bluuebunny/crossref_metadata_2025 huggingface dataset:
Squat and VIM Effects on a Towed Gravity-Based Structure With Low Under Bottom Clearance
Authors: Ciortan Cosmin , Johnsgard Helge , Rognebakke Olav , Ruth Eivind , Overstake Meg
Published: September 2018

Computational Fluid Dynamics (CFD) are used to predict the behavior of a massive Gravity-Based Structure (GBS) during open-water towing. Generally, it is poorly understood how the hydrodynamic interaction between the shaft (slender portion of the GBS structure at the waterline), free surface and caisson (lower, storage portion of the GBS) influences vortex shedding and potential VIM response. There is also limited ability to quantify the expected squat response of a structure of the substantial mass and bottom profile of a GBS being towed through areas of decreasing under bottom clearance. In order to properly capture all relevant physical effects, the Detached Eddy Simulations (DES) turbulence model is used. The resulting mesh is somewhat coarser and temporal resolution lower than in a previous study [1] which focused on accurate pitch and roll damping estimates. However, new physical decay test data confirm that the current model reproduces the damping in the relevant amplitude range. The planned towing speed results in reduced velocities for the shaft in the range where Vortex Induced Motions (VIM) typically occur. The numerical set-up is qualified by studying the shaft alone in a uniform current; self-sustained VIM is triggered, with behavior similar to known results. Two towing velocities and loading conditions are studied; a higher speed with a large under bottom clearance and a lower speed where the under bottom clearance is just a few meters. When no VIM is simulated for these conditions, a higher speed corresponding to the VIM range for the shaft is studied. Due to the presence of the caisson and the significant total mass of the GBS, no material VIM occurs. This is explained by a simplified one-degree of freedom model combining the excitation force on the shaft with the damping characteristics of the caisson and total GBS added mass and inertia. The simulations are also used to assess if large pitch or roll motions may be triggered.
The Spring-Like Air Compressibility Effect in OWC Wave Energy Converters: Hydro-, Thermo- and Aerodynamic Analyses
Authors: Falcão António F. O. , Henriques João C. C.
Published: September 2018

The oscillating-water-column (OWC) wave energy converter consists of a hollow (fixed or floating) structure, open to the sea below the water surface. Wave action alternately compresses and decompresses the air trapped above the inner water free-surface, which forces air to flow through a turbine coupled to a generator. The spring-like effect of air compressibility in the chamber is related to the density-pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is normally not accounted for in model testing at reduced scale. Three theoretical models of increasing complexity are analysed and compared: (i) the incompressible air model; (ii) the isentropic process model; (iii) and the (more difficult and rarely adopted) adiabatic non-isentropic process model in which losses due to the imperfectly efficient turbine are accounted for. The air is assumed as a perfect gas. The hydrodynamic modelling of wave energy absorption is based on linear water wave theory. The validity of the various simplifying assumptions, especially in the aero-thermodynamic domain, is examined and discussed. The validity of the three models is illustrated by a case study with numerical results for a fixed-structure OWC equipped with a Wells turbine subject to irregular waves.
Numerical Modelling of a Variable-Pitch, Vertical Axis Tidal Turbine Incorporating Flow Acceleration
Authors: Mannion Brian , Leen Seán B. , McCormack Vincent , Nash Stephen
Published: September 2018

This paper presents details of the numerical modelling of a novel vertical axis tidal turbine that incorporates localised flow acceleration and variable-pitch blades. The focus of this research is to develop a computational fluid dynamics model of a 1:20 scale model of the device using ANSYS® Fluent®. A nested sliding mesh technique has been developed, with an outer sliding mesh being used to model the turbine and additional inner sliding meshes being used for each of the six blades. The turbine sliding mesh is embedded in an outer static domain which includes the flow accelerating bluff body. The variable pitch of the blades is specified in the model using a user-defined function (UDF) which faithfully reproduces the blade pitch during operation of the 1:20 scale model. Modelled power performance and velocity data are compared with experimental results obtained from scale model tests in a recirculating flume. The modelled power curves show good agreement with the measured data; the difference in maximum CP, for example, is just 5.7 %. The model also accurately reproduces measured flows downstream of the turbine. The high model accuracy means that it can now be used for design optimisation studies.
Investigation of Effect of Twisted Surface on Suppression of Vortex-Induced Motion of a Square Column
Authors: Wu Chih-Hua , Lim Teck-Bin Arthur , Kang Chang-Wei , Ma Shengwei
Published: September 2018

Suppression of vortex-induced motion (VIM) of multi-columns semisubmersibles is an important factor for the safety design and operating environment of floating platforms. Vortex generation around square columns is a key issue to cause VIM, and thus, an essential point for suppressing VIM is to desynchronize vortex shedding frequency and natural frequency of the system. In our work, we investigate the effect of a square column with the twisted surface on the suppression of VIM. The numerical tool we used in this work is an open source package, OpenFOAM, to solve transient flow pattern around a vibrating square column. The numerical validation of the present study is done by benchmarking with the reported experimental work at moderate Reynolds numbers (Re). In real sea state, the flow around floating structure is turbulent and is in high Re region. We implemented k-ω SST DES turbulence model in the present numerical model to solve complex turbulent flow around the square column with the twisted surface at high Re. By comparison with VIM of a square column, the VIM reduction of a twisted column is significant, which is up to 86% of VIM reduction. The vortex structure, flow separation points and vortex shedding frequency are varied in the spanwise direction of the twisted column, which causes the effect of vortex desynchronization on the VIM. The detailed flow pattern, 3D vortex structure (Q criterion) around the twisted column is demonstrated as well. A remarkable conclusion is made based on the present numerical findings.
FSI Analysis the Dynamic Performance of Composite Propeller
Authors: Zhang Fanchen , Ma Jianjun
Published: September 2018

The marine propeller is regarded as critical component with regard to the performance of the ships and torpedoes. Traditionally marine propellers are made of manganese-nickel-aluminum-bronze (MAB) or nickel-aluminum-bronze (NAB) for superior corrosion resistance, high-yield strength, reliability, and affordability. Since the composite materials can offer the potential benefits of reduced corrosion and cavitation damage, improved fatigue performance, lower noise, improved material damping properties, and reduced lifetime maintenance cost, Many researches on the application of the composite materials for marine propeller had been conducted. In this work, the INSEAN 1619 large screw 7 bladed propeller is analyzed, to explore the hydrodynamic and structural performance of composite materials effect on propeller’s performances, The commercial software ANSYS Workbench was used in this research. The coupled FSI method was used to analysis the dynamic performance of INSEAN 1619 large screw 7 bladed propeller made of different materials. The simulation results show that the effect of fluid–structure interaction in the analysis of flexible composite propellers should be considered.
Seakeeping Prediction of a Survey Vessel Operating in the Caspian Sea
Authors: Burlacu Elisabeta , Domnisoru Leonard , Obreja Dan
Published: September 2018

This paper presents the numerical and experimental analysis of the seakeeping performances of a survey vessel operating in the Caspian Sea. For the numerical analysis we have developed our own code, based on a linear hydro-dynamic strip theory formulation. The irregular waves are modelled by short-term power density spectra JONSWAP, for the spectral parameters significant wave height and peak period corresponding to the Caspian Sea scattering diagram. The experimental study is developed at the towing tank from Naval Architecture Faculty of Galati, using a semi-captive scaled model 1:16 of the survey vessel, made of fibre glass and wood, being recorded the heave, pitch, roll motions and wave elongation. The experimental tests are carried out for two speeds and several significant heading cases: head, follow and beam regular waves. This study delivers the prediction of the survey vessel seakeeping capabilities and the validation of the numerical response amplitude operators by experiment.
Wax Deposition Prediction and Control in Waxy Crude Oil Tieback Flowlines for South China Sea Deepwater Oil Development
Authors: Yu Xichong , Wang Chunsheng , Li Qingping , Li Yan , Zhu Xiaosong , Wang Qing , Cheng Bing , Qing Yaqi
Published: September 2018

Crude oils are generally very complex chemical systems consisting predominantly of hundreds to thousands of hydrocarbon compounds from simple low-molecular-weight paraffin to high-molecular-weight waxes. There are many factors effecting wax deposition such as the fluid composition, pressure and temperature and so on. Wax deposition is complex dynamic process related to transient heat and mass transfer, therefore it is very difficult to predict wax deposition velocity accumulation amounts on the wall in waxy crude oil tieback flowlines. There exist many waxy crude oil tieback flowlines for South China Sea deepwater oil development, which can cause many flow assurance problems. It is very important to carry out wax deposition prediction calculation and effective measures are adopted to solve flow assurance problems. In this study, based on certain waxy crude oil tieback flowlines for South China Sea deepwater oil development, OLGA software is used to simulate transient flow characteristics in waxy crude oil tieback flowlines such as wax precipitation rate, thickness of wax layer deposited at wall, pigging frequency and so on. OLGA software simulation results is coincident with field pigging operation results, finally obtaining a good simulation methods to predict transient flow characteristics in waxy crude oil tieback flowlines and giving good suggestions to determine pigging periods and frequency. The study results show two 10 inch non-insulation steel flowlines are recommended to develop deepwater waxy crude oil field and pigging periods should comprehensive consider thickness of wax layer deposited at wall and the increase of pressure caused by wax deposition. It is reasonable to 35 days pigging periods during 2 year after put into production, 60 days pigging periods after 2 years. The results show maximum liquid flow rate for a 10 inch flowline is 7000m3/d, and it is reasonable that one flowline is used to transport when flow rate is lower than 7000m3/d, and two flowlines are used to transport when flow rate is more than 7000m3/d. The pump pressure at FPSO is kept at 6000kPaA, and required maximum liquid flow rate of pigging at FPSO is 4200m3/d (175m3/h).
Subsea Cable Stability on Rocky Seabeds: Comparison of Field Observations Against Conventional and Novel Design Methods
Authors: Griffiths Terry , Draper Scott , Cheng Liang , Tong Feifei , Fogliani Antonino , White David , Johnson Fraser , Coles Daniel , Ingham Stephen , Lourie Caroline
Published: September 2018

As offshore renewable energy projects progress from concept demonstration to commercial-scale developments there is a need for improved approaches beyond conventional cable engineering design methods that have evolved from larger diameter pipelines for the oil and gas industry. New approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of power transmission and increase operational reliability.

This paper reports on subsea cables that MeyGen installed for Phase 1a of the Pentland Firth Inner Sound tidal stream energy project. These cables are located on rocky seabeds in an area where severe metocean conditions occur. ROV field observation of these cables shows them to be stable on the seabed with little or no movement occurring over almost all of the cable routes, despite conventional engineering methods predicting significant dynamic movement.

We cite recent research undertaken by the University of Western Australia (UWA) to more accurately assess the hydrodynamic forces and geotechnical interaction of cables on rocky seabeds. We quantify the conformity between the cables and the undulating rocky seabed, and the distributions of cable-seabed contact and spanning via simulations of the centimetric-scale seabed bathymetry. This analysis leads to calculated profiles of lift, drag and seabed friction along the cable, which show that all of these load and reaction components are modelled in an over-conservative way by conventional pipeline engineering techniques. Overall, our analysis highlights that current cable stability design can be unnecessarily conservative on rocky seabeds. Our work foreshadows a new design approach that offers more efficient cable design to reduce project capex and enhance through-life integrity management.
Hydrodynamics Around a Deep-Draft Semi-Submersible With Various Corner Shapes
Authors: Liang Yibo , Tao Longbin
Published: September 2018

A numerical study on flow over a stationary deep-draft semi-submersible (DDS) with various corner shapes was carried out to investigate the corner shape effects on the overall hydrodynamics. Three models based on a typical DDS design with different corner shapes were numerically investigated under 45° incidence. The present numerical model has been validated by an experimental test carried out in a circulating water channel. It is demonstrated that, as the corner shape design changed, the hydrodynamic characteristics alter drastically. In addition, the flow patterns were examined to reveal some insights of the fluid physics due to the changing of different corner shape designs. The detailed numerical results from the geometric study will provide a good guidance for future practical designs.
VIV Response and Fatigue Damage of Flexible Cylinders With Time-Varying Axial Tension
Authors: Yuchao Yuan , Hongxiang Xue , Wenyong Tang , Jun Liu
Published: September 2018

The time-varying effect of axial tension has recently attracted increasing focus when investigating vortex-induced vibration (VIV) for flexible cylinders. This paper applies an alternative time domain force–decomposition model to predict VIV response, in which the structural stiffness will be updated at each time step to take the tension variation into account. Firstly, the adopted numerical model is compared against the latest published experimental results of a small-scale cylinder with constant and time-varying tensions. Then, extensive cases of a long flexible cylinder are designed to investigate the tension time-varying effect on structural response and fatigue damage respectively. Several new response characteristics different from the constant tension case are analyzed from the VIV mechanism level. Fatigue analysis also reveals the influence laws of the amplitude and frequency of varying tension. Mathieu-type resonance between VIV and time-varying tension excitation is captured, under which structural response as well as fatigue damage will enlarge significantly. Some conclusions drawn by this research can provide reference at the engineering design stage of marine slender structures.
Finite Element Analysis of a Container Ship Struck by Rigid and Deformable Bows
Authors: Liu Bin , Chen Lin , Liao Xianting , Zhu Ling , Guedes Soares C.
Published: September 2018

A rigid striking bow is often applied in the analysis of ship-to-ship collisions. In this paper, a deformable striking bow is used to investigate the influence of deformed bow on the resistance of struck ship double-hull structure. The numerical simulations manage to describe the respective energy dissipated by the deformed striking bow and struck side structures. Furthermore, detailed information of the energy dissipated by each structural component is provided. As the material failure criterion strongly affects the numerical results, three constant-strain failure criteria are used and compared: PES, GL and LIU criteria. It is of considerable practical importance to estimate the extent of structural deformation and rupture during collision accidents. More collision simulations are performed to analyze the influence of collision locations on the structural crashworthiness of the ship.
The Effect of Fatigue Loading Spectrum on Crack Propagation in a Ship Detail
Authors: Huang Xiaoping , Yan Xiaoshun , Ong Muk Chen , Huang Yingcai
Published: September 2018

The assessment of the loading sequence on fatigue crack growth of cracked details is essential when the ship hull is subjected to random wave-induced loading during the service time. In this paper, the effect of loading sequence on the crack growth life is investigated in order to find a physical engineering method to generate fatigue loading spectrum for ship fatigue assessment. The long term distribution of fatigue loading and the unique crack growth rate curve model have been employed in the analysis. The results show that the shape parameter affects the fatigue life significantly if the long-term distribution of wave-induced stress range is modeled by two-parameter Weibull distribution. Moreover, the spectral-based method provides a reasonable fatigue loading spectrum and avoid the confusion in determining the shape parameter for different empirical formulas, which are recommended by several main ship classification societies. An example of fatigue assessment for a cracked detail in a container ship is demonstrated as a reference for fatigue assessment of a ship hull based on crack growth.
Development of a Scaled Rotor Blade for Tank Tests of Floating Wind Turbine Systems
Authors: Schünemann Paul , Zwisele Timo , Adam Frank , Ritschel Uwe
Published: September 2018

Floating wind turbine systems will play an important role for a sustainable energy supply in the future. The dynamic behavior of such systems is governed by strong couplings of aerodynamic, structural mechanic and hydrodynamic effects. To examine these effects scaled tank tests are an inevitable part of the design process of floating wind turbine systems. Normally Froude scaling is used in tank tests. However, using Froude scaling also for the wind turbine rotor will lead to wrong aerodynamic loads compared to the full-scale turbine. Therefore the paper provides a detailed description of designing a modified scaled rotor blade mitigating this problem. Thereby a focus is set on preserving the tip speed ratio of the full scale turbine, keeping the thrust force behavior of the full scale rotor also in model scale and additionally maintaining the power coefficient between full scale and model scale. This is achieved by completely redesigning the original blade using a different airfoil. All steps of this redesign process are explained using the example of the generic DOWEC 6MW wind turbine. Calculations of aerodynamic coefficients are done with the software tools XFoil and AirfoilPrep and the resulting thrust and power coefficients are obtained by running several simulations with the software AeroDyn.
A Comparative Study of Fatigue Damage Assessment Methods to a Rigid Planar Jumper
Authors: Igeh Laila Aarstad , Liu Zhenhui , Wu Jie , Ong Muk Chen
Published: September 2018

A rigid jumper is an important part of the subsea production system, it may experience significant vortex induced vibrations (VIV) if subjected to current. It has normally non-straight geometry shape in three-dimensional space. Consequently, the response of a rigid jumper under VIV is much more complicated compared to straight pipeline structures. Currently, there are very limited studies and design guidelines including methods on how to assess the fatigue damage of rigid jumpers under VIV. The methodology used for straight pipelines is often applied by ignoring the non-straight geometry characteristics and the multi-axial stress states (coexisting of flexural and torsional stress). However, both experimental and numerical results show that the torsional stress does exist besides the flexural stress for rigid jumpers under VIV. On the other side, the response of the rigid jumper under VIV is also challenging. The objective of this study is to do a fatigue assessment practice based on state-of-the-art calculation methods to a rigid jumper on model scale. The VIV response is inherited from experimental tests and numerical calculations by either force or response model methods. The influence of torsional stress on fatigue assessment is demonstrated. Two approaches have been investigated. In the first method, the flexural and torsional stresses are evaluated separately. The second method uses the 1st principle stress to calculate the fatigue damage, thus the flexural and torsional stresses are evaluated together.

It is shown that the use of the 1st principle stress gives higher fatigue damage if the torsional stress contribution is significant. Further, the principle stress method is also less time-consuming on processing the results. Detailed discussions based on results have been performed, which could be also applied to general real scale rigid jumpers.
Validation of Seismic Streamer Simulator and Sea Current Estimator Using Full Scale Data
Authors: Grindheim Jan Vidar , Revhaug Inge , Welker Ken , Solheim Peder , Pedersen Egil
Published: September 2018

Survey optimization and cost reductions are ongoing challenges for the marine seismic petroleum exploration industry. The present research intends to improve seismic streamer steering as well as to reduce time on line-changes, where the streamer spread is turned 180 degrees. Sea current estimation and prediction of future streamer state are important aspects of improved streamer steering, which will increase survey efficiency and quality.

A seismic streamer simulator has been validated against full scale proprietary data from two seismic survey line-changes. The streamer spread has 12 cables of slightly more than 4 km length each, with 13 control birds for each streamer. Interpolated Acoustic Doppler Current Profiler (ADCP) measured sea current is introduced at the streamer nodes. Two streamer tension cells are installed at each streamer, giving validation of simulated tension.

Secondly, a sea current estimation algorithm has been validated against the interpolated ADCP current data for the two line-changes.
Design of a Tidal Turbine Array for the Bohai Strait, China
Authors: Chen Lei , Bonar Paul A. J. , Adcock Thomas A. A.
Published: September 2018

In this paper, we consider array design strategies to maximise the power available to turbines placed in the Bohai Strait, which is considered to be one of China’s most promising candidate sites for tidal stream power. The discontinuous Galerkin version of the open-source hydrodynamic model ADCIRC is used to simulate flow through the strait and tidal turbines are introduced using a sub-grid scale actuator disc model. New design algorithms based on key theoretical results are used to build large arrays, which are then compared in terms of both the collective power output and the power produced per turbine. The results of the analysis are used to draw general conclusions about the optimal design of tidal turbine arrays.
A Theoretical Method to Estimate the Fatigue Life of Tensile Armors of Flexible Pipes
Authors: Jiang Kaien , Lu Yutian , Bai Yong
Published: September 2018

This paper mainly focuses on a theoretical methodology to calculate the fatigue life of tensile armor of flexible pipes. This approach employs the local model of flexible pipe that converts forces and moments obtained in time-domain global analyses into stresses in the spiral tendons of tensile armor layer. The stresses are then processed by rainflow counting methods, and S-N curves are adopted to evaluate the fatigue damage of tensile armors. Finally, Miner linear cumulative damage theory is used in order to calculate the accumulated fatigue damage. A case study on the fatigue life of a flexible pipe employing this methodology is presented, and the fatigue life of flexible pipe is obtained. The main points addressed in the study are the effect of mean stress and friction coefficients. The results indicate that the inner tensile armor at suspension point is the most prone to fatigue damage, in addition, mean stress correction and friction coefficients strongly influence the fatigue life of flexible pipes.
Mooring of Semi Submersibles in Extreme Sea States: Simplified Models for Wave Drift Forces and Low Frequency Damping
Authors: Larsen Kjell , Vigesdal Tjerand , Bjørkli Rune , Dalane Oddgeir
Published: September 2018

This paper presents results from extensive small-scale model testing of three semi submersibles together with an overview of damping contributions of low frequency motions. The objectives of the model tests were to verify empirical correction formulas for viscous wave drift forces and to recommend and validate theoretical low frequency damping models. The main parameters of the semis such as displacement, number of columns and diameter of columns were intentionally varied in order to assess the effects on total wave drift forces and corresponding damping. The results show that viscous effects significantly increase the total wave drift forces in extreme sea states. The presence of current increases the effect. As expected, the viscous contribution to wave drift is especially important for semis with slender columns. A revised empirical correction formula for wave drift forces is proposed based on model test results.

An overview of the different low frequency damping effects is given. Damping from viscous forces on the hull and damping from the mooring system are the most important sources of damping for the moored semis. A simplified model to calculate mooring system damping is proposed. For accurate prediction of low frequency motions of moored semi submersibles in extreme sea states, a damping level in the range 40–70% of critical damping should be applied for surge and sway when the empirical correction formulas for wave drift forces are applied.
Insulation Performance of Sandwich Pipe
Authors: Yang Jiankun , Estefen Segen F. , Souza Marcelo Igor Lourenço , Wang Yuxi , Hong Cheng
Published: September 2018

As the tendency of the offshore oil industry is going deeper and further, the subsea pipeline is exposed under tougher condition combining lower temperature with higher hydrostatic pressure. The severe condition creates a challenge towards flow assurance, which often results in a high cost solution. Reducing the cost while providing a qualified insulation performance is of great significance to deepwater development. For ultra-deepwater beyond 1500m, single-wall pipe usually fails to meet the flow assurance requirements or requires a huge amount of insulation material. Pipe-in-pipe configuration can provide a good insulation performance but comes with a high cost associated. Sandwich pipe is a new concept composed of two concentric steel pipes separated by a cementitious composite annulus that provides a combination of high structural strength with thermal insulation. It is reported to be a promising alternative for both flexible and rigid conventional pipes in applications for long distance pipelines. In order to further investigate its feasibility in deep waters, a subsea production system with depth at 2200m was used as a case study for a comprehensive evaluation of insulation performance of the sandwich pipe, including both steady-state and shut-in working conditions. For a comparative study, scenarios using single-wall pipe (SW), pipe-in-pipe (PIP) and flexible pipe (FP) were also considered separately. The results showed that (i) sandwich-pipe performs better in steady-state but worse in between shut-in and the restart period (ii) sandwich-pipe with larger diameter performs better than it with smaller diameter. The reasons for the sandwich pipe behavior were discussed and suggestions to improve the performance are presented.
A New Method in Identifying Critical Void Volume Fraction of GTN Model for NVA Mild Steel in Ship Collision and Grounding Scenario
Authors: Song Zijie , Hu Zhiqiang
Published: September 2018

The NVA mild steel is a commonly used material in shipbuilding, which possesses good ductility character. However, the description of ductile fracture process for NVA steel in numerical simulation is still a challenging task. A new method to predict the critical void volume fraction fc of Gurson-Tvergaard-Needleman (GTN) model is introduced in this paper.

GTN-model is one of the well-known micromechanical models for ductile fracture. The traditional plasticity theory assumes that the plastic volume is incompressible and that the yield of the material is independent of the hydrostatic stress, whereas the yield surface of the GTN-model takes the effect of the macroscopic hydrostatic stress into account. The yield surface is reduced with the increase of the void volume fraction, which can reflect the deterioration characteristics of the material with development of damage during the deformation process. Therefore, GTN-model is a promising mathematical model for describing the ductile fracture process of the ship structures during accidental scenarios of collision and grounding.

The traditional way to determine fc of GTN-model is using the inverse method directly, which has a high degree of uncertainty. A new method based on Hill, and Bressan & Williams’s assumptions proposed in this paper solve this problem effectively. Besides, the combined of Voce and Swift constitutive model is used to describe the mechanical property of the NVA material.

Furthermore, numerical simulations were also conducted with code LS_DYNA by developing the user-defined subroutine. It is found that the model can predict the structural damage quite accurately, which proves its feasibility of being applied in the research of structural responses in ship collision and grounding accidents.
The Set-Down and Set-Up of Directionally Spread and Crossing Surface Gravity Wave Groups in Severe North Sea Storms
Authors: McAllister Mark L. , Adcock Thomas A. A. , van den Bremer Ton S. , Taylor Paul H.
Published: September 2018

Recent work by McAllister et al. (2018) [1] has experimentally confirmed that the set-down of the wave-averaged free surface, first described by Longuet-Higgins and Stewart (1962) [2], can turn into a set-up when wave groups are sufficiently spread or cross at large angles. Experimental results were shown to agree well with second-order theory, including frequency-sum and frequency-difference terms, where the latter are responsible for the wave-averaged free surface. In this paper, we review these experimental results and examine theoretically the magnitude of the wave-averaged free surface in realistic extreme North Sea conditions. Specifically, we examine the role of the shape of the spectrum, water depth, and the relative magnitude of the peak frequencies of the two crossing groups. We find that having a realistic spectrum (JONSWAP vs. Gaussian) considerably enhances the magnitude of the second-order contribution, the total second-order signal increases with decreasing depth and can display a maximum provided the water depth is shallow enough for small to moderate degrees of spreading or crossing angles and is larger for spectral peaks that are further apart.
Effect of Welding Residual Stress on Stress Intensity Factors for Semi-Elliptical Surface Cracks in a Butt-Welded Steel Plate
Authors: Qiang Bin , Wang Xin
Published: September 2018

The through-thickness distribution of welding residual stress in a 30-mm-thick butt-welded Q345qD steel plate has been investigated through experimental measurements and finite-element simulations. In this paper, the weight function and finite element methods are used to investigate the stress intensity factors (SIFs) at the surface and deepest points of the semi-elliptical surface cracks, subjected to a combination of external tensile load and through-thickness welding residual stress. Different crack aspect ratios and relative depths are analyzed. The results reveal that the longitudinal residual stress is always tensile through the plate thickness, which makes the SIFs of the surface and deepest points larger than those without considering the longitudinal residual stress. However, the transverse residual stress through the thickness presents tension–compression–tension, with the tensile transverse residual stress causing the SIFs to increase. When the crack tip enters the compressive stress region, the compressive stress offsets the external load and causes the SIFs to decrease.
An Experimental Comparison of Horizontal Resistance of Single Suction Pile and Group Suction Piles
Authors: Lee Juhyung , Kim Hakman , Cho Jinwoo
Published: September 2018

In this study, a new type of suction pile foundation for floating structures, namely group suction piles, was proposed to improve the shortcomings of conventional single suction piles. Small-scale model tests were performed to estimate the horizontal behavior of single suction piles and group suction piles with different pile spacing (2, 3 and 4 times the pile diameter). The horizontal behavior of group suction piles with different pile spacing was analyzed for various loading locations based on the model tests. For the given group pile configurations (a group pile having 9 component piles with a pile formation of 3 × 3), the horizontal resistance increased with increasing pile spacing. The maximum ultimate horizontal resistances were found at the loading locations of 50% of the embedded depth. Unlikely in the single suction pile case, the significantly maintained residual resistances were found for group suction piles. The residual resistances of group suction piles were at least higher than 40% of the corresponding ultimate horizontal resistances.
Numerical Simulation and Innovative Structure of Drainage Cover
Authors: Zhang Shengbiao , Wang Zhandong , Yan Yan , Sun Guifang , Ni Zhonghua
Published: September 2018

A fluid simulation was conducted on the flow states of water and drainage gas from the drainage cover in underwater local dry laser welding. The results of the simulation of gas volume fraction are obtained. Drainage cover was developed based on the drainage cover model. Experiments of drainage in this machined drainage cover was carried out, and were compared with the numerical simulation. The results show high similarity, which verifies the feasibility. Then, innovative structure featuring an uneven buffer around the exit of drainage cover are proposed to improve the performance of drainage. A group of 3D models of fluid domain related to differently characterized drainage covers was designed and the flow states were simulated. They are N-model with a narrow outlet, S-model with a smooth buffer added additionally based on N-model, U-model with an uneven buffer compared with S-model, and W-model with a wide outlet and the diameter of outlet twice than N-model. The gas volume fracture distributions were recorded and in comparison with each other. The results indicate that U-model with the uneven buffer realized the deepest drainage depth.
Validation of Applicability of Low Frequency Motion Analysis Theory Using Observation Data of Floating Offshore Substation
Authors: Yoshimoto Haruki , Yoshida Hisafumi , Kamizawa Ken
Published: September 2018

In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind power generation are being implemented and planned around the world. In Japan, a demonstration project named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is carried out by the Ministry of Economy, Trade and Industry. The project is the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation.

This floating offshore substation is installed in order to observe meteorological and oceanographic data and motion data as well as boosting the generated electric power. Since the installation in 2013 it continues to record various kinds of continuous data. The substation is an advanced spar type floater moored by four spread catenary mooring lines.

In the design of the mooring system for offshore structure, the motion of the structure under environmental external force is very important. The motion of the moored floating structure is divided into wave frequency motion, which is a motion of a relatively short period, and low frequency motion caused by mooring restoring force and variable external force, both of which are important elements in the design. Among them, wave frequency motion is known to be accurately estimated by potential theory as a result of research on various types of structures. On the other hand, in addition to the existence of various calculation methods including time domain analysis, its statistical characteristic and applicability are entirely depending on the target structure. Also, observation data of low frequency motion have been very few.

In this paper, long-term data observed at the floating offshore substation in Fukushima FORWARD was analyzed with focusing on low frequency motion and its statistical properties were clarified. Furthermore, we analyzed the low frequency wave force spectrum and motion by conventional low frequency motion theory using the wave drifting force calculated by the potential theory. And, we compared the calculated value with the analysis result of the observation data and validated the applicability of the simplified low frequency motion theory.
Similarity Analysis for Downscaling a Full Size Drill String to a Laboratory Scale Test Drilling Rig
Authors: Ambrus Adrian , Skadsem Hans Joakim , Mihai Rodica G.
Published: September 2018

The drill string used in drilling oil and gas wells is a long and slender structure that is confined by the wellbore wall and subject to significant axial, lateral and torsional vibrations while drilling. Detection and mitigation of drill string vibrations are especially important, as vibrations can be hard to detect at the surface, yet cause significant damage to the drill bit, downhole tools and the formation being drilled.

Study of the drilling process by downscaling to laboratory conditions is an attractive prospect as it is a cost-efficient alternative to dedicated large-scale testing and it can be instrumented to provide vibration measurements from different locations along the string. However, the extreme geometrical length scales, the complex wellbore trajectories and the large mechanical strains on the drill string lead to challenges when attempting to downscale the drilling process to manageable laboratory conditions.

Downscaling based on similarity analysis provides consistent scaling laws for predicting large-scale dynamics based on observations from the downscaled test drilling rig. We perform a similarity analysis for an example full scale drill string that illustrates these challenges in terms of similarity criteria for an equivalent laboratory model of the drill string. We focus particularly on the geometric and mechanical properties of the drill string and consequences of downscaling on the time scale and the forces in the string. We illustrate the downscaling criteria through numerical simulations by solving the governing equations of motion at different scales, and provide recommendations for downscaling based on widely available material types.
Uniaxial Capacities of Sand-Filled Large Cylindrical Structures in Soft Foundation
Authors: Xiao Zhong , Li Zongquan , Liu Ying
Published: September 2018

Large diameter cylindrical structures are a kind of large-diameter thin-walled structures which are suitable for a verity of coastal projects such as embankments, breakwaters, wharfs and sea cofferdams. The amount of cylinder material per meter is independent of the cylinder diameter, and the bearing capacities of the structure, especially the horizontal and moment bearing capacities, increase with the increase of its diameter. This kind of structure is economic, reliable and easy to construct. In construction, multiple vibration hammers are installed at the upper part of the cylinder, and as the hammers vibrate synchronously, the soil adjacent to the cylinder wall is disturbed, allowing the large cylinder to sink under its dead weight. The prospect for applying this kind of structure in soft foundation is bright. However, due to the complexity of the interaction mechanism between the large cylinder, internal sand and saturated soft soil foundation, the bearing characteristics of sand-filled large cylindrical structures in soft foundation, especially the effect of the internal sand on the bearing capacities, are not clear. In this paper, the effects of internal sand filling height, foundation depth and soil undrained shear strength on the uniaxial capacities and the failure mechanism of sand-filled large cylindrical structures are studied via finite element numerical analysis. Meanwhile, uniaxial capacities of sand-filled large cylindrical structures are compared with those of suction bucket foundations and non-sand-filled large cylindrical structures in identical conditions. In addition, approximating expressions for calculating the bearing capacities of sand-filled large cylindrical structures are proposed to provide reference for the design of such structures.
Fluid Structure Interaction Simulations Involving Free Surface
Authors: Díaz-Ojeda H. R. , González L. M. , Huera-Huarte F. J.
Published: September 2018

The aim of this paper is to evaluate how much affects the presence of gravity and free-surface to a flexible structure in a classical fluid structure interaction (FSI) problem typically found in off-shore problems and other oceanic applications. The base problem selected is the Turek benchmark case where a deformable plate is attached to the wake of a circular cylinder. To focus on the differences of considering free surface, a simple geometry has been selected and two different situations have been studied: the first one is the classical Turek benchmark, the second is a similar geometry but adding gravity and free surface. The free surface problem was studied placing the structure at different depths and monitoring the deformation and forces on the structure.
Numerical Evaluation of Ship Maneuvering Performance in Waves
Authors: Seo Min-Guk , Nam Bo Woo , Kim Yeon-gyu
Published: September 2018

This paper considers a numerical computation of ship maneuvering performance in waves. For this purpose, modular-type maneuvering model (MMG model) is adopted and wave drift forces and moments are included in maneuvering equation of motion. Wave drift forces ware calculated using a seakeeping program based on higher-order Rankine panel method. When calculating the wave drift force acting on a ship, the forward speed, wave heading, wave period and drift angle of the ship are considered as key parameters. It means that ship’s lateral speed is also included to calculate wave drift force. Numerical simulations are carried out in regular waves using S175 containership and computation results are validated by comparing them with results of free-running model test. Using the developed program, numerical simulation in irregular waves are, also, conducted and discussion is made on the sensitivities of time signal of wave elevation on turning performance.
Study on the Porpoising Phenomenon of High-Speed Trimaran Planing Craft
Authors: Yuan Yuming , Wang Chao
Published: September 2018

This paper presents the application of Computational Fluid Dynamics (CFD) simulations to the heaving and rolling motion of the planing craft under different speeds and centers of gravity. Comparing the flow lines, the pressure distribution at the bottom of the boat, the heave and the trim angle before instability with those elements after instability, a critical trim angle results in the early separation of the air in the bow. Meanwhile, due to effect of aerodynamic lift, the bow is lifted, which eventually leads to instability of the hull. Forward or upward movement of the center of gravity may eliminate or postpone the porpoising, the backward center of gravity may result in the unstablity of the ship. Serious porpoising is random and irregular. It will damage the structure of the hull, affect the maneuverability of the ship and threaten the safety of the crew.
Structural Integrity Assessment of Maritime Transport Equipment
Authors: Jiang Xiaoli
Published: September 2018

Structural integrity assessment is the study of the safe design and assessment of components and structures under loads, and has become increasingly important in engineering design. The technology and applications of structural integrity widely range from transportation, oil and gas, power generation to petrochemical, nuclear sectors, etc. In this paper, the character of structural integrity assessment in the maritime field is discussed, the latest approach and techniques are classified in three sub-topics, damage diagnosis, damage prognosis & maintenance strategy and then illustrated using a few practical studies. Finally, the future challenges introduced by new material, new exploitation field and new technology, i.e., IOT, big data, etc., are discussed and the potential development of structural integrity assessment in maritime industry is explored.
On the Use of the White-Noise Approximation for Modelling the Slow-Drifts of a FOWT: An Example Using FAST
Authors: Simos Alexandre N. , Souza do Carmo Lucas Henrique , Carlos Camargo Ewerton
Published: September 2018

When designing the mooring system of a floating unit, performing extensive time-domain simulations in several sea conditions is common practice. For this, the second-order wave induced forces, expressed by QTF matrices, are most often precomputed in frequency domain diffraction codes. However, the computation of the full QTFs is quite demanding and it is also not uncommon for the designer to be in doubt as to the frequency limits and resolution required for their construction. Among the approximations that can be used to ease this burden, the most well-known is Newman’s approximation, which performs quite well as long as the natural periods of drift and the water depth are sufficiently large. The white noise approach, on the other hand, leads to an approximation of a different kind. Taking advantage of the fact that the slow-drift response is narrow-banded, it approximates the second-order force spectrum where it contributes the most, and in a way that is independent of the natural periods and depth. However, its original formulation, based on the force spectra, is certainly more convenient in frequency domain. This article presents an easy way to make use of the white noise approach in time domain simulations. For this, the well-known OC4 semi-submersible FOWT is taken as a case-study. Simulations in different wave conditions are performed with the software FAST using both, the original full QTFs and new ones, simplified according to the principle of the white noise approximation. It is shown that, with the latter, the simulations can be performed without significant loss of accuracy, indicating that the white-noise approach indeed is an interesting option for preliminary design stages.