bluuebunny/crossref_metadata_2025 huggingface dataset:
Study on Effects of the Scouring on Horizontal Bearing Capacity of the Composite Bucket Foundation
Authors: Hongyan Ding , Xing Zhao , Puyang Zhang , Conghuan Le
Published: September 2018

Composite bucket foundation is a new kind of wide-shallow foundation for offshore wind turbines, and can be made on land, transported and installed by one step. Compared with other deep pile foundations, the erosion caused by waves and currents is more obvious and this would cause more risks for the overall safety of the wind turbine. In order to know more about it, a 3D FEM composite bucket foundation - soil model is established using Abaqus in this paper. In this study, the lateral bearing capacity of composite bucket foundation in different situation is analyzed by FEM and the results of FEM have been compared with that of physical model test. And through anther model test, the size parameters of scouring pit are obtained. Then the emergence and development of scouring pits are simulated by continuously excavating soil. The horizontal bearing capacity of a practical project is calculated considering the influence of scouring on the CBF by the displacement loading mode. The calculation results show that the horizontal bearing capacity of the bucket foundation is significantly reduced after scouring, and the influence of scour depth is more significant than the scouring extent.
Model Test of Integrated Floating Transport Technology of Offshore Wind Turbines With Composite Bucket Foundation
Authors: Ding Hongyan , Meng Lingqian , Zhang Puyang , Le Conghuan
Published: September 2018

With continuous innovation and development of the wind power technology, the offshore wind turbine is rapidly developing. However, it also has difficulties in construction at sea and other shortcomings. One-step installation technique of the composite bucket foundation (CBF) provides a new way to solve the difficulties in the construction of offshore wind turbine at sea. And the integrated floating transport technique of the offshore CBF is the key link in the one-step installation technique. In this paper, by controlling three factors, such as draft, wave height and speed, the contact force between the vessel and CBF in the integrated floating transportation process is studied with experimental method. The relationship and the effect between the three factors and the contact force is analyzed. The experimental results are collected and analyzed to verify the safety of the integrated floating transportation of offshore CBF.
Electromechanical Impedance Method for Damage Detection of Typical Joint on Jacket Platform
Authors: Zhang Boying , Hameed Hamad , Xu Yuxin , Zhang Chonglin , Bai Yong
Published: September 2018

Health monitoring of welded structural joints is a very important factor of the engineering community. Electro-mechanical impedance (EMI) technique allows the direct evaluation of structural dynamics by evaluating its E/M impedance or admittance signatures. This paper first gives a brief introduction of the theoretical background on the described method. Then, the described EMI technique is applied to recognize the presence of damage by executing experimental works where the damage in the form of crack is simulated with an impedance analyzer at various distances. Four typical welded metallic joints on a jacket platform successfully produced submillimeter cracks under cyclic loading and root mean square deviation (RMSD) is used to evaluate the degree of crack damage. Finally, an outcome of laboratory measurements performed with developed structural health monitoring system based on the electromechanical impedance phenomenon is presented.
Dynamic Cuttings Slip Velocity Evaluation in Non-Newtonian Fluids Using a Temperature Dependent Transient Drift-Flux Model for Directional Wells
Authors: Wiktorski Ekaterina , Sui Dan , Fjelde Kjell Kåre , Langåker Vebjørn
Published: September 2018

The objective of drilling a well is to prepare a clean hole without obstructions for further casing and production tubing running. Cuttings transport has always been important, but challenging process, especially when drilling long directional wells. Poor hole cleaning causes severe problems, as stuck pipe, extreme torque and drag, difficulties in casing landing, cementing, etc. Extensive studies of cuttings transport, both theoretical and experimental, have been performed to estimate, for example, cuttings concentration and cuttings slip velocity to determine optimal conditions for effective hole cleaning.

This paper presents a dynamic analysis of cuttings transport in non-Newtonian fluids based on a transient drift-flux model and an associated numerical scheme AUSMV (advection upstream splitting method) developed by Evje and Fjelde 2002. In this paper, the scheme is modified to simulate cuttings transport dynamically taking into account effects related to pressure, temperature and cuttings slip.

During drilling, the heat is transported from the formation into the wellbore and up to the surface. In this paper, the energy balance is enhanced by introducing an analytical temperature model into the AUSMV scheme. The temperature distribution along the well is calculated at the beginning of simulation and kept constant throughout the simulation.

Additionally, the AUSMV scheme is improved by considering drilling fluid’s transport- and thermal properties. Transport properties of an oil-based mud, such as viscosity and density, are obtained from experiments. The experimental results were used to determine the coefficients in a linear density model used in the study to investigate the effect of non-Newtonian behavior on the heat transfer, cuttings transport and downhole pressure. Furthermore, a model to calculate the apparent viscosity at various pressures and temperatures was developed based on the experimental results and used to evaluate the impact of viscous forces on the cuttings distribution in the well.

Presented numerical scheme solves dynamic cuttings transport problems taking into account the slip velocity variation with wellbore geometry, operational (controllable) parameters and formation properties. In comparison to the traditional steady-state models, the transient cuttings transport model with integrated depth-dependent parameters gives a possibility to achieve a more realistic simulation of cuttings transport, distribution and accumulation along the wellbore through the time.
Numerical Study on the Migration of Two Spheres in Upward Pipe Flow
Authors: Liu Lei , Lu Haining , Yang Jianmin , Tian Xinliang , Peng Tao , Li Jun
Published: September 2018

Migration of particles in pipe flow is commonly seen in offshore engineering, such as vertical transport of ores in deep sea mining. As the basis of the investigation on fluid-particle two-phase flow, the interaction of two spheres in upward pipe flow is studied by direct numerical simulations in this paper. The pipe flow is set as Poiseuille flow; the Reynolds number is no more than 1250. The dynamic responses of the spheres and the flow pattern are analyzed at different flow velocity. When compared to the sedimentation of two spheres in quiescent flow, the trailing sphere in Poiseuille flow will never surpass the leading one in Poiseuille flow. As the flow velocity increases in the pipe, the spheres are easier to separate after collision. When the flow velocity exceeds a critical value, the spheres will never collide.
Bivariate Distribution Modelling for Wave Height and Period in Jiaozhou Bay
Authors: Huang Weinan , Tao Shanshan , Bai Qiang , Dong Sheng
Published: September 2018

Ocean waves with one-peaked and double-peaked spectra are the main wave patterns in Jiaozhou Bay, China. This paper discusses the joint distribution of the wave heights and the periods of the two wave patterns. Maximum entropy distribution, lognormal distribution, GEV distribution and Weibull distribution are adopted to construct bivariate distribution using conditional probability formula. In order to compare the four joint distribution models, squared Euclidean distance and AIC criterion are applied to measure the goodness of fit. The result shows that bivariate distribution constructed by GEV distribution have better performance in fitting wave height and period to double peak case, while that by maximum entropy distribution or Weibull distribution is better to single peak case.
Definition of Efficiency and Safety Criteria for Icebreaker in Ice Management Operations
Authors: Karulin Evgeny , Karulina Marina , Kazantsev Mikhail , Proniashkin Aleksander , Zaikin Dmitry
Published: September 2018

Ice management (IM) is often required to support offshore production of oil and gas in freezing seas. It helps to mitigate ice impact on marine structures and thus minimize risks of accidents as well as to increase weather windows for marine operations. One of the IM tactics is to use an icebreaker for producing a zone of managed ice for ensuring safe and efficient operation of marine facilities: platforms, offloading terminals, tankers, etc. The choice of the right icebreaker which is best capable to cope with the IM jobs is quite a challenging task. This paper suggests an approach to objectively compare operational efficiency of different icebreakers in performance of some typical IM tasks. This approach made it possible to work out universal criteria for assessing the efficiency of these ships. The criteria of icebreaker efficiency and operational performance have been derived from actual ice breaking and maneuvering data including safety aspects of required icebreaker maneuvers. The paper contains case studies with estimation of the said criteria for a number of IM icebreakers expected to be used for ice management in the south-eastern part of the Barents Sea.
Application of Special-Purpose Equipment for Management of Ice Buildups Around Prirazlomnaya Platform
Authors: Karulin Evgeny , Karulina Marina , Kazantsev Mikhail , Proniashkin Aleksander , Zaikin Dmitry
Published: September 2018

From experience, OIRFP Prirazlomnaya (offshore ice-resistant fixed platform) installed in the Pechora Sea was seen to accumulate sustained piles of rubble ice in winter time. These features were formed along the platform walls and rested on the seabed (berm) obstructing operation of supply ships and crew evacuation. Researchers from the Krylov State Research Centre, St. Petersburg, were studying the ice piling processes and behavior under variation of environmental conditions (e.g., changes in ice drift headings) during design as well as service life of the platform. Moreover, the Krylov Centre ice basin was engaged in development of various ice management techniques to reduce the ice buildup as well as to remove rubble ice piles. The results of these investigations, which are presented in the paper, have led to a relatively simple solution for removing the rubble ice piles with a special-purpose shovel (excavator) mounted on the deck of a supply vessel. The paper describes details of the suggested solution and its implementation. The efficiency of this ice pileup management method has been proven in actual operating conditions.
UHB Model Uncertainty for Structural Reliability Analysis of Pipeline OOS Design
Authors: Liu M. , Cross C.
Published: September 2018

Design load factor structural reliability analysis is critical for pipeline postlay OOS design to mitigate global UHB for a trenched and buried subsea pipeline configuration operating at elevated temperature and pressure. During the detailed engineering phase it is necessary to evaluate and define any measure available to be finalised for UHB mitigation such as deep trenching selection, enhanced blanket or spot rockdumping. In order account for inherent uncertainties in the design variables, a pre-emptive SRA is normally performed for the probabilistic UHB design load factors prior to pipeline installation according to the typical trench imperfection statistics and some specified survey accuracy. As per the current practice the semi-analytical universal design curve method is used in the limit state for design load factor predictions. The SRA results will be updated once the OOS survey data become available. A rockdump schedule can then be established by FEA incorporating appropriate safety or load factors to address uncertainties in the design parameters and as-built pipeline OOS survey measurement accuracy.

This paper examines the UHB model uncertainties in the load factor and backfill cover assessment with a view to improving the SRA OOS analysis. Sources of uncertainties and variability in the UHB design are discussed first. Some disparity and inconsistency arising between the SRA and FEA models for the limit state are considered. Alternative UHB models are investigated by taking Timoshenko shear stiffness and associated deformation with pipe-soil interactions into consideration. A comparison is made with the conventional universal design curve method, the improved model and FE modelling to demonstrate the findings and conclusions. Of these, the pipe-soil interaction and its representation in the SRA limit state assessment are identified as a significant factor.
Walking of Long Pipelines With Multiple Buckles: Can a Trend Be Trusted?
Authors: Carneiro Daniel , Rathbone Andrew
Published: September 2018

Walking of long pipelines with multiple buckles is usually self-limiting. The buckles break the ‘long’ pipeline into multiple ‘short’ ones that are prone to walk. However, as temperature decays over the length of the pipeline, the ‘short’ sections further downstream might become cyclically constrained and eventually anchor the full pipeline length. Walking of the hot end would then slow down and cease.

This tapering down can take a large number of cycles, and not seem obvious when after a fair number of cycles, a small value of accumulated axial displacement per cycle is still observed in FEA. Often, designers would stop the analyses at some stage and assume the small rate will continue indefinitely. This can be overconservative, as a limit will often exist — which is demonstrated using first principles in the paper.

On the other hand, extrapolating without full understanding of the underlying processes can be dangerous. For some particular conditions, the trend can suddenly change after continuing unaltered for many cycles. This paper illustrates such change in behavior with the example of a fictitious pipeline seeing a gentle, gradual increase in operational temperature over time. The exercise shows that, after the trend has apparently settled, at a given point the rate of walking can increase again. The conditions that trigger it are shown to be predictable.
RANS Study on Hydrodynamic Characteristics of Flapped Rudders
Authors: Liu Jialun , Hekkenberg Robert , Zhao Bingqian
Published: September 2018

Ships that equipped with flapped rudders have better manoeuvring performance than ships fitted with traditional spade rudders. Moreover, this advantage is achieved without significantly affecting the ship’s resistance during normal cruising. Flapped rudders are, therefore, favourable for ships that require high manoeuvring performance and sail long distance. Nowadays, there is a trend of using twin flapped rudders on newly built inland vessels in the Yangtze River. To properly design these ships and analyse their manoeuvring performance, the hydrodynamic characteristics of the flapped rudders are required. In this paper, a RANS study is performed to analyse the impacts of the three main properties of a flapped rudder on its hydrodynamic coefficients. The target properties are the rudder profile, the flap-linkage ratio (the flapped angle relative to the rudder chord line divided by the applied rudder angle), and the flap-area ratio (the sectional area of the flap divided by the total sectional area). The RANS simulations are carried out with commercial meshing tool ANSYS Meshing and CFD solver ANSYS Fluent.
Hydroelastic Analysis of a Submerged Horizontal Plate Using a Coupled SPH-FEM Model
Authors: Gao Xifeng , He Ming , Xu Wanhai , Wang Hongshu
Published: September 2018

A coupled SPH-FEM model is established to study the hydroelastic interactions between waves and submerged horizontal plates. The fluid motion is governed by the continuity equation and Navier-Stokes (NS) equations, which are solved based on the smoothed particle hydrodynamics (SPH). The solid boundary condition is implemented with the dynamic boundary particles (DBPs), and the multi-layer DBPs technique is used to describe the thin plate. As for the structural deformation, it is governed by the structural dynamic equation and solved using the finite element method (FEM). Four-node square elements are used for mesh generation and the Newmark scheme is employed for time integration. To couple these two models, a novel attachment algorithm for the fluid-solid interface is proposed, and the Dirichlet–Neumann boundary condition is enforced in the weak coupling regime. The accuracy and robustness of the coupling model are checked through a numerical test of a hydrostatic water column on an elastic plate. Then the model is used to compare the hydroelastic performances of submerged horizontal plates with different rigidities. An equivalent conversion method is introduced to avoid using different spatial resolutions and excessive computational resources in the comparison. On this base, some valuable conclusions that have not been previously reported are drawn.
Simulation of Working Posture of Deepwater Gravity Sampling Device Using OpenFOAM
Authors: Ma Zhe , Kong Danya , Wang Yin , Ren Nianxin , Zhai Gangjun
Published: September 2018

Computational fluid dynamics (CFD) has been widely used in the field of marine engineering recent years. In this work, the free falling process and working posture of gravity sampler have been studied using OpenFOAM. The overset grids have been used to simulate the movement of the sampler in the flow field. Main works are as follows: (1) verifying the accuracy of the overset grid method; (2) establishing a numerical model using the Newmark method to analyze the responses of sampler vertical and slant drop; (3) investigating the relationship among the deflection angle, falling speed and sampling rate of sampler. Finally, the best fall distance of GPS-01 sampler has been recommended in combination with the falling velocity and the initial falling angle.
A Towing Test of a Floating Type VAMT With Cycloidal Pitch-Controlled Blades
Authors: Ikoma Tomoki , Eto Hiroaki , Masuda Koichi , Oguchi Atsuhiro
Published: September 2018

Sea areas around the Japanese Islands which is feasible for tidal current generation are not a lot because sea sites where tidal current velocity is above 2.0 m/s are a few. We can find such sea sites at a west side of the Kyushu Island especially. However, we would earn electrical energy to be generated if it is able to generate electricity long time using around 1.0 m/s in current velocity. A vertical axis turbine should be better than horizontal axis types because VATs can take relatively higher torque. It is very useful that we can set and control a marine turbine to be higher performance in various current velocity. The present study introduce variable pitch-control system to a vertical axis turbine for tidal current generation. The pitch-control system adapts a cycloidal mechanism so that to vary pitch angle of turbine blades is conducted mechanically. The study developed a vertical axis marine turbine with cycloidal pitch-controlled three blades which was based on previous studies and experimental data. The diameter of the turbine is 1.0 m, length of a blade is 1.3 m. The turbine was set on a floating structure in order to carry out towing tests at a sea. We obtained several kinds of data from the towing tests, which were turbine torque, the number of rotation of the turbine, output power from an electrical generator and acceleration of the floating structure. As a result, the turbine made 50 W power from the generator. Although the PTO was not so large, the pitch-control was effective very much. Some issues were found at the same time. We need to consider and develop more useful gears, assemble methods to be feasible of variable pitch system.
Mechanical Behaviors of Steel Strip Reinforced Flexible Pipes Under Bending
Authors: Jin Shan , Yuan Shuai , Liu Ting , Han Peihua , Bai Yong
Published: September 2018

Steel strip reinforced flexible pipe (SSRFP) is a kind of unbonded composite pipe, which has more application foreground in offshore engineering due to its excellent mechanics and the considerable flexibility. In practical application, SSRFP will inevitably experience bending during reeling process and installation. In this paper, the mechanical behavior of SSRFP subjected to pure bending are studied both experimentally and numerically. A four-point bending equipment is utilized to conduct the full-scale laboratorial tests of SSRFP. Furthermore, the commercial software ABAQUS is employed to simulate its ovalization instability. The results acquired from the ABAQUS simulation are compared with the ones from verification bending experiment, which are in good coincidence with each other. The proposed model and the relative results may be of interest to the manufacture factory engineers.
A Time Domain Prediction Method for Vortex-Induced Vibrations of a Flexible Pipe With Time-Varying Tension
Authors: Zhang Mengmeng , Fu Shixiao , Ren Haojie , Li Runpei , Song Leijian
Published: September 2018

Prediction of vortex induced vibration (VIV) for a long-flexible pipe has always been an important concern for the design of risers. Currently, VIV prediction methods are mainly based on the linear beam theory, where the axial tension is treated as time-independent, and the couples between VIV and axial tension are totally ignored. However, experimental results have illustrated strong couples between the axial tension and VIV [1–2]. The purpose of this paper is to develop a time domain VIV prediction model. This model consists of pipe’s structural non-linearity, couplings between axial force, cross-flow/in-line (CF/IL) VIV responses, and the hydrodynamic forces. The hydrodynamic forces are further divided into vortex-induced force in CF and IL directions, and drag force in IL direction. The former one is determined via empirical force model based on forced oscillation test of rigid cylinders. The IL drag coefficients model considering the effects of VIV developed by Song [3] is adopted. VIV responses under these hydrodynamic forces at each time step are solved by Newton-Raphson method. Comparison between present method and the experimental results under uniform flows and shear flows are conducted, which verified the feasibility and reliability of the proposed method. In addition, by comparing the results under constant tension and time-varying tension, it is proved that the time-varying tension has a significant effect on VIV responses, especially under the case of high flow velocity and high vibration mode.
Microbiologically Influenced Corrosion on Seabed Chain in the North Sea
Authors: Gabrielsen Øystein , Liengen Turid , Molid Solfrid
Published: September 2018

The last years Statoil has replaced some of our seabed mooring chain segments. Some of these chains have corrosion pits caused by Microbiologically Influenced Corrosion (MIC).

In 2016 and 2017 one full length of a seabed chain segment, including anchor, was retrieved from a SEMI at approximately 300m water depth in the North Sea. The chain has been 20 years on the seabed. The corrosion on the chain was carefully documented, and showed significant levels of MIC. The extent of the MIC showed a strong dependency on seabed contact and how well the chain was buried in the sediments. The observed MIC is caused by Sulphate Reducing Bacteria (SRB). After corrosion identification, the chain has also been subject to full scale fatigue testing.

This paper presents the technical condition of the seabed mooring chain, describing the different levels of MIC, typical SRB corrosion attacks, and the results from the fatigue testing.
Numerical Study of a Fishing Vessel Operating in Partially Ice Covered Waters
Authors: Aarsæther Karl Gunnar , Su Biao , Kristiansen David
Published: September 2018

The Arctic ocean has been the focus of increasing activities in oil and gas, marine traffic and fisheries as the resources in the Arctic area becomes more attractive for exploitation. There have been several studies on the response of ships and structures in ice covered waters, mainly for oil and gas applications. This paper presents a scenario simulation model for fisheries, crab pot retrieval, in partially ice covered waters. Snow crab fisheries in the Barents Sea is an ongoing commercial activity where partial ice covers may drift over crab pots which need retrieval. This scenario is unique in the sense that the ship and ice can be expected to experience wave forcing in addition to the ice–structure interaction. The complexity of such scenarios favor simplified models and a coupled simulation model consisting of ship hydrodynamics, ice hydrodynamics, ice–ice and ice–ship interactions. A case vessel is presented together with a scenario simulation model which is used to assess the ice impact forces during operations and the amount of ice interaction which can be expected in the region where the pot string is retrieved from the ocean.
In-Situ Investigation of Initiation and Propagation Behavior of Bainitic Steels in Sour Environment
Authors: Yasuda Kyono , Ishikawa Nobuyuki , Matsui Yutaka , Mizuno Daisuke , Yokota Tomoyuki
Published: September 2018

Hydrogen induced cracking (HIC) occurs by the poisoning effect of hydrogen sulfide (H2S) which promotes hydrogen absorption and entry at steel surface. Therefore, it is important for linepipe steels to have sufficient HIC resistance in sour environments. The HIC resistance is usually evaluated by measuring cracks after the standardized immersion test such as NACE TM0284. However, the general evaluation method cannot investigate HIC initiation and propagation behavior separately. It is necessary to understand the effect of metallurgical factors on the cracking behavior of sour service linepipe. In this study, in-situ ultrasonic inspection equipment was applied to the HIC test for the several linepipe steels with bainitic microstructure in order to clarify crack initiation and propagation behavior quantitatively. The three dimensional (3-D) distribution of cracks in the specimen was successfully captured as time sequence, and the temporal change of the crack area ratio (CAR) was investigated. It was revealed that the CAR-time curves are consist of four stages with different CAR increment rate. The first stage is the incubation of crack initiation. In the second stage, cracks occur and grow, and adjacent cracks coalesced rapidly. Regarding the first and second stages, sensitivity for the HIC initiation was well correlated with the hydrogen diffusion coefficient and the density of crack initiation site, such as MnS and Nb inclusions. In the third stage, the coalesced cracks propagate along the center segregation region. From the investigation of individual crack behavior, the crack along harder region showed higher propagation rate. In the fourth stage, the crack propagation rate was decreased to be in stasis. It can be stated that crack growth in the final stage is strongly affected by the hardness of base material and the crack easily propagate when HIC occurs in high strength steels.
Analysis of Yielding and Buckling Strength of a Minimal Ballast Water Crude Oil Tanker
Authors: Zhou Xueqian , Song Hexing , Li Chenfeng , Ren Huilong , Wang Siyu
Published: September 2018

The minimal ballast water crude oil tanker has the design of a trapezoidal-shape inclined bilge entirely instead of the regular U-shaped tanker, also known as the trapezoidal tanker, which reduces the amount of the ballast water. This type of ship does not only reduce the cost on ballast water management, but also is beneficial to the environment for reducing the risk of water pollution. Since it is a new design, there are no applicable specifications for the assessment of structural strength at present. In order to find out characteristics of the yielding and buckling strength of this type of ship, the strength of a conventional tanker and a trapezoidal tanker are calculated by a finite element method and then compared with a variety of cases. It can be seen that the trapezoidal tanker has lots of advantages in strength and lighting weight.
Predicting the Heading Misalignment of a Vessel-Shaped Offshore Fish Farm Under Waves and Currents
Authors: Li Lin , Jiang Zhiyu , Wang Jungao , Ong Muk Chen
Published: September 2018

A vessel-shaped fish farm concept for open sea applications has been proposed recently. The whole system consists of a vessel-shaped floater, fish cages positioned longitudinally along the floater, and a single-point mooring system. The whole system weathervanes; this feature increases the spread area for the fish waste. However, the downstream cages may experience reduced water exchange when the vessel is parallel to the currents. This situation may jeopardize the fish health. A dynamic positioning (DP) system may be necessary to improve the flow conditions. This paper investigates the misalignment angle between the heading of the vessel-shaped fish farm and the currents under combined wave and current conditions. The misalignment angle is critical for the estimation of the DP system consumption. A numerical model of the fish farm system with flexible nets is developed. Current reduction factors are included to account for the flow velocity reductions between the net panels. The heading of the system is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. An integrated method using metamodels is proposed and applied for the prediction of the misalignment angle for a reference site. The probability distribution of the misalignment angle between the vessel heading and the currents is calculated using the Kriging metamodel for the reference site. Based on the prediction, the requirement for the DP system to improve the flow condition in the fish cages is discussed.
Influence of a Taut Cable on the Performance of a Point-Absorber Wave Energy Converter
Authors: Wang R. , Wei Y. , van Rooij M. , Jayawardhana B. , Vakis A. I.
Published: September 2018

In recent years, wave energy converters (WECs) have received considerable attention as an efficient way to harvest alternative energy sources. Within this class of systems, point-absorbers are popular and have become one of the most widely used renewable energy harvest designs all over the world, at least in the preliminary R&D stage, with many relevant research works having been published as well. However, unlike the single buoy and PTO systems which already have a comprehensive research basis, the connection cable has received little attention. The traditional taut cable analysis approach, initiated from the needs of the oil&gas industry, has been applied for WEC investigations. However, this approach utilizes an essential assumption that the oscillating term (PTO force) is much smaller than the static term of the cable force (pre-tension) and could be neglected, which may not be proper for WEC applications. In this work, a conventional frequency domain model is utilized to test and verify the validity of the previously mentioned assumption by presenting the ratio between two force terms. Then the ratio could be applied in combination with sea state contours to reveal the critical state of the cable. Then, a fully nonlinear time domain method of a numerical solution of the point-absorber wave energy converter is presented. According to the critical states obtained from the frequency domain analysis, an improved model of a slack cable is proposed. Its influence on the energy extraction performance is investigated using the open source code — WEC-Sim. This work provides insight into simulating a proper model of the cable and how the design of the cable influences the WEC performance.
A Fourier Approximation Method for the Multi-Pump Multi-Piston Power Take-Off System
Authors: Wei Y. , Barradas-Berglind J. J. , Almuzakki M. Z. , van Rooij M. , Wang R. , Jayawardhana B. , Vakis A. I.
Published: September 2018

In this work, a frequency-domain method for the numerical solution of the nonlinear dynamics of a wave energy converter with a pumping system is presented. To this end, a finite Fourier series is used to describe the nonlinear force components, i.e., the pumping force. The dynamics of the buoy and the piston are obtained by solving a set of linear motion equations. The numerical model is validated by comparing it with experimental results. The dynamic characteristics of the pumping system are investigated by performing a series of numerical simulations for several approximation orders under various wave conditions. The advantages and limitations of this method are also discussed in the paper. This work provides great insight into the mechanism of a WEC with a pumping unit under various sea states, which can guide the design of a multi-piston pump (MPP) system. This model will be applied to investigate the optimal configuration for the MPP system in the future.
Effect of Moonpool Shape and Dimensions on Drillship Operability
Authors: Chalkias Dimitris , Krijger Jan Willem
Published: September 2018

All modern drillships are equipped with moonpools, through which drilling and other subsea operations are performed. The operability of the vessel, is the percentage of time that the vessel can perform a specific operation in a specific location not limited by environmental conditions. Specific operations such as lowering a BOP or X-tree through the splash zone, have strict operability criteria and often a drillship is waiting on weather to perform these operations.

In this paper, the underwater shape of the moonpool of a drillship is varied and the operability of the vessel is calculated and compared to the original shape. Ten moonpool configurations are used for the study. The GustoMSC “Galene” moonpool shape is used which is comprised of an upper and a lower chamber. The lower chamber or cutout step dimensions such as length, breadth and height are the variables for this study.

An analytical method originally proposed by Newman [1] and extended by Chalkias and Krijger [2] is used in this optimization study. The method is comprised of a potential flow radiation/diffraction solver, where the moonpool modes are accounted as additional separate generalized modes. In this way each mode can be damped separately.

In order to calculate the damping factors for the moonpool modes used in the potential flow solver, free decay CFD calculations are performed and a P-Q analysis is applied on the resulting time-traces. Additional regular wave CFD calculations are performed for method validation purposes. The efficiency and ease of the proposed method is demonstrated by calculating the responses and operability of multiple shape variations in the frequency domain.
Turbulence and Wake Effects in Tidal Stream Turbine Arrays
Authors: Nuernberg Martin , Tao Longbin
Published: September 2018

Electricity generation from tidal current can provide a reliable and predictable addition to a reduced carbon energy sector in the future. Following the deployment of the first multi-turbine array, significant cost reduction can be achieved by moving beyond demonstrator projects to large scale tidal turbine arrays. The interactions between multiple turbines installed in close proximity can affect the total electricity generation and thus require knowledge of the resulting flow field within and downstream of the array.

Results are presented for experimental and numerical studies investigating the flow field characteristics in terms of velocity deficit and turbulence intensity in a staggered tidal turbine array section. Multiple configuration with varying longitudinal and transverse spacing between devices in a three-turbine array are tested.

Comparison between numerical and experimental flow characteristics shows that open source numerical models with dynamic mesh features achieve good agreement and can be used for the investigation of array wake effects. The standard k–ω SST shows good agreement with experiments at reduced computational efficiency compared to higher order turbulence models (RSM). The importance of mixing with ambient flow is highlighted by identifying areas of significantly reduced velocity recovery within closely spaced arrays where ambient flow does not penetrate between adjacent wakes.
Drilling Data Quality Management: Case Study With a Laboratory Scale Drilling Rig
Authors: Geekiyanage Suranga C. H. , Sui Dan , Aadnoy Bernt S.
Published: September 2018

Drilling industry operations heavily depend on digital information. Data analysis is a process of acquiring, transforming, interpreting, modelling, displaying and storing data with an aim of extracting useful information, so that the decision-making, actions executing, events detecting and incident managing of a system can be handled in an efficient and certain manner. This paper aims to provide an approach to understand, cleanse, improve and interpret the post-well or realtime data to preserve or enhance data features, like accuracy, consistency, reliability and validity.

Data quality management is a process with three major phases. Phase I is an evaluation of pre-data quality to identify data issues such as missing or incomplete data, non-standard or invalid data and redundant data etc. Phase II is an implementation of different data quality managing practices such as filtering, data assimilation, and data reconciliation to improve data accuracy and discover useful information. The third and final phase is a post-data quality evaluation, which is conducted to assure data quality and enhance the system performance. In this study, a laboratory-scale drilling rig with a control system capable of drilling is utilized for data acquisition and quality improvement. Safe and efficient performance of such control system heavily relies on quality of the data obtained while drilling and its sufficient availability. Pump pressure, top-drive rotational speed, weight on bit, drill string torque and bit depth are available measurements.

The data analysis is challenged by issues such as corruption of data due to noises, time delays, missing or incomplete data and external disturbances. In order to solve such issues, different data quality improvement practices are applied for the testing. These techniques help the intelligent system to achieve better decision-making and quicker fault detection. The study from the laboratory-scale drilling rig clearly demonstrates the need for a proper data quality management process and clear understanding of signal processing methods to carry out an intelligent digitalization in oil and gas industry.
Hydrodynamics on Circular Cylinder Close to a Wall: Effects From Wall Boundary Layers
Authors: Tong Feifei , Cheng Liang , An Hongwei , Griffiths Terry
Published: September 2018

We present a study on the hydrodynamics of a circular cylinder close to a wall, which is an idealized representative of subsea pipelines and cables used in both offshore oil and gas and marine renewable energy industries. This research utilizes Computational Fluid Dynamics (CFD) modeling method to investigate the influences of the boundary layer turbulence and the cylinder-to-floor distance on hydrodynamic forces. A significant jump in hydrodynamic forces is observed over the range of Reynolds number that coincides with boundary layer transition from laminar to turbulence. This transition gives rise to a drop in the size of the thickness of the boundary layer at the location of the cylinder. The force jump is also characterised by a pressure increase on the upwind surface of the cylinder, while the pressure decreases on the leeward surface.
Design Optimization of Floating Structure for a 100 MW-Net Ocean Thermal Energy Conversion (OTEC) Power Plant
Authors: Adiputra Ristiyanto , Utsunomiya Tomoaki
Published: September 2018

This paper presents a design procedure based on optimization to contrive a floating structure for a commercial scale of OTEC power plant. In the aim to get a safe yet economical floating structure, a commercial oil tanker ship was converted as the plantship. The process was started by defining independent variables, constraints and fix parameters. The independent variables included the velocity of seawater transport and type of oil tanker ship. The next step was breaking down the fix parameters which were kept constant during the iteration process. These parameters were about the general requirements and the necessary equipment to produce 100 MW-net power output. Some constraints were also introduced as permissible borders to determine whether the particular case was acceptable or not. The constraints included the constraint due to provided space, allowed weight, net power output and fluid phenomena on the riser. During the iteration process, a spiral model was developed as analysis guideline. Based on the result of the optimization, it could be concluded that the typical Suez-max oil tanker ship was the best option and the most optimum seawater transport velocity was 3 m/s. Finally, the general arrangements and the base layout design were also conceptualized in this paper.
A Transient Flow Model for Investigating Parameters Affecting Kick Behavior in OBM for HPHT Wells and Backpressure MPD Systems
Authors: Gomes Dalila , Nilsen Marius S. , Frøyen Johnny , Bjørkevoll Knut Steinar , Lage Antonio C. V. M. , Fjelde Kjell Kåre , Sui Dan
Published: September 2018

Gas kicks can occur during conventional drilling operations, in which case the well has to be shut in and the kick circulated safely out of the system. In back pressure managed pressure drilling (MPD) systems, one can tolerate minor influx sizes before the well is shut in.

However, when using oil based mud, solubility issues can complicate the picture making both kick detection and safe handling of kicks more complex. For sufficient large pressures the solubility can be infinite. A kick can also be taken undetected and reach the riser before the free gas suddenly emerge with a corresponding volume expansion that can unload the riser. Managed pressure drilling can be based on having a rotary control device on top of the riser, which makes it possible to add a backpressure on top to dampen the volume expansion, but possibly also change the depth where free gas emerges. It has been shown in another paper how one can use transient models to predict what kind of surface pressure is needed to maintain a constant bottomhole pressure when free gas emerge.

In this paper, a fully transient model developed for a research cooperation between a research institute and Academia will be presented. The model includes behavior of kicks in oil based mud and solubility issues. The model will be used to study the behavior of kicks in riser when considering a backpressure MPD system with special focus on MPD from floaters. One objective will be to show how the results can be impacted by accuracy in the calculation method due to the amount of numerical diffusion present. Another objective will be to show how the PVT model adopted can affect the results. In addition, one will study the impact on results when varying physical input parameters like kick size, magnitude of pore pressure and the geometry of the riser.
Multiaxial Fatigue Analysis of Mooring Chain Links Under Tension Loading: Influence of Mean Load and Simplified Assessment
Authors: Martinez Perez Imanol , Bastid Philippe , Constantinescu Andrei , Venugopal Vengatesan
Published: September 2018

Current standards (as an example, DNV-OS-E301[1] and API-RP-2SK[2]) do not account for mean load in the fatigue assessment of mooring chains. Both standards, provide S-N curves derived from experimental work without specifying the mean load for which they have been obtained or proposing a mean load correction function. This paper reports a fatigue analysis study of mooring chains under Tension Loading using a multiaxial fatigue criterion for two different mean loads. Multiaxial fatigue criteria enable to account directly for complex phenomena, such as residual stresses, non-proportionality of the stress tensor, among others. This paper presents an example of the implementation of the Dang Van fatigue criterion for studying the fatigue behavior of mooring chains under tension. It quantifies the effect of the mean load on the fatigue lifetime and the failure location. Furthermore, it also proposes a simplified approach to reduce the complexity and the computational time of the fatigue analysis using Dang Van fatigue criterion. The paper is organized as follows: in the first part an example of the fatigue assessment is reported. Two different loading conditions with the same load amplitude but different mean loads are studied. The assessment method is based on two steps: a mechanical analysis and a fatigue analysis. In the second part, a simplified fatigue assessment method is proposed. As part of this method, a ratio between the fatigue lifetimes of two loading conditions, which have the same load amplitude but different mean load, is formulated. This ratio has been obtained analytically using the geometric representation of the Dang Van fatigue Criterion. Finally, the paper ends with a discussion, based on recent works, regarding the formulation of the locus of the Dang Van criterion and the fatigue properties used for the calibration of this criterion.
Study on Wave Load Prediction and Fatigue Damage Analysis of River-Sea-Going Ship
Authors: Guo Guohu , Wang Yiwen , Gan Jin , Wu Weiguo , Ye Yunling
Published: September 2018

With the consideration of shipping efficiency and the limited conditions of the Yangtze River, flat and wide ship become the preferred type for River-Sea-Going ship. This kind of structure is adverse to the ship’s stiffness under longitudinal bending, transverse bending. And two-stage repeated loading brings to the fatigue assessment of the ship structure new uncertainties, so accurate cumulative fatigue damage is a matter of necessary. Based on the theory of three-dimensional potential flow, the hydrodynamic analysis of River-Sea-Going container ship under specific route is carried out. The hull motion and the load transfer function of each section are obtained. The finite element model is used to calculate the amplitude-frequency response Operator RAOs. The influence of special routes and speed factors are also considered. Based on the frequency domain spectral analysis method combined with the scatter diagram of E1 sea area, the characteristics of wave-induced load are analyzed under 10−7.5 probability level. The stress response spectrum and the short-term distribution of stress range are obtained by combining the structural stress response calculation with the whole ship finite element structure model. The long-term distribution of stress response spectrum and stress range is also obtained. The cumulative fatigue damage of River-Sea-Going container ship under the special route is evaluated by using the S-N curve recommended by the criterion. It can provide the basis for the fatigue assessment of the new type of River-Sea-Going container ship.