bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance Assessment of Davit-Launched Lifeboat
Authors: Prat Laurent , de Vries Leo , Vredeveldt Alex W. , Khattab Omar , Maisonneuve Jean-Jacques , Boekholt Thomas , Andersen Ole , Cummings David , Block Jan
Published: July 2009

In response to the recent changes to SOLAS, which further encourage the alternative design of life-saving appliances, the European Union has funded a Framework Programme 6 (FP6) project called SAFECRAFTS that was initiated by the Netherlands Research Organisation TNO. The aims of the project are two fold: to develop a methodology for assessing the performance of life saving systems (made up of life saving appliances or LSA); and to develop novel concepts. This paper focuses on the first objective and presents the results for conventional davit-launched lifeboats. The proposed performance indicator is the Success Rate of the evacuation process, which compares the final Human Health Status (the number of persons in good health, injured or deceased at the end of the process) with the initial Human Health Status (number of persons in good health onboard the mother ship). The Human Health Status calculation follows a step-by-step approach for both the human and the hardware as they progress along an obstacle course, which represents the evacuation sequence specific to the design. Associated to each obstacle is a transformation function that characterises the degradation of the Human Health Status. The determination of the transformation functions is based on first principle methods, risk analysis methods, and human factor methods some of which are inspired by the automotive industry. A scenario-based approach is applied to account for increasingly severe environmental and damaged ship conditions. In addition, the systems performance is represented along the evacuation and rescue route, so that it is possible to identify and quantify the contribution of individual obstacles to the overall degradation. This is essential to select the critical areas for improvement and support the development of innovative designs. Finally, the paper advocates that the set of scenarios and their associated Success Rates represent performance criteria for LSA which can be used for approving alternative designs and arrangements.
Numerical and Experimental Studies of Liquid Sloshing in Rectangular Tanks Using a Particle Method
Authors: Gao M. M. , Koh C. G. , Luo C.
Published: July 2009

The sloshing waves in rectangular tanks are studied experimentally and numerically based on the fully nonlinear wave theory. A moving-particle semi-implicit (MPS) method belonging to the category of the particle method is utilized to solve the Navier-Stokes equation that is the governing equation of the incompressible fluids. The motion of each particle is calculated through interactions with neighboring particles covered with the kernel function. The governing equations are solved by Lagrangian approach and no grid is needed in the computation. The incompressibility is satisfied by keeping the particle number density constant. When the tank undergoes two-dimensional motion, the numerical results obtained are found to be in good agreement with other published data as well as our experimental results obtained by shake-table tests. Experiments with rectangular tanks have been conducted to validate the numerical results, for which favorable agreement is shown. Results will also be presented for the study which is currently extended to three dimensional sloshing by exciting the tank at an inclined angle.
Development of Simplified Structural Design Formulation for Slamming Loads
Authors: Sinha Satyaranjan , Kar Suman , Sarangdhar D. G.
Published: July 2009

Impact Loads due to wave slamming on horizontal members of the hull structure are of considerable interest in the context of very light and optimized structure, as found on Naval Vessel and other high speed vessels. These Slamming Loads are highly dynamic in nature, which is characterized by very short duration of the load, usually 10–20 milli-seconds, with a peak value for even a shorter duration. Therefore, a dynamic analysis is required to study the response of the structure, which is susceptible to slamming. Also, considering the extreme load response with a small probability of occurrence, certain amount of permanent deformation can be accepted. This paper presents the systematic transient dynamic analysis carried out for almost 2000 cases and proposes a method based on this study, for design of structures subjected to slamming loads. A Dynamic Load Factor, which converts the dynamic load into an equivalent static load, is introduced, after which a simple static analysis would yield the same results as that obtained from the dynamic analysis. Further, the paper also provides a method for calculating the plate thickness, given any slamming load, acceptable value of permanent deformation, and in-plane stress.
The Effects of Fish Cages on Ambient Currents
Authors: Gansel Lars C. , McClimans Thomas A. , Myrhaug Dag
Published: July 2009

Experiments were carried out to measure forces on and wake characteristics downstream from fish cages. Cylinders made from metal mesh with porosities of 0%, 30%, 60%, 75%, 82% and 90% were tested in a towing tank. The drag force was measured with strain gauges and the flow field downstream from the models was analysed using particle image velocimetry (PIV). The Reynolds numbers ranged from 1000 to 20000 based on the model diameter and 15 to 300 based on the diameter of the strings of the mesh as an independent obstacle. High porosities (here 82% and 90%) lead to low water blockage and allow a substantial amount of water to flow through the model. The data indicate, that the wake characteristics change towards the wake characteristics of a solid cylinder at a porosity just below 75%. The drag force is highly dependent on the porosity for high porosities of a cylinder.
Tidal Boundary Layers
Authors: Holmedal Lars Erik , Myrhaug Dag
Published: July 2009

The tidal boundary layer has been investigated by solving the boundary layer equations using a constant eddy viscosity and by using a k–ε model, and the difference between these two methodologies is shown. The real particle trajectories have been estimated for a given realistic case.
PRA-1: An Innovative 4-Leg Launched Jacket Concept
Authors: Maciel Vitor G.
Published: July 2009

This paper presents an innovative 4-leg launched jacket concept, originally developed as a more economical solution for PETROBRAS PRA-1 platform in Brazilian Campos Basin. The concept goal is structural and functional optimization, focusing mainly in minimization / elimination of the usual drawbacks associated with the addition, in conventional 4 leg jackets, of launch trusses and associated structures that are specifically designed for the skidding and launching operations, and that are useless and cumbersome after the completion of the installation, as they do not contribute to the in-situ overall strength and increase the dead weight and environmental loads acting on the structure. Pile cluster design optimization is also addressed, leading to improved load sharing between piles. The concept consists of a structure with four main legs, two of them, located in the rear face, slanted and straight from the top to the base, like in a conventional jacket, and the two other main legs, forming the front face, parallel in the upper region and slanting outwardly in a lower and a transition regions, constituting a widened base, endowed with dihedral external lateral faces. The parallel legs in the upper region are extended into transition and lower regions by reduced diameter ramifications referred as secondary legs, making up the launch rails on which the structure can be loaded out in a barge from the construction site and later launched at sea from the barge. The paper describes the concept main features and performance and its application to the PRA-1 project, a hub platform with a topside operating weight of 16,200 metric tons (mton), recently installed in 106m WD. Advantages, like 20% reduction of steel weight over a conventional light weight jacket, developed in parallel to a same stage of preliminary design for comparison purposes, are quantified. Descriptions of installation and in-situ static, dynamic, foundation and fatigue analyses are also presented.
A Numerical Study of Liquefaction Induced Deformation on Caisson-Type Quay Wall Using a Partially Coupled Solution
Authors: Arablouei A. , Ghalandarzadeh A. , Gharabaghi A. R. M.
Published: July 2009

A partially coupled effective stress analysis is assessed in predicting seismic performance of caisson-type quay wall. Using a nonlinear finite difference program, a numerical study of shaking table tests, carried out at Tokyo University, is performed. In this paper, the formulation of employed computational code is described and the results of numerical simulation are compared to the measured records. The results demonstrate that the trend and magnitude of vertical and horizontal displacement of quay wall appears to be predicted reasonably well. The overall tendency is to overpredict the horizontal movement but the vertical movement is underpredicted.
A Stern Slamming Analysis Using Three-Dimensional CFD Simulation
Authors: Kim Kunho , Shin Yung S. , Wang Suqin
Published: July 2009

A stern slamming analysis based on three-dimensional computational fluid dynamics (CFD) simulation is presented with an application to a liquefied natural gas (LNG) carrier with twin skegs. This study includes; seakeeping analysis, statistical analysis for relative motions and velocities, three-dimensional slamming simulation by a CFD software, and structural assessment for plates and stiffeners. The stern areas are divided into panels in which relative velocity/motion and pressure coefficients are to be calculated. Seakeeping calculations are carried out in full load and ballast loading conditions at ship speeds of 0 and 5 knots. A series of equivalent 20-year return sea states in a wave scatter diagram are selected for environmental conditions. Extreme velocities are then evaluated from the loading conditions and the speeds considered with reference to the probability of slamming occurrence. Slamming simulations are carried out in a three-dimensional domain with a CFD software to calculate pressure coefficients. Two-phase flow with water and air is to be adopted in conjunction with free surface capturing method. Viscous laminar flow is assumed in simulation. Slamming design pressure is calculated by the pressure coefficients and the extreme velocities. Based on computed design pressure, an ultimate strength analysis is performed for the determination of required plate thickness. Also, required stiffener dimensions are determined by analytic formulas. As mentioned above, this approach has been applied to an LNG carrier with twin skegs. In the application, two-phase flow with water and air was adopted in conjunction with the volume-of-fluid method for free surface capturing. Mixed hexahedral and tetrahedral grids were employed. The computational case was determined from simulations of global ship motion. Maximum slamming pressure was found near the end of a skeg. Large pressure also can be observed in the stern overhang area. Generally slamming pressure decreases away from the stern.
A Fundamental Study on the Influences of Submarine Groundwater Discharge on Coastal Water Environment
Authors: Tabeta Shigeru , Kinoshita Tsuguki
Published: July 2009

It has been considered that most of the terrestrial water and contained substances are supplied into the ocean through the rivers. However, recent studies show that the submarine ground-water discharge (SGD) is also significant as the route of water and substances from the land to the ocean. In order to evaluate the influences of SGD on the coastal water environment, it is important to investigate the amount of SGD flux into the sea as well as the behaviour of discharged water and substances in the coastal sea. In the present study, the total SGD flux is estimated by the budget analysis of the target sea area, in which a parameter for water exchange between the coastal sea and the ocean is calculated by means of a numerical simulation by a hydro-dynamic model and a dynamic budget model. The behaviour of discharged groundwater is also investigated by hydraulic experiments and numerical simulations using non-hydrostatic model. It is found that the early-stage behaviour of the discharged submarine groundwater as buoyant plume could be categorized into three types due to the ratio of discharge velocity to the background horizontal current velocity.
Dynamic Response of a Porous Seabed and an Offshore Pile to Linear Water Waves
Authors: Lu Jian-Fei , Jeng Dong-Sheng
Published: July 2009

In this study, a coupled model is proposed to investigate dynamic response of a porous seabed and an offshore pile to ocean wave loadings. Both the offshore pile and the porous seabed are treated as a saturated poro-elastic medium, while the seawater is considered as a conventional acoustic medium. The coupled boundary element model is established by the continuity conditions along the interfaces between the three media. In the system, wave force is considered as an external load and it is evaluated via the wave function expansion method in the context of a linear wave theory. Numerical results show that the increase of the modulus ratio between the pile and the seabed can reduce the horizontal displacement of the pile and the pore pressures of the seabed around the pile. Furthermore, the maximum pore pressure of the seabed usually occurs at the upper part of the seabed around the pile.
Stress Concentration Factors of Longitudinal and Transverse Plain Dents on Steel Pipelines
Authors: Pinheiro Bianca C. , Pasqualino Ilson P.
Published: July 2009

The objective of this work is to evaluate the stress concentration induced by longitudinal and transverse plain dents on steel pipelines under cyclic internal pressure. This work is within a study to propose a new methodology to assess the fatigue life of dented steel pipelines based on the current high cycle fatigue theory. This methodology employs stress concentration factors induced by plain dents, which are used to modify material S-N curves of metallic structures under high cycle fatigue loadings. The proposed assessment methodology was validated according to small-scale fatigue test results of steel pipe models with spherical dents under cyclic internal pressure. Here, stress concentration factors induced by longitudinal and transverse plain dents on steel pipes under internal pressure are obtained from a previously developed finite element model. Several finite element analyses are carried out in a parametric study. Analytical expressions are developed to estimate stress concentration factors for these two different dent geometries as function of pipe and dent geometric parameters. With the inclusion of these expressions, the proposed assessment methodology is improved and is now able to deal with three different plain dent geometries: spherical, longitudinal and transverse dents.
Direct Design of Large Ice Class Ships With Emphasis on the Midbody Ice Belt
Authors: Daley Claude , Kendrick Andrew
Published: July 2009

The future development of oil and gas reserves in remote Polar Regions areas will require a new generation of highly ice-capable vessels. Many may need to be capable of operating at all times of the year. These ships will need to be able to travel faster in heavy ice than all but the largest icebreakers, which poses challenges for both hull and machinery design. The American Bureau of Shipping (ABS), BMT Fleet Technology Limited (BMT) and Hyundai Heavy Industries (HHI) are currently undertaking a joint project aimed at addressing these design challenges. Because of the unique and innovative aspects of large fast ships for Polar ice development, new methodologies for direct calculation of loads on all areas of the hull are needed. The project is also addressing the need for new techniques for the analysis of the outer hull, double hull and gas containment systems of these ships under design and accidental loads; areas in which ‘rule design’ can only provide a starting point. This paper focuses on the midbody ice loads that may results from both ice pressures and from glancing collisions in the midbody area. The paper highlights some of the challenges of direct design.
Numerical Simulation and Practical Sand Silting in Waterways of Langya Bay
Authors: Zhao Lin , Liang Bingchen , Shi Hongda , Liu Xiangzhu
Published: July 2009

Dongjiakou Harbor is located at the Langya Bay in the city of Qingdao, Shandong Province. It is a multi-functional harbor of heavy passing capacity under planning in China. The sediment distribution and dispersion in the waterways and harbor basin water areas is of great importance to the construction and operation of the harbor. This article is based on the measurement of waves and currents as well as sediment suspension characteristics on site, and using numerical methods to predict the dispersion and deposition rules in this area. A combined wave-current-sediment model of COHERENS-SED is created through the combination of hydrodynamic model COHERENS and wave model SWAN as well as a sedimentation model SED developed by the authors. Inside COHERENS-SED, SWAN is regarded as a subroutine and it gets time and space varying current velocity and surface elevation from COHERENS. COHERENS gets time and space varying wave relevant parameters calculated by SWAN. Wave-enhanced bottom stress, wave dependent surface drag coefficient and radiation stress are introduced to COHERENS. Then a fully coupled hydrodynamic–sediment model COHERENS-SED accounting for interaction between the waves and currents is obtained and adopted to simulate these hydrodynamic conditions and the sedimentation processes in Langya Bay area. The open boundary of waves and currents is obtained through nesting from running a wider model which includes the Bohai Sea and the North Yellow Sea with coarser solution and contains coastal regions of Shandong Peninsula which includes the whole area of Langya Bay. Generally, the values of time series of current velocities, current directions and water levels as well as sediment concentrations have good agreements with observation data. The study shows the currents in the waterways and harbor basin water areas are relatively weak due to the narrow water width at the port mouth and the current directions parallel to the wharf directions. Also, sediment dispersion scales and strength are predicted according to the computation. The study also estimates the average sediment deposition amount and seabed erosion in this area. Besides, significant wave height and wave period obtained by COHERENS-SWAN shows that simulation result with wave-current interaction is better agreed with the measurement than the case without current.
Influence of the Wind Fields on the Accuracy of Numerical Wave Modelling in Offshore Locations
Authors: Rusu Liliana , Bernardino Mariana , Guedes Soares C.
Published: July 2009

The influence of the wind field resolution in the accuracy of the wave predictions is studied using spectral numerical models in highly non-stationary situations. The main area of interest is the port of Sines, located in the Portuguese continental coastal environment south of Lisbon. An implementation of the MM5 atmospheric model was developed for the area of study, starting from a large area of the Atlantic Ocean and nesting successively with finer grid towards the point of interest, so as to provide wind fields with increasing level of detail. The SWAN model was forced with wind fields simulated by different models and with different resolutions. A wave rider type directional buoy located offshore the Sines port at approximately 100 meters water depth was used as check point. In a first approach, two different wind fields simulated by REMO and MM5 were considered for forcing the SWAN based transformation module. Both wind fields have a spatial resolution of 0.5 degrees and were obtained using the global NCEP reanalysis data as wind driver. Initial wave boundary conditions are provided by WAM simulations for the entire North Atlantic basin. Afterwards, three successive SWAN areas were implemented. The three spatial resolutions defined for these SWAN computational domains were 0.05, 0.02 and 0.005 degrees. Corresponding special resolutions for the wind fields produced by MM5 ranged from 0.15 to 0.015 degrees. Simulations were performed from the beginning of December 2000 till the end of February 2001, which is one of the most energetic periods registered close to this coast and the results in terms of significant wave height, mean period and wave direction were compared with the in situ measured data.
Use of Satellite SAR Systems to Support Ocean Wind and Wave Farming
Authors: Schulz-Stellenfleth Johannes , Lehner Susanne , Ko¨nig Thomas , Brusch Stephan
Published: July 2009

It is well known that satellite radar systems as flown on the ERS-2 or ENVISAT satellite are capable of providing high resolution information on the near surface ocean wind field and ocean waves independent of light or cloud conditions. In this presentation an overview is given of the retrieval of different geophysical parameters which are relevant for the optimal siting, the design and the operation of offshore wind farms and wave energy plants. Different examples of high resolution wind fields acquired over the wind parks “Horns Rev” and “Butendiek” in the North Sea are presented. Both image mode scenes of 100 km by 100 km size as well as wide swath images of 500 km by 500 km size are used for the investigation. Special emphasis is put on the use of the radar data for the analysis of small scale atmospheric phenomena like shadowing or turbulence as well as the estimation of ocean wave parameters like the significant wave height. High resolution maps of ocean wave parameters like significant wave height or wave period are presented. The presentation will also include the application of more sophisticated techniques like the use of Dual-Polarisation data available from ENVISAT. The final part of the presentation will deal with the next generation radar satellite TerraSAR-X launched in 2007. The spatial resolution of this instrument is in the order of 1 m, which is an order of magnitude better than the ENVISAT and ERS satellites used up to now. It has furthermore interferometric capabilities, which enable the derivation of ocean current information. The potential as well as the limitations of this new instrument with respect to the offshore windfarming and wave farming sector will be discussed.
Experimental and Numerical Studies of the Wave Energy Hyberbaric Device for Electricity Production
Authors: Estefen Segen F. , Esperanc¸a Paulo de T. T. , Ricarte Eliab , da Costa Paulo R. , Pinheiro Marcelo M. , Clemente Cristiano H. P. , Franco Davide , Melo Eloi , de Souza Jovani A.
Published: July 2009

Small scale experiments and numerical simulations have been performed to evaluate a new concept of converter to produce electricity from wave energy. The models, tested in scale 1:10, have been submitted to different regular waves, with height and period representatives of the Brazilian coast, to evaluate the power generation and efficiency. The tests were carried out in a wave basin able to simulate in scale the wave conditions. Software especially written for the simulation of the converter behavior under regular waves has been initially correlated with the experimental results and, later on, it will be employed in parametric studies to estimate the best parameters of the system, in particular the prescribed pressure to be set for the best performance in terms of electricity production.
Methodologies on Hull Girder Ultimate Strength Assessment of FPSOs
Authors: Wang Xiaozhi , Sun Haihong , Yao Tetsuya , Fujikubo Masahiko , Basu Roger
Published: July 2009

Floating production, storage and offloading systems (FPSOs) have been widely used for the development of offshore oil and gas fields because of their many attractive features. They are mostly ship-shaped, and either converted from existing tankers or purpose-built. The hull structural scantling design for tankers may be generally applicable to FPSOs. However, FPSOs have their own unique characteristics, including various operational requirements. In addition to that, the expectation of safety and economic aspects of FPSOs require an optimized structure to be designed. This calls for reliable structural assessment methodologies. One of the most important aspects of FPSO structural design and assessment is the hull girder ultimate strength. In this paper, different methodologies of hull girder ultimate strength assessment will be introduced and numerical calculations of hull girder ultimate strength will be presented based on six different FPSO designs. The results will then be analyzed in terms of their differences, and conclusions will be made based upon reliable methodologies for hull girder ultimate strength assessment of FPSOs.
Structural Analysis of a Blastwall
Authors: Ye Hao , Sun Liping , Deng Zhongchao
Published: July 2009

A study about the nonlinear property of blastwall is carried on by using FEM software ABAQUS/Explicit. The study considers different corrugated barrier sections, distinguished by the depth of the sections in accordance to the Technical Note 5 (TN5) issued by the Fire and Blast Information Group, and subjected to dynamic loadings. A comparison of this 3 different profile barrier sections is made. And then a comparison on the responses of the blastwalls using two different materials is made. The design overpressures and duration are selected so as to represent realistic values that results in either an elastic and plastic response of the blast barrier. A simple research on the sensitivity of peak displacements, plastic strains and dissipated energy to mesh density and loading are also investigated.
Ship Motions in Shallow Water As the Base for a Probabilistic Approach Policy
Authors: Vantorre Marc , Laforce Erik , Eloot Katrien , Richter Jan , Verwilligen Jeroen , Lataire Evert
Published: July 2009

A calculation tool has been developed for determining tidal windows for deep-drafted ships approaching and leaving the Belgian harbors according to probabilistic criteria. The calculations are based on a database containing response functions for the vertical motions in waves and squat data for a selection of representative ships. The database contains both results of model tests carried out in the Towing tank for maneuvers in shallow water – co-operation Flanders Hydraulics Research & Ghent University in Antwerp (Belgium), as well as calculated values. During the experiments, draft, trim, under keel clearance (7 to 20% of draft) and speed have been varied. The tests were performed in regular waves with lengths which are small compared to ship length, and in wave spectra that are typical for the Belgian coastal area. For given input data (ship characteristics, speed, tide, directional wave spectra, bottom, trajectory, current, departure time), the tool calculates the probability of bottom touch during the transit, so that a tidal window can be determined. Other restrictions, such as penetration into fluid mud layers and current, are taken into account as well.
Fatigue of Tubular Joints: Hot Spot Stress Method Revisited
Authors: Dong Pingsha , Hong Jeong K.
Published: July 2009

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.
A Wake Oscillator With Frequency Dependent Tuning Coefficients for the Modeling of VIV
Authors: Ogink R. H. M. , Metrikine A. V.
Published: July 2009

A wake oscillator for the modeling of vortex-induced vibration is presented in which the coupling term between the oscillator and the structural equation is written in the form of a convolution integral. In this way the potential to tune the model to measurements is extended greatly. The model is tuned to forced vibration measurements. Then, without changing the tuning parameters, the model is applied to simulate free vibration experiments. A good prediction of the maximum amplitude and the width of the lock-in range is found over a wide range of mass and damping ratios.
A Lattice Model to Simulate Ice-Structure Interaction
Authors: Dorival O. , Metrikine A. V. , Simone A.
Published: July 2009

The interaction between ice sheets and offshore structures is of key importance in the design of offshore platforms in the Arctic. Unfortunately, due to the complexity of ice material the use of small-scale experiments is problematic if one aims at drawing some conclusions about the forces exerted on large-scale structures such as oil rigs. As a preliminary work this paper proposes a discrete numerical model to investigate the interaction between an ice sheet and a rigid structure. The behavior of ice is modelled by a two dimensional lattice model, in which inhomogeneity and possible failure of ice are incorporated. According to the numerical results such a model seems able to capture the main trends involved in ice sheets cracking. This is an essential step towards predicting interaction forces between ice and structure.
Effect of Icing on Ship Maneuvering Characteristics
Authors: Falzarano Jeffrey M. , Lakhotia Chandan
Published: July 2009

In this paper the effect of icing forward on the maneuvering characteristics of a small offshore supply vessel hull form, which was used for acoustic surveys in the North Atlantic by the US Navy during the Cold War, is studied. Icing is well known to compromise the roll motion stability of vessels but its effect on maneuvering and specifically path stability is not as well known. Using available empirical formulas for maneuvering coefficients and steady turning ability the effect of icing on the path stability and steady turning ability of this vessel are approximately evaluated. The results show that icing does affect the maneuverability but more study is required to more precisely quantify this effect.
New Techniques for Integrity Management of Flexible Riser-End Fitting Connection
Authors: Marinho Mauro G. , Camerini Claudio S. , Morikawa Sergio R. , Pipa Daniel R. , Pires Gustavo P. , Santos Joa˜o M. , Moreira Agildo B.
Published: July 2009

As the operation water depth for flexible risers increases, the stress level in these structures also increases. This higher load, associated with stress concentration close to end fitting, can induce fatigue damage in tensile armors at riser top section. Progressive rupture of wires due to fatigue, which may be accelerated by corrosion, is an important indication of deterioration in riser-end fitting connection, and has become a typical failure mechanism for flexible riser top section. In order to mitigate the progression of these damages, apart from the inspection program and the surface monitoring procedures already implemented, techniques for the early detection of failure mechanisms at the riser-end fitting connection, specially at the tensile armors, are under development. This paper describes and evaluates these techniques, such as visual torsion monitoring, detection through acoustic emission of tensile armor wire rupture, electromagnetic tensile armor stress measurement and monitoring through optical fiber sensor extensometers, as well as reports the results obtained from field experience.
Two Tempests: A Case Study on Capturing Nearshore Hurricane Wave Conditions
Authors: McGehee David D. , Winer Harley S. , Herbers Thomas H. C.
Published: July 2009

Measurements of the shallow water hydrodynamic characteristics of major hurricanes are required to develop appropriate design and policy responses and to minimize impacts from future storms. Several agencies manage measurement programs that had operational systems within the nearshore zone of influence of major hurricanes in recent years. These gages represent widely different approaches to the challenge of capturing extreme event data and each exhibited both successes and shortcomings. In general, the closer the instrument site was to the storm path and the shallower the water, the more likely the system was to experience problems with the sensor, the platform, and/or data recovery. The various systems and their results are described. All methods were partially successful in that some of the systems operated during the hurricanes and useful data were eventually recovered, yet in each of the cases examined the gages either failed or exhibited data quality issues during the peak conditions. Nevertheless, they are the only available measured time series of wave conditions in shallow water close to a major, landfalling hurricane. Lessons learned for making these types of data less rare in the future are discussed. Options for improving utility of extreme measurements to the practicing engineer are presented.
Statistical Modelling of Current Profiles for a Riser Reliability Design
Authors: Mendoume Minko Ignace D. , Prevosto Marc , Damy Gilbert
Published: July 2009

In the design of risers, current profiles modelling is of great importance. In this paper, various aspects to derive a reliable statistical modelling of currents profiles are considered. First, the reduction of current profiles is treated by mean of the EOF (Empirical and Orthogonal Function) method. In contrast to other studies that have utilized this technique, the information of direction is considered. Secondly, we propose a technique based on B-splines smoothing to identify time series of the different time scales. This can be of utility for a better understanding of some specific events or to improve numerical models. Finally, the efficiency and accuracy of the proposed approach is studied by comparing measured current profiles with those reconstructed from EOF method, by mean of extreme value analysis, using two simple criteria. The first one is based on static calculation and the second is derived from a VIV analysis.
The Use of Inspection and Monitoring Reliability Information in Criticality and Defect Assessments of Ship and Offshore Structures
Authors: Brennan Feargal , de Leeuw Bart
Published: July 2009

This paper describes the use of inspection reliability information in fitness-for-service and criticality assessments for ship and offshore structures. Assessments of components that have never been inspected should assume a defect distribution from manufacturing quality assurance reports taking into account any propagation of damage that might have occurred. By understanding how to incorporate Probability of Detection (POD) and Probability of Sizing (POS) information with associated confidence measures into damage modelling, operators can appreciate the benefit of conducting inspections and the resulting implications for quantitative risk assessments particularly where no defects are found. The paper illustrates the use of POD and confidence levels for predicting remaining life due to corrosion and fatigue and also how to incorporate sizing statistical performance characteristics of the inspection system into remaining life assessments. In addition, the paper addresses the emerging trend towards monitoring with inspection and how operators and designers can benefit from future trends in structural health monitoring.
Distribution of Maxima of Non-Linear Rolling in Case of Coupled Sway and Roll Motions of a Floating Body in Irregular Waves
Authors: Mendoume Minko Ignace D. , Prevosto Marc , Le Boulluec Marc
Published: July 2009

The so-called Linearize & Match (L&M), which gives a good approximation of the exact distribution of maxima roll angle of non-linear systems, was studied some years ago by Armand and Duthoit (1990) and by Prevosto (2001). The developments within this method were made in the case of single degree of freedom dynamic systems. Moreover, the terms (mass, damping, stiffness) of the non-linear transfer function did not depend on the circular frequency. In this paper, first, the L&M method is improved by adding a last step in the procedure which correct the Gaussian closure technique of the method, secondly is generalized to a coupled sway and roll dynamic system in which the hydrodynamic coefficients are frequency-dependent. The system is modelled by a set of ordinary differential equations in which the non linearity is only in the roll motion. In order to validate the results obtained in this case by the L&M method, a Monte Carlo method with long simulations of the response of the system was carried out. Hence, some aspects of the time domain simulation, based on Cummins equations, are also discussed.
3D Numerical Modelling of the Wave-Induced Response Around the Circular Caisson Founded on the Seabed
Authors: Chan Andrew H. C. , Ou Jian-Hua
Published: July 2009

Wave-induced liquefaction is one of the main factors influence the stability of marine structures. However, the investigation on this phenomenon is complicated as the dynamic interaction between soil, pore fluid and the structure is closely coupled. In order to obtain a better understanding of the wave-induced response around the circular caisson founded in the seabed, three dimensional numerical analyses have been performed using the 3D finite element program DYNE3WAC in order to investigate the wave-induced response around the circular caisson.
Higher Order Modal Response of Riser Fairings
Authors: Braaten Henning , Lie Halvor , Skaugset Kjetil
Published: July 2009

Riser fairings are designed to rotate freely about the riser axis and to passively align with the direction of incident flow so they will effectively streamline the flow and eliminate VIV (Vortex-Induced Vibrations). This rotational degree of freedom introduces the possibility of a complex dynamic phenomenon involving coupling between the hydrodynamic forces and the fairing / riser motions (e.g. cross-flow translation and rotation). Slocum et al. reported a scaled model test of a long flexible riser model with a freely-rotating riser fairing conducted at MARINTEK at OTC-2004. At low flow speeds, the test showed the fairings to be effective. However, at higher towing velocities they became unstable resulting in high displacements at its first bending mode. This paper presents the work related to a study of effectiveness of fairings and is one of several VIV research activities NDP (Norwegian Deepwater Programme) has conducted at MARINTEK in 2002–2007. The present work is partly a follow up activity to the work reported by Slocum et al. The purpose of the present work was to study possible higher order modal response of faired risers and in particular to find out if such response can occur in higher bending modes than the first. A vertically towed instrumented riser was tested with 2 different fairing designs in uniform current profile with different towing speeds. Fairing II was identical to the one used in Slocum et al (2004) while Fairing I represents an alternative design. The riser model was 9.32m long, had diameter of 20mm and was flexible. Both bare riser configuration and full coverage of the two fairing sets were tested. This study documented first-, second- and third-mode responses at high amplitudes (instability behavior) for Fairing II. Tests with Fairing I showed that the riser was stable, but the riser vibrations were found to be similar with respect to displacement amplitudes and frequencies to the bare riser VIV.
The Response of a Flexible Sheet Under Axial Tension Immersed in Parallel Flow
Authors: Morris-Thomas Michael T. , Steen Sverre
Published: July 2009

This paper explores the fluid-elastic response of a cantilevered flexible sheet in the presence of uniform airflow. The leading edge of the sheet is clamped, while at the trailing edge, tension is applied to provide additional rigidity to the sheets small but finite bending stiffness. We outline a series of experiments performed in a wind tunnel with the purpose of examining fluid-elastic instabilities. In particular, we examine the role of tension induced rigidity and how it influences static divergence and convected wave instabilities. The flow is characterised by Reynolds numbers of order 105–106 and we specifically examine a sheet with an aspect ratio of L/l = 1.33. A unique aspect of this present work, is the direct measurement of sheet displacements through an optical tracking method with a grid of passive markers placed on the sheet surface. We show the evolution of the sheet surface from stability, through to divergence, and then finally into flutter. The frequency composition of the flutter event shows higher harmonic components that suggest significant nonlinearities. Tension induced rigidity plays a crucial role in the response of the sheet to the fluid in both postponing and suppressing instabilities.
Finite Element Modelling of Impact Loads on Offshore Containers Conforming to DNV 2.7-1 Regulations
Authors: Nejad Amir Rasekhi , White Paul
Published: July 2009

Impact load analysis is an essential part of structural calculations for offshore containers resulting from shocks during lifting, shipping, loading & offloading. The impact loads are classified as non harmonic dynamic loads acting in short time. Calculating the dynamic nature of impact loads requires complicated calculation; therefore, a practical quasi-static approach can be very useful in the industry. DNV based on many years of experience in marine and offshore industry, has introduced certain procedures to calculate, check and test the offshore container’s structures against the impact loads under the DNV certification standard no. 2.7-1, offshore container, issued on Apr. 2006 [1]. In this paper we will determine the maximum vertical impact load as a function of the mass for free falling motion and compare with the DNV design load factor. We will detail a method to model the impact loads in FEA programs as per DNV recommendation and we will compare the results with the conservative method of individual beam calculations. The results will determine the accuracy and speed of this improved FEA calculation method.
Physical Model Test of Water Intake Including Huge Pump Station and Drum Screen
Authors: Barzegar Sadegh , Banae Mohammad , Hamed Mohammad Ali , Qaheri Badr Mohammad
Published: July 2009

A physical model, built at an undistorted scale of 1:15 tested the original design of the six drum screen and nineteen cooling water pump intake connected to header bay. The capacity of origin water intake including huge pump station and drum screen is 400,000 m3/hr. The study objectives were to evaluate as-designed screen bay and pump bay performance and to propose design modifications to optimize intake flow conditions with respect to head-losses, uniformity of the approach flow, evenness of pump throat velocity distribution, and free and subsurface vortex formation. The model was built and operated in accordance with froude-number similitude. It allowed accurate representation of complex flow patterns caused by the physical geometry of the approach bay and pump bays. The major factors that can affect the selection of a concept and design development for a water intake are: a) The occurrence of dead water zones, flow separation or reverse flow b) Vortex building and air entrainment in the pump compartments c) Submerged vortices building in the pump compartments d) Low velocity area e) Strong rotational flow f) Strong cross flow appear in front of pump units g) Pre rotation in the pump suction lines. Dye injection was used to examine the stratified flow behavior along water. The existing design of the pump bays was found to produce a uniform, symmetrical flow distribution in the approach flow, weak but persistent floor and side-wall-attached submerged vortices, avoiding cross flow and reverse flow in front of the pumps and negligible swirling motion in the pump suction. Modified design includes (i) profiling low velocity area (ii) adding flow deflectors along inner walls (iii) infill area of low velocity (iv) adding suspended baffle in front of drum screens (v) adding diffuser block in front of pumps (vi) provision of floating booms in front of pumps.
How Should We Combine Long and Short Term Wave Height Distributions?
Authors: Forristall George Z.
Published: July 2009

Estimating the maximum wave or crest height that will occur in a long return interval is one of the fundamental problems for ocean engineers. Long time series of individual wave heights are not available. The calculations must start with measured or hindcast time series of significant wave heights. An extreme value distribution is fit to that data. The resulting long term distribution is then combined with a short term distribution for the individual heights. This study is concerned with finding the most accurate methods for that calculation. The basic tool is the Borgman integral, but it has been applied in many different ways. Theoretical derivations do not clearly indicate which method is most accurate, and time series of measurements long enough for accurate tests do not exist. These problems were circumvented in this study by constructing very long simulated time series with known distributions. Both initial value and storm based methods were tested. The correct method of calculation depends on what question is being asked. The distribution of the maximum wave heights in a six hour interval is different than the distribution of the maxima of all of the waves. The distribution of the maxima in a storm is different than the distribution of the maxima in an interval. We believe that the finding the maximum in a storm is the most appropriate question for ocean engineering design. The Tromans and Vanderschuren (1995, Proc. Offshore Tech. Conf., OTC 7683) method accurately matches the results from our storm simulations.
Analysis of the Behavior of Moored Tankers
Authors: Taveira Pinto F. , Veloso Gomes F. , Rosa Santos P. , Guedes Soares C. , Fonseca N. , Santos J. Alfredo , Moreira A. Paulo , Costa P. , Bro´gueira Dias E.
Published: July 2009

Moored ship behavior inside harbors and, therefore, the operational and security conditions at a port terminal does not have a straightforward relationship with local environmental conditions. Due to the diversity and complexity of the phenomena involved it is important to use a methodology that combines physical model tests with numerical simulations, taking advantage of potential synergies. Results of prototype measurements are also a key element to making the validation and calibration of both physical and numerical models possible. This paper focuses on studying the behavior of moored tankers using combined methodology. Aspects related with the inclusion in the numerical models of shallow water effects, non-linear characteristics of mooring lines and fenders, the influence of harbor boundaries and viscous damping are analyzed and discussed. The role of physical modeling as a tool to address/quantify some of the conditions and to provide data for the calibration of numerical models is presented, as well as the methodology defined for the study of the operational conditions at an existing berth. This methodology includes the development of a computer vision system to measure ship motion at the port terminal (prototype). Actual operational conditions at the berth are also described in the paper.