Abstract UFOWTs (Unmoored Floating Offshore Wind Turbines) are a new concept of floating offshore wind turbine in which mooring lines are replaced by thrusters. This approach has several advantages, e.g it may open exploitation of far offshore wind, but only if the thrusters power consumption is kept low enough. This research aims at estimating the annual production of UFOWTs. To do so, a steady state model is implemented, taking into account environmental loads from wind and waves, based on realistic meteocean data from ERA-5 database. The results are promising, as they indicate that a UFOWT can achieve 56 % to 69 % of the production of a conventional FOWT depending on how far the UFOWT is allowed to drift away from its initial position.

This study investigates the dynamic behavior and power performance of an energy ship designed to harvest offshore wind energy through wind propulsion and onboard hydrokinetic turbines. A nonlinear dynamic model, validated at model scale, is used to assess the influence of waves and wind gusts on power generation. Simulations accounting for wave effects show that, while wave-induced dynamics generate power fluctuations of up to approximately 2%, their impact on mean power production remains limited, with a reduction of less than 1%. The most constraining conditions are associated with wave periods close to the surge resonance of the vessel. Simulations taking into account wind gusts reveal that transient overproduction may arise under nominal operation, exceeding up to 120% the maximum allowable power. Thus, a combined control strategy-adjusting both the hydrokinetic turbine rotational speed and ship heading is introduced. It is shown that it can successfully maintain power, torque, and rotational speed within safe operating limits, while enabling an energy gain of 6-10% during gust events. The results highlight the need to consider unsteady environmental effects when designing energy ship control systems and demonstrate the effectiveness of the proposed strategy in ensuring safe and efficient operation under realistic sea conditions.

Abstract. This paper investigates the effect of rotor design on energy performance and cost of a Stationary Unmoored Floating Offshore Wind turbine (SUFOWT). A SUFOWT is a Floating Offshore Wind Turbine (FOWT) for which a dynamic positioning (DP) system is used in lieu of a mooring system for station-keeping. It is particularly well suited for deployment in the far-offshore. Previous studies have shown that positive net power production can be achieved with SUFOWTs depending on number and size of thrusters, and wind turbine characteristics. However, they did not consider rotor design. This gap is addressed in the present paper. The study is based on a physical engineering model. The wind turbine rotor design is represented by its rated induction factor. Results show that the optimal rated induction factor is smaller than the usual value of 1/3 both from the perspective of energy performance and cost of energy. Thus, wind turbine rotors designed for SUFOWTs should be developed to optimize their cost. However, results show that the cost of energy reduction is somehow limited, of the order of 2.5 to 4.3 % for the considered designs.

This paper investigates the effect of rotor induction and peak shaving on energy performance and cost of a stationary unmoored floating offshore wind turbine (SUFOWT). A SUFOWT is a floating offshore wind turbine (FOWT) for which a dynamic positioning (DP) system is used in lieu of a mooring system for station-keeping. It is particularly well suited for deployment in the far-offshore. Previous studies have shown that positive net power production can be achieved with SUFOWTs depending on the number and size of thrusters and on wind turbine characteristics. However, they did not consider the effect of rotor induction or peak shaving. This gap is addressed in the present paper. The study is based on a physical engineering model, the wind turbine rotor design being modelled using the actuator disc theory. Results show that the rotor induction which maximizes net power production (which takes into account the thrusters' power consumption) is smaller than the value of 1/3 which maximizes wind turbine power production. However, the increase in the annual energy production or capacity factor brought by rotor induction optimization is rather small, of the order of a few percent. The effect of peak shaving was also found to be small with respect to energy production and capacity factor. Both rotor induction and peak shaving were found to be able to significantly reduce the power ratio (the ratio of the thrusters' nominal power to the wind turbine rated power), which can be expected to be beneficial for the cost of energy.

This article investigates the impact on power production of an energy ship of two types of heading controllers. The first controller includes a filter which eliminates the wave-frequency dependent motion of the ship, while the other aims at compensating the effects of waves on the motion of the ship. The study is based on a numerical model of the energy ship which is presented in the paper. Results show that waves compensation reduces the generated power variation but increases rudder oscillations. In the opposite, the waves filtering controller allows for a slightly greater power production at the expense of greater power variations.

Potential flow theory-based solvers are commonly used in ocean engineering to investigate the interactions between ocean waves and floating bodies. Depending on assumptions, several methods have been proposed. Among them, the Weak-Scatterer method is an interesting trade-off in the sense that this approach is not limited in theory by the small wave amplitudes and small body motions assumptions of linear methods. Moreover, this approach is in practice more stable than the fully non-linear methods. An implementation of the Weak-Scatterer method is the WS-CN code (Letournel, 2015; Chauvigné, 2016; Wuillaume, 2019).The computational time of the WS-CN code which is considered in the present study is relatively long for engineering purposes. In order to reduce it, the present paper presents an implementation of the Parareal method in the WS-CN code. The Parareal method is an algorithm for parallelizing a simulation in time that can accelerate the complete simulation (Lions, 2001) . This is a key difference in comparison to other acceleration techniques which have been studied in the literature (e.g. the Fast Multipole Method (FMM), the precorrected Fast Fourier Transform (pFFT) method, … ). To the authors’ knowledge, the present study is the first to couple the Parareal method to a potential flow theory-based wave-structure interaction solver. It is shown that the method can significantly reduce the computational time for small wave steepness, but that the performance decreases rapidly with increasing steepness.

This article presents the principles and technologies for converting the main renewable ocean energy resources (ocean thermal energy, wave energy and tidal energy) into electricity. Their energy production potential is put into perspective in terms of the challenges of the energy transition.

Dans cet article, on présente les principes et technologies permettant de convertir en électricité les principales ressources d’énergies océaniques renouvelables (énergie thermique des mers, énergie des vagues et énergie marémotrice). Leur potentiel de production d’énergie est mis en perspective par rapport à l’enjeu de la transition énergétique.

This paper deals with the seakeeping of a catamaran propelled by Flettner rotors. The case study is an energy ship. The prediction of the roll motion of such a ship is of particular interest since she will mostly sail in beam seas. We used a numerical model based on the boundary element method to deal with the interactions of the ship with waves. It is supplemented by a model for the aerodynamic damping effect due to the rotors, the gyroscopic effects from the rotors and by the ITTC correction for roll damping. In the present study, it has been assumed that the Flettner rotors can be modelled as a distribution of elementary airfoils whose lift and drag depend on the local apparent wind speed. Interaction effects between the rotors and between the hull and rotors have been neglected. Regular waves have been investigated. For the considered case study, it is found that the Flettner rotors can have a small destabilizing effect or a small stabilizing effect on the ship motion depending on the ship speed and the rotors’ rotational speed.

This article presents a wave energy converter exploiting the pitch of a floating body moored to the seabed. When the floating body tilts under the action of an incoming wave, a movable mass, placed inside the hull, moves relative to the floating body and actuates an electrical generator. Most devices of this type have the drawback that the moving mass sequentially accelerates, slows down, stops and then repeats this sequence in the opposite direction. This generates an irregular instantaneous power output. The proposed concept consists of (at least) two eccentric bodies having the same mass and revolving atopposite speeds around a vertical axis. In this « counterrotating » solution, the oscillations of the float result in the continuous circular motion of the direct-drive PTO, though the global centre of gravity of the eccentric bodies moves back-and-forth along the symmetry axis of the device. If the eccentric bodies move at constant speed, their global centre of gravity moves in a sinusoidal manner along its pathway. The present study aims to investigate, through modelling and numerical simulations, the influence of the main parameters, such as the phase and the PTO mass moment, on the performance of a counter-rotating device exposed to waves of various heights and various wavelengths. Optimal phase and mass moment are determined numerically. The resulting output power is close to the theoretical maximum power that can be harvested by the floating body.

Wave energy converters (WECs) energy production estimates are key metrics for performance predictions. This study compares four methods for energy production assessment: power matrix, interpolated power matrix, capture length matrix, and a reference method based on the exact omnidirectional spectra for every sea state. Two deployment sites are considered and their wave resource is derived from hindcast databases. The WEC chosen for this study is a two-body self-referenced heaving device characterised using a boundary element method (BEM) numerical model run in time-domain and accounting for some non-linearities. The model also includes power take-off capping, in terms of power capacity and a force cap, independently. A novel metric is introduced to assess the shape similarity between two spectra and it is used to assess the impact of approximating raw spectra with standard ones on energy production estimates. The study shows that the power take-off capping approaches and values and the way the exact resource spectra are approximated have a significant impact WEC energy estimation methods accuracy. Indeed, relative differences in yearly production estimates with respect to the benchmark method vary from 2.4% to 8.3% across capping values and estimation methods. It also shows that there is little difference in yearly averaged energy production estimates between the different “matrix based” methods. These differences are of the order of tens of percent for a given power take-off capping configuration and a given site.

Optimizing the production of wave energy converters using Model Predictive Control (MPC) requires a real-time, deterministic prediction of the waves arriving at the device. This study presents a new method for deterministic sea wave prediction, using the horizontal velocity profile over the water column as a boundary condition for a dedicated nonlinear wave model. However, direct measurement of the horizontal velocity component over the whole vertical column is hardly achievable at sea. A method to reconstruct this profile from measurement devices currently at use, such as ADCPs, is thus presented and evaluated. The performance of the prediction method itself is then tested using synthetic numerical data. First, the reconstruction of the horizontal velocity profile as a boundary condition is evaluated. Then, the whole prediction procedure is assessed. In both these stages, the simulations are based on synthetic numerical data and the outcomes are compared with numerical reference solutions. The results show that the method is promising enough to justify further investigation through wave tank experiments.

In this paper, a method is introduced which enables the forcing of any degrees of freedom in 6 DoFs ship simulator. It is based on the introduction of an extra force in the equation of motion of the ship and on the forcing of the second derivatives of the forced degrees of freedom rather than the forced degrees of freedom themselves. The method is explicit which makes it easy to implement in existing software. Examples of its application to oblique towing tests and forced heading in wind and waves are presented.

Wind propulsion is envisioned as one of the solutions for the decarbonisation of maritime transport, as it offers high efficiency in terms of primary energy comsumption. Many wind propulsion systems already exist at various development stages, but the uncertainties over their performance is a strong obstacle to their adoption by ship owners. This paper presents a general method for the assessment of steady and unsteady performances of wind- propelled ships with 6 degrees of freedom, as implemented in the open-source program xWASP_CN. Inspired by system-based modelling, the method consists in the independent modelling of the forces acting on the ship, as functions of the ship’s 6 degrees of freedom and environmental conditions. An original root-finding algorithm that leverages the specifics of the physical problem to find the steady equilibrium is presented. The method works either like a Power Prediction Program (PPP) or like a Velocity Prediction Program (VPP). As a PPP, the forward speed and course are fixed while the required propulsive power, leeway angle, heel, trim and sinkage are solved. As a VPP, only the course is fixed and the attained forward speed, leeway angle, heel, trim and sinkage are solved. This makes the method suitable for both hybrid propulsion and pure wind propulsion. The force models can be semi-empirical (usually requiring very little input data), based on preliminary experimental or numerical results (such as forward resistance curves, lift/drag coefficients and frequency-domain sea-keeping coefficients), or full-fledged flow solvers (e.g. potential theory, CFD). Thus the method is suitable for all design stages as each force can be modelled with several levels of accuracy depending on the input data available. Comparisons of an intermediate-level model with experiments on a 18-ft catamaran fitted with a Flettner rotor and a water turbine show good agreement for steady-state results.

This paper deals with a new concept for the conversion of far-offshore wind energy into sustainable fuel. It relies on autonomous sailing energy ships and manned support tankers. Energy ships are wind-propelled ships that generate electricity using water turbines attached underneath their hull. Since energy ships are not grid-connected, they include onboard power-to-X plants for storage of the produced energy. In the present work, the energy vector X is methanol. In the first part of this study, an energy ship design was proposed, and its energy performance was assessed. In this second part, the aim is to update the energy and economic performance of such a system based on design progression. In collaboration with ocean engineering, marine renewable energy and wind-assisted propulsion experts, the energy ship design of the first part has been revised. Based on this new design, a complete FARWIND energy system is proposed, and its costs (CAPEX and OPEX) are estimated. Results of the models show (i) that this FARWIND system could produce approximately 70 000 t of methanol per annum (approximately 400GWh per annum of chemical energy) at a cost in the range EUR1.2 to 3.6/kg, (ii) that this cost may be comparable to that of methanol produced by offshore wind farms in the long term and (iii) that FARWIND-produced methanol (and methanol produced by offshore wind farms) could compete with gasoline on the EU transportation fuel market in the long term.

A consistent frame for the numerical simulation of lowering operations is investigated in this paper from a new wave-structure coupling. The mechanical modeling is based on the Composite-Rigid-Body Algorithm, which is able to simulate the nonlinear dynamics of multibody systems. The hydrodynamic model is based on the weak-scatterer approach, which allows the computation of unsteady hydrodynamic loads without being limited by the classical hypotheses of the linear potential flow theory. The coupling of these two models leads to the numerical simulation of articulated multibody systems with large relative motions in waves. The coupling equation is derived in this paper. This new numerical modeling is compared to the classical linear potential flow theory in the case of a lowering operation with a payload in the water. The impact of the lowering velocity is studied. Results show that this new model matches the classical approach for small lowering velocities but as soon as nonlinearities arise, differences between the two models appear.