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Unique system for generating electricity from ocean waves

Connect, no. 160, May/June 2009, p.3

Pelamis WEC during sea trials Photograph courtesy of Pelamis Wave Power Ltd
Pelamis WEC during sea trials Photograph courtesy of Pelamis Wave Power Ltd

Edinburgh-based Pelamis Wave Power produces a unique system for generating electricity. TWI's design review of a key component from the next generation of Pelamis wave energy converter focused on maximising manufacturing efficiency and operational endurance.

Held in position by a mooring system, the Pelamis Wave Energy Converter (WEC) is a semi-submerged, articulated structure composed of cylindrical sections, linked by hinged joints. The wave-induced motion of these joints is resisted by hydraulic rams, which pump high-pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive generators to produce electricity, which can be fed to the National Grid via a seabed cable. Several WEC scan be connected to provide more power.

Pelamis Wave Power is working on its next generation of WEC and wanted to ensure that the design was fully optimised for the cylindrical sections that fit between the power-producing hinged joints. The company asked TWI to carry outa design review through Joining Forces Scotland: this was to focus on maximising manufacturing efficiency and fatigue strength.

Like most fabrications for offshore operation, the cylindrical sections are subject to fatigue loading.

Ensuring that the proposed design provides good fatigue performance and achieves the required design life involved detailed consideration of a number of factors including:

  • the effect of weld residual stresses, treatment of compressive stress in the fatigue cycle;
  • treatment of low stress cycles in fatigue assessment
  • post-weld techniques for improving fatigue performance
  • development of custom S-N curves
  • use of computer-generated stress data

Charles Taylor, Structural Engineering Manager at Pelamis Wave Power, said 'Joining Forces Scotland is an excellent mechanism for accessing TWI's professional technical support, which will directly benefit our design and manufacturing processes.

'Our experience has been very positive and we anticipate seeking further assistance from TWI in the future.'

Design reviews to maximise manufacturing efficiency and achieve service performance requirements are typical of the work carried out by TWI engineers. Performed on products of all shapes and sizes from diverse sectors of industry,design reviews help companies innovate and bring new products to market faster and with reduced risk.

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