bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled Dynamic Analyses of Deep-Water Semi-Submersible With New Spread Mooring System
Authors: Chandrasekaran S. , Jain Arvind Kumar , Uddin Syed Azeem
Published: December 2020

Offshore complaint structures dominate the deepwater oil exploration and production due to their adaptive geometric form and well-established construction practices. Semi-submersible is one of the widely preferred, floating production systems due to its form-dominant ability, better stability characteristics, and best constructional features. It is usually position-restrained using a dynamic-positioning system (active-restraining) or mooring system (passive-restraining); being less-sensitive to freak ocean environment is an added advantage. The Semi-submersible, chosen for the present study is based on a similar configuration of a 6th generation deep-water Hai Yang Shi You (HYSY) – 981 platforms, commissioned by the China National Offshore Oil Corporation (CNOOC) in 2012. A sixteen-point, spread catenary-mooring without submerged buoy (case-1) in the form of chain-wire-chain type configuration is used for position-restraining. Response behavior of the semi-submersible with a conventional spread catenary-mooring system with a submerged buoy (case-2) is compared. API spectrum is used for computing wind loads, while the JONSWAP spectrum is used to represent irregular waves for various directions of wave heading. The effect of non-linearly varying current is considered up to 10% of water depth. Numerical analyses of the semi-submersible are carried out under 10-years, and 100-years return period events using Ansys Aqwa. Under wind, wave, and current loads, motion responses of the Semi-submersible at 1500 m and 2000 m water depths are investigated for both the cases in time-domain. Dynamic mooring tension variations arise from the environmental loads are further investigated for a fatigue failure using the S-N curve approach. It is found that the fatigue life of the mooring lines after the inclusion of the buoy is enhanced. It was also observed that, during failure of mooring lines there is an increase in tension of the mooring lines which are adjacent to the failed mooring lines and this is due to the transfer of mooring load and hence reducing their fatigue life.
Cavitation Performance of Low Speed Ice-Classed Propeller
Authors: Wan Churui , Liu Zhenghao
Published: December 2020

In the ice breaking condition, on account of the low speed and heavy propeller load, the ship resistance is large, which will aggravate the propeller cavitation and the propeller-induced pressure. In this paper, the cavitation performance of the ice-classed propeller is analyzed by numerical simulation and model experiment. Commercial CFD software was used for the numerical simulations, in which the cavitation flow is solved by Schneer & Sauer cavitaiton model based on a single-fluid multiphase mixture flow approach. Model tests to measure cavitation flow on an ice-classed propeller were carried out in SSSRI K15 Cavitation Tunnel. The size of the test section of SSSRI K15 Cavitation Tunnel is 600mm*600mm. The propeller performances in uniform flow over a range of advance coefficients were carried out in open water test in a towing tank. The diameter (D) of the model propeller was 248mm in this research. Firstly, the open water performance of propeller is numerically studied. Near the design conditions, the numerical results are almost consistent with the test results, with an error of less than 1%. In the case of ice breaking, the blocking effect of ice in front of a propeller is studied. The experiment results show that with the ice block close to the propeller, one or more vortex tube structures are generated between the propeller blade and the ship bottom while the vortex cavitation occurs. Such phenomenon is also found between the propeller and the ice block. When the blocking effect is significant, the stable vortex tube structure will appear and significantly change the cavity shape near the blade. When the distance between the ice and the blade disc exceeds 0.72D, the vortex tube structure will disappear.
Reliability Analysis of FPSO Oil and Gas Processing System Based on Petri Net
Authors: Kang Jichuan , Geng Xinyuan , Sun Liping , Jin Peng
Published: December 2020

This paper aims to introduce a novel method in terms of risk analysis and control of FPSO oil and gas processing system. The Failure Mode and Effect Analysis (FMEA) is implemented to identify the critical equipment and typical failure modes, in order to improve the accuracy and pertinence of model establishment. A Petri Net model is then developed based on FMEA results and the correlation analysis between different components. The input reliability data are primarily collected from Offshore and Onshore Reliability Data (OREDA), and the maintenance information is assessed by industry experts, using fuzzy synthetic evaluation method. This paper focuses on the accuracy, rapidity and feasibility in the modeling and solution process, and considers the influence of weather factors on the maintenance operation and limited maintenance crew number, forming a complete set of reliability and maintenance strategy optimization analysis method of FPSO oil and gas processing system. The results contain system reliability, availability, and maintenance information. The proposed approach can reveal the risk feature of the system and provide corresponding risk control scheme.
Innovative Mooring in the Port of the Future: Scale Model Testing of the ShoreTension System
Authors: Reijmerink S. P. , Bruinsma N. , van der Hout A. J. , de Jong M. P. C. , Clement C.
Published: December 2020

Moored vessels often experience low-frequency vessel motions when moored in a port due to wave excitation. Under such conditions the loading and offloading of vessels may be hampered when these movements become too large [1,2,3]. Innovative mooring techniques can be used for reducing issues with excessive motions of moored vessels in waves [4,5,6]. Considering applying such techniques as part of the design of mooring facilities and ports is expected to make different approaches to port or mooring facility designs possible. Such techniques, like the ShoreTension (ST) system, are already applied successfully worldwide in ports [7,8,9], however the application and performance limits of such systems under extreme conditions are not well known.

This paper describes the results of a research project using physical scale modelling to systematically verify and extend the applicability and performance limits of innovative mooring systems. It resulted in a solid validation database for validating numerical models. The knowledge developed in this research will benefit developers of mooring facilities (including ports) to significantly reduce costs by limiting the need for structures providing shelter from waves. Furthermore, this may also help lowering the impact of port infrastructure on the coastal system when using less invasive infrastructure.
Rig-Assisted Snubbing: Drillpipe Pressure Control in MPD/UBD
Authors: Aasen Jan Aage , Svensson Joakim Haram
Published: December 2020

An important pressure control issue during live well work is the expulsion force acting on the drillpipe. Wellhead pressure multiplied by drillpipe sealing area in the blowout preventor must be overcome before entering the well. The highest snubbing force (compression) is applied when running the first joint into the well since pipe weight is at its lowest at this time. Focus of this paper is the mechanical analysis of the drillpipe as a well barrier element during live well entry. We look at normal operation (primary well barrier) and contingency (secondary well barrier). Load cases include critical unsupported buckling, helical buckling inside tubing guide, collapse and burst. In critical unsupported buckling there is no radial confinement of the pipe and the critical buckling limit is determined at the onset of lateral deflection. On the other hand, inside the tubing guide the pipe is allowed to buckle into a helix and the buckling limit is related to permanent corkscrewing of the pipe.

Technical contributions in this paper include engineering design formulas for unsupported buckling and helical buckling. Also presented are experimental buckling results from pipe up to 3½ inch (8.9 cm) diameter. Design calculations for primary and secondary well barriers are explained and analyzed using a field case. All load calculations are based on zero neutral axial stress as stress reference datum, which produces a yield circle that is conveniently deployed for three-dimensional well tubular design. The use of the dimensionless yield circle is found to be an efficient method to assess helical buckling loading and the combined effects of pressure and axial stresses.
An Efficient Importance Sampling Method for the Long-Term Mooring Lines Response Estimation Considering Wind Sea and Swell
Authors: Simão Marina Leivas , Videiro Paulo Maurício , de Oliveira Mauro Costa , Sagrilo Luis Volnei Sudati
Published: December 2020

Escalating demand in the oil and gas industry has led offshore structures to be installed in ever deeper waters and under severe environmental conditions. As the mooring system is a crucial element in floating offshore structures, a reliable estimation of its long-term response is a decisive step in any usual design procedure. In the long-term scenario, the environmental actions to which these structures are subjected to, such as waves, wind and current, are non-stationary processes. However, this long-term behavior is usually modeled as a series of short-term stationary conditions (typically 3-h). In a full long-term analysis approach, an estimate of the long-term N-year response can be obtained through a multidimensional integration over all these short-term environmental conditions. In this paper, this multidimensional integral is numerically evaluated by means of the Importance Sampling Monte Carlo Simulation (ISMCS) method, where the uniform distribution is used as the sampling function. Thus, all short-term environmental conditions have the same probability of being sampled, which assures that conditions with very low original probability of occurrence, but with knowingly higher contributions to the long-term response, are efficiently accounted for. The random variability of the short-term environmental parameters and their interdependencies are represented by a simplified joint probabilistic model which comprehends both wind sea and swell waves. The methodology is numerically validated for an idealized single-degree-of-freedom (SDOF) model and later investigated for a mooring line connected to an FPSO installed in Brazilian deep waters. It is shown that ISMCS provides good estimates for the long-term N-year response with a moderate amount of required simulations and can be a powerful tool in order to account for simultaneous occurrence of wind sea and swell waves in structural response evaluations.
Dynamic Ultimate Strength of a Container Ship Under Sagging Condition
Authors: Jagite George , le Sourne Hervé , Cartraud Patrice , Bigot Fabien , Derbanne Quentin , Malenica Šime
Published: December 2020

When technically specifying ships for the future, the following aspects are examples of what we will have even more focus on than today: bigger, lighter, and faster. Thereby, the hydro-elastic type of structural response will be more and more significant. In addition, some of the recent container ships’ designs are with very low values of the “minimum hogging still water bending moment.” Combined with high whipping induced sagging moments, it casts some doubts on the probability of buckling appearance in the upper structure. Therefore, the objective of the research work presented in this paper is to investigate the dynamic ultimate strength of the entire hull girder section, subjected to sagging bending moment associated with wave loads and whipping response. The dynamic ultimate strength is computed and compared with the quasi-static ultimate strength, in order to derive the dynamic load factors, which can be used as an estimator of the dynamic collapse effect.
A New Approach to Compute the Non-Linear Whipping Response Using Hydro-Elastoplastic Coupling
Authors: Jagite George , le Sourne Hervé , Cartraud Patrice , Malenica Šime , Bigot Fabien , de Lauzon Jérôme , Derbanne Quentin
Published: December 2020

In the last ten years, the importance of whipping on the extreme hull girder loads has received much attention, but its consequence on the hull girder’s collapse is still unclear. The most common practice is to consider the structural behavior as linear-elastic in the hydro-elastic coupling, and as non-linear elasto-plastic in the ultimate strength evaluation. In order to investigate the influence of the non-linear structural behavior on the hydro-structure interaction responses, a new hydro-elastoplastic model is proposed to compute the non-linear whipping response. The structural part is modeled as two beams connected by a non-linear hinge, which follows the collapse behavior of a ship’s hull girder. The hydrodynamic problem is solved using the three-dimensional boundary element method, and the exact coupling between the structural model and the hydrodynamic one is made by making use of the shape function approach. Finally, the fully-coupled hydro-elastoplastic problem is solved directly in time-domain by numerical integration.
Four-Phase Flow of Oil, Gas, Water, and Sand Mixtures in Pipelines
Authors: Odan Mohamed , Ben Rajeb Faraj , Rahman Mohammad Azizur , Aborig Amer , Imtiaz Syed , Zhang Yan , Awad Mohamed M.
Published: December 2020

This paper investigates issues around four-phase (Oil/CO2/water/sand) flows occurring within pipelines. Multiphase flows are the norm, as production fluid from reservoirs typically include sand with water. However, these multi-phase flow mixtures, whether three- or four-phase, are at risk of forming slug flows. The inclusion of sand in this mixture is concerning, as it not only leads to increased levels of pipeline erosion but it also has the potential, to accumulate sand at the bottom of the pipe, blocking the pipe or at the very least hindering the flow. This latter impact can prove problematic, as a minimum fluid velocity must be maintained to ensure the safe and regulated flow of particles along a pipeline. The presence of low amounts of sand particles in oil/gas/water flow mixtures can serve to reduce the pressure exerted on bends. The sand volume fraction must in this case, be relatively low such that the particles’ resistance causes only a moderate loss in pressure. Therefore, the study aims to gauge the impact of oil/gas/water/sand mixtures on various pipeline structures as well as to further investigate the phenomenon of flow-induced vibration to determine the optimal flow variables which can be applied predicting the structural responses of pipelines.
Research on Lateral Buckling Mechanism of Tensile Armor Wires in Unbonded Flexible Pipe
Authors: Yang Zhixun , Yan Jun , Zhang Liang , Shi Dongyan , Lu Qingzhen
Published: December 2020

The buckling behavior and critical value are analyzed to explain the mechanism of the lateral buckling. Moreover, a 3D numerical model of four tensile armor wires with symmetrical arrangement is established with the interaction between armor wires and inner structure. The lateral buckling is discussed through a relationship between critical value and axial displacement. Then the sensitivity of critical value on friction coefficient is figured out. The results show that the lateral buckling resistance of tensile armor wires performs better with the friction coefficient in the range of 0.25 to 0.3. Finally, this research provides mechanical support for the lateral buckling resistance design of the unbonded flexible pipe.
Research on Intelligent Loading of Bulk Carriers
Authors: Fang Sheng , Chen Shunhuai
Published: December 2020

With the rapid development of computer technology in recent years, the shipbuilding industry is accelerating its intelligent process. Intelligent cargo management is an important functional module of intelligent ships, and intelligent loading is an important part of intelligent cargo management. Cargo loading is an important link in the process of cargo transportation. The loading effect affects the sailing performance, safety performance and loading efficiency of the ship, so it is necessary to improve the intelligence of ship loading. Based on the calculation and research of the floating state, stability, structural strength and loading efficiency of the bulk carrier during loading, a general mathematical model for the intelligent loading of bulk carriers is established. The model comprehensively considers the influence of loading rounds, loading capacity and loading order. It is solved using an improved genetic algorithm to obtain optimized loading results. The loading case calculation was carried out using a 20000t bulk carrier as an example, and an optimized solution was obtained, which proved the effectiveness of the algorithm. The results show that the method can effectively improve the loading efficiency of the ship while ensuring that the ship has sufficient strength and suitable floating state.
Convolutional Neural Networks Applied to Flexible Pipes for Fatigue Calculations
Authors: Ribeiro Machado da Silva Vinicius , Serrano de Araujo Breno
Published: December 2020

The industry standard approach for the design of flexible pipes makes use of non-linear finite element analysis (FEA) in time domain to simulate the physical responses of the structure in different environmental conditions. Wave dynamics can be represented either by an irregular wave (IW) or an equivalent regular wave (RW) approach, which simplifies the analysis. Irregular wave modeling approximates better the structural responses, due to the stochastic nature of the environmental loading, having the drawback of being more computationally expensive. The computer processing time of IW-FEA often becomes intractable due to the large number of scenarios that need to be simulated, for different values of Hs (significant height), Tp (peak period) and different wave directions. Reducing the time needed to simulate each scenario would reduce significantly the total processing time. In order to achieve this, alternative hybrid methods have been proposed in the literature, combining FEA with machine learning models. This paper proposes the use of nonlinear autoregressive exogeneous convolutional neural networks (NARX-CNN) to predict tension and curvature responses along the length of a flexible riser. Experimental results show that the proposed model can generate more accurate responses than previous models. This work also extends the region analyzed by forecasting responses beyond the bending stiffener level, going down to the end-fitting and touch down zone locations. It is the first time that such regions, prone to fatigue issues, are evaluated with these types of algorithms for flexible pipes, as per authors’ knowledge.
A Numerical Investigation of Steep Irregular Wave Properties With a Mixed-Eulerian Lagrangian HOS Method
Authors: Fouques Sébastien , Pákozdi Csaba
Published: December 2020

The design of structures at sea requires knowledge on how large and steep waves can be. Although extreme waves are very rare, their consequences in terms of structural loads, such as wave impact or ringing, are critical. However, modelling the physical properties of steep waves along with their probability of occurrence in given sea states has remained a challenge. On the one hand, standard linear and weakly nonlinear wave theories are computationally efficient, but since they assume that the steepness parameter is small, they are unable to capture extreme waves. On the other hand, recent simulation methods based on CFD or fully nonlinear potential solvers are able to capture the physics of steep waves up to the onset on breaking, but their large computational cost makes it difficult to investigate rare events. Between these two extremes, the High-Order Spectral (HOS) method, which solves surface equations, is both efficient and able to capture highly nonlinear effects. It may then represent a good compromise for long simulations of steep waves. Unfortunately, it is based on a perturbation expansion where the small parameter is the wave steepness, and consequently, simulations tend to become unstable when steep wave events occur.

In this work, we investigate the properties of irregular waves simulated with a modified HOS method, in which the sea surface is described with a Lagrangian representation, i.e. by computing the position and the velocity potential of individual surface particles. By doing so, nonlinear properties of the surface elevation are simply captured by the modulation of the horizontal and vertical particle motion. The same steep wave is then described more linearly with a Lagrangian representation, which reduces the instabilities of the HOS method. The paper focuses on bi-chromatic waves and irregular waves simulated from a JONSWAP spectrum. We compare simulations performed with the standard HOS and the modified Lagrangian methods for various HOS-orders.
State of the Art in Determination of Crack Direction in Shell Structures
Authors: Heidari Hossein , Walters Carey L.
Published: December 2020

Considerable work has been done on the modelling of fracture and necking for accidents in the maritime and offshore industry, but very little has been done to find the crack orientation during this failure. Such information will be required in future analyses which use the eXtended Finite Element Method (XFEM), rather than the element deletion approach which is typically used. It also offers a more realistic portrayal of the extents of damage, which will be useful in assessments of salvageability and residual strength analysis in maritime structures. This paper reviews the current state of the art on the direction of necking and fracture relative to the main in-plane loading axes. It was found that necking and fracture can both evolve under different angles relative to the first principal stress, depending on the ratio of inplane strains, material anisotropy, and other effects. Traditional analysis has given an easy relationship between necking orientation and strain ratio for negative second principal strains. Anisotropy is found to have a very important role for the necking angle for strain ratios (ε2/ε1) greater than 0.5. Fracture theory predicts a fracture angle perpendicular to the first principal stress for states of stress between uniaxial tension and equi-biaxial tension, but it predicts in-plane slant fracture for strain ratios less than −0.5.
Computational Fluid Dynamics Simulation of the Transient Behavior of Liquid Loading in Gas Wells
Authors: Khaled Mohamed , Rahman Mohammad Azizur , Hassan Ibrahim , Sultan Rasel A. , Hasan Rashid
Published: December 2020

Liquid loading is one of the major flow assurance challenges in gas wells, causing production problems and reducing the ultimate recovery. Liquid loading is defined as the inability of a well to carry all the co-produced liquid up the tubing. This leads to liquid accumulation in the well resulting in increased bottomhole pressure and decline of gas flow rate. Although many studies have been performed on liquid loading phenomena, available models generally lack the ability to capture transient behavior of liquid loading in gas wells. We have developed a computational fluid dynamics (CFD) model using Ansys Fluent 19.1 R3 version to model the transient features of liquid loading.

In this study, the CFD model is developed and validated with data from 42 meter long vertical pipe lab at Texas A&M University. The Eulerian multiphase approach combined with volume of fluid approach (VOF) - Multi-fluid VOF model with realizable k-Є turbulence closure is used to study the flow behavior. In addition, hexahedral mesh is utilized and compared to tetrahedron mesh to test accuracy and computational time.

The developed CFD model has unique parameters combinations that shows an acceptable agreement with the experimental work. Model accuracy and computational time is improved by using hexahedral mesh. Liquid film flow reversal mechanism is expected to be the root cause of liquid loading in gas wells rather than droplet fall back mechanism. The CFD model captures the transition from one phase to another that is crucial for determining well end life. Model novelty is based on the ability to be a reliable predictive tool that can help in the remediation of liquid loading and give a precise representation of liquid loading transient behavior in gas wells.
Investigations Into Tidal Current Turbine System Faults and Fault Tolerant Control Strategies
Authors: Yan Xuhua , Norman Rosemary , Elgendy Mohammed A.
Published: December 2020

In recent years, there has been a growing interest in tidal current energy as it is a potential source for green electricity generation and the most predictable form of ocean renewable energy. Due to the harsh marine environment, the Tidal Current Turbine (TCT) system has to be designed to be robust and to work reliably with high availability to minimize the need for intervention. Thus, fault tolerant control strategies are needed to enable the system to continue operating under some fault conditions, this will reduce the power generation cost and also increase the system robustness. This paper introduces some of the different fault conditions that may occur in TCT systems such as sensor faults, especially tidal current sensors. Potential solutions for these faults are then introduced. The paper then presents a standalone TCT generation system model with perturb and observe (P&O) control; this control aims to solve the tidal current speed sensor fault problem, ensuring that the system operates near the maximum power point (MPP) without the tidal current speed sensor. The control system is simulated using MATLAB/Simulink, for a TCT, utilizing a permanent synchronous generator (PMSG) and a boost converter.
Structural Connection Fatigue Evaluation Methodology Using Time Domain Approach
Authors: Samaria Sagar , Zhang Bob , Tallavajhula Sudhakar , Kyoung Johyun
Published: December 2020

There is an ever-increasing demand for life extension of existing floating platforms worldwide. To adequately support these life extension projects there is a need to predict fatigue life of floating structures more accurately using a time domain approach. However, structural fatigue damage calculations using time domain response analysis can be very time consuming and challenging. An efficient and effective structural analysis methodology is developed to calculate accumulated fatigue damage for structural connections in a floating offshore platform using a response-based time domain routine. The methodology discussed in this paper can be applied to estimate fatigue life for hull critical connections in floaters such as Spars, TLPs or Semis as well as local structural supports such as mooring foundations and riser foundations. It also provides the option to generate stress histograms that can be utilized for Fracture Mechanics Evaluation (FME) of welds in structural connections.

To calculate the accumulated fatigue damage at desired locations on a floating platform, the time domain analysis employs a Stress Intensification Factor (SIF) which correlates global loads with local stresses. In cases where a crack initiation is observed on a structural connection, fracture mechanics is used to evaluate the structural integrity of the weld. The FME requires fatigue stress range histograms as one of the input parameters. The stress ranges and cycles that are calculated and used to compute the fatigue damage using this methodology can be converted to stress range histograms which can then be used in the FME. The standard method to compute fatigue damage for a structural connection is by using an S-N fatigue approach under the assumption of linear cumulative damage (Palmgren-Miner rule). The methodology discussed in this paper uses a rainflow counting algorithm to effectively calculate the stress range and cycles which are then utilized for computing the fatigue damage.

This methodology can be applied to green field projects involving a new design or for life of field studies of an existing platform requiring life extensions. It is particularly beneficial for brownfield projects where more accurate re-evaluation of fatigue life is needed. It can also provide Clients with reliable Engineering Criticality Assessments (ECA) and enable them to plan in-service inspections and repair work. As an application, a typical truss connection for a Spar platform is used to evaluate structural fatigue damage and generate the stress range histogram for FME. Also, a comparative study is performed for a typical truss connection fatigue damage result between the traditional approach used and the method discussed in this paper.
RANS Simulation of the Flow Around a Ship Advancing in Shallow Water
Authors: Zhang Yan-Yun , Zou Zao-Jian , Yao Jian-Xi
Published: December 2020

This paper simulates the viscous flow about a ship advancing in calm water of different water depths using Reynolds-Averaged Naiver-Stokes (RANS) method. A Wigley hull is taken as the study object, and the hull is free in sinkage and trim in the simulations. The fluid domain is discretized into hexahedral structured grids. The overset grid method and the deforming grid method are applied in different cases to capture the ship’s sinkage and trim motion. The grid independence analysis and validation of numerical method are carried out under deep water condition. Then, systematic simulations are carried out under shallow water condition at different ship speeds. The resistance performance and the wave pattern characteristics are compared with deep water condition to demonstrate the shallow water effect. Furthermore, resistance coefficient results under water depth-to-draft ratios of 2.0 and 1.5 are presented and compared. All simulations show great consistency with the theoretical and other potential theory based numerical results.
Study on the Consideration Method of Damage Stability Criteria Corresponding to IEC 61400-3-2 for Floating Offshore Wind Turbine
Authors: Chujo Toshiki , Haneda Ken , Komoriyama Yusuke , Kokubun Kentaroh , Yamada Yasuhira , Fujiwara Toshifumi , Inoue Shunji
Published: December 2020

The development of FOWTs (Floating Offshore Wind Turbines) is going into the commercial stage from the demonstration one. One of the subjects for success in the commercial stage is how to reduce the construction cost. For example, watertight bulkheads in narrow space of FOWTs are obstacles for the cost reduction from the perspective of structural complexity and manufacturing. On the other hand, new IEC 61400-3-2 standard describes the unnecessity of the damage stability requirement under certain conditions. To satisfy enough safety without the damage stability requirement according to the IEC 61400-3-2, an examination method with a probability theory and the FEM analysis is proposed in this paper. The collision probability with cruising ships, P1, around a FOWT is decided to use by the AIS data, and the probability of structural total loss, P2, due to the ship collision is indicated according to the total loss curve which is delivered from the FEM collision analysis. The joint probability which is represented by the product of P1 and P2 is compared to the target damage probability, 10−4 per year. The detailed information about this method and the results of trial analyses for a FOWT off the coast of Nagasaki Prefecture are also introduced in this paper.
Analytical Formulae for the Lateral Buckling Behaviour of Pipelines Installed With Residual Curvature
Authors: Teigen Martin , Ibrahim Malik
Published: December 2020

Residual curvature installation of subsea pipelines has become a popular method for lateral buckling management because of its low-cost implementation and high reliability. The method is foreseen to remain attractive due to the positive operational feedback made available to the public domain.

On the design methods, previous research has predicted the behaviour of pipelines installed with residual curvature mainly via finite element analysis (FEA). These analyses include lateral buckling, installation, reeling etc. Further to this, Teigen and Ibrahim have put an effort into quantifying design uncertainties using structural reliability analysis (SRA). Analytical approaches have also been explored, such as pipeline rolling, and other effects during pipeline installation. However, there is little published work on analytical approaches for the lateral buckling behaviour.

Therefore, this paper suggests analytical formulations for the lateral buckling behaviour of pipelines installed with residual curvature. For predicting the critical buckling force, the Palmer formulation was used as a basis. For predicting the pipeline integrity post buckling while accounting for non-linear effects and residual plasticity in the system, the formulation is derived using a combination of dimensional analysis, regression analysis and a modified Hobbs formulation. The resulting analytical formulation is calibrated to a database of finite element solutions.

The suggested approach is assessed for a configuration that applies model parameters based on the Skuld pipeline. A validation has been performed and the errors have been assessed to verify the suitability of the proposed analytical approach.
An Optimized Thrust Allocation Algorithm for Dynamic Positioning System Based on RBF Neural Network
Authors: Tang Ziying , Wang Lei , Yi Fan , He Huacheng
Published: December 2020

The thrust allocation of Dynamic Positioning System (DPS) equipped with multiple thrusters is usually formulated as an optimization problem. Hydrodynamic interaction effects such as thruster-thruster interaction results in thrust loss. This interaction is generally avoided by defining forbidden zones for some azimuth angles. However, it leads to a higher power consumption and stuck thrust angles. For the purpose of improving the traditional Forbidden Zone (FZ) method, this paper proposes an optimized thrust allocation algorithm based on Radial Basis Function (RBF) neural network and Sequential Quadratic Programming (SQP) algorithm, named RBF-SQP. The thrust coefficient is introduced to express the thrust loss which is then incorporated into the mathematical model to remove forbidden zones. Specifically, the RBF neural network is constructed to approximate the thrust efficiency function, and the SQP algorithm is selected to solve the nonlinear optimization problem. The training dataset of RBF neural network is obtained from the model test of thrust-thrust interaction. Numerical simulations for the dynamic positioning of a semi-submersible platform are conducted under typical operating conditions. The simulation results demonstrate that the demanded forces can be correctly distributed among available thrusters. Compared with the traditional methods, the proposed thrust allocation algorithm can achieve a lower power consumption. Moreover, the advantages of considering hydrodynamic interaction effects and utilizing a neural network for function fitting are also highlighted, indicating a practical application prospect of the optimized algorithm.
Towing Experiment of a Hydro-Structural Container Ship Model in Bow-Quartering Modulated Wave Trains
Authors: Houtani Hidetaka , Komoriyama Yusuke , Matsui Sadaoki , Oka Masayoshi , Sawada Hiroshi , Tanaka Yoshiteru , Waseda Takuji
Published: December 2020

We experimentally investigated the influence of the geometries of a modulated wave train on the vertical-bending and torsional moments acting on a container ship in bow-quartering sea conditions. We conducted a towing experiment with a hydro-structural container ship model in the Actual Sea Model Basin (ASMB) (80 m deep, 40 m wide, and 4.5 m deep) at the National Maritime Research Institute. The ship model is made of urethane foam and was designed to have similar vertical bending and torsional vibration mode shapes to an actual ship. A modulated wave train was generated in the ASMB by the higher-order spectral-method wave generation (HOSM-WG) method such that the maxim crest appeared at the center of the basin. The ship model was towed in the modulated wave train with a relative heading angle of 120 degrees. A series of tests was performed by varying the encounter timing of the ship model and the maximum crest of the modulated wave train. In the experiment, fiber-Bragg-grating strain gauges successfully measured whipping vibrations of the ship model due to a slamming impact. The experimental results revealed that the rear wave height Hr and the ratio of the rear and front wave heights Hr/Hf were the dominant parameters governing the maximum sagging and torsional moments of a ship in bow-quartering modulated wave trains.
Electrokinetics Application in Concrete and Well Construction
Authors: Jafariesfad Narjes , Geiker Mette Rica , Sangesland Sigbjørn , Gawel Kamila , Torsæter Malin
Published: December 2020

Electrically induced or coupled transport processes including electrophoresis, electroosmosis and electromigration in solutions and porous media under an external electric field have been extensively studied and employed in many disciplines.

For protection and rehabilitation of concrete structures, cathodic protection, electrochemical realkalization, and chloride extraction are extensively used. Other electrokinetic techniques are developed for the concrete industry, but have not been widely used so far, including electrokinetic treatment processes, for corrosion mitigation, recovery from sulfate attack, crack healing, and porosity and permeability reduction. These processes can improve the microstructure of the cement-based systems resulting in an improved performance in long-term and can be applied to repair failed structures.

Application of electrokinetic processes are rapidly extended in well construction due to the increased interest in techniques enabling manipulation of micro- and nanosized particles. The techniques could be beneficial in building a robust cement sheath in oil and gas wells. Additionally, electrokinetic remediation techniques can possibly be introduced for repairing damaged structures in oil and gas wells.

This review provides an overview of electrokinetic-based techniques, which has been introduced to cement-based materials, mainly reinforced concrete. The potential application of these techniques in oil well construction is discussed.
Research on Protective Performance of Protective Liquid Tank to Shock Wave Generated by Fragment
Authors: Hong Yang , Li Xiaobin , Bao Yuchen
Published: December 2020

In this paper, the simulation method is used to study the installation of metal plates in the liquid tank as a protective structure. The process of shock wave propagation in the liquid tank is analyzed by finite element software, and the metal plate deformation caused by shock wave is discussed. In addition, the pressure changes of different measuring points are compared when installing aluminum plate, steel plate and no protective structure in the liquid tank, and the effects of different protective structures on weakening shock wave are analyzed. Through the simulation method, the protective effects of the three basic parameters (the layer number, position and angle) of the protective structure are analyzed, and the effects of different forms of protective structures on the ability for weakening shock wave are also discussed. The results of this paper can provide references for the optimal design of liquid tank protection and its application in ship protection.
Application of Two Phase Eulerian CFD Model to Simulate High Velocity Jet Induced Scour
Authors: Tavouktsoglou Nicholas S. , Dimakopoulos Aggelos , Spearman Jeremy , Whitehouse Richard J. S.
Published: December 2020

Submerged water jet causing soil excavation is a typical water-soil interaction process that occurs widely in many engineering disciplines. In hydraulic engineering for instance, a typical example would be scour downstream of headcuts, culverts, or dam spillways. In port and waterway engineering, erosion of the channel bed or quay wall by the propellers of passing ships are also typical water jet/soil interaction problems. In ocean engineering, trenching by impinging high-velocity water jets has been used as an efficient method for cable and pipeline burial. At present, physical modelling and simple prediction equations have been the main practical engineering tool for evaluating scour in these situations. However, with the increasing computational power of modern computers and the development of new Computational Fluid Dynamics (CFD) solvers, scour prediction in such engineering problems has become possible.

In the present work three-dimensional (3D) numerical modelling has been applied to reproduce the capability of a pair of water jets to backfill an excavated trench. The simulations are carried out using a state-of-the-art three-dimensional Eulerian two-phase scour model based on the open source CFD software OpenFOAM. The fluid phase is resolved by solving modified Navier-Stokes equations, which take into consideration the influence of the solid phase, i.e., the soil particles.

This paper first presents a validation of the numerical model against vertical jet erosion tests from the literature and conducted at HR Wallingford. The results of the model show good agreement with the experimental tests, with the numerical model predicting the scour hole depth and extent with good accuracy. The paper then presents a validation of the model’s ability to reproduce deposition which is evaluated through a comparison with settling velocity data and empirical formulations found in literature, again with the model showing good agreement.

Finally, the model is applied to a prototype cable burial problem using a commercially available controlled flow jet excavator. The study found that the use of water jets can be effective (subject to confirmation of the time-scale required for real operations) for performing backfill operations but that the effectiveness is closely related to the type of sediment and selection of an appropriate jet discharge. As a result, in order for the water jet method to be effective for backfill, there is a requirement for a good description of the variation in sediment type along the trench and a requirement for the jet discharge to be varied as different sediment types are encountered.
Air Gap Assessment of Semi-Submersibles: Efficient Utilization of Model Test Data
Authors: Machado-Damhaug Ulla , Hanssen Finn-Christian W. , Brunborg Maren , Storheim Håkon , Haver Sverre
Published: December 2020

In linear air gap analysis of semi-submersibles, the surface elevation is modified by an asymmetry factor to account for the crest-to-trough asymmetry related to wave nonlinearity and the effect of nonlinear diffraction. The asymmetry factor varies with numerous conditions including the position relative to the semi-submersible, the sea state, the relative wave direction and the semi-submersible’s loading condition. Although simplified values for the asymmetry factor are suggested in rules and guidelines such as DNVGL-OTG-13, model tests should ideally be performed in order to get accurate values for each specific design. When considering nonlinear responses, model tests are generally done according to the contour line approach, where several realizations (seeds) of a few critical sea states are performed using the 3-hour maximum from each of these to fit a Gumbel distribution. We here seek to explore if the number of seeds can be optimized when considering the relative wave elevation (upwell) by using the less data-wasteful Peaks-Over-Threshold method.
Estimation of Environmental Contours Using a Block Resampling Method
Authors: Mackay Ed B. L. , Jonathan Philip
Published: December 2020

A new method for estimating joint distributions of environmental variables is presented. The key difference to previous methods is that the joint distribution of only storm-peak parameters is modelled, rather than fitting a model to all observations. This provides a stronger justification for the use of asymptotic extreme value models, as the data considered are approximately independent. The joint distribution of all data is recovered by resampling and rescaling storm histories, conditional on the peak values. This simplifies the analysis as much of the complex dependence structure is resampled, rather than modelled explicitly. The storm histories are defined by splitting the time series into discrete blocks, with the dividing points defined as the minimum value of a variable between adjacent maxima. Storms are characterised in terms of the peak values of each parameter within each discrete block, which need not coincide in time. The key assumption is that rescaling a measured storm history results in an equally realistic time series, provided that the change in peak values is not large. Two examples of bivariate distribution are considered: the joint distribution of significant wave height (Hs) and zero up-crossing period (Tz) and the joint distribution of Hs and wind speed. It is shown that the storm resampling method gives estimates of environmental contours that agree well with the observations and provides a rigorous method for estimating extreme values.
Experimental Study of Slam Forces on External Turrets With Different Top Angles
Authors: Ishihara Yuki , Murai Motohiko , Nakanishi So , Kato Shunji , Nakajima Yasuharu
Published: December 2020

The external turret is widely used for mooring FPSOs. However, external turrets can be exposed to slamming and it leads to generate FPSO’s whipping. To analyze the fatigue strength of the FPSOs, probability distributions of slam forces acting when slamming happens are necessary. In this study, we focus on slam force filtered by 10Hz low-pass filter. Then, we examined the probability distribution of peak slam forces, the effect of the top angle on slam forces, and the difference of two types (fixed and oscillator-mounted) tests. For three turret models with different top angle (180°, 160°, and 140°), two types of model tests were carried out.

To confirm the experiment’s validity and reliability, we compared these model tests with the previous study. The slam forces measured on a conical (140°) turret were consistent with the estimated curve in the previous study. The comparison of two types of tests indicated that fixed turret tests were an easy and valid way of measuring the slam forces on external turrets.

3-hour return values calculated by five methods were used to examine the characteristic of each turret. And these values were compared with 3-hour mean maxima to examine the difference between the five methods and experimental results. As a result, the use of Generalized Extreme Values and Generalized Pareto distributions is more suitable than using Rayleigh distribution in estimating return values of slam forces on external turrets.