the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the seventies. Therefore it was suggested to investigate the pile weld at the sea bed in detail to learn about the actual fabrication, the residual stress at the weld after pile driving and 30 years of in service life and residual fatigue life of the pile. 6 test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.
Deterministic and Stochastic Prediction of the Hydrodynamics of a Three-Dimensional Body Falling Through Water
Authors: Zhu Xuemei , Liu Yuming
Published: July 2009

We investigate the dynamics of a three-dimensional mine-shaped body falling through water deterministically and stochastically. A physics-based deterministic model, MINE6D, is developed for the prediction of the six degree-of-freedom motion of the body falling freely through water. In MINE6D, the hydrodynamic load due to the added inertia effect is obtained exactly by using a boundary-element method while the viscous drag associated with flow separation and vortex shedding is modeled using a quasi-steady approach. Since the mine motion is found to be highly sensitive to varying the physical parameters such as body geometry, mass distribution, and initial releasing conditions, we develop a stochastic model using Monte-Carlo MINE6D simulation for the statistical analysis of mine motions in practical applications. The statistical prediction is compared with available field measurements both qualitatively and quantitatively. The characteristic features and dependence on physical parameters of the statistical prediction of mine motions are investigated. The present study is of importance to the prediction of mine burial in seabed and the design of mines.
In-Place Free Span Assessment Using Finite Element Analysis
Authors: Pereira Antonio , Bomfimsilva Carlos , Franco Luciano , Tardelli Luciano , Eigbe Uwa
Published: July 2009

The DNV RP 105 standard provides guidelines for evaluating the fatigue damage of pipelines over free spans, where guidance is provided for calculating the approximate Vortex Induced Vibration (VIV) response, considering the pipe properties, the span geometry, the pipe-soil interaction and the effective axial force. The approximate response models are limited, however, to single spans with leveled shoulders, short length, i.e. span lengths less than 140 times the pipe diameter, and bar buckling not influencing the pipeline dynamic response. To overcome these limitations, specifically for “long” spans and multi-spanning pipelines, RP F105 recommends that eigen-value analysis be performed using Finite Element (FE) method to calculate the natural frequencies, mode shapes and corresponding stresses associated with the mode shapes considered for VIV fatigue assessment. In this sense, a methodology and suite of Finite Element (FE) based tools for multi-mode/multi-span VIV fatigue assessment have been developed. The FE methodology accounts for the initial static equilibrium configuration of the pipeline in the as-laid condition followed by application of the subsequent load steps, such as flooding, hydrotesting, dewatering, start-up, etc. It also considers the non-linearity of the seabed stiffness and the effect of geometric non-linearity/large deflections on the dynamic response of the pipeline. In addition, the FE approach allows the determination of the dynamic response at every location of the free span for the different mode shapes and hence offers the ability to calculate the distributed fatigue damage along the spanning section of the pipeline instead of assuming all damage occurring at a single location. The methodology was applied in recent projects to allow for a better estimate of the requirements for free span correction with significant cost savings anticipated. The proposed methodology also has the potential for post-lay assessment of the pipelines and for through-life in-service assessment of existing pipelines, where new free spans are often observed during inspection due to soil movements along the lifetime of the pipeline. This paper addresses the in-place FE methodology, the validation process and the tools that were developed to speed up the fatigue assessment procedure, which is a key factor especially when analyzing post-lay survey in real-time for determination of requirements for free span correction in the field.
Finite Element Analysis of Clamp-On Buckle Arrestor for Pipe-in-Pipe Flowlines by Reel-Lay Installation
Authors: Sun Jason , Jukes Paul
Published: July 2009

Development of deep water oil reservoirs are undertaken in the Gulf of Mexico (GoM) where the flowlines are installed in the water depths in excess of 3,050m (10,000ft). Deepwater external pressure becomes so significant that it makes local buckling or accidental collapse propagate along the pipeline. Such propagation will not stop until it reaches a region where the external pressure falls below the propagating pressure or where the pipe wall is strengthened. Field data indicates that once a buckle happens, the flowline could collapse many kilometers instantly. It concludes that buckle propagation could cause substantial economical impact if left uncontrolled. For pipe-in-pipe (PIP) flowline, due to lack of pressure differential, the outer pipe becomes a fragile component in terms of buckle propagation. One way to prevent the propagation of local buckling or collapse is to utilize the buckle arrestors of various types. Clamp-on buckle arrestor is so far the best choice for the flowlines to be installed by the Reel-Lay method. The objective of this paper is to present the results of a finite element (FE) study, to reveal the phenomena of collapsing/propagating of the pipe-in-pipe flowline, and to investigate the effectiveness of Clamp-on buckle arrestor for deep water flowlines. Sensitivities of key design parameters are explored with the purpose of guiding detail mechanical design of the clamp-on buckle arrestor.
Numerical Analysis of Soil Response to Ice Scouring
Authors: Evgin E. , Fu Z.
Published: July 2009

Icebergs and ice ridges frequently scour the surface of seabed deposits. Ice scouring can be problematic for offshore pipelines and other seabed installations. In order to reduce the risk of failure, pipelines are buried in the seabed. However, a stationary or moving ice feature could cause a significant increase in stresses and deformation in the seabed soil deposits below the contact surface between the soil and the ice, and consequently, might result in structural failure of buried pipeline. Safe burial depth for pipelines has been the subject of both experimental and numerical studies. In this paper, two and three dimensional analyses are conducted using PLAXIS and ADINA. In these analyses, geometric and material nonlinearities are considered. In order to establish the validity of the finite element calculations, the experimental results reported in the literature and the numerical results obtained in the present study are compared. The emphasis is placed on determining the importance of (1) using interface elements between different materials such as soil and ice, soil and pipelines; (2) using the soil model correctly, and (3) using a three dimensional analysis rather than a two dimensional analysis. The changes taking placed in the deformation pattern and the stress states in the seabed soils resulting from ice scouring are determined. The effects of pipeline burial depth, the shape of the ice feature, and the characteristics of seabed soils on the stresses acting on the pipeline are evaluated.
Research on Methods of Preventing Children From Getting Lost at Beaches
Authors: Miyazaki Wataru , Kondo Takeo , Yamamoto Kazukiyo , Koga Takahiro
Published: July 2009

At Oarai Sun Beach, one of the most popular bathing beaches in Japan, more than 500 children get lost every year. However, the only method employed at bathing beaches in Japan, including Oarai Sun Beach, to prevent children from getting lost is supervision of children by their guardians. Thus, a method that can be followed by beach management personnel is required. This study reports the results of a survey and analysis conducted with regard to developing a method of preventing children from getting lost and a method of quickly finding lost children as well as clarifies the reasons for children getting lost. It was found that in many cases, children get lost because they are distracted and lose sight of their guardians in specific areas, e.g., near the water and on the sandy shore, as well as in crowded areas.
Experimental and Numerical Investigation Into the Effects of Fluid-Structure Interaction on the Sloshing Impact Loads in Membrane LNG Carrier
Authors: Jung Jong-Jin , Lee Hyun-Ho , Park Tae-Hyun , Lee Young-Woo
Published: July 2009

The hydro-elasticity effect of sloshing loads in LNG cargo tank has been studied through experiments and numerical simulations regarding the fluid-structure interaction between sloshing impact pressures and tank structures. Sloshing model tests with 1/50 scale membrane type tanks were carried out for 1-D regular harmonic motion to investigate variations of impact pressures due to elasticity differences of the tank structure. Numerical simulations were performed and validated for the same case. Additionally, wall impinging jet flow was simulated by numerical simulation to verify the relation between elasticity of structure and impact pressure. It was commonly observed that the elasticity of the tank structure had significant influence on the height and shape of the impact pressure peak. Numerical study showed that the ratio between the structural natural period and the duration of the impact pressure is important for the influence of impact pressure on the tank structure.
Liquid Sloshing Dynamics in a Container Subjected to Coupled Mode Excitation
Authors: Nasar T. , Sannasiraj S. A. , Sundar V.
Published: July 2009

An experimental work has been carried out to study the phenomena of sloshing of liquid in a partially filled tank mounted on a barge exposed to regular beam waves. Liquid fill level with aspect ratio (hs/l, where hs is the static liquid depth and l is the tank length) of 0.325 is studied. The time histories of sloshing oscillation are measured along the length of container at predefined locations. The nonlinear behaviour of sloshing oscillation is observed for the regular wave excitation. The spectra of the sloshing oscillation and their qualitative assessment are reported. The individual sway and heave analytical model have been studied in order to substantiate the importance of coupled mode of excitation. Attempts are made to evaluate the harmonics present in the sloshing oscillation and compare with the results of earlier studies. In the present interaction study, it was found that the nonlinear response of the floating body also plays a role to induce violent sloshing oscillation. The effects of wave excitation frequency on the sloshing oscillation are reported.
Statistical Properties of a Directional Wave Field: Direct Simulations of the Euler Equations and Second-Order Theory
Authors: Toffoli A. , Bitner-Gregersen E. M. , Onorato M.
Published: July 2009

It is well established that the modulational instability enhances the probability of occurrence for extreme events if waves are long crested. Recent studies, however, have shown that the coexistence of directional wave components can substantially reduce its effects. Here, direct numerical simulations of the Euler equations are used to investigate whether the modulational instability may produce significant deviations from second-order statistical properties of surface gravity waves when short crestness (i.e., directionality) is accounted for. The case of a broad-banded directional wave field (i.e. wind sea) is investigated. Results will show that the distribution proposed by Forristall [1] provides a good estimate of the simulated crest height also at low probability levels.
Safety of Mega Container Ship Focusing on Brittle Crack Initiation and Arrest Behavior of Heavy Thickness Plate
Authors: Toyoda Masanobu , Kiji Noboru , Nakajima Yoshiyuki , Okada Tetsuo , Nakanishi Yasumasa , Inose Koutarou
Published: July 2009

The enlargement of containership requires the higher tensile strength steel and thicker plate around the hatch coaming more than the prescribed classification rules. These call the concerns about the safety of brittle crack initiation and arrestability of hull structure. It is known that increasing of stress and thickness of plates will reduce the toughness of base metal and welding joints. Therefore, the authors conducted to assure the toughness of base metal and welding joint for new higher tensile strength steel of 460MPa yield strength (YP460) and 60mm thickness. It has been considered that weld assembled steel plate structure has some arrestability at intersections of plates, though there are no prescription on the rules. And it was reported that very thicker mother plate has not enough toughness to arrest brittle crack recently. The authors investigated the structural arrestability by model testing and FE analysis and established it. Furthermore, structural details for intersections were also established. Both preventing the brittle crack initiation and arresting the brittle crack of weld structure were described, focusing on developing higher yield strength and heavier thickness of YP460 for containership in this paper.
Stern Slamming of an LNG Carrier
Authors: Oberhagemann Jan , Holtmann Michael , el Moctar Ould , Schellin Thomas E. , Kim Daewoong
Published: July 2009

Rational assessment of stern slamming of a large twin screw LNG carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behavior of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ship’s stern. Reynolds-averaged Navier-Stokes (RANS) computations based on the volume of fluid (VOF) method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were coupled to a nonlinear motion analysis of the ship in waves. At every time step of the transient computation, the finite volume grid was translated and rotated, simulating the actual position of the ship. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Slamming-induced pressure peaks, typically lasting for about 0.5 s, were characterized by a steep increase and decrease before and after the peak values. Shape and duration agreed favorably with full-scale measurements and model tests carried out on other ships, indicating that computed results captured all essential physical phenomena. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. Short-duration impact-related slamming loads excited the ship structure to vibrations in a wide range of frequencies. Excitation of the lowest fundamental eigenmode contributed most to additional stresses caused by hull girder whipping. Although longitudinal and shear stresses caused by quasi-steady wave bending were uncritical, we obtained a significant amplification (up to 25 percent) due to the dynamic structural response.
Assessment of Thermal Protection of Life Rafts in Passenger Vessel Abandonment Situations
Authors: Mak Lawrence , Kuczora Andrew , DuCharme Michel B. , Boone James , Brown Rob , Farnworth Brian , Evely Kerri-Ann , Basset Fabien A. , MacKinnon Scott
Published: July 2009

Inflatable life rafts are currently used on almost all passenger, fishing and commercial vessels, and offshore oil installations. Worldwide, life rafts are the primary evacuation system from fishing vessels with relatively small crews to large Roll on/Roll off passenger vessels with over a thousand passengers and crew. While International Maritime Organization (IMO) standards currently require inflatable life raft components to “provide insulation” or “be sufficiently insulated”, there are no performance criteria for these requirements (IMO, 1996). In a passenger ship abandonment situation in cold water, passengers may be wearing very little personal protective clothing. Therefore, life rafts provide the only significant thermal protection against the cold ocean environment while they await rescue. Manufacturers equip life rafts with an insulated floor to reduce heat loss from direct contact with the cold ocean water. The insulation provided is critically important for life raft occupants who have little protective clothing. The heat loss of unprotected persons is drastically increased if there is a layer of water on the floor as would likely be the case when someone climbs into the life raft from the ocean or if water is splashed into the life raft in heavy weather. Experiments were conducted in mild cold (16°C water temperature and 19°C air temperature) and cold conditions (5°C water temperature and 5°C air temperature) to assess the thermal protection of a 16-person, Safety of Life at Sea (SOLAS) approved, commercially available life raft. This paper presents results in the mild cold condition only. It has been found that the wave height effect may be ignored as a first approximation to reduce the number of environmental variables because the results demonstrated that wave height effect is less important with leeway. Heat conductance decreases considerably with floor inflation. Heat conductance is about the same with floor inflated 50% and 100%. The CO2 concentration in the 11-person test exceeded 5000 ppm in less than an hour inside the life raft, with closed canopy and no active ventilation. This hostile microclimate inside the life raft suggests that active ventilation at a known rate is required to keep the CO2 level at a safe controlled level when longer duration tests are to be conducted in the future. Wet clothing has a significant effect on occupant heat loss.
Dynamic Behaviour of a Container Ship Using Two- and Three-Dimensional Hydroelasticity Analyses
Authors: Basaran I. , Belik O. , Temarel P.
Published: July 2009

The symmetric (vertical bending) and antisymmetric (coupled horizontal bending and twisting) dynamic behaviours of a feeder container ship travelling in regular oblique waves are investigated using two- and three-dimensional hydroelasticity theories. Different three-dimensional FE structural models are generated in order to investigate the influence of hatch coamings and on-deck container cargo. Corresponding beam structural idealizations are obtained from these models. Comparisons are carried out between the dynamic characteristics and wave-induced loads predicted by both methods.
Combined Acoustic and Hydrodynamic Experimental Data Acquisition in Towing Tank: Moonpool Experiments
Authors: Sadiq Salman , Yao Xiong-Liang
Published: July 2009

Time and cost effective experimental verification and accuracy procedures are the first and utmost requirements in ship building research conducted in towing tank. This paper presents towing tank experimental setup and procedures to acquire acoustic and hydrodynamic data at the same time for a moonpool experiment. The same technique can be applied on a variety of configurations to acquire combined acoustic and hydrodynamic data. In present work the experimental technique possesses the ability to monitor pressure fluctuations, acoustics, free surface wave height inside a moonpool and structural vibration of a moonpool model at the same time with a better estimation and accuracy. The results are compared with numerical modeling, empirical relations and a detailed collected data comparison analysis is given in light of ‘Empirical Mode Decomposition’ (EMD) method. Collected data from sensors are proved to be in engineering practice limits proving the importance and authenticity of the data acquisition procedure discussed here. We believe that the results and methods given in this paper have surely a permanent space in the technical literature.
Multi-Dimensional Numerical Free Surface VOF Modeling With Moonpool Experiments
Authors: Sadiq Salman , Yao Xiong-Liang
Published: July 2009

The homogeneous multiphase incompressible flow past a moonpool was investigated to determine the shape of vortex, flow field and pressure distributions. In this approach, a homogeneous flow model together with the VOF method for interface capturing is used to compute the entire flow field within the moonpool. The turbulence is represented via fully homogeneous buoyant model with air and water as continuous fluids. Numerical results are verified by conducting towing tank experiments. Simulation ship moonpool applications are verification of capabilities in ANSYS CFX multi-physics code by two and three dimensional circular and square shaped moonpool subjected to flow in a channel. The numerical results indicate that cavity location with a unit factor of 1.667(L/L1), where ‘L’ is length of cavity and ‘L1’ is the distance from inflow edge; was proved to be appropriate for such cavities to have optimum performance related to moonpool hydrodynamics. Distance ‘L2’ had a negligible effect on cavity. Free surface height ‘H’ inside the cavity was maintained at 0.4 m both in numerical calculations then in experiments. Numerical and experimental comparison of results reveals that due to multiphase modeling; results diverge from actual value near phase coupling. Shape factor like internal curvature, has a great influence on vortex shape and hydrodynamic forces inside the moonpool. By analyzing these numerical results a better understanding is established for VOF models and moonpool piston phenomenon. Square shaped moonpool results and numerical results agree very well till half of the moonpool depth from free surface height. These results can be very well used for the basis of designing complex shaped moonpool having free surface. The agreement with the experimental data is within the accuracy of other simulations. Further studies and the development of Best Practice Procedures are required as a next step to reduce/avoid numerical and set-up errors and establish CFD as an industrial tool.
Onboard Screening of Forecast Weather During Installation
Authors: Passano Elizabeth , Gjo̸steen Janne K. O̸. , Sævik Svein
Published: July 2009

Pipelines and umbilical are key elements in subsea infrastructure for energy transport. Offshore installation of long pipelines and umbilicals on uneven seabeds in deep waters are critical operations. Uncertainties in weather conditions and currents add to the challenge. During the planning of an installation, many detailed static and dynamic analyses are performed in order to determine the weather conditions under which the operation may take place: the operational weather window. Weather windows may either be established for the whole operation, or different weather windows may be established for different phases of the operation, e.g., depending on water depth, bottom conditions or prevailing current along the route. To allow these weather windows to be established, representative conditions must be chosen for each part of the route. If the weather deteriorates during the operation so that forecasted weather conditions are not within the weather window, the operation must be halted until the weather conditions improve. Considering the day rates for laying barges, the increased costs may be significant. A delay may also be detrimental to the total project. An alternative is to base the decision to proceed or halt on updated analyses at the present location. The correct water depth, soil characteristics, local seafloor geometry and possible free spans in the laid pipeline may thus all be included in the analyses along with the most up-to-date weather and current forecasts. The most correct estimate of the response will then be available as a basis for the decision to continue or halt the operation. Other “what if” scenarios may also be investigated; e.g., allowing the operator to explore the effect of changed operation or deviations from the predicted weather. Estimates of fatigue damage rates may be used to evaluate continued operation in deteriorating conditions. A system for onboard screening of forecast weather condition is presented along with examples from a prototype application. Using knowledge of the system, the extent of non-linear simulations is limited to where they will give the most contribution to the extreme response in the forecasted conditions.
Multi-Objective Optimization Technique Applied to Preliminary Design of a Tanker
Authors: Martins Marcelo R. , Burgos Diego F. S.
Published: July 2009

This paper shows one rational process of selecting the optimal dimensions and coefficients of form of tankers using the technique of genetic algorithm in the early stage of design. Two objective attributes are used to evaluate each design: Total Cost and Mean Oil Outflow. It is proposed a procedure to balance the designs in weight and useful space and assesses their feasibility. A genetic algorithm is implemented to search optimal design parameters and identify the non-dominated Pareto frontier. A real Suezmax vessel is used as case study.
Evaluation of Key Hydraulic Tensioner Performance Parameters for Ultra Deep Water Applications
Authors: Yu Alan , Cheng Yongming , Bhat Shankar
Published: July 2009

Hydraulic tensioners are widely used to provide tension and stiffness to vertical risers on spar and tension leg platform floating production facilities. Their performance directly affects dynamic response of the risers and the global motions of the floating system. The coupling effect of the tensioners, risers and vessel becomes increasingly important as the water depth increases into the ultra deep. This paper evaluates key performance parameters of long-stroke high-tension hydraulic tensioners for ultra deepwater application. It derives the fundamental equations and evaluates key performance parameters and their impact on the riser and vessel responses. This paper also proposes data measurement locations and methods to collect field data for calibrating these key performance parameters for future hydraulic tensioner design. The key tensioner performance parameters are the gas constant and the cylinder friction. In addition for a typical spar riser configuration, the keel joint friction also plays an important role in the overall riser performance. Their importance increases as water depth increases to 8,000 feet and beyond due to the nature of the coupling effect, long stroke and high tension requirements. Gathering of full scale-test data for these parameters has been costly. With several 7000 to 8000 feet risers and tensioners to be installed in the next few years, a method of collecting field data and its analysis for calibrating against the current assumptions of these parameters is proposed.
Design Through Simulation: Finite Element Capabilities for Ocean Engineering
Authors: Steinke Dean M. , Nicoll Ryan S. , Buckham Bradley J.
Published: July 2009

Design optimization and testing of marine technology and offshore structures, such as risers, moorings, or manned and unmanned submersibles is a challenge. This is due to many factors including weather, costly ship time, and the need for experienced off-shore personnel. Nevertheless, early stage design optimization is critical to a project’s success. There is a need for simulation facilities that can capture the complexity and the non-linear dynamics of large mechanical and structural systems, and provide accurate assessment of design variations. This article outlines the development of a nonlinear simulation tool for modeling mechanical systems and structures in the ocean. The framework design and simulation set-up procedures are discussed. The main components of the simulator, a nonlinear finite element cable model and a rigid body model, are discussed. Next, this paper shows how these fundamental models are used to simulate risers, remotely operated vehicle (ROV) umbilicals, and mooring lines. In addition, a module that produces the effects of vortex induced vibration (VIV) based on recent developments on the wake oscillator model is presented. Payin and payout simulations of a ROV tether are also presented to demonstrate the use of the variable-length capabilities of the cable model. Lastly, this paper discusses how ROV instrumentation can be simulated, permitting the design and refinement of instrumentation processing algorithms, such as a Kalman filter, or controllers.
Simulation of Residual Stress Effect on Stress Corrosion Crack Growth in Gas Pipelines
Authors: Fourozan M. R. , Olfatnia M. , Golestaneh S. J.
Published: July 2009

In this paper, a quantitative study on stress corrosion crack growth in large diameter gas pipelines is presented. Finite element method is applied for determining stress intensity factor at the crack tip. First a small semi-elliptical axial surface crack is assumed. Then internal gas pressure and residual stress, induced from welding process, are considered. Applied forces and crack growth rate are calculated as a function of stress intensity factor based on an empirical equation. Crack front shape is determined by calculating stress intensity factor distributions along the crack tip. As a result, the effect of residual stress on stress intensity factor and therefore crack growth is determined. In addition, minimum crack size that activates the stress corrosion cracking mechanism is determined. It is shown that the applied method could be used to estimate the reliable life of pipeline and the suitable time for inspection of the pipeline’s surface.
Reliability Based Assessment for Fatigue Integrity Management
Authors: Manzocchi M. , Shetty N. , Stacey A.
Published: July 2009

Reliability analysis can be used to assess the risk of structural failure in ageing offshore installations and to subsequently optimise inspection plans. This paper describes the findings of a joint industry project to develop and implement a methodology for system reliability updating based on the results of in-service inspections. The effects of different combinations of inspection techniques, namely flooded member detection (FMD) and magnetic particle inspection (MPI), and inspection intervals on the component reliability were examined. Additionally, a case study was performed to demonstrate the system reliability of actual structures with inspection histories. A comprehensive methodology for updating the assessment of system reliability of offshore structures based on in-service inspection results was developed. The methodology allows several important features associated with system reliability, i.e. redundancy, stress redistribution and correlation between the random variables, to be accounted for. The structural reliability is given in terms of an overall reliability for combined fatigue and pushover. The study demonstrated that the methodology may be used to target inspections to those areas of the structure with low fatigue life and low redundancy.