bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Response of Articulated Tower Platforms to Random Sea Environment
Authors: Zaheer Mohd Moonis , Islam Nazrul
Published: February 2010

Articulated tower motions have been characterized by rigid body mode of vibrations falling in the wind excitation frequency range due to its compliant nature. Dynamic response analysis of a multi hinged articulated tower platform to random wind and wave forces are presented in this paper. The wave forces on the submerged elements of the tower are calculated by using Morison’s Equation. The fluctuating wind is modeled with Ochi and Shin wind gust spectrum. The effect of wave age (young, intermediate and fully developed waves) on the wind gust spectrum is incorporated by adopting the wave age dependent Volkov, and wave age independent Charnock sea surface roughness models. The response of the tower is determined by a time domain iterative method. An example of results demonstrates the clear effect of wave age on the nonlinear dynamic response on the system. The mean wind modifies the mean position of the surge response to the positive side, causing an offset. Moreover, for high mean wind speeds the total wind response is much smaller than the wave response, but for low wind speeds the wind appears to be more important.
Application of Bayesian Networks in the Analysis of Human Contribution in Collision Accidents
Authors: Martins Marcelo R. , Maturana Marcos C.
Published: February 2010

Analysis, assessment, and risk management are vital in the oil and gas sector due to the potential severity of accidents with respect to human life, to the environment, and patrimony. As most of the accidents in this area are driven by human factors, the purpose of this work is to present an efficient methodology and technique for the analysis of these factors in this industry. The applied quantitative technique is based on Bayesian Networks. Finally, this article presents a study of an oil tanker operation, focusing on the quantification of the human factor contribution in collision scenarios.
Optimisation of FRP Structures for Marine Vessel Design and Production
Authors: Sobey Adam J. , Blake James I. R. , Shenoi Ajit R.
Published: February 2010

Structural integrity is an important factor in design and production of any vessel. The design and production of the structure will permeate throughout the other subsystems of a vessel. These interactions require that the structure must be light, hydrodynamic and easy to build allowing an efficient and low cost design to be constructed. During the design process engineers use iterative design methods to improve optimum designs. The ability to have effective reliability data will allow this iteration to be a more quantitative process. The addition of these methods to optimisation techniques will allow a powerful tool that optimises design while producing data for other designers to determine possible changes that could take place. This paper looks at using reliability methods to increase the power of design for production within concurrent engineering and structural optimisation.
Motions and Mooring Loads of an LNG-Carrier Moored at a Jetty in a Complex Bathymetry
Authors: Weiler Otto , Cozijn Hans , Wijdeven Bas , Le-Guennec Stephane , Fontaliran Franc¸ois
Published: February 2010

The Yemen LNG Company Ltd. is working on the design and construction of an LNG plant in the Republic of Yemen. The LNG plant, located at Balhaf on the Gulf of Aden, includes a jetty approximately 750 m from the shore to allow loading of LNG carriers. The bathymetry around the jetty is very complex and includes a large variation in water depth along the berth. Furthermore a cape near the jetty affects the incoming wave conditions. Deltares (formerly WL | Delft Hydraulics), together with MARIN, carried out a study of combined hydrodynamic scale model tests and computer simulations. The aim of the project was to determine the limiting environmental conditions for safe mooring of the LNG carriers at the jetty. The hydrodynamic scale model tests at Deltares focused on an accurate modeling of the wave conditions at the jetty and the motion response of the moored ship. To achieve this, the bathymetry around the jetty was modeled in detail, including the cape partially shielding the jetty from incoming waves from the open sea. Wind was applied to the moored ship as constant forces. The time-domain computer simulations were carried out by MARIN, using their TERMSIM simulation model. After calibration against scale model test results, the numerical model was used to quantify the effect of gusting wind for all environmental conditions and all ships as tested in the basin. The results of the scale model tests, corrected for the effect of gusting wind, indicated that the vessel can stay safely moored at the jetty in quite severe conditions. This leads to a high jetty availability, which is a favorable outcome of the project. After describing, in a general way, the methodology and results of the project, this paper focuses on the comparison of the results of the model tests with those of the computer simulations. This comparison showed that the low frequency effects, both excitation and response, in the complex bathymetry that was considered here, are very complex and beyond the present numerical modeling capabilities. Therefore, in the short term, physical model testing will remain necessary for an accurate prediction of the moored ship’s response in such situations. For the longer term the development of additional analysis and simulation methods is required.
A Phenomenological Model for Vortex-Induced Motions of the Monocolumn Platform and Comparison With Experiments
Authors: Rosetti Guilherme F. , Gonc¸alves Rodolfo T. , Fujarra Andre´ L. C. , Nishimoto Kazuo , Ferreira Marcos D.
Published: February 2010

Vortex-Induced Motions (VIM) of floating structures is a very relevant subject for the design of mooring and riser systems. In the design phase, Spar VIM behavior as well as Semi Submersible and Tension Leg Platform (TLP) flow-induced motions are studied and evaluated. This paper discusses flow-induced behavior on the Monocolumn concept by presenting a phenomenological model and comparing its results with a set of experiments that took place in the IPT Towing Tank - Brazil (September 2008). The experimental results have shown some fundamental differences from previous VIM tests on other units such as Spars. This numerical model attempts to identify these disparities in order to better understand the mechanics of this phenomenon. The model is based on a time-domain, two degree-of-freedom structural model coupled with a van der Pol type wake oscillator. The comparison was performed in order to calibrate the model, to study and better understand the tests results, and finally to identify important aspects to investigate in further experiments.
Reconstruction and Reinforcement of Pile Driving Barges
Authors: Wang Guozhi , Liu Zhouyi
Published: February 2010

With the increasing demand of petroleum exploitation as well as the construction of bridges and ports, the size of pile driving barges becomes much greater with longer piles. Considering of the economy, many old pile driving barges face to the reconstruction through the strengthening and heightening. In determining the reconstruction scheme, the traditional method for loads estimation is the combination of static load with dynamic coefficient, but it is perhaps not reasonable. The reconstruction design of a pile driving barge is described in this paper. Various of loads were considered including the normal working load, the wind load, the wave load as well as the added dynamic loads especially the transient load during hoisting and pile sliding. The loads transfer between the main hook and the auxiliary hook were simulated experimentally. The FE method was adopted to obtain the stress contribution and find the position with maximum stress. A reconstruction scheme was then put forward to strengthen the weak position of the frame. The upper platform and the main stanchion need strengthened by increasing the outer diameter and the wall’s thickness locally. The connection structure of the upper platform and the main stanchion should be strengthened through the structural optimization. The stress distribution and the maximum stress were then calculated and the safety was verified according to the allowable stress value of the materials. The transient movement of the pile was investigated. The pile driving barge was then reconstructed based on the reasonable reconstruction scheme from the load simulation and structural optimization.
Hierarchical Modeling of Pipeline Defect Growth Subject to ILI Uncertainty
Authors: Maes Marc A. , Faber Michael H. , Dann Markus R.
Published: February 2010

Pipeline deterioration arises chiefly as the result of various types of internal and external corrosion processes, which are typically subject to several uncertainties. They include material uncertainties, uncertainties in external influences such as loading and environmental variations, uncertainties in operating conditions, various spatial and temporal uncertainties, inspection uncertainties, and modeling uncertainties. Typically, the metal loss time-path at one defect feature may be quite different from the metal loss time-path in a neighboring location even when subject to supposedly similar loading, material and environmental circumstances. On top of that, in-line inspections (ILI) of pipeline systems affected by deterioration are performed infrequently and suffer from considerable uncertainty due to sizing errors and detectability. The present paper provides a Hierarchical Bayes framework for corrosion defect growth. While a full Hierarchical Bayes analysis is practical only for selected critical defect features, we also develop a simplified method based on multi-level generalized least squares. The latter method is useful for scanning large defect inspection data sets. Two detailed examples of the approach are presented.
TLP Model Lines Truncation Using the Magnetic Base Concept
Authors: Fernandes Antonio Carlos , Salhua Ce´sar Augusto , Almeida Jose´ Carlos
Published: February 2010

Ultra deep water scenarios are still showing up offshore Brazil. New recent oil discoveries in the Tupi and Jupiter oil fields are over more than 3000m water depth. These new scenarios require new technology for their development. A useful methodology is though the use experimental test in ocean basins. However, the depth of all world ocean basins is not enough to accommodate recommended scales for floating bodies and moorings and risers for such water depth. Hence, line truncation is necessary. The present paper proposes a new idea to surpass this limitation. The work suggests the use of the so called magnetic bases to avoid some problems that occur with a blunt truncation of lines. These magnetic base devices are projected to obtain with the truncated lines, the same full scale properties such as restoring forces and also the same viscous dissipation. This paper describes the procedure followed to study a magnetic base application. Due to availability of a ITTC semisubmersible, the first step was to make this ITTC model to work as TLP. The next task was to design the truncation of its lines using the concept of magnetic base. Subsequently, the TLP project with full and truncated lines mooring system in what concern theirs natural frequency of response, the decay behaviour and the response in irregular waves were addressed numerically using a non-linear time domain program.
Viscous Drift Forces and Responses on a Semisubmersible Platform in High Waves
Authors: Berthelsen Petter Andreas , Baarholm Rolf , Pa´kozdi Csaba , Stansberg Carl Trygve , Hassan Amany , Downie Martin , Incecik Atilla
Published: February 2010

In this paper the significance of viscous effects on a moored semisubmersible platform in various sea state conditions is explored. Experimental data from a 1:50 model scale tests in a 50m × 80m wave basin are compared with numerical time domain simulations. Both regular and irregular waves are included, and some tests are also run with current. This paper presents results from a horizontally moored semisubmersible, where we focus on the hull hydrodynamics alone. The emphasis here is low frequency surge responses. Use of conventional potential theory shows large discrepancies when compared with experimental results in high sea states. For surge motion, they are believed to be due to viscous forces in the wave zone. Viscous forces and damping in the numerical model are included by the drag term of Morison’s equation using a total relative velocity approach, which is integrated up to the instantaneous free surface elevation. A common challenge is to choose a suitable Cd coefficient which provides for sufficient excitation force without introducing excessive motion damping. It is found that using a larger Cd coefficient in the wave zone gives larger excitation without influencing the total damping significantly. This way, applying the drag term of Morison’s equation can give results that compare well with measurements. Also, the coefficient is found to be lower in waves with current than in waves only.
Shallow Water STL Mooring and Riser System
Authors: Yang Qinzheng , Chezhian Muthu , Hovde Geir Olav
Published: February 2010

A shallow water disconnectable STL turret mooring and riser system has been developed for water depth between 30 and 50 m. This technology is based on APL’s disconnectable STL (Submerged Turret Loading) and STP (Submerged Turret Production) technologies which had been widely applied for water depth between 70 m to 2600 m for FPSOs and LNG offshore terminals. The advantage of disconnectable system is that the mooring and riser system can be designed to a preferred sea state. When the sea state is higher than design sea state (like hurricane), the vessel can be disconnected and sail away. The shallow water STL system consists of STL buoy, mooring lines, riser system and landing pad. The interface with vessel is the same as traditional STL system. The mooring and riser system are connected to the vessel through STL buoy and can be pulled into vessel by using ship winch. Unlike traditional STP and STL buoys, the shallow STL buoy has a net weight and will stay on the landing pad when disconnected from vessel. The landing pad is designed to support the impact load from STL buoy and supply enough friction for the STL buoy to stay in position during 100-year storm. The mooring system design has taken the advantage of directionality of weather when close to the shore by using different mooring line length in different directions. Further an innovative Hold-Back-Wave riser configuration has been developed for shallow water system. The riser configuration has a larger flexibility compared to traditional wave configuration and has proved to be feasible for significant wave height at least 7 m when connected to the vessel and 10+ m when disconnected from the vessel. Model test for the disconnectable shallow water turret mooring and riser system had been performed in MARINTEK, Trondheim with a LNG re-gasification vessel model at 30 m water depth. For connected system, significant wave height Hs = 6 m and 8 m has been tested. The mooring and riser system perform well, as predicted. For disconnected system (when the buoy sitting on the landing pad), significant wave height Hs = 10 m has been tested. The STL buoy is sitting on the landing pad without significant movement and the riser system performs well. SIMO program has been used to calibrate the model test results with numerical simulations. By adjusting surge, sway, yaw damping and 2nd order wave drift force, the calibrated SIMO model agrees well with model test results and can be used for similar development.
Wave Kinematics and Seakeeping Calculation With Varying Bathymetry
Authors: de Hauteclocque Guillaume , Rezende Fla´via , Giorgiutti Yann , Chen Xiao-Bo
Published: February 2010

Diffraction/Radiation theory is used to calculate the wave kinematics and the motions of a floating body in area of varying bathymetry. The bathymetry is modeled as a second body, which, without special measures, leads to spurious reflection at the edge of the mesh. A modified formulation of the Boundary Element Method is introduced to model partially transparent panels. Those panels, when properly used to smoothly extend the actual (opaque) bathymetry, allow much more accurate computation. The efficiency of the method is tested with regards of several parameters concerning the bathymetry size and the way to smooth the truncation. Numerical results are satisfactorily compared with a 3D shallow water code based on Green-Naghdi theory. The sensitivity to the slope on the ship response is then investigated (motion, added mass, radiation damping and second order loads). The differences with the constant depth calculations are significant, due to the modified incident wave field, but also due to modified added mass and radiation damping terms. The method presented here could be useful in the context of LNG terminals where the depth is quite shallow and the bathymetric variations significant.
Efficient Computations of Second-Order Low-Frequency Wave Load
Authors: Chen Xiao-Bo , Rezende Fla´via
Published: February 2010

As the main source of resonant excitations to most offshore moored systems like floating LNG terminals, the low-frequency wave loading is the critical input to motion simulations which are important for the design. Further to the analysis presented by Chen & Duan (2007) and Chen & Rezende (2008) on the quadratic transfer function (QTF) of low-frequency wave loading, the new formulation of QTF is developed by the series expansion of the second-order wave loading with respect to the difference-frequency upto the order-2. It provides a novel method to evaluate the low-frequency second-order wave loads in a more accurate than usual order-0 approximation (often called Newman approximation) and more efficient way comparing to the computation of complete QTF. New developments including numerical results of different components of QTF are presented here. Furthermore, the time-series reconstruction of excitation loads in the motion simulation of mooring systems is analyzed and a new efficient and accurate scheme is demonstrated.
Wave Slamming on External Turrets of FPSOs
Authors: Buchner Bas , Voogt Arjan
Published: February 2010

The paper discussed the possible slamming loads on the underside of the (chaintable of) external turret moored F(P)SOs. For this problem of turret slamming, a combined exceedance of the airgap between the underside of the chaintable and the relative wave is needed, as well as a certain relative wave velocity. A model test series was carried out to determine these loads on a typical external turret. The model test results provide insight into the complex loading process of external turret type structures. A clear quadratic relation between submergence velocity and impact load, as typically observed for slamming problems, has not been identified. This is probably a result of local effects, such as the strong disturbance of the free surface (giving a locally different deadrise angle) and air entrapment effects. The maximum pressure on the conical turret (chain table) is significantly reduced compared to the flat turret, which was expected based on the reduced rate of change of momentum with the conical turret. Based on linear 3D diffraction analysis and the empirical relations derived from the model tests, a method is presented to determine first design estimates of the pressures.
DP Pipe-Laying and Crane Barge: Procedure for Defining Operational Window and Capability Plots Using Dynamic Simulations
Authors: Tannuri Eduardo A. , Silva Joa˜o Luis B. , Oshiro Anderson T. , Saad Arthur C.
Published: February 2010

Special station-keeping requirements must be defined for a safe operation of a DP pipe-laying and crane barge. When a pipe is being laid in shallow waters, small displacements of the launching ramp may induce large forces on the pipe or even to deviate it from the defined route. Offshore crane operations are performed in close proximity with other vessel or platform, and large loads are transported in a pendulum configuration. Again, a precise positioning of the barge is required, in order to avoid unsafe relative motions, as well as keep the load being transported on a stable position. Due to these special DP requirements, it has been shown in the present work that a simple static analysis of the DP System is not adequate in this case. Dynamic effects related to wind gusts, slow drift forces, propellers response, DP filtering and time-delay must be considered since the initial stages of DP specification. The common approach of considering an amount of 20% extra power to compensate for dynamic effects may underestimate the necessary power. A fully non-linear dynamic simulator was then used to carry out a complete analysis of the barge. Thrust utilization capability plots were obtained for the DP design environmental condition, considering the occurrence of a single failure. After that, an analysis of the environmental conditions in the Brazilian waters was carried out, and a comprehensive set of more than 1700 conditions were obtained. Dynamic simulations were then used to define the operational window of the barge as well as the estimated downtime for each operation. The barge is also able to operate in a DP-mooring assisted mode. The simulations were used to define under which operational and environmental conditions that such mode must be used.
Beyond Waves and Spectra: Euler Characteristics of Oceanic Sea States
Authors: Fedele Francesco , Sampath Prasanna , Gallego G. , Yezzi A. , Benetazzo A. , Tayfun M. A. , Forristall George Z. , Cavaleri L. , Sclavo M. , Bastianini M.
Published: February 2010

We present an application of a novel variational Wave Acquisition Stereo System (WASS) for the estimation of the space-time dynamics of oceanic sea states. WASS technology, if combined with statistical techniques of the Euler Characteristics of random excursion sets, provides a new paradigm for the accurate prediction of the largest crest expected over an area.
Wave-Deck-Load Analysis for a Jack-Up Platform
Authors: Schellin Thomas E. , Peric´ Milovan , el Moctar Ould
Published: February 2010

The paper analyzes freak wave-in-deck load effects on a typical three-leg jack-up platform stationed in the North Sea. Considered were cases where the air gap is small and the hull is subject to impact-related wave-in-deck loads. Wave load predictions were based on the use of a validated CFD code that solves the Reynolds-averaged Navier-Stokes equations. The code relies on the interface-capturing technique of the volume-of-fluid type to account for highly nonlinear wave effects. It computes the two-phase flow of water and air to describe the physics associated with complex free-surface shapes with breaking waves and air trapping, hydrodynamic phenomena that had to be considered to yield reliable predictions. Stokes 5th order wave theory was used to initialize volume fraction of water and velocity distribution in the solution domain, as well as to prescribe time-dependent boundary conditions at inlet and outlet boundaries. It was demonstrated that CFD methods can be used to predict the load phenomena under extreme wave conditions and thus serve as a valuable tool for both initial design and subsequent assessment of safety aspects. In particular, we demonstrated that effects of different operating and design parameters on wave-in-deck loads, such as wave direction, wave height, wave period, and wind speed, can be evaluated with an affordable computing effort using personal computers.
Downtime Analysis for Offloading Operation: DP X Non-DP Shuttle Tanker
Authors: Cueva Marcos , Matos Vinicius L. F. , Correa Sylvio H. , Tannuri Eduardo A. , Mastraˆngelo Carlos
Published: February 2010

The Brazilian oil company PETROBRAS will install the first FPSO ever in the Gulf of Mexico (GoM). This unit will be installed in a water depth of 2.500m, and in addition to several challenges to develop such a project, one has received special attention: the offloading to be performed by a shuttle tanker in tandem with the FPSO. In a general offloading analysis, the motions of the shuttle tanker and the hawser tension are evaluated only in the maximum operational environmental conditions. This approach has limitations, the most important being the fact that the worst results do not always happen with the most severe environmental conditions and that it does not provide an indication of the operational downtime. In this work, an analysis is performed to evaluate the downtime of a shuttle tanker, with and without dynamic positioning (DP) assistance, under the scatter environmental data taken from the GoM METOCEAN technical specification. Due to the large amount of possible environmental combinations of wave, wind and current, a reduced selection of 60 conditions has been chosen based on statistical procedures. The offloading analysis is performed for a turret moored FPSO, connected with two types of shuttle tankers: a non-DP shuttle tanker (ST) or a DP shuttle tanker (DPST). The DPST uses 2 tunnel thrusters in the bow, 1 in the aft and the main propeller, with a total power of 12,500kW. The ST is assisted by one tug in tandem, which applies at least 10t of force at the ST stern. The calculations are performed with the in-house PETROBRAS software DYNASIM, a fully coupled time domain simulator. In the analysis the position of the moving shuttle tanker is monitored within the green zone, defined as ±45° from the FPSO bow-stern axis, and the mean and maximum hawser tensions, for all defined environmental conditions. The downtime for each loading condition is obtained by the summation of the occurrence probabilities of the environmental conditions under which the ST or DPST results did not stay inside the defined limits of position in the green zone and by the hawser tensions, i.e. the offloading cannot be performed. Machinery failure probability is not considered for the evaluation. As a result a downtime smaller than 3%, or 11 days per year, was obtained for the ST and smaller than 2.4%, or 9 days per year, for the DPST, with the hawser tension limit exceedence being the main cause.
Diffraction/Radiation of 135,000M3 Storage Capacity LNG Carrier in Shallow Water: A Benchmark Study
Authors: Naciri Mamoun , Sergent Emmanuel
Published: February 2010

The HAWAI (sHAllow WAter Initiative) JIP was launched in 2005. The objective was to improve the reliability of Offshore (LNG) Terminals by combining the expertise of offshore hydrodynamics and coastal engineering to better address key issues regarding motion and mooring prediction methods in shallow water. One of the key issues identified was the diffraction/radiation calculation as this is the main foundation of all motion and mooring analyses. Comparisons of second order wave drift load transfer functions predicted by leading diffraction/radiation software for a typical 135,000m3 storage capacity LNG Carrier (LNGC) had shown notable differences (see Ref [1]). A benchmark study was launched for a standard LNGC in 15m water depth. Seven leading commercial diffraction/radiation software were used for this comparison (AQWA, DELFRAC, DIFFRAC, DIODORE, HYDROSTAR, WADAM and WAMIT). Comparison was first done by specifying the hull form in CAD format and then by specifying the mesh. First and second order results are presented and conclusions are drawn regarding the robustness of these codes.
Ultimate Bearing Capacity of Hull Girder Considering the Effects of the Local Loads
Authors: Ren Huilong , Li Chenfeng , Feng Guoqing , Bai Baoqiang , Zhang Jian , Liu Xiaobo
Published: February 2010

In order to evaluate the ultimate moment behavior of hull girder reasonably, especially considering the influence of the local loads such as water pressure and cargo pressure, a method was given to calculate ultimate bearing capacity of hull girder using nonlinear finite element method. Some initial imperfections such as the welding residual stress and initial deflection which affect ultimate longitudinal strength were also considered. Nishihara (MST-3) was taken as example, calculation results indicates that the effects of local loads cannot be neglected, the ultimate bearing capacity of hull girder not only related to self-structure of ship hull, but also has something to do with local loads.
Fatigue Strength Assessment of Ship Structures Based on the Crack Propagation Theory
Authors: Niu Song , Feng Guoqing , Ren Huilong , Zhang Jian
Published: February 2010

Fatigue damage is one of the main failure modes of ship structures. At present, the fatigue assessment of ship structures is mainly based on S-N method, such as the simplified procedures by classification societies or the spectral fatigue analysis method, etc. Because the initial flaw or crack can not be taken into account in S-N method, the fatigue assessment of ship structures based on fracture mechanics or crack propagation theory is worth to be studied. In this paper, the crack propagation theory based fatigue assessment of ship structures is studied. First, the calculation of stress intensity factor based on finite element analysis and weight function method are given, then Paris model is used to predict the crack propagation life. Finally, a numerical example is given to show the procedure of fatigue assessment of ship structures.
Dry Disconnectable Riser System for Low Keel Clearance Floaters
Authors: Murray John J. , Mukundan Harish , Gupta Apurva , Choi Guibog
Published: February 2010

Fixed structures operating in extreme offshore environments (eg: arctic) have to resist and survive the challenging conditions like large ice forces. On the other hand, the floating systems in such environments benefit from their ability to be evacuated and transported in the event of severe condition. However, such an ice-management scheme requires the moorings and risers to be disconnected in the severe conditions and reconnected during more clement conditions. Among the floater designs typically considered for extreme offshore applications is the Spar. The Spar offers low extreme motion responses than other shallower draft floaters and as a result, low fatigue damage on the risers. However, this design has versions with open centerwells that can contain leaked product and cause a potential hazardous condition. Furthermore, most designs that require the risers to be disconnected below the mean water line (wet-disconnection mechanism) not only carry the risk of leakage but have poor access for visual inspection. One solution is to use a continuous flexible riser without in-line connections or terminations in the flexible riser between the seafloor pipe line end manifolds (PLEM) and the production deck manifolds (dry-disconnection mechanism). The risers disconnect at an elevation above the water line and the termination point is lowered to a disconnect buoy supported at the keel. Subsequently the floater is moved away. The main difficulty is that the lowered flexible riser has to be suspended from the disconnect buoy and at the same time avoid contact with the seafloor. This paper describes and discusses a design of a dry-disconnectable flexible riser system comprised of a buoy supporting arches to control the bending in the risers during operation and disconnect. The system is particularly effective when the clearance between the keel and the seafloor is restricted. The rationale is based on strength, control of the minimum allowable bend radius and interference among the risers.
Sea-State Monitoring Via X-Band Marine Radar Images Sequences: A New Approach for an Accurate Surface Currents Estimation
Authors: Serafino Francesco , Lugni Claudio , Soldovieri Francesco
Published: February 2010

This work deals with the sea state monitoring starting from marine radar images collected on a moving ship. For such a topic, one of the key factors affecting the reliability of the reconstruction procedure is the determination of the equivalent surface current that also accounts for the speed of the moving ship. Here, we propose a method able to evaluate also high values of the sea surface current. The reliability of the proposed procedure is shown by a numerical analysis with synthetic data. Finally, we present some preliminary results with measurements collected on a moving ship.
Numerical Simulations of the Rolling of a Ship in a Stochastic Sea: Evaluations by Use of MCS and FORM
Authors: Nielsen Ulrik D. , Jensen Jo̸rgen J.
Published: February 2010

The paper elaborates on the probabilistic assessment of a simplified model for the rolling of a ship in a stochastic seaway. The model can be easily integrated with a probabilistic tool which enables evaluations of numerical simulations by the first order reliability method (FORM) and by Monte Carlo simulation (MCS). Results are presented for synchronous roll as well as parametric roll, where e.g. mean outcrossing rates have been calculated. FORM offers an efficient approach for the computations, although the approach should be applied with care in cases of parametric roll. The paper also touches on issues such as ergodicity and transient versus stationary stages in the roll realisations.
A Comparison Between CFD, Potential Theory and Model Tests for Oscillating Aircushion Supported Structures
Authors: van Kessel J. L. F. , Fathi F.
Published: February 2010

This contribution presents a comparison between Computational Fluid Dynamics (CFD), potential theory and model tests for an oscillating aircushion supported structure. The linear method was developed at Delft University of Technology and uses a linear adiabatic law to describe the air pressure inside the cushion. In this method, the structure and the water surface within the aircushion are modelled by means of panel distributions representing oscillating sources. The CFD solver is the commercial software CFX which solves the whole flow field using Reynolds Averaged Navier Stokes Equations (RANSE). The free surface is modelled by a Volume of Fluid (VOF) approach. The results in this paper show a good agreement between experimental results and numerical results of both methods for aircushion pressure variations, added mass, damping and wave elevations inside the aircushion. As such it is validated that the behaviour of an aircushion supported structure subjected to forced heave oscillations can be well predicted by both CFD and potential theory.
Steepness of High Ocean Waves in Quasi-Determinism Theory
Authors: Anta˜o Ewa , Arena Felice , Guedes Soares C. , Romolo Alessandra
Published: February 2010

This paper investigates the steepness of large ocean waves. Different steepness and asymmetry parameters of the highest wave in a time series recorded in severe storms as well as nonlinearity of the records were investigated in Guedes Soares et al (2003, 2004). Here the steepness of high waves will be investigated by considering the ocean wave groups, both to the first and to the second order in a Stokes expansion, and some conclusions are obtained with wave data generated in an offshore basin. In particular, some parameters of wave steepness defined in Guedes Soares et al. (2004) are obtained with an analytical approach, by starting from the Boccotti’s linear Quasi-Determinism theory, including the second-order component derived by Arena (2005) for wave groups in undisturbed wave field. Furthermore, the wave field is analyzed in the space domain, to determine the actual steepness of the highest waves. These results, achieved starting from either the time or the space domain, will be analyzed for different wave spectra.
Ultimate Strength Analysis of Ship Hull Girder Under Random Material and Geometric Properties
Authors: Vhanmane Suhas , Bhattacharya Baidurya
Published: February 2010

The ultimate strength of a ship’s hull depends on its material and geometric properties, some or all of which may be random in nature. In addition, initial imperfections in the form of the initial deflection and residual welding stresses in the plating between stiffeners, can significantly affect the hull ultimate strength. In this paper, the effect of randomness in yield strength, member thickness and in the initial imperfections on ultimate hull girder strength is determined. Different levels of statistical dependence between the yield strength and initial imperfection of all stiffeners and plating between stiffeners have been considered. The methodology is applied on a bulk carrier and a VLCC tanker.
New Standard for Design of Free Fall Lifeboats
Authors: Ronold Knut O. , Nestega˚rd Arne , McGeorge Dag , Ertsgaard Aage , Skjæveland Rolf , Narten Frode
Published: February 2010

A new standard for site-specific design of free fall lifeboats has been developed, aiming at providing for sufficiently safe lifeboat designs. The objective has been to develop a standard that follows the same reliability-based safety philosophy and design principles as those implemented and used for design of conventional fixed and floating offshore structures. The standard is intended to cover all aspects involved in the design process for a free fall lifeboat, providing requirements that shall be met in design as well as guidance on how to meet these requirements. The following aspects are covered: Safety philosophy and design principles, metocean conditions, loads, materials, structural design, operational requirements, occupant safety and comfort, model testing and full scale testing, installation, equipment, and qualification of lifeboat concepts. The new standard is published both as an OLF Guideline by the Norwegian Oil Industry Association OLF and as a DNV Offshore Standard, DNV-OS-E406, by Det Norske Veritas. The paper presents the highlights of the new standard with emphasis on topics which are critical for design of free fall lifeboats, first of all structural safety, human safety and comfort, and headway.
Strength Assessment of Membrane LNG Tank Structure Based on Direct Calculation of Structural Response
Authors: Mravak Zoran , de Lauzon Je´roˆme , Chung Yun-Suk , Diebold Louis , Baudin Eric
Published: February 2010

Expanding LNG market reinforces the demand for new concepts of LNG transportation. Membrane LNG vessel design widely applied until now, encounters new challenges due to requirement for larger vessel’s capacities and more flexible operation in partially filled conditions. Present assessment procedures of LNG tank structure usually combine small scale sloshing loads measurement and containment system structural strength assessment, on a comparative base for the reference and target vessels. For the new LNG design, more rational methods become essential in the assessment procedure. Some improvements in the strength assessment procedure of membrane LNG tank structure is presented in this paper, combining small scale sloshing load measurements with direct FEM calculation of structural response. The complexity of problem is the consequence of: stochastic nature of impulsive sloshing loads, material used for the cargo containment system at cryogenic temperature and strong hydro-elastic interaction during impacts. Disadvantages of small scale testing and limits of today’s numerical methods require that further in the future certain simplifications and assumptions should remain. In the paper, method for the design loads selection from the small scale sloshing measurements is described and discussed. The impulse, transferred over the corresponding impacted surface, is the base for the comparison of successive violent sloshing loads. The stochastic nature and statistics of measured loads are discussed. The structural analysis of LNG tank structure under selected design sloshing loads, using on-linear and time-dependant explicit FE calculations, is described. This paper presents Bureau Veritas recent developments and their applications in the field of sloshing assessment.
Return Period of a Sea Storm With at Least Two Waves Higher Than a Fixed Threshold
Authors: Arena Felice , Barbaro Giuseppe , Romolo Alessandra
Published: February 2010

The analytical solution for the return period R≥2′ (H) of a sea storm having at least two individual waves higher than a fixed level H, is obtained. This solution is based on the application of the Boccotti’s Equivalent Triangular Storm model for the representation of actual storms. One of the corollaries of the solution gives the exact expression for the probability that at least two waves higher than level H are produced during the lifetime of a structure. The previous solution of R≥2′ (H) and the relative probability of exceedance can be effectively applied for the risk analysis of ocean structures. The results obtained show an excellent agreement with those achieved by considering the different approach, proposed by Arena and Pavone (2009), for the calculation of the return period R≥2 (H).
Behaviour of an Innovative Universal Structural Connection Under Monotonic and Cyclic Shear Loading
Authors: Khonsari S. V. , England G. L. , Mohammadi A. R.
Published: February 2010

A new structural connection with special unique features was developed. While under bending it showed very high rotational capacity, in shear, unlike other existing connections, it also demonstrated a large shear deformation capacity. The ductile response of this connection stems from its innovative geometry as well as the ductility of the elements embedded in it. Since the previous shear tests on the specimens of this connection were carried out under ‘unrestricted’ conditions, the tests reported here were all under ‘restricted’ conditions. These shear tests consisted of ‘monotonic’ as well as ‘cyclic’ tests on mild steel specimens. Due to the restrictions imposed on the specimens during the test, the stiffness increased, compared with that of their unrestricted counterparts. Also, the transition from shear phase to tensile phase of deformation took place at an earlier stage compared with its non-restricted counterpart. The monotonic tests proved the high shear deformation capacity of the connection which exists alongside its high stiffness as well as strength. The cyclic tests, however, showed the large number of hysteresis cycles the specimens could endure before failure. Both types of tests, monotonic and cyclic, proved the ability of the connection to dissipate energy in shear under either loading regime in a very efficient manner.
Reliability-Based Design and Optimization of Self-Twisting Composite Marine Rotors
Authors: Motley Michael R. , Young Yin L. , Baker Jack W.
Published: February 2010

The objective of this work is to develop a reliability-based design and optimization methodology to improve the efficiency of self-adapting composite marine rotors. The goal is to quantify the influence of material and operational uncertainties on the performance of self-adapting marine rotors, and to present a design and optimization scheme to maximize the performance and reliability of these structures.
Frequency and Time Domain Analyses of Vessel Motions During Tropical Cyclones
Authors: Whelan J. R. , Drobyshevski Y. , McConochie J. D.
Published: February 2010

A permanently turret moored floating facility located off the North West coast of Australia is likely to be exposed to tropical cyclones, which exhibit extreme wave height, wind and surface currents. Furthermore the cyclonic eye and fringes are characterised by rapidly varying metocean conditions, particularly wind speed and direction. It is necessary to understand the weather vaning and motion responses of the vessel during these transient conditions for successful facility development. This paper presents weather vaning and motion analyses of a Floating Storage and Offloading (FSO) vessel during the passage of tropical cyclones. A synthetic tropical data base developed by Woodside Energy Limited was used to generate the cyclonic conditions. The data base contains storms with return periods ranging from 10 to 100,000 years. Time histories of wave spectrum, wind and current at half-hourly intervals for several cyclones were examined. The weather vaning and motions responses of the FSO were computed using two methods: (1) a frequency domain quasi-stationary approach, and (2) a time domain approach. In the frequency domain analyses the metocean conditions were treated as stationary for each half-hour interval and the mean vessel heading and most probable amplitudes of motions were calculated. In the time domain analyses, the time histories of metocean conditions were closely matched to the synthesised cyclonic conditions, and time histories of vessel heading and motion responses were generated. Multiple realisations were simulated for each cyclone to assess variability of results associated with wave train random seed. A key finding of the study is that the worst roll response tends to occur after the eye of the cyclone has passed, at which time the wind and waves were highly non-collinear. At this time the vessel weather vanes so as to experience waves which are substantially beam-on. When the cyclonic eye is directly over the site, there is a reduction in wind speed and as a consequence the vessel weather vanes into the sea. This causes a corresponding reduction in the roll response. Results from both analysis methods have been compared and good agreement is observed. The applicability and limitations of the two analyses methods are also considered.
Wave Drift Forces in Directional Seas in Shallow Water
Authors: Pinkster J. A.
Published: February 2010

Wave drift forces in shallow water are dominated by low frequency bound wave effects. The effect of short-crestedness of the waves on the wave drift forces is investigated based on direct evaluation of the near field pressure equations in irregular directional seas. The background to the method is treated and examples of results showing some of the details of the computational method are given. Some trends found for the effect of the short-crestedness on the second order wave drift forces on a typical LNG carrier moored in shallow water are presented. Finally, results are given of low-frequency horizontal motions of the LNG carrier in directionally spread head, bow-quartering and beam seas for a range of directional spreading values.
Investigation of Interaction Between Oil Spills and Hydrostatic Changes
Authors: Tavakoli Mohammad Taghi , Amdahl Jo̸rgen , Leira Bernt J.
Published: February 2010

In this paper an assessment of the potential oil outflow from tankers in the case of grounding is presented. The purpose of the study is to investigate how the introduction of double sides, double bottom and damage location will influence the amount of oil escaping from a damaged tank. Furthermore, the objective is to analyze the interaction between oil outflow and hydrostatic changes. In a previous paper, OMAE2008 [1], the pressure integration technique has been utilized to estimate the spill rate and the potential amount of spill without considering the hydrostatic changes. The results were confirmed by CFD simulations. The principles of this technique were presented and its advantages and disadvantages were discussed. In this paper interaction between oil leakage and hydrostatic changes is studied. In order to investigate possible additional oil spill caused by hydrostatic changes two procedures are utilized. In the first procedure, it is assumed that the oil flows out from the cargo tank or water flows in till hydrostatic equilibriums is attained, and subsequently the buoyancy changes due to oil outflow or water inflow are calculated. The additional oil spills caused by the hydrostatic changes are then calculated. The computations are carried out iteratively and continued until the difference between oil loss volumes obtained from two consecutive computations reaches a prescribed tolerance. In reality, there is no time lag between the buoyancy changes and oil loss volume. In order to eliminate the lag, time domain procedure has been applied. In the second procedure, the oil spill and hydrostatic changes are calculated simultaneously in each time step.