Modeling and Dynamic Simulation of a Hybrid Wind-Wave System on a Hexagonal Semi-Submersible Platform

TL;DR

The paper addresses the challenge of jointly harvesting wind and wave energy by proposing a hybrid floating platform that integrates a central wind turbine with three flap-type WECs on a hexagonal semi-submersible. The authors develop an integrated modeling pipeline using Capytaine for wave–body interaction, WEC-Sim for wave-energy damping and dynamics, and MOST for aero–hydro–mooring co-simulation, under site-specific wind and wave data. Key findings show that slurry ballast yields robust hydrostatic stability across flap configurations, while geometry (notably flap dimensions and tower length) strongly influences stability, energy capture, and structural loads; time-domain analyses reveal how wave incidence angle affects power sharing among flaps, and flap-angle sweeping can modulate platform pitch. Annual energy production estimates indicate wind contribution of 16.86 GWh and wave contribution of 3.65 GWh, with wave energy accounting for about 18% of total output, highlighting the practical potential of integrated wind–wave offshore platforms and guiding future optimization and advanced control work.

Abstract

Offshore renewable energy systems offer promising solutions for sustainable power generation, yet most existing platforms harvest either wind or wave energy in isolation. This study presents a hybrid floating offshore platform that integrates a wind turbine with three oscillating surge wave energy converters (WECs) into a hexagonal semi-submersible structure. In this configuration, the flaps are integrated with the platform geometry to provide both energy extraction and hydrodynamic stability. A modeling and simulation framework was developed using WEC-Sim and benchmarked against the NREL 5 MW semisubmersible reference. Metacentric height analysis confirmed hydrostatic stability across a range of prescribed flap angles. Sensitivity analysis of twelve geometric variables identified flap dimensions and tower length as dominant drivers of stability, energy capture, and tower stress. Time-domain simulations revealed dependence on wave incidence angle, with variations in flap power sharing, capture width ratio (CWR), and platform response. The feasibility of using flap sweeps to modulate pitch motion was also demonstrated. Annual energy production (AEP) estimates based on site-specific data indicate 16.86 GWh from wind and 3.65 GWh from wave energy, with WECs contributing about 18% of the total. These results highlight the potential of integrated wind-wave platforms and point toward future studies on structural modeling and advanced control.

Figures (17)

• Figure 1: Top views of platform footprints used for scaling. (a) NREL 5 MW semi-submersible reference platform. (b) Proposed hexagonal platform with equivalent footprint area. (c) Overlay comparison of both platforms, shown to the same scale.
• Figure 2: Geometric variables of the hybrid hexagonal platform. (a) Full assembly showing the hexagonal platform, central cylinder, wind turbine tower, rotor–nacelle assembly, and three flaps. (b) Key geometric variables of the platform, including side length ($l_s^p$), width in the $xy$-plane ($w_{xy}^p$), and central cylinder diameter ($d_c^p$). (c) Dimensions in the $xz$-plane, including freeboard height ($z_{fr}^p$), draft ($z_{dr}^p$), and platform height in the vertical direction ($w_z^p$); the flap is mounted 2.38 m above the platform to prevent collision during rotation. (d) Tower geometry, including base and top outer diameters ($d_b^t$, $d_t^t$), base and top thicknesses ($b_b^t$, $b_t^t$), and total tower length ($l_t$). (e) Flap geometry including width ($w^f$), height ($h^f$), and length ($l^f$). Superscripts $p$, $f$, and $t$ denote platform, flap, and tower, respectively.
• Figure 3: Examples of automatically generated boundary-element method (BEM) meshes for the hybrid platform and flap system under different geometric design configurations. The meshes are created using a Python-based procedure within Capytaine, allowing rapid updates in response to changes in geometric variables from the platform vector $\mathbf{x}_p$ (e.g., platform diameter) and the flap vector $\mathbf{x}_f$ (e.g., flap length), and include only the submerged portions of the structures, which are relevant for hydrodynamic analysis.
• Figure 4: Flowchart for the buoyancy and mass allocation procedure used before the BEM solver. The platform ($x_p$), flap ($x_f$), and tower ($x_t$) design vectors are inputs that determine buoyancy balance, total required mass, and its distribution between platform and flaps, while ensuring the metacentric height ($GM$) remains positive.
• Figure 5: Examples of flap and platform rotation configurations used to compute metacentric heights and evaluate hydrostatic stability. The top row shows configurations with flap angles of $-65^\circ$, $0^\circ$, and $-40^\circ$, while the platform pitch varies from $0^\circ$ to $15^\circ$ and $-15^\circ$. The bottom row shows flap angles of $40^\circ$, $-50^\circ$, and $30^\circ$, with platform pitch angles of $0^\circ$, $10^\circ$, and $-10^\circ$. Each image illustrates the submerged portion of the hybrid platform under distinct combinations of flap and platform rotations.