Pull and Lock Marine (PALM)

Overview

The Pull and Lock Marine (PALM) connector proposes robust, reliable and repeatable connections of the Wave Energy Converter (WEC) in a short, simple marine operation.

The PALM is based around a submerged buoy mooring with an integrated electrical riser. Tugs are used to position the WEC, engage the PALM and complete the mechanical and electrical connection. The core innovation is the design of a formed plug component and catcher arm. Coarse and fine guide slots ensure tight alignment, so that electrical connection can be completed simultaneously. The resulting linkage is fully articulated, while riser tension ensures continuous connection in all sea states.

The PALM achieves a robust, close fit mechanical and electrical connection in a single pulling action from a tug's winch. It comprises an upper ‘receptacle’ attached to the WEC, and a lower ‘plug’ element on top of a mooring system. During connection, the tug pulls the plug through a series of guides inside the receptacle via a pre-reeved fairlead. These orientate and direct the plug into its locked position, while electrical connection is achieved through wet-mate stabs. Disconnection is similarly achieved by a single winch wire pull.

Hinged joints release the required degrees of freedom for operations and the system is designed to be rugged for harsh environments. The central innovation is the passive mechanical operation of effecting the connection or disconnection. No hydraulic or servo actuators are required. The reliance only on tug action and proven mechanical components draws on learning from other sectors. It is anticipated that the system may also be applicable in floating wind and tidal stream devices.

All components are based on proven marine technology. The number of moving parts is kept to a minimum, and load-bearing components are rugged.

In Stage 1, Apollo assessed the technical and commercial feasibility of the PALM connector. We evolved the mechanism with engineering calculations and 3D modelling, while making initial decisions on materials and fabrication options. CAPEX and OPEX cost estimates were prepared and an overall risk assessment undertaken. The feasibility report includes a concept specification.

Stage 2 activities include:

• Development of the PALM for specific WEC designs, including design modelling, drawings and engineering calculations.
• Working with manufacturers to specify electrical connectors and slip rings.
• Survival analyses of moorings, mechanism and WEC interface.
• Analysis of operational methods, seastate limits and potential workability.
• Investigation into materials, coatings, and cathodic protection.
• Component-level FMECA.
• Detailed operational plans, including storyboards and HIRA.
• A development strategy and cost model for future commercialization.
• A plan for the stage 3 including the physical testing of a prototype.

The analysis considered two test case implementations of the PALM on actual wave energy converter (WEC) designs. These were:

• Mocean’s Blue X device, which includes a combined mechanical and electrical connection mounted underneath the WEC.
• Carnegie’s CETO 6 device, with a mechanical seabed-mounted connection that experiences much higher mechanical loads but does not include an electrical connection.

Stage 3 saw the design, build, and testing of a demonstrator PALM prototype. The testing regime has demonstrated the mechanical functionality and trialled the marine operations.

The ultimate aim of the PALM development project is to propose a solution that will lower the LCOE for grid-scale WECs and potentially other pieces of offshore energy infrastructure, such as offshore wind installations.