bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Including the Bauschinger Phenomenon in the Formula for Collapse Pressure of Thin-Walled Pipes
Authors: Walker Alastair , Selker Ruud , Liu Ping , Jurdik Erich
Published: December 2020

The method presented by DNVGL in DNVGL-ST-F101 [1], “Submarine pipeline systems”, 2017, for calculating the collapse pressure of submerged pipelines is well-known for design of pipes intended to operate in very deep water. Such pipes are regarded as quite thick-walled with diameter to wall thickness ratio in the range of 15 to 30. There is now substantial experience in the practical manufacture, installation and operation of such pipes. Recently there has been a growing use of large diameter pipelines to transport high volumes of gas over great lengths at moderate water depths. The pipes are considered to be thin-walled with ratios of diameter to wall thickness in the range of 30 to 45. This paper assesses the validity of the DNVGL design method when applied to the design of such thin-walled pipes.

A particular aspect of the buckling pressure of large diameter pipes is the effect of the Bauschinger phenomenon. The phenomenon occurs when pipes made using the UOE method are subjected to internal pressure, to provide expansion of the pipe during manufacture, thus reducing the out-of-roundness of the pipe wall, and subsequently subjected to external hydrostatic pressure during pipeline operation. To date the Bauschinger phenomenon has been recognised as resulting in a reduction of the circumferential compressive yield of the pipe material. This reduction is accommodated in the DNVGL design formula. Recent research into material properties has shown that the Bauschinger effect also has the effect of reducing the modulus of steel materials over a range of values of applied circumferential compressive stresses.

The paper reviews the basis of the Bauschinger phenomenon and presents results from very detailed accurate testing of UOE pipe material. The tests determine the levels of modulus for pipes subject to circumferential compressive stresses. Although results for compressive stress-strain values have previously been available for pipes subject to high levels of hydrostatic pressure it has been considered that the Bauschinger effect is not generally significant for thick-walled pipes. The tests reported here consider the calculation of material modulus levels for low levels of stress that correspond to the buckling stress of thin-walled pipes. The calculated collapse pressure for such pipes is examined in this paper and compared to corresponding results from the DNVGL design formula to provide guidance on the effect of design levels of pipe wall thickness due to inclusion of the Bauschinger effect. The comparisons are for example pipe wall thickness and material conditions. Conclusions are drawn that including the Bauschinger effect in the calculated pipe wall thickness can have a beneficial effect with regard to pipe manufacturing and installation costs for pipe subjected to mild heat treatment.
Stability and Chaos Analysis of Nonlinear Roll Motion of Trimaran Ship With Variable Lay-Out Under Wind and Waves
Authors: Zhang Yihan
Published: December 2020

The combined action of wind and waves has a great influence on the dynamic stability of roll motion of a trimaran ship, which may get into chaotic situation even capsizing. The lay-out of the trimaran is the main factor influencing the roll performance and its dynamic stability. In order to study the stability performance of the roll motion, firstly, the nonlinear roll motion equations under transverse wind and beam waves are established, in which the main coefficients are obtained by CFD method combined with model test. Then, the Hamilton system is used to analyze the phase portraits of the homoclinic and heteroclinic orbits under different transverse spacing. Finally, the Melnikov function is used to calculate the critical wave threshold of the asymmetric system under the combined action of wind load and wave force, and the Lyapunov exponent based on RHR algorithm was used to verify it. A series of significant conclusions are obtained by comparing the calculation models of different transverse spacing, which can provide references for the design of the trimaran ship.
Nonlinear Roll Damping and Roll Motion Study of Asymmetric Catamaran With Variable Layouts
Authors: Zhang Yihan , Hu Jingfeng
Published: December 2020

Because of the interference between the main hull and side hull, the layout of asymmetric catamaran has a great influence on the seakeeping performance. In order to assess the characteristics of roll damping and roll motion of this kind of ship, firstly, a numerical prediction method of roll damping is established by CFD with overlapping grids to simulate the roll decay curves at different transverse and longitudinal spacing and ship speed. The roll damping property is analyzed by energy method and flow field monitoring. Then, based on the correction of nonlinear roll damping, the roll motion response in waves is calculated by 3D potential method. Finally, model tests are carried out to verify the numerical methods. The change of transverse layout has a great influence on the roll damping and roll motion of the asymmetric catamaran, while the longitudinal layout has a little influence on the roll performance. The research results of this paper can provide some useful reference for the design of asymmetric catamaran.
Experimental Investigation of the Effects of the In-Line Top-Motion on the Vortex-Induced Vibration Response of a Flexible Riser
Authors: Yang Wei , Ma Chuanzhen , Kang Zhuang , Zhang Cheng , Li Shaojie
Published: December 2020

In order to understand the relation between top-motion and VIV of flexible risers, this paper presents an experimental investigation on concomitant vortex-induced vibration and top-motion excitation with flexible risers. The riser can was mounted vertically, with the diameter of 2 cm and the length of 5 m. The responses of amplitude, frequency and other parameters were analyzed in detail under conditions of different excitation amplitude and frequency in uniform flow. It was found that the concomitant VIV and top-motion excitation significantly affects the flexible cylinder response when compared to the pure VIV tests. The amplitude analysis results show that when the reduced velocity is small (less than about 15), the top-motion excitation has an important influence on amplitude of in-line directions. However, the excitation amplitude and frequency of in-line direction have a little influence on amplitude of cross flow direction. The frequency analysis results show that when the reduced velocity is small (less than about 5), the riser motion amplitude is small and irregular in different excitation and when the reduced velocity is large (5 < Ur < 55), the in-line vibration frequency is two times the cross-flow vibration frequency. A strong connection between the top-motion excitation frequency and the vibration frequency was also found. Overall, some phenomena and characteristics observed in the VIV considering top-motion excitation are different from those in classic VIV, which may provide basic reference for the VIV investigation involving the effect of floating bodies.
Parametric Study of Catenary Mooring System for a Semisubmersible Floating Wind Turbine in Intermediate Water Depth
Authors: Li Jiawen , Zhang Qiang , Du Jiali , Jiang Yichen
Published: December 2020

This paper presents a parametric design study of the mooring system for a floating offshore wind turbine. We selected the OC4 DeepCwind semisubmersible floating wind turbine as the reference structure. The design water depth was 50 m, which was the transition area between the shallow and deep waters. For the floating wind turbine working in this water area, the restoring forces and moments provided by the mooring lines were significantly affected by the heave motion amplitude of the platform. Thus, the mooring design for the wind turbine in this working depth was different from the deep-water catenary mooring system. In this study, the chosen design parameters were declination angle, fairlead position, mooring line length, environmental load direction, and mooring line number. We conducted fully coupled aero-hydro dynamic simulations of the floating wind turbine system in the time domain to investigate the influences of different mooring configurations on the platform motion and the mooring tension. We evaluated both survival and accidental conditions to analyze the mooring safety under typhoon and mooring fail conditions. On the basis of the simulation results, this study made several design recommendations for the mooring configuration for floating wind turbines in intermediate water depth applied in China.
A Decision Support Framework for Cost-Effective and Energy-Efficient Shipping
Authors: Bui Khanh Q. , Perera Lokukaluge P.
Published: December 2020

Stringent regulations regarding environmental protection and energy efficiency (i.e., emission limits regarding NOx, SOx pollutants and the IMO greenhouse gases reduction target) will mark a significant shift to the maritime industry. In the first place, the shipping industry has strived to work towards feasible technologies for regulatory compliance. Nevertheless, life cycle cost appraisal attaches much consideration of decision-makers when it comes to investment decisions on new technologies. Therefore, the life cycle cost analysis (LCCA) is proposed in this study to evaluate the cash flow budgeting and cost performance of the proposed technologies over their life cycles. In the second place, environmental regulations may support innovation especially in the era of digitalization. The industrial digitalization is expected to revolutionize all of the aspects of shipping and enable the achievement of energy-efficient and environmental-friendly maritime operations. The so-called Internet of things (IoT) with the utilization of sensor technologies as well as data acquisition systems can facilitate the respective maritime operations by means of vessel operational performance monitoring. The big data sets obtained from IoT should be properly analyzed with the help of Artificial Intelligence (AI) and Machine Learning (ML) approaches. Our contribution in this paper is to propose a decision support framework, which comprises the LCCA analysis and advanced data analytics for ship performance monitoring, will play a pivotal role for decision-making processes towards cost-effective and energy-efficient shipping.
Feasibility Study of S-Lay Installation for Deepwater SCRs and Pipelines
Authors: Hou Jing , Yu Chenteh Alan , Cheng Yongming , Bangalore Gurudutt , Song Hao
Published: December 2020

The J-lay and S-lay are two common methods for SCR and pipeline installations. When using the S-lay installation method, onboard welded pipe joints leave the vessel horizontally and are guided to the seabed over a stinger structure. The pipe is lowered using tensioners. With the advantage of high production rate, Slay can be a cost-effective solution for deepwater riser and pipeline installation. Based on HYSY201 installation vessel, this paper investigates the feasibility of S-lay installation for deepwater SCRs and pipelines to be used in South China Sea.

It first introduces the SCRs and pipelines to be used for a deep draft semi-submersible for the Lingshui 17-2 project. It then presents the S-lay installation vessel HYSY201 and S-lay configuration. The hydrodynamic motion analysis for a Response Amplitude Operator (RAO) was computered for HYSY201 in different environmental headings. With the site-specific metocean design basis, this paper presents an installation procedure, analysis methodology, and acceptance criteria.

The study covers different sizes of SCRs and pipelines to investigate the feasibility of S-lay installation. The study starts from the static installation analysis of SCRs and pipelines and includes different installation steps. The acceptance criteria are examined for the pipes at over bend and sag bend regions. The support reactions load on the stinger structure are reported at each step. The dynamic analysis is selectively performed to evaluate Dynamic Amplification Factors (DAFs) of support reaction loads especially for roller box supports on the stinger structure. The sensitivity of DAFs to wave parameters such as wave height and peak period is analyzed as well.

The extreme support reaction loads are computed for evaluating the strength performance of the stinger structure. The feasibility of S-lay installation for deepwater SCRs and pipelines is determined by the global performance of SCRs and pipelines, installation vessel hold back tension and A&R winch load capacity, and performance of the stinger structure.

Based on the study work, this paper finds the feasibility of S-lay installation of deepwater SCRs and pipelines for Lingshui 17-2 project using the installation vessel of HYSY201.
Nonlinear Time-Domain Simulation of the Heaving-Buoy Type Wave Energy Converter by Using Three-Dimensional Potential Numerical Wave Tank Under Irregular Wave Conditions
Authors: Kim Sung-Jae , Koo Weoncheol , Kim Moo-Hyun
Published: December 2020

The aim of this paper is to evaluate the hydrodynamic performance of a heaving buoy type wave energy converter (WEC) and power take-off (PTO) system. To simulate the nonlinear behavior of the WEC with PTO system, a three-dimensional potential numerical wave tank (PNWT) was developed. The PNWT is a numerical analysis tool that can accurately reproduce experiments in physical wave tanks. The developed time-domain PNWT utilized the previously developed NWT technique and newly adopted the side wall damping area. The PNWT is based on boundary element method with constant panels. The mixed Eulerian-Lagrangian method (MEL) and acceleration potential approach were adopted to simulate the nonlinear behaviors of free-surface nodes associated with body motions. The PM spectrum as an irregular incident wave condition was applied to the input boundary. A floating or fixed type WEC structure was placed in the center of the computational domain. A hydraulic PTO system composed of a hydraulic cylinder, hydraulic motor and generator was modeled with approximate Coulomb damping force and applied to the WEC system.

Using the integrated numerical model of the WEC with PTO system, nonlinear interaction of irregular waves, the WEC structure, and the PTO system were simulated in the time domain. The optimal hydraulic pressure of the PTO condition was predicted. The hydrodynamic performance of the WEC was evaluated by comparing the linear and nonlinear analytical results and highlighted the importance accounting for nonlinear free surfaces.
CFD Simulation on Dynamic Installation of the Light-Weight Gravity Installed Plate Anchor in Clay
Authors: Liu Jun , Bu Nanqiao , Han Congcong
Published: December 2020

A light-weight gravity installed plate anchor (L-GIPLA) is put forward in the present study to secure offshore floating structures, such as floating wind turbines, floating net-cages, wave and tidal energy converters, and floating oil and gas platforms. The L-GIPLA is installed with the aid of a booster, which can be connected at the tail of the anchor during dynamically installation and retrieved after installation. The L-GIPLA owns the advantages of dynamically installation, deep penetration depth and high capacity efficiency. In the present study, the dynamic installation process of the L-GIPLA with a booster in clay is modelled by conducting three-dimensional large deformation analysis in commercial software ANSYS CFX, which is a computational fluid dynamics (CFD) software based on the finite volume method (FVM). Non-Newtonian fluid model, incorporating the strain-rate and strain-softening effects, is used to simulate the undrained clayey soil. Various factors influencing the final penetration depth of the anchor have been studied. These factors include the soil strength characterizations (strain-rate and strain-softening effects), and impact velocity. The results show that the L-GIPLA can obtain a relatively deep penetration depth in the seabed, indicating the L-GIPLA is an efficient alternative in offshore engineering.
A Study of Detecting for Dragging Anchor
Authors: Saito Akira
Published: December 2020

Frequent attack of typhoon leads to crucial accidents caused by anchor dragging. In 2018, 21st typhoon hit west of Japan and the vessel, which was fallen into anchor dragging, collided with the bridge connecting Kansai International Airport and opposite shore. This accident had serious damage to the transportation of people and logistics. Moreover, when 15th typhoon in 2019, which gave crucial damages such as house damages and suspension of the electric power and the water supply, hit Chiba prefecture located in east of Japan, 107 vessels which were equivalent to one third of all vessels anchored in Tokyo Bay had the symptom of anchor dragging.

Seafarers must take measures as precaution to anchor dragging. However, missing its opportunity causes irreparable disasters. As the attack of typhoon becomes more frequent and serious, the means, which can evaluate the risk quantitatively to anchor dragging and help them make a decision, are necessary because seafarers have possibilities to fall into unexpected situation even if they are skilled and familiar with ship’s characteristics.

This study aims at detecting the signal of earlier stage of anchor dragging and visualizing the level of danger, as a mean to make a decision for taking precautions in an appropriate opportunity. With the acceleration meter, which can measure quantitatively ship’s lists and vibrations what human cannot recognize, acceleration data of ship’s motion were measured and evaluated. However, they don’t always show necessary factors for analyzing ship’s motion because they include all motions on the ship, such as wind, wave force and vibrations from its generator.

Then, the system, sorting of each range of frequency from measured accelerations and extracting necessary data, was created. This enables extraction of ship’s motion at anchor. The analysis results between the outer force affecting to the ship at anchor and the data extracted by this system, specified the signal of anchor dragging. In addition, the new system was built for indicating the risk of anchor dragging, by evaluating relationship between the signal of anchor dragging and the effect by outer force.

Measuring their data on actual ship showed the validity of this system. Thus, with measuring acceleration data at anchor and analyzing them, detecting ship’s motion, which indicates the signal of anchor dragging, proved to be possible.
A New Type of Robot Used for Deep Stratum Drilling in Seabed
Authors: Zhang Peihao , Chen Jiawang , Tian Zhenwei , Ren ZiQiang , Ge Yongqiang , Gao Qiaoling , Gao Feng
Published: December 2020

With the increasing scale of the exploitation of ocean oil and gas resources, the decomposition of natural gas hydrate and the exploitation of submarine oil will damage the engineering mechanical structure of submarine sediments, leading to submarine collapse, landslide, even earthquake and other geological disasters, seriously threatening the exploration and exploitation of Marine resources. Therefore, it is necessary to develop a deep-seabed drilling robot to carry out real-time long-term monitoring of changes in the seabed environment by carrying sensors and detection devices. This paper will describe the design structure of the drilling robot, and explain the working process through that the drilling robot is released from the sea bed into the stratum of the mining area, and can achieve autonomous drilling and steering functions in the seabed stratum. In this paper, theoretical analysis and simulation verification will prove the feasibility of the robot moving in the stratum.
Quantitative Wear Estimation for Floating Structures by Using 3-D Geometry of Mooring Chain
Authors: Takeuchi Takaaki , Utsunomiya Tomoaki , Gotoh Koji , Sato Iku
Published: December 2020

For reducing the maintenance cost of floating offshore wind turbine structures, it is necessary to establish a quantitative wear estimation method for the mooring chains. In this paper, attempts have been made to improve the accuracy of the estimation method in terms of the mooring chain model. These investigations were performed about a spar-type floater moored with three catenary mooring lines at Goto, Nagasaki prefecture, Japan.

Up to now, the mass-spring model had been used for the mooring chain in response analysis and the relative angle between two spring lines was considered as only a sliding angle without friction. However, there is also rolling in the motion between mooring links, which should cause less wear than by sliding. In this study, the detailed motion of the link in response analysis is calculated and applied to the wear estimation by using a 3-D model in MSC. Adams. This enables the wear estimation considering link motion closer to a real phenomenon. A Contact analysis between the 3-D chain model requires some contact properties (e.g. contact stiffness and friction). In this paper, these properties are calculated based on the Hertzian contact method and FEM analysis.

As a result, the wear amounts overestimated by using the mass-spring model in the previous investigation, especially at the point located clump weight and touchdown point, decrease getting closer to the measurements. In addition, by tracking the contact points it is found that the major motion caused between links is the rolling. For future works, there remains a need for further validation and the consideration of elasticity between mooring links, impressions caused by proof load test and the effect of corrosion.
Numerical Study on the Hydrodynamic Performance of the DARPA Suboff Submarine for Steady Translation
Authors: Takahashi Kenshiro , Sahoo Prasanta K.
Published: December 2020

This study was built upon previous works conducted by the authors in a series of numerical studies on submarine hydrodynamics and is aimed at enhancing the accuracy of computational fluid dynamics (CFD) application processes, which estimate the hydrodynamic performance of underwater vehicles for steady translation conditions in the horizontal and vertical planes. In an earlier work, the computed straight-ahead resistance of a submarine agreed with those of experiments within a comparison error of 2%. However, a maximum comparison error of approximately 20% was obtained for sway force under a steady translation condition. The Defense Advanced Research Projects Agency (DARPA) Suboff submarine model was adopted as a benchmark, and the computational modeling was based on the Reynolds-averaged Navier–Stokes (RANS) turbulence model for steady simulations. The curvature correction approach was tested to improve the computation of circumferential flow around the cylindrical hull, in particular. The dominant maneuvering coefficients were calculated using the computed forces and moments as a function of the yaw and pitch angles along with simplified equations of motion by fitting a curve to the plots. The hydrodynamic forces and moments exerted on the stern plane were individually computed using a locally refined mesh around the tail section.

It was confirmed that the curvature correction approach improved the computational accuracy for the steady translation conditions, and general trends were captured over the tested yaw and pitch angles. However, some data points had notable comparison errors. Some of the estimated maneuvering coefficients agreed well between the CFD simulation and the experiments, whereas others had considerable comparison errors. The individually computed forces and moments exerted on the stern plane that had attack angles were inconsistent with those obtained in experiments. Those comparison errors may have been amplified by the complexity of configuration and arisen from differences in the experiments, such as the presence of a free surface and supporting strut to mount the hull to a carriage, and, perhaps, the geometrical differences owing to machining accuracy.

The investigation of flow field at the propeller plane revealed that the wake distributions inside the nozzle were significantly affected by the angled stern planes; the reduced velocity area was expanded and shifted. Furthermore, harmonic analysis of the wake fraction was conducted, and several primary nth harmonics were observed, which were associated with the struts and stern planes. This result suggests the risk of higher noise levels associated with the number of blades.
Line Type Optimization of the Flexible Jumper for New Generation Subsea Suspended Manifold Production System
Authors: An Weizheng , Li Zhigang , Luo Wentao , Wang Yingying , Duan Menglan
Published: December 2020

A new generation of subsea production system with the suspended manifold as the major characteristic was proposed to solve the disadvantages for hard to be discarded and recovered for the traditional subsea manifold fixed in seabed. Here, the flexible jumpers connecting the dry trees in the subsea functional chamber to the suspended manifold, can not only provide enough mooring forces as the mooring system, but also transport oil and gas from dry trees, which is an indispensable part of a complete new generation of subsea production system. So how to optimize the flexible jumpers to guarantee a good hydrodynamic performance is quite essential. In this paper, a steep wave type of flexible jumper is optimized by changing the suspended height, connection width, and position and diameter of the buoyancy block. The result shows that the location and the size of the buoyancy block both have a great influence on the distribution of the mechanical property and the line type of the flexible jumper while the influence of suspended height and connection width is very small. Calculations and analysis demonstrated that changing the position of the buoyancy block has no effect on the maximum tensile force of the flexible jumper, but the farther the buoyancy block is from the seabed, the larger the minimum bending radius of the flexible jumper is. Meanwhile, the larger the diameters of buoyancy block becomes, the larger the maximum tensile force is, and the smaller the minimum bending radius will be.
Study on Estimation of Flow Around Marine Propeller by Large-Scale LES and Affection of Grid Resolution and Reynolds Number
Authors: Fujisawa Takeharu , Tsubokura Makoto , Tanaka Hisao
Published: December 2020

To develop a ship with higher propulsion performance, accurate estimation method is required. Tthe performance of ships is estimated by both experimental methods using scale models and numerical methods based on computational fluid dynamics (CFD) using Reynolds average Navier-Stokes equation (RANS) models. Experiments with scale models have limitations regarding fast execution and cost reduction due to physical constraints, and we believe it is important to improve numerical methods. Adjustment of turbulent models can improve the estimation accuracy of numerical methods, but it is necessary to continuously compare and verify the numerical and experimental results. There is a limit to the improvement of the estimation accuracy of the numerical method with RANS models.

Direct numerical simulation (DNS), which can be solved without modeling the turbulence field, can acquire an accurate flow field, but it is difficult to implement because it requires a huge number of grids. In a large eddy simulation (LES) model, if the grid resolution (filter size) is fine, the limitations imposed by the model itself are reduced. LES performed with a sufficiently fine grid resolution may be able to achieve the comparable estimation accuracy as DNS without a high computational load. We considered this method to be promising for accurately estimating ship performance.

This is an initial study of a ship performance estimation calculation method using large-scale LES. We performed LES calculations with up to 6.4 billion cells for propeller open water test and obtained basic knowledge about large-scale LES calculations. We carried out calculations with different grid resolutions under the same operating conditions, and with different Reynolds numbers at the same resolution to show how the grid resolution and Reynolds number affect the estimation of propeller performance and flow around the propeller.
Water Wave Diffraction and Radiation by a Submerged Horizontal Circular Cylinder in Front a Vertical Wall
Authors: Li Ai-jun , Liu Yong
Published: December 2020

This article studies water wave diffraction and radiation by a submerged horizontal circular cylinder in front of a vertical wall under the assumption of linear potential flow theory. Based on the image principle, the hydrodynamic problem of a horizontal cylinder in front of a vertical wall is transformed into an equivalent problem involving symmetrical cylinders in a horizontally unbounded fluid domain. Then, analytical solutions for the present physical problem are developed using the method of multipole expansions combined with the shift of polar coordinate systems. The wave exciting forces on the cylinder as well as the added mass and radiation damping due to the cylinder oscillation are calculated. The analytical solutions converge very rapidly with the increasing truncated number of multipoles. Calculation examples are presented to examine the effects of different parameters on the hydrodynamic quantities of the cylinder. Results indicate that the hydrodynamic quantities of the cylinder in front of a vertical wall greatly differ from those in a horizontally unbounded fluid domain.
Model Tests on the Parametric Resonance of the Deep Draft Semi-Submersible Under Regular and Irregular Waves
Authors: Li Chenglin , Yu Liwei , Wang Shuqing , Tian Zhe , Wang Xujie
Published: December 2020

Model tests on the parametric resonance of the deep draft semi-submersible (DDS) under regular and irregular waves are conducted in the wave flume of the Ocean University of China (OUC). Model tests with and without mooring lines are conducted under various regular and irregular waves with different wave angles. From the results of the model without mooring lines, it is found that parametric resonance generally occurs when wave period is about the natural period of heave and about half of the natural period of pitch or roll. The steady amplitude of the parameter resonance is larger under larger wave height. In heading waves, parametric resonance occurs in the roll direction, while parametric resonance occurs in both roll and pitch direction for the beam wave. In irregular waves, parametric resonance is practical non-ergodic. For the model without mooring lines, parametric resonance does not occur under the constraints of the mooring system.
Experimental and Numerical Study of an Obliquely Towed Ship Model in Confined Waters
Authors: Terziev Momchil , Elsherbiny Khaled , Tezdogan Tahsin , Incecik Atilla
Published: December 2020

In this study, the forces and moments acting on the KCS ship model as a result of oblique towing at 10 and 20 degrees drift angles are evaluated experimentally and numerically via a commercial Reynolds averaged Navier-Stokes solver. For the purposes of this work, the KCS hull is modelled both experimentally and numerically at a scale factor of 1:75. The adopted case-studies feature both horizontal and vertical restrictions. Thus, the subject of this work is the oblique motion of a ship in a narrow canal with a depth of h/T = 2.2. The relative impact of turbulence modelling is assessed by comparing the computed integral quantities via several eddy-viscosity closure strategies. These include significant variants of the k-ϵ and k-ω models as well as a widely used one-equation closure. Multiphase numerical simulations are performed at several of the experimentally investigated depth Froude numbers for each drift angle condition in order to fully capture the physics of the problem at hand. The present study aims to provide a quantitative evaluation of the performance of the adopted turbulence models and recommended the best closure strategy for the class of investigated problems.
Investigation of Thermal Effects on Gas-Oil Stratified Flow Wax Deposition
Authors: Chi Yuandao , Daraboina Nagu , Sarica Cem
Published: December 2020

Two-phase flow wax deposition is a flow-pattern-dependent phenomenon. The thickness and hardness of the deposit vary along the pipe circumference. In this work, two-phase gas-oil stratified flow wax deposition experiments at various liquid and gas flow rates have been conducted systematically using Garden Banks condensate and natural gas in a 2-inch I.D. multiphase flow loop under the pressure of 350 psi. Both deposit mass and wax content increased as superficial gas and liquid velocities increased. The local deposits were observed to be thinner but harder at the sides compared to the bottom of the pipe.

Meanwhile, the cross-sectional deposits were crescent-shaped with an increasing local wax mass flux along the circumferential direction. The local multiphase hydrodynamic and heat transfer characteristics are known to play an essential role in the wax deposition process, and the temperature gradient is critical for establishing the concentration gradient. Thus, it is paramount to have a proper understanding of the local momentum and heat transfer to predict wax deposition in multiphase flow accurately.

Therefore, numerical simulations with an SST (Shear Stress Transport) k∼ω turbulent model was implemented to understand local heat transfer in two-phase gas-oil stratified flow. After each simulation, the ANSYS CFD-Post was used to export, visualize, and analyze the simulated results. A total of 19 locations were selected for circumferential sampling to analyze the local heat transfer in the model. Detailed information on liquid volume fraction, shear stress, and temperature were analyzed. It has been observed that the local shear stress, temperature gradient, and inner wall temperature decrease with increasing θ. The thickness of the thermal boundary layer increases as θ increases due to reduced Nuθ. The comparison between the localized Nuθ and Nu from unified heat transfer model has revealed that variation in Nuθ is critical in the circumferential heat transfer calculation and wax deposition modeling.
Fatigue Analysis for Mooring System of a Spar-Type Floating Offshore Wind Turbine
Authors: Xue Xutian , Liu Xiaoyong , Chen Nian-Zhong , Gao Xifeng
Published: December 2020

This paper aims to perform a time-domain mooring fatigue analysis for a Spar-type floating offshore wind turbine operated in the South China Sea. Tension ranges of mooring lines are achieved from a hydrodynamic analysis where the effects of wind, wave and current are considered. A rainflow counting method is used to calculate the number of mooring tension cycles with corresponding ranges. The fatigue lives of mooring lines are then predicted by Palmgren-Miner’s rule according to T-N & S-N curves. A comparison of fatigue lives predicted by T-N & S-N curves-based approaches with/without considering safety factors is made. The results show that the T-N curves-based approach is more conservative than the S-N curves-based approach if safety factors are not considered in the two approaches, while the fatigue lives predicted by both approaches are in general comparable when the safety factors suggested by API and DNVGL are applied in the two approaches. A comparative study of three kinds of R4 grade studless mooring chains with different diameters (2.5-inch, 4-inch, 5-inch) is also conducted and the results show that the design with the 2.5-inch chain does not meet the fatigue requirements.
Influence of Torpedo Piles Parameters When Assessing Final Penetration Depths
Authors: Sant’Anna do Nascimento Leonardo
Published: December 2020

This paper has been influenced by a basic question: assuming a torpedo pile launching in a base case scenario, which parameters, when not well controlled, would affect more the pile final penetration depth? Aiming to provide an answer to this question, the methodology adopted in this study assumes the application of numerical simulations and statistical analyses in order to quantify the influence of the uncertainties of such parameters in the final penetration depth. From the assessment, one may expect responses in terms of which parameters shall be better controlled during a launching procedure in order to reduce the uncertainties associated to the target depth.

For the development of the study, a simplified approach for the dynamic numerical analyses has been adopted in order to allow a massive data generation for the statistical treatment. The main parameters associated to the torpedo pile and the launching scenario have been generated taking into account their uncertainties. The Monte Carlo method has been considered and statistical treatment has been applied to quantify the influence of the parameters in the final penetration depth.

The results demonstrate that apply the simplified method described in the paper is adequate to simulate both the subsea free fall and the pile driving phases to obtain the final penetration depths. The torpedo mass has been found as the physical parameter which influences more the final penetration depth. However, the study demonstrates that, given a standard launching procedure assessed by numerical simulations, uncertainties on physical parameters are much less significant for the final penetration depth than the uncertainties inherent to the assumptions and data applied to simulate the soil damping.
Nonlinear Analysis of an Oscillating Wave Surge Converter in Frequency Domain via Statistical Linearization
Authors: da Silva Leandro S. P. , Sergiienko Nataliia Y. , Cazzolato Benjamin S. , Ding Boyin , Pesce Celso P. , Morishita Helio M.
Published: December 2020

Wave energy devices operate in resonant conditions to optimize power absorption, which leads to large displacements. As a result, nonlinearities play an important role in the system dynamics and must be accounted for in the numerical models for realistic prediction of the power generated. In general, time domain (TD) simulations are employed to capture the effects of the nonlinearities. However, the computational cost associated with these simulations is considerably higher compared to linear frequency domain (FD) methods. In this regard, the following work deals with the nonlinear analysis of an oscillating wave surge converter (OWSC) in the FD via the statistical linearization (SL) technique. Four nonlinearities for the proposed device are addressed: Coulomb-like torque regulated by the direction of motion, viscous drag torque, nonlinear buoyant net torque, and parametric excitation torque modulated by the flap angle. The reliability of the SL technique is compared with nonlinear TD simulations in terms of response probability distribution and power spectrum density (PSD) of the response and torque; and mean power produced. The results have demonstrated a good agreement between TD simulations and SL, while the computation time of the SL model is approximately 3 orders of magnitude faster. As a result, SL is a valuable tool to assess the OWSC performance under various wave scenarios over a range of design parameters, and can assist the development of such wave energy converters (WECs).
Column Interference Effect of Multi-Column Floating Structures on Vortex-Induced Motion Using Numerical Prediction Model
Authors: Toyoda Yosuke , Hirabayashi Shinichiro , Gonçalves Rodolfo T. , Suzuki Hideyuki
Published: December 2020

In this study, a numerical prediction model for vortex-induced motions (VIM) of 4-circular-column floater was developed from a database of hydrodynamic forces acquired from the forced oscillation tests. A set of combinations of oscillation frequencies and amplitudes in the transverse direction (1 degree of freedom) were carried out. The numerical results were validated with experimental results of VIM performed in a towing tank. The effect of column interference was discussed by analyzing the hydrodynamic forces on each column separately and also the motion behavior of the floater for spacing ratios between columns, S/D = 2, and 3, where S is the distance between column centers and D is the column diameter.
Experimental Study on Fish-Harvest Performance of the Flexible Hose Net
Authors: Li Qiao , Li Yue , Mizukami Yoichi , Dong Shuchuang , Yoshida Takero , Kitazawa Daisuke
Published: December 2020

Set net fishery is one of the efficient fishery methods and has a history of several centuries in Japan. In 2017, fish harvest amount of set net fishery was 326,200 tons, about 10.2% of the total amount in Japan. The traditional method for harvesting fish is to use two fishing vessels to haul up the box chamber and obtain the fish in it, which requires supplementary equipment and human power. For reducing human power and safety, the automated net-hauling system has been developed. This system used a flexible hose net, with air injected from one side of the hose net, then the net could float up gradually and harvest the fishes. A new hose net was developed by reducing the number of air injection tubes from seven to two in comparison to the previous hose net. The paper presents the results of the animal experiment using 1/60 model of the flexible hose net. The fish-harvest effect in the flow are experimented by the Horizontal Circulating Water Channel. The fish-harvest effect was observed directly. The floating up velocity of the hose net can affect the fish-harvest rate. If the floating up velocity is too fast, the fish are easy to be stuck on the net and cannot swim forward to arrive at the catching spot (trap net). Therefore, the floating up velocity of the hose net not only determines the operational efficiency, but also affects the fish-harvest rate to some extent. Additionally, the deformation of flexible hose net was considered not only during the air injection, but also during the water intrusion.
Characteristic Analysis for Spectrum of Swell Dominated Seas With In-Situ Measurements
Authors: Gao Xiang , Ma Xiaozhou , Ma Yuxiang , Dong Guohai , Gao Junliang
Published: December 2020

The characteristics of wave spectrum in the swell dominated seas (i.e., the Chancay Bay on the western coast of Peru) are studied based on directional wave spectra measured using the technique of acoustic Doppler current profiler (ADCP). According to the wave rose diagram and the measured wave spectra, this sea area is mainly affected by the swell of the southwest direction and the significant wave height is between 0.7m-2.6m. Moreover, the wave spectra feature the single-peaked spectra and the double-peaked spectra. The JONSWAP spectrum model is adopted to fit single-peaked swell spectra. A close agreement could be found between the observed and fitted spectrum parameters. Hence, the JONSWAP model can be used to describe the single-peaked swell spectra in the sea area studied in this work. As for the double-peaked spectrum, Ochi-Hubble double peak spectrum model and simplified Torsethaugen double peak spectrum model are adopted to fit the wave spectra, respectively. Comparing the fitting results of two double-peaked spectrum modal, it’s possible to conclude that the measured double-peaked spectra can be well represented by the Ochi-Hubble double peak spectrum model.
Numerical Prediction of Cavitation Performance of Controllable Pitch Propellers With Different Pitch Adjustment Velocities
Authors: Xue Ying , Dong Xiao-Qian , Yang Chen-Jun
Published: December 2020

The turbulent flow around a cavitating controllable pitch propeller (CPP) is simulated by solving the Reynolds-Averaged Navier-Stokes (RANS) equations, to investigate the dynamic effects on cavitation when the pitch of propeller blades is changed at different pitch adjustment velocities (PAVs). The process of changing the pitch at prescribed PAVs is controlled by a user-defined function (UDF) in the software FLUENT, and during the process, the time-dependent flow domain is re-discretized at each time step with dynamic meshes. The SST k-ω turbulence model and the cavitation model proposed by Schnerr and Sauer are employed in the simulation. The numerical simulation approach is first validated against model experiments for a fixed pitch propeller (FPP) working in the open water. A grid dependence study is carried out to determine a proper mesh resolution for the simulation of such cavitating flows; then the hydrodynamic performance as well as the extent and volume of the sheet cavities obtained from the RANS simulations are compared with experimental data. Then influences of the PAV on the hydrodynamic performance and cavity geometry are investigated. The CPP blades are rotated around the spindle axes to change the pitch, and the movement is controlled by a UDF. The PAV is prescribed and kept constant in the process of adjusting the pitch. At different PAVs, the unsteady thrust and torque, pressure distributions on blade surfaces and propeller disk, cavity geometry, as well as cavitation volume of the cavitating flow are compared with each other to assess the dynamic effects of the PAV.
State Estimator for Floating Offshore Wind Turbines and Performance Evaluation
Authors: Tanabe Hiroki , Hara Naoyuki , Konishi Keiji
Published: December 2020

Floating offshore wind turbines (FOWTs) are considered as a promising technology for installing turbines in deeper sea sites. One of the difficulties is that the maintenance cost tends to be high because of the limited accessibility to the site. Therefore remote monitoring and estimation of various physical variables of a wind turbine in real-time is important for health assessment and fault diagnosis before a critical failure occurs. One promising way to do this is to have a wind turbine model in computer and estimate the various physical variables of the turbine in real-time by using the available noisy sensor measurements. In this paper, we consider a 5MW FOWT and design a Kalman filter to deal with the estimation problem of the turbine state variables and effective wind speed. A blade pitch controller that uses the estimated state feedback and wind feedforward is also designed. The proposed estimation and control scheme is evaluated using the turbine simulation code, FAST, and results show that the state variables are well estimated except for the blade flapwise moment.
Heave Motion Performance of Multiple Wave Energy Converters Arrayed in a Water Channel Resonator
Authors: Kim Jeongrok , Cho Il-Hyoung
Published: December 2020

The performance of multiple wave energy converters (WECs) arranged in a Y-shaped water channel resonator (WCR) for amplifying the wave energy with low density was investigated. The WCR consists of a long channel and waveguide installed at the entrance. If the period of the incident wave coincides with the natural period of the fluid in the WCR, then resonance occurs, and the internal fluid is greatly amplified in the form of standing waves. The WECs were positioned at the anti-nodes of standing waves formed in the WCR to maximize energy extraction. We dealt with the heave motion, time-averaged power, and capture width ratio (CWR) of WECs, which are composed of a heaving cylinder and a linear generator. For this purpose, we used the boundary element method and WAMIT commercial code. In parallel, systematic model tests were conducted at the 2D wave tank in Jeju National University to validate the numerical solution. Both results were in good agreement. WECs with a short draft are efficient in energy extraction compared with WECs with a long draft. Numerical and experimental results reveal that the WECs arranged in a WCR have higher efficiency over a wide band of periods than a single WEC without a WCR. Therefore, the wave energy with low density can be amplified by the resonance of the internal fluid in the WCR.