bluuebunny/crossref_metadata_2025 huggingface dataset:
Frequency Domain Fatigue Analysis for a Unbonded Flexible Riser: Damage Induced by Dynamic Tension
Authors: Yuan Jiabei , Hou Yucheng , Tan Zhimin
Published: November 2019

Evaluation of fatigue damage of offshore flexible risers is critical in flexible riser system design. For deepwater application, irregular wave time domain approach is often adopted as the state of practice to avoid excessive conservatism due to its better representation of the stochastic offshore environment. The approach can indeed fully capture the non-linear behaviors of the system at a significant cost of computational time. For example, computational time typically takes over 3∼4 weeks for a deep water free hanging riser system with thousands of fatigue load-cases and the full 3-hour simulations. On the other hand, the same scope of simulation can be completed in frequency domain within day(s), which will enable the designer to accelerate the optimization of riser system design. This paper presents an analysis method in frequency domain for assessing the fatigue damage of tensile armour wires inside the top end fitting (EF), which is induced by dynamic tension variation and often governs the riser service life in deep water applications. A validation measurement is also implemented to ensure the accuracy and practicability of this frequency domain approach in riser system design.
Pure Collapse Behavior of Pipelines With D/t Ratio Below 10: Real Scale Experimental Tests and Numerical Studies
Authors: Vilas Bôas Ana Carolina , Jorge Gabriel , e Silva Júlio Márcio , de Souza Ana Paula F. , Solano Rafael F.
Published: November 2019

This work addresses pure collapse experimental tests conducted with DNVGL-450 seamless pipe samples with D/t ratio equal 7.7 and 8.6. Interesting results were achieved, showing that the pipes presented a significant plastic deformation before collapse. There is a limited amount of available technical publications addressing real scale tests with thicker pipes, D/t < 20, that makes the results available quite important for the better understanding of thicker pipes. FEM was conducted and a good correlation with experimental test results were obtained. The experimental and numerical results were correlated with the DNVGL-ST-F101 pure collapse limit state equation, one of the main the rule used in the design of ultra-deep-water subsea pipelines. Conclusions and recommendations for future work based on the results achieved are presented.
A Preliminary Study on the Development of a Novel Marine Growth Cleaning Robot for Jacket Platforms
Authors: Jiang Zhe , Sun Tao , Luo Gaosheng , Wang Biao , Guo Wei
Published: November 2019

As the underwater structures of offshore Jacket platforms are always immersed in seawater, the marine growth, which include various forms of algae, slime, and seaweed, barnacles, mussels and other species of adhesive shellfish, will attach to steel-pipe surface and will accelerate structural corrosion and impair structural safety. Currently, routine cleaning and inspection task is undertaken by divers using cleaning jets in normally every 3∼5 years. The cleaning duration for one single platform will take up more than two months, even up to half a year, due to the constraints of weather windows and limited working hours of divers. It is a risky job for divers not only because of huge pressures that water-jets produce, but also the harsh working conditions of poor visibility, unexpected vortex and waves around platform, and etc.

Underwater robots are being developed for various applications in offshore oil industry ranging from inspection to maintenance and cleaning of submerged surfaces and constructions. This paper introduces a novel underwater robot specializing in cleaning marine growth for offshore Jackets. Since the diameter of steel-pipes varies from about 600mm to 2000mm, a self-adapted mechanism is designed. The self-adaption mechanism makes the robot travel on pipes in different directions with high mobility and clean continuous region of underwater pipes’ surface at the same time. Two key issues have been studied in this paper. The magnetic adhesion method is adopted in the robot. A sensitivity study on the distance between steel-pipes and thickness of steel pipes with the adhesion force are conducted both using finite element method and experiments. Besides, the flushing capability for various nozzles has been simulated using computational fluid dynamics method. The proposed underwater robot is needed in the inspection and maintenance of offshore Jacket platforms. Compared with traditional maintenance by divers, it is more efficient, economic and safe.
How Emulation Improves Offshore Operations
Authors: McGuire Leslie
Published: November 2019

Subsea 7 is supporting its offshore fleet by incorporating emulation. Virtualising the actual control system software used offshore and integrating a high resolution functional 3D “digital twin” model of its vessels and equipment. Subsea 7 can effectively test control systems under as near to live conditions as possible by also incorporating historical dynamic positioning archives of vessel sea states and other external influences such as temperatures, loads and faults into the system. Using the Subsea 7 emulation techniques means operatives can be trained on exact virtualised control system software in advance of actual construction and as a result, software faults can be highlighted, de-bugged and rectified using the emulator system. The digital twin models can also be used to facilitate efficient planning of deck operations. This paper demonstrates how technology helps improve Subsea 7’s offshore operations.

Methods, Procedures, Process: A vessel emulation room is in use for control system testing and familiarisation of construction equipment, including for instance, lay, crane or diving systems and is currently developing its library of vessel functional 3D models with full integration to the control system software. By use of physics engine feedback into the functional 3D model and control systems, projects operations can be verified.

Results, Observations, and Conclusions: It is proven using emulation from an office base, for vessels operating anywhere in the world, that Subsea 7 are able to save significant costs and time. The emulations can be re-used and expanded for the full life cycle of the equipment. Over 90% of software bugs can be ironed out prior to installation, a significant advantage for Operations.

Novel/Additive Information: This paper describes a significant step towards the digitalisation of the Subsea 7 fleet and equipment incorporating the actual control system software used to operate the equipment.
Numerical Convergence on the Hydroelasticity of a Large Containership
Authors: Lu Ye , Temarel Pandeli , Jin Qiu , Wu Yousheng , Ni Xinyun , Tian Chao
Published: November 2019

Nowadays, more and more 20,000 twenty-foot equivalent unit (TEU) class ultra large container ships (ULCS) have been built in service across the worldwide. It is paramount that hydroelastic specialists become paying close attention to structural responses and loads predictions due to up to a 400m length of the ships. First of all, mesh convergence by finite element analysis is necessary to determine in the numerical calculation. In this paper, based on the three-dimension linear frequency domain hydroelasticity theory, the hydrodynamic meshes convergence is discussed when modelling the hull surface of the ULCS. Ascribe to the Sunway TaihuLight, rank 3 in the current TOP500 supercomputer list, the Message Passing Interface and the multi-level parallel programming model are used aimed to the wetted panels, the wave frequencies and so on. Several sets of different global grid density and grid distribution along the ship’s length for the containership are calculated to compare the hydrodynamic coefficients such as added mass, damping, wave exciting force, ship motions and exterior loads with several typical service speeds in the head regular wave. It has been concluded that sensitivity of numerical modelling converges to a stable state with increasing the panel numbers per ship. Therefore, one set of grid division optimised, and superposed elastic modes numbers are recommended in the hydroelastic analysis of numerical hydroelastic prediction of springing and whipping.
Long-Term Extreme Response of a Straight Floating Bridge Across the Bjørnafjord
Authors: Giske Finn-Idar Grøtta , Fredriksen Arnt
Published: November 2019

In this paper, long-term extreme response analysis is performed for a straight floating bridge across the Bjørnafjord, using a recently developed inverse first-order reliability method (IFORM) approach. Full integration of the long-term extreme response formulation is also performed for comparison. Two different environmental models are estimated based on a scatter diagram of significant wave height and peak period for the given location. The IFORM method is seen to provide reasonable estimates of the long-term extreme response, at a significantly reduced computational effort.
Studies Toward the Development of Accurate Directional Spectrum Estimation Method Using Field Observation Data
Authors: Hashimoto Noriaki , Mitsui Masao , Kawaguchi Koji , Fujiki Takashi
Published: November 2019

An upgraded submerged Doppler-type directional Wave Meter (DWM) can measure 31 wave quantities related to directional wave motions, i.e., water surface elevation, 3 components of water particle velocities at each layer of 10 different water depths ranging from shallow to deep. In this study, accuracy of directional spectrum estimation is investigated for various cases where directional spectra are estimated with various sets of different number of wave quantities measured with DWM. As a result, accuracy improvement is confirmed in some wave conditions when more quantities are applied to the estimations, compared with the cases where a few wave quantities are applied to the estimations. On the other hand, it was found that when the directional spectra are estimated by adding the water particle velocity components at the deep water depths, the energy concentration of the direction function tends to be estimated higher in proportion to the number of observation layers of the deeper water particle velocity components. Since this feature should be clarified in investigating characteristics of directional spectra observed with DWM, we will continue further investigation for more accurate and reliable directional spectrum estimation.
Two-Dimensional Numerical Simulation of Vortex Shedding of Multiple Stranded Ropes
Authors: Wang Xinxin , Huang Liuyi , Tang Yanli , Zhao Fenfang , Sun Peng
Published: November 2019

The stranded rope is one of the important components of the fishery aquaculture equipment. We investigate the fluid flow through two-dimensional stranded rope by direct simulation of the Navier-Stokes equations. We show that for different kinds of stranded rope structures, there are significant differences in hydrodynamic performance. This paper established a numerical model of unsteady flow past the stranded rope based on the Navier-Stokes equation and Morison formulas to study the hydrodynamic characteristics of three-stranded rope, four-stranded rope, and seven-stranded rope, respectively. The turbulence flow was simulated using Standard k-ε model and Shear-Stress Transport k-ω (SST) model. The flow distribution strongly depends on the Reynolds number, a range of 3,900 and 30,000. With increasing Reynolds number, the alternate eddy formation and shedding were repeated behind the stranded ropes. Such parameters of hydrodynamic characteristics of multiple stranded ropes were calculated as the lift and drag coefficients, and vortex shedding frequencies. The numerical simulation results presented flow performances of different cross sections (a, b, c, d) at different Reynolds numbers. However, Reynolds number has no significant impact on the Strouhal number for the same attack angle of the stranded rope.
Numerical and Experimental Research on Residual Ultimate Strength of Hull Plates Under Uniaxial Cyclic Loads
Authors: Xia Tian , Yang Ping , Cui Cong , Peng Ziya , Ma Li
Published: November 2019

The present paper aims at obtaining insights on the strength behaviors of ship hull plates under uniaxial cyclic loads. The residual ultimate strength of ship hull plates under uniaxial cyclic load is studied by numerical analysis using nonlinear finite element method. A series of tests are also conducted for comparison and verify the reasonability of numerical results. The accumulated plastic damage and crack propagation from the low cycle fatigue damage during cyclic load are both considered in present study. From the numerical results and tests some significant conclusions are drawn to the residual ultimate strength of plates under uniaxial cyclic loads.
Cuttings Transport Simulation in Large-Diameter Inclined Borehole
Authors: Ignatenko Yaroslav , Gavrilov Andrey , Bocharov Oleg , May Roland
Published: November 2019

An Eulerian mixture model of the two-phase flow was used for cuttings transport simulation. The model was tested using experimental data for particles transport in pipes. Three types of problem statements were analyzed: steady-state flow, non-stationary flow in a short-length channel with periodic boundary conditions, and non-stationary flow in a long channel.

Simulation of cuttings transport by Herschel-Bulkley fluid through an inclined 21-inch borehole/–6.5-inch drillpipe annulus was performed. All problem statements showed very close results, even for unsteady flow. These results demonstrated the applicability of 2D steady-state problem formulation for cuttings transport simulation.

The unsteady flow was observed for an inclination of less than 20 degrees. Slow downward sliding of cuttings in the lower part of inclined boreholes was observed simultaneously with upstream dunes movement. Drill pipe rotation significantly decreased the cuttings concentration and pressure gradient, and shifted the maximum cuttings transport downward sliding rate from a 20- to 40-degree inclination.
Numerical Simulation for Predicting Ship Resistance and Vertical Motions in Regular Head Waves
Authors: Bekhit Adham S. , Lungu Adrian
Published: November 2019

The present study is concerned with predicting the resistance and vertical motions of the surface combatant DTMB5512 ship model in regular head waves. A series of numerical simulations are performed for various wave lengths, heights and different ship speeds. Computations are performed by making use of the ISIS-CFD solver of the commercial software Fine™/Marine provided by NUMECA, where the discretization in space is based on finite volume method using unstructured grid. The unsteady Reynolds-Averaged Navier-Stokes equations are numerically solved while the turbulence is modeled by making use of the k-ω SST model. The free-surface is captured through an air-water interface based on the Volume of Fluid (VOF) method. Computed results are validated through direct comparisons with the experimental data provided by IIHR test cases. For the sake of numerical results verification, a grid convergence study is performed on four computational grids and a time step convergence test is also included. Validation of the numerical results shows a reasonable agreement with the experimental data.
Hybrid Method for Predicting Ship Manoeuvrability in Regular Waves
Authors: Mei Tianlong , Liu Yi , Tello Ruiz Manasés , Vantorre Marc , Lataire Evert , Chen Changyuan , Zou Zaojian
Published: November 2019

The ship’s manoeuvring behaviour in waves is significantly different from that in calm water. In this context, the present work uses a hybrid method combining potential flow theory and Computational Fluid Dynamics (CFD) techniques for the prediction of ship manoeuvrability in regular waves. The mean wave-induced drift forces are calculated by adopting a time domain 3D higher-order Rankine panel method, which includes the effect of the lateral speed and forward speed. The hull-related hydrodynamic derivatives are determined based on a RANS solver using the double body flow model. The two-time scale method is applied to integrate the improved seakeeping model in a 3-DOF modular type Manoeuvring Modelling Group (MMG model) to investigate the ship’s manoeuvrability in regular waves.

Numerical simulations are carried out to predict the turning circle in regular waves for the S175 container carrier. The turning circle’s main characteristics as well as the wave-induced motions are evaluated. A good agreement is obtained by comparing the numerical results with experimental data obtained from existing literature. This demonstrates that combining potential flow theory with CFD techniques can be used efficiently for predicting the manoeuvring behaviour in waves. This is even more true when the manoeuvring derivatives cannot be obtained from model tests when there is lack of such experimental data.
Added Resistance CFD Analysis of the KVLCC2 With the Naval Hydro Pack
Authors: Vukčević V. , Gatin I. , Kim G.-H. , Jasak H.
Published: November 2019

A CFD analysis of added resistance of a KVLCC2 ship model is presented in this paper. The Naval Hydro Pack, an open source software library for computational naval hydrodynamics based on OpenFOAM is used to perform the simulations.

Ten head wave cases are considered in this study ranging from short waves to long waves (wave length to length between perpendiculars ranging from 0.3 to 2). During the initial stages of our research, we had noticed significant over-prediction of added resistance compared to experimental results. After thorough analysis, the issue was found to be related to inadequate turbulence modeling using the standard k-ω SST model. Using the free surface sensitised model, the prediction of the added resistance improves significantly Compared to the experimental results, majority of the cases with different wave lengths have errors smaller than several percent. In addition to added resistance, heave and pitch motion amplitudes are compared to recent experimental results by Park et al. [1], showing good agreement. CPU time required to perform the computations is also discussed.
Development of an Image Processing Module for Autonomous Underwater Vehicles Through Integration of Object Recognition With Stereoscopic Image Reconstruction
Authors: Lin Yu-Hsien , Chen Shao-Yu
Published: November 2019

This study investigated the development of visual recognition and stereoscopic imaging technology, applying them to the construction of an image processing system for Autonomous Underwater Vehicles (AUVs). For the proposed visual recognition technology, an optical flow algorithm was used to detect the linear features and movement speeds of dynamic images; the proposed stereoscopic imaging technique employed a Harris corner detector to estimate the distance of the target. A physical AUV was constructed with a wide-angle lens camera and a binocular vision device mounted on the bow to provide image input. Subsequently, a simulation environment was established in Simscape Multibody and used to control the post-driver system of the stern, which contained horizontal and vertical rudder planes as well as the propeller. Finally, the dynamic testing results were combined with a fuzzy controller to output the real-time responses of the vehicle regarding the angles, rates of the rudder planes, and the propeller revolution speeds at various distances.
Estimating Operational Weather Downtime: A Comparison of Analytical Methods
Authors: Lambkin David , Wade Ian , Stephens Robin
Published: November 2019

Weather downtime (WDT) is a logistical and financial risk when planning operations or offering services. Such risk is typically identified and managed in advance using statistical predictions based on historical weather data. Estimates of programme and cost for offshore construction work may vary, not because of the nature of a task, or the environment at the location, or the capability and price of a vessel, but because estimates of WDT have been calculated in different ways.

Estimates of WDT are required in order to develop a realistic programme for complex and long duration projects. Therefore, a good understanding of the analytical options and a feel for the implications of the many and varied approaches is key to finding optimal solutions regarding WDT assessments.

In this paper we consider a number of variants to the two principal approaches (namely ‘Weather Windows’ and ‘Simulation Based’ WDT analysis) to the derivation of WDT statistics. WDT estimates calculated using the same environmental input data, but alternative approaches are presented. The presentation highlights the potential variation in downtime statistics that can result from the alternative analyses, aiming to improve awareness of the application of such statistics when estimating project programme and cost at the planning stages.
Method Evaluating Axial Response of Vertically-Loaded Piles During Spudcan Penetration
Authors: Fan Yifei , Wang Jianhua
Published: November 2019

Large soil deformation can be induced during spudcan penetration into stratum near an existing piled foundation of a fixed offshore platform, and some research has been done on the effects of spudcan penetration on the lateral response of an adjacent pile. However, available centrifuge model tests found that the maximum vertical soil displacements are manifested in the same magnitude compared with those in the lateral direction, leading to large negative friction along the pile shaft. Also, the combined loading effects of transmitted loads from the upper structure should be taken into account in axial analysis during spudcan penetration, which has been proved important in prediction of axial pile response due to soil movement. This paper investigates a method to quantitatively evaluate the effect of spudcan penetration on vertical response of an adjacent pile. The vertical soil displacement is found by first simulating a free field spudcan penetration by Coupled Eulerian-Lagrange (CEL) method. The flow field is then analyzed to calculate the displacement components at specific pile location. Then the predicted vertical soil displacement profile is input into the nonlinear foundation pile model to determine the axial response of an adjacent pile subjected to spudcan penetration-induced axial and pile head loads. The proposed method is adopted to analyze centrifuge model tests in sand and clay, and predicted results prove to be basically in agreement with measured ones.
Evaluation of Impact Loads on Offshore Jacket Platform During Float-Over Mating Operation
Authors: Kagita Gurumurthy , Addala Mahesh B. , Achary Gudimella G. S. , Sripada Subramanyam V. R.
Published: November 2019

In the mating phase of float-over operation, the topsides deck load from the vessel is transferred onto the jacket either by ballasting the vessel or by the combination of ballasting and hydraulic jacking system. During this phase of operation, the topsides and jacket experience impact loads through the contact points in a short duration of time. To evaluate the impact loads and to capture the transient effects precisely, a non-linear time domain hydrodynamic analysis is required. To obtain the design loads, generally the numerical jacking simulation is initiated at the time instant of maximum wave height when the jacking system is used. However, the conservative response may also depend on the relative velocity between the jacket and topsides legs. In this paper, a series of non-linear time domain as well as linear frequency domain hydrodynamic analyses are performed to evaluate the impact loads between 9000 tonne integrated topsides deck and a 4-legged jacket in a water depth of 50 m during float-over mating operation. The simulations are performed using MOSES software. The float-over hardware such as LMUs (leg mating unit), DSUs (deck support unit), Jacks, Fenders and Mooring lines are modelled as appropriate linear / nonlinear springs. The principle of the mating operation is considered through a combination of vessel ballasting and jacking operation. This paper discusses about random wave seed selection, effect of vessel response and wave headings on the impact loads of LMUs and Jacks/DSUs.
Sensitivity Analysis of Different Parameters of Taut Mooring System of a Truss Spar
Authors: Kang Zhuang , Chang Rui , Kang Youwei , Liu Shanchuan
Published: November 2019

The taut mooring system is widely used for some advantages, such as smaller mooring radius, lighter total weight and better anti-corrosion performance. In this paper, the taut mooring system of a Truss Spar platform which was taken as the research object was investigated under the condition of 2000 m water depth in South China Sea. Firstly, the main body of the platform was analyzed in frequency domain based on the 3-d potential theory, and then the nonlinear solutions of platform displacement and mooring line force were obtained by using coupling analysis method in time domain, which determined the preliminary design parameters of mooring system. The sensitivity of the taut system is studied by changing several design parameters such as the top angle of mooring line, cable hole position and method of mooring disposal. In summary, the variation of the motion and dynamic response of the platform and mooring system has been explored and summarized by studying the design process and influential parameter of dynamic characteristics of mooring system and optimizing ideas of relevant parameters, which can further provide technical support and engineering reference for the design and application of the taut mooring system of deepwater Truss Spar platforms.
Impacts of Reliability on Operational Performance and Cost of Energy Evaluation of Multi-Megawatt, Far-Offshore Wind Turbines
Authors: Dao Cuong D. , Kazemtabrizi Behzad , Crabtree Christopher J.
Published: November 2019

Wind energy is growing at a fast pace around the world. According to a report published by WindEurope, 55% of total power capacity installations in the EU came from wind in 2017. In this context, offshore wind plays a decisive role, with countries such as the UK leading the development of large-scale offshore wind projects within Europe and around the world. It is essential that the cost of energy from offshore wind remains competitive with other sources of energy to encourage further investment in offshore wind developments. One way to maintain and further reduce the cost of offshore wind energy is to take advantage of economies of scale by increasing the megawatt ratings of offshore wind turbines. On the other hand, the operational expenditure of the turbines could also be reduced significantly. In this paper, we present a new integrated operation simulation framework for performance evaluation of multi-megawatt direct drive wind turbines suitable for use in far offshore wind farms. The operation simulation considers several essential wind turbine data such as component reliability, i.e. failure rates and downtimes per failure, historical wind speed, turbine information, and repair cost per failure to estimate the operational and economic performance of the wind turbine in its entire lifetime. In the proposed operation simulation, component reliability models and a wind power model are coupled together to simulate wind turbine operation over its entire lifetime using a time-sequential Monte Carlo simulation. Since the reliability data for large-scale offshore wind turbines are scarce and/or restricted to only direct stakeholders, a range of operational profiles for the turbines based on different level of reliability are simulated. In addition, the economic performance of the turbine is measured by defining an index for levelised cost of energy as a function of component reliability. In this way, the wind turbine reliability, power output, failure cost and levelised cost of energy are estimated under the variation of input reliability data. The results of this paper can inform wind turbine performance depending on the reliability of its components, and provide useful information for critical components identification and economic assessment of future far offshore wind turbines.
Hydrodynamic Forces on Intermittently Spanning Pipelines in Steady Currents
Authors: Teng Yunfei , Cheng Liang , An Hongwei , Tong Feifei , Griffiths Terry , Sun Wei , Chi Jiawei , Xiong Zhijian
Published: November 2019

Experimental investigations on the hydrodynamic forces on an intermittently spanning pipeline exposed to steady currents were carried out. The effect of intermittent local spanning sections on the global hydrodynamic behavior was studied by changing the ratio between the non-spanning length (B) and the total length (L), namely the blocking ratio B / L. A range of gap height (G) to diameter (D) ratios, i.e. gap ratio G / D, and 4 different Reynolds numbers (Re) in the subcritical region were tested in the experiments. The results show: i) for a certain gap ratio, the mean drag increases gently with the decreasing blocking ratio at Re = 5.5 × 104, whereas the mean lift decreases significantly with the decreasing blocking ratio at all values of Re tested; and ii) for a certain blocking ratio, increasing the gap ratio leads to an increase in mean drag and decrease in mean lift. Further, simple approaches are proposed based on the present dataset for estimating the global effects on hydrodynamic drag and lift forces due to local spanning geometry.
Hydrodynamics Interactions on Vortex-Induced Motions of a Multi-Body Floating System
Authors: Liang Yibo , Tao Longbin
Published: November 2019

In this study, numerical analysis has been carried out to investigate the hydrodynamic interactions of two multi-columns platforms. The objective of the work is to preliminarily evaluate the feasibility of a tension-leg-platform (TLP) dry tree unit (DTU) with tender assisted drilling (TAD) from the aspect of vortex-induced motions, characterized by the current. Two multi-columns floating platforms with a small gap ratio (28% of the overall platform width) are numerically simulated with the scenario of 3 degree-of-freedom (DOF) on the horizontal plane (including transverse, in-line and yaw motions). A comprehensive numerical simulation was conducted to examine the hydrodynamics interactions due to the flow over two floating platforms. Horizontal plane motions including transverse, in-line and yaw motions as well as drag and lift forces on both structures are discussed. The numerical data on the multi-body VIM interactions within the “lock-in” region will serve as a preliminary study for future coupled motions analysis of the TLP-TAD system design.
Fatigue Tests on Corroded Mooring Chains Retrieved From Various Fields in Offshore West Africa and the North Sea
Authors: Ma Kai-tung , Gabrielsen Øystein , Li Zhen , Baker David , Yao Aifeng , Vargas Pedro , Luo Meng , Izadparast Amir , Arredondo Alberto , Zhu Linfang , Sverdlova Nina , Skutle Høgsæt Ingrid
Published: November 2019

When an aged mooring system seeks a life extension, it is necessary to assess the remaining fatigue life of the corroded mooring chain. This paper summarizes the results of fatigue tests performed on mooring chain samples retrieved from six different fields in West Africa and North Sea. The impacts of corrosion on fatigue life on the samples were researched. The tests were managed under a Joint Development Project, “Fatigue of Corroded Chains (FoCCs JDP)”. The objectives of the JDP are (1) to derive a methodology for assessing the remaining fatigue life of corroded chain, (2) to develop guidance for performing reliable FEA of chain links to assess remaining fatigue life, and (3) to provide more rational basis to improve industry guidance on mooring line replacement criteria for life extension. Fatigue test procedure was defined by the fifteen (15) participating members. The procedure specified the testing parameters, including mean tension, tension range, and test frequency. Six sets of fatigue tests have been completed in seawater with the number of cycles to failure recorded. These chain samples were retrieved from floating production and storage units, e.g. FPSOs and FSUs, that were still in service. Fatigue data obtained from the tests were plotted against the design SN curves and results from fatigue testing of new chain. It was found that most of these samples have limited amount of fatigue capacity remained. Most interesting finding is that the sharpness of the surface feature on the corroded chain link has a significant impact on the remaining fatigue life. Another interesting finding is that the surface feature created by corrosion can be quite distinct and unique depending on the geophysical locations where the sample came from. These findings and test results may serve as references for life extension assessment of an aged mooring system.
Staggered Grooves for the Suppression of Vortex-Induced Vibration in Flexible Cylinders
Authors: Law Yun Zhi , Jaiman Rajeev K.
Published: November 2019

Vortex-Induced Vibration (VIV) remains a challenge to the offshore structures such as deepwater riser and subsea pipelines, which require a robust and cost-effective control to circumvent the impact of the dynamic loads and the fatigue damage. While the state-of-the-art helical strakes are effective in the suppression of VIV amplitudes, they cause a higher drag force and bending moment on the submerged structure. In this work, we numerically investigate the recently proposed staggered groove concept to reduce both the VIV amplitudes and the drag force. The staggered groove is constructed by aligning the square grooves alternatively along the spanwise (axial) direction of the cylinder. The performance of the staggered groove concept is examined in three dimensions for two VIV configurations at subcritical Reynolds number (Re) namely: (i) two-degree-of-freedom elastically mounted rigid cylinder (Re = 3000–10000), and (ii) pinned-pinned flexible cylinder in a uniform current flow at Re = 4800. Their characteristic responses and the vortex dynamics are compared to their plain cylinder counterparts. For the two VIV configurations, our results show a remarkable reduction of both the peak vibration amplitude and the drag force up to 40% and 20%, respectively. Further analysis has shown that such reduction is related to the diminishing of the spanwise correlation of hydrodynamic forces due to the alternating alignment of the grooves. Such effect on the spanwise correlation leads to the broadening of the frequency spectra of the forces, thereby reduces the average power transferred to the cylinder and leads to the VIV suppression.
Effect of Buoyancy and Inertia on Viscoplastic Fluid-Fluid Displacement in an Eccentric Annulus With an Irregular Section: Part 2 — Displacements in Vertical Annulus
Authors: Skadsem Hans Joakim , Kragset Steinar
Published: November 2019

Casing strings and liners are important subsurface structural components in petroleum and in geothermal wells. After the casing string has been run in hole, it is cemented to the formation by pumping a sequence of spacer fluids and cement slurry into the annulus outside the string. Spacer fluids are usually pumped ahead of the cement slurry in order to displace the drilling fluid from the annulus that is to be cemented, and thereby avoid contamination of the cement slurry. Fluid displacements are governed by inertia, buoyancy and viscosity effects, in addition to being strongly influenced by the annular geometry. Poor centralization of the casing or irregularities such as washouts can influence the displacement flows both locally and over long axial distances.

We present three dimensional numerical simulations of the displacement flow involving two viscoplastic fluids in the vicinity of a symmetric local hole enlargement. We focus on laminar flow regimes in the regular part of the annulus and investigate how the volumetric flow rate and the mass density difference between the fluids affect the displacement efficiency in the regular and the irregular parts of the annulus. This study considers viscoplastic displacement flows in a near-vertical, irregular annulus and is an extension of a previous publication that focused on a near-horizontal annulus.

We contextualize our simulations by comparison to industry guidelines for effective and steady laminar displacements in the regular, near-vertical annulus. Here, eccentricity favors flow in the wider sector of the annulus while a positive density difference between the fluids generates secondary, azimuthal flow toward the narrow side of the annulus. In the enlarged and irregular section, both the axial bulk velocity and casing eccentricity decrease sharply and buoyancy becomes more pronounced compared to in the regular annulus. We quantify and discuss the effects of local hole enlargements on displacement efficiencies. Simulations of cementing flows can aid in optimizing fluid properties and pump rates, including when the wellbore has suspected or confirmed zones of irregular geometries.
Wind and Current Loads on Barges and Ships
Authors: el Moctar Ould , Schellin Thomas E. , Neugebauer Jens
Published: November 2019

Increased waterborne trade has led to the construction of ever larger ships and barges as oversized modules are transported by sea. The provision of tugs for towing such vessels has become a serious issue, especially in restricted areas often characterized by coastal regions of limited water depth. Wind loads are most relevant for towing operations because large forces act on the sail area and submerged hull of the towed vessels, such as construction barges when carrying oversized modules or fully laden containerships. Systematic steady and unsteady numerical simulations were performed using a RANS-based field method to predict wind and current forces and moments acting on representative construction barges, containerships, tankers, and passenger ships. Aero- and hydrodynamic computations were carried out separately. Aerodynamic computations considered various deck load configurations to represent realistic loading conditions; hydrodynamic computations accounted for finite water depth. Agreement with available wind tunnel experimental data was generally favorable.

Our purpose was to provide a reference for wind and current loads on different barge and ship types. The objective was not to investigate flow details needed for, e.g., smoke propagation, helicopter landing, etc. This has been covered by other researchers.
Analysis of Wake Interaction of Oscillating Platform With Four Columns
Authors: Hirabayashi Shinichiro , Cicolin Murilo M. , Gonçalves Rodolfo T. , Assi Gustavo R. S. , Suzuki Hideyuki
Published: November 2019

The effect of wake interaction on the hydrodynamic force was experimentally investigated. A scaled platform model with four columns was subjected to a current in the circulating water channel with constant velocity. The diameter of each column was 75 mm, and the aspect ratio (defined by the ratio between the column draft and diameter) was 1.5. The spacing ratios, (defined by the ratio between the distance of column centers and diameter) were 3 and 4. The platform was forced to oscillate with different frequencies and amplitudes. Flow around columns was measured by using Particle Image Velocimetry (PIV). The difference of hydrodynamic forces on each column was discussed concerning the effect of wake interaction among columns.
Estimating Fracture Toughness From Charpy Data
Authors: Pisarski Henryk , Bezensek Bostjan
Published: November 2019

Circumstances arise when direct determination of fracture toughness, necessary for conducting Engineering Critical Assessments (ECAs), is not possible but Charpy data are available. These situations can arise, for example, when assessments are needed for existing equipment to demonstrate avoidance of fracture or preliminary assessments are required when only specification properties are available. Some of the empirical procedures that may be used to estimate fracture toughness of steels are described. These are based on a recent revision and update of Annex J of BS 7910; the latter provides an integrated method for conducting ECAs. Procedures for estimating fracture toughness from Charpy data representing lower shelf (energies less than 27J), transitional (based on T27J or T40J (Charpy temperatures for 27J or 40J, respectively)) and upper shelf Charpy behaviour are described. In addition, a method is described for estimating T27J when determining fracture toughness from transitional Charpy behaviour where an incomplete transition curve or only single temperature data are available. The thinking behind the procedures is described and examples for their validation (i.e. predictions of fracture toughness compared with actual data) are provided.
CFD for Vortex-Induced Motions and Line Forces of a Floating Caisson With Complex Geometry
Authors: Decrop Boudewijn , Kulkarni Rohit , Breugem Alexander , Villaverde Vega Damian
Published: November 2019

Two caissons with complex geometry need to be towed to an installation site in a straight with potentially strong currents, fairly strong wind and moderate wave conditions. In order to design the mooring system, current, wind and wave forces need to be assessed. Both time-averaged and oscillating components are important. A methodology in which the degree of complexity is gradually increased has been conceived. In this methodology, first an assessment of the order of magnitude of each force is determined, after which it is decided whether in-depth computation of higher accuracy is required. This led to a methodology in which wind forces have been determined analytically, current forces and Vortex Induced Motions (VIM) have been determined using CFD and wave forces by means of a panel method. The resulting forces have been combined in a numerical mooring analysis. The focus of the presented paper is on the CFD model applied to determine VIM and a comparison with resonance frequencies determined analytically.
Fatigue of Welded Tubular X-Joints in Offshore Wind Platforms
Authors: Papatheocharis Theocharis , Sarvanis Gregory C. , Perdikaris Philip C. , Karamanos Spyros A.
Published: November 2019

The paper is part of the European research program JABACO (2015–2018), on the optimization of design and construction of offshore jacket platforms for supporting large wind turbines (5–10 MW) in water depths ranging from 30m to 80m. In particular, the paper describes an experimental investigation on the high-cycle fatigue performance of welded tubular connections, subjected to in-plane bending loading. Experimental results from seven (7) X-joint specimens are presented. The specimens were manufactured with 18-inch-diameter tubes and a brace-to-chord-diameter ratio equal to 1. Furthermore, the brace-to-chord-thickness ratio is equal 0.6, and the brace-chord angle is 90-degrees. The specimens are made of regular carbon steel grade 355, and have been fabricated using two different welding techniques: (a) manual (semi-automatic) welding (5 specimens); and (b) robot (automatic) welding (2 specimens). The comparison of the fatigue design life of those welding methods is a major objective of the present study. Prior to testing, numerical simulations have been performed to determine the critical locations around the weld toe, for proper instrumentation of the tubular specimens in terms of strain gage locations. This research work aims at a critical evaluation of available design standards, towards the development of more reliable design tools and reduction of the construction cost of the platform.
Quasi-Static and Dynamic Compressive Behaviors of Closed-Cell Stochastic Foams Based on Voronoi Model
Authors: Chen Jianyong , Liu Jun , Cheng Yuansheng , Zhang Pan
Published: November 2019

Closed-cell stochastic foams are widely used in engineering field due to the excellent energy absorption capability. In this paper, the crushing response of three dimensional closed-cell foams is investigated under quasi-static and dynamic compression loading. Voronoi tessellation is employed to generate the mesoscale geometric models of closed-cell stochastic foams, and subsequently numerical analysis is carried out using LS-DYNA software. Results reveal that the plateau stress of Voronoi model under quasi-static compression linearly increases with the increase of relative density. In addition, the mechanical properties of Voronoi model under quasi-static and dynamic compression are related to a shear band collapse mode and a layer-wise collapse mode, respectively. The plateau stress and the densification strain under high-speed loading are higher than those under quasi-static and low-speed loading.
On the Hydrodynamic Interaction Between Ship and Free-Surface Motions on Vessels With Moonpools
Authors: Ravinthrakumar Senthuran , Kristiansen Trygve , Ommani Babak
Published: November 2019

Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quasi three-dimensional settings, where the models are studied in forced heave and in freely floating conditions. The two-dimensional setups are with a recess, while the quasi three-dimensional setups are without recess. One configuration with recess is presented for the two-dimensional case, while three different moonpool sizes (without recess) are tested for the quasi three-dimensional setup. A large number of forcing periods, and three wave steepnesses are tested.

Boundary Element Method (BEM) and Viscous BEM (VBEM) time-domain codes based on linear potential flow theory, and a Navier–Stokes solver with linear free-surface and body-boundary conditions, are implemented to investigate resonant motion of the free-surface and the model. Damping due to flow separation from the sharp corners of the moonpool inlets is shown to matter for both vessel motions and moonpool response around the piston mode.

In general, the CFD simulations compare well with the experimental results. BEM over-predicts the response significantly at resonance. VBEM provides improved results compared to the BEM, but still over-predicts the response.

In the two-dimensional study there are significant coupling effects between heave, pitch and moonpool responses. In the quasi three-dimensional tests, the coupling effect is reduced significantly as the moonpool dimensions relative to the displaced volume of the ship is reduced.

The first sloshing mode is investigated in the two-dimensional case. The studies show that damping due to flow separation is dominant. The vessel motions are unaffected by the moonpool response around the first sloshing mode.
Optimising Fracture Assessment of Welded Structures Using BS 7910, R6 and FEA
Authors: Hadley Isabel , London Tyler
Published: November 2019

The fracture clauses of BS 7910 and R6 present a hierarchy of assessment methods. Depending on the data available, the user may adopt the simplest approach (Option 1), or the higher Options (2 & 3), allowing increasing accuracy and decreasing conservatism. Additional assessment procedures are available via the Annexes of BS 7910 and via Chapter III of R6, which address the inclusion of welding residual stress, crack tip constraint, weld strength mismatch and warm prestress.

This paper illustrates the application of both basic and advanced fracture assessment procedures to a set of welded wide plate test data. The tests featured extensive materials testing, along with detailed characterisation of welding residual stress both in the as-welded condition and after a warm prestress treatment.

The study shows how the accuracy of the assessment increases as the more advanced assessment methods are employed. A tailored assessment of the uniaxial tests using elastic-plastic FEA was also carried out, allowing a comparison between the analytical formulae given in BS 7910/R6, numerical analysis and experimental results.