Leonard LeBlanc Houston Wave and current powered electrical generators include the hydropiezoelectric (HPE) generator, the seabed wave power converter, the mid-water propeller turbine, and the Wells air oscillation plant. [40027 bytes]

Leonard LeBlanc

Wave power competing with
wind and solar power offshore

One thing designers of offshore wave power generator systems have in common - they never give up. The vision of developing a prodigious renewable power supply with low maintenance and high operating efficiency is constantly just out of reach. The chloride environment and variable physical motion of the waves limits mean-time-between-failure for most mechanical systems, and others do not provide sufficient power consistently to warrant wider interest.

There are low-power requirements specific to offshore petroleum industry functions that can be filled by solar and wind power. Solar energy panels have powered data transmission and some control functions on unmanned production structures offshore and wellheads onshore. Presently, wind generators are being installed on several North Sea platforms to power electronics and data transmission functions.

Wave power generation has not been considered until now because of the high maintenance requirement on rotating equipment. Virtually everything corrodes or erodes. However, Ocean Power Technologies of Princeton, New Jersey (US) says its wave power unit features hydropiezoelectric motion, thus has no prime mover parts in motion. The firm will install a 1 kW hydropiezoelectric generator on an undisclosed platform in the US Gulf of Mexico early next year, and expects to have operating a 1,000 kW unit by 1997.

The piezoelectric effect is the conversion of mechanical energy to electrical energy through use of a material with piezoelectric properties. Ocean Power's system uses the differential between an anchored element and a moving element to generate the motion. A cam moves up and down past piezoelectric elements (plastic-like polyvinyledene fluoride), creating electricity. The units reach optimal output at 1-meter wave heights with a frequency of 0.15 Hz.

Other wave power designs have been tried in past years. However, developers have found that wave tank testing has not imposed the complete spectrum of offshore wave and seabed conditions, resulting in less-than-adequate performance or actual problems in trial. In addition to the hydropiezoelectric device, other systems are in various trial stages:

  1. Seabed piston pump: This system involves a float at the surface connected to a piston pump and valved chamber on the seabed. Up-and-down wave forces on the float are transferred through a rope to the piston pump on the seabed, which pulls water through a turbine generator.

    A working system was installed off Denmark in 1994, after five years of prototype deployment and testing. The unit was removed earlier this year for repair of pinhole leaks in the float, which was considered to be a fabrication problem. The system has operated continuously through numerous North Sea gales, and is considered to be a success.

    The advantages of this type of system are that wave energy is higher offshore than nearshore, and the unit can be deployed in most shallow water regions.

  2. Oscillating water column: This system operates best in a confined inlet along the shoreline, although platform-based models with an air chamber can be fabricated if the economics are justified. The principle is that wave oscillation in a confined space creates air oscillation, which in turn, drives a bi-directional turbine positioned vertically or horizontally over the air chamber. A bypass helps smooth the flow regime. The turbine can operate with or without guide vanes. Greater efficiency can be created with linked upstream and downstream guide vanes, but they can be a source of wear.

    Most of the oscillating air systems make use of the Wells turbine, which deploys symmetrical aerofoil blades around a hub at a 90 angle to the axial air flow. Lift and drag forces on the blades generate blade velocities of up to 2,500 rpm. Lately, contra-rotating blades have been examined to improve efficiency.

    After extensive trials with plants in Islay, Scotland and Trivandrum, India, three more air oscillation plants will be built at shoreline locations in Scotland, Ireland, Portugal.

  3. Mid-water propeller: This system consists of an axial flow propeller mounted on an underwater monopile. The propeller is driven by mid-water current flows in coastal estuaries, between islands, or in strong offshore gyres. The propeller rotates about the pile to face current flow. The entire assembly can be lifted to the surface for repairs. The drawback to this system is the size of the propellers and possibility of blade impact with marine junk, marine mammals, or trawling gear.

Further details on some of these systems are available in the technical proceedings on offshore power systems presented at the ISOPE '95 conference, held at The Hague, The Netherlands in June (Dr. Jin Chung; International Society of Offshore and Polar Engineers; P. O. Box 1107, Golden, Colorado, 80402, US)

Pipelines carry data signals over 35 km

A data system being developed in the UK utilizes pipeline walls as a transmission medium. The system was developed by Flight Refueling of Wimborne (Dorset), in association with the OSO, Mobil, Enterprise Oil, British Gas, and Marathon.

The unit couples a subsea electronics module to an acoustic transmitter at one end of the pipeline. An acoustic sensor, at distances of up to 35 km away, picks up the transmitted signal. Attenuation of the signal in pipelines is considerable, but sensitive electronics can pick out the needed data.

The system eliminates use of umbilicals or water acoustics. Originally, water acoustics were the focus of considerable investigation for deepwater operations, but thermoclines and salt saturation variances created major signal distortion.

Immediate applications for pipeline signaling are subsea stepout well control and telemetry, monitoring of cathodic protection potential, integrity and thickness monitoring for pipelines and J-tubes, product transfer, anchor and mooring line monitoring.

The first trial for the system, held in Morecambe Bay, was successful. A signal was transmitting over a pipeline distance of four km.

New developments in electronics, software:

Robot driller: Shell UK is developing a robot drilling system with real-time downhole information especially for horizontal and slim-hole drilling programs.

High visibility: A hand-held infrared camera has been developed by Texas Instruments (Dallas, Texas) to allow expanded vision capabilities at night and in conditions of smoke and fog.

Cellular control: A system that uses cellular telephone communications to monitor and control a number of valves at non-electrified remote sites has been developed by Daniel Industries of Houston.

Monitoring/control: An expandable Windows based monitoring and control software package that contains communications protocols for over 60 instrument manufacturers has been developed by Ci Technologies of Fairport, New York.

Electrochemistry: A Windows-based graphical package developed by Solartron Instruments of Houston allows the simulation of electrochemical reactions, particularly for corrosion analysis.

Copyright 1995 Offshore. All Rights Reserved.

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