bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural Analysis of Node Cut-Outs in a Semi-Submersible Offshore Platform
Authors: Ringsberg Jonas W. , Karlsson Daniel , Forser Mårten
Published: October 2016

The connection between column and deck-box on a semi-submersible platform is often subjected to high stress concentrations that can be governing for the structural strength. One common design used to withstand the high stresses is a cast integral in high tensile material, which is both heavy and expensive to manufacture, which is why an alternative cut-out design is investigated. This study presents an analysis of a node connection with regard to yield stresses, buckling, fatigue damage, weight and cost in order to propose an alternative cut-out design using the same criteria. The cast integral, which is analysed as a reference case and used for comparison of designs, is of a typical design used today. A parametric study shows that the use of a cut-out design may be advantageous in the centre bulkhead connection between the column and the deck-box. The adjacent geometry is of simple character and the cut-out does not affect the overall structural design. The cut-out is found to be a feasible alternative to the cast integral, with yield stresses and fatigue damage in the same range. Further refinements of the cut-out design could, however, decrease both yield stresses and fatigue damage. A suggested cut-out solution leads to a decrease of 4.5% in weight for a reference structure. For the same reference structure the production cost is estimated to decrease by 9%, hence, the cut-out design is recommended for the centre bulkhead connection.
Hydrodynamic Performance Effect of Steel Catenary Risers on Wave Frequency Motions of a Semi Submersible
Authors: Gang Chen , Nan Zhao , Wei Zhang , Hongtao Yuan , Chunhui Li , Yong Yang
Published: October 2016

The analysis of the influence of risers on the motions of a floating platform is often conducted and simplified by uncoupled method. As the number of risers and water depth increase, this method would not predict system motion accurately. Coupled analysis method in time domain becomes a very convenient approach in response calculation since it automatically includes the interaction among platform, mooring lines and risers.

This paper introduces a full coupled approach by AQWA-NAUT to include viscous damping of the semi submersible and effects of steel catenary risers on the wave frequency response of platform in time domain motion analysis.

The main conclusion of this paper is that full coupled method can accruately predict semi submersible Response Amplitude Operator (RAOs) comparing to the case without risers. Other conclusions are that risers have an important effect on the wave frequency motion of semi submersible and also lead to a phase lag with respect to platform motions.
Data Compression of Ship Performance and Navigation Information Under Deep Learning
Authors: Perera Lokukaluge P. , Mo Brage
Published: October 2016

Various emission control regulations enforce vessels to collect performance and navigation data and evaluate ship energy efficiency by implementing onboard sensors and data acquisition (DAQ) systems. These DAQ systems are designed to collect, store and communicate large amounts of performance and navigation information through complex data handling processes. It is suggested that this information should eventually be transferred to shore based data analysis centers for further processing and storage. However, the associated data transfer costs introduce additional challenges in this process and enforce to investigate cost effective data handling approaches in shipping. That mainly relates to the amount of data that are transferring through various communication networks (i.e. satellites & wireless networks) between vessels and shore based data centers. Hence, this study proposes to use a deep learning approach (i.e. autoencoder system architecture) to compress ship performance and navigation information, which can be transferred through the respective communication networks as a reduced data set. The compressed data set can be expanded in the respective data center requiring further analysis. Therefore, a data set of ship performance and navigation information is analyzed (i.e. compression and expansion) through an autoencoder system architecture in this study. The compressed data set represents a subset of ship performance and navigation information can also be used to evaluate energy efficiency type applications in shipping. Furthermore, the respective input and output data sets of the autoencoder are also compared as statistical distributions to evaluate the network performance.
Reliability Analysis of Redundant Dynamic Positioning Control System With Human Factor Involved
Authors: Wang Fang , Bai Yong , Xu Feng
Published: October 2016

Deepwater oil and gas explorations bring more safety and reliability problems for the dynamically positioned vessels. With the demands for the safety of vessel crew and onboard device increasing, the single control architecture of dynamic positioning (DP) system can not guarantee the long-time faultless operation for deeper waters, which calls for much more reliable control architectures, such as the Class 2 and Class 3 system, which can tolerate a single failure of system according to International Maritime Organization’s (IMO) DP classification. The reliability analysis of the main control station of DP Class 3 system is proposed from a general technical prospective. The fault transitions of the triple-redundant DP control system are modeled by Markov process. The effects of variation in component failure rates on the system reliability are investigated. Considering the DP operation involved a human-machine system, the DP operator factors are taken into account, and the human operation error failures together with technical failures are incorporated to the Markov process to predict the reliability of the DP control system.
Perforation Modes of Double-Layered Plates With Air Gap Struck by a Blunt Rigid Projectile
Authors: Xiaowei Chen , Bing Liu
Published: October 2016

The failure modes of the perforation of double-layered or multi-layered plates may be quite different. Further experimental analyses are conducted on the perforations of single and double-layered Weldox E steel plates with air gap to discuss the different perforation modes. It indicates that, in case of higher initial striking velocity, the failure mode of the first layer plate is adiabatic shear plugging, whilst the failure mode of the second layer plate is adiabatic shear plugging or shear plugging as the striking velocity of the projectile slows down. It concludes that the final failure mode of the double-layered plates is the hybrid of shear plugging and adiabatic shear plugging.
A Frequency-Domain Model of Bitumen-Coated Armor Wires in Subsea Power Cables, Umbilicals, and Power Umbilicals
Authors: Komperød Magnus
Published: October 2016

Bitumen is commonly used as anticorrosion protection for armor wires in subsea power cables, umbilicals, and power umbilicals. Bitumen’s viscoelastic behavior influences the cable’s mechanical properties. The present paper derives a simple, analytical model of bitumen-coated armor wires. The model calculates the axial stresses of the armor wires and the armor wires’ contribution to the cable’s bending stiffness. The model shows that there is a phase shift between the sinusoidal curvature oscillations and the corresponding armor wire stresses and cable bending moment. Two examples show that the armor wire stresses and the cable’s bending stiffness are strongly temperature-dependent. The purpose of the novel model is to calculate bending stiffness, fatigue stresses, and capacity (allowed combinations of axial cable tension and cable bending curvature) more accurately and to study these variables’ sensitivity to temperature and frequency. The model may also be included in calculations of bitumen’s influence on VIV damping.
CFD Driven Drillship Design
Authors: Krijger Jan Willem , Chalkias Dimitris
Published: October 2016

Drillships have specific features which make ship resistance and propulsion characteristics difficult to predict with simple empirical methods. In this paper, RANSE CFD was used to optimize the hull and appendages of a drillship for speed or fuel consumption. An innovative thruster arrangement was initially designed for dynamic positioning and its performance in transit was verified with the use of CFD. Furthermore a special hydrodynamically shaped moonpool has been developed to eliminate moonpool sloshing during transit, further reducing resistance and increasing safety on board.

CFD was also used to determine the propulsion efficiencies of the vessel. An innovative way to use an actuator disc approach was used to calculate effective wake factors.

The various CFD calculations and the resulting design modifications are verified and confirmed with calm water resistance and self-propulsion tests.

The bow shape modifications reduce the bare hull resistance by 12% and the headbox modifications reduce the added headbox resistance by 26% of the bare hull resistance. The Callirrhoe moonpool design reduces the added moonpool resistance by 37% compared to a conventional moonpool while at the same time eliminates sloshing in transit. The propulsion efficiency is above 60% when the aft thrusters are used, which is deemed high for such a vessel.
CO2 Plume Migration With Gravitational, Viscous, and Capillary Forces in Saline Aquifers
Authors: Lee Hyesoo , Jang Youngho , Jung Woodong , Sung Wonmo
Published: October 2016

In this study, we attempt to consider three forces of gravitational, viscous and capillary forces, simultaneously when CO2 is injected in saline aquifers. In order to conduct it, we propose a dimensionless group in the form of combination of Capillary number and Bond number. This dimensionless group is a function of CO2 saturation, in which the pattern of CO2 flow regimes can be determined. By the analysis of the acquired each trapping mechanism from flow regimes, optimum CO2 injecting scheme can be designed for maximizing the solubility and residual trappings as stable mechanism. With utilizing the proposed dimensionless group, we conducted several numbers of simulations using 2D vertical heterogeneous system with respect to CO2 flow rate, interfacial tension between CO2 and water, and brine salinity. From the simulation results, when gravitational and viscous forces with respect to capillary force are described by two individual dimensionless groups of Capillary number and Bond number, CO2 saturation profiles are variously generated. These are not satisfactory correlations in the dependence of CO2 saturation on Capillary number and Bond number for the variable terms of density differences between CO2 and water and CO2 injection rates. With the proposed dimensionless group, the universal profile of CO2 saturation was obtained in describing CO2 flow behaviors for the variables. Thus, considering two variables of density differences and CO2 injection rates simultaneously, that is, when three forces considering at the same time, more realistic CO2 flow behavior can be analyzed. This study helps to determine the most secure conditions of CO2 injection and storage according to building the pattern of CO2 flow regimes which is classified by the range of a dimensionless group.
Computation of the Diffraction Transfer Matrix and the Radiation Characteristics in the Open-Source BEM Code NEMOH
Authors: Fàbregas Flavià Francesc , McNatt Cameron , Rongère François , Babarit Aurélien , Clément Alain H.
Published: October 2016

Until now, widely available boundary element method (BEM) codes did not allow the calculation of two non-conventional hydrodynamic operators, which characterize the way a body diffracts and radiates waves, known as Diffraction Transfer Matrix and Radiation Characteristics respectively. When embedded into the finite-depth interaction theory developed by [1], they drastically speed up the computation of the added mass, damping and excitation force coefficients of a group (“farm”) of floating bodies.

This paper presents the implementation of their computation in the open source BEM solver NEMOH using the methodology proposed by [2]. Results for two different geometries, a cylinder and a square box, are presented and compared to an alternative computational approach developed by [3]. A very good agreement between them is found. In addition, the hydrodynamic operators of the cylinder are compared to a semi-analytical solution available in the literature showing a good match. Results obtained using the finite-depth interaction theory are shown for a generic multi-body wave energy converter (WEC) demonstrating how the capabilities added to the BEM software NEMOH can facilitate the numerical modeling of the hydrodynamic interactions in large arrays of bodies.
Experimental and Numerical Investigation on the Relative Motions of an FLNG System During Side-by-Side Operation
Authors: Xu Qiaowei , Hu Zhiqiang
Published: October 2016

This paper addresses a study on the relative motions of the floating liquefied natural gas (FLNG) systems during side-by-side offloading operation through experimental method and numerical calculation method. Different filling levels and sea state conditions were adopted both in model test and numerical calculation. Model test results were used to validate the numerical models, which was then used to identify the influence of the filling levels and wave/wind directions on the relative motions of the vessels. It is indicated that the numerical calculation results show satisfactory agreement with the experimental results. More severe relative motions of vessels occur when the displacement of the LNGC reduces. When the wind and wave come from the side of the LNGC, the relative sway and heave of the two vessels become more severe while the relative surge is not affected by the shelter effect. The investigation will be beneficial to the further numeral research and FLNG side-by-side offloading operation.
Dune Erosion Behind Revetments due to Overtopping of Irregular Waves With Different Angles of Wave Incidence
Authors: Bonakdar Lisham , Oumeraci Hocine , Thorenz Frank , Blum Holger
Published: October 2016

The wave-induced erosion processes involved in the interaction of revetments and dunes are investigated by means of small scale experiments performed in the 3D wave basin of Leichtweiss-Institute for Hydraulic Engineering and Water Resources (LWI) in Braunschweig, Germany. Two adjacent revetments (R1 and R2) were constructed with an angle of δ = 22° in plane. Irregular JONSWAP waves were generated at two water depths, which resulted in two different wave overtopping rates. The influences of the wave attack angle and wave overtopping rate on dune erosion behind the revetments are investigated by means of laboratory tests and the results are discussed.
On Approximations of the Wave Drift Forces Acting on Semi-Submersible Platforms With Heave Plates
Authors: Molin Bernard , Lacaze Jean-Baptiste
Published: October 2016

The horizontal wave drift force acting on a vertical floating column, without then with a heave plate, is considered. Computations are performed with a diffraction-radiation code and through the Morison and Rainey equations. Focus is on wave frequencies around the heave resonance where the drift force may be significant, even though the scattered wave-field being weak. It is found that the Morison equation overpredicts the drift force while Rainey equations perform rather well.
Niobium-Bearing Offshore Plate Steel Process Metallurgy, Design and Quality
Authors: Jansto Steven G.
Published: October 2016

Offshore structural steel design and material requirements continue to present increasing challenges for the steelmaker and fabricators. Niobium-bearing steels currently play a key role in meeting these objectives through the Nb-grain refinement mechanism of the microstructure and cost effective steelmaking. These steels possess a combination of exceptional properties with high strength, excellent weldability, high toughness at low temperature, good ductility, excellent corrosion resistance, and high formability. Reduced variation of Charpy toughness through the thickness of heavy plates is imperative in these offshore platforms to enhance reliability and performance. Toughness variation can be reduced through the proper continuous casting and hot rolling mechanical metallurgy process. These high-performance steels (HPS) possess an optimized balance of these properties to provide maximum cost effective performance in offshore structures at strength levels from 355 to 700MPa with excellent corrosion resistance. This combination of good strength-toughness balance, excellent weathering properties and reduced preheat temperatures for welding in these low carbon Nb-structural steels result in significant cost savings. These enhancements provide structural engineers the opportunity to further improve the structural design and offshore platform performance. Lower carbon Nb-alloy designs have exhibited reduced operational production cost at the steel mill as well, thereby embracing the value-added attribute Nb provides to benefit both the producer and the end user throughout the supply chain.
Idealized Structural Unit Method: A Review of the Current Formulation
Authors: Oksina Anna , Lindemann Thomas , Kaeding Patrick , Fujikubo Masahiko
Published: October 2016

The evaluation of the maximum load-carrying capacity of structures is an essential issue in determining their safety. Only if the prediction of the ultimate strength is ensured, their probability of survival in extreme load conditions can be estimated. Particularly for safe dimensioning of ship structures, the determination of structural strength against buckling and yielding is required by Common Structural Rules (CSR). For ultimate strength calculations of ship structures, including ultimate hull girder capacity and ultimate strength of plates and stiffeners, the Finite Element Method (FEM) is a feasible tool. Despite the enormous development in computer technology, nonlinear finite element analyses (FEA) are still very time consuming and not easily to perform as well. Therefore the Idealized Structural Unit Method (ISUM) can be applied for collapse analysis of large structures.
The Role of Software Quality in Maritime Safety and Security
Authors: Card David N.
Published: October 2016

This article discusses the importance of achieving software quality to ensure the safety and security of maritime systems. Software controls the behaviour of many maritime and offshore systems, making it an essential component of such systems. Software defects have been the primary cause of several accidents and have contributed to many more. All the good work of mechanical engineers and marine architects can be undone by a simple programming error. Unfortunately, standards and risk analyses that address software currently are not part of standard industry practice.

This article reviews the motivation for addressing software as part of maritime systems assurance and summarizes the standards and risk analyses available to the industry. Both international standards and classification rules are beginning to be adopted, as part of an increasing recognition of the role of software in all complex systems.

This article introduces a framework for examination of software quality, including process product, operational, and customer perspectives. Important elements of this article were introduced in Card [1]. This article focuses on software process quality. Getting the software right the first time is much cheaper than testing and fixing it to reach acceptability. The cost of rework for a typical software development project ranges from 20% to 50% of total project costs. This article identifies some common software process reference models and explains how they help to achieve software quality. While, there is no specific national or international regulation addressing Software Quality Assurance (SQA) in the Maritime and Offshore industry, some recommendations and guidelines have emerged recently. Three classification societies (DNV GL, ABS, and LR) have published software process reference models (standards) as optional rules for offshore vessels. These standards are based on widely accepted ISO/IEC standards that have been endorsed by national standards bodies. Regulations and liability concerns will eventually force the maritime and offshore industry to accept greater requirements for software quality assurance, as has happened in other major industries. For now, if a customer wants quality, he/she must ask for it. These new optional standards make that easier.
Analysis of Clump-Weight Interference With Offshore Pipelines: Mechanisms and Actions
Authors: Alsos Hagbart S. , Wendel Åsta O. , Kvarme Stig Olav , Sævik Svein
Published: October 2016

The article addresses challenges and actions from interference between trawl gear and offshore pipelines. Interaction from twin trawlers and their clump weights presents a particular challenge for offshore pipelines, especially in free-spans. The design loads are typically derived from DNV-RP-F111 and are of such magnitude that protection by rock installation is often required. This is especially the case for small and medium sized diameter pipelines, and has an obvious cost aspect tied to it.

The paper investigates the detailed behavior of clump weights interfering with pipelines by breaking down the kinematics of the clump weight and its interaction with an offshore pipeline to a set of differential equations. The system of equations is solved numerically and verified with tests. Next, the response of the clump weight and pipeline is assessed for various parametric variations. This involves assessing the sensitivity of the response for variations such as: interaction with free spans versus flat seabed, effect of varying the pipeline diameter, effect of contact friction, effect of trawl drag and warp line stiffness, effect of clump weight rigging, and the effect of lateral dragging resistance of the pipeline. Finally, the clump weight response solver is applied in case study to provide the trawl pullover design load for an FE pipeline model. The response and consequence for the pipeline is then finally compared with the equivalent scenario generated from the DNV-RP-F111 design pullover load.
Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) FEA Results and Multiaxiality Study Results
Authors: Rampi Lucile , Bignonnet Andre , Le Cunff Cedric , Bourgin Francois , Vargas Pedro
Published: October 2016

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.

A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.

This paper summarizes the computational Finite Element Analysis (FEA) scope of work that provided the understanding and validation of the OPB mechanism through correlation with the test program results on chains. In addition, a multiaxial assessment of the fatigue stresses is studied and the main results are presented in this paper.
Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) Fatigue Test Program Results and Methodology
Authors: Rampi Lucile , Gerthoffert Arnaud , Francois Michel , Bignonnet Andre , Vargas Pedro
Published: October 2016

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon.

A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the OPB mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations.

This paper summarizes the full scale fatigue tests on chains and also the tests on small samples addressing the environmental influence on fatigue initiation and crack propagation stages. This paper also addresses the major step that was achieved: the implementation of a multiaxial fatigue criterion to address OPB hotspots as a standard practice in offshore industry. Moreover, the paper presents the first Industry OPB based S-N curves and its comparison to the existing industry fatigue S-N curve. Lastly, this paper provides a summary of the main steps in a framework for OPB fatigue calculation guidelines.
Extreme Response Prediction for Fixed Offshore Structures by Monte Carlo Time Simulation Technique
Authors: Abu Husain M. K. , Mohd Zaki N. I. , Johari M. B. , Najafian G.
Published: October 2016

For an offshore structure, wind, wave, current, tide, ice and gravitational forces are all important sources of loading which exhibit a high degree of statistical uncertainty. The capability to predict the probability distribution of the response extreme values during the service life of the structure is essential for safe and economical design of these structures. Many different techniques have been introduced for evaluation of statistical properties of response. In each case, sea-states are characterised by an appropriate water surface elevation spectrum, covering a wide range of frequencies. In reality, the most versatile and reliable technique for predicting the statistical properties of the response of an offshore structure to random wave loading is the time domain simulation technique. To this end, conventional time simulation (CTS) procedure or commonly called Monte Carlo time simulation method is the best known technique for predicting the short-term and long-term statistical properties of the response of an offshore structure to random wave loading due to its capability of accounting for various nonlinearities. However, this technique requires very long simulations in order to reduce the sampling variability to acceptable levels. In this paper, the effect of sampling variability of a Monte Carlo technique is investigated.
Derivation of Morison’s Force Coefficients by Three Alternative Forms of the Method of Moments
Authors: Mohd Zaki N. I. , Abu Husain M. K. , Najafian G.
Published: October 2016

Morison’s equation is the most widely used method of predicting wave forces on slim cylindrical members of offshore structures. The equation assumes that the wave force is composed of two components: a drag force and an inertial force, where the drag component is due to water particle velocity and the inertial component is due to water particle acceleration. Morison’s equation has two empirical coefficients, which are usually referred to as the drag and inertia coefficients. The values of these empirical coefficients are determined from laboratory and/or field experiments. In a typical wave load investigation, the wave force together with corresponding water particle velocity and acceleration are measured. The measured data is then analysed to calculate constant values for drag and inertia coefficients. One of the methods used in derivation of these coefficients is the (conventional) method of moments. However, the coefficients obtained from this method show considerable scatter due to large sampling variability. The purpose of this paper is to compare the sampling variability of drag and inertia coefficients from the conventional method of moments with those derived from two alternative forms of the method, i.e. methods of linear and low-order moments. Simulated data has been used to compare the efficiency of the three methods of moments. The results indicate that in most cases, the method of linear moments is superior to the other two methods. This is particularly true for drag-dominated forces.
Numerical Models as Enabling Tools for Tidal-Stream Energy Extraction and Environmental Impact Assessment
Authors: Yang Zhaoqing , Wang Taiping
Published: October 2016

This paper presents a modeling study conducted to evaluate tidal-stream energy extraction and its associated potential environmental impacts using a three-dimensional unstructured-grid coastal ocean model, which was coupled with a water-quality model and a tidal-turbine module. The unstructured-grid tidal-turbine model was first applied to investigate the effects of different tidal farm configurations on tidal energy extraction and the effects on the system flow field as well as biogeochemical transport processes in an idealized bay with a narrow channel connecting to the coastal ocean. Model results indicated that a large number of turbines are required to extract the maximum tidal energy and cause significant reduction in the volume flux. Model results also showed that tidal energy extraction has a greater effect on flushing time than on volume flux reduction. In the idealized tidal channel, a 10% reduction of volume flux caused by tidal energy extraction would result in an approximately 50% increase in flushing time in the bay. The flushing time increases exponentially as a function of flow reduction. A water-quality model simulation was conducted to investigate the dynamic effect of tidal energy extraction on water quality in a stratified tidal channel and estuary system. Model results showed that deployment of tidal turbines in the channel would increase vertical mixing in the bay. However, extraction of tidal energy also would result in a decrease in bottom dissolved oxygen in the bay during summer, which may cause hypoxia in fish. Finally, the tidal-turbine model was applied to a real-world site in Puget Sound — a highly energetic estuary on the US Pacific Northwest coast. The model application of tidal energy extraction in Puget Sound demonstrated the advantage of using an unstructured-grid modeling approach with high grid resolution near the tidal-turbine farm within a large model domain. This study showed that a numerical model can be a useful tool for assessing tidal energy extraction and its environmental impacts and for informing regulatory and policy processes for tidal energy development.
Study of Seabed Trench Induced by Steel Catenary Riser and Seabed Interaction
Authors: Wang Kunpeng , Low Ying Min
Published: October 2016

Seabed trench profile has significant effect on the fatigue damage of steel catenary riser near touchdown point. This study briefly demonstrates an approach in literature to determine the seabed trench induced by wave frequency response based on the cubic polynomial model. In this approach, a criterion for the matching between catenary riser and seabed trench is proposed, which is an optimization problem, and needs iterative static analysis of catenary riser. Based on the criterion, the sensitivity of the trench length and position to three parameters is parametrically studied: riser mass per unit length, ratio of horizontal span to vertical span of catenary part, trench depth. The obtained data are employed to fit the equations of trench length and position, which is taken as surrogate model since the iterative static analysis is very complicated. For completeness, the validation against data obtained from hysteretic seabed model is also illustrated. Based on the surrogate model, this study investigates the effect of trench depth on the fatigue damage near touchdown and the effect of the low frequency response on the seabed trench, and some useful conclusions are obtained.
Reliability of Offshore Wind Turbine Support Structures Subjected to Extreme Wave-Induced Loads and Defects
Authors: Yeter Baran , Garbatov Yordan , Guedes Soares C.
Published: October 2016

The probability of existence of defects, fatigue damage and crack growth in the offshore wind turbine support structures subjected to extreme waves and wind-induced loads is very high and may occur at a faster rate in a low cycle fatigue regime and crack growth, leading to a dramatic reduction in the service life of structures. It is therefore vital to assess the safety and reliability of offshore wind turbine support structures at sea. The aim of the present study is to carry out a low cycle fatigue and crack growth reliability analysis of an offshore wind turbine support structure during the service life. The analysis includes different loading scenarios and accounts for the uncertainties related to the structural geometrical characteristics, the size of the manufacturing and during the service life defects, crack growth, material properties, and model assumed in the numerical analyses. The probability of failure is defined as a serial system of two probabilistic events described by two limit state functions. The first one is related to a crack initiation based on the local strain approach and the second one on the crack growth applying the fracture mechanic approach. The first and second order reliability methods are used to estimate the reliability index and the effect of low cycle fatigue and crack growth on the reliability estimate of the offshore wind turbine support structure. The sensitivity analysis is performed in order to determine the degree of the significance of the random variables and several conclusions are derived.
Residual Strength of Pitted Mild Steel Plates Subjected to Biaxial Compression
Authors: Jiang Xiaoli , Guedes Soares C.
Published: October 2016

The present paper focus on the residual strength of pitted mild steel rectangular plate under biaxial compression. This paper aims to propose a general and practical formula to predict the residual strength of pitted rectangular plates under biaxial compression starting from the classic formula for intact rectangular plates and assessing whether it can be applicable to pitted plates, where the degree of pitting corrosion is modelled as one key parameter. Firstly, the numerical model is verified with an existing case study. Afterwards, a series of nonlinear FEM analysis are performed, changing geometrical attributes of both pits and plates, i.e., the radius and location of pits and the slenderness of plates. Based on those simulation results, it is found that the classic formula for intact rectangular plates can be applied reasonably well for pitting corroded plates. A unique parameter DOP (degree of pitting), which is easily determined, is employed to evaluate the effect of pitting corrosion with adequately accuracy and without bias to either longitudinal or transverse compressive stress. The proposed formula can provide guidance during the process of ship structural maintenance decision-making and strength reassessment conveniently.
Effect of Welding Sequence on the Residual Stress Distribution in a Stiffened Plate
Authors: Chen Bai-Qiao , Guedes Soares C.
Published: October 2016

This work investigates the temperature distribution, deformation and residual stress in steel plates as a result of different sequences of welding. The single-pass gas tungsten arc welding process is simulated by a three dimensional nonlinear thermo-elasto-plastic approach. It is observed that the distribution of residual stress varies through the direction of plate thickness. It is concluded that the welding sequence affects not only the welding deformation but also the residual stress mainly in the lower layer of the plates. An in-depth discussion on the pattern of residual stress distribution is presented, especially on the width of the tension zone. Smaller residual tension zone and slightly lower compressive stress are found in thicker plate.
Fatigue Design and Analysis of Offshore Pipelines and Risers Subjected to Waves and Currents
Authors: Liu M. , Cross C.
Published: October 2016

The industry consensus would appear that the effect of currents on wave induced fatigue damage accumulation is assumed as insignificant and can be ignored. Only when dealing with stability, ultimate limit state design and vortex induced vibration, the recommended industry practice is to consider both currents and waves simultaneously, but except for fatigue design.

This paper presents a study on how environmental loads should be considered in terms of currents and waves for the fatigue life design of offshore pipelines and risers. The study is intended as a spur to redress the misapprehension by focusing on the coupling effect of direct waves and currents in the context of fatigue damage assessment.

It is demonstrated unequivocally that waves and currents cannot be decoupled for fatigue design assessments. Wave induced fatigue with the inclusion of currents is manifested twofold, not only the increased mean stress correction effect but also higher total damage accumulation due to elevated stress ranges. The practice of using wave histograms while ignoring currents is shown to result in an unacceptable non-conservative fatigue design. Both effects should be accounted for in the engineering assessment. A first order correction factor involving the ratio of current and wave velocities is introduced to evaluating the environmental load coupling effect. It is recognized that fatigue associated specifically with VIV phenomena is well understood and documented elsewhere, its discussion is thus outwith the aims of this paper.
An Alternative Approach to Validation of Ship Manoeuvring Models
Authors: Gavrilin Sergey , Steen Sverre
Published: October 2016

This paper is devoted to validation of ship manoeuvring models. An alternative approach to validation is proposed. It is based on comparison of accelerations measured in a validation experiment and calculated using a simulation model. It can be applied to arbitrary trials and in-service collected data. The approach is applied to assess three versions of a manoeuvring model of a twin-screw, twin-rudder vessel. Case trials are the 20° turning circle and the 20°/20° zigzag, performed with a free sailing model. Full simulations are performed as well. Conclusions made from two approaches are compared.
Life Extension and Management of Ageing FPSOs
Authors: Salleh Hilman
Published: October 2016

FPSOs have been a popular choice for deep water oil and gas production with many installations worldwide. Many of these floating production systems were tanker conversions and they are now approaching their mid-life or end of life hence, facing ageing issues relating to asset integrity.

Concurrently, there are also requirements for these floating production systems to operate to operate beyond the design life. As most of this maintenance and refurbishment work is to be done while on station, there needs to be a structured process to ensure that all key areas of concerns are reviewed.

This paper outlines the strategy available and addresses the issues and possible solutions to manage the life extension and ageing of FPSOs.
VIM Model Test and Assessment on a Semi-Submersible Type Floater With Different Column Intervals
Authors: Fujiwara Toshifumi , Nimura Tadashi , Shimozato Kohei , Matsui Ryosuke
Published: October 2016

Semi-submersible type offshore floating structures are expected to be used in the Japanese coastal area and at sea off Japan for promoting resource exploitation and development in near future. As a moored offshore floating structure may be suffered from current, Vortex-induced Motion (VIM) effect should be assessed in an appropriate manner since the VIM may cause fatigue damage of the floating structure’s mooring lines. VIM phenomenon on semi-submersible type floating structures, however, is not clear, and its comprehension is insufficient since there are only small number of open research papers with lack specifications of the structures. Moreover, a standardized assessment method for the VIM on them has not ever been proposed.

At first in this paper, the results of VIM measurement test using some large semi-submersible floating structure models in relatively high Reynolds number flow are presented, and afterwards simplified estimation method, that is useful in the design stage for a semi-submersible type offshore floating structure, is introduced as one trial.
Short-Term Prediction of Ship Pitching Motion Based on Artificial Neural Networks
Authors: Huang Bai-Gang , Zou Zao-Jian
Published: October 2016

Due to the random nature of ship motion in an open sea environment, the ship related maritime operations such as landing on an aircraft carrier, ship-borne helicopter recovery, cargo transfer between ships and so on, are usually very difficult. An accurate prediction of the motion will improve the operation safety and efficiency on board ships. This paper presents a research on the application of artificial neural network methods in the short-time prediction of ship pitching motion. The radial basis function (RBF) neural network is applied to develop a model for short-time prediction of ship pitching motion, and the other two kinds of artificial neural networks, i.e., back propagation (BP) neural network, Elman neural network are also applied for the same purpose. A comparative analysis among them is presented. It is shown that RBF neural network provides a more effective and accurate tool for predicting the ship pitching motion.
Stopping Test Method for Free-Running Model Ship Equipped With Auxiliary Thruster
Authors: Ueno Michio , Tsukada Yoshiaki
Published: October 2016

The authors report how to estimate stopping ability of full-scale ships using free-running model equipped with an auxiliary thruster. Theoretical analysis based on a modular mathematical model clarifies the similarity of three-degree-of-freedom stopping manoeuvre of a model ship to full-scale cannot be ensured by the use of auxiliary thruster. The authors, however, propose JSC, propeller advance ratio J and speed correction, ensuring the equality of J and the similarity of speed response of free-running model ship to full-scale during stopping manoeuvre. JSC is a control method of free-running model ship equipped with an auxiliary thruster for estimating stopping ability of full-scale ship from the viewpoint of safety. Numerical simulation confirms JSC gives safe side estimates of stopping ability with regard to the track reach and stopping time with acceptable margin. The analysis shown here proves the advantage of JSC comparing with possible other control methods of free-running model ships.
A Comparative Study on Displacement Response Between Air-Backed and Water-Backed Condition for Flexible Plate Structures Subjected to Underwater Shock Load
Authors: Wadadar Debashis , Samanta Asokendu , Ambetkar Vighnesh
Published: October 2016

For ships operating in combat situations, estimating the response of the bottom and side shells subjected to non-contact underwater blast load is of paramount importance. The damage severity in underwater explosion (UNDEX) depends not only on the shock factor but also on the type of fluid behind the structure (air or water). The response of a free-standing air-backed (AB) and water-backed (WB) plate has already been studied analytically by Liu and Young [1]. However, analysis for AB or WB conditions for fixed flexible plate structures has not been given due importance. In the present study, the equation of motion for the generalized single degree of freedom (SDOF) model for AB or WB plate, which accounts for its flexibility, is formulated from the principle of virtual work. However, since this model is constructed to have a global overview of the present problem without taking account of its inherent complexities, a detailed numerical investigation using MSC.DYTRAN solver is carried out for bare and stiffened plates. The results obtained from both the analytical model and numerical simulations clearly show significant reduction in displacement in case of WB condition compared to AB condition for equal shock factors. This study emphasizes the fact that WB condition can be used to our advantage in order to reduce damage associated with UNDEX in case of doublebottom or double-hull naval vessels.
The Fracture Resistance Approach in Order to Prevent Brittle Failure of Offshore Structures Under Arctic Environments
Authors: Horn Agnes Marie , Østby Erling , Moslet Per Olav , Hauge Mons
Published: October 2016

This paper is concerned with the challenges related to steel design under Arctic conditions where both loading and temperature have been discussed in relation to material requirements. Today there is a lack of rules and standards for selecting steel materials for bulk engineering for a lower design temperature than −10°C (NORSOK N-004 [1] allows down to −14°C). Both ISO 19902 Steel Structures [2] and NORSOK N-004 Design of steel structures make reference to EN10225 “Weldable structural steels for fixed offshore structures technical delivery conditions [5]” where steel materials are Charpy tested at a lowest test temperature of −40°C and proven for a design of −10°C. Hence, one major challenge for designers are to specify adequate toughness requirements at an early stage of the design process for low temperature applications.

Both NORSOK N-004[1] and ISO 19902[2] provide requirements to load combinations that need to be fulfilled, however the relationship between various load types and temperature is not mentioned in any of these standards. Thus, in the design stage the material needs to demonstrate adequate toughness where loading and temperature are treated independently. For the offshore industry, the main question is the balance between materials requirements and cost-effective solutions, and how to address this within an overall design perspective in order to avoid brittle failure. This paper discusses some of these challenges with the aim of starting a focused process leading up to a clear interpretation of the implications of overall design philosophies, necessary in order to define consistent materials requirements ensuring that brittle fracture is not going to represent a significant threat to the structural integrity. The material recommendations provided in the paper are based on the latest research results from the Arctic Materials project (2008–2017) managed by SINTEF and supported by the industry.