bluuebunny/crossref_metadata_2025 huggingface dataset:
Frequency and Time-Domain Models for a Two-Body Wave Power Device That Extracts Energy From Sea Waves by Relative Pitch Motion
Authors: Justino Paulo , Amador Jose´ C. , Morais Tiago , Hadden Marc , Ferreira Nuno
Published: July 2009

In this paper analytical and numerical tools are used to describe the behaviour of a new wave power device. This device extracts energy from sea waves by relative pitch motion between their two independent parts. Frequency-domain analysis is carried out assuming that its hydrodynamic performance and its power take-off equipment behaviour may be considered linear. For regular waves optimal mechanical damping coefficients are computed assuming restrictions to hold for relative pitch motion between the two device parts. Useful power, capture length as well as rotations and displacements are computed. Time-domain analysis is necessary if one needs to compute the device’s performance under more realistic operational conditions. In order to get device’s variables trajectories for irregular sea conditions, a time-domain approach is adopted since the hydraulic power take-off equipment exhibits a non-linear behaviour. Results are obtained for irregular waves, i.e. for different sea states, and thus trajectories for several device variables are presented. Power available to the hydraulic machine is computed and presented.
A Finite Element Analysis for Unbonded Flexible Risers Under Axial Tension
Authors: Bahtui Ali , Bahai Hamid , Alfano Giulio
Published: July 2009

Recent developments on the numerical analysis of detailed finite element models of unbonded flexible risers using ABAQUS are presented. Several analytical methods are studied and combined together, and their results are compared with those obtained in the finite element model for two different tests, the second one involving cyclic loading. In the finite element model all layers are modeled separately and contact interfaces are placed between each layer. A fully explicit time-integration scheme was used on a 16-processor cluster. The very good agreement found from numerical and analytical comparisons validates the use of our numerical model to provide benchmark solutions against which further detailed investigation will be made.
Optimization of Offshore Structures Based on Linear Analysis of Wave-Body Interaction
Authors: Birk Lothar , Clauss Gu¨nther F.
Published: July 2009

This paper discusses new developments in automated hull shape optimization spearheaded by the authors. The use of the linear diffraction-radiation panel code WAMIT® as a design tool is highlighted. Early optimization results yielded bodies with extreme shapes. A series of studies has been performed comparing model test data and numerical computations. The presented comparisons reinforce the numerical results. The authors connected the reliable hydrodynamic analysis provided by WAMIT® with a newly developed parametric hull design methodology. This allows the automated generation of hull shapes without requiring user interaction. Single- and multi-objective optimization algorithms are available to solve a wide range of nonlinear programming problems. The integrated system optimizes hulls by minimizing motions and forces in waves. This is especially important for innovative systems when prior design experience is missing. The results of an optimization run provide a wealth of information which can be utilized to support rational design decisions.
A Weakly Nonlinear Coupled-Mode Model for Wave-Current-Seabed Interaction Over General Bottom Topography
Authors: Belibassakis K. A. , Gerostathis Th. P. , Athanassoulis G. A.
Published: July 2009

A weakly nonlinear, coupled-mode model is developed for the wave-current-seabed interaction problem, with application to wave scattering by steady currents over general bottom topography. Based on previous work by the authors (Athanassoulis & Belibassakis [1], Belibassakis et al [2]), the vertical distribution of the scattered wave potential is represented by a series of local vertical modes containing the propagating mode and all evanescent modes, plus an additional term accounting for the bottom boundary condition when the bottom slope is not negligible. Using the above representation, in conjunction with Luke’s [3] variational principle, the wave-current-seabed interaction problem is reduced to a coupled system of differential equations on the horizontal plane. If only the propagating mode is retained in the vertical expansion of the wave potential, and after simplifications, the present system is reduced to an one-equation model compatible with Kirby’s [4] mild-slope model with application to the problem of wave-current interaction over slowly varying topography. The present coupled-mode system is discretized on the horizontal plane by using a second-order finite difference scheme and numerically solved by iterations. Numerical results are presented for two representative test cases, demonstrating the importance of the first evanescent modes and the sloping-bottom mode. The analytical structure of the present model facilitates its extension to treat fully non-linear waves, and it can be further elaborated to study wave propagation over random bottom topography and general currents.
Time-Domain Models and Wave Energy Converters Performance Assessment
Authors: Ricci Pierpaolo , Saulnier Jean-Baptiste , Falca˜o Anto´nio F. de O. , Pontes M. Teresa
Published: July 2009

To evaluate the performance of a Wave Energy Converter (WEC) with realistic Power Take-Off (PTO) configurations, moorings, control systems and other contributions, time-domain models are required to deal with the non-linearities arising from the different elements of the energy chain. Future developers, in order to give a correct estimation of the expected power output of their devices, will have to apply these models and will be asked about the accuracy they can provide, particularly on what concerns the performance of the device in a determined location. A general mathematical outline of this approach was firstly proposed by Cummins by using, under linear assumptions, a classical way of representing the equation of motion of a floating body with a system of integro-differential equations with convolution terms that involve frequency-dependent coefficients. Many methods have been proposed, in literature, to solve this system in the most efficient and accurate way. Some of them relied on a direct numerical integration using standard methods for the solution of Ordinary Differential Equations, while, in turn, others are based on the approximation of the radiation convolution term with a determined number of linear sub-systems or properly chosen transfer functions. This paper presents a general scheme for a simple heaving single-body WEC, whose hydraulic Power Take-Off is coupled to a gas accumulator that serves as a storage device. Different time-domain methods will be used and compared. Particular attention will be paid to the accuracy of the performance calculation of this WPA. It is expected that the results of the simulations provide deeper understanding of the importance of the numerical parameters used in the estimation of the device performance and in this way will constitute an additional suggestion for the choice of a time-domain model for the evaluation of a WPA performance.
Separation of Wave Systems in Time Series of Combined Sea States
Authors: Nunes Luis Manoel Paiva , Guedes Soares C. , Lima Jose Antonio Moreira
Published: July 2009

In some oceanographic applications it is necessary to separate the individual sea state components, or wave systems, that are present in complex wave conditions represented by multipeaked wave energy spectra. Later on, it may be required to realign the original sea states having only the calculated parameters of the splitted spectrum. As an example, this situation may arise when trying to make correlation between time evolution of the sea states with meteorological forcings. To solve this problem a method was developed to follow the time evolution of a given initial sea state using a mathematical tracking procedure. It is based in a “similarity index” between one point of the time series and three previous points. The criterion uses wave parameters Hs (significant wave height), Tp (spectral peak period) and Dir (spectral peak wave direction) to calculate the similarity index. The method selects points that have the highest similarity index as a part of the same type of spectral component, following the time sequence of waves.
Wind-Generated Sea Wave Climate Characterization at the Tagus Estuary
Authors: Capita˜o Rui , Santos Joa˜o Alfredo , Pinheiro Liliana , Fortes Conceic¸a˜o , Coli Alexandre
Published: July 2009

In order to forecast both the short and the medium-term evolution of estuarine beaches, as the ones that exist in the Tagus Estuary, there is a need to both define the sea wave climate generated by local wind in those confined areas and to understand the mechanism that enables the wave propagation at the estuary up to wave breaking at the beach. In this paper, the SWAN model, [1], is applied to study wind generated waves at an area near Alfeite beach to characterize the wave conditions at the Tagus estuary considering the actual wind and tide conditions as observed on the 3rd of October, 2006. A comparison of SWAN numerical results with the wave elevation measurements collected at the site with resistive wave gauges and also with a pressure transducer, taking into account the influence of the ferries crossing the study area, is made and discussed.
System Reliability of Jack-Up Platforms Under Fatigue and Fracture Failures
Authors: Shabakhty Naser
Published: July 2009

Jack-up is a rig for oil drilling and exploration in shallow water. The ability of movement from one location to another place, in addition to the possibility of lifting up the hull during operation, increase demand for the use of this platform in deeper water and harsher environments. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In practice, some uncertainties are included in loads and characteristics of capacity demand. It will be reasonable to take these uncertainties into account within reliability framework. Based on the crack propagation approach and fracture mechanics, a reliability of structural element under fatigue degradation can be estimated. However, the first failure of structural element under fatigue degradation may not cause system failure. This paper presents a new approach to estimate structural system reliability of jack-up platforms in combination of fatigue and fracture failure modes. The probability of failure of each component is firstly estimated using fatigue limit state and the subsequent failure probability is calculated extending fracture modes. In fracture mode, the crack size is required. In this paper a new approach based on Monte Carlo Simulation is presented to estimate the statistical crack size in accordance to the fatigue limit state. Important sequences of failure are then identified utilizing branch and bound technique and finally the system reliability through combination of important failure paths leading to system failure has been calculated. The advantage of this method is that the FORM or SORM technique can be applied to compute each failure path individually and finally determine system reliability based on combination of significant failure paths identified in branch and bound search technique. It is shown the system failure in this sequence has higher probability than the entirely first and second failure under fatigue failure modes.
A Case Study of High Pressure/High Temperature Pipeline for Ice Scour Design Using 3D Continuum Modeling
Authors: Fredj Abdelfettah , Comfort George , Dinovitzer Aaron
Published: July 2009

Offshore pipelines in ice environments are increasingly required to operate at higher pressure and temperature conditions. These offshore pipelines can be subjected to stress and strains resulting from loading condition and ice scour created in the area. This paper describes a study performed to investigate the mechanical behavior of High Pressure/High Temperature (HP/HT) pipelines to withstand ice scouring events. A parametric study was completed to define the governing parameters for HP/HT pipeline design. This study was based on 3D continuum modeling with an ALE (Arbitrary Lagrangian Eulerian) formulation that was developed and run using LS-DYNA. The model incorporated large deformation theory, non-linear pipe-soil interaction and non-linear pipe material behavior to define ice interaction induced pipe deformations.
Comparing Fatigue Reliability of Jack-Up Platforms Based on Selected Nonlinear Stress Models
Authors: Shabakhty Naser
Published: July 2009

Jack-up is typically an exploration and production platform in shallow water operation. The possibility of moving this type of offshore structure from one location to other place and lifting up the deck above sea level made it attractive for offshore industry to use in deeper water and harsher environments. The jack-up structure consists of various components and behaves in a nonlinear and dynamic manner. Degradation phenomena such as fatigue may influence the structural performance and reduce strength of platform during its lifetime. Appropriate estimation of fatigue degradation strongly related to the suitable assessment of stress fluctuation, however, there are some uncertainties both in fatigue characteristics and environmental loads. These uncertainties can be taken into account in fatigue reliability framework. In this paper, several nonlinear models proposed by Wirsching and Light (W&L), Zhao and Baker (Z&L), Winterstein (Hermite model), in addition to the typical narrow banded model are utilized to specify nonlinearity in the stress fluctuation. The fatigue reliability is calculated based on these models and are compared with more advanced technique of rain flow counting method. The result shows a dependency between nonlinear behaviour of stress response and the selected model.
Predicting Spar VIM Using CFD
Authors: Holmes Samuel
Published: July 2009

Recent work toward predicting spar vortex induced motion (VIM) with computational fluid dynamics (CFD) suggests that such simulations can anticipate many aspects of spar response and thus supplement tow tank experiments and other design methods. However, the results also highlight a number of challenges as well. The spar VIM problem is characterized by very high Reynolds numbers, geometric complexity including the presence of numerous external appendages and the presence of very rough surfaces. In this paper, we first review recent work on spar VIM where CFD was used to simulate tow tank experiments. This work suggests that CFD methods give good results in most cases but also points to some exceptions. In particular, in simulations of small scale vortex induced motion tests of spars, good agreement between analysis and experiments is usually obtained when the flow separates from the spar hull at the strakes. The CFD simulations are sometimes less successful at predicted VIM when flow separation occurs at the spar hull. We then examine our own recent practice in simulating tow tank experiments with CFD with the objective of finding possible modeling deficiencies. The focus is on the resolution of the large eddies in the wake which most influence the fluctuating loads on the spar, but we are also concerned with the use of wall functions to model the boundary layer. All of the calculations use detached eddy simulation (DES). In order to test the method, we make use of wind tunnel experiments at on a fixed truncated cylinder without strakes. The wind tunnel experiments are performed at Reynolds numbers (Re) that are about the same as those used in scale model spar VIM experiments. Wake particle image velocimetry (PIV) and other data from wind tunnel experiments published in the open literature are used for comparison. The comparisons are used to examine requirements for grid resolution in the wake. Finally, it is suggested that specific wind tunnel experiments might be used to gather needed data on the effects of rough walls and appendages at very high Reynolds numbers.
Calculation Method of the Residual Capability of Damaged Warships
Authors: Ren Huilong , Li Chenfeng , Feng Guoqing , Li Hui
Published: July 2009

The residual capability of damaged warships is one of the important aspects of its residual strength assessment. The calculation method of ultimate longitudinal bending moment was based on the Smith method. Then, a method to calculate the statistical characteristic values of residual capability of damaged warships was presented, in which some uncertainties, such as the size of broken holes caused by weapons, material mechanical properties of the steel and the size of structures, etc, were considered. The ultimate longitudinal bending moment of Reckling No.23 model was calculated and compared with the result from the literature, the result is quite satisfactory. With a warship as an example, the statistical characteristic values of residual capability of the damaged ship hull were calculated, and some factors which may evidently affect the residual capability were investigated.
Causes and Influence of the Ice Heterogeneity on Loads on Offshore Structures
Authors: Shkhinek Karl N. , Blanchet Denis , Jilenkov Alexander G. , Thomas Graham A. N.
Published: July 2009

The influence of heterogeneity of ice properties on loads exerted on offshore structures is examined analytically in the present work. The methodology proposed permits considering the influence of heterogeneity induced by different factors: local factors such as ice strength or ice thickness variations, and large-scale factors such as the existence of ice fields of different ages. Local areas with parameters (strength, thickness, etc) different from parameters of the surrounding ice are treated as inclusions and their influence on loads is investigated. Results of the investigation show that under some conditions the heterogeneity influence is negligible whereas sometimes it can be important and leads to the size effect formation. The methodology of the heterogeneity influence determination and conditions when it is important are discussed in this paper.
Study of the Characteristic and Possibility for Applying Composite Materials to the Blades of Tidal Power Generation
Authors: Uzawa Kiyoshi , Kageyama Kazuro , Murayama Hideaki , Ohsawa Isamu , Kanai Makoto , Nishiyama Tatsuya , Shichiri Akihisa
Published: July 2009

Recently, several power plants from the rotation of turbine with tidal current have been tried. Since the density of seawater is 800 times as much as that of the air, the loading of water on a turbine strictly requires much more strength and stiffness of blade compared with the wind turbine. Neither wind turbine nor standard hydroelectric dam turbines can simply be submerged into an ocean current. There are some formidable technical challenges to be overcome compared with the wind turbine. Key issues are cost effectiveness, structural integrity and workability in access and installation. The metal blade has enough strength, but is too heavy to install and handle easily. The light weight and extreme strength are essential to the blade. The objective of this work is to determine the mechanical properties of the tidal turbine, and to examine the availability of the turbine blade of composite materials for an approach to eliminate the above problems. The study was conducted in the preliminary study of the demonstration plant, which will be settled in Oma Promontory, Aomori Prefecture in Japan, whose maximum power output is 300kW and turbine diameter is 11 meters. A number of materials were considered, i.e. comprised rolled steel, aluminum bronze, GFRP for blade. We made two models for structural study based on the propeller blade shape with thin section and the wind turbine blade shape with thick section. The FEM analysis were conducted as follows, Aluminum-Bronze solid model with propeller shape; the real model at the present moment in the Oma plant. Composite material solid model; same shape as propeller but applied with composite materials. Composite material shell model with wind turbine blade Shape; structured by monocoque construction with changing the thickness by 10mm from 10mm to 50mm. The properties of GFRP for the structural study were measured from the ISO-laminates, which were fabricated by VaRTM, of multi-axial non-crimp fabrics and epoxy. Furthermore, the vibratory cavitation erosion tests of Composite materials were conducted. In order to compare with the aluminum bronze and composite, each cavitations weight loss in fresh-water were measured and observed. As the result, the multi-axial GFRP for propeller type blade was insufficient in rigidity and strength of shear. It is necessary to use not GFRP but CFRP for the propeller type blade. In contrast, as for wind turbine type blade, it was led to the conclusion GFRP blade is workable. As for erosion, the durability of composite materials is remarkably inferior to metals.
HF Radar Directional Spectra Validation With Buoy and Model Data
Authors: Wyatt L. R. , Green J. J. , Middleditch A. , Waters J. C.
Published: July 2009

Examples of the measurement of the directional wave spectrum using HF radars are presented. These are compared with both buoy and model data and good agreement is demonstrated. Work in progress on partitioning these spectra for more detailed comparisons and for assimilation into models is discussed.
A Practical Method to Study Over and Under-Pressurizing of Ship Tanks During Ballasting and Deballasting
Authors: Kumar Manoj
Published: July 2009

There have been many incidents of hull damage due to over and under-pressurizing of ship tanks during ballasting and deballasting. A safety relief hole of 6 to 8mm diameter is generally provided in Air-pipes to prevent accidental over and under-pressurizing of ship tanks during ballasting and deballasting. This paper investigates the pressure rise and drop inside the tank assuming the air-pipe to be closed. A practical method based on Computational Fluid Dynamics has been presented to find out the maximum pressure rise or drop. A parametric study, based on varying relief hole diameter, has been carried out. The investigation brings to prospective the extent of pressure rise or drop, and hence the damages that can occur due to poor operation of the Air-pipes during ballasting and deballasting and a need for automated opening and closing of the Air-pipes for a safer ship.
Structural Analysis for a Panamax Bulk Carrier
Authors: Pacuraru-Popoiu Sandita , Iancu Paulina , Crudu Liviu I.
Published: July 2009

This paper is devoted to the development of a structural analysis for a bulk carrier vessel. According to the CSR requirements for bulk carriers, an assessment of the hull structure using FEA (Finite Element Analysis) on a model extended over 3 cargo holds is presented. This method is used in order to assess the structural integrity of the cargo holds under the considered loads. The selected vessel is a PANAMAX bulk carrier with double hull and longitudinal uniform structure. There are three main priorities for the FE-analysis: one is to perform a fine mesh necessary to capture the stressed induced by the considered loads. The second priority is to apply the right boundary conditions in order to approach the hull girder bending and stress distribution on the cargo holds. The stress distribution is induced by the cargo weight, the hydrostatic pressure and the external water considered as dynamic pressure. The dynamic pressure was computed using an in-house code, neglecting the inertia forces induced by the ship motions and the horizontal accelerations. Also shear forces and bending moments were obtained for head angles of 0, 45 and 180 degrees.
Comparing Energy Yields From Fixed and Yawing Horizontal Axis Marine Current Turbines in the English Channel
Authors: Blunden Luke S. , Batten William M. J. , Bahaj ‘Bakr S.
Published: July 2009

At some sites with high tidal stream velocities there is an appreciable change in flow direction (‘swing’) away from 180 degrees between the two maxima of flow speed. In order to assess the performance of horizontal axis marine current turbines in non rectilinear currents, measurements of a model rotor have been made in a towing tank. Curve fits have been calculated as a function of the cosine of the yaw angle squared and the thrust as cosine of the yaw angle. The curve fits have been used in a case study to investigate the impact of fixed-orientation or yawing rotor designs on average annual energy output, at three locations in the English Channel. All three sites are of the type where flow is accelerated around a headland or cape, but their tidal streams vary in deviation from rectilinearity. For two of the sites — Portland Bill (Dorset, UK), Race of Alderney (Alderney, Channel Islands/Normandy, France) and St. Catherine’s Point (Isle of Wight, UK) — available data consisted of tidal stream diamonds printed on Admiralty navigational charts. These rely on local tidal elevations for interpolation of tidal streams. At the other site — St. Catherine’s Point, Isle of Wight, Hampshire — current meter measurements of duration one month were available from the British Oceanographic Data Centre (BODC) at the location of a tidal diamond, allowing a direct tidal analysis. For the three sites, the available data were analyzed into harmonic constituents and then extrapolated into the future. For each year’s worth of predictions, the cubed speeds as a function of time were sorted into bins to form a histogram. The annual power output for each design of turbine was then calculated using the known performance at each value of cubed speed. This process was repeated for each year over an 18.6-year lunar nodal cycle in order to ascertain the inter-annual variation in power output.
Transverse Stability of a Cargo Ship at Parametric Rolling on Longitudinal Waves
Authors: Obreja Dan C. , Nabergoj Radoslav , Crudu Liviu I. , Pacuraru-Popoiu Sandita
Published: July 2009

Unfavourable environmental conditions associated with an insufficient stability margin adopted in the design process may determine ship capsizing. One of the most critical situations is generated when the ship moves on longitudinal waves, at parametric resonance condition. In this paper, the theoretical and experimental investigations on transverse stability of a cargo ship on regular longitudinal waves are considered. During the seakeeping model tests on longitudinal waves, the occurrence of induced roll motions were observed and measured. The restoring moment on longitudinal waves was determined based on theoretical and experimental methods. A computer code has been implemented in order to calculate the main parts of the restoring moment i.e. Froude-Krylov, diffraction and radiation components. In order to reproduce the real physical phenomenon, captive and semi-captive model tests were correlated. The radiation and diffraction forces and moments and the restoring moment on regular longitudinal waves were measured. The agreement of the calculated solutions with the experimental results indicates the possibility of using the adopted hydrodynamic model in the design process.
Nonlinear Seismic Analysis of Free Spanning Submarine Pipelines: Effects of Pipe-Water Interaction
Authors: Zeinoddini M. , Sadrossadat S. M. , Parke G. A. R.
Published: July 2009

The seismic response of free spanning submarine pipelines has received little attention in the literature. This paper deals with this subject and its main focus is on water-pipeline interaction during the event. The added mass approach provides a simple water-pipeline interaction model which is commonly used for the dynamic analysis of submarine pipelines. With free spans, however, earthquake components normal to the seabed also contribute to the dynamic pressures inputs to the pipeline. The line, therefore, becomes subject to loads which might be overlooked in a simplified added mass approach. To provide a more accurate estimation for the water-pipeline interaction, the pipe body, the sea bed, the free-spanning and the surrounding water have been incorporated in a numerical finite element model. The accuracy of the model has been verified by correlating the dynamic characteristics of the model with some analytical solutions. The models have then been analyzed under both harmonic excitations and extreme ground motion records. The site effects have been ignored. Both the horizontal and vertical excitations have been considered. A non-linear incremental direct integration dynamic analysis approach has been employed. Deviations up to 14% have been observed between the model predictions and those from the simple added mass method. The maximum deviation has been found to occur when the frequency contents of the excitation has been close to the first natural frequency of the free spanning pipe.
Real-Time Ocean Monitoring Through Lagragian Drifters During an Offshore Drilling Operation
Authors: Fragoso Mauricio da Rocha , Alves dos Santos Francisco , Marques da Cruz Leonardo M. , Pellegrini Ju´lio A. C. , Mafra Tatiana , Assireu Arcilan Trevenzoli , Cerrone Bruna Nogueira
Published: July 2009

MONDO Project is a surface ocean monitoring program for offshore operations never done before in Brazilian waters. It consisted on the periodic deployment of satellite tracked ocean drifters that provides real-time surface ocean currents and temperature data. Throughout September–November 2007, 40 satellite tracked ocean drifters were deployed at Santos Basin, located at the Brazilian Southeastern Ocean Bight. The results of this project can be used to study a wide range of subjects about ocean dynamics such as eddy activity, predictability, lagrangian integral scales, diffusivity, wind influence, etc. This paper is focused on eddies activities and ocean dynamics predictability. It can be observed that the number of anticyclonic eddies is a little higher than cyclonic ones during 2007 spring and that they present high values of relative vorticity (O(±10−5 s−1)). The rotation period lies between 16 days (anticyclonic) and 21days (cyclonic), with tangential velocities around 0.4 m/s. Comparison with data measured by drifters and altimetry showed good results, which indicates that a more extensive analysis about eddies in this region using altimetry data should provide relevant results. The methods of non-linear analysis applied to MONDO Project data allowed to estimate the predictability of the phenomena within Rossby radius scale (∼30 km). The results indicate that the prediction errors tend to double for periods greater than 3 days. It is suggested also that at least 4 variables should be considered in modeling studies on this region.
Riser Analysis for a Dry Tree Semi-Submersible
Authors: Gupta Apurva , Yang Chan K. , Murray John , Shen Guoan , Hussain Anis , Nah Edwin
Published: July 2009

There is currently ongoing interest in the offshore industry to develop a dry tree production semi submersible that has motion characteristics similar as a Spar, but with the functionality of a Tension Leg Platform (TLP) or a Semi in terms of fabrication, installation and commissioning. It has been proposed that the required motions can been achieved by adding a heave plate to an existing deep draft semi with external extendable columns supporting the heave plate. The ESEMI II is such a concept with telescopic structure to lower the Second Tier Pontoon (STP). The riser technology presently used on Spars can be ported to a dry tree semi design assuming the motions are similar. These designs consist of a tensioning system using either buoyancy cans or hydraulic/pneumatic tensioners. The main components of the risers for such a semi-submersible are similar to those of the Spar including Keel Joints and stress joints, where the heave plate on the dry tree semi replaces the soft tank on the Spar. The Top Tensioned Risers (TTR), however, can not be tensioned using simple buoyancy cans and have to be tensioned by non-linear pneumatic tensioners because of exposure in the wave zone. The stroke of the risers in the dry tree semi is estimated to be similar to that of the Spar. The heave plate connectors have to be designed considering not only the hydrodynamic loadings but also horizontal riser loads at the keel as well, which would give enormous moments at the keel and stress joints. The stroke of the riser should also be considered while designing the heave plate connectors. The riser, unlike a Spar, will not be shielded in the hull area, and will be subjected to high currents, and hence the Vortex Induced Vibration (VIV) effects may also be significant. This paper discusses a methodology for riser and VIV analysis, and mitigations, on a dry tree semi-submersible for two water depths.
Effect of Hurricanes on the Cumulative Fatigue Life Consumption of a GOM TLP
Authors: Aalberts Pieter , van Dijk Radboud , Zheng Weizhong
Published: July 2009

Accurate assessment of cumulative tendon fatigue is of importance for evaluation of the overall design life of TLPs. This cumulative fatigue damage can be derived from direct tendon load measurements. However, in some cases the quality of the tendon load measurements decreases or direct measurements are not available and assessment of cumulative tendon fatigue is not possible. An alternative approach to derive cumulative tendon fatigue is presented in this paper, based on the measured accelerations of the topsides. The quality of this approach is verified using data measured on board the Marco Polo TLP. To investigate the dynamic behavior of a Tension Leg Platform (TLP) in real life conditions and therewith provide insight in the physical mechanisms, as well as data to verify and validate the current design methods, a full-scale monitoring campaign on the Marco Polo Tension Leg Platform (TLP) has been conducted as part of a Joint Industry Project (JIP). Within this JIP measurements of the loads in four tendons have been collected continuously since June 2004. In addition to the tendon loads the wave frequency TLP motions and wave elevations were also recorded. The alternative approach to derive the cumulative tendon fatigue is verified using Marco Polo measurements recorded during 2005 and 2006. From the Marco Polo TLP motions recorded during the two year period the axial loads were calculated in all tendons. Statistical and spectral comparisons were made between the calculated and measured tendon loads. Based on those results a frequency dependent correction function was computed and applied on the computed tendon loads. Next the fatigue consumption based on the corrected calculated tendon loads was computed and verified with the fatigue consumption based on the actual measured tendon loads. This comparison provides the accuracy of the fatigue calculation when the computations are based on the measured TLP motions.
Quality Control Issues in Estimating Wave Climate From Hindcast and Satellite Data
Authors: Bitner-Gregersen Elzbieta M. , de Valk Cees
Published: July 2009

Accuracy of an estimated design wave condition such as the 100-year significant wave height is important information for risk assessment in design of marine structures. Assessment of accuracy of design conditions is already not simple when they have been derived from in-situ wave measurements at the project site. When using wave hindcast data this task is even less straightforward due to often a lack of sufficiently reliable measurement data near the project site for comparison. Different hindcasts may give significant discrepancies in prediction of extremes. Also, the quality of the hindcast may vary over time. In practice, we often do not much more than illustrating the quality of the data, for example by scatter plots of hindcast data versus measurements far away from the project site. In order to assess the quality of a design wave height from a hindcast systematically, errors in both the local hindcast data and in the extrapolation from these data need to be addressed. The paper discusses the overall idea and some building blocks for such an approach, while realising that there will not be one simple recipe applicable in all situations. One of the issues is the selection and use of observational data in quality control. Satellite wave data are an attractive data source for this purpose. For illustration, data from the seas around Norway are used.
Net Scantlings and Structural Reliability
Authors: Boon Bart
Published: July 2009

Vessel structures deteriorate over time. These ageing effects mean that ship strength and structural reliability diminish over time. In the past this was implicitly taken into account in the rules of classification societies. Today’s assessment of ship structures is based upon the use of first principles. This presupposes a realistic description of the deteriorated structure as well. Present rules mainly do this by reducing scantlings of plates and stiffeners by an assumed amount of uniformly distributed corrosion. Some experimental, phenomenological and analytic results are presented that illustrate failure behaviour of local and global ship structures quite different from that using net scanlings only. Typical aspects that play a role are non-uniform degradation, such as groove corrosion. Energy absorption may be the governing instead of ultimate strength as is normally assumed. It is shown that assessment of the reliability of aged ship structures requires additional information and analysis different from that normally performed today.
Numerical and Experimental Study on Aircushion Supported Structures
Authors: van Kessel J. L. F.
Published: July 2009

The use of aircushions for very large floating structures has been investigated in recent years at Delft University of Technology. Model tests were performed to validate the results of numerical calculations based on a linear three-dimensional potential method. A linear adiabatic law was used in the numerical approach to describe the air pressures inside the cushions. It is assumed that air cannot escape from the cavity underneath the structure. The water surface within the aircushions and the mean wetted surface are modelled by panel distributions representing oscillating sources. Experimental results and numerical calculations of two configurations of aircushion supported structures at zero speed are presented in this paper. The results show that model tests of different aircushion supported structures can be well predicted by means of 3D diffraction calculations.
On Sequence of High Waves in Nonlinear Groups
Authors: Arena Felice , Guedes Soares C.
Published: July 2009

The structure of wave groups in time domain depends upon the spectrum. For narrow-band spectra, if a wave with a very large height H occurs, the waves that precede and follow the highest one have height (H′ and H″ respectively) very close to H. For real spectra, the difference between H and either H′ or H″ depends on the bandwidth of the spectrum: it increases as larger is the spectrum. For bimodal spectra, the authors have shown [1] that the wave groups are strongly modified and they are function of the energy associated to the swell and to the wind wave components, as well as of the distance between the two peaks. This paper analyzes the structure of the succession of three waves, given by the high waves and the two that come before and after the highest one, in time domain. The results are analyzed, up to the second-order, for wind wave unimodal and for bimodal spectra, by comparing the height H with H′ and H″.
Computation of the Three-Dimensional Nonlinear Flow Around a Body in or Near the Free Surface
Authors: Yang Chi , Lu Haidong , Lo¨hner Rainald , Sandberg William C.
Published: July 2009

An unstructured grid-based, parallel incompressible flow solver has been developed for solving the three-dimensional non-linear flow around a body in or near the free surface. The incompressible Euler/Navier-Stokes equations are solved together with or without the free surface equation. The overall scheme combines a finite-element, equal-order, projection-type three-dimensional incompressible flow solver with a finite element, two-dimensional advection equation solver for the free surface equation. The solution is marched in time until a steady state is reached. The computer code developed based on the method described above has been applied to the simulation study of the three-dimensional nonlinear flow around a flying fish when it swims underwater, taxis on the free surface, and glides above water.
Analytical Prediction of Oil Spill From Grounded Cargo Tankers
Authors: Tavakoli Mohammad Taghi , Amdahl Jo̸rgen , Ashrafian Alireza , Leira Bernt J.
Published: July 2009

The objective of this paper is to study the oil spillage process which corresponds to various tank designs. For this purpose, analytical calculations and time domain simulations were applied. Numerical simulations have been performed to verify the models. Good agreement has been found between the results obtained from the simplified analytic model and those from the CFD calculations for predicting the oil spill from different hull designs. The volume and efflux rates of oil that are based on the suggested model are comparable to those obtained by means of numerical simulations.
Autonomous Underwater Vehicle Control in Presence of Waves
Authors: Moreira L. , Guedes Soares C.
Published: July 2009

In this paper, the 6 degrees of freedom equations of motion of an autonomous underwater vehicle (AUV) are described as a linear model and divided into three non-interacting (or lightly interacting) subsystems for speed control, steering and diving. In addition to the model of the AUV dynamics, the first and the second order wave force disturbances, i.e. the Froude-Kriloff and diffraction forces are introduced. Based on the principle of superposition it is possible to represent the AUV dynamics as the sum of low and high frequency motions. An algorithm of non-linear regression for the rationalization of the sub-surface sea spectrum is provided. Two different control designs, based on H2 and H∞ methodologies, were applied to the diving and course control of the vehicle considering the presence of the wave disturbances. The work is based on the slender form of the Naval Postgraduate School AUV, considering that the subsystems can be controlled by means of two single-screw propellers, a rudder, port and starboard bow planes and a stern plane. The wave effect on the corresponding motions of the underwater vehicle is analyzed and evaluated considering the AUV operating at different depths and different sea states using both controllers. The model presented here can be a useful simulation tool to predict the underwater vehicles behavior in different mission scenarios.
Analysis of Wave Drift Forces on a Floating Wave Energy Converter
Authors: Fonseca Nuno , Pascoal Ricardo , Marinho Joa˜o , Morais Tiago
Published: July 2009

Wave drift forces acting on floating wave energy converters (WEC) are often the most important loading component for the design of the mooring system. On one hand these forces may be, at least, one order of magnitude larger than wind and current forces, and on the other hand the floating structure and mooring system may respond dynamically to the slowly varying wave drift forces. The paper presents an analysis of the wave drift forces on an articulated floating wave energy converter. Particular attention is given to the effects of the wave energy extraction on the time history of the horizontal drift forces. The hydrodynamic calculations are carried out by a frequency domain Green function panel method, resulting on the transfer function of the WEC motions as well as the transfer function of the mean drift forces. The power takeoff system is represented by a simple linear model where the extracted power is related to the relative velocity in the articulation and the damper of the PTO. With the transfer function of the mean drift forces, the variance spectrum of these same forces is calculated for stationary irregular seastates, and finally time histories of the drift forces are produced for typical operational conditions.
Calculation of Second Order Drift Forces on a FLNG Accounting for Difference Frequency Components
Authors: Fonseca Nuno , Pessoa Joa˜o , Guedes Soares Carlos
Published: July 2009

The usual procedure for calculation of slow drift forces is to simplify the quadratic transfer function by representing the difference frequency components in terms of the zero difference results. In this way the second order problem is much simplified as well as the computational effort. However this approximation has some limitations and in particular for the slow drift oscillations problem it may be important to consider correctly the difference frequency components. The paper presents an analysis of the slow drift exciting forces on a FLNG (Floating Production Storage and Offloading system for production of LNG). The complete second order forces are computed by WAMIT V6.1s which is based on a boundary element method. Results of the horizontal second order drift forces are presented for several difference frequency values. Deep water and two shallow water depths are considered, since water depth seems to have an important effect on the quadratic transfer functions.
Global Buckling Design of Submarine Pipelines: Design Performed Based on DNV-RP-F110
Authors: Ro̸neid Sigbjo̸rn , Collberg Leif
Published: July 2009

DNV-RP-F110 was issued in October 2007 and gives design criteria for expansion caused by internal pressure and temperature. Three different scenarios are considered in the RP; the first and second scenario give criteria for situations where the expansion forces are released by global buckling on even and uneven seabed, respectively, while the third scenario is when the expansion forces are restrained. This is the first public document ever to provide design criteria for situations where the expansion forces are released by global buckling. The first revision of the JIP project specific guideline that constitutes the basis of the RP was issued in 2001 and it is hence “Field proven”. This paper will illustrate different design impacts based on the RP for the even and uneven seabed scenarios. Global buckling design is associated with large uncertainties in important design parameters which include pipe-soil resistance, stress-strain curve and possible interference loads (trawling). These uncertainties are treated systematically in DNV-RP-F110, and the approach leads to a consistent safety level in line with the general requirements in DNV-OS-F101. This paper elaborates how the design may be performed by adopting the principles in DNV-RP-F110. Key aspects in the design process is the calibration of the load condition factor γc for asymmetric response from variation in soil resistance, establishing of maximum distance between the buckles and finally performing the seabed intervention design in order to fulfill the design criteria. The finite element model is also discussed in general and the pipe-soil resistance in particular.
Investigation of Vertical Slamming on Point Absorbers
Authors: De Backer Griet , Vantorre Marc , Victor Sam , De Rouck Julien , Beels Charlotte
Published: July 2009

This paper focuses on the impact of vertical slamming on floating point absorber systems and the associated pressures that might be expected when these phenomena occur. In a first part it will be shown how the occurrence probability of slamming can be reduced by implementing a slamming restriction, i.e. by controlling the motion of the point absorber. The impact of these slamming restrictions on power absorption will be discussed. Secondly an investigation is made of the pressures that occur when the buoys are subject to vertical bottom slamming. Analytical results are presented, which give a pressure prediction of an impacting body with conical and hemispherical shape, using Wagner theory. Laboratory experiments have been carried out at Ghent University. Impact pressures were measured during drop tests with both hemispherical and conical buoy shapes. These pressures were measured by ICP pressure sensors with a range up to 345 kPa with small membrane and very high resonance frequency (> 250 kHz). Analytical and physical results are compared and conclusions are drawn.