Irregular Wave Conditions Research Articles

Numerical Study on Unified Seakeeping and Maneuvering of a Russian Trawler in Wind and Waves

The maneuvering performance of a ship on the actual sea is very different from that in calm water due to wave-induced motion. Enhancement of a ship’s maneuverability in waves at the design stage is an important way to ensure that the ship navigates safely. This paper focuses on the maneuvering prediction of a Russian trawler in wind and irregular waves. First, a unified seakeeping and maneuvering analysis of a Russian trawler is proposed. The hydrodynamic forces acting on the hull in calm water were estimated using empirical formulas based on a database containing information on several fishing vessels. A simulation of the standard maneuvering of the Russian trawler was conducted in calm water, which was checked using the International Maritime Organization (IMO) standards for ship maneuvering. Second, a unified model of seakeeping and maneuvering that considers the effect of wind and waves is proposed. The wave forces were estimated by a three-dimensional (3D) panel program (ANSYS-AQWA) and used as a database when simulating the ship maneuvering in wind and irregular waves. The wind forces and moments acting on the Russian trawler are estimated using empirical formulas based on a database of wind-tunnel test results. Third, standard maneuvering of a Russian trawler was conducted in various directions under wind and irregular wave conditions. Finally, the influence of wind and wave directions on the drifting distance and drifting angle of the ship as it turns in a circle was found. North wind has a dominant influence on the turning trajectory of the trawler.

A Phenomenological Study of Lab-Scale Tidal Turbine Loading under Combined Irregular Wave and Shear Flow Conditions

Tidal devices are likely to faced with shear flows and subjected to various wave climates. The paper presents an experimental study of the combined impacts of shear profile and irregular waves on the loading of a 1/20th scale device operating at peak power extraction. The experiments presented were conducted at various depths to facilitate analysis of the effects of the shear flow and wave impact on the device at various positions in the water column. The fluid field was measured at three different upstream positions and at three depths (top, middle and bottom of the rotor) for each experiment; in doing so, data from the device were captured three times. The fluid measurements were of a high quality and were analysed to present the structure flow upstream of the device, which contained velocity and turbulence profiles. The upstream measurement was utilised to understand the development of flow structures in the approach to the device, and the impact of the flow structures measured was confirmed via cross-covariance calculations. The long datasets gathered were used to produce full rotational probability density functions for the blade-root-bending moments for three blades. The spectral characteristics were also considered, and showed that rotor loading quantities are less reactive to smaller scale flow structures.

Experimental study of the influence of the rope material on mooring fatigue damage and point absorber response

The present work investigates the influence of mooring material on the hydrodynamic responses of a point absorber by experimental tests. Three taut mooring systems are considered with different materials, namely wire, nylon and polyester. In order to study the influence of mooring material on point absorber hydrodynamic responses, the other factors that may influence system dynamics are kept as unchanged as possible. During the model tests, the pretension of the three mooring systems are set to be similar and the parameters of wave conditions are identical. Decay tests are conducted to determine the system natural periods and linear damping coefficients in different modes. Totally seventeen regular wave cases and two irregular wave conditions are tested to study the motion responses of the buoy as well as mooring dynamics. The response amplitude operators of motion responses and mooring tensions are obtained by the regular wave model tests. Three wideband spectral methods as well as two simplified spectral methods are applied to study mooring fatigue damage in the random wave conditions, and the results are compared with the estimations of rainflow counting approach. It is shown that the α0.75 method provides the most precise estimations of fatigue damage in most mooring cases among spectral methods discussed this study.

Analysis of oscillating-water-column wave energy converter configurations for integration into caisson breakwaters

Energy production from ocean waves remains in the research and development phase, due in part to the lack of maturity of the technology, as well as the economical unfeasibility of large-scale projects. Integration of wave energy converters into breakwaters is a strategy to improve the economic viability of the energy conversion system. The cost of electricity is reduced through the sharing of construction, installation, maintenance and operation activities. This work focuses on the design of an oscillating-water-column device to be integrated into a caisson used for breakwaters. A numerical model based on linear potential flow theory was developed. The viscous flow effects in the duct and the nonlinear turbine damping characteristic were linearized for the application of a frequency domain analysis. Furthermore, the device performance was estimated under irregular wave conditions using stochastic modelling for three wave climates and the influence of tidal variability is studied. The design and performance optimization of the submerged duct, air chamber and turbine are considered for the following oscillating-water-column duct configurations: conventional; U-shape; and L-shape. The results show all devices have better power conversion performance for the lower wave periods observed in the Mediterranean Sea than for the studied North Atlantic Ocean wave climates. The U-shaped converter outperforms the other configurations in all three locations, with a maximum theoretical annual pneumatic power of 46.8kW/m when compared with 39.4kW/m and 38.0kW/m for the L-shape and conventional device, respectively. The tidal level variation does have some influence on the device performance, but the impact is minor.

Model testing of a floating wave energy converter with an internal U-shaped oscillating water column

The development of wave energy converters is centred on the combination of two factors: cost-effectiveness of energy extraction and system survivability on extreme sea conditions. Despite advances in numerical modelling, wave tank model testing still presents the most reliable option to evaluate these two factors. This paper presents an experimental study on the hydrodynamic responses of the UGEN, a wave energy converter consisting of an asymmetric floater with an internal U-shaped tank partially filled with water. The device absorbs energy through the oscillating-water-column (OWC) motion inside the U-shaped tank, induced by the wave action on the floater. The experimental testing of a bottom-moored 1:24th-scale model was performed at the COAST Laboratory, University of Plymouth, UK, considering regular and irregular wave conditions. The wave tank test measurements report six-degree-of-freedom rigid-body motions, mean drift forces, OWC motion, structural stresses and mooring loads. Results present the characterization of the energy absorption at the internal OWC induced by the floater’s sway, heave and roll. The occurrence of low-cycle auto-parametric resonance under certain wave conditions was detected and induced large roll motions, which affected power extraction and increased mooring line loads, particularly for large wave amplitudes.

Numerical modelling of hydrodynamic responses of Ocean Farm 1 in waves and current and validation against model test measurements

With the continuous growing of the aquaculture industry and increasingly limited fish farming sites at close to shore areas both in Norway and worldwide, there is a need to develop fish farms suitable for aquaculture production in typical offshore environments. For this purpose, SALMAR has developed and deployed the Ocean Farm 1 facility for offshore fish farming.The main purpose of this paper is to develop a reliable numerical model and investigate the motion responses of the Ocean Farm 1 structure in waves and current. The established numerical model consists of the Ocean Farm 1's frame structure (with rigidly-connected circular column components), the net and the mooring system. The hydrodynamic external loads and coefficients of the frame structure are obtained by using potential flow theory. The quadratic drag load on the individual circular columns of the frame structure is formulated by a given drag coefficient. The loads on the net are formulated by using the screen model, where the Reynold number dependent lift and drag forces are formulated as a function of the solidity ratio Sn of the net, relative inflow angle and velocity. The hydrodynamic loads on the mooring lines are formulated using the Morison's equation and the structural responses of the mooring lines are obtained using a nonlinear FE model.With the developed numerical model, time domain simulations are performed. The simulation results are firstly validated against measured data from the decay tests, current tests, and regular wave tests. After the validation, numerical simulations are performed in different irregular wave and current combined weather conditions and the obtained motion response of Ocean Farm 1 are discussed and compared with available measurement data.

Unveiling the potential of using a spar-buoy oscillating-water-column wave energy converter for low-power stand-alone applications

Applications including the offshore aquaculture, remotely operated vehicles, data acquisition systems, and desalination can be either supplemented or completely powered by renewable energy. The spar-buoy oscillating-water-column wave energy converter concept, typically studied for large scale wave energy production, can be re-designed to meet the power requirements of such applications, which results in a much smaller device working outside resonance conditions under typical sea states, and therefore with a smaller energy conversion efficiency. Experimental results are presented for a 1:10th scale model of a spar-buoy oscillating-water-column, undertaken in a wave channel. Calibrated orifice plates were used to model the flow characteristic of an impulse turbine, which will be used in the full-scale system. The system performance is examined for regular and irregular wave conditions. The results show the presence of non-linear effects, caused by viscous flow and turbine damping. A capture width ratio matrix is generated through stochastic modelling using regular-wave data and the performance of the device is assessed for two locations with different wave energy resource. The device presents an annual-averaged power of approximately 1kW for an Atlantic wave climate, and an annual-averaged power of approximately 0.4kW for a Mediterranean wave climate, which makes it adequate for the supply of specific oceanographic equipment in the first location, while for the second location a larger buoy should be considered.

Experimental investigation on the infragravity wave on different reef systems under irregular wave action

This paper presents the results of a laboratory investigation into the role of infragravity motions in reef hydrodynamics over different reef systems. Laboratory experiments are performed with reef profiles of a platform reef and a fringing reef under irregular wave conditions. The propagation and the distribution of the infragravity wave are different over these two reef systems. Analysis of the measured time histories of water surface elevation shows that shorter sea-swell (SS) waves, longer infragravity (IG) wave and mean water level on the reef flat are significantly larger in the fringing reef system. The IG wave height is much larger on the fringing reef than that on the platform reef. This marked increase of the IG wave is considered to be due to: (1) the superposition of incoming IG wave and reflected IG wave from shoreline; (2) more violent wave breaking in the surf zone that may enhance the transfer of wave energy from the frequency band of the shorter wave to IG wave. In this experiment, the wave breaking mechanism controls the relationship between IG wave height and water depth.

A Numerical Study on Hydrodynamic Energy Conversions of OWC-WEC with the Linear Decomposition Method under Irregular Waves

A numerical study was performed to investigate the applicability of the linear decomposition method for the hydrodynamic energy conversion of an oscillating-water-column type wave energy converter (OWC-WEC). Hydrodynamic problems of the OWC chamber were decomposed into the excitation and radiation problems with the time-domain numerical method based on the linear potential theory. A finite element method was applied to solve the potential flow in the entire fluid domain including OWC chamber structure. The validity of the linear decomposition method was examined by comparing with the direct interaction method for the turbine–chamber interaction based on the linear pressure drop characteristics. In order to estimate the hydrodynamic energy conversion performance under the irregular waves, the response spectrum method was applied with the transfer function based on the linear decomposition method. Under the various irregular wave conditions, the pneumatic power of OWC-WEC calculated by the response spectrum based on the linear decomposition method agreed well with the direct irregular wave simulation results.

Alternative configurations to optimize tension in the umbilical of a work class ROV performing ultra-deep-water operation

The utilization of ROV (Remotely Operated Vehicle) for subsea operation has been increasing in the past few years alongside the increasing use of subsea technologies in deep-water oil and gas activities and other maritime operations. While survey class ROVs have reached the deepest depth in the ocean, the work class ROVs are mostly limited to 3 km depth. For a work class ROV operating in deep waters, the excessive static tension due to self-weight of the ROV and its umbilical system leaves small room before the dynamic tension exceeds the safe working load of the umbilical. Snap loads can also exist in the low-tension area of the umbilical when the ROV system is under dynamic motions which can lead to a large tensile load in the line. Therefore, there is a need for a practical solution to ensure that the safe working load is not exceeded during ultra-deep-water operation.This study focuses in reducing the tension in the umbilical hence increasing the operational depth of a work class ROV from deep to ultra-deep waters. Three different configurations, namely the hook, hook & top floaters, and mid-depth hook are numerically investigated and compared with the conventional free-flying ROV configuration. Firstly, deep-water operation of a 3 km ROV system is simulated under combination of a real extreme vertical current profile and various irregular wave conditions. This shows negligible effect of the current profile on umbilical maximum tension.The study also shows that for 3 km ROV operation, the suggested hook configuration is sufficient to effectively reduce the tension load as well as reducing the possibility of snap in the umbilical. Using hook configuration in the umbilical has significant benefits in minimizing ROV motion and also filters out the destructive snap induced high frequency tension fluctuations in the umbilical. For 6 km operation, either hook & top floaters or mid-depth hook can be utilized whereas the mid-depth hook configuration provides a broader environmental operating window for ROV operation. However, installation of floaters on the umbilical to produce the mid-depth hook configuration increases the ROV offset.

Experimental study for characteristics of slamming loads on bow of a ship-type FPSO under breaking and irregular wave conditions

An experimental investigation of the characteristics of the slamming impact loads on the bow of a ship-type Floating Production Storage and Offloading (FPSO) under breaking and irregular wave conditions was conducted. The breaking waves were generated by the focusing wave method and three different heading angles were applied. To examine the repeatability of the slamming loads, the model experiments were repeated 20 times for each heading condition. The slamming impact loads were measured using 15 force sensors attached to the bow of the FPSO unit model, while force panels were used to measure the local impact loads. The locations of the largest slamming impact loads were observed to vary with the heading angle. Two types of slamming impact loads were identified and damping was also observed in the slamming impact load with two peaks. Irregular wave tests conducted under oblique wave conditions were further used to investigate the characteristics of the slamming impact loads, and the velocities of the directional sway and roll motions were found to be significant determinants of the slamming impact load.

Experimental Observation on Beach Evolution Process with Presence of Artificial Submerged Sand Bar and Reef

For observation on the influence mechanism of environmentally and aesthetically friendly artificial submerged sand bars and reefs in a process-based way, a set of experiments was conducted in a 50 m-long flume to reproduce the cross-shore beach morphodynamic process under four irregular wave conditions. The beach behavior is characterized by the scarp (indicating erosion) and the breaker bar (indicating deposition), respectively, and the scarp location can be formulated as a linear equation regarding the natural exponential of the duration time. Overall, main conclusions are: (1) the cross-shore structure of significant wave height and set-up is mainly determined by the artificial reef (AR); (2) the cross-shore distribution of wave skewness, asymmetry, and undertow (indicating shoaling and breaking) is more affected by the artificial submerged sand bar (ASB); (3) the ASB deforms and loses its sand as it attenuates incident waves, which leads to a complex sediment transport pattern; (4) the scarp retreat is related to the beach state, which can be changed by the AR and the ASB; (5) the AR, the ASB, and their combination decrease wave attack on the beach. In conclusion, this study proves positive effects of the AR and the ASB in beach protection through their process-based influences on beach behaviors and beach states for erosive waves.

Long-term extreme response analysis for semi-submersible platform mooring systems

The accurate assessment of long-term extreme responses of floating-structure mooring system designs is important because of small failure probabilities caused by long-term and complex ocean conditions. The most accurate assessment would involve considering all conceivable sea states in which each sea state is regarded as a stochastic process and performing nonlinear time-domain numerical simulations of mooring systems to estimate the extreme response from a long-term analysis. This procedure would be computationally intensive because of the numerous short-term sea states involved. Here, a more feasible approach to evaluate the long-term extreme response is presented through immediate integration combined with Monte Carlo simulations. A parameter fitting procedure of the short-term extreme response distribution under irregular wave conditions is employed to solve the long-term response integration. Case studies were conducted on a semi-submersible platform using environmental data measurements of the Gulf of Mexico and a joint distribution model of the environmental parameters was considered. This approach was observed to be effective and the results were compared with those of traditional methodologies (univariate extreme value design and environmental contour methods). The differences were reflected using a reliability analysis of mooring lines, which indicated that the design standards must be stricter when using long-term analysis.

Power Performance Analysis According to the Configuration and Load Control Algorithm of Power Take-Off System for Oscillating Water Column Type Wave Energy Converters

A power take-off (PTO) system for an oscillating water column (OWC) wave energy converter comprises a turbine-generator-power converter. In this study, only the topologies of the power converter that affect the load control algorithm are compared. A power converter for renewable energy is composed of a diode-dc/dc converter and a pulse-width modulation (PWM) converter operating at small and large capacities, respectively. However, selecting a power converter according to the capacity based on the characteristics of the wave energy converter, in which the input energy is highly fluctuating, can significantly reduce the power performance. Thus, to verify load control characteristics according to the topology of the power converter, the turbine-generator-power converter was incorporated in the modeling, and the power performance based on the power converter topology under various wave conditions was analyzed. Further, torque control to obtain the maximum power among load control algorithms was applied under irregular wave conditions, and the power performance and PTO system characteristics according to the torque coefficient were analyzed. The results of this study suggested an increase in the torque coefficient of the maximum power control for the operational stability of the OWC-WEC, and it was confirmed that the RPM characteristics of the PTO system were reduced.

Confined-crest impact: Forces dimensional analysis and extension of the Goda's formulae to recurved parapets

In this paper, an extended numerical analysis of the “confined-crest impact” (hereinafter referred as “C–CI”), induced by non-breaking waves on recurved parapet walls, is presented to better understand the physics and characteristics of this impulsive wave phenomenon, which significantly depends on the geometry of the parapet (here made by a sector of circumference) as well as on non-breaking wave steepness. A dimensional analysis of the forces has been carried out and used to analyse the results obtained by numerical CFD computations. The numerical simulations, performed for different radii of curvature, opening angles and incoming wave characteristics (changing both the wave height H and wave period T), have shown that the maximum impulsive pressure: (i) takes place on the top of the recurved parapet and decreases moving towards the SWL, where it tends to vanish; (ii) tends to decrease as the radii of curvature and the incoming wave characteristics (wave height H and period T) increase. However, this reduction in the maximum pressure, is always accompanied by an increase of the forces acting on the recurved parapet and a decrease of the wave overtopping volumes. Moreover, the maximum wave induced pressures have been examined by means of regular and irregular wave conditions in order to analyse the possibility of schematising the effects (i.e. forces) of an irregular sea state with those of a regular one, providing an equivalent force and therefore saving computational time. Based on the new results, a schematization of the pressure diagram induced by the C–CI impact has been proposed, used to extend the Goda-Takahashi pressure diagrams, valid for vertical walls in non-breaking wave conditions, including recurved walls.

Experimental Study on the Influence of an Artificial Reef on Cross-Shore Morphodynamic Processes of a Wave-Dominated Beach

Artificial reefs are being implemented around the world for their multi-functions including coastal protection and environmental improvement. To better understand the hydrodynamic and morphodynamic roles of an artificial reef (AR) in beach protection, a series of experiments were conducted in a 50 m-long wave flume configured with a 1:10 sloping beach and a model AR (1.8 m long × 0.3 m high) with 0.2 m submergence depth. Five regular and five irregular wave conditions were generated on two types of beach profiles (with/without model AR) to study the cross-shore hydrodynamic and morphological evolution process. The influences of AR on the processes are concluded as follows: (1) AR significantly decreases the incident wave energy, and its dissipation effect differs for higher and lower harmonics under irregular wave climates; (2) AR changes the cross-shore patterns of hydrodynamic factors (significant wave height, wave skewness and asymmetry, and undertow), leading to the movement of shoaling and breaking zones; (3) the beach evolution is characterized by a sandbar and a scarp which respectively sit at a higher and lower location on the profile with AR than natural beach without AR; (4) the cross-shore morphological features indicate that AR can lead to beach state transformation toward reflective state; (5) the scarp retreat process can be described by a model where the scarp location depends linearly on the natural exponential of time with the fitting parameters determined by wave run-up reduced by AR. This study demonstrates cross-shore effects of AR as a beach protection structure that changes wave dynamics in surf and swash zone, reduces offshore sediment transport, and induces different morphological features.

Multi-floater-mooring coupled time-domain hydro-elastic analysis in regular and irregular waves

In this study, a time-domain hydro-elastic model is developed to solve the floater-connector-mooring coupled system. The frequency-domain model is first devised with the discrete-modulus-based method in which the deformable floating structure is modeled with multiple rigid bodies connected by high-order beam elements. The corresponding multi-body hydrodynamic coefficients and loads are estimated by 3D potential theory with the coupling stiffness matrix based on the theory of the Euler-Bernoulli beam and Saint-Venant torsion. The time-domain hydro-elastic model with mooring dynamics is then subsequently developed with multi-body Cummins equation with high-order rod-FE (Finite Element) mooring elements connected to relevant floaters. The present frequency- and time-domain models are validated through comparisons with published experimental and frequency-domain results for a VLFS (Very Large Floating Structure) without mooring lines. The present method directly solves the hydro-elastic problem with mooring lines without using the conventional modal superposition method, for which wet modes with mooring need to be obtained in a priori through cumbersome iterations. The developed hydro-elasticity model is also very useful when complex inner structure-connection conditions, such as discontinuity by multiple hinged joints, are involved. Such an example is also numerically tested, and reliable results are obtained. Vertical flexible displacements, bending and torsional moments, shear forces, and mooring tensions in regular and irregular wave conditions are systematically analyzed and compared with rigid-body and without-mooring cases.

Numerical Study on a Multibuoy-Type Wave Energy Converter With Hydraulic PTO System Under Real Sea Conditions

This study aims to investigate the power takeoff (PTO) performance of a two-buoy-type wave energy converter (WEC) with a PTO system under real sea conditions. The two-buoy-type WEC is composed of two hemispherical heaving buoys with a hydraulic PTO system. The hydrodynamic analysis considering two-body interaction is carried out based on the wave green function and Cummins equation. In addition, a hydraulic PTO system is described as an approximate Coulomb damping force and applied to the equation of motion for a WEC. First, by comparing the extraction power on various irregular wave conditions, the PTO performance of an isolated-buoy WEC, and the two-buoy-type WEC is estimated. Through this, it is found that the maximum extraction power occurred when the wave peak period is 1.2 times longer than the heave natural period of a WEC. By comparing the external force and wave elevations near the WEC, three incident wave directions are applied for the maximum or minimum extraction power. Based on these results, the two-buoy-type WEC is applied to the Korean coastal area (Uljin). The PTO force is calculated from the polynomial regression analysis to produce maximum extraction power on various irregular wave conditions. Finally, the annual extraction power and capture width of the WEC in Uljin are calculated. The relation between the design wave period and heave natural period of the WEC is investigated. The annual extraction power is maximum when the design wave period is 20% longer than the WEC's heave natural period and the incident wave is 90°.

Wave-flume tests of a model-scaled OWC chamber-turbine system under irregular wave conditions

The rapid development of the oscillating water column (OWC) wave energy converter proposes a demand to predict its overall performance accurately. Traditional experimental and numerical studies only employed the orifice or porous media as the power take-off substitutes, causing an inaccurate evaluation of the OWC performance. In this study, an impulse turbine model was used directly in a generic OWC model for the secondary-stage energy conversion. The wave-flume tests were conducted under irregular wave conditions and the rotation speeds of the turbine were fixed. Typical time histories of some performance parameters were presented. The primary-stage and secondary-stage efficiencies were analyzed under various test conditions. It was found that the distribution characteristics of the overall efficiency of the chamber-turbine system is significantly determined by the secondary-stage efficiency. The different distribution characteristics of the efficiencies for the regular and irregular wave conditions indicated that the irregular wave tests are preferred for the shape parameter and operating mode design of the OWC plant.

Experimentally Verifying the Generation Characteristics of a Double-Sided Linear Permanent Magnet Synchronous Generator for Ocean Wave Energy Conversion

This paper presents an evaluation system to experimentally verify the performance of a double-sided linear permanent magnet synchronous generator (PMSG) prototype. In general, research on wave power generation systems has concentrated on the optimum design technology of PMSGs, power generation performance analysis technology, loss analysis, and performance evaluation technology. Despite active research in this field, the performance of a system wherein a wave generator and buoy are linked has never been evaluated. Therefore, in this study, a linear PMSG was used in a performance evaluation system to predict a buoy's motion and derive the parameters for optimal power generation under regular and irregular wave conditions.