bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Multiple Member Failure on the Predicted Reliability of Fixed Jacket Structures
Authors: Sanderson D. , Nelson A. , Stacey A.
Published: July 2009

Current offshore inspection practice relies on the detection of gross damage, such as flooded member detection. Such an inspection philosophy relies upon the structure’s ability to withstand gross damage for an inspection interval based on the large reserve factors on the jacket design strength in both the undamaged and damaged conditions. This requires a demonstration of the structure’s strength in all of the possible damaged conditions with the most critical member removed. However, the load originally carried by the critical member is redistributed to other neighbouring members which must consequently have an impact on the design fatigue lives of other members and increase the probability of more members failing. Ignoring the effects of stress redistribution due to member failure could therefore result in under-prediction of the probability of a second member failing and hence an optimistic prediction of platform reliability. This paper aims to quantify the effects of stress redistribution on the prediction of platform reliability. Stress and ultimate strength analyses were performed on three platforms in 45m water depth with bracing configurations of varying structural redundancies, namely, single diagonal, inverted K and X-braced, to calculate the stress enhancement in all members due to the individual failure of all other members. These stress enhancements were input into probabilistic fracture mechanics models to calculate the increase in failure probability of these second members due to the failure of a first member. More than 1000 pushover analyses were performed to calculate the ultimate strength of the structures in all single-member failed conditions and many hundreds of dual member failed permutations for each jacket. A cross-correlation of the most significant stress redistribution results and pushover results was used to perform platform reliability assessments. The effect of including stress redistribution and dual member pushover results was shown to reduce the predicted platform reliability for all bracing configurations, mainly due to a number of dual-member failed conditions that weakened the jackets significantly. It is recommended that, in addition to performing single member failed pushover analyses, there is a need to assess dual-member failed conditions that drastically reduce platform strength.
High-Cycle and Low-Cycle Fatigue Resistance of Girth Welds in Sour Service
Authors: Buitrago Jaime , Hudak Stephen , Baxter David
Published: July 2009

The fatigue performance of fracture-critical production lines, such as risers and flowlines, has been shown to significantly degrade in the presence of sour hydrocarbon production caused by water injection of reservoirs. To ensure the reliability of the fatigue design under such conditions, experimental verification of the degradation effect on fatigue life due the presence of H2S is required. To that end and over the past several years, ExxonMobil has developed new testing methodologies to evaluate the riser fatigue performance for both in-air and sour conditions. This paper reviews the general elements of the fatigue qualification process and presents new sour fatigue data aimed at assessing performance at the high-cycle fatigue (HCF) and low-cycle fatigue (LCF) regimes. These new data are relevant to that seen in steel catenary riser (SCR) and flowline thermal responses, respectively. Testing methodologies for each regime are discussed and results presented. The new data are interpreted within the context of previous data in the intermediate-cycle fatigue (ICF) to provide a more robust basis for riser design. The main finding is that the new data support a constant slope S-N curve for the practical domain of fatigue lives to which offshore lines are typically designed under sour conditions.
Prediction of Longshore Sediment Transport Using Soft Computing Techniques
Authors: Bakhtyar Roham , Barry David Andrew , Ghaheri Abbas
Published: July 2009

An important task for coastal engineers is to predict the sediment transport rates in coastal regions with correct estimation of this transport rate, it is possible to predict both natural morphological or beach morphology changes and the influence of coastal structures on the coast line. A large number of empirical formulas have been proposed for predicting the longshore sediment transport rate as a function of breaking wave characteristics and beach slope. The main shortcoming of these empirical formulas is that these formulas are not able to predict the field transport rate accurately. In this paper, an Adaptive-Network-Based Fuzzy Inference System which can serve as a basis for consulting a set of fuzzy IF-THEN rules with appropriate membership functions to generate the stipulated input-output pairs, is used to predict and model longshore sediment transport. For statistical comparison of predicted and observed sediment transport, bias, Root Mean Square Error, and scatter index are used. The results suggest that the ANFIS method is superior to empirical formulas in the modeling and forecasting of sediment transport. We conclude that the constructed models, through subtractive fuzzy clustering, can efficiently deal with complex input-output patterns. They can learn and build up a neuro-fuzzy inference system for prediction, while the forecasting results provide a useful guidance or reference for predicting longshore sediment transport.
Tapering Effects on the Installation of Suction Caissons in Clay
Authors: Zeinoddini Mostafa , Shariati Woorya H. , Nabipour Mahmood
Published: July 2009

This paper reports results from an investigation on the tapering effects on the installation and pull-out performance of suction caissons. A numerical finite element approach has been used for the study. The finite element models have first been calibrated/verified against several available experimental data for the installation of the upright suction caissons in clay. The verified models have then been used to examine the behaviour of the tapered suction caissons during the pull-out and installation phases. Numerical results indicate that tapered caissons present considerable enhancement in their pull-out capacity comparing to those from corresponding upright caissons. Also it has been noticed that in general tapered caissons of positive wall slopes need extra forces, in comparison to their equivalent upright caissons, to achieve a full penetration. However, at least with those models studied, these extra forces have found to be less than twenty five percent when the wall slope varies from zero (upright) to 15%. This is while the additional pull-out capacities that might be achieved from these tapered suction caissons could reach to several hundred percents. An almost linear relationship has been observed between the total installation force and the caisson’s wall slope.
Analysis of Numerical Oscillation Problems in a Non Linear Time Dependent Mild Slope Model and First Developments for the Implementation of Wave Breaking
Authors: Zo´zimo Ana Catarina , Fortes Conceic¸a˜o
Published: July 2009

In this paper, a description of the numerical model NMLSE is presented. This model solves the time dependent non linear mild slope equation, without including energy dissipation due to wave breaking [1]. Some modifications are made in the boundary conditions of the original version of the model in order to overcome the numerical oscillation problems detected in the work done by [2]. To evaluate the effectiveness of the new versions of the model, they are applied to test cases of the bibliography and to a bar-trough profile beach for which there are data from physical model tests. The basic theoretical formulation of a new momentum equation that includes energy dissipation due to wave breaking is also presented. The energy dissipation due to wave breaking is included through the addition of a dissipative term based in the eddy viscosity concept.
Validation of Wave Run-Up Calculation Methods for a Gravity Based Structure
Authors: Danmeier Donald G. , Seah Robert K. M. , Finnigan Timothy , Roddier Dominique , Aubault Alexia , Vache Michel , Imamura John T.
Published: July 2009

During the design of a Gravity-Base Structure (GBS) for harsh environments, it is essential to account for the maximum wave run-up in operational and extreme weather conditions. Linear diffraction theory and empirical correction factors are typically used in the early design phase of a project in which wave run-up is a concern. As the project nears final design, model tests are usually used to assess wave run-up and air gap requirements. This paper addresses the use of alternative methods for prediction of run-up around a GBS in approximately 100 m water depth. Results from a second-order diffraction code (WAMIT) and a fully nonlinear CFD program (ComFLOW) are compared to assess the importance of nonlinearities, which are shown to depend on incident wave steepness and wavelength. Extending diffraction theory to second-order significantly improves linear predictions and produces more realistic spatial patterns of maximum run-up. However CFD simulations are required to accurately predict run-up associated with very steep incident waves and highly nonlinear characteristics. In addition to regular wave computations, linear and second-order potential flow calculations are also compared against model test results for an irregular sea.
Methodology to Evaluate Slamming Loads on Ocean Platforms
Authors: Maia Junior Marcio Domingues , Fernandes Antonio Carlos , Trindade Marcela , Ramiro Andre
Published: July 2009

The purpose of the study is suggest a methodology to be applied in ocean platforms and ships in order to appraise the maximum impact pressure due to the slamming occurrence in the hull shape near its bottom or horizontal regions. This methodology uses a theory based on potential flow. However, there are some phenomena such as creation of a compressible air pocket between the body and free surface at the impact moment that requires a more complete theory and or experimental methods. This gives rise to experimental coefficients to reduce the theoretical errors. The procedure presented here goes by the platform motion dynamics and “impact topology” to allow the potential to be used. Due to the complexity of the phenomenon studied and need for certifying accuracy and precision of the results, tank tests at the LabOceano model basin were carried out. The results showed a good fitting between numerical results and experiments. It should also be pointed out that the pressure sensor used in these experiments gives a pressure distribution over the instrumented area what brings more reliability on the results and a better visibility to the slamming phenomenon. Lastly the methodology in this work stands out as an important tool to evaluate slamming loads.
A Comparative Numerical Study on the Bending Response of Aluminium Stiffened Plates With Fixed/Floating Frames
Authors: Khedmati Mohammad Reza , Rastani Mehran
Published: July 2009

In this paper, different structural arrangements of the transverse frames in an orthogonally stiffened plate are investigated from the bending response point of view. The transverse frames are assumed to be either fixed or floating. Other alternate placements of the transverse frames are also included in the comparative calculations. Stress and deflection contours are obtained via finite element analyses. Finally, some recommendations are outlined comparing the results of maximum stress and deflection with the allowable limits.
Spatial vs. Temporal Evolution of Nonlinear Wave Groups: Experiments and Modeling Based on the Dysthe Equation
Authors: Shemer Lev , Dorfman Boris
Published: July 2009

The evolution along the tank of unidirectional nonlinear wave groups with narrow spectrum is studied both experimentally and numerically. Measurements of the instantaneous surface elevation within the tank are carried out using digital processing of video-recorded sequences of images of the contact line movement at the tank side wall. The accuracy of the video-derived results is verified by measurements performed by conventional resistance-type wave gauges. An experimental procedure is developed that enables processing of large volumes of video images and capturing the spatial structure of the instantaneous wave field in the whole tank. The experimentally obtained data are compared quantitatively with the solutions of the modified nonlinear Schro¨dinger (MNLS, or Dysthe) equation written in either temporal or spatial form. Results on the evolution along the tank of wave frequency spectra and on the temporal evolution of the wave number spectra are presented. It is demonstrated that accounting for the 2nd order bound (locked) waves is essential for getting a qualitative and quantitative agreement between the measured and the computed spectra.
Effect of Transformation Temperature of Weld Metal on Welding Distortion
Authors: Morikage Yasushi , Kubo Takahiro , Yasuda Koichi , Mikami Yoshiki , Mochizuki Masahito , Watanabe Hirohisa
Published: July 2009

Welding distortion during welding is an inevitable outcome of weld metal shrinkage due to non-uniform heating and cooling in welded joints. Several studies have investigated methods of estimating and controlling welding distortion in order to construct steel structures correctly, easily and efficiently. In the present study, the effect of transformation expansion of the weld metal on welding distortion was investigated in order to develop a welding wire that reduces welding distortion. Concretely, experiments and numerical simulations were performed on the effect of the transformation temperature of the weld metal on angular distortion of a horizontal fillet welded joint. The results showed that angular distortion of the welded joint decreases when the martensitic transformation start temperature of the weld metal decreases in the range from 350°C to 450°C. Based on this result, a welding wire that is effective in reducing welding angular distortion was developed considering usability, weldability and the mechanical properties of welded joints.
New Wake Models With Capability of Capturing Nonlinear Physics
Authors: Marzouk Osama A. , Nayfeh Ali H.
Published: July 2009

We developed reduced-order models for the wake (in terms of the amplitude of the induced lift force) of a forced circular cylinder using analytical techniques. We first considered several forced oscillators (existing and proposed) and evaluated their suitability to model the synchronized wake, where the lift frequency is synchronized with the forcing frequency. These forced oscillators differ in the nature of the forcing terms used to represent the influence of the cylinder motion on the lift. Existing wake oscillators for a moving cylinder do not capture the important nonlinear phenomena, such as the multi-valuedness and the abrupt changes that occur as the parameters (e.g., the reduced velocity) change. These phenomena occur when one mode of the wake loses stability through a bifurcation to another mode. We then proposed two new oscillators that are capable of generating the correct wake response over the entire frequency range of synchronization even across the discontinuity. The forcing terms are a combination of external and parametric terms. Current models use only external (ideal) forcing, which we show to be insufficient. We then investigated the effect of the oscillators parameters on the modeled lift.
Bivariate Distributions of Significant Wave Height With Characteristic Wave Steepness and Characteristic Surf Parameter
Authors: Myrhaug Dag , Fouques Se´bastien
Published: July 2009

The paper provides a bivariate distribution of significant wave height and characteristic wave steepness, as well as a bivariate distribution of significant wave height and characteristic surf parameter. The characteristic wave steepness in deep water is defined in terms of the significant wave height and the spectral peak period, and is relevant for e.g. the design of ships and marine structures. The characteristic surf parameter is given by the ratio between the slope of a beach or a structure and the square root of the characteristic wave steepness in deep water. The characteristic surf parameter is used to characterize surf zone processes and is relevant for e.g. wave run-up on beaches and coastal structures. The paper presents statistical properties of the wave parameters as well as examples of results typical for field conditions.
On the Assessment of Redundancy of Ship Structural Components
Authors: Blagojevic´ Branko , Zˇiha Kalman
Published: July 2009

This paper firstly sums up some of the views on structural redundancy with particular emphasis on ship and marine structures. Next, it places the engineering decision process in the event space and, consequently, applies the representation of operational modes by systems of events. Furthermore, the paper takes some of the relations from the entropy concept in information theory. The entropy concept in probability theory is employed in the paper to redefine the structural redundancy in terms of conditional entropy of operational modes. The redundancy modeling is presented by systems of operational modes in which some of the transitive events may lead to new operational states. Finally, a ship substructure example of a stiffened panel with a girder is elaborated. The conclusion supports the thesis that the efficient structural redundancy can be comprehended as the most uniform distribution of the operational modes probabilities. Moreover, the efficient structural redundancy can be maximized.
Dense Ocean Floor Network for Earthquakes and Tsunamis Around the Nankai Trough Mega Thrust Earthquake Seismogenic Zone in Southwestern Japan: Real Time Monitoring of the Seismogenic Zone
Authors: Kaneda Yoshiyuki , Kawaguchi Katsuyoshi , Araki Eiichiro , Matsumoto Hiroyuki , Nakamura Takeshi , Kamiya Shinichiro , Ariyoshi Keisuke , Hori Takane
Published: July 2009

The Nankai Trough is well known as the mega thrust earthquake generating tsunamis, with the interval of 100–150 years. The 1944 Tonankai and the 1946 Nankai earthquakes around the Nankai trough, each hypocenter was located off the Kii peninsula. Based on structural and simulation researches, we proposed and have been starting to deploy the dense ocean floor observatory network system around the Tonankai seismogenic zone, to monitor crustal activities using broadband seismometer, accelerometer and precise pressure gauges. The ocean floor network is significant ant important to monitor the crustal activities around mega-thrust earthquakes.
Overview of the European FITNET Fitness-for-Service Procedure
Authors: Hadley Isabel , Koc¸ak Mustafa
Published: July 2009

The European FITNET consortium was convened in 2002 with the remit of preparing a procedure for fitness-for-service (FFS) evaluation of flawed engineering components. The procedure is intended to be used by a broad range of industries across Europe, and can be used at any stage in the life of an engineering component, eg design, fabrication, operation, failure analysis or life extension. This paper presents an overview of the structure of the procedure. There are four main modules, each covering a particular failure or damage mechanism: fracture, fatigue, creep and corrosion (including environmentally-assisted cracking). These are linked by the use of a common terminology and a single set of reference compendia (annexes), eg for stress intensity, plastic collapse and residual stress, so that a particular flaw can be rapidly analysed for more than one failure mechanism. The FITNET fracture assessment procedures in particular represent a significant advance compared with current published FFS procedures such as API 579-1/ASME FFS-1 and BS 7910. There is a hierarchy of different approaches, designated Options 0 to 5, the choice between them depending on the quality of information (in particular, materials property data) available to the user. This could range from Charpy and tensile data only (Option 0) through to the constraint-dependence of fracture toughness (Option 5). Other Options allow crack driving force to be calculated directly from FEA (Option 4), or permit weld metal strength mismatch to be taken into account (Option 2). The fatigue analysis module likewise contains several alternative approaches, termed Routes. Some (Routes 1–3) are based on the concept of a nominally flaw-free structure, whilst Route 4 is based on cycle-by-cycle integration of the Paris law, and Route 5 addresses non-planar flaws. FITNET also set itself the goal of providing training in FFS techniques, both through a series of seminars held during the project (2002–2006) and through provision of lasting training material (slides, tutorials, case studies and a validation document), which are now publicly available.
A Methodology for the Analysis of Physical Model Scale Effects on the Simulation of Wave Propagation up to Wave Breaking: Preliminary Physical Model Results
Authors: Fortes Conceic¸a˜o , Neves Maria da Grac¸a , Santos Joa˜o Alfredo , Capita˜o Rui , Palha Artur , Lemos Rute , Pinheiro Liliana , Sousa Isaac
Published: July 2009

This paper describes the experiments performed at the National Laboratory for Civil Engineering (LNEC) aiming at simulating, in a flume, the wave propagation along a constant slope bottom that ends on a sea wall coastal defence structure, a common structure employed in the Portuguese coast. The objective of these tests is to calibrate the parameters of FUNWAVE, a Boussinesq type model, for wave propagation in coastal regions. This is the first step in the validation of a methodology to combine numerical and physical models in the study of the interactions between beaches and structures. This work is performed in the framework of the Composite Modelling of the Interactions between Beaches and Structures (CoMIBBs) project, a joint research activity of the HYDRALAB III European project.
Energy Conversion OWC Devices With Additional Vertical Ducts
Authors: Camporeale Sergio M. , Filianoti Pasquale G. F.
Published: July 2009

In breakwaters embodying an OWC connected to the sea through a vertical duct or through a small opening, the oscillations of the water column are due to the wave pressure acting on the outer opening of the vertical duct or on the small opening. In fact, in neither of the two cases waves can enter the plant, like it happens in conventional OWCs. The additional vertical duct extends along the wave-beaten wall, giving the device the characteristic form of a U-conduit; for this reason they were also named U-OWC. Experiments on a small-scale breakwater embodying a U-OWC were carried out in the natural laboratory of Reggio Calabria. The plant is a 1:10 scale model of a breakwater suitable for the North-East Pacific coast. The paper describes new experiments on the U-OWC device connected to a monoplane Wells turbine. During an intensive measurement campaign, more than 260 sea states, 5 min long were recorded in order to characterize the energy conversion process. From the experiments, the analysis of the energy conversion shows that: 1) the system is able to absorb a share up 80% of the incident wave energy; this result is similar to that obtained in previous experiments carried out without the turbine; 2) a large fraction of the energy entering the U-OWC is converted in pneumatic power acting on the turbine, being head losses in the water flow limited; 3) the efficiency of conversion of the pneumatic power in turbine power is relatively low (about 36% of the pneumatic power) due to the small dimensions of the turbine that lead to low Reynolds number and large influence of secondary losses.
Full Stochastic Fatigue Analysis of Stiffened Panels Subjected to Wave Slamming
Authors: Schmidt Dilnei , Bardanachvili Carlos A. , Videiro Paulo M.
Published: July 2009

The aim of the present work is the evaluation of the fatigue lifespan of a horizontal stiffened panel of an offshore structure subjected to random wave slamming. In order to achieve this objective, a methodology for time-domain slamming prediction was developed. Time series for the relative distance and velocity between the bottom panel and sea surface and the angle between the panel and the sea surface are simulated to provide a way to evaluate when the slamming occurs and the loads associated with slamming, using factor calibrated after a model test on ocean basin. With the loads obtained from these simulations, the structural analysis is then performed, considering all the shell plating, main stiffeners as well as other main supporting structures, by using a finite element structural analysis. The fatigue lifespan is estimated in a complete stochastic analysis, considering all possible sea states during the lifetime of the offshore structure as well as each probability of occurrence associated. All phases of the methodology used for the evaluation of the slamming loads and the fatigue analyses are presented.
A Wave Impact Parameter
Authors: Stansberg Carl Trygve
Published: July 2009

Critical events in random wave trains that can lead to impact on offshore structures are addressed. Experience from model testing has shown that impact is correlated with steep and energetic waves, characterized by high crests, wave heights, orbital velocities, slope, or a combination of all. A new impact alert parameter derived directly from a wave record, unifying these properties in a physically consistent way, is proposed. Hilbert transform analysis is applied. The analysis is demonstrated through application on numerical and laboratory wave records. Impact responses from model tests with floating offshore structures in steep storm sea states are compared to predictions made by the new simple alert parameter derived from the calibrated wave records. It is found that a large majority of events identified by the new parameter turn out to be real critical events also in the response records.
Flexible Riser Fatigue Re-Evaluation to Extend the Service Life
Authors: Lemos Carlos A. D. , Sousa Fernando J. M. , Sousa Jose´ R. M.
Published: July 2009

Some PETROBRAS fields are near mature now, around 15 years of production, and their production still important to the company portfolio, the possibility of extending the service life of these flexible risers becomes extremely attractive. This work addresses the re-evaluation of the fatigue life of old flexible risers aiming to extend their fatigue life at the same environment conditions or at new and less challenging ones. To fulfill this condition a special procedure is being applied to stretch the service life of the installed flexible risers, considering irregular wave analysis conditions, distributions of damage around the circumference and along the bend stiffener area and phase difference between tension and bending and in some cases a coupled analysis of the ship, mooring and risers systems. This kind of new fatigue procedure could also become of paramount importance to Petrobras to allow the design of conventional flexible risers for ultra deep waters.
Accumulated Plastic Strain Evaluation of Steel Tube for Umbilical Cables
Authors: dos Santos Marcelo Corac¸a , Balena Rosianita , de Paula Fernando , Smith Matt
Published: July 2009

This paper presents the tests performed for the qualification of the level of accumulated plastic strain (APS) to be respected by manufacturing and installation of SAF 2507 steel tube umbilicals. The manufacturing and installation processes for steel tube umbilicals are carried out through repetitive cycles of coiling and uncoiling the tubes or umbilical during manufacturing. During installation the umbilical is also coiled and uncoiled in installation reels or vessel’s baskets (turn tables or carrousels) as well as straightened and bent under tension over sheaves or chutes. These operations impose tensile and compressive strains to tube, which means that the tube is submitted to work hardening and have its mechanical properties modified to a certain extent during its manufacturing and installation processes. Therefore, it becomes necessary to quantify the level of APS expected to occur during these processes and compare it to the maximum level of APS qualified for the tube material being used in order to ensure that the mechanical properties of the steel tube are maintained at a high quality level. The scope of this paper is to present the tensile and fatigue tests performed by Marine Production Systems do Brasil (Brazilian Oceaneering Factory) in strained and unstrained tubes and define a maximum acceptable level of APS. Both strain hardening and fatigue life influence effects are assessed. Based on the results of these tests, the maximum acceptable level of APS to be respected during the manufacturing and installation of umbilicals was defined and associated with this level, the S-N curve determined by the tests must be used for the umbilical service life fatigue analysis.
Extreme and Freak Waves: Results of Measurements and Simulation
Authors: Lopatoukhin Leonid J. , Boukhanovsky Alexander V.
Published: July 2009

Main statistical characteristics of wave climate are considered in respect of offshore and ship design. Sophistication of ships and marine platforms and expansion of offshore activities to non-investigated regions means increasing of probability of being damaged by high waves. Hindcasting of wave fields, using the hydrodynamic models is main approach to wave climate investigation. Offshore wave measurements are used, mainly, for model verification. In compliance with existent regulatory documents and accepted practice applied statistical characteristics of wind waves are prescribed to operational and extreme. Operational statistics describe wind and wave conditions for the life span of a ship or an offshore structure. Extreme characteristics determine the so-called “structure survival regime”. There are a lot of approaches to calculations of extreme wave heights at a point (classical unconditional extremes). Their comparison shows the advantages and disadvantages of each of them. Freak (rogue) waves have some principal difference from extreme wave, mainly due to their form and asymmetry. In this sense freak wave is a multidimensional extreme. Contaminated distribution may be used for probability density approximation of joint extreme and freak wave. The example of recent freak wave event is the loss of ship “Aurelia” (Class of Russian Register of shipping) in February 2005 in the North Pacific. “Aurelia” sunk during passing of atmospheric front with veering wind, changing wind waves. Any wave has at least three dimensions: height, length, and crest length. The last parameter in mean is 3 times greater than wave length. Any information about three dimensional waves is of interest, as such measurements are unique. Some results of unique stereo wave measurements in the South Pacific where the wave as high as 24.9m was fixed (probably still almost the highest measured in the World Ocean), is presented and discussed.
Achievements in Applications of Marine Hydrodynamics
Authors: Nestega˚rd Arne , Hansen Vigleik , Rognebakke Olav , Cerup-Simonsen Bo
Published: July 2009

The paper gives a review of the achievements in the applications of marine hydrodynamics to problems in offshore and maritime engineering. In particular focus will be on numerical methods for analysis of ships and floating production systems and how the work of Professor Newman has influenced the development of computational tools for use by the industry. The paper also presents some recent applications of such numerical tools for analysis of fixed and floating production systems, seakeeping of ships with forward speed including problems related to sloshing and moonpool dynamics. The challenges for each of these applications will be discussed from a physical viewpoint and how the numerical tools are applied to solve the problems. Validation of computer tools in light of their intrinsic limitations will be discussed. Finally the paper presents some of the most important engineering challenges today with respect to modelling and simulation of loads and response of ships and floating production systems.
On the Modeling of Nonlinear Waves for Prediction of Long-Term Offshore Wind Turbine Loads
Authors: Agarwal P. , Manuel L.
Published: July 2009

In the design of wind turbines—onshore or offshore—the prediction of extreme loads associated with a target return period requires statistical extrapolation from available loads data. The data required for such extrapolation are obtained by stochastic time-domain simulation of the inflow turbulence, the incident waves, and the turbine response. Prediction of accurate loads depends on assumptions made in the simulation models employed. While for the wind, inflow turbulence models are relatively well established, for wave input, the current practice is to model irregular (random) waves using a linear wave theory. Such a wave model does not adequately represent waves in shallow waters where most offshore wind turbines are being sited. As an alternative to this less realistic wave model, the present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on an offshore wind turbine, with a focus on the fore-aft tower bending moment at the mudline. We use a 5MW utility-scale wind turbine model for the simulations. Using, first, simpler linear irregular wave modeling assumptions, we establish long-term loads and identify governing environmental conditions (i.e., the wind speed and wave height) that are associated with the 20-year return period load derived using the inverse first-order reliability method. We present the nonlinear irregular wave model next and incorporate it into an integrated wind-wave-response simulation analysis program for offshore wind turbines. We compute turbine loads for the governing environmental conditions identified with the linear model and also for an extreme environmental state. We show that computed loads are generally larger with the nonlinear wave modeling assumptions; this establishes the importance of using such refined nonlinear wave models in stochastic simulation of the response of offshore wind turbines.
Performance Characteristics of a Tightly Moored Piston-Like Wave Energy Converter Under First- and Second-Order Wave Loads
Authors: Mavrakos Spyros A. , Katsaounis George M. , Chatjigeorgiou Ioannis K.
Published: July 2009

The paper deals with the presentation of a model to predict performance characteristics of a tightly moored piston-like wave energy converter which is allowed to move in heave, pitch and sway modes of motion. The WEC’s piston-like arrangement consists of two floating concentric cylinders, the geometry of which allow the existence of a cylindrical moonpool between the external cylinder, the ‘torus’ and the inner cylinder, the ‘piston’. The first-order hydrodynamic characteristics of the floating device, i.e. exciting wave forces and hydrodynamic parameters, are evaluated using a linearized diffraction-radiation semi-analytical method of analysis that is suited for the type of bodies under consideration. According to the analysis method used, matched axisymmetric eigenfunction expansions of the velocity potentials in properly defined fluid regions around the body are introduced to solve the respective diffraction and radiation problems and to calculate the floats’ hydrodynamic characteristics in the frequency domain (Mavrakos et al. 2004, 2005). Based on these characteristics, the retardation forcing terms are calculated, which account for the memory effects of the motion. In this procedure, the coupling terms between the different modes of motion are properly formulated and taken into account (Cummins, 1962; Faltinsen, 1990). The floating WEC is connected to an underwater hydraulic cylinder that feeds a hydraulic system with pressurized oil. The performance of the system under the combined excitation of both first- and second order wave loads is here analyzed. To this end, the diffraction forces originated from the second order wave potentials are computed using a semi-analytical formulation which, by extension of the associated first-order solution, is based on matched axisymmetric eigenfunction expansions.
Effects of Aging on the Mechanical Properties of Pipeline Steels
Authors: Hukle Martin , Newbury Brian , Lillig Dan , Regina Jonathan , Horn Agnes Marie
Published: July 2009

The intelligent design of a given pipeline system intended for operation beyond the elastic limit should incorporate specific features into both the base material (line pipe) and girth weld that enable the affected system to deform safely into the plastic regime within the intended strain demand limits. The current paper focuses on the mechanical properties known to influence the strain capacity of the base material (i.e., line pipe steel independent of the girth weld). Line pipe mechanical properties of interest include: longitudinal yield strength, tensile strength, yield to tensile strength ratio, reduction of area, elongation and uniform elongation. Of particular interest (in consideration of the conventional thermally applied corrosion protection coating systems to be employed), are the longitudinal mechanical properties in the “aged” condition. The present study investigates six (6) different pipeline steels encompassing grades X60 (415 MPa) to X100 (690 MPa), and includes both UOE Submerged Arc Welded - Longitudinal (SAW-L) and seamless (SMLS) forming methods.
Sea State Statistics and Extreme Waves Observed by Satellite
Authors: Lehner S. , Schulz-Stellenfleth J. , Ko¨nig Thomas , Li X.
Published: July 2009

For the design of ships as well as for the investigation of ship accidents it is important to have knowledge about both the two dimensional spectral wave properties as well as extreme value statistics of ocean waves. Although numerical wave models have reached a high level of accuracy, they still have weaknesses with respect to the details of the 2-D wave spectrum. Furthermore standard models like WAM provide estimates of the 2-D wave spectrum, i.e., second order sea state statistics and therefore lack information on individual wave properties and the occurrence of extreme events. In this study the potential of global Synthetic Aperture Radar (SAR) wave mode data acquired by the European satellites ERS-2 and ENVISAT to investigate ship accidents is discussed and compared to altimeter data and ECMWF model results. These data are acquired independent of light and weather conditions on a global scale. A historic data set of ERS-2 wave mode data acquired between 1998 and 2000 is co-located with accidents which occurred during that time. ENVISAT ASAR wave mode data acquired since 2002 are considered, too. Different ocean wave parameters like significant wave height and wave periods are derived from the SAR data. The potential role of the respective wave conditions for some recent accident is discussed in detail. This includes in particular the analysis of cross sea conditions, groupiness and extreme events.
Dynamic Simulation of Immersion of Tunnel Elements for Busan – Geoje Fixed Link Project
Authors: Chakrabarti Partha , Chakrabarti Subrata K. , Olsen Tommy , Sig Koo Im , Whan Kim Chang , Wook Huh Jin
Published: July 2009

Construction of an 8.2km motorway between Busan — Korea’s southernmost, second largest city — and the island of Geoje, called the Busan-Geoje Fixed Link Project, is currently underway. As part of this four-lane fixed link, there will be a submerged tunnel section of 3.2km in length. COWI and Daewoo Engineering Co. have formed a joint venture (CDJV) to carry out the design of the immersed tube tunnel (Busan-Geoje Fixed Link Immersed Tunnel). The Immersed Tunnel consists of 18 Tunnel Elements and will be constructed from East to West. The immersed tunnel will become one of the longest in the world to date, especially in an area of significant seismic activity. Each of the 18 elements is identical, although they will be placed in water depths varying from 15m up to 50m subjected to waves and current. The prefabricated tunnel section will be brought to the site in a submerged condition, where it will be suspended from immersion rigs by four wire ropes. Each tunnel section will be lowered and connected to the previously installed element. The immersion rig pontoons and the submerged tunnel element will be moored to stay in position using as many as 14 wire ropes. This paper describes the unique immersion concept specially developed for this project, the salient features of each phase of analysis and the major conclusions. The findings in this study are expected to aid designers of similar immersion process concepts to ensure safe installation.
A Joint Probability Model for Environmental Parameters
Authors: Sagrilo Lui´s Volnei Sudati , de Lima Edison Castro Prates , Papaleo Arnaldo
Published: July 2009

The joint probabilistic models (JPM) of the environmental parameters of wave, wind and current are nowadays extremely needed in order to perform reliability analyses of offshore structures. These JPM are also essential steps for the design of offshore structures based on long-term statistics and to perform dynamic response analysis of floating units that are strongly dependent on the directionality of the environmental actions, such as turret-moored FPSOs. Recently, some JPM have been proposed in the literature to represent the joint statistics of a reduced number of environmental parameters. However, it is difficult to find a practical and fully operational model taking into account the complete statistical dependence among all the environmental parameters intensities and their correspondent directions. In this paper, it is presented a straightforward methodology, based on the Nataf transformation, to create a JPM of the environmental parameters taking into account the dependence between the intensity and direction of all variables. The proposed model considers the statistical dependence of ten short-term variables: the significant wave height, peak period and direction of the sea waves, the significant wave height, peak period and direction of the swell waves, the amplitude and direction of the 1-h wind velocity and, finally, the amplitude and direction of the surface current velocity. The statistical dependence between them is evaluated using concepts of linear-linear, linear-circular and circular-circular variables correlation. Some results of the proposed JPM methodology are presented based on simultaneous environmental data gathered in a location offshore Brazil.
The Design and Implementation of FPSO Mooring Management System
Authors: Ma Gang , Sun Liping
Published: July 2009

Now FPSO (Floating Production Storage and Offloading) has been widely used in exploitative engineering of offshore petroleum/gas reservoir and becoming one of the most important production facilities. FPSO which is a floating structure has the virtue of sea keeping, permanent mooring, large oil storage, offshore unloading and recursive and so on. FPSO can be used in lots of kinds of oil fields. The Paper mainly introduces software FPSO Mooring Management System which is designed for CNOOC Oil Base Group. There are 17 FPSO serving in China Sea area, so that it is becoming more and more serious on FPSO mooring system. The main failures of mooring system are as follows: broken of main bearing, fracture of YOKE transverse beam, crack of anchor chain, leak of swivel head, local damage under typhoon etc. FPSO Mooring Management System is designed to track and control FPSO mooring situation and to keep detailed information on FPSO mooring management, operation, maintenance etc. This system puts chief data of 136 FPSO together which are serving, under construction, maintaining in the world. This system also provides technical support for company decision. FPSO Mooring Management System is made with JSP, using ORACLE as database, so it can run at any operating system. This system offers access to upload folders and EXCEL files to database to save time and decrease errors in managing lots of data. There are a lot of kinds of files which should be classified by subject. In order to satisfy the need to get different united information not only by certain classification, this system is designed with open structure, it allows users to store files by any classification and own the power to change it. The system offers different searching ways, by which user can get what they want exactly, then the transfers corresponding programs in the computer to play these files. This system meets the requirement of users to get what they want efficiently.
Innovative Ice Protection for Shallow Water Drilling: Part III — Finite Element Modelling of Ice Rubble Accumulation
Authors: Gu¨rtner Arne , Konuk Ibrahim , Gudmestad Ove Tobias , Liferov Pavel
Published: July 2009

The concept of the Shoulder Ice Barrier (SIB) has previously been presented in a companion paper under the same title at OMAE 2006 (Gu¨rtner et al., 2006), whereas ice model tests of the SIB are presented in an accompanying paper at this year’s OMAE (Gu¨rtner and Gudmestad, 2008). The present paper investigates a computational model for simulating ice-SIB interactions. This involves the simulation of rubble accumulations and accordingly the exerted ice forces. The computational model is developed within the framework of finite elements. Characteristic fracture of ice is handled by introducing the Cohesive Zone Approach (CZA), wherein cohesive elements are placed in-between the finite element grid of the ice. Fracture may thereby occur along element boundaries with due regard to fracture properties such as traction and separation. Fracture and plastic deformation of the ice are hence co-existing, though competing, mechanisms, while accounting for the dynamics of the ice mass. We compare the computational results with the ones obtained during the model testing.
Numerical Simulations of Integrated Aircraft-VLFS-Water Interaction Systems Excited by Aircraft Landing Impacts
Authors: Xing J. T. , Jin J. Z.
Published: July 2009

The developed mixed mode function-boundary element method to analyse the dynamic behaviour of integrated aircraft-VLFS-water interaction systems exited by impacts of aircraft landing is summarised and further examined by numerical investigations. In this mathematical model, the floating structure and aircraft are considered as two elastic bodies with different complex elastic modulus to represent their respective internal material damping properties. The landing gear of the aircraft is modelled by three support units each of which consists of a spring and a damper. The coefficients of the spring and the damper can be obtained using the experimental data. The water is assumed incompressible, inviscid and satisfying a linear free surface wave condition and an undisturbed condition at infinity. Governing equations are derived to consider the transient dynamic interactions between aircraft, VLFS and water. Based on a substructure approach, the motions of aircraft and floating structure are described by their respective mode functions. The fluid domain is modelled using a boundary element approach. The numerical matrix equation describing the integrated dynamics of this complex interaction system is developed. The numerical method and the corresponding computer code are provided. Examples are presented to illustrate and demonstrate the mathematical model and the numerical method.