bluuebunny/crossref_metadata_2025 huggingface dataset:
Design Challenges of Gas Export Pipelines for Pre-Salt Area Offshore Brazil
Authors: Solano Rafael F. , de Azevedo Fábio B. , Oazen Eduardo
Published: November 2013

Recently the pre-salt development is a great challenge for Petrobras regarding the offshore exploration and production activities. The pre-salt area comprises several ultra-deepwater fields located far off the Brazilian coast.

In order to flow the produced gas of these pre-salt fields, Petrobras has planned to install large diameter gas export pipelines as trunklines. The design of large diameter gas pipelines in ultra-deepwater of about 2200m has hydrostatic collapse and installation loadings being the major issues to be faced by the technical team. Reduction of pipeline wall thicknesses may improve the technical and economic feasibility of its installation in ultra-deepwater and sometimes can also increase the number of laying barges / vessels able to perform the installation. However, reduction in wall thickness is only feasible if the pipeline integrity under external pressure as well as under combined loadings is ensured in accordance with the applicable design code and the engineering best practices.

This paper presents the assessment of some design parameters able to reduce the wall thicknesses along the pipeline length, while keeping the commitment to the engineering good practices and complying with all requirements of the DNV-OS-F101 design code. In order to illustrate the assessment, this paper presents results of two gas pipelines designed in accordance with DNV standards, one of them being optimized. The benefits of this optimization are emphasized in this work.
Reliability and Robustness Based Design Attributes for Multi-Criteria Decision Making
Authors: Žanić Vedran , Pirić Karlo , Kitarović Stanislav
Published: November 2013

Novel design methodology with inclusion of reliability and robustness-based design criteria is presented. Robustness is defined as the insensitivity of a design attribute to uncontrollable design parameters. The developed design procedure for the concept design phase is divided into two basic, coordinated tasks: (1) multi-criteria topology/geometry optimization of the ship structural model; (2) scantlings / material multi-criteria optimization of structural panels.

Reliability criteria and robustness of design attributes are applied as relative measures of quality, besides standard design attributes such as costs and weight. They are used in generation of Pareto-optimal design variants. Reliability attributes used for the panel design are compared with respect to fidelity and computational efficiency. A novel method for fast reliability calculations is presented using dimension reduction method (DRM) as implemented into FASTREL software. The method is verified with respect to accuracy and speed on the box girder design and panel design with CalREL methods (MC, FORM).

The design procedure steps are executed in the predefined sequence of design sub-problems, using the fast and balanced collection of analysis and synthesis modules/methods of the MAESTRO/OCTOPUS design system. They are as follows: • Probabilistic determination of design loads; • Calculation of the structural serviceability and ultimate strength criteria on the panel (macro-element) level; • Calculation of the cross section ultimate longitudinal strength criterion; • Calculation of reliability and robustness measures on the panel level (safety) and on the global level. Other design attributes (initial cost, structural weight, etc.) are also determined; • Generation of the Pareto frontier for the selected test structure based upon the cost–safety design paradigm; • Generation of insight into the multilevel optimization process with graphic presentation of designs in design and attribute spaces.

Practical application of the developed concept design methodology and of the design environment to the structural design of modern multi-deck ship elements (panels) is presented for verification/validation of accuracy and speed of FASTREL module.
Wind Measurements Near the Surface of Waves
Authors: Babanin Alexander V. , McConochie Jason
Published: November 2013

Wind-over-wave in situ measurements are typically conducted in two different fashions: either by means of wave-riding buoys, or by placing anemometers well elevated above the surface. Routinely, concept of the constant-flux layer is invoked to convert one into another as necessary. In the paper, comparisons of mean wind speeds and wind-momentum fluxes are conducted, based on measurements throughout the wave boundary layer, including wave-follower measurements very near the surface. Significant deviations from the constant-flux expectations are found. Near the surface, the fluxes are less than those obtained by extrapolation within the logarithmic-layer assumption, and the mean wind speeds are correspondingly larger. Such results have significant implications for modelling the wind-generated waves and for calibrations of remotely sensed surface wind conditions.
Investigation to Air Compressibility of Oscillating Water Column Wave Energy Converters
Authors: Sheng Wanan , Thiebaut Florent , Babuchon Marie , Brooks Joseph , Lewis Anthony , Alcorn Raymond
Published: November 2013

It has been suggested that for full scale oscillating water column (OWC) devices, the pressure in and the volume of the air chambers can be large to create air compressibility in the air chamber. Due to compressibility, its density and temperature are different from those in atmosphere. When in exhalation, the pressurized air is driven out of the air chamber and mixes into the atmosphere outside the air chamber; whilst in inhalation, the atmosphere is sucked through the power take-off (PTO) system into the air chamber, and mixes with the de-pressurized air in the chamber.

This paper presents a study on air compressibility in OWC air chambers by theoretical analyses and the relevant experimental studies. The theoretical analysis is based on the first-order differential equation for the flowrate and the chamber pressure, which has been derived for the air flow under the assumptions of the isentropic process and the known power take-off characteristics. In the study, an orifice type of PTO and a porous membrane type PTO, which are supposed to represent a typical nonlinear and linear PTO for small models, respectively, are both investigated. The investigation has shown the feasibility of the theoretical method on the air compressibility and the possible power loss due to the air compressibility.
Global Buckling of a Pre-Pressurized Flexible Flowline
Authors: Yang Zhengmao , Karunakaran Daniel
Published: November 2013

For the protection from object drop/fishing trawl impact, flexible flowline is normally trenched or rock-dumped. And hence, upheaval buckling is promoted by the elevated temperatures and high pressures. In order to reduce the rock cover requirement for mitigation of upheaval buckling, rock-dumping or trenching while the flexible pipe are pressurized has been performed successfully in several north sea projects.

The temperature and pressure induced elongation of flexible pipe are design dependent. For high pressure flexible flowline, the pressure expansion is significantly higher than conventional rigid pipelines. Due to the low bending stiffness and high pressure expansion, a flexible flowline will buckle laterally when it is pre-pressurized in hydro-test before trenching or rock-dumping. As a consequence, lateral imperfections are induced and will be kept after trenching or rock-dumping due to lateral resistance and bending stress relaxation of the flowline. In these locations, the flowline tends to deform laterally in operating. On the other hand, when the flowline is de-pressurized after trenching or rock-dumping the contraction of the flowline is restrained by the surrounding soils or rocks, and hence axial tension force can be obtained. When the flowline starts to operate, this tensile force will neutralize part of the compressive axial force, and therefore the required upheaval resistance is reduced.

In this paper, global buckling of a pre-pressurized flexible flowline has been studied, and the influence on the requirement of rock covers is presented.
Sediment Transport Calculation Considering Laminar and Turbulent Resistance Forces due to Infiltration/Exfiltration and its Application to Tsunami-Induced Local Scouring
Authors: Nakamura Tomoaki , Mizutani Norimi
Published: November 2013

A sediment transport calculation which consistently considers the effect of laminar and turbulent resistance forces due to infiltration/exfiltration was proposed. From a comparison of the non-dimensional bed-load sediment transport rate, it is found to be essential to consider laminar resistance force as well as turbulent resistance force when formulating the effect of infiltration/exfiltration in sediment transport calculations. The proposed sediment transport calculation was incorporated to improve a three-dimensional coupled fluid-structure-sediment interaction model, and the improved model is applied to tsunami-induced local scouring around an inland structure. Numerical results show that the consideration of infiltration/exfiltration improves the computational accuracy of a scour hole formed around the seaward edge of the structure, and accordingly the improved model can capture the evolution of the scour hole with sufficient accuracy. This suggests that the improved model is expected to be a useful tool for assessing tsunami-induced local scouring.
Mathematical Modelling of a Flap-Type Wave Energy Converter
Authors: Renzi Emiliano , Dias Frederic
Published: November 2013

A mathematical model for a flap-type wave energy converter is derived in the framework of a linear inviscid potential flow theory. The wave field resulting from the interaction of an incident plane wave with the flap is determined by applying Green’s integral theorem to the fluid domain. The hydrodynamic parameters of the system are determined, allowing to solve the equation of motion of the flap and to obtain the capture factor of the device. Asymptotic analysis in the far field yields new relations between the hydrodynamic parameters of the system, which reveal the peculiarity of a flap-type device with respect to traditional point-absorber converters. Finally, comparison is made between the behaviour of the device in the open ocean and in a channel. It is shown that the effect of the channel lateral walls on the system can concur to increase its performance.
Dynamic Seabed Response Around Wind Turbine Foundation: Donghai Offshore Wind Farm China
Authors: Chang K. T. , Jeng D.-S.
Published: November 2013

Donghai offshore wind farm, the first and largest commercial operating offshore wind energy system in China, adopted a novel foundation–high-rising structure foundation. In this paper, a three-dimensional porous model, based on Reynolds-Averaged Navier-Stokes equations and Biot’s poro-elastic theory, was developed by integrating 3D wave and seabed models to simulate wave-induced seabed response around the high-rising structure foundation. Then, a parametric study for the wave and seabed characteristics on the foundation stability was conducted. The numerical results concluded from the numerical analysis were as follows: (i) the existence of structure had a significant effect on the wave transformations and the distributions of wave-induced pore pressures; (ii) the magnitude of wave-induced pore pressure increased as wave height or wave period increased; (iii) the dissipation rate of pore pressure increased as the degree of saturation decreased.
Measurement and Modelling of Residual Stresses in Offshore Circumferential Welds
Authors: Zhang Yan-Hui , Smith Simon , Wei Liwu , Johnston Carol , Stacey Alexander
Published: November 2013

Residual stresses (RS) are unavoidably generated in components after welding. Tensile residual stresses in engineering structures generally have an adverse effect on structural integrity, with detrimental effects on brittle fracture, corrosion properties and fatigue performance.

Residual stress measurements for offshore welded joints are generally very limited. In this study, experimental measurements and numerical modelling of residual stresses were carried out and the through-thickness residual stress distributions in circumferential butt welds are presented. Pipes of two different dimensions and steels were investigated: a seamless X70 steel pipe with an outside diameter of 406mm and a seam-welded X65 steel with an outside diameter of 508mm. The measurement methods used included the centrehole air abrasion drilling, the deep-hole drilling (DHD) and the block removal, splitting and layering (BRSL). The numerical modelling was undertaken to determine whether modelling could provide a satisfactory prediction of the final residual stresses. Finally, the new experimental data were combined with the BS 7910 database to derive revised upper bound through-thickness residual stress distributions for girth welds and to provide an assessment of the impact of residual stresses on the structural integrity of circumferential welds in offshore structures.
Optimal Vibration Control of Floating Wind Turbines in the Presence of Nonlinearities
Authors: Ramos Roberto
Published: November 2013

This paper investigates the effects of nonlinearities on the design of a constrained optimal controller for spar-type floating wind turbines. The considered nonlinearities are due to wind speed variation and actuator saturation. The collective blade pitch actuator, usually employed for aerodynamic rotor speed regulation, is adopted for the mitigation of platform pitch vibrations as well. The wind speed effects and the control law are obtained from a linearized rigid-body dynamic model, taking into account persistent disturbances related to wind turbulence and wave induced loads, which are estimated using the Blade Element Momentum aerodynamic theory and Morison equation, respectively. A performance analysis is carried out for several operating points in the above rated wind speed region, considering the reduction of the platform pitch motion as the main control objective. The simulation results show that the designed fixed-gain multivariable controller can yield significant vibration reduction in comparison with a baseline gain-scheduled proportional-integral controller in the presence of the considered nonlinearities.
Sour Resistant High Strength Seamless Pipes for Risers
Authors: Ortolani Matteo , Anelli Ettore , Novelli Paolo , Bagliani Emanuele Paravicini
Published: November 2013

In case of a Weld On Connector’s riser using ASTM A182 F22 forged joints, high strength (SMYS of 80 ksi) steel pipes for sour service (hardness below or equal to 250 HV10) suitable for welding to the connectors are required. Welding is challenging because of the Post Weld Heat Treatment (PWHT) needed to reduce the hardness in the F22 HAZ while maintaining the required strength in the pipe. Theoretical evaluations were performed by means of metallurgical models and a potential solution was identified in grade P22-like steel (2¼ Cr - 1 Mo), with minor modifications with respect to the standard ASTM A335 and supplied in Q&T condition. A trial heat was cast and hot-rolled into pipes. After water quenching, the response to tempering was assessed by means of laboratory heat treatments and subsequent mechanical testing, together with metallurgical examination. Simulated PWHTs were also performed on Q&T material. 80 ksi grade P22 seamless pipes were finally produced and qualification involved mechanical testing before and after simulated PWHT: SMYS of 80 ksi and HV10 ≤ 250 requirements were met. The material also exhibited excellent toughness and resistance to HIC and SSC cracking.
VIV of Flexible Cylinder in Oscillatory Flow
Authors: Fu Shixiao , Wang Jungao , Baarholm Rolf , Wu Jie , Larsen C. M.
Published: November 2013

VIV in oscillatory flow is experimentally investigated in the ocean basin. The flexible test cylinder was forced to harmonically oscillate in various combinations of amplitude and period. VIV responses at cross flow direction are investigated using modal decomposition and wavelet transformation. The results show that VIV in oscillatory flow is quite different from that in steady flow; novel features such as ‘intermittent VIV’, amplitude modulation, mode transition are observed. Moreover, a VIV developing process including “Building-Up”, “Lock-In” and “Dying-Out” in oscillatory flow, is further proposed and analyzed.
Improvement of Wave Power Take-Off Performance due to the Projecting Walls for OWC Type WEC
Authors: Ikoma Tomoki , Masuda Koichi , Omori Hikaru , Osawa Hiroyuki , Maeda Hisaaki
Published: November 2013

This paper describes a method in order to improve the performance of the primary conversion of wave power take-off. A corresponding wave energy convertor (WEC) is an oscillating water column (OWC) type. The method of the improvement has been proposed and its usefulness has been confirmed in past researches. In the method, projecting walls were attached onto front of inlet-outlet of OWC.

The prediction method of hydrodynamic behaviors for the projecting walls installed OWC type WEC is explained in the paper. The boundary element method with the Green’s function is applied and influence of air pressure and free surface within every an air-chamber was directly taken into consideration in the prediction method based on a linear potential theory. Validity of the prediction method was proved comparing with results of model experiments.

Series calculations are performed with the prediction method. Behaviors of air pressure, water elevation and the efficiency of primary conversion of wave power are investigated. From the calculations, length of the projecting walls was shown to affect the efficiency of primary conversion. It was available to equip the projecting walls for the improvement in oblique waves to beam sea condition as well as head sea condition. As well as only the projecting walls, application and effects of the end walls with the slit were investigated in the paper. The end walls were very useful to improve the efficiency.
Evaluation of Safe and Failure Zones of Risers and Mooring Lines of Floating Production Systems
Authors: Cruces Girón Aldo Roberto , Corrêa Fabrício Nogueira , Jacob Breno Pinheiro
Published: November 2013

In recent years, the design procedures of risers and mooring system for floating production systems (FPS) have had more feedback. In this way mooring and risers designers can identify, even in an early stage, the constraints imposed by one system over the other.

This work presents an evaluation of the crossing of the information obtained from the analyses of risers and mooring system. Different riser and mooring analysis procedures are applied to a typical FPS for deep water applications. First, failure zones of the riser system are identified, so a safe operating limit zone can be defined. Then, the excursions of the platform are calculated taking into account the global response of the coupled system (hull, mooring lines and risers). Finally, the results are crossed in order to verify if the excursions of the platform are within the safe operating area.

The evaluation presented here shows the important of correctly defining the safe operational zones and how the crossing of information can be conservative or not within the design process of mooring lines and risers.
Metallurgical Design and Performance of HFW Linepipe With a High-Quality Weld Seam Suitable for Sour Services
Authors: Toyoda Shunsuke , Goto Sota , Okabe Takatoshi , Kimura Hideto , Sato Shuichi , Kumazawa Shintaro , Yabumoto Satoru , Sato Akio , Inoue Tomohiro
Published: November 2013

To clarify the effects of inclusions on the sour resistance properties of X60- to X70-grade steel, their resistance to hydrogen-induced cracking (HIC) was numerically simulated. The steel was assumed to have a yield strength of 562 MPa and a tensile strength of 644 MPa for the simulation. To estimate the effect of nonmetallic inclusions, a virtual inclusion was situated at the center of a 10-mm-thick HIC test specimen. Tests were performed using NACE test solution A. The crack initiation criterion was determined as a function of the diffusible hydrogen concentration, the diameter of the inclusion, the edge radius of the inclusion, and the fracture toughness of the matrix after hydrogen absorption. The crack propagation was calculated as a function of the diffusion coefficient of hydrogen in the steel matrix and the gasification reaction ratio of hydrogen at the interface of the steel matrix and the inclusion.

Based on the results of the numerical estimation, high-frequency electric resistance welded (HFW) Linepipe with a high-quality weld seam was developed. Controlling the morphology and distribution of oxides generated during the welding process by means of temperature and deformation distribution control is the key factor for improving resistance to HIC.
Experimental Investigations on the Hydrodynamic Characteristics of Heave Plate
Authors: Tian Xinliang , Yang Jianmin , Li Xin , Peng Tao
Published: November 2013

Heave plates are installed in Spar platforms to improve the heave motion performance of the platforms. The heave plates not only increase the equivalent added mass of the platform but also provide extra damping to the system. This article presents an experimental study on the hydrodynamic characteristics of an isolate heave plate. The plate is forced to oscillate in the water at rest. The added mass and damping of the plate are used to represent its hydrodynamic characteristics. Influences of the thickness ratio, the plate shape, the corner radius and the porosity on the added mass and the damping coefficients of the heave plate are evaluated at various KC and β numbers.
Deformation Analysis and Reliability Assessment of the Ship Hull in Irregular Waves
Authors: Yu Pengyao , Feng Guoqing , Ren Huilong , Zhao Xiaodong
Published: November 2013

When the ship navigates in the sea, the dynamic deformation of the ship hull will be induced by the waves. The relative large deformation of the ship hull induced by the waves may affect the operation of some certain equipment. In order to keep the equipment operating normally, the influence of the ship deformation should be evaluated. The method for the calculation and analysis of the ship deformation is discussed here. The wave loads of the ship in unit regular wave amplitude are calculated based on 3-D linear potential flow theory. The sea pressure and inertial force of the ship are obtained and applied to the global finite element model of the ship. Under the quasi-static assumption, the structural deformation response in unit regular wave amplitude is calculated with the use of finite element analysis. Then, the amplitude frequency response of the relative deformation between two arbitrary positions in the hull is achieved. The history of the deformation can be obtained based on the simulation of deformation response in irregular waves or the modal superposition method. With the help of spectral analysis method, the spectrum of the relative deformation between two arbitrary positions in the hull may be obtained. The statistical analysis of ship hull deformation in the short-term sea state is realized. Considering the critical value of ship deformation, the reliability analysis method is adopted to assess the ability of hull to resist the deformation.
Lifting Operation Simulation of a Dynamic-Position Crane Vessel
Authors: Ai Shangmao , Sun Liping , Cheng Chuanyun
Published: November 2013

A time domain method, weakly coupling the block and crane vessel in time sequence, has been developed for lifting operation of the dynamic-position crane vessel. Restoring thrust is distributed by the dynamic positioning PID control system and directly acted on the crane vessel’s control equation as external force; the weak coupling simulation between the block and crane vessel is realized through the sling elastic deformation. Lifting operation simulation of a dynamic-position crane vessel in irregular waves shows that the posture of crane vessel relative to the wave direction in the lifting operation should be adjusted as soon as possible, and had better be made in ahead sea state to improve the security of the suspension stage.
The Research of Statistic Prediction for Wave Loads Based on Three Dimensional Method
Authors: Zhang Yihan , Ren Huilong , Li Hui , Li Xiaoyu
Published: November 2013

The exact prediction of wave loads for ship or other marine structure is the key to its design and the assessment of structural strength, reliability and security. The short-term and long-term prediction of wave loads are always used in direct calculation for structural strength, fatigue strength assessment and so on based on spectral analysis method. In this paper, the numerical calculation method for statistic prediction is discussed firstly, including the Weibull distribution fitted method and the stack method. Further more, it is necessary to find a quick solution in order to improve the efficiency to compute the nonlinear equation in the second method. Then, some main factors that may influence the long-term or short-term prediction are discussed, such as wave spectrum, wave scatter diagram, incident wave angle interval and frequency interval. Finally, the wave loads prediction for a series of typical bulk carriers and oil tankers are calculated by the uniform predict method discussed above base on three dimensional wave loads calculation theory. The results showed that the method used in this paper can predict the statistic value of wave loads induced by irregular incident waves conveniently and efficiently. A rule to choose a series of uniform factors is confirmed for statistic prediction and some empirical formulas for long-term value of wave bending moment are concluded which are very useful in marine engineering.
Experimental Analysis of Higher-Order Sliding Mode Control Applied to Dynamic Positioning Systems
Authors: de Sousa Tadeu F. , Tannuri Eduardo A.
Published: November 2013

The control algorithm normally used in Dynamic Positioning (DP) Systems is based on linear control theory (proportional-derivative or linear quadratic MIMO controller), coupled to an Extended Kalman Filter (EKF) to estimate the environmental forces and wave filtering. Such controllers and estimators have problems of performance and stability related to large variations of loading (for tankers for example) or environmental conditions. The adjustment of controller gains and parameters of EKF is a complex process. Therefore, other techniques are being applied.

An investigation into the area of control of mechanical systems was made, carrying out theoretical and experimental studies involving nonlinear robust control techniques applied to dynamic positioning of floating vessels. Two robust control techniques were applied and compared: first order sliding mode control (SMC) and higher order sliding mode control (HOSM). It is known that the main drawback of SMC is the presence of high-frequency oscillations called chattering. This undesirable effect can be eliminated by using HOSM. In order to ascertain the performance of the controller under the DP system, time-domain simulations were done. Furthermore, the technique of sliding mode requires higher order derivatives of the vessel’s position signal. Therefore was developed an exact real-time differentiator, a mathematical technique used to obtain the signal derived from the position signal in real time. To validate the simulated controller, experimental tests were performed considering a small-scale model of a DP tanker.

The results confirmed the robustness of the HOSM controller, the good performance of the differentiator and the elimination of the chattering problem.
Comparison of RANS and Potential Flow Force Computations for One DOF Heave and Pitch of the ONR Tumblehome Hullform
Authors: Field Parker L. , Neu Wayne L.
Published: November 2013

The commercial CFD software STAR-CCM+ is applied as a RANS solver for comparison with several potential flow codes in calculating time varying forces and moments resulting from prescribed heave and pitch motions of the ONRTH hullform. Combinations of amplitude and frequency of oscillation ranging from small to large are considered. A verification study is completed to ensure that the mesh and time step used by the RANS code are acceptable. Differences in the force and moment results from the various potential flow and RANS methods are attributed to particular code characteristics. RANS and nonlinear potential flow methods are shown to agree well for all cases considered.
Fatigue Crack Growth Rate and Fracture Toughness of Non-Sour Service Grade X65 and X80 in Sour Environments
Authors: Nakamura Jun , Hisamune Nobuyuki , Fukuba Tetsuya , Thodla Ramgopal , Scott Colin Stuart , Gui Feng
Published: November 2013

Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion.

There has been some work performed to understand the performance of carbon steels in dynamic applications like risers and flow lines in sour service. This work is focused on exploring a range of environmental and loading conditions that are relevant to offshore production to evaluate the non-sour service grades X65 and X80 to gain greater insight into the role of chemistry and thermo mechanical processing on the sour service fatigue and fracture behavior of line pipe steels. Two different grades of non-sour service grade seamless pipe X65 and X80 were investigated in a range of sour environments. The concentration of diffusible hydrogen in the range of environments to relate the environment assisted cracking properties to the concentration of hydrogen. Fracture toughness measurements indicated that crack initiation and tearing modulus were dependent on the concentration of diffusible hydrogen. The lowest toughness properties was obtained in pH = 5, 4.6psia H2S. FCGR exhibited a1/√f dependence in all conditions suggesting that hydrogen diffusion was likely the rate limiting step before reaching a plateau. X65 and X80 exhibited the highest FCGR in pH = 5, 4.6psia H2S. The FCGR of X80 was lower than X65 under the same environmental conditions in all cases while the plateau frequency was very similar for both X65 and X80. Paris law measurements performed at constant Kmax and decreasing ΔK at plateau frequencies exhibited elevated FCGR levels above the in-air values consistent with the frequency scan data.
Free Surface Effects on Hydrodynamic Analysis of Flapping Foil Thruster in Waves
Authors: Filippas E. S. , Belibassakis K. A.
Published: November 2013

The analysis of an oscillating wing located beneath the ship’s hull is investigated as an unsteady thruster, augmenting the overall propulsion of the ship and offering dynamic stabilization. The unsteady thruster undergoes a combined oscillatory motion in the presence of waves. For the system in horizontal arrangement the vertical heaving motion is induced by the motion of the ship in waves, essentially ship heave and pitch, while the rotational pitching motion of the flapping propulsor about its pivot axis is set by an active control mechanism. Our method is based on coupling the seakeeping operators associated with the longitudinal and transverse ship motions with the hydrodynamic forces and moments produced by the flapping lifting surfaces, using simplified unsteady lifting line theory. First numerical results presented in Belibassakis & Politis [1],[2] indicate that high levels of efficiency are obtained in sea conditions of moderate and higher severity, under optimal control settings. For the detailed investigation of the effects of the free surface in the present paper a potential-based panel method has been developed for the hydrodynamic analysis of 2D hydrofoil operating beneath the free surface, undergoing heaving and pitching oscillations while moving with constant forward speed. The instantaneous angle of attack is influenced by the foil oscillatory motion and by the incident waves. At a first stage of development we consider moderate submergence and relatively low speeds permitting us to approximately neglect effects due to breaking waves and cavitation. Numerical results are presented concerning the numerical performance of the developed BEM. Also results concerning the thrust coefficient and the efficiency of the system over a range of motion parameters, including reduced frequency, Strouhal number, feathering parameter and compared against other methods. Our analysis indicates that significant efficiency can be obtained under optimal operating conditions. Thus, the present method can serve as a useful tool for assessment and the preliminary design and control of such systems extracting energy from sea waves for marine propulsion.
Noise Prediction and Control of Floating, Production, Storage and Offloading (FPSO)
Authors: Song Keun Bok , Kim Hyung Taek , Kim Sung Hoon , Joo Won Ho
Published: November 2013

Recently, the noise regulations are being reinforced to restrict the noise levels in the workplace and to protect the operator on the offshore structures. So, the noise levels of indoor and outdoor are one of the crucial barometers for determining the quality of offshore structures.

In this paper, an efficient and accurate noise analysis method, which is used to calculate the indoor and outdoor noise levels for Floating, Production, Storage and Offloading (FPSO) vessel, is introduced. The indoor noise level is calculated through summing up structure-borne noise, airborne noise from machinery, HVAC noise and transmitted noise from topside. Each noise level is predicted using appropriate methods by HHI’s procedures, which consider room acoustic theories, numerical methods like SEA and measured database at various ships and offshore structures. The outdoor noise level is predicted by using ray tracing method. Noise control procedures to improve acoustic properties of cabin/machinery room and to reduce the indoor/outdoor noise levels of offshore structures are also introduced: these procedures are produced by numerous experiments at the large scale acoustic test facilities like reverberant rooms and cabin mock-up.

The established method has been applied to various types of real projects for FPSOs. The predicted results are confirmed to agree well with measurements results. It is expected to be very helpful to design the low noise FPSO.
Steel Plates With Excellent HAZ Toughness for Offshore Structure
Authors: Ichimiya Katsuyuki , Hase Kazukuni , Endo Shigeru , Yuga Masao , Hirata Kenji , Matsunaga Naoki , Suzuki Shinichi
Published: November 2013

In the field of offshore structure, the strength and the toughness required for steel plates used for oil resource development become higher as the installation areas of the structures move into arctic and deep water areas. High strength steel plates for offshore structures, which meet the low temperature specification, have been developed. Excellent properties of the steel plates have been achieved by micro-alloying, the latest controlled rolling and accelerated cooling technology. Excellent properties of weld joints have been also achieved by advanced metallurgical techniques, which are grain-refinement of the coarse grain heat affected zone (CGHAZ), reduction of Martensite-Austenite (M-A) constituent in inter-critically CGHAZ (ICCGHAZ), and improvement of the matrix toughness. These steels are designed for excellent weldability due to low weld cracking parameter (PCM) value up to 550 MPa class in yield stress, and also up to 101.6 mm in thickness with 420 MPa class in yield stress, and satisfying −40°C of crack tip opening displacement (CTOD) temperature specification for offshore structure.
Roll Damping Coefficients Assessment and Comparison for Round Bilge and Hard Chine Hullforms
Authors: Begovic Ermina , Bertorello Carlo , Prpic Orsic Jasna
Published: November 2013

The sustainable design of small passenger vessels and large size pleasure craft indicates new trends: lower speed, reduced fuel consumption and better seakeeping in all range of velocities. Small amplitudes of roll motion are considered one of the most important comfort parameter. These trends call for reviewing some commonly used concepts and designer practice. Fundamental parameters as metacentric height and midsection form affect comfort as well as ship safety in counteracting way.

Round bilge, due to good seakeeping characteristics and to large amount of available data from systematic series is the most common hull form choice when cruising speed implies displacement or semi displacement mode. Hard chine hull forms are generally chosen when relative speed leads to some hydrodynamic lift, although recently they have been considered for low relative speed. They allow simplified and cheaper construction and among their interesting features is higher roll damping. This appears very attractive for craft that are generally fitted with stabilizing systems and ask for the most reduced roll motions at any and zero speed.

This paper presents the results of experimental tests relative to roll decay of two ship models of the same dimensions, displacement and GM values: they have the same warped bottom hull form developed at University of Naples, the first is hard chine and the second has round bilge from transom to midship. Both of them are suitable for displacement and semi-displacement craft operating at medium-low relative speed. The tests have been carried out at University of Naples towing tank, in calm water, at Fn ranging from 0.0 to 0.45. Natural roll frequencies and linear and non linear damping coefficients are reported for all tested speeds allowing a fair comparison of the roll characteristics due to the variation in hull form.
2D and 3D CFD Investigations of Seabed Shear Stresses Around Subsea Pipelines
Authors: Shen Wenwen , Griffiths Terry , Xu Mengmeng , Leggoe Jeremy
Published: November 2013

For well over a decade it has been widely recognised that existing models and tools for subsea pipeline stability design fail to account for the fact that seabed soils tend to become mobile well before the onset of pipeline instability. Despite ample evidence obtained from both laboratory and field observations that sediment mobility has a key role to play in understanding pipeline/soil interaction, no models have been presented previously which account for the tripartite interaction between the fluid and the pipe, the fluid and the soil, and the pipe and the soil.

There are numerous well developed and widely used theories available to model pipe-fluid and pipe-soil interactions. A challenge lies in the way to develop a satisfactory fluid-soil interaction algorithm that has the potential for broad implementation under both ambient and extreme sea conditions due to the complexity of flow in the vicinity of a seabed pipeline or cable. A widely used relationship by Shields [1] links the bedload and suspended sediment transport to the seabed shear stresses. This paper presents details of computational fluid dynamics (CFD) research which has been undertaken to investigate the variation of seabed shear stresses around subsea pipelines as a parametric function of pipeline spanning/embedment, trench configuration and wave/current properties using the commercial RANS-based software ANSYS Fluent. The modelling work has been undertaken for a wide range of seabed geometries, including cases in 3D to evaluate the effects of finite span length, span depth and flow attack angle on shear stresses.

These seabed shear stresses have been analysed and used as the basis for predicting sediment transport within the Pipe-Soil-Fluid (PSF) Interaction Model [2] in determining the suspended sediment concentration and the advection velocity in the vicinity of pipelines. The model has significant potential to be of use to operators who struggle with conventional stabilisation techniques for the pipelines, such as those which cross Australia’s North West Shelf, where shallow water depths, highly variable calcareous soils and extreme metocean conditions driven by frequent tropical cyclones result in the requirement for expensive and logistically challenging secondary stabilisation measures.
The Effect of Wave Energy Spreading on the Mooring and Riser System Response of a Spread-Moored FPSO
Authors: Caire Marcelo , Schiller Rafael Vergara
Published: November 2013

Floating unit, mooring lines and risers comprise an integrated dynamic system that respond to environmental loading due to wind, waves and currents in a complex way. Correct consideration of wave energy spreading may be extremely significant for the floating unit response and the mooring and riser system integrity assessment as the directional spectrum induce vessel coupled responses. The riser and system response of a spread-moored Floating, Production, Storage and Offloading unit (FPSO) is assessed employing a cos2s function for the wind-generated wave spreading acting together with a unidirectional swell system. A coupled dynamic analysis is performed considering simultaneously the dynamics of the vessel and of the slender structural system. A case study is presented and the results assessed considering typical offshore Brazilian conditions.
Likelihood-Weighted Method of General Pareto Distribution for Extreme Wave Height Estimation
Authors: Wada Ryota , Waseda Takuji
Published: November 2013

In designing ocean structures, estimating the largest wave height it may encounter over its lifetime is a critical issue, but wave observation data is often sparse in space and time. Because of the limited data available, estimation errors are inevitably large. For an economical and robust structure design, the probability density function of the extreme wave height and its confidence interval must be theoretically quantified from limited information available.

Extreme values estimations have been made by finding the best fitting distribution from limited observations, and extrapolating it for the desired long period. Estimations based on frequentist method lack of generality in confidence interval estimations, especially when the data size is small. Another technique recently developed is based on Bayesian Statistics, which provides the inference of uncertainty. Previous studies use informative and non-informative priors and Markov Chain Monte Carlo (MCMC) simulation for estimation.

We have developed a “Likelihood-Weighted Method (LWM)” to objectively evaluate probability density function of the extreme value. The method is based on Extreme Theory and Bayesian Statistics. Our attempt is to use the ignorant prior to relate each parameter set’s likelihood to its probability. This method is pragmatic, because the numerical implementation does not require the use of MCMC.

The theoretical background and practical advantages of LWM are described. Examples from randomly produced data show the performance of this method, and application to real wave data reveals the poor estimations of previous methods that do not use the Bayesian theorem. The quantification of probability for each extreme value distribution enables the probability-weighted evaluation for inference such as maximum wave height probability density function. The new inference derived from this method is useful to change structure design methodologies of ocean structures.
An Exploration of a PASSIVE Articulated Fish-Like System
Authors: Hu Jianxin , Xiao Qing , Nguyen Vinh Tan
Published: November 2013

This study is aimed to investigate the bio-mimetic swimmer propulsion mechanism with the use of its internal muscle force. A simplified articulated model, with multiple rigid segments connected by the joints (imitating the muscle force), is adopted to represent the real swimmer body. To modeling the problem, a Computational Fluid Dynamics (CFD) method is coupled with the system dynamic equations. Some preliminary results obtained showed that the numerical methods developed are able to predict the self-propelled articulated multi-body system with multiple degrees of freedom.
CAISSON: A Suction Pile Design Tool
Authors: Kennedy Justin , Oliphant John , Maconochie Alasdair , Stuyts Bruno , Cathie David
Published: November 2013

The two main suction pile design methods that are generally applied and accepted within the industry are 3D Finite Element analysis and limit equilibrium. The limit equilibrium method involves assuming a number of failure mechanisms with the mechanism offering the least resistance adopted for design. The limit equilibrium suction pile design software CAISSON has been developed and validated by Cathie Associates for Technip. It is currently in use for rapidly and reliably determining the critical failure mechanism and ultimate holding capacity of initiation, mooring and hold back suction piles in clay. CAISSON has been developed as a stand-alone program written in Visual Basic with a user-friendly program interface implemented to allow for efficient computations. The failure mechanisms employed in CAISSON were identified initially using 2D FE results from PLAXIS. The failure mechanisms identified were further calibrated using 3D FE modelling in ABAQUS and FLAC to account for the influence of side shear within the limit equilibrium equations adopted in CAISSON. The current version of CAISSON can analyse suction piles with L/D aspect ratios from 0.5 to 5 installed in clay of uniform or linearly increasing undrained shear strength. Additional program features include computation of inverse catenary shapes for anchor chains, anisotropic undrained shear strength profiles, pile tilt and pile misalignment. The development and validation of CAISSON is presented in this paper along with a case study and a short parametric study to identify the significance of the CAISSON input parameters that govern the ultimate holding capacity of suction piles. Planned upgrades to CAISSON will also be presented.