Richard Yemm explains how a ‘sea snake’ could soon lead the way in commercial, offshore wave energy production

Ocean waves represent a major renewable energy resource, estimated by the World Energy Council to be in excess of 2TW worldwide. Waves act as an efficient storage and transport medium for the energy in wind as it blows over 70% of the earth’s surface. Wave energy is therefore largely immune to short term local climatic effects. Hourly and diurnal variations are much less than for wind or solar and accurate, medium term forecasting of sea-state is already a reality within the marine and offshore industries. Offshore wave energy offers perhaps the lowest environmental impact of any of the true renewables with minimal visual, noise and habitat disturbance.

Until recently wave energy was viewed as a long term option but technological developments now indicate that commer-

cially competitive systems are not only feasible but are only a few years away.

Significant breakthroughs have been made over the last five years that will result in a range of competing pre-commercial systems by 2002. These will have generation costs comparable to those for offshore wind and biomass schemes. Costs will be further reduced (to levels competitive with conventional generation) through the installation of significant capacity, once the core wave energy technology has been fully demonstrated on a commercial scale.

Pelamis wave energy converter

ocean-power-delivery of the UK has performed preliminary studies on a number of novel concepts, in addition to studying existing proposals for offshore wave energy converters (WECs). The lessons learnt have resulted in a set of conceptual criteria that are thought to be necessary characteristics of a promising offshore WEC concept:

  • Survivability before power capture efficiency: the device

must be designed with survivability as the key objective,

and then effective ways of improving power capture must be found.

  • Choice of source of reference: the device must react

against itself, rather than against a fixed reference frame

such as the sea bed. The preferred source of reaction is

‘length-down-wave’ as this allows significant de-

referencing in the long wavelengths associated with storm

waves and presents the minimal cross-sectional area to the

passage of extreme waves in which drag loads are significant.

  • Inherent load shedding: the device must incorporate

features that inherently limit loads and motions once the

rated power wave amplitude has been reached.

  • Resonant motion: a degree of resonant response is essential

to improve power capture in small waves. However, the

device must be capable of de-tuning in large waves to

prevent excessive loads and motions.

  • Hydrodynamic added damping: the device must have a

significant amount of added damping in relation to its

displacement. The device must be capable (when

compared to its displacement) of being an effective wave-

maker if run in reverse.

  • Power smoothing: delivered power must be smooth to

reduce grid connection problems.

  • Non site-specific: the device must be able to be installed in

a range of water depths and sea bed conditions.

  • Minimal on-site work: the complete device must be

constructed, assembled and tested off site with minimal

installation work required on site.

  • Prototypes and initial production devices must use 100%

available technology.

The Pelamis, or ‘sea snake’, concept was conceived to embrace all of these criteria and preliminary work showed that the concept was sound.

Ocean Power Delivery (OPD) was formed in January 1998 to develop, demonstrate and commercialise the Pelamis concept. The company was successful in its bid for a power purchase agreement within the 1999 Scottish Renewables Order (SRO) and now holds a 15-year power purchase agreement for power from the first commercial installation of Pelamis off the west coast of Scotland. The company has secured major partners from the offshore industry and is working towards full scale demonstration of the technology by early 2002.

Design

The Pelamis device 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 electrical generators to produce electricity.

Power from all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single sea bed cable. A 750kW device will be 130m long and 3.5m in diameter. Prototypes and initial series production devices will use all available technology.

A novel joint configuration is used to induce a tuneable cross-coupled resonant response that greatly increases power capture in small seas. Control of the restraint applied to the joints allows the resonant response to be ‘turned-up’ in small seas where

capture efficiency must be maximised or ‘turned-down’ to limit loads and motions in survival conditions. The complete device is moored flexibly so as to swing head-on to the incoming waves and derives its reference from spanning successive wave crests. The device is designed to be manufactured, assembled and tested at a suitable facility such as a shipyard.

A core theme of the Pelamis concept is survivability. The fundamental survivability mechanisms are the use of length as the source of reaction to allow the system to de-reference in long storm waves in conjunction with a finite diameter to induce full submergence and emergence (hydrostatic-clipping) in large, steep waves. The moorings offer little restraint and have a motion envelope large enough to accommodate extreme wave motions in addition to the low frequency wave-group induced response. Survivability in high and steep waves has been demonstrated by 80th, 35th and 20th scale tank test experiments. (The 20th and 35th scale experiments are pictured below. For the 80th, see main image on p41).

The wave-induced motion of each joint is resisted by sets

of hydraulic rams configured as pumps. The rams pump

oil into smoothing accumulators which then drain at a

constant rate through a hydraulic motor coupled to an electrical generator. The accumulators are sized to allow continuous,

smooth output across wave groups. Output smoothness from

the complete device will be comparable with that of a diesel

generator set.

Each joint has an independent power conversion system

to provide redundancy and to avoid large flow losses through long hydraulic manifolds. The generator sets are linked

by a common 415V, three-phase bus running the length of the device. A single transformer is used to step-up the voltage to

an appropriate level for transmission to shore, high voltage power is fed to the seabed by a single flexible umbilical

cable and then to shore via a conventional sub-sea cable.

Installation of the Pelamis WEC requires minimal on-site work. The moorings, sea bed power cable and flexible umbilical are placed before the device arrives. Installation only involves connection of mooring lines and the power umbilical and therefore requires the shortest possible weather window. The system can be completely removed at the end of its service life.

Operation

The device is autonomous in operation and includes all

supervisory control and protection systems required to ‘cut in’ as wave power levels rise, match captured and output power

in operation and ‘cut out’ under fault conditions and when

wave power levels fall below the required minimum. Operation is similar to that of a modern wind turbine in most respects.

The structure and power conversion systems are designed or selected to minimise in-service maintenance. Estimates of

service requirements have been derived through discussion with component suppliers and it is currently envisaged that any maintenance required will be limited to a structural, mooring and systems check once a year with a mid-life partial refit to replace hoses and seals. Actual maintenance requirements will be determined through close monitoring of the initial prototype and

production devices.

The staged development programme to commercialisation is

as follows:

  • Core technology programme from October 2000 –

June 2001.

  • Full scale technology programme from May 2001 – March 2002.

  • Full scale demonstration in March 2002.

  • SRO system commissioned by August 2002.

OPD is committed to a responsible, staged development path towards full scale demonstration of the technology by early 2002. The nine-month core technology phase will take the Pelamis to seventh scale for systems development. The seventh scale model will be used to develop and test all aspects of

the full scale hydraulic and control systems on a cheap, rugged platform. The model will allow extensive testing in extreme

conditions and will allow simulations of partial and full system failures that would be impossible to conduct safely at full scale. The ten-month full scale technology phase will prove key

elements of the full scale system on the shore before moving to

the first prototype device by early 2002.