bluuebunny/crossref_metadata_2025 huggingface dataset:
Support Vector Regression-Based Multidisciplinary Design Optimization for Ship Design
Authors: Li Dongqin , Guan Yifeng , Wang Qingfeng , Chen Zhitong
Published: August 2013

The design of ship is related to several disciplines such as hydrostatic, resistance, propulsion and economic. The traditional design process of ship only involves independent design optimization within each discipline. With such an approach, there is no guarantee to achieve the optimum design. And at the same time improving the efficiency of ship optimization is also crucial for modem ship design. In this paper, an introduction of both the traditional ship design process and the fundamentals of Multidisciplinary Design Optimization (MDO) theory are presented and a comparison between the two methods is carried out. As one of the most frequently applied MDO methods, Collaborative Optimization (CO) promotes autonomy of disciplines while providing a coordinating mechanism guaranteeing progress toward an optimum and maintaining interdisciplinary compatibility. However there are some difficulties in applying the conventional CO method, such as difficulties in choosing an initial point and tremendous computational requirements. For the purpose of overcoming these problems, Design Of Experiment (DOE) and a new support vector regression algorithm are applied to CO to construct statistical approximation model in this paper. The support vector regression algorithm approximates the optimization model and is updated during the optimization process to improve accuracy. It is shown by examples that the computing efficiency and robustness of this CO method are higher than with the conventional CO method. Then this new Collaborative Optimization (CO) method using approximate technology is discussed in detail and applied in ship design which considers hydrostatic, propulsion, weight and volume, performance and cost. It indicates that CO method combined with approximate technology can effectively solve complex engineering design optimization problem. Finally, some suggestions on the future improvements are proposed.
3-Dimensional Numerical Simulation of Ship Motion in Pack Ice
Authors: Zhan Dexin , Molyneux David
Published: August 2013

In OMAE2010, the authors presented a two dimensional numerical simulation for predicting ship maneuvering in pack ice. This paper is an expansion of this analysis to include a three dimensional numerical simulation for an arctic drill ship moving in pack ice (with ice concentration up to 90%) using a discrete element modeling program (DECICE3D) combined with a ship maneuvering code (SML). The mathematical models of ship motion, ice motion and ship-ice interaction are introduced in the paper. Ship motions of steady drift angle, turning about a turret, turning circle and zigzag maneuvers in pack ice are simulated. The results of the simulations are compared with experimental data or results of simulations for ice free conditions. A sensitive study for the effects of the mass damping term used in the equations of motion is conducted.
Design Aspects of the Zap-Lok® Connection for Onshore and Offshore Applications
Authors: Chapman Ben
Published: August 2013

The Zap-Lok® joint is a mechanical interference connection that, under certain conditions, utilises applied loads to energise a metal-to-metal seal face. One end of the pipe is cold-formed to form a ‘bell’ and the other is formed into a spigot or ‘pin’ as shown in figure 1. These are then subsequently joined in the field by means of a hydraulic press.

The technology has been applied in numerous permanent oil and gas applications both onshore and offshore as an alternative to conventional welding where its inherent speed of construction and mechanical integrity provide a useful means of performing well hook ups.

The purpose of this paper is to draw from the wealth of testing that has been conducted on the Zap-Lok® joint over recent years and present a fully verifiable analytical modelling technique that describes the stress-strain behaviour under typical static operational loading conditions as an orthotropic material. This shall be put forth in the form of a series of stress strain curves as well as a maximum strength model.

In addition, certain requirements shall be put into place in order that pipeline engineers and designers may put into context the various load parameters that exist can be interpreted under the ASME B31 suite of pipeline design codes in order that design and installation shall comply with the aforementioned codes.
Application of a Wave Devouring Propulsion System to Ocean Engineering
Authors: Terao Yutaka , Sunahara Shunji
Published: August 2013

A WDPS (wave devouring propulsion system) is a device that generates thrust directly from wave power and, at the same time, generates large damping forces. This phenomena is same as the thrust generation by the oscillating hydrofoil which are commonly used as the animal propulsion system such as the sea mammals or birds or fish propulsion. Relative flow acting on the hydrofoil generates thrust and damping force respectively.

It is quite simple and consists of hydrofoils positioned in front of the bow. If the WDPS is installed in a ship hull, it can drive the hull (even against waves). One example is shown in Photo 1-1, the small WDPS vessel named Mermaid II, which succeeded in a historical trans-Pacific voyage from Hawaii to Japan (about 7800 km) in 2008 using only wave power. It proves that the WDPS may have some potential in the field of ocean engineering. In this paper we will propose two WDPS applications.

One potential application is a midsized floating-type ocean wind turbine generator, which is composed of a single wind turbine on a catamaran hull with a set one-point mooring system. For this application, a new hull form and WDPS are developed based on Mermaid II. The hull needs to be stabilized because the ocean wind energy absorption efficiency is affected by the hull’s motion. Additionally, the mooring forces acting on the hull need to be reduced to keep the construction and power generation costs down. Therefore, the WDPS is installed in front of the hull to reduce/overcome the wave drifting force and the wind drag force. Another anticipated function of the WDPS is motion stabilization, particularly for pitch and roll motions. A suitable hull form is determined from wave tank experiments and is discussed herein.

The other proposal is a multi-function WDPS that functions as both a wave energy absorption device and a wave thruster. Many hydrofoil-type wave energy conversion systems with forward propulsion have been proposed, but no advanced speed-type systems with a multi-function WDPS. A newly designed forced heave-pitch oscillator is also introduced, and tests are performed in the wave tank. The optimum hydrofoil control method in waves is discussed, and the thrust control is also tested and discussed.
Novel Hybrid System for Reducing Hydroelastic Response of Very Large Floating Structures
Authors: Wang Chien Ming , Gao Rui Ping , Koh Chan Ghee , Kitipornchai Sritawat
Published: August 2013

This paper presents a novel hybrid system for reducing the hydroelastic response of pontoon-type, very large floating structure (VLFS) under wave action. The hybrid system comprises flexible connectors and “gill cells” which are compartments in VLFS with holes or slits at their bottom surfaces for allowing water to enter or leave freely. The gill cells are modeled by eliminating the buoyancy forces at their locations. In the hydroelastic analysis, the water is assumed to be an ideal fluid and its motion is irrotational so that a velocity potential exists. The VLFS is modeled as an isotropic plate according to the Mindlin plate theory. In order to decouple the fluid-structure interaction problem, the modal expansion method is adopted for the hydroelastic analysis which is carried out in the frequency domain. The boundary element method is used to solve the Laplace equation for the velocity potential, whereas the finite element method is employed for solving the equations of motion of the floating plate. It is found that by appropriately positioning the flexible line connector and a suitable distribution of gill cells in the VLFS, the hydroelastic response and stress resultants of the VLFS can be significantly reduced.
Risk Assessment of Transportation and Installation of Offshore Jacket
Authors: Sun Hai , Bai Xu , Li Shangzhan , Sun Liping
Published: August 2013

The risk assessment in the process of transportation, installation and operation is becoming more and more important with the increasing offshore structures accident frequency, the surveys shown that the transportation and installation consists only 5% of the total cost but 80% of the risk. In this paper, the risk assessment for offshore jacket platform in transportation and installation is presented: firstly, the main task profile was identified according to the concrete operation process, the components and risk factors related to the assessment; secondly, the failure models of task profiles are analyzed and coefficient is assigned according to the platform accident database in recent years and the principle of As Low As Reasonably Practicable (ALARP). Finally, the FMEA/FMECA of the transportation process is established based on RELEX Studio 2011, and Fault Tree Analysis (FTA) was proposed to further safety assessment of the implantation process with respect to significant failure models of jacket This paper apply risk assessment in platform transportation, the work in this paper has great significance to the evaluation and future design of platform.
New Polyamide With Improved Hydrolysis Resistance
Authors: Do A. T. , Chicheportiche S. , Chollet C. , Coutarel A.
Published: August 2013

Polyamide, particularly PA11, has been used in flexible pipes for number of years, and has demonstrated an excellent behavior for containment layer application because of its good mechanical properties. However, Polyamides are subjected to hydrolysis, a phenomenon accelerated by elevated temperatures and the acidity of the fluid. Polyamides can thus be used in a temperature domain which must be consistent with the level of ageing acceptable for flexible pipe applications (static or dynamic). The methodology to calculate PA11 life time is dependent on the temperature, acidity and the application detailed in API 17TR2. For several years, Technip has been working on the development and the qualification of a new polyamide with enhanced hydrolysis resistance. Therefore, a full qualification program including tests recommended by API 17 J, extrusion process validation and full-scale dynamic tests, has been performed. Moreover, in line with API 17 TR2, hydrolysis curves have been designed from a full set of ageing test results. These curves show that this new PA can be used at higher temperatures than those presented in the API. Technip has now completed qualification of this new High Performance Polyamide (HPPA) for the inner sealing layer in applications where the conveyed fluid is hot (up to 90°C) and contains a significant water cut during the whole life of the flexible pipe.
Non Parametric Probabilistic Approach of Ice Load Peaks on Ship Hulls
Authors: Suyuthi Abdillah , Leira Bernt J. , Riska Kaj
Published: August 2013

Due to its complex phenomenon, ice-induced load process could not be physically possible to be modeled by a specific well established probabilistic model. Therefore, a non parametric approximate probabilistic approach should be performed based on the available data. This paper describes the procedure and also the extrapolation to get the short term and long term extreme values. Comparison with the classical approach, where the initial distribution of ice-induced load is assumed to be one of the well established probabilistic models, was made. The comparisons discussed in this paper were explored by using the same available data of full scale measurement on board a coastal guard vessel KV Svalbard during the winter 2007. There was a tendency that the non parametric approach produces more conservative results.
Study on Residual Strength Estimation Methods of Corroded Pipelines Under Internal Pressure
Authors: Li Xin , Bai Yu , Wang Amin
Published: August 2013

A significant effort over the past 30 years has been made to study the behavior of metal-loss defects of corroded pipeline under static internal pressure using experimental, numerical and theoretical methods. According to DNV RP F101, the real corrosion defects can be divided into three types, namely single defect, interacting defect and complex shaped defect. A single defect is one that does not interact with a neighboring defect; an interacting defect is one that interacts with neighboring defects in an axial or circumferential direction; a complex shaped defect is a defect that results from combining colonies of interacting defects, or a single defect for which a profile is available.
LQ Structure Design and Verification for FPU
Authors: Song Yooseok , Choi Bowon , Park Taeyoon , Jeon Sangbae
Published: August 2013

In this paper, Living Quarter (LQ) structural design and verification is carried out for Floating Production Unit (FPU). There is much requirement at LQ area such as ceiling height, escape route and outfitting line for human living and operation, so it is quite difficult to get an optimum structural design. Generally, for the LQ design, the following requirement should be considered.

- Company Specification Requirement

- Class Requirement

- Means of escape and access to lifeboats

- Material Handling

- Outfitting routing space such as Duct, piping, MCT (Multi Cable Transit) and Cable tray etc

- Structural fire protection

- Structural Weight and VCG for FPU stability

- Structural safety – Main Structure, Helideck and Platform

- Temporary living quarters

To meet structural safety criteria under above requirements, global LQ structural analysis and local hole penetration evaluation such as door opening, duct, piping, cable tray and etc is performed. From the analysis results, structural reinforcement is applied at some area and outfitting design and routing is often changed at other areas. To get the final design, this kind of design spiral is repeated. So, close clash checks between structure, outfitting and HES department are essential for LQ design and evaluation.

This paper explains how to proceed the LQ design at Shipyard and be a good example.
Risk Assessment of Hoisting Aboard and Installation for Offshore Wind Turbine
Authors: Bai Xu , Sun Liping , Sun Hai
Published: August 2013

With the accidents of offshore structures occurring frequently, the significance of risk assessment in the process of transportation, installation and operation is increasing. In this paper, the risk assessment of offshore wind turbine in transportation and installation is presented. Firstly, the main task profiles are identified according to the concrete operation process, the components and risk factors related to the assessment. Secondly, FTA of hoisting aboard and FMEA of installation process are established based on RELEX Studio 2011. The failure modes of task profiles are analyzed and coefficients are assigned according to the principle of As Low As Reasonably Practicable. Finally, using Risk Priority Number method to evaluate their severity rank, and some recommended operations and management measures are given. Risk assessment is applied in offshore wind turbine transportation in this paper and the work has great significance to the evaluation and future design of offshore structures.
Effect of Dissociation of Gas Hydrate on the Stability of Submarine Slope
Authors: Xiong Zhongsheng , Zhang Jianhong
Published: August 2013

Gas hydrate is a promising energy source because of its great storage under the sea. However, it is not easy to exploit as it becomes unstable and is easy to dissociate when it is disturbed and may trigger a submarine slide and cause a great impact to the safety of human beings and facilities in the sea. In this paper, changes of strength and pore pressure in a soil caused by the dissociation of gas hydrates are discussed. The shear strength of the soil containing gas hydrates decreases significantly with the dissociation of gas hydrates, which reduce the safety factor of a slope but not sufficient to cause failure. During a quick dissociation of gas hydrates in soil, gas or water in pores can hardly escape, and the released gas is greatly compressed and the pore pressure can be calculated. The strength and the pore pressure changes were considered in an infinite slope model. The result shows that only a small amount of gas hydrates has dissociated before the failure of the slope occurs. It is the increasing pore pressure rather than the reduction of soil strength that causes the failure of the slope. Parametric study was also carried out on gradient of slope, water depth and embedment depth of gas hydrates in the seabed. It was found that the gradient of a submarine slope is not an important factor to the failure of the slope while the water depth and the embedment depth affect the stability of the slope significantly.
Time-Domain Simulation of Second-Order Wave Diffraction in Irregular Wave
Authors: Xu Gang , Hamouda A. M. S.
Published: August 2013

A time-domain second-order method is presented to simulate three-dimensional (3D) wave-body interaction. In the approach, Taylor series expansions are applied to the free surface boundary conditions, and Stokes perturbation procedure is then used to establish corresponding boundary value problem at first-order and second-order on the time-independent surfaces. A Boundary Element Method (BEM), based on Rankine source, is used to calculate wave field at each time step. Multi-Transmitting Formula coupled with Damping Zone method (MTF+DZ) is employed as radiation condition to minimize the wave reflection. A stable Integral form of Free surface Boundary Condition (IFBC) is used to update velocity potential on the free surface. The present method is applied to compute the second-order Stokes wave diffraction of bottom-mounted circular cylinder first, and then to compute the irregular second-order Stokes wave diffraction of truncated cylinder in infinite water depth with three wave components. It is shown that long time simulation can be done with stability, and the model can be used to time-domain simulation of nonlinear irregular wave-body interaction.
Experimental Investigation on Added Mass Coefficient of a Truss Spar Subjected to Vortex-Induced Motions
Authors: Zhang Hui , Yang Jianmin , Peng Tao , Lu Haining
Published: August 2013

The prediction of response frequency of Truss Spars is important in the estimation of fatigue damage rates. Response frequency depends on the natural frequency of the structure which is influenced by the added mass.

The 1:60 scale Truss Spar model tests have been conducted in towing tank at Zhejiang Ocean University (ZOU). The Truss Spar model consists of a hard tank with removable helical strakes, a truss section and a square soft tank. Model tests were carried out with simplified mooring system.

In this study, results from the experiment subjected to uniform current were reported. The Reynolds numbers varied from 8E04 to 2E05. The mean added mass coefficient and the time-variable added mass coefficient were calculated. The results shown that the predicted natural frequency based on the measured added mass coefficient was approximately equal to the measured mean oscillation frequency. The added mass coefficient calculated from one oscillation cycle to the next varied considerably. The oscillation frequency from one oscillation to the next corresponded to the natural frequency including the added mass coefficient for the same cycle.
Characterization of Dynamic Slug Flow Induced Loads in Pipelines
Authors: Reda Ahmed M. , Forbes Gareth L. , Sultan Ibrahim A.
Published: August 2013

The flow of a liquid mass, i.e. a ‘slug’, inside thin-walled spanning pipelines, produces a lateral traversing force. This moving force initiates dynamic stresses within the structure and is often critical when assessing structural fatigue.

Moving slugs in spanning pipelines may be modeled as either a moving concentrated force or a moving mass when investigating the vibration response of the pipeline under the passage of a slug flow. The moving concentrated force model only yields accurate results when the mass of the slug is small in relation to that of the pipeline; although, the moving mass model should be used instead when the slug’s mass cannot be regarded as small in relation to the mass of the pipeline.

The modeling of a moving concentrated force is much more readily implemented than that of a moving mass. Thus, the aim of this paper is to identify those situations where the simplification of considering a moving concentrated force can be made, or indeed if dynamic analysis is even required. Results are given in this paper to quantify when the two modeling techniques begin to differ significantly.

It is intended that this paper will assist pipeline engineers discriminate between which appropriate conditions to use for either of the two different models of a traversing concentrated force/mass over a structure.
Investigation on Smeared Thickness Method for Plating Stiffeners on Prediction of Grounding Character of Double Bottom Tanker Over Obstacles With Large Contact Surface
Authors: Liu Yi , Hu Zhiqiang , Amdahl Jørgen
Published: August 2013

An investigation on smeared thickness method used in prediction of grounding characters for double bottom tanker over obstacles with large contact surface and trapezoidal cross-section was conducted in this paper. Present analytical methods predicting grounding characters mostly simplify the stiffeners’ effect, and consider it by smeared thickness method. Paik proposed a method to calculate the smeared thickness of the plating on the results of model tests of axial crushing of square tubes. This method is used frequently for its simple calculation work. In his method, Paik calculated the smeared thickness by adding a parameter k to take the property of stiffeners on bottom plating into account, and took it for granted that the parameter k can be used as 1.0. However, taking the parameter as 1.0 is not sufficient to consider the stiffeners’ roles during the sliding grounding course, which has been proved by Hu. What value should the parameter k be? It is answered in this paper.

Numerical simulation technique is used to do the investigation. A hold of a double bottom tanker is chosen as the object. Two finite element models are built, one with stiffeners and the other without stiffeners but with smeared thickness on plating. Structural components within the double bottom are built with fine mesh, and the rest are built with coarse mesh. LS_DYNA is used to do the numerical simulation. Grounding resistances and energy dissipation are obtained, to do the comparison and verification. Cases with different indentations are defined. The results of the simulations for the model with stiffeners are taken as true values. The results of simulations for the model without stiffeners but with smeared thickness plating are used to do the comparison, on purpose of determining the value of the parameter k.

With the comparison, the feasibility of the smeared thickness methods is verified. The suitable values of parameter k in smeared thickness method are determined for different indentations cases, for the double bottom tanker during sliding grounding scenario with seabed obstacle with large contact surface and trapezoidal cross section.
Concept Design of a Multi-Column TLP for a 5MW Offshore Wind Turbine
Authors: Zhao Yongsheng , Yang Jianmin , He Yanping
Published: August 2013

Currently, floating wind turbine may be the most economical and suitable equipment to exploit offshore wind energy in deep water. According to the environmental conditions which are as same as OC3-Hywind (NREL), a new 5MW offshore wind turbine on a multi-column TLP foundation (Windstar TLP) is proposed. The general arrangement, main structure and mooring system are also designed. Aero-hydro-servo-elastic coupled analysis based on IEC-61400-3 requirements is carried out in the time domain with the numerical tool FAST. Statistics of the key parameters are obtained and analyzed. It is shown that the design requirements are satisfied. The new concept holds potential for further development.
Challenges in Inspection Planning for Maintenance of Static Mechanical Equipment on Ageing Oil and Gas Production Plants: The State of the Art
Authors: Ratnayake R. M. Chandima
Published: August 2013

Although the design life of many of the oil and gas (O&G) production and process facilities on the Norwegian Continental Shelf (NCS) has been exceeded, the same physical assets are still under exploitation as a result of extended life based on the information gathered by inspection, maintenance, modification and replacement history. Nevertheless, pressure systems, which comprised of static mechanical equipment such as piping components (valves, separators, tanks, vessels, spools, etc.), undergo continuous inherent deterioration (fatigue, corrosion, erosion, etc). Often the deterioration rates vary over the lifetime following no specific pattern due to the changes in product quality of the well stream, varying environmental conditions and unexpected cyclical loading. These necessitate effective inspection planning to repair, modify or replace those components that reach the end of their design life. This enables the integrity of the physical assets to be retained at a tolerable level. The inspection planning has traditionally been driven by prescriptive industry practices and carried out by human experts, based on risk-based inspection (RBI) and risk-based maintenance (RBM) philosophies. The RBI and RBM involve the planning of inspections on the basis of the information obtained from risk analyses of a particular system and related equipment. This manuscript reviews the evolution of inspection and maintenance practices. Then it provides a conceptual framework to mechanize the inspection planning process in order to reduce the effect arising from human involvement, whilst improving the effective utilization of data from different sources.
Asset Management Challenges of Offshore Installations in Arctic Regions
Authors: Bencomo Alejandro , Markeset Tore
Published: August 2013

In the last decade we have seen an increase interest by the oil companies in developing oil and gas fields in arctic regions. Companies are trying to adapt conventional procedures and technologies used in more tempered offshore regions into subzero environments. However, some all these proven technologies and techniques are not quite efficient in extreme cold environments.

In this paper the authors discuss the main challenges inherent to operation and maintenance of offshore facilities in arctic regions. Moreover a set of conventional and innovative solutions for these challenges is presented.
Hydrodynamic Analysis of Tension Based Tension Leg Platform
Authors: Bhaskara Rao D. S. , Panneer Selvam R. , Srinivasan Nagan
Published: August 2013

Tension Leg Platforms (TLPs) are one of the best options for offshore industry in deep waters due to proven motion response characteristics. These are water depth sensitive structures and the motion responses in vertical plane motions (heave, roll and pitch) are critical for a TLP. Tension Based TLP (TBTLP) is a new concept and finds application in much deeper waters. A provision of a tension base at mid-depth results in an economical design of TLP. In fact, the TLP installed at a certain depth without any modifications can be made to be deployed in much deeper water depths by means of a tension base. In this paper, the concept of TBTLP is highlighted and hydrodynamic analysis of the chosen platform has been carried out using ANSYS AQWA package. The motion responses in terms of Response Amplitude Operators (RAOs) of TBTLP with one Tension Base in surge, heave and pitch have been obtained and compared with a TLP without a tension base.
Motion of an Air-Cushion Vehicle in Transforming Nearshore Head Seas
Authors: Dhanak Manhar R. , Kindel Michael
Published: August 2013

Pitching and heaving motions of an air-cushion vehicle, such as a surface effect ship (SES), in response to wave encounters as it transitions through a transforming wave-field such as in the near-shore zone are considered. The vehicle maintains a pressurized air cushion between its two side hulls and the deck hull. In head seas, the passage of waves across the length of the ship results in variations in the air cushion pressure and in associated compressibility effects. In view of the shallow draft of the vehicle, the primary forcing is considered to be due to the compressible dynamics of the air in the cushion in response to the evolving wave field. Through solution of equations governing the response of the vehicle to unsteady changes in the air cushion pressure due to encounter with transforming waves in head seas is described. Coupled unsteady equations for heave and pitch are solved to determine response to passage through transforming head seas. Complementary laboratory experiments are described. Wave transformation from deep waters to the beach is determined using the model of Dally et al. (1985) and the shallow-water SWAN model. The transformations of a regular wave over a beach slope and of an offshore JONSWAP spectrum are computed and used as incident wave fields to determine the response of the vehicle in transitioning at speed U in head seas. The heave and pitch motions for a specific SES are computed as functions of time and frequency in a coordinate system moving with the transitioning vehicle. The characteristics of the vehicle response to a regular transforming monochromatic wave and to transforming irregular waves are determined. Heave and pitch motion of a selected SES, driven by cushion dynamics suggest how the vibrational energy is spread over a range of frequencies as the vehicle proceeds through this evolving wave field. A test platform involving a scaled model SES instrumented with an inertial motion unit to measure six-degrees-of-freedom motions of the vehicle, a pressure gauge to record the air cushion pressure and a custom flex sensor to measure bow skirt deflections due to wave impacts is used to study the vehicle response to transforming waves in laboratory experiments. Results of low Froude number experiments show that the heave, pitch and air cushion excess pressure fluctuations increase as mean air cushion excess pressure increases. The results are compared with the computations described above.
Calibration of UWA’s O-Tube Flume Facility
Authors: Luo Chengcai , An Hongwei , Cheng Liang , White David
Published: August 2013

The O-tube facility, designed and established at the University of Western Australia, is an innovative closed loop flume in which a random storm sequence can be reproduced via control of a large pump system. The O-tube facility is capable of simulating hydrodynamic conditions near the seabed and the interaction with seabed sediment and any infrastructure that is resting on it.

The purpose of carrying out the O-tube calibration described in this paper is to obtain the relationship between the motor rotation movement and the flow velocity generated in the O-tube, such that any required storm history within the performance envelope of the O-tube can be reproduced. A range of flow velocities and the corresponding pump speeds were measured under steady current, oscillatory flow and combined flow conditions. It was found that the relationship between the pump speed and the flow velocity varies with the oscillatory flow period. Based on the pump characteristic curves and O-tube system curves, the correlation between the motor speed and the flow velocity was derived by applying hydraulic theory and the principle of energy conservation.

The derived correlation is validated by reproducing a wide range of target storm series, including a (1:5.8) scaled 100-year return period storm from the North West Shelf of Western Australia in 40 m water depth.
Fatigue Variation in Ships due to the Variability of Environmental Loads
Authors: Mao Wengang , Li Zhiyuan , Ringsberg Jonas W. , Rychlik Igor
Published: August 2013

The design and analysis of structural strength against fatigue failure always includes large uncertainties. It is crucial to understand and identify the most important uncertainties that affect the performance, functionality and service life of an engineering structure — in particular when it comes to the safety aspect, which may involve the risk of loss of human lives. In maritime industry, it is known that due to various sources of uncertainties in ship fatigue design, some ships may survive (the occurrence of fatigue cracks which may endanger the structural integrity) much longer than their designed life, while other ships develop fatigue cracks far too early. The current investigation presents some of the most important uncertainties and their effects on the accuracy of fatigue assessments in a container vessel.

The study emphasizes the analysis of the fatigue damage variation when the ship is sailing on different routings between two ports. A fatigue model developed by the authors for ship fatigue routing application is employed to estimate the long term fatigue damage. In this model, only a few parameters, i.e. the encountered significant wave height and operational profiles, are needed. The procedure of using this model for a ship fatigue routing design is described in detail. Its potential use and benefits are demonstrated in a case study by a 2,800 TEU container ship using both full-scale measurements and hindcast wave data. It is shown that awareness and careful fatigue routing design can reduce fatigue damage significantly by up to 50%!
The Bulldozer Effect When Cutting Water Saturated Sand
Authors: Miedema Sape A.
Published: August 2013

In the last decennia a lot of research has been carried out into the cutting of water saturated sand at small cutting angles, especially at the Delft University and Deltares. Because of tunnel boring machines there was also interest in larger cutting angles in the 90’s. Now this can also be applied to the bulldozer effect in front of drag heads or the dragging of ice keels resulting in soil displacements under gouges. At small cutting angles the sand will flow over the blade according to the flow type of cutting mechanism, however at large angles a wedge will occur in front of the blade, while at very large cutting angles the sand will be pushed under the blade. Based on FEM calculations of the pore pressures a method has been developed named the parallel resistor method, in order to determine the pore pressures in the water saturated sand. Once these pore pressures are known, the forces and moments can be determined and it can be predicted at which cutting angle a static wedge will start to occur and at which cutting angle the sand will start to move under the blade resulting in much larger soil deformations. The paper will describe the model and also give a recipe on how to determine when the static wedge will occur.
Lateral Buckling Induced by Trawl Gears Pull-Over Loads on High Temperature/High Pressure Subsea Pipeline
Authors: Herlianto Iswan , Chen Qiang , Karunakaran Daniel
Published: August 2013

Lateral buckling has become a challenge to deep water and high pressure/high temperature (HP/HT) flowlines. In areas that support major commercial fishing industries (e.g. the North Sea in Norway and Atlantic Margin in UK), there is high possibility of interaction between flowlines and fishing trawl gears. This interaction can expose the flowlines to substantial loads and induce lateral buckling.

This paper presents global response of subsea pipeline as a result of trawl gear pull-over loads. The external interference from trawl gear pull-over loads can create substantial imperfection or out-of-straightness on the pipeline and may also generate global lateral buckling. The pull-over loads can also induce excessive bending moments and strains in the buckle region.

To be able to understand the global response of the pipeline under pull-over loading conditions, a Finite Element (FE) analysis is carried out based on DNV OS F101 [1], DNV RP F110 [2] and DNV RP F111 [3] using general FE analysis software ANSYS v13. Non-linear transient analysis is used to incorporate the non-linear effects, such as the pipeline material nonlinearity, and the response of a structure under the action of pull-over time-dependent loads [8].

The FE analysis covers two periods of duration, i.e. during pull-over duration and post pull-over duration. The analysis during pull-over duration deals with the pipeline global response as a result of trawl gear pull-over loads. The pipeline is subjected to substantial horizontal and vertical pull-over forces from the trawl gear. For post pull-over duration, the FE analysis was carried out for an additional five seconds. In this period, the pull-over loads are no longer applied. However, the pipeline may expand further due to temperature and pressure loads on pipeline.

The FE analysis result shows that the pull-over loads induce out-of-straightness on the pipeline and may cause lateral buckling. The pipeline deforms laterally at the pull-over location. The DNV displacement condition code check is used to check the integrity of the pipeline. The pipeline may in the risk under the trawl gears pull-over loads.

This paper also shows the development of lateral buckling on the pipeline under different magnitudes of trawl gear pullover loads and lateral soil frictions. Further work should also take into account different dimension of pipeline, as the variation of operating temperature and pressure and variation of lateral and axial soil friction combinations to obtain better conclusions.
Wellhead Fatigue Analysis Method: The Effect of Variation of Lower Boundary Conditions in Global Riser Load Analysis
Authors: Reinås Lorents , Russo Massimiliano , Grytøyr Guttorm
Published: August 2013

Subsea wellhead mechanical fatigue can potentially result in a gross structural failure of barrier elements in the upper part of the well, potentially resulting in loss of well control. Several major E&P operators have acknowledged the importance of wellhead fatigue and are participating in the JIP “Structural Well Integrity”. It is within the scope of this JIP to develop a recommended practice for wellhead fatigue analysis methodology. The analysis methodology currently being investigated by the JIP is a decoupled approach, with modifications of the lower boundary to account for the stiffness of the conductor, soil and template interface. A detailed local wellhead model is used to generate the lower boundary condition for a decoupled global riser load analysis model. This lower boundary condition definition is intended to capture the overall non-linear stiffness of a site specific well in order to achieve best possible global riser loads estimate.

In this article the effect of varying the lower boundary conditions on a global load estimate is studied. Global load estimates are generated from a typical North Sea case and various lower boundary conditions are introduced as the only change to the global riser model. A fixed lower boundary condition is used as a reference and load estimates generated from riser models with various lower boundary conditions are compared.

The different lower boundary conditions selected for comparison in this study has been derived from the following cases:

1. Fixed at WH

2. As per ISO 13624-2

3. As per JIP “Structural Well Integrity” -Current

4. As per JIP “Structural Well Integrity” -Modified

Comparing the analysis results gives indications that the lower boundary condition modelling approach affect global riser load estimate. The fixed lower end boundary conditions did not yielded the most conservative load history in a fatigue context. Modelling well specific flexibility at the riser lower end increased the total number of wellhead fatigue load cycles. This finding support the current approach suggested by the works of the JIP “Structural Well Integrity”. Ensuring that riser load results are still conservative places a higher importance on precise local modelling of the well system.
On the Dynamic Response of Flexible Risers Caused by Internal Slug Flow
Authors: Ortega Arturo , Rivera Ausberto , Nydal Ole Jørgen , Larsen Carl M.
Published: August 2013

Slug flow through flexible risers is a frequent phenomenon which occurs during production of a mixture of oil and gas. The dynamic nature of the slug pattern induces time varying forces, which leads to structural vibrations of the riser. These vibrations can produce large deflections and stresses, which can leave it to fail by fatigue, excessive bending or local buckling.

In this work the influence from slug flow on the structural dynamic response of a lazy wave flexible riser is analyzed using a computational tool consisting of one program for calculation of slug flow dynamics, and another program for structural dynamic response. Both programs apply a time integration method, and since slug flow will lead to dynamic motion response of the riser, and riser motion dynamics will influence slug flow dynamics, the two codes need to exchange information during the integration process. Information exchange is established by making a federation based on High Level Architecture (HLA).

The federation is composed of SLUGIT and RISANANL. SLUGGIT is a two-phase flow code written in C++ which simulates dynamic slug flow through pipes and riser using a Lagrangian tracking model. RISANANL is a FORTRAN program for static and dynamic structural analysis of slender marine structures based on a finite element formulation. Using the HLA standard these two programs can carry out synchronized time integration and exchange information for each time step.

In this work the structural analysis code accomplishes the dynamic response using a linear finite element (FE) formulation. Hence, forces from centripetal acceleration of the internal flow, relative velocity between the riser and surrounding water, and varying gravity of the pipe and content will be accounted for in the dynamic analysis. Displacements, stresses, internal pressure, and outlet flow rates of liquid and gas will be accounted for. The results encourage us to carry out a fully non-linear finite element analysis, in order to have a better understanding of the dynamic behaviour of flexible risers undergoing an unsteady internal two-phase flow.
The Use of Flexible Pipe for CO2 Enhanced Oil Recovery Applications
Authors: Taravel-Condat C. , Epsztein T.
Published: August 2013

The use of CO2 rich gas mixtures as injection fluid is nowadays considered in order to enhance the oil recovery process. Flexible pipes, which have been used to transport oil and gas in offshore applications for more than 35 years, are attractive products for the transportations of such fluids. The CO2 content encountered for these applications can be higher than 90%. This value being superior to what is found in standard applications (<15%), and CO2 being known to behave as a supercritical fluid as soon as the temperature and the pressure are higher than 31.1°C, 72.8 atm, the compatibility of the pipe materials with the CO2 rich gas mixture had to be verified.

This paper will present the qualification work carried out so far on several thermoplastic materials used for flexible pipe pressure sheathes and stainless steel carcasses. This qualification work covers application as high as 90°C/600bar/100%CO2.

The formation of blisters (blistering) is known as a possible degradation mode of thermoplastic materials when subjected to rapid gas decompression. This phenomenon is directly linked to the fluid solubility in the polymer. Different materials have been tested for their ability to withstand rapid gas decompression in CO2 rich gas mixtures.

Long time exposure tests have been carried out. The objectives of these tests were to check the impact of CO2 on the physical, mechanical and chemical properties of the polymer grades. A special attention has been brought to the plasticizer effect of CO2. The impact of supercritical CO2 on the hydrolysis degradation mode has also been assessed.

Finally, CO2 being a small molecule, it will permeate through the pressure sheath to reach the annulus. The metallic layers which are constitutive of this annulus are potentially sensitive to corrosion phenomenon in acidic conditions. A good knowledge of the permeation properties of the polymer materials when subjected to high pressure CO2 is necessary in order to assess the severity of this annulus and to select the appropriate steel grade. The results of such tests will also be presented.
The Effect of a Fatigue Failure on the Wellhead Ultimate Load Capacity
Authors: Reinås Lorents , Sæther Morten , Aadnøy Bernt Sigve
Published: August 2013

A subsea well will experience external loading during drilling operations that can lead to the development of a fatigue fracture in the primary load bearing structural members of the upper well construction. Such a fatigue fracture can occur at several fatigue hotspots which all are located in the upper part of a subsea well. There are two main load sharing structural members; the outer tubular string named the conductor (structural) casing and the next tubular string named the surface casing. Both these strings have a circumferential load bearing weld close to the top. The load sharing between these 2 tubular strings are affected by the supported weight from further tubular strings placed inside the well.

This paper discusses the residual ultimate load capacity of a typical North Sea subsea well assuming that a fatigue fracture has developed. The discussion is based on FEM analysis results where a fully developed fatigue fracture has been introduced to the analytical model of a typical well either to the conductor part of the well or to the surface casing string. Then the residual ultimate load capacity is evaluated assuming a fully developed fatigue fracture. Evaluations presented herein can be important and necessary tools in considering the consequences of a possible fatigue failure of a subsea well.

A reduction in ultimate load capacity due to a fatigue fracture may reduce the safety margin should an accidental or extreme loading occur. The results indicate that the location of the potential fatigue failure is important when assessing the residual ultimate load capacity. If the factored fatigue life of a subsea well is approaching its limit the presence of a fatigue fracture should be assumed. The most prudent approach would then be to perform a permanent P&A operation of the well. Planning of such operations should comprehend the possibility of reduced structural capacity of the well due to a fatigue fracture. This paper also discusses the results in an operational context. The applied methodology is outlined and illustrative results are presented from a typical North Sea well.
An Optical Tool for Measuring Vortex-Induced Vibrations of Long Flexible Cylinders
Authors: Huera-Huarte Francisco J.
Published: August 2013

We describe the design and performance of a novel optical measurement system that can be used for measuring vortex-induced vibrations (VIV) of long flexible cylinders. We believe this is the first time this technology has been used to measure VIV in a long flexible cylinder. The emphasis is given to the measurement technique itself. It can be used either in air or in water and we believe it is a better alternative to other traditional vibration response measurement techniques used to study VIV in the laboratory. The measurement system is based on the Defocusing Digital Image Particle Velocimetry (DDPIV) concept, proposed by Willert and Gharib in [1] for measuring velocity flow fields.

A set-up, which was built to conduct vortex-induced vibration measurements in a wind tunnel facility with a cross-section of 1.8 × 1.2 m, has been used in a laboratory to demonstrate the capabilities of the optical measurement system when known displacements are produced in the mappable or measurement volume. The same supporting structure designed to provide suport for the cylinder models inside the wind tunnel test section, has been used to hold a target which has been move to different known locations to measure the performance of the system. The design, description and performance of the measurement technique are discussed.