bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluating Constant DC-Link Operation of Wave Energy Converter
Authors: Ekström Rickard , Kurupath Venugopalan , Boström Cecilia , Waters Rafael , Svensson Olle , Leijon Mats
Published: August 2013

A Wave Energy Converter (WEC) based on a linear generator and a point-absorbing buoy has been developed at Uppsala University. Interconnecting an array of WECs in parallel requires a point of common coupling, such as a common DC-bus. The DC voltage level seen by the generator is directly linked to the electromagnetic damping of the generator. A lower DC-level results in a higher damping factor and is important for increased absorption of the wave power. The drawback is increased losses in generator windings and cable resistance. There will be an optimal DC-level for maximum power output. This is a function of not only generator and buoy characteristics, but the current sea state. Experimental results of the full-scale system have been carried out, and used as validation of a simulation model of the system. The model is then used to evaluate how the DC-level seen by the generator influence the power output. The results indicate that higher DC-levels should be used at higher sea states, and power output may vary by up to a factor five depending on which DC-level is chosen.
Worldwide Approximations of Current Profiles for Steel Riser Design: The WACUP Project
Authors: Prevosto Marc , Forristall George Z. , Jeans Gus , Herry Christelle , Harte Gavin , Harrington-Missin Liam , Dooley Patrick
Published: August 2013

This paper describes the objectives and methodology of the WACUP (Worldwide Approximations of Current Profiles) Joint Industry Project. The project goal is to establish best practice for reducing large current profile databases into a smaller, representative set of profiles for riser design. We studied the use of the following three techniques for comparing and reducing measured in-situ databases: Empirical Orthogonal Function (EOF), Self Organizing Maps (SOM) and classical Current Profile Characterisation (CPC). We evaluated the skill of these three techniques in estimating Vortex Induced Vibration (VIV) fatigue damage to a Steel Catenary Riser (SCR). Our measure of skill was the comparison of the results from the reduced data sets with results from the original complete database. The standard techniques were modified and accuracy was improved. The role of numerical current models to complement in-situ measurements in riser design was also assessed.
Study of Fatigue Strength of Copper Conductor Considering Irregularities Surfaces by Experimental Testings and FE-Analysis
Authors: Nasution Fachri P. , Sævik Svein , Gjøsteen Janne K. Ø.
Published: August 2013

The objective of this work was to investigate the fatigue performance of a 95 mm2 copper conductor. The compacting procedure applied during the manufacturing process causes geometrical irregularities in the wires. Specimens from different layers have been tested in tension-tension mode with a stress-ratio, R = 0.1. The irregularities have been measured and through numerical studies applied to assess the resulting bending stresses in tension loading. When this is accounted for, the fatigue behavior of the core, the inner layer and the outer layer seems to be similar. This indicates that the difference in fatigue data observed between different layers of wires can be explained by surface irregularities. However, this needs to be supported by more data.
An Empirical Nonlinear Model to Estimate FPSO With Extended Bilge Keel Roll Linear Equivalent Damping in Extreme Seas
Authors: de Oliveira Allan C. , Fernandes Antonio Carlos
Published: August 2013

Although FPSO roll damping is well known nonlinear, most of the analysis which depends on platform motion evaluations, as riser, mooring and structural analysis are based on frequency domain approach results and transfer. Due to the nonlinearity, RAOs for roll, for instance, are dependent on FPSO motion amplitude, being different for each sea state of interest. Recent researches, however, have detected a saturation level in roll damping with extended bilge keels, which means a constant damping level for larger rolling amplitudes, where a linear coefficient (viscous based) can be used in platform motion analysis. Based in model tests where this saturation occurs, a nonlinear model was fitted to this damping data in order to predict the roll damping in extreme sea conditions. Those tests have taken into account aspects which have strong influence on viscous damping, loading conditions, hull form and bilge keel characteristics. The nonlinear model implemented could provide a safe level of damping comparing the results with extreme irregular wave model tests, becoming interesting for early design phases of such structures.
Wireless System for Tidal Effect Compensation in the Lysekil Research Site
Authors: Castellucci Valeria , Eriksson Markus , Waters Rafael , Ferhatovic Senad , Leijon Mats
Published: August 2013

This paper describes, firstly, the rope adjustment device for wave energy converters (WECs) to minimize the tidal effect on the electricity production and, secondly, a wireless communication network between point absorbing WECs in the Lysekil Research Site and a computer station at the Department of Engineering Sciences at Uppsala University. The device is driven by a motor that activates when the main water level deviates from the average. The adjustment is achieved through a screw that moves upwards during low tides and downwards during high tides. For the purpose of testing the device in the research site, a wireless connection between the buoy in the sea and a computer on land will be designed. A sensor located close to the research site monitors the sea water level and, every time a significant variation is registered, it sends wirelessly a signal to the data logger that controls the power to the motor. The position of the screw is observed by a second sensor and the measurements are retrieved back to Uppsala via GSM connection. The full scale device is tested in the lab and it is demonstrated to work properly, requiring less than 750 W to lift and lower different loads. Moreover, the wireless communication network is designed and once it will be built, it will allow to recall and store data, send information from one node of the system to another, monitor the proper functioning of the device and modify the control as desired.
SPH Simulations of Ship Propeller Induced Harbour Bed Erosion
Authors: Ulrich Christian , Rung Thomas
Published: August 2013

The paper reports on the predictive prospects of Smoothed-Particle-Hydrodynamics (SPH) for simulations of ship propeller induced scours in harbours. Such erosions represent unpleasant phenomena, especially if they occur close to quay walls, and generate cost intensive counter measures. These measures are usually based on a rather weak background knowledge. SPH simulations can help to analyse the erosional processes and to understand the interaction between ship, water, soil and structure. In the present research, a body-force propulsor model based on the open water characteristics is used to represent the ship’s propeller. The evolution of the liquid and granular phase particles is obtained from an SPH-integration of the continuity and momentum equations. The fluid is considered to be Newtonian and the viscosity of the soil-phase is modelled in line with the Mohr-Coulomb yield stress criterion. Water and soil particles interacting in a suspension layer are assigned to a viscosity that is derived from a Chézy-relation between the shear stress and the local flow velocity. A variable particle resolution strategy is applied to handle large domains, in which the areas around the ship hull demand a fine resolution. A complex full-scale application example included refers to the starting sequence of a container ship propeller.
Multi-Physics SPH Simulations of Launching Problems and Floating Body Interactions
Authors: Ulrich Christian , Rung Thomas
Published: August 2013

The paper reports on the predictive prospects of the Smoothed-Particle-Hydrodynamics (SPH) method for jacket launching and foundation installation simulations. The type of considered applications usually involves the interaction between floating structures (e.g. barges and jackets) as well as structures and seabeds. Such scenarios pose challenges to mesh-based solvers, particle methods like SPH therefore are of advantage. The presented procedure captures floating body motions using a quaternion based 6DOF motion solver. Fluid/Soil interaction is predicted by an adequate suspension model. An efficient parallelization strategy is applied together with a dynamic variable resolution approach to handle large computational domains and structural details requiring a rather fine discretization.
Application of the Contour Line Method for Estimating Extreme Response in Mooring Lines of a Two-Body Floating Wave Energy Converter
Authors: Muliawan Made Jaya , Gao Zhen , Moan Torgeir
Published: August 2013

One of design criteria that have been used for the mooring system design for floating platforms in the oil and gas industry is the Ultimate Limit State (ULS). The 100-year level response in the mooring line should be applied for this ULS design check, which is ideally estimated by taking into account the dynamic mooring line tension in all sea state available in the operational site. This is called a full long-term response analysis using the all sea state approach. However, this approach is time consuming. Therefore, it is proposed to use the contour line method to estimate the 100-year response by primarily studying the short term response for the most unfavorable sea states along the 100-year environmental contour line. Experiences in the oil and gas industry confirmed that the method could give good prediction if the response at higher percentile than the median is used. In this paper, the mooring system of a two-body wave energy converter (WEC) is considered. Since this system involves interaction between two bodies, the estimation of the ULS level response by using the all sea state approach will be even more time consuming. Therefore, the application of the contour line method for this case will certainly be beneficial. However, its feasibility for a WEC case needs to be documented first. In the present paper, the ULS level response in the mooring tension that is predicted by the contour line method is compared with that estimated by taking into account all sea states. It is achieved by performing the coupled time domain mooring analyses using SIMO/Riflex for six cases with different mooring configurations and connections between two bodies. An axi-symmetric Wavebob-type WEC is chosen as the object of investigation and the Yeu site in France is assumed to be the operational site of the WEC. Hydrodynamic loads including 2nd order forces are determined using WAMIT. Finally, the applicability of the contour line method to predict the ULS level mooring tension for a two-body WEC is assessed and shown to yield accurate results with proper choice of percentile level for the extreme response.
Experimental Investigation of Irregular Wave Cancellation Using a Cycloidal Wave Energy Converter
Authors: Siegel Stefan G. , Fagley Casey , Römer Marcus , McLaughlin Thomas
Published: August 2013

The ability of a Cycloidal Wave Energy Converter (CycWEC) to cancel irregular deep ocean waves is investigated in a 1:300 scale wave tunnel experiment. A CycWEC consists of one or more hydrofoils attached equidistant to a shaft that is aligned parallel to the incoming waves. The entire device is fully submerged in operation. Wave cancellation requires synchronization of the rotation of the CycWEC with the incoming waves, as well as adjustment of the pitch angle of the blades in proportion to the wave height. The performance of a state estimator and controller that achieve this objective were investigated, using the signal from a resistive wave gage located up-wave of the CycWEC as input. The CycWEC model used for the present investigations features two blades that are adjustable in pitch in real time. The performance of the CycWEC for both a superposition of two harmonic waves, as well as irregular waves following a Bretschneider spectrum is shown. Wave cancellation efficiencies as determined by wave measurements of about 80% for the majority of the cases are achieved, with wave periods varying from 0.4s to 0.75s and significant wave heights of Hs ≈ 20mm. This demonstrates that the CycWEC can efficiently interact with irregular waves, which is in good agreement with earlier results obtained from numerical simulations.
Environment–Induced Cracking in Weld Joints in Subsea Oil and Gas Systems: Part I
Authors: Dodge M. F. , Dong H. B. , Milititsky M. , Barnett R. P. , Marques V. F. , Gittos M. F.
Published: August 2013

In subsea oil and gas systems, steel linepipes (typically API X65) have been welded to 8630M low alloy steel hubs, using a buttering technique and nickel alloy filler materials (e.g. Alloy 625). After buttering, postweld heat treatment (PWHT) is carried out to temper the heat-affected zone of the low alloy steel. However, while the vast majority of subsea joints provide successful service, a small number of in-service failures has had significant ramifications. During PWHT of the buttered joints, carbon migrates from the ferritic steel to the high alloy ‘austenitic’ weld metal. The resulting microstructures and consequent localised hardening cause sensitivity to hydrogen induced cracking.

Subsea systems employ cathodic protection (CP) using aluminium based anodes in order to prevent corrosion. However, while cathodic protection has proved successful as a means of preventing corrosion of the steel components, complications can arise due to hydrogen charging.

This paper describes the detailed characterisation of such dissimilar interfaces from several joint types, including examples recovered from the seabed after exposure under CP and newly fabricated joints, using electron microscopy, microanalysis and nanohardness testing.
Review of Approximations to Evaluate Second-Order Low-Frequency Load
Authors: de Hauteclocque Guillaume , Rezende Flávia , Waals Olaf , Chen Xiao-Bo
Published: August 2013

The second order low-frequency loads are one of main sources of excitation for moored systems. These loads are usually decomposed into the quadratic part, contributed only by first order quantities and potential part contributed by the second order potentials. In shallow water the second order incoming and diffracted potentials give a significant contribution to the low frequency forces. Therefore, the accuracy on the determination of this parcel of the low-frequency loads is a key issue for the assessment of mooring lines and operability of systems moored in shallow water area, as for example LNG terminals.

Due to the complexity in computing the second order diffraction potential, which would involve a non-homogeneous free surface boundary condition, the so-called Pinkster approximation has been proposed. This approximation is based on the assumption that the major contribution to the potential part of low-frequency loads is given by the second order potential of the undisturbed incoming waves. The methods to compute the wave forces related to the second order potentials are based on scaling of the first order wave induced forces.

Another approximation recently formulated in Chen and Rezende consists of developing the second-order bi-frequency load into a series of different orders of the difference frequency. The potential contribution to the term proportional to the difference-frequency can be evaluated efficiently by involving an integral over a small zone on the free surface around the body.

In the present paper, the existing approximations are revisited and compared to analytical solution of exact second-order load on a vertical cylinder and for the case of floating body (LNG) in shallow water. Some guidelines in the practical use of different approximations will be derived.
First and Second Order Wave-Current Interactions for Floating Bodies
Authors: Monroy Charles , Giorgiutti Yann , Chen Xiao-Bo
Published: August 2013

The influence of current in sea-keeping problems is felt not only for first order quantities such as wave run-ups in front of the structure, but also mainly for second order quantities. In particular, the wave drift damping (which is expressed as the derivative of drift force with respect to the current) is of special interest for mooring systems. The interaction effects of a double-body steady flow on wave diffraction-radiation is studied through a decomposition of the time-harmonic potential into linear and interaction components. A boundary integral method is used to solve the first order problem. Ultimately, a far-field method is proposed to get access to second order drift forces.
Methodology for Determining Remnant Fatigue Life of Flexible Risers Subjected to Slugging and Irregular Waves
Authors: Gundersen Paul , Doynov Krassimir , Andersen Torgrim , Haakonsen Rune
Published: August 2013

Operational experience has shown that flexible risers producing different combinations of oil, gas and water can be subjected to increased dynamic motions due to slugging — a cyclic accumulation of finite volumes of liquids at a low point of the riser (e.g. sag point of a lazy-S riser) until sufficient pressure is built up behind the slug to push the liquids up through the riser. It has been observed that the slug induced dynamic riser motions can cause riser displacements larger than those generated by moderate and some extreme waves in the absence of slugging. A major impact of the slug induced riser motions is the increased fatigue damage of the tensile wires — the cross-sectional component that most frequently defines the fatigue resistance of flexible riser systems. While international standards like ISO 13628-2 & -11 require and recommend that the effects of slug flow on riser response are considered, they provide no guidance on how to practically incorporate potential slugging effects in pipe design or analysis.

A methodology has been developed to determine the remnant fatigue life of a riser subjected to slug induced motions combined with the normally considered vessel motions and wave loading. The methodology is based on using commercially available global and local riser analysis tools. The global analysis tool is used to determine the riser response induced by continuous and regular slug loading combined with loading from different irregular waves, vessel offsets and motions. The slug loading parameters are determined through an iterative process calibrating riser displacements and frequencies with those observed in the field. The local analysis tool is used to determine wire stress transfer functions, which in turn are used to derive wire stress time series from the riser tension and curvature time histories. Stress ranges are identified through rain-flow counting applied on all the calculated stress time series and fatigue performance is estimated using the Palmgren Miner summation of damage using an appropriate wire S-N curve. In a case study, the combined slug and first order wave induced fatigue damage increased by a factor of approximately two compared to the wave induced damage alone.

This methodology can be used for: a) riser fitness for service assessments by bounding the impact of slug-induced riser motions observed in the field, and b) new riser design when slugging parameters are adequately bounded by flow assurance calculations.
Hybrid Spool to Meet Deepwater Hybrid Riser Bundle Tower Critical Design Drivers
Authors: Zhao Tao , Lee Dan , Willis Neil , Qiu Hui , Puliafito Marco
Published: August 2013

“Vertical type” spool configuration is commonly adopted as riser base spool for deepwater field development. With increasing water depths and risers subject to harsher environments, the excessive lower riser assembly motions and slugging induced fatigue damage have emerged as the governing design criteria for deepwater riser base spool design. Conventional vertical spool has inherent shortcomings to handle such design conditions. A new “hybrid” concept is therefore proposed, which extends beyond the “traditional” spool concept by integrating both vertical and horizontal spools into a 3D configuration. It inherits the vertical spool’s strength on handling large spatial expansion and utilise the pipe-soil interaction to dampen the potential resonance caused by slug flow.

Several prototypes have been examined and the hybrid spool expansion-mechanism is discussed together with an optimisation procedure proposed. An advanced FEA technique using both Abaqus/Flexcom has been employed as part of the hybrid spool development, which consists of rigorous 3D dynamic analysis, bespoke non-linear soil interaction model and utilising the global riser dynamic behaviour. An in-house spool automation tool is developed to optimise the iterative analyses required to obtain a satisfactory hybrid spool configuration. This paper described a successful case study in recent deepwater hybrid riser bundle (multibore hybrid riser) tower development project, where the need to accommodate large lower riser assembly motion and slugging fatigue damage are the two main design drivers. This presentation provides a creative insight into this innovative technology.
Application of a Particle Filter-Based Subset Simulation Method to the Reliability Assessment of a Marine Structure
Authors: Guédé Zakoua , Tantar Alexandru , Tantar Emilia , Del Moral Pierre
Published: August 2013

The present study aims at investigating advanced subset simulation techniques, which are based on the theory of particle filter, for the assessment of the failure probability of a marine structure under extreme loading conditions. Three approaches are considered, namely the classical particle filter method, the subset simulation with a branching process and one using the minimum values of the samples as levels. They are, first, intensively applied on a simple example for which a known analytical solution is available, in order to investigate their parameter settings. Then, they are applied, with good performance, using their respective best parameter settings, to the assessment of failure probability of a FPSO subjected to extreme roll motion.
Numerical Simulation of a Floater in a Broken-Ice Field: Part II — Comparative Study of Physics Engines
Authors: Metrikin Ivan , Borzov Andrey , Lubbad Raed , Løset Sveinung
Published: August 2013

Numerical simulation of a floater in ice-infested waters can be performed using a physics engine. This software can dynamically detect contacts and calculate the contact forces in a three-dimensional space among various irregularly shaped bodies, e.g. the floater and the ice floes. Previously, various physics engines were successfully applied to simulate floaters in ice. However, limited attention was paid to the criteria for selecting a particular engine for the simulation of a floater in broken-ice conditions.

In this paper, four publicly available physics engines (AgX Multiphysics, Open Dynamics Engine, PhysX and Vortex) are compared in terms of integration performance and contact detection accuracy. These two aspects are assumed to be the most important for simulating a floater in broken ice. Furthermore, the access to code, documentation quality and the level of technical support are evaluated and discussed. The main conclusion is that each physics engine has its own strength and weaknesses and none of the engines is perfect. These strength and weaknesses are revealed and discussed in the paper.
Development of a Wing Section Design Code Including Inviscid/Viscous Interaction
Authors: Steinbach Christoph Michael , Krueger Stefan
Published: August 2013

For wing design purposes the value of maximum lift angle is an important quantity. At the high Reynolds Numbers in naval architecture flows the onset and development of turbulent separation is the deciding value for the maximum lift angle. For the calculation of separated turbulent flows usually fully viscous flow solvers, like e.g. Reynolds averaged Navier Stokes (RANS) Solvers, are used. Instead of this kind of solvers, which are expensive by means of computational time, also interacting boundary layer (IBL) methods can be used. Due to the viscous-inviscid coupling, these methods are able to compute flows with limited separation up to the maximum lift angle and represent a cheap and robust alternative to higher value viscous solvers. In this paper a turbulent boundary layer method solving the integral momentum equation together with the integral energy equation of the boundary layer in an inverse formulation is described. The method is combined with an existing inviscid flow solver for 2D wing section flows and a laminar boundary layer method code including transition forecast.
On the Collision Safety of a Gas Fuelled 6500 TEU Container Vessel
Authors: Uhlenbrock Santiago , Scholz Benjamin , Würsig Gerd-Michael
Published: August 2013

Scheduled international environmental regulations are resulting in an increasing interest to use Liquid Natural Gas (LNG) as fuel for ships. However, the well known potential of LNG to reduce emissions must not increase the potential risks compared to conventional fuels.

The Maritime Safety Committee has published an Interim Guidelines MSC.285(86) which includes a collision safety rule for gas fuelled vessels which requests a minimal distance of the LNG storage tanks from the ship outer shell (B/5-Rule). However, research work is needed regarding the actual crashworthiness of alternative ship designs with an LNG tank position closer to the ship shell than the B/5-Rule.

A minimum safety concept actually discussed requires that LNG fuelled vessels shall have a collision resistance at least similar to the one of LNG carriers. In the present paper a case study is presented where the collision resistance of a gas fuelled 6500 TEU Container Vessel is investigated based on the GL-Rules I-1-33. The Container Vessel has an alternative design to the B/5-Rule exploiting more cargo hold space for LNG bunckering by placing the LNG tanks at 8% of B from the side shell. The crashworthiness of the Container Vessel is compared to the one of an LNG tanker of approximately the same size.

Numerical computations indicate that the critical collision speed up to penetration of the inner hull is larger for the LNG carrier. For the more relevant case of LNG spill the Container Vessel with alternative design has an increased crashworthiness compared to the LNG tanker. It is shown that the critical collision speed for the Container Vessel is almost twice the one of the LNG tanker in case of an LNG spill.
DYPIC - Dynamic Positioning in Ice: First Phase of Model Testing
Authors: Haase Andrea , van der Werff Solange , Jochmann Peter
Published: August 2013

DYPIC (Dynamic Positioning in Ice) is a research and development project within the MARTEC ERA-NET project of the European Union. Its objective is to contribute to the closure of the gap between DP in open water being an industry standard, and DP in ice which has some extra challenges to tackle.

Two phases of model testing in ice form the back bone of the project and are facilitated by HSVA (Hamburg Ship Model Basin, Germany). The first test phase, which was executed from May to July 2011, involved two different model ships. Both were tested in free floating mode (where the model sailed solely by its own propulsion system) and fixed mode (where the model was connected to a carriage). In the free floating mode the controlling was performed by a prototype DP system scaled to model parameters.

Four different managed ice fields with systematically varied ice concentration and ice floe size were prepared in the ice tank in order to investigate the influence of the relevant parameters. Tests were executed for several velocities and headings with respect to the approaching ice floes.

In the free floating case ice loads on the hull were derived from the measured loads on the thrusters. The behavior of the model ship was captured by the position and heading tracking system Qualisys and several installed video cameras. The fixed mode tests serve well as a reference measurement.

The results will be used to develop a model scale DP system for ice that is adjustable to different kinds of vessels and ice conditions and eventually to develop testing procedures for the assessment of the DP performance of a vessel in managed ice. A second phase of model testing for fine tuning and benchmarking the developed system will be carried out in August 2012.

Within the scope of the paper is the description of the performed tests speaking of test setup and ice conditions. Analyses of results are not covered.
Higher-Harmonic Response of a Slender Cantilever Beam to Fully Nonlinear Regular Wave Forcing
Authors: Bredmose Henrik , Schløer Signe , Paulsen Bo Terp
Published: August 2013

The higher-harmonic response of a vertical cantilever beam to fully nonlinear wave loads is investigated. Such responses are also known as ‘ringing’ and is of practical interest in the context of offshore wind turbine foundations which, in contrast to the classical incidents of ringing at deep water, are placed at intermediate or shallow water.

The purpose of the study is to provide generic results which can be of later use for the interpretation of more complex cases of irregular wave forcing. To this end, the problem parameters are defined and reduced by dimensional analysis. A simple numerical model is proposed, based on linear beam theory and fully nonlinear regular stream function waves. The hydrodynamic forces are determined from the extended Morison equation.

Parametric studies of the response dependence to relative forcing period, wave height and depth are presented and discussed. A central finding of the paper is that for waves of 85% maximum height, the third-harmonic response increases substantially when the depth is reduced from deep-water conditions into intermediate depth.
High Toughness API 5L X70MS Pipe Grade Development for Deep and Ultradeep Water Application
Authors: Ponciano Marcos , Chad Luis Claudio C. , Teixeira Marcelo Jose B. , de Abreu Lima Vinicius R. , Ferreira Helder Heleno , Ribeiro Monica C. Riccio
Published: August 2013

The offshore deep and ultra deep water explorations summed to the distant points of the Brazilian coast bring to national industry important technological challenges. Considering production and transportation need of large volumes of natural gas and oil in a safe condition from offshore installations, the use of large diameter steel pipes produced by UOE-SAWL process is an important alternative and already applied in some important projects. However, due to the hostile environment in which the new fields are placed, it is necessary to develop new products in order to meet the rigorous mechanical properties and Sulfide Stress Cracking (SSC) and Hydrogen Induced Cracking (HIC) resistance requirements. This work presents the evaluation of mechanical properties and SSC resistance of OD 20 inches, WT 25,4 mm APL 5L X70MS line pipe, for both pipe material and girth weld joints.

For this work, plates were produced with very restrict metallurgical control, including chemical control and state of art rolling as Thermo-Mechanical Controlled Process with Accelerated Cooling — TMCP-ACC. Very good CHARPY and CTOD values were achieved at −20°C and −40°C respectively, in both conditions, and the pipes presented good (SSC and HIC) resistance even for the girth weld joint performed with a low and high heat inputs simulating the range of conditions that an operator would find in offshore welding operations.
Calibration of Partial Safety Factors for Suction Caissons in Connection to Catenary and Taut-Leg Mooring Systems
Authors: Silva-González Francisco L. , Heredia-Zavoni Ernesto , Valle-Molina Celestino , Sánchez-Moreno Jorge , Gilbert Robert B.
Published: August 2013

A risk and reliability based calibration of partial safety factors for the ultimate limit state of suction caissons subjected to inclined tension loads from floating production systems is presented. The formulation is for the case of normally consolidated clay deposits with undrained loading conditions.

The load capacity analysis is carried out using the plastic limit model proposed by Aubeny et al. [1–3]. A procedure to calibrate the plastic limit model based on finite element numerical results is described. Line tensions from two mooring systems of an FPSO designed for two sites in deep water at the Bay of Campeche, Gulf of Mexico, are used.

A procedure to characterize probabilistically the load capacity of the caisson at mudline is presented. The physical lower limit of the load capacity and soil-chain interaction are taking into account. The mean and dynamic tensions of mooring lines are modeled through response surfaces in terms of uncertain metocean variables describing extreme sea-states. Reliability analyses are performed using FORM and considering both mooring line tensions and load capacity of the soil-caisson system at the mudline, rather than at the padeye. Partial safety factors for the design of caissons in connection to catenary and taut-leg mooring systems are calibrated separately. This is considered to be most appropriate taking into account their differences in terms of relative contributions of the mean and dynamic tension components, failure mechanisms controlling the loading capacity, and lower bound capacity. Calibration of safety factors is performed for a target reliability index equal to 4.2.
Calibration of Design Conditions Based on Long Term Top Tension for Catenary Risers
Authors: de Sousa Fernando Jorge Mendes , Sudati Sagrilo Luís Volnei , de Lima Edison Castro Prates , Papaleo Arnaldo
Published: August 2013

Response based approaches are not common in riser design. Due to the high computational costs associated to these methodologies, it is usual to replace the calculation of extreme long term responses by the calculation of responses to a few number of artificial sea states, supposed extreme. However, this hypothesis may not always be applicable. The extreme response of a riser is influenced by several factors. For instance, vessel response motions resonance can occur for waves of periods lower than the ones associated to the desired long term period. In this way, this work has two main objectives. The first is to propose a computationally feasible methodology to calculate long term extreme responses; the second is to calibrate loading conditions, based on the long term responses, to be used when designing catenary risers. The parameter selected to represent the response is the centenary (100y) riser top tension. The utilization of the proposed methodology is illustrated by a case study where three possible positions for a turret in a FPSO hull were compared. The obtained results indicate that this methodology can contribute to substantial changes in the way risers are designed, focusing on the response instead of on the occurrence of extreme sea states.
Nonlinear Structural Consequence Analysis of Blast Wall Structures Under Hydrocarbon Explosive Loads
Authors: Sohn Jung Min , Kim Byoung Hoon , Paik Jeom Kee , Schleyer Graham
Published: August 2013

Many accidents that occur on offshore structures, especially explosions, are extremely hazardous. Violent explosions can have serious consequences for health, safety, and the marine environment. The topsides of offshore platforms are the most likely areas to be exposed to hazards such as hydrocarbon explosions. Therefore, profiled barriers are being increasingly used as blast walls in offshore topsides modules to provide a safety barrier for personnel and critical equipment.

The aim of this study is to develop a practical procedure for the nonlinear structural response analysis of corrugated blast walls under explosion. Within the framework of quantified risk assessment and management of offshore installations, more refined computations are required to assess the consequences or hazardous action effects of explosions.

In addition, appropriate guidance will be presented on the use of the finite element numerical tool for the above purpose. The structural response has been computed using commercial nonlinear finite element analysis (NLFEA) code and the results compared with the single degree of freedom (SDOF) method. The relationships between blast pressure and the impulse of corrugated blast walls are developed. This study’s insights into modeling techniques and procedures will be applicable to the explosion risk assessment of offshore structures.
Pipeline Stabilisation Using Pre-Trenching and Sand Backfill
Authors: Chamizo David J. , Campbell Dean R. , Erbirch Carl T. , Jas Eric P. , Cheng Liang
Published: August 2013

Stabilizing large diameter natural gas pipelines on the seabed against extreme hydrodynamic loading conditions has proven to be challenging in the northwest of Australia. Tropical storms, which affect the area annually between November and April, can generate wave heights exceeding 30 m and on-bottom steady-state currents of 2 m/s or more. Consequently, in shallow water depths, typically less than 40–60 m, subsea pipelines can experience very high hydrodynamic loads, potentially causing significant lateral movement. If the seabed is rugged, or at locations where the pipeline approaches a point of fixity, this can lead to the pipeline suffering mechanical damage, which is undesirable.

In many places on the Northwest Shelf of Australia, there is a layer of minimum 3 m deep marine sediments. The sediments predominantly comprise of relatively stable, fine to medium sized carbonate silts and sands, sometimes with some clay content. Traditionally, in Australia and other parts of the world, post-trenching techniques such as ploughing and jetting have been applied in such areas. These techniques can successfully lower the pipeline into the seabed. However, in many situations on the Northwest Shelf of Australia, post-trenching has had limited success. This has in part been due to the unpredictable levels of cementation of the carbonate sand, which has often resulted in an insufficient trench depth, with the need to implement costly and time consuming remedial works to ensure pipeline stability.

The uncertainties in the success of post-trenching tools lead to the development of the pre-trenching and sand backfill method, which was first applied in Australia in 2003 on a 42-inch diameter natural gas trunkline. This technique has several advantages compared to post-trenching and other conventional pipeline stabilization methods such as rubble mound pipeline covers or gravity anchors.

This paper presents an overview of the pre-trenching and sand backfill method, its design principles, benefits, and risks and opportunities.
A Study on Gap Influence on Slamming Experiment
Authors: Park C. W. , Shin J. Y. , Kwon S. H. , Chung J. Y. , Lee S. B. , Yang Y. J. , Jung J. H. , Yoon H. S.
Published: August 2013

This study presents an investigation of gap effect on slamming experiment. Two gaps were considered; a gap between transverse side of a model and tank wall and a gap between longitudinal end and end of a model. The deadrise angle was fixed 0°. The pressure was measured at the central location and compared. Three different drop heights were chosen to see the difference. A High speed camera was used to record the flow field. Therefore pressure at the center, and flow field around corner of the specimen were analyzed to investigate the gap effect in slamming experiment. The results showed that there was clear influence of the gap in slamming experiment.
An Experimental Study on Hydro-Elasticity in Sloshing
Authors: Choi Hu In , Park Jung Ho , Kwon Sun Hong , Lee Ki Hyun , Lee Sang Beom , Yang Young Jun
Published: August 2013

This research presents an experimental investigation on hydro-elasticity in sloshing experiment. Two models were tested. One of the models is ordinary, which is considered to be rigid wall made of plexi-glass. The other model has a flexible wall made of stainless steel. The two models were operated with identical excitation so that the influence of the elasticity can be detected. Four different filling levels were tested. The magnitude of excitation was 10% of the model longitudinal length. The models were operated only in surge motion. The models were tested with several excitation frequencies. The pressures were measured at 12 locations. It was shown that the effect of hydro-elasticity increases the magnitude of impact pressures.
Development of the BS 7910 Failure Assessment Diagram for Strain Based Design With Application to Pipelines
Authors: Smith Simon
Published: August 2013

Engineering Critical Assessment (ECA) uses J estimation schemes to derive the crack tip loading of complex structures to determine their tolerance to crack-like flaws. The methods currently being used were derived in the 1980s for structures with primary stresses below the material yield strength. These are now being extensively used for loads beyond this level for what has been called Strain Based Design (SBD). Some papers have shown the standards BS7910:2005 and R6 Revision 4 can be unconservative when used for SBD. A possible reason has been identified and a suitable modification proposed. The proposed modification is briefly reviewed in the present paper together with a comparison of the method with suitable crack driving force data.
Plasticity Induced Residual Stress in Pipes
Authors: Zhou Daowu , Mirzaee-Sisan Ali
Published: August 2013

An analytical model for predicting residual stress introduced in a pipe (away from girth weld) by plastic bending is examined. Stresses for a specific case where the pipe is bent, reverse bent and straightened are compared with results from a non-linear finite element analysis. The analytical model is then used to determine the through-thickness plasticity induced residual stress in the axial direction for a wide range of pipe geometries which are typically used offshore. It is found that the magnitude of extreme axial residual stress depends significantly on the ratio of pipe diameter to wall thickness. A simplified formula for estimating the extreme fibre tensile residual stress is derived and presented.
Fatigue Analysis of a 6” Flexible Pipe With Broken Tensile Armor Wires
Authors: de Sousa José R. M. , de Sousa Fernando J. M. , de Siqueira Marcos Q. , Sagrilo Luís V. S. , Campello George , Vardaro Eduardo , de Lemos Carlos A. D.
Published: August 2013

This work focuses on a methodology to predict the fatigue life of flexible pipes with wires broken in their tensile armors. Initially, the mechanical behavior of these pipes is discussed. Relying on this discussion, a simple set of equations is proposed in order to calculate the stresses in the armors of these pipes. These equations employ pre-estimated linear coefficients to convert forces and moments that act on the pope into stresses. These stresses are then processed by well-known cycle counting methods and S-N curves are finally used to evaluate fatigue damage at several points in the pipe’s cross section. The use of this methodology is exemplified by the assessment of the fatigue life of a 6” flexible pipe in which 0 up to 5 wires of its outer tensile armor are broken. The results indicate a substantial reduction in the fatigue life of the pipe with the increasing number of wires broken.
Permeability of Marine Sediments and Tropical Volcanic Soils
Authors: Brandes Horst G.
Published: August 2013

Permeability values for a range of fine-grained deep-sea sediments are presented and evaluated in terms of index properties such as plasticity, grain size and carbonate content. It is found that whereas clay-rich sediments have similar permeabilities to those of equivalent land-based fine-grained soils, the presence of volcanic, carbonate and other non-clay fractions tends to increase permeability somewhat. Volcanic silty-clayey soils from Hawaii have comparable permeability values, although they can be slightly more permeable.
The Influence Research of Ultimate Strength to Deepwater Semi-Submersible Platforms Structure Under Corrosion Damage
Authors: Zhang Jian , Shen Zhongxiang , Ji Chunyan , Yin Qun
Published: August 2013

Ultimate strength is an important indicator of the adaptability to changing environment of semi-submersible platforms. As corrosion damage has significant effects on the ultimate strength, the corrosion damage factor should be taken into account in the platforms structural safety evaluation process. The basic theories and researching methods of ultimate strength are expounded and the corrosion forecasting mathematical model is established in this paper. Taking 3000m deep sea semi-submersible platforms as research objects, the parameterized model of typical component and node is established by taking corrosion thickness as variable using the finite component software. Based on the gradual damage calculation method and finite component analysis method, the load is increased progressively according to proportion increment theory. The ultimate bearing capacity of typical component and node is calculated under the corrosion damage. The ultimate bearing capacity evolution law of typical component and node under various failure modes and various service years is summarized.
Rock Berm Design for Pipeline Stability
Authors: Chamizo David J. , Campbell Dean R. , Jas Eric P. , Ryan Jay R.
Published: August 2013

Stabilizing large diameter natural gas pipelines on the seabed against extreme hydrodynamic loading conditions has proven to be challenging in the northwest of Australia. Tropical storms, which affect the area annually between November and April, can generate wave heights exceeding 30 m and storm steady state currents of 2 m/s or more. Consequently, in shallow water depths, typically less than 40–60 m, subsea pipelines can be subjected to very high hydrodynamic loads, potentially causing significant lateral movement. To mitigate the risk of the pipeline suffering mechanical damage due to excessive lateral movement, quarried and graded rock is often dumped over the pipeline as a secondary stabilization solution.

In order to satisfy functional requirements, the rock berm must comprise of a sufficiently large rock grading size and berm volume to withstand the design hydrodynamic loading such that the pipeline cannot break out of the berm. The design of rock berms for pipeline secondary stabilization has traditionally followed a deterministic approach that uses empirical equations for preliminary rock sizing, followed by small-scale physical modeling for design verification and optimization. Whilst the traditional approach can be effective in producing a robust rock berm design, opportunities for further optimization are inhibited by a lack of available data and an imperfect understanding of the failure mechanisms.

This paper presents an overview of an improved approach for rock berm design optimization. A general overview of rock berms, the design principles, benefits and risks are also presented.
Fatigue Life Estimation in Complex Geometries and Loadings
Authors: Cravero Sebastian , Mantovano Luciano , Ernst Hugo
Published: August 2013

Components with complex geometries and loading conditions frequently serve under cyclic loadings. This situation makes the evaluation of fatigue life in mechanical parts a fundamental issue. Particularly, for notched geometries and loadings involving plasticity, the cyclic stress and strain fields should be determined using finite elements models. The difficulties are increased since the constitutive equations of material under cyclic plasticity employed in the finite element codes is still an active research field and simplifications must be assumed always. In this paper a comprehensive study of strain-life method for fatigue estimation is presented. In addition, the difficulties found to establish a reliable methodology are enumerated.