Realistic Sea States Research Articles

Impact of Limited Degree of Freedom Drag Coefficients on a Floating Offshore Wind Turbine Simulation

The worldwide effort to design and commission floating offshore wind turbines (FOWT) is motivating the need for reliable numerical models that adequately represent their physical behavior under realistic sea states. However, properly representing the hydrodynamic quadratic damping for FOWT remains uncertain, because of its dependency on the choice of drag coefficients (dimensionless or not). It is hypothesized that the limited degree of freedom (DoF) drag coefficient formulation that uses only translational drag coefficients causes mischaracterization of the rotational DoF drag, leading to underestimation of FOWT global loads, such as tower base fore-aft shear. To address these hydrodynamic modeling uncertainties, different quadratic drag models implemented in the open-source mid-fidelity simulation tool, OpenFAST, were investigated and compared with the experimental data from the Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project. The tower base fore-aft shear and up-wave mooring line tension were compared under an irregular wave loading condition to demonstrate the effects of the different damping models. Two types of hydrodynamic quadratic drag formulations were considered: (1) member-based dimensionless drag coefficients applied only at the translational DoF (namely limited-DoF drag model) and (2) quadratic drag matrix model (in dimensional form). Based on the results, the former consistently underestimated the 95th percentile peak loads and spectral responses when compared to the OC5 experimental data. In contrast, the drag matrix models reduced errors in estimates of the tower base shear peak load by 7–10% compared to the limited-DoF drag model. The underestimation in the tower base fore-aft shear was thus inferred be related to mischaracterization of the rotational pitch drag and the heave motion/drag by the limited-DoF model.

URANS prediction of ship performance: An analysis of propulsion system failure on turning behaviour of a ship in waves

The loss of ship propulsive power has been recognised as the most frequently occurring incident at sea. Given the fact that the manoeuvrability of a ship is highly reliant on the propulsive power produced by a rotating propeller, propulsion loss may pose potential threats to navigational safety in a real seaway by resulting in further serious incidents, such as collision, contact, and grounding. The key objective of this paper is to analyse the propulsion loss impacts on the turning capability of the KRISO Container Ship (KCS) model in different wave height conditions, using unsteady Reynolds-Averaged Navier-Stokes (URANS) computations. It is expected that this work will be of great interest to Master Mariners and navigating officers who are in charge of ship handling and help enhance navigational safety in real sea states.

Self-Tuning WEC Controller for Changing Sea States

A self-tuning proportional-integral control law prescribingmotor torques was tested in experiment on a threedegree-of-freedom wave energy converter. The control objectivewas to maximize electrical power. The control law relied uponan identified model of device intrinsic impedance to generate afrequency-domain estimate of the wave-induced excitation forceand measurements of device velocities. The control law was testedin irregular sea-states that evolved over hours (a rapid, butrealistic time-scale) and that changed instantly (an unrealisticscenario to evaluate controller response). For both cases, thecontroller converges to gains that closely approximate the postcalculatedoptimal gains for all degrees of freedom in a sufficientlyshort-time for realistic sea states. In addition, electricalpower was found to be relatively insensitive to gain tuning overa broad range of gains, implying that an imperfectly tunedcontroller does not result in a large penalty to electrical powercapture. Because the controller relies on an identified model ofdevice intrinsic impedance, the sensitivity of power capture wasevaluated with respect to uncertainty in the constituent termsof intrinsic impedance. Power capture is found to be relativelyinsensitive to uncertainty of 20% in constituent terms of theidentified intrinsic impedance model. An extension of this controllaw that allows for adaptation to a changing device impedancemodel over time is proposed for long-term deployments, aswell as an approach to explicitly handle constraints within thisarchitecture.

Phase-resolved wave prediction in highly spread seas using optimised arrays of buoys

In many offshore operations, including the operation of wave energy converters, phase-resolved wave prediction is essential for safety and efficiency. However, for sea-states with large directional spreading angles, wave prediction becomes increasingly difficult. This study investigates the benefit of exploiting concurrent surface-displacement time histories measured by a buoy in three degrees of freedom (DoF) to facilitate prediction in such highly spread sea-states. A direct comparison is made with arrays of sensors measuring in a single DoF using the same formulation. The arrays considered are extensively optimised along with multiple sets of representative angles used to describe wave propagation, which is assumed to be normally distributed. The performance of the scheme is tested on a range of synthetically generated, realistic sea-states comprising single and crossing swells prevalent along the south coast of Australia. Additional tests involve wind-sea and swell waves described using Ewans models. Accurate predictions are obtained for times up to three wave periods into the future, in typical cases. With the multiple sets of optimal angles obtained for each optimal array, we further enhance the capability of the scheme by aggregating and averaging the different predictions from these sets. We observe a marked increase in both performance with large spreading and tolerance to possible variation in mean wave directions.

Space-time statistics of extreme ocean waves in crossing sea states

The study of extreme ocean waves has gained considerable interest in recent years, due to their importance for offshore design and navigation safety, and several theoretical approaches have been developed for their statistical description. However, in the case of crossing seas, where two or more wave systems of different characteristics are present, a full understanding of the main physical mechanisms responsible for the occurrence of very high individual waves is still lacking. As a consequence, the prediction of extremes in such conditions currently relies on integrated parameters of the total sea state, such as the spectral wave steepness. In this study, to gain further insight into the role of the crossing wind sea and swell wave systems in producing extreme individual waves, we investigate realistic sea states during typhoon Kong-rey (2018) using an ensemble of numerical simulations obtained from a phase-resolving wave model based on the High-Order Spectral (HOS) method. The reliability of the numerical fields is assessed for the first time with stereo wave measurements of the sea surface elevation field collected from an offshore platform in the area of interest. We show that, in specific conditions, space-time extreme crest heights in crossing seas can be larger than in unimodal seas due to second-order bound wave interactions between the wind sea and the swell. To improve existing prediction capabilities, we propose a novel formulation for the wave steepness in crossing seas, which includes nonlinear effects up to the second order and accounts for the spectral parameters of the interacting wave systems.

Ship design for real sea states under uncertainty

A ship is designed under inescapable uncertainty. Neglecting important uncertainties might lead to suboptimal designs. Furthermore, to achieve the best possible performance, the ship design should be optimized for the actual operational conditions, not just for calm water at design speed. This paper aims to propose a two-stage method for the optimal design of ships under uncertainty, operating at various speeds in real sea states, addressing both aleatory uncertainties due to the weather and epistemic uncertainties due to the model and methodology. A general cargo ship is used as a test case. A comparison with different cases representing traditional approaches for optimizing the ship design is provided. The effect of the slow steaming speeds on the design is studied. Results indicate the effectiveness of the adopted approach to deal with the epistemic and aleatory uncertainties in all cases studied. Verification with in-service data confirms the effectiveness of the suggested approach. Furthermore, epistemic and aleatory uncertainties resulted in up to 26.91% reductions in the average attainable speed of different cases compared to the optimal case.

Numerical Analysis of the Available Power in an Overtopping Wave Energy Converter Subjected to a Sea State of the Coastal Region of Tramandaí, Brazil

The present work proposes a numerical study of an overtopping wave energy converter. The goal of this study is to evaluate the theoretical power that can be converted by an overtopping device subjected to sea waves in the coastal region of Tramandaí, Brazil. For this, realistic irregular waves were generated using the WaveMIMO methodology, which allows numerical simulation of sea waves through the imposition of transient discrete data as prescribed velocity. For the numerical analysis, a two-dimensional computational model was employed using Fluent, where the device was inserted into a wave channel. The volume of the fluid multiphase model was used for the treatment of the air–water interaction. The results indicated that the free surface elevation obtained using the WaveMIMO methodology, which converts a realistic sea state into a free surface elevation series, was adequately represented. The evaluation of the theoretical power of the overtopping device during around 45 min indicated that 471.28 W was obtained. In addition, a monthly generation projection showed that this device would supply 100% of the electricity demand of a school in the city of Tramandaí. These results demonstrated that the conversion of sea wave energy into electrical energy can contribute to supplying electricity demand, especially for coastal cities.

A Fully Coupled CFD-DMB Approach on the Ship Hydroelasticity of a Containership in Extreme Wave Conditions

In this paper, we present a fully coupled computational fluid dynamic (CFD) and discrete module beam (DMB) method for the numerical prediction of nonlinear hydroelastic responses of a ship advancing in regular and focused wave conditions. A two-way data communication scheme is applied between two solvers, whereby the external fluid pressure exported from the CFD simulation is used to derive the structural responses in the DMB solver, and the structural deformations are fed back into the CFD solver to deform the mesh. We first conduct a series of verification and validation studies by using the present CFD–DMB method to investigate the global ship motion, vertical bending moments (VBMs), and green water phenomenon of the ship in different regular wave conditions. The numerical results agreed favourably with the CFD–FEA model and experimental measurements. Then, the extreme ship motions are studied in focused wave conditions to represent extreme sea conditions that a ship may experience in a real sea state. According to the conclusion drawn from the numerical simulations, it is founded that the focused wave case will lead to the increase of the longitudinal responses of the hull compared to regular wave condition, i.e., the heave, pitch, and total VBMs rise about 25%, 20% and 9%, respectively. In focused wave conditions, intensive ship responses and severe waves cause stronger slamming phenomena. It is found that the instantaneous impact pressure from the focused wave is higher and sharper compared to the regular waves and comes along with the obvious green-water-on-deck phenomena.

Numerical Analysis of an Overtopping Wave Energy Converter Subjected to the Incidence of Irregular and Regular Waves from Realistic Sea States

The present study aims to evaluate the difference in the fluid-dynamic behavior of an overtopping wave energy converter under the incidence of irregular waves based on a realistic sea state when compared to the incidence of regular waves, representative of this sea state. Thus, the sea data of three regions from the Rio Grande do Sul coast, Brazil, were considered. Fluent software was employed for the computational modeling, which is based on the finite volume method (FVM). The numerical generation of waves occurred through the imposition of the velocity boundary conditions using transient discrete values through the WaveMIMO methodology. The volume of fluid (VOF) multiphase model was applied to treat the water–air interaction. The results for the water amount accumulated in the device reservoir showed that the fluid-dynamic behavior of the overtopping converter has significant differences when comparing the two proposed approaches. Differences up to 240% were found for the water mass accumulated in the overtopping device reservoir, showing evidence that the results can be overestimated when the overtopping device is analyzed under the incidence of the representative regular waves. Furthermore, for all studied cases, it was possible to approximate the water volume accumulated over time in the overtopping reservoir through a first-degree polynomial function.

A new method for optimal ship design in real sea states using the ship power profile

Typically, ships are designed for calm water and a constant design speed. However, a ship experiences different speeds and sea conditions over her lifetime. Following the significant correlations between environment and ship behavior at sea, efforts are needed to design energy-efficient and safe ships with good performances in realistic sea conditions. This article proposes a method for optimization of the design of a ship with respect to different levels of propulsive power and different operational conditions using a probabilistic approach. The objective of the optimization is the average ship speed, averaged over all propulsion power and environmental condition combinations based on a long-term analysis. The proposed approach is tested on a Post-Panamax container ship operating in the route from Busan to Hamburg port. The decision variables are the ship and propeller main dimensions. Comparisons with traditional methods used in ship design are also presented. Comparing the proposed method with the traditional methods indicates that 7.89% increase in average ship speed is obtained, demonstrating that the proposed method is effective in designing better ships for actual sea states. Better performance in rough sea for the ship designed with the proposed method is also reported.

Enhanced energy harvesting of wave energy converters in site-specific wave climates: A hybrid approach by geometric shape optimization and power take-off control

Wave energy converters are devices that capture the energy carried by ocean waves and convert it into electricity, and their performance is significantly influenced by the geometric shapes of harvesters and the control strategies of power take-off systems. This paper presents a hybrid methodology to improve conversion efficiency in realistic sea states measured from an intended deployment site, taking into account the full coupling of ocean waves, geometric shapes of wave energy harvesters, and control strategies of power take-off systems. The proposed hybrid approach employs a control-strategy-informed optimization routine to select optimal/suboptimal geometries at the design stage, then employs different control strategies for the power take-off system at the operation stage. In numerical case studies, the proposed hybrid methodology is applied to a direct-driven point-absorber type wave energy converter, where its geometric shape is optimized, informing an optimal linear passive control strategy at the design phase. Simulation results indicate that the concave shape has the best energy harvesting performance. A high capture width ratio of 428% is achieved for the found optimum concave shape that employs an optimal linear active control strategy, in a sea state with a significant wave height of 1.6 m and an average period of 5.5 s.

A coupled frequency and time domain approach for hydroelastic analysis of very large floating structures under focused wave groups

This paper presents a Python-based scheme for simulating the hydroelastic response of very large floating structures (VLFS) under extreme conditions which are represented by focused wave groups. Applying the modal expansion method, we decouple the fluid-structure problem into a hydrodynamic problem in terms of velocity potentials for the fluid and a mechanical problem for the structure. The hydrodynamic problem is solved by the higher-order boundary element method in the frequency domain, while the structural analysis is performed by a time-domain finite element model. In order to validate the proposed scheme, numerical simulations are carried out on a scaled model of VLFS in regular and random waves. Both analytical and discrete numerical modal shape functions are used for representing the elastic deformations. The Mindlin plate theory is used for the finite element model. Extensive convergence studies of finite element model meshes, hydrodynamic model meshes and elastic mode numbers are carefully carried out. Good agreements of the vertical displacement, bending moment and shear force amplitudes of the floating plate are found between the predictions of the present model, and experimental data and numerical results in the literature for regular and random waves. The hydroelastic responses of the scaled model in focused wave groups are then investigated. A modified NewWave model is proposed to generate focused pressure in linear wave groups, and the results show the effectiveness of the proposed model. Influence of the focal position, water depth, incident wave angle, and peak spectrum frequency and bandwidth of the focused wave groups on the hydroelastic response of the plate is also studied. The proposed approach is shown promising for hydroelastic analysis for more complex VLFS in realistic sea states.

A frequency-time domain method for annual energy production estimation in floating wind turbines

A new method is proposed to estimate a floating wind turbine’s annual energy production (AEP) using frequency and time-domain design techniques. The approach demonstrated herein estimates the AEP by performing a convolution between the floating platform response and the response power operators (RPOs) that map the average power produced by the turbine as a function of the amplitude and frequency of the platform motions. One advantage of this approach is that it can be performed early in the conceptual design phase to help discover design space trade-offs between the platform and rotor design. The methodology is applied to the IEA Wind 15 MW WindCrete spar-buoy model using OpenFAST. The RPOs are obtained by prescribing single-DOF platform motions to the turbine with a given amplitude and frequency. This methodology is then validated by comparing the AEP estimation from the RPOs with the AEP estimation from fully-coupled simulations. The results indicate that the method is able to estimate the value of AEP for a realistic sea-state and regular waves. However, further validation is needed as, in the first case, the turbine is moving too little and, in the second case, the contribution of the controller may be dominant.

Study on the wave energy capture spectrum based on wave height take-off

With the progress in wave energy research, large-scale development has become a trend. In real sea states, the energy extraction characteristic of each unit is significant to the array's composition. Only focusing on the mean absorbed power for optimization cannot provide guidance for realizing the internal mechanism of the energy capture behavior. Therefore, by extending the concept of Wave Height Take-off (WHTO) to random waves, this paper proposed and defined the Wave Energy Capture Spectrum (WECS), which derives from the change of wave height to describe the energy extraction characteristics of wave energy converters in real sea states. Simulations were conducted in a validated 2D numerical wave flume with a cuboid heaving buoy under different sea states. Linear Power Take-off (PTO) damping coefficient was taken as an example for a parametric study of the WECS. The results verified the concept and showed that the optimization method of WECS can reveal the capture characteristics of the device from both the perspective of frequency and energy. The comparison of capture efficiency between WECS and the conventional PTO method indicated that the difference is not significant, which also examined the method. This manuscript aims to validate the concept and promote the method.

CFD-based hydrodynamic analyses of ship course keeping control and turning performance in irregular waves

Ship manoeuvrability studies are usually carried out in calm and regular seas; however, an irregular sea state can better present the real operational conditions at sea, compared to both calm and regular seas. In this study, the manoeuvrability of a well-known benchmarking ship in an irregular sea state was investigated using a fully nonlinear unsteady RANS model. The JONSWAP spectrum was used to generate long-crested irregular seas with a significant wave height of 5m and a peak period of 12.4s in full scale, representing sea state 6. Comparisons with the ship manoeuvrability in both calm and regular seas were also made with a view to identifying the changes in the manoeuvring characteristics of the ship. The generated regular waves were characterised by the height and period equivalent to the average height and period of the irregular waves applied in this work. The ship is equipped with a single propeller with a semi-balanced horn rudder, and the propeller is simulated by an actuator disk model based on the body force method. In analysing the correlations between the ship manoeuvrability and the irregular waves, the findings of this study have demonstrated that the irregular waves may cause substantial changes in the course keeping capability and turning performance when compared to the inherent manoeuvring qualities in calm water. It is expected that the results of this work can provide a deeper insight into ship manoeuvrability in irregular waves as well as help masters and navigation officers in decision-making for ship handling actions in real sea states.

Research on the statistical characteristic of freak waves based on observed wave data

Freak waves are characterized by high wave, great destructive power, abrupt behavior, and a significant threat to offshore structures. In this study, the observed wave data of multiple sea areas are utilized, the relation between the occurrence of freak waves at real sea states and the statistical characteristics of wave trains are investigated, and the probable reasons for freak wave generation at real sea conditions is explored. It can be found that for the wave trains with higher kurtosis, the occurrence probability of huge wave height is underestimated by the Rayleigh distribution. It can be accurately predicted by the MER distribution and GC distribution. Generally, the occurrence probability of freak wave increases linearly with the increase of kurtosis from the statistical results, and the MER model is capable of describing the relation between occurrence probability of freak wave and kurtosis. Moreover, the generation types of freak waves are analyzed based on the wavelet energy spectrum and time process of wave surface for typical freak waves in the observation data. Freak waves at real sea states are attributed to the modulation instability and wave energy superposition.

Numerical study of a point absorber wave energy converter with different power take-off systems

Wave energy resources are huge and widely distributed all over the world. Wave energy converters (WECs) are employed to harness this energy. In this paper, a heaving point absorber WEC with three kinds of Power Take-Off (PTO) systems is investigated. A fully nonlinear numerical model of the WEC has been developed, taking into consideration viscosity. A Computational Fluid Dynamics (CFD) analysis is performed in order to determine the quadratic viscous term coefficient. A physical model test has also been carried out and used to validate the numerical model. The response and energy capture characteristics of the WEC with various PTO systems are analyzed in both regular and irregular wave. The differences among the WECs with different PTOs are assessed. Finally, the annual wave power assessment and suggestions for optimizing the WEC in the real sea state are given.

Unsteady RANS CFD simulations of ship manoeuvrability and course keeping control under various wave height conditions

Ships encounter different wave heights in real sea states. Various wave heights can lead to substantial changes in the behaviour of a ship during manoeuvring, and hence a ship's manoeuvrability in waves should be accurately evaluated to ensure navigational safety at sea. The aim of this study is to estimate wave height effects on the manoeuvring behaviour of the benchmarking KRISO Container Ship (KCS) model by performing a fully nonlinear unsteady RANS simulation. Three types of manoeuvres were simulated with the free-running KCS model appended with an actuator disk and a moving semi-balanced horn rudder using a dynamic overset technique: self-propulsion, course keeping control, and turning circle manoeuvres. Manoeuvring analyses were carried out in the bow quartering waves of a range of wave heights for constant wave length, as coupled with the fifth-order Stokes wave model. In order to control the rudder deflection based on manoeuvre mechanisms, a feedback controller was embedded in the present CFD model. The results clearly revealed that wave heights have a strong effect on the manoeuvring behaviour of the KCS, including ship's speeds, seakeeping behaviour, and critical turning indices through comparative analyses under different wave height conditions. One important finding is the standard manoeuvring parameters in waves, which would help navigation officers in decision-making for manoeuvring actions in waves.

Numerical Evaluation of the Hydropneumatic Power of the Oscillating Water Column Wave Energy Converter Submitted to Regular and Irregular Waves

The purpose of this study is to computationally analyze the hydropneumatic power available in the air duct of an Oscillating Water Column (OWC) Wave Energy Converter (WEC) device when subject to realistic sea state data (irregular waves) and when submitted to the regular waves representative of this sea state. The OWC WEC is mainly composed of a hydropneumatic chamber and an air duct where a turbine and electric generator are coupled. The chamber is open below the free surface while the duct is open to the atmosphere. The oscillating movement of the water-free surface inside the chamber causes the air to flow, moving the turbine and generating electricity. To execute this study, a bi-dimensional computational model was considered and numerical simulations of wave generation were carried out using ANSYS Fluent, which is a Computational Fluid Dynamics (CFD) software based on the Finite Volume Method (FVM). The Volume of Fluid (VOF) multi-phase model was applied in the treatment of the water-air interaction. To evaluate the average hydropneumatic power available in the duct, the static pressure, velocity, and air mass flow rate were monitored. The results were analyzed, showing that the available power is 250% greater when the device is subject to realistic irregular waves rather than subject to representative regular waves.

WaveMIMO Methodology: Numerical Wave Generation of a Realistic Sea State

This paper presents a methodology that allows the numerical simulation of realistic sea waves, called WaveMIMO methodology, which is based on the imposition of transient discrete data as prescribed velocity on a finite volume computational model developed in Fluent software. These transient data are obtained by using the spectral wave model TOMAWAC, where the wave spectrum is converted into a series of free surface elevations treated and processed as wave propagation velocities in the horizontal (x) and vertical (z) directions. The processed discrete transient data of wave propagation velocity are imposed as boundary conditions of a wave channel in Fluent, allowing the numerical simulation of irregular waves with realistic characteristics. From a case study that reproduces the sea state occurring on March 31st, 2014, in Ingleses Beach, in the city of Florianopolis, state of Santa Catarina, Brazil, it was concluded that the WaveMIMO methodology can properly reproduce realistic conditions of a sea state. In sequence, the proposed methodology was employed to numerically simulate the incidence of irregular realistic waves over an oscillating water column (OWC) wave energy converter (WEC). From these results, the WaveMIMO methodology has proved to be a promising technique to numerically analyze the fluid-dynamic behavior of WECs subjected to irregular waves of realistic sea state on any coastal region where the device can be installed.