Feasibility study of a flexible floating solar concept as energy supply for Sleipnir during operations

Abstract

The electricity demand increases globally and requires a shift toward renewable sources to prevent the exhaustion of the planet. The shipping industry is responsible for 2-3 % of the global Greenhouse Gas emissions and Heerema Marine Contractors (HMC) identified floating solar as a promising solution to reduce the emissions, of their crane vessel Sleipnir, during operations. This study is the starting point of a technical feasibility study as a temporary energy supply for Sleipnir. The design is strongly focused on the temporary deployment and limited occupied deckspace during transit. Current floating solar systems are commonly designed for permanent deployment as supporting structure for rigid glass photovoltaic (PV) panels. Furthermore, modular constructions are used to limit the transportation costs however they are not designed for temporary deployment. A new PV innovation is flexible lightweight films which allow a more flexible supporting structure. A flexible thin sheet can be spooled on a drum to make temporary deployment possible. The influence of the wave loading on the coupled hydrodynamic behaviour is evaluated since wind and current loading are predictable based on previous research.

The structural design parameters of the thin sheet and drum are designed to mimic the excitation motion since wave structure interaction has been minimized to reduce the mooring force. Therefore, the draft must be low and the characteristic length related to the bending stiffness of the sheet should be smaller than the excitation wavelength. The draft of the drum should be low to have a natural heave frequency higher than the excitation frequency.

The coupled hydrodynamic response for head loading is evaluated with model tests in a towing tank. The concept is scaled according to Froude to ensure the surface waves, which are gravity-driven, are properly scaled. Regular waves are chosen based on the workability wave spectrum of Sleipnir. The roll and heave response over the frequency domain is indicated by analyzing the stable response at certain frequencies. The motions of the drum are obtained with the use of object tracking based on video recordings. The force within the connection of the system was measured with a force transducer whereas the mooring force was measured with a newly developed 3D-sensor.

It turns out that the heave motion of the system mimics the excitation motion over the wavelengths resulting in small drift forces. Significant rotations of the drum were observed for the longer wavelengths leading to water pumping over the sheet. The overturning moment is driven by the dynamic pressure over the drum diameter and the measured force in the connection generates a counteracting moment. The connection force is proportional to the buoyancy required to submerge the sheet and the acceleration of the free-floating sheet.

The feasibility of an OFPV concept for Sleipnir is demonstrated but the rotations have to be reduced by lowering the natural roll frequency. The drum dominates the coupled hydrodynamic behaviour compared to the sheet. Either the dimensions of the drum should be lowered or the thickness of the sheet must be increased. Decreasing the drum diameter is favourable over a thicker sheet since that would increase the characteristic length. Another option is to adjust the geometry of the drum to a shape where increased water displacement is required for the roll motion.