Lightweight Industrial Gas Turbines In Offshore Installations
Authors: Biggs K.J. 
Published: December 2007

Biggs, K.J., Regional Market Manager Rolls-Royce (1971) Limited, United Kingdom

The industralised (long life) version of an aircraft gas turbine was first introduced as a land-based industrial prime mover in 1958, at a power station in the United Kingdom, driving an alternator for peak lopping duty using diesel oil as the fuel. The first unit specifically designed as a base load duty machine for use in the oil, gas and petrochemical industry, was installed in late 1960, driving a gas compressor on a natural gas pipeline in Kentucky, U.S.A. The output of the unit was 10,600 b.h.p., pipeline in Kentucky, U.S.A. The output of the unit was 10,600 b.h.p., utilising the pipeline gas as fuel.

Since this time over 2500 industrial and marine jet type units have been installed worldwide, approximately 25% of these for base load duty in the oil and gas industry.

The operational experience of the long life industrialised aircraft derivative has resulted in the type being widely accepted in the oil and gas industry as a reliable and efficient prime mover.

Operational records show that the type has demonstrated both reliability and on load availability in excess of 99%; where reliability is defined as equal to the sum of hours in operation plus standby hours plus planned maintenance hour divided by the total possible hours of operation normally 8760 i.e. one year. Availability is defined as the sum of hours in operation plus standby hours divided by the total possible hours.

The current trend towards oil and gas exploration and production offshore has produced a market outlet for which the jet gas turbine is (theoretically) the ideal prime mover. This paper presents the jet gas turbine as a long-life industrial product, lists the gas generators currently available together with the package suppliers and details the technical and maintenance concepts particularly associated with offshore operation.

The jet gas turbine consists of two major components. an aircraft derivative gas generator and a free power turbine (see Fig. 1). The gas generator serves as a producer of gas energy which is classified as exhaust gas horsepower. It is derived from an aviation gas turbine incorporating modifications to the fuel system and its controls to enable applicable industrial fuels to be burnt. Design innovations are incorporated to ensure the required long life characteristics of the land-based environment.

The (free) power turbine converts the gas energy to mechanical rotating energy or brake horsepower. Closely coupled to the gas generator by a transition duct, the power turbine expands the gas generator combustion products through one or more turbine stages and the residual gas energy is exhausted to atmosphere, or can be utilised further by waste heat recover), process, thus increasing the overall thermal efficiency of the process, thus increasing the overall thermal efficiency of the unit. The rotative power produced by the power turbine is then available for mechanical coupling to the driven equipment. The efficiency of power turbine designs currently on the market vary from 80% to 90%.

The world's three major manufacturers of aircraft gas turbines. Rolls-Royce. Pratt and Whitney and General Electric, have designed and sold power turbines. and have packaged complete gas turbine sets. However. the majority of the power turbines are not aircraft derived and are generally designed and manufactured by the package suppliers. The package supplier becomes the prime contractor in furnishing the complete jet gas turbine which includes baseplates, oil systems, fuel and governor systems and the control panels for operation and protection. The equipment and auxiliaries are then factory assembled and mechanically tested as a packaged prime mover to deliver to the installation site. As can be prime mover to deliver to the installation site. As can be seen from the list of package suppliers. these are mostly internationally renowned manufacturers of compressor or electrical equipment.

Gas Generator Development

As previously stated, the long life industrial gas generator is derived from an aircraft engine and its principal design features and configurations are predetermined by the requirements for flight engines. These features have been found to be of great advantage for the industrial version. Specific design criteria for a flight engine which are also important in an offshore environment are:

Light weight; ease of replacement: modular engine design: high power to weight ratio. very rapid response to control: low starting power; low oil consumption: high reliability. high availability; high cyclic life; ability to start rapidly and frequently; simple control system and instrumentation: built-in engine monitoring and engine systems: compact size.

In the industrial long life engine derivative most of these features remain unchanged and it is largely because of this background that this type of prime mover has achieved. and is continuing to achieve, operational success.