Abstract Flow-Induced Motions (FIM) is an issue for multi-column platforms, and the phenomenon can decrease the fatigue life of the mooring, riser, and cable systems. In the past, FIM was studied mainly for platforms installed in deep waters. The new concepts of Floating Offshore Wind Turbines (FOWT) have multi-column design and may therefore observe FIM. However, FOWTs have been installed in shallow water and, in this case, the FIM remains insufficiently investigated. To address this issue, FIM model tests were performed for the – Semisubmersible (SS) Floating System design developed for the DeepCwind project (OC4 Phase II). The goal of this paper is to investigate the presence of FIM for this type of system to show the importance of FIM in the design of FOWT. Three different incidence angles of the current were tested, namely 0, 90 and 180 degrees. For each heading, thirty reduced velocities were tested. The results showed amplitudes in the transverse direction of around 70% of the diameter of the platform column, which is similar to the ones observed for the deep-draft (DD) SS with circular columns and larger than for the platforms with square columns. The results showed that FIM was present for this specific FOWT SS investigated and that it may thus be essential to consider when designing the mooring system, as an increment in the total cost of the platform may make the system economically unfeasible. When extrapolating the results for the full-scale configuration, the FIM synchronization occurred for current velocities from 0.5m/s up to 1.2m/s, and the maximum nondimensional nominal amplitudes for the motions in the transverse direction reached 70% of the external column diameter.

Abstract Over the past decade the use of Unmanned Aerial Vehicles (UAVs) for the inspection of turbine blades has been registering steady progress and is fast becoming a well-established inspection methodology especially at offshore wind farms. A UAV operating in the open field is subject to varying ambient conditions which have an effect on the power required to maintain stable flight. This may have an impact on the flight endurance of the UAV, especially when operating in windy conditions. Simulations are a very useful tool for estimating the impact of such ambient conditions on the performance and flight endurance of a UAV. However, it is extremely difficult to accurately model all the dynamics at play in the open field where flow conditions are highly stochastic. Few open field studies necessary to validate such simulation models have been carried out to date in this regard. In this study, the impact of open field wind conditions on the flight endurance of a hovering UAV is investigated. The test vehicle used in this study is a quadrotor UAV, which was fitted with an array of sensors to monitor power consumption parameters of the propulsion motors whilst the vehicle is hovering at a fixed altitude above the ground. The quadrotor was also fitted with an ultrasonic wind sensor in order to measure the relevant wind parameters that the quadrotor was being subjected to during the hovering study. The test UAV was flown in different ambient conditions to establish the impact on the UAV flight endurance when subjected to different wind speeds. Results from a series of UAV test flights in the open field indicated that the power required by the UAV to maintain hovering flight decreases as the wind speed increases.

Abstract This paper summarizes the assessment of the structural analysis and design of a floating foundation for offshore floating wind turbine (FWT) based on DNVGL standard and Eurocode in terms of economy and reliability. The wind loads are calculated using empirical equations. The wave loads are obtained and verified using various methods including hand calculation, AQWA and Flow-3D. It is found that the shell thickness could be reduced significantly by introducing the stiffeners (stringer or ring), which can decrease the weight of the hull and lower the cost. While DNVGL and Eurocode yield similar design solutions if using plane shell structures, Eurocode significantly underestimates the buckling resistance of stiffened cylindrical shells.

Abstract Offshore wind system encountered wind, wave, current, soil, and other environmental loads. The support structure is randomly loaded for a long time, which is more likely to cause fatigue damage. In this paper, the NREL 5MW wind turbine and OC4 jacket support structure is selected to perform the time domain fatigue analysis. Commercial software Bladed and SACS are used to perform the required structural responses and fatigue strength calculations. The Stress Concentration Factors (SCF) and S-N curves for the stress calculations of tubular joints are adopted based on the recommendation of DNV GL guidelines. The magnitude of the stress variation range and the corresponding number of counts are obtained by using the rain-flow counting algorithm. Finally, the Palmgren-Miner’s rule is adopted to calculate the cumulative damage ratio and the fatigue life can then be estimated. Fatigue damage ratio and structural fatigue life of each joint during 20 years of operation period are evaluated.

Abstract Continually increasing demands on aquaculture products drive the current monoculture to upgrade and upscale because of not only economic but environmental sustainability reasons. Over the past decade, open water integrated multi-trophic aquaculture (IMTA) practiced as a potential alternative has been demonstratively illustrated from both scientific and public attention. Basing on previous studies of this synergistic aquaculture system, we, here, studied the physical environment in Onagawa Bay as the cornerstone for further IMTA implementation. Onagawa Bay locates in Miyagi Prefecture, Japan, and because of its mature practice on polyculture, it is recognized as a suitable site for IMTA. Unfortunately, the earthquake and tsunami in 2011 caused a huge uncertainty on physical environment changes. Still insufficient researches have been conducted on physical environment study, especially through modelling method. Here, adopting the three-dimensional Marine Environmental Committee (MEC) ocean model, we described the setup and validation for Onagawa Bay in this research. At the present stage, simulation results can best fit observation data on the tidal elevation with the correlation coefficient between observed and simulated tidal elevation reaching 0.96, captured the main characteristic of flow velocity, and exhibited homogenous tendency towards water temperature. Furthermore, through the plot of the residual velocity field and statistical seasonal velocity distribution, potential aquaculture configuration has been discussed spatio-temporally on the hypothesis that high current speeds contribute to the further implementation.

Abstract Japan Oil, Gas and Metals National Corporation (JOGMEC) has been conducting R&D of production system for Seafloor Massive Sulfides (SMS) mining. In order to reach commercialization stage in SMS development, the pipe wear due to slurry flow is one of many technical issues to be solved. In the present paper, the authors conducted the large scale experiment using full scale pipes to obtain the various data on pipe wear, and investigated the pipe wear amount in commercial operation. In the experiment, SUS 304 and UHPE were used for the inner materials of test pieces. In addition, the test pieces were installed in the vertical, horizontal and inclined section. Then the authors estimated the thickness reduction of pipe due to slurry flow based on the experimental results.

Abstract When a tsunami attacks in harbors, a vessel moored at a wharf is occurred a serious damage. The first wave of the tsunami has two forms and those are called the leading wave and the backwash. And, it is difficult to predict the form of the first wave of tsunamis. Authors have been studying about damage protection measures against the leading wave of vessels moored at wharves. However, it is also necessary that a damage protection measure against the backwash of vessels moored at wharves is considered. In this study, it is examined the damage status of a vessel at a wharf due to the backwash. Then, it is examined the influence of the backwash to the author’s proposal tsunami protection measures. The three dimensional MPS (Moving Particle Semi-implicit) method is used. The Backwash is simulated as the dam break phenomenon. Outflow angles, flow velocities, mooring conditions and with and without of tsunami protection measures are considered.

Abstract A new riser system concept is being investigated for deep-water application. This is the Branched Riser Systems with its three types: the Branched Steel Catenary Riser (BSCR) System, the Branched Steel Lazy Wave Riser (BSLWR) System and the Branched Lazy Wave Hybrid Riser (BLWHR) Systems. The branched riser system consists of a large bore pipe, which is terminated at an optimized water depth considering the minimum wall thickness requirement from burst and collapse criteria. The larger bore riser pipe is extended from this cut off water depth to the seabed by two smaller riser pipes via a connecting component (connector). The concept draws benefits from the performance of smaller riser bore pipe in strength and fatigue performance besides the opportunities provided by larger bore pipe for maximum flow throughput and vessel top decongestion. Preliminary results from ongoing investigation of type 1 (BSCR) of the branched riser systems, using a 12-inch pipe in combination with two 8-inch riser pipes in a water depth of 3,000m for a cut off water depth of 1500m, indicates a reduction in overall riser weight, stress utilization and fatigue damage at the riser hang off (HO) and touch down zone (TDZ). Results obtained indicate that possible benefits can be achieved through a combination of advantages of smaller and larger bore riser pipe and justifies further research interest on the novel riser concept.

Abstract The on-bottom stability design of subsea pipelines and cables is important to ensure safety and reliability but can be challenging to achieve, particularly for renewable energy projects which are preferentially located in high energy metocean environments. Often these conditions lead to the seabed being stripped of all loose sediment, leaving the cables to rest on exposed bedrock where roughness features can be similar in size to the cables. As offshore renewable energy projects progress from concept demonstration to commercial-scale developments, new approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of export power transmission and increase operational reliability. These same considerations also apply to the cables and small diameter pipes — such as umbilicals — required by oil and gas projects located on rocky seabeds. Recent experimental testing using the University of Western Australia’s unique Large O-tube has enabled the experimental measurement of hydrodynamic forces on small diameter cables and pipes in proximity to smooth and rough beds. The tested conditions extend well beyond the existing published parameter range including much higher KC conditions together with seabed roughness which is comparable in size to the diameter. The results provide design data of great relevance to the ongoing development of marine renewable and conventional oil and gas projects, especially on rocky seabeds. This paper presents a summary of the existing knowledge on the subject as a preface to preliminary test results and gives tentative conclusions on the likely outcomes from this work.

Abstract Indus river basin’s irrigation system in Pakistan is extremely complex, spanning over 90,000 km. Maintenance and monitoring of this extensive network demand enormous resources. This paper describes the development of a streamlined and low-cost autonomous underwater vehicle (AUV) for the monitoring of irrigation canals including water quality monitoring and water leak detection. The vehicle is a hoveringtype AUV, designed mainly for monitoring irrigation canals, with fully documented design and open source code. It has a length of 0.43 m, and a radius of 0.09 m with a depth rating of 4m. Multiple sensors have been installed onboard the AUV for monitoring water quality parameters including pH, turbidity, total dissolved solids, temperature, and electrical conductivity. A 9-DOF IMU, GY-85, is used, which incorporates an accelerometer, a gyroscope, and a magnetometer. The readings from these three sensors are fused together using direction cosine matrix algorithm, providing the AUV with the roll, pitch and yaw angles. A water pressure sensor, MS5837, gives the depth of the AUV. Four thrusters control the vehicle’s surge and heave, providing 3 DOF. The thrusters are controlled using a proportional-integral-derivative feedback control system, with IMU data and water depth being the controller’s input and the thruster’s speed as the output. The AUV is controlled by a single onboard processor, Arduino Mega 2560. A flow sensor has been installed beneath the main hull, for monitoring the changes in canal water flow and detecting potential water theft in the irrigation system. For recording underwater sounds, an indigenously developed hydrophone is placed on the AUV. The vehicle also provides information on water quality, providing the capability to identify the potential source(s) of water contamination. Detection of such events can provide useful policy inputs for improving irrigation efficiency and reducing water contamination. The AUV, being low cost and of small size, is suitable for autonomous maneuvering and water quality monitoring in the irrigation canals and can be used for monitoring irrigation networks at a large scale.

Abstract The steel lazy wave riser has been used in deep-water oil and gas field development because it has good adaption to the movement of the upper platform and economic efficiency. The typical design criterion and design flow of steel lazy wave riser are introduced in this paper. The design method and the equivalence principle of distributed buoyancy modules are given. The formulas of equivalent hydrodynamic parameters are derived in this paper. The influences of distributed buoyancy modules (DBM) and the buoyancy factor on the configuration of the riser, the top tension, and the bending moment distribution are discussed and summarized. The distribution law of effective stress response along the pipe can be analyzed by dynamic analysis, and it provides reference for the global design of steel lazy wave riser.

Abstract Multiphase conditions can lead to high vibration in flexible pipe systems due to high dynamic flow induced forces. To better model the dynamic forces, large scale 6” air-water, near-atmospheric experiments have been done with different bend radii and upstream configurations. This paper gives an overview of the obtained results with respect to the overall dynamic force and force power spectral density description parameters. Scaling rules based on these experiments are given.

Abstract In this paper, the propulsive performance of a caudal peduncle-fin swimmer mimicking a bio-inspired robotic fish model is numerically studied using a fully coupled FSI solver. The model consists of a rigid peduncle and a flexible fin which pitches in a uniform flow. The flexible fin is modeled as a thin plate assigned with non-uniformly distributed stiffness. A finite volume method based in-house Navier-Stokes solver is used to solve the fluid equations while the fin deformation is resolved using a finite element code. The effect of the fin flexibility on the propulsive performance is investigated. The numerical results indicate that the compliance has a significant influence on the performance. Under the parameters studied in this paper, the medium flexible fin exhibits remarkable efficiency improvement as well as thrust augment, while the least flexible fin shows no obvious difference from the rigid one. However, for the most flexible fin, although the thrust production decreases sharply, the efficiency reaches the maximum value. It should be noted that by non-uniformly distributing the rigidity across the caudal fin, our model is able to replicate some fin deformation patterns observed in both the live fish and the experimental robotic fish.

Abstract Time-domain analysis is important for the development of performance-based criteria for the intact stability of ship (level 3 criteria in the 2 nd Generation Intact Stability Criteria). It can be implemented to assess directly the vulnerability of ships against various modes of intact stability failure and ensure a sufficient level of safety. In this context, ELIGMOS, a novel time domain simulation code combining a maneuvering model and a seakeeping model, is under development. As the maneuvering model has been the subject of previous paper, the seakeeping part is presented herein and validated in terms of linear and nonlinear vertical motions in head seas. Heave and pitch motions are augmented near the resonant periods, testing the ability of the numerical tool to capture accurately the ship’s behaviour, as it plays a dominant role for her direct stability assessment afterwards. Surge motion is considered uncoupled and it is excluded from the system of equations, keeping the forward speed constant. Comparison regarding the seakeeping performance of KVLCC2 is presented against experimental results and results obtained by a 3D-potential flow, frequency domain software as well. Ship’s geometry is modelled by means of quadrilateral panels whilst radiation forces were incorporated by means of memory functions by adopting the well-known concept of convolution integrals. The ability of ELIGMOS to capture the effect of geometrical nonlinearities on the vertical motions is demonstrated through preliminary simulation of large amplitude motions of a C11-class containership.

Abstract To determine the optimal trimaran configuration for best calm-water transportation efficiency, a Deep Neural Network (DNN) is trained with sufficient computational results provided by, as an example, an in-house developed potential-flow code called Multi-hull Simple-source Panel Method (MSPM). Even though the computational method is extremely efficient in accurately establishing the mapping relation between the key design parameters governing the trimaran configuration problem and the resulting calm-water transportation cost, the modeling efforts are non-trivial since the number of geometric and configuration parameters in a typical situation is large. In this work, we demonstrate how the “Big Data” of computational results can be effectively utilized in training a DNN. An optimal trimaran configuration solution within a specified design space, subject to realistic range constraints, can be quickly determined in a minimal amount of time with the DNN. A demonstrative case study is provided for illustration.

Abstract Hydrodynamic behavior of wave responses and wave forces induced by piston-modal resonance in the narrow gap formed by a ship section in front of a bottom mounted terminal is investigated based on the OpenFOAM ® package. Numerical simulations suggested that the free surface piston-modal oscillations in the narrow gap have closely relationship with the vertical velocity along the gap bottom, implying the velocity flow around the gap bottom is able to affect the wave amplitude in the narrow gap, significantly. The horizontal wave forces can be decomposed into static water forces due to the difference of water level besides the box and the dynamic water forces due to the velocity flow. The amplitudes of static water forces are larger than that of horizontal wave forces, implying that the dynamic water forces can counteract part action of the static water forces. The dynamic wave forces approach to the maximal values at resonant frequencies, indicating that the extreme flow velocity around resonant frequency is useful for reducing the horizontal wave forces.

Abstract Experimental investigations of hydrodynamic performance of mini-AUV in non-uniform flow field were conducted in the basin of Harbin Engineering University, the revolved body and flat body of mini-AUV model were tested respectively. The three dimensional flow fields were generated by local jet of the underwater pump, and circulated in the basin. The three dimensional velocity distributions at different positions were measured by a Doppler current profiler. The three component balance was used to measure the drag, lateral force and yawing moment acting on the mini-AUV models depending on drift angle in the flow field, and the influence of complex flow field to the hydrodynamic performance of mini-AUV indicated that drag was not sensitive to drift angle and yawing moment was increased obviously. The conducted experiments could supply reference to the maneuverability research of mini-AUV in real marine environments in the future.

Abstract Although there is a significant body of research devoted to the shallow water hydrodynamic aspects of ships, several unexamined topics remain. Among these is that of critical outer flow in a dredged channel and its influence on parameters of interest. While empirical methods can be used with ease to resolve this, they can provide results with reliability sufficient only for an early design stage. On the other hand, more sophisticated potential flow theories are either inapplicable or do not perform well at the critical limit. However, RANS (Reynolds Averaged Navier-Stokes) – based tools can accurately capture all underlying phenomena without relying on limiting assumptions. This paper presents an attempt at comparing some results obtained via a CFD-based RANS solver and the slender body theory for critical outer flow in a dredged channel.

Abstract Forced harmonic oscillations of seven configurations consisting of horizontal side by side plate elements are performed experimentally and numerically. The configurations are oscillated in vertical direction and represent generalized mudmats of subsea structures. The tests are performed for Keulegan-Carpenter (KC) numbers relevant for force estimation during lifting operations. Hydrodynamic added mass and damping coefficients are presented. The coefficients are found to be amplitude dependent for all configurations tested. The interaction effects between the plates increase with increasing amplitude and decreasing distance between the plates. For oscillation amplitudes small compared with the gap between the plates, the plates behave approximately like individual plates. A study of the relation between the damping force and the added mass force for the tested structures illustrates the importance of applying representative damping coefficients in numerical analysis of marine operations. Numerical results are obtained using a potential flow solver (BEM) and a viscous flow solver (CFD). Low-KC added mass coefficients predicted with the BEM are in accordance with the experiments. There is acceptable agreement between the CFD and the experiments. Best agreement is obtained for small KC numbers. For increasing KC numbers, the differences are, in general, larger. This is possibly due to the CFD being based on two-dimensional laminar flow.

Abstract A two-dimensional numerical analysis on the hydrodynamic force of perforated plates in oscillating flow is presented, and a new semi-analytical force model is proposed. Plates with ten different perforation ratios, τ, from 0.05 to 0.50 are simulated. The Keulegan–Carpenter numbers in the simulations cover a range from 0.002 to 2.2 when made nondimensional with the width of the plates. Resulting hydrodynamic added mass and damping coefficients are presented. All perforated plates with perforation ratios greater than or equal to 10% are found to be damping dominant. The numerical results are obtained using a two-dimensional Navier–Stokes solver (CFD), previously validated against dedicated 2D experiments on perforated plates. Furthermore, we present verification of the code against the analytical solid flat plate results by Graham. The presently obtained hydrodynamic coefficients are compared with the state-of-the-art semi-analytical method for force coefficient calculation of perforated plates by Molin, as well as the recommended practice for estimating hydrodynamic coefficients of perforated structures by DNV GL. Based on the CFD results, a new method for calculating the hydrodynamic force on perforated plates in oscillating flow is presented. The method is based on curve fitting the present CFD results for perforated plates, to the analytical expressions obtained for solid plates by Graham. In addition to its simplicity, a strength of the method is that coefficients for both the added mass and damping are obtained.