Share reportStructural Materials
This programme investigated if it is possible to make a step change impact in LCOE by constructing typical WEC devices from alternative materials to those traditionally used, such as steel.
Lead contractor
Tension Technology International Ltd
Sub-contractor(s)
Black & Veatch Ltd
Quoceant Ltd
Optimus (Aberdeen) Ltd
University of Strathclyde
TTI Testing
Griffon Hoverwork Ltd
Kelvin Hydrodynamics Laboratory
International Centre for Island Technology (Heriot Watt University - ICIT
Orkney Campus)
PSG Marine & Logistics Ltd
Overview
The NetBuoy concept encompasses impermeable fabric buoyant modules encapsulated by fibre rope ‘load nets’. The buoyant modules form a compliant, load shedding, peak load resistant, primary structure while the ‘load nets’ apply distributed restraint loads that are agglomerated to one or more structural points, connecting the NetBuoy to the other parts of the WEC. The combination reduces the overall structural mass significantly (compared to an equivalent steel structure), while the inflatable nature of the buoyant module affords significant advantages at the manufacturing, transportation and installation phases.
A qualification process has been followed to address identified technical risks. This involves laboratory material and fatigue tests and has culminated in an extended sea trial of a NetBuoy system.
Scale-model tests demonstrate the applicability of the NetBuoy system to a broad range of wave energy converter architectures.
More information on the NetBuoy system can be found on a dedicated website www.netbuoy.co.uk which includes a design tool for WEC developers to assess the suitability of and cost estimate for a NetBuoy based prime mover when integrated into their technology of interest.
Stage 1
May 2018
The NetBuoy project focusses on two strands on the path towards cost competitive wave energy – impermeable fabrics to provide compliant and thus load shedding/peak load resistant buoyant modules and fibre rope ‘load nets’ to encapsulate the buoyant modules, applying distributed restraint loads and agglomerating the distributed load back to a single or number of structural points to connect to the other parts of the WEC system such as the PTO. The load net is seen as essential in enabling the use of fabric buoyant modules as they cannot easily be restrained otherwise – the restraint must be distributed over the surface of the buoyant module.
The WEC technology case study for this project has been a generic ground referenced heaving point absorber. Three scales of Netbuoy were considered at the concept design stage with characteristic swept volumes of 10m3, 100m3 and 300m3. The medium sized Netbuoy100 was ultimately chosen for more detailed engineering and cost assessment This was benchmarked against a steel equivalent buoy to demonstrate the cost benefits and potential impact on LCoE, using the WES costing tool. TTI also consider tow forms of Netbuoy 1) assuming machine room is on the seabed 2) assuming machine room is integrated and forms part of the Netbuoy.
Stage 2
May 2020
The Stage 2 NetBuoy project encompassed two areas Tension Technology International Ltd (TTI) sees as being key to the path towards cost competitive wave energy – impermeable fabrics to provide compliant and thus load shedding/peak load resistant buoyant modules, and fibre rope ‘load nets’ to encapsulate the buoyant modules, applying distributed restraint loads and agglomerating the distributed load back to a single or number of structural points to connect to the other parts of the WEC system such as the PTO.
The medium-sized NetBuoy100 was ultimately chosen for more detailed engineering and cost assessment. This was benchmarked against a steel equivalent buoy to demonstrate the cost benefits and potential impact on LCoE (Levelised Cost of Energy), using the WES costing tool.
Stage 3
March 2022
NetBuoy integrates two enabling technologies for cost competitive wave energy – impermeable membranes to manufacture buoyant modules and fibre rope nets to encapsulate the buoyant modules. The net applies distributed restraint loads and agglomerates them back to structural ‘hard’ point(s). This is essential in enabling the use of membrane buoyant modules as they cannot be restrained otherwise.
This combination significantly reduces prime mover mass when compared to a steel; the density of the materials are around one-seventh that of steel and are much more compliant with strain at break of around 15% for the membrane. This compliance means it can locally deform under extreme waves whereas the steel structure must carry excess mass (and therefore cost) to provide the required rigidity to avoid plastic deformation.
Stage 3 has progressed the NetBuoy system to a high Technology Readiness Level and demonstrated its suitability for integration into commercial scale wave energy collector (WEC) technology. A qualification plan, addressing the residual technical risks identified at the end of Stage 2, was executed and included an extended sea trial of a NetBuoy system (to assess the effect of long-term exposure to seawater, environmental loading, biofouling and UV and ozone), laboratory material and fatigue tests, and further scale-model tests to demonstrate the applicability of the NetBuoy system to a broad range of WEC architectures. Further design engineering activities resulted in further improvement in the cost of energy.
The inflatable module affords significant advantages in all phases of the lifecycle. The buoyant pods can be manufactured globally in an existing supply chains. The deflated buoyant pod can be cheaply transported in standard shipping containers. CAPEX and LCOE calculations show very encouraging potential for step-change cost reductions.
The long-term vision is the deployment of large WEC arrays utilising the NetBuoy for their prime mover water plane, swept volume and buoyancy requirements.
More innormation on the NetBuoy system can be found on a dedicated website https://www.netbuoy.co.uk which includes a design tool for WEC developers to assess the suitability of and cost estimate for a NetBuoy based prime mover when integrated into their technology of interest.
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