OSPREY and LIMPET

by George Hagerman, SEASUN Power Systems, Alexandria, Virginia
ART's associate in the United States

Background

Applied Research & Technology Ltd (ART) is a private company formed in 1991 with the sole objective of developing and commercially exploiting wave energy systems . The company is committed to developing and supplying means of providing to Island and Coastal communities at minimum cost supplies of electricity and fresh water ( via desalination) derived from renewable energy sources. The means are based on fundamental scientific principals so as to ensure the optimum use of resources in manufacture and minimal environmental impact in service. The technical stimulus for the company stems from the innovations of Professor Alan Wells, formerly of Queens University, Belfast and the commercial background is provided by Allan Thomson a businessman well established in the field of offshore fabrication. The company has offices with access to workshop and test facilities in Inverness in the Highland Region of Scotland.

ART have developed two types of wave energy collector in parallel. Namely the nearshore OSPREY (Ocean Swell Powered Renewable EnergY) collector unit and the shoreline LIMPET (Locally Installed Marine Powered Energy Transformer) unit. The OSPREY is designed to be deployed in the nearshore zone in water depths of between 15 to 20 metres. The OSPREY unit is currently rated at 2MW, this power capability can be further augmented as the unit has been designed for fitment of a wind turbine (rated at 1.5MW). The LIMPET which has been designed in collaboration with Queens University Belfast is classed as a shoreline device. The LIMPET (rated at 0.5MW) is designed for installation on a cliff face or as part of a breakwater. The larger OSPREY unit is designed to be constructed at a shipyard (or similar facility) whereas the smaller LIMPET is designed to be transported in modular kit (‘Flatpack’) form to installation sites worldwide. The choice of unit depends on a number of factors which include: onshore constraints such as access to coast; geotechnical considerations (cliff type and local hydrography); local infrastructure/regional resources (construction facilities, skilled manpower, marine equipment, tugs etc).

A major step in the utilization of ocean wave energy was taken in August of this year with the offshore installation of OSPREY 1 by Applied Research and Technology Ltd. (ART) of Inverness, Scotland. With a generating capacity of 2 megawatts (MW), OSPREY 1 is the largest wave power plant ever built. When connected to Scottish Hydro-Electric’s mainland utility grid, it will more than quadruple the worldwide amount of grid-connected wave power, which now totals 685 kilowatts (kW), including a 350 kW Tapered Channel plant in Norway, a 150 kW caisson-based system on India’s southwest coast, a 74 kW gully-based unit on the Scottish island of Islay, and a handful of smaller projects in Japan.

Unlike these earlier wave power plants, the OSPREY is not based on shore or attached to a breakwater. It is a stand-alone unit that can be towed offshore and installed in water depths of 10 to 20 m. A single design can be used for a variety of project locations, and large projects can benefit from the economy of mass-producing identical modules. Fashioned from welded steel plate, the OSPREY lends itself to shipyard fabrication, providing alternative employment for yards that in the past have relied on now-dwindling orders for naval vessels or commercial fishing craft.

The ART OSPREY Concept

Based in Inverness, ART is located near the Aberdeen centre of Scotland’s offshore oil and gas activity and has derived its technology in part from that industry’s proven fabrication and deployment methods, and in part from a long history of national government-sponsored wave energy research in the United Kingdom. OSPREY is the latest addition to the British aquatic bestiary of wave energy device names that includes Dr Stephen Salter’s nodding Duck (developed at the University of Edinburgh) and Dr Norman Bellamy’s SEA Clam (developed at Coventry Polytechnic). Although scale models of these floating devices have been tested on the waters of Loch Ness, the trials did not include electric power generation equipment, and neither the Duck nor Clam has been tested at sea.

The only British wave energy device to generate electric power in an ocean environment is the 75 kW shoreline project on Islay, developed at the Queen’s University of Belfast under the direction of Dr Trevor Whittaker. This plant has a concrete capture chamber placed over the head of a natural shoreline gully, which traps a pocket of air over an oscillating water column (OWC) that rises and falls in response to incident wave action. When the column rises, it forces air out of the chamber; when the column falls, it draws air in.

Several different self-rectifying turbine designs have been developed to harness the reversing air flow from such an OWC. The most widely used is that invented by Dr Alan Wells when he was Professor of Civil Engineering at the Queen’s University of Belfast. Wells turbine blades have a symmetric aerofoil section and are mounted on the generator shaft in such a way that they have no inclination to the plane of rotation. The rotation of the blades and the wave-driven axial air flow combine to produce an apparent angle of incident air flow that generates a lift force on the blade sections. Although the axial component of this lift force periodically reverses direction, the in-rotational-plane component always acts to drive the rotor forward, regardless of which way the axial air flow is directed.

The Islay project is a useful test bed and has enabled a better understanding of how to match Wells turbine design parameters to a given wave climate so that power output is maximised over the range of naturally occurring sea states. As a commercial project, however, the economic viability of such a shoreline device depends greatly on local rock and soil mechanics, as well as the shape and size of the gully and the presence of wave dissipating rock formations. Civil works, which account for more than half the capital cost, must be tailored to each project site, and even mechanical and electrical components may have to be differently sized for the variable capture chamber geometry expected among natural gullies along a given section of coast.

Free of shoreline variability, an offshore wave energy project can be composed of identical structural modules, each housing a standard mechanical and electrical plant that can be fitted into the module when it is built. This maximises the benefits of mass production and makes large projects easier to finance, since a block of ten or twenty modules can be brought on-line within a year of order, earning revenue towards the financing of additional blocks. Building up large-scale generating capacity in such an incremental manner also reduces the risk of utility commitment to excess supply.

ART’s strategy has been to develop a wave absorber that is not tied to the shoreline, but which is a fixed rather than floating structure so as to avoid deep-water mooring costs and power cable bending dynamics. The unit is designed for installation in a water depth of 10 - 20 m, where the day-to-day wave energy resource is still relatively good, but where the most extreme storm waves break seaward of the plant. Design wave heights for platform survival are thus depth-limited and not nearly as extreme as would be experienced by a floating device in deeper water, such as the Duck or Clam.

Unlike the massive concrete caissons that have been used for fixed OWC projects in Japan and India, ART’s caisson design is a lightweight, stiffened steel structure that can be welded from flat and simply curved plate sections. It thus can be fabricated at conventional shipyards without major retooling.

Key components of the OSPREY are diagrammed in Figure 1. Wave energy is harnessed by means of an OWC within the inverted dome of a collector chamber. Water in the chamber rises and falls in response to passing waves, causing a reversing air flow through two vertical stacks. Each stack is 3 m in diameter and contains two 500 kW Wells turbines that are directly coupled to induction generators, which operate at variable speed. AC-DC-AC power electronics convert the generators output to constant voltage and frequency. Only two turbine/generators are operated in low waves, while all four are brought on-line in high waves. This enables the OWC to be optimally damped over a wider range of sea states, which is further enhanced by real-time microprocessor control of turbine rotational speed. ART has filed patent applications worldwide to protect its commercial rights to these innovative aspects of OSPREY technology.

Although not yet installed on OSPREY 1, the caisson is designed to also support a wind turbine. Offshore wind speeds are generally greater than those on land, but the high capital cost of piers to support wind turbines in the open ocean has prevented the utilization of this resource. The OSPREY caisson, however, can support a wind turbine of up to 1.5 MW in size without significant cost penalty, since the additional wind loading is negligible compared with the wave loading that the caisson has already been designed to withstand. The OSPREY also has an existing electrical infrastructure, so that the incremental capital cost of adding a wind turbine is much less than if the same turbine was installed offshore on its own.

Design, Fabrication, and Deployment of the OSPREY 1

In 1991, the Commission of the European Communities, through its Directorate General for Science, Research, and Development (DGX11), launched the Commission’s first wave energy research program. With funding of $1.5 million under its JOULE 1 program (Joint Opportunities for Unconventional or Longterm Energy), four Preliminary Actions were carried out over an 18 month period, including a survey of European wave data, a review of wave energy conversion technology, and initial feasibility studies of candidate pilot plant project sites. The results of this effort were reported at the first European Wave Energy Symposium held July 1993 in Edinburgh, Scotland. That same month, proposals were accepted for the main research and development program under JOULE 11. With $3.5 million from the Commission and matching funds from government, academic, and industrial sources in member countries, this two-year program began in January 1994.

ART was one of three device teams to receive funding from the Commission for the design and construction of a wave energy pilot plant. The other two projects are shoreline OWC systems, one developed by the Queen’s University of Belfast for a second unit at Islay, and the other developed by the Technical University of Lisbon under the direction of Prof Antonio Falcao, for the island of Pico in the Azores. In addition to being larger than its counterparts and the only offshore project, the OSPREY 1 is the first European pilot plant to be deployed; the other two projects are not expected to come on-line until late next year.

In February 1995, British Steel began delivery of 750 tonnes of structural steel to UiE Scotland’s 30 acre shipyard in Clydebank, near Glasgow, where the OSPREY 1 was built on the same slipway that launched the Queen Mary and QE11. Although this yard had been dormant for several years, the simplicity of the OSPREY structure made it possible to get the yard operating quickly. The OSPREY 1 fabrication contract employed an average of 60 workers over a six-month period, with a peak work force of nearly twice that number.

Ballast tank panels were fabricated into sections weighing up to 180 tonnes apiece. These were then assembled on the slipway, along with the capture chamber roof and back wall. The finished caisson that was launched off the ways has a seafloor “footprint” area of approximately 40 m by 60 m. The capture chamber has a waterplane area 24 m wide by 19 m deep.

The curved seaward face of the collector chamber is designed to safely absorb breaking wave impact pressures of up to 10 bar. Caisson corrosion protection is provided below water by sacrificial aluminum/zinc anodes and above water with a plasma arc sprayed aluminum coating that has been sealed with glass flake epoxy paint.

Once the caisson had been launched, the power modules, each weighing 6 tonnes, were installed from pierside. The power module deflector hoods rise to an elevation nearly 30 m above the ballast tank mudmats.

The OSPREY 1 was officially christened with a bottle of local Orcadian Highland Park whisky on Wednesday, August 2, 1995. Trimmed with concrete ballast to float at a draft of about 3.5 m amidships, OSPREY 1 was then towed down the Clyde and north to the project installation site near the town of Dounreay, southwest of the Orkney Islands. The four day tow, covering a distance of nearly 360 nautical miles, was accomplished by the Schmit Wysman tug Tempest, at an average bollard pull of 60 tonnes.

Once on the site, the integral ballast tanks of OSPREY 1 were flooded in a carefully controlled air-venting process whereby the tank toes touched bottom before the tanks were allowed to free flood. Final placement occurred 300 m offshore in a water depth of 14.4 m. In its present position, the OSPREY 1 has an immersed weight of approximately 6000 tonnes, provided primarily by sediment ballast pumped from the surrounding seafloor. Stability of the caisson has been verified by 1:40 scale model testing in an independently audited wave tank, which was required by Lloyds of London, insurance underwriters for this project.

With the OSPREY 1 now firmly anchored on site, installation of measuring instrumentation will be completed, and a 2.5 MVA submarine power cable will be laid to connect the device to the onshore substation at Dounreay; connection to the utility grid is expected next spring*.

Today’s winds are tomorrow’s waves, and when funding permits, the OSPREY 1 will be fitted with a 500 kW or larger wind turbine, in order to learn more about combining both renewable energy technologies in a single plant. These data will enable the optimization of a hybrid wind/wave configuration, which will then be the basis of future commercial projects. ART also has entered into a cooperative agreement with the Queen’s University of Belfast to reduce the construction costs of shoreline OWC systems by developing 500 kW modules that can be transported in kit form to project installation sites worldwide. A 1 MW pilot plant, consisting of two such modules, will be built next summer (1996) alongside the existing gully-based test plant on Islay. Once proven under ocean conditions, these modules can be mass produced for installation on new or existing harbour breakwaters.

As the world’s appetite for energy grows with increasing population and industrialization, so do concerns about global warming and acid rain. ART believes that wave power can supply a significant portion of the increasing demand for sustainable energy sources that do not further deplete the Earth’s finite fossil fuel reserves or cause irreparable damage to the environment.

There is no energy source that has zero environmental impact, and during construction, the OSPREY will have environmental impacts similar to those associated with the fabrication of an offshore platform or ship. Once installed, the environmental impacts of OSPREY technology are likely to be overwhelmingly positive. Breakwater-based systems will have a visual impact consistent with existing harbor development, and to the degree that oil imports are displaced, pollution from spills and evaporation during pierside fuel transfers will be reduced. This may be particularly significant in island and rural coastal communities now dependent on diesel electric power generation.

* Everything did not quite go according to plan. The machine was damaged when it was launched from the UiE yard and this may have caused two out of the nine ballast tanks to spring a leak when it was lowered into place off the Caithness coast. This structural damage was made worse by poor weather conditions and it became impossible to refloat the machine. The turbines and other equipment were removed before it eventually sank on 27 August. ART insist the design was not at fault and work on building OSPREY II has already begun.

The SITC ‘Islands Network’ was contacted by John Jurgensen, General Manager, of Something yoU Need (SUN) in Antigua seeking detailed information about OSPREY and LIMPET. This was needed in order for SUN to assess their potential for possible use in their proposed National Renewable Energy Demonstration Program for Antigua and Barbuda.

This program is being prepared by SUN and will be submitted to their Government, the Danish Ministry of Foreign Affairs, European Union and World Bank for possible funding. The intention is to demonstrate various RE systems and their direct/indirect applications at six sites around the island. These are:

• A. Rural demonstration centre - small windmill and solar modules with battery storage
• B. SUNPARK urban demonstration centre - solar, wind, biomass, hydro, solid waste
• C. The Cove - offshore wind farm with windmills mounted on anchored rafts with mariculture pens beneath
• D. Rum Distillery - combined heat and power plant utilising solid waste
• E. Crabbs peninsula - combined wind, diesel and desalination plant.
• F. Indian Town Point - OSPREY

SITC has put John Jurgensen directly in touch with ART and George Hagerman of SEASUN Power Systems to discuss the technical, socio-economic and environmental pros and cons of adopting OSPREY in relation to local factors prevailing in Antigua. Jurgensen acknowledges that RE is not the complete answer to ameliorating the devastating effects of global warming (see box section) and additional problems brought about primarily by man’s consumption of fossil fuels, but it is a major start.