Serious power with a gas turbine package

Non-reciprocating turbogenerators are the first choice on many off-grid oil and gas sites - initial cost is high but the benefits massive

Diesel gensets provide for most oil/gas operators' off-grid power needs, but where a really big supply is needed – to pump hydrocarbons ashore 24/7, for example - a modern gas turbine connected via a hefty gearbox to a high-capacity alternator is usually specified. These gas-fuelled turbogenerators typically produce output in the range 10,000-100,000kW.

Widely used on ocean-going ships too, these lightweight high-power packages offer many advantages over a bank of lumbering diesels which have to be synchronised to meet the demand. Reliability and flexibility are the most welcome characteristics in challenging West African conditions, especially offshore, but other advantages of gas turbine power include:

- a very high power-to-weight ratio, especially if an 'aeroderivative' type of prime mover is chosen
- fits much more handily into the tight space
- available on a rig, production platform or drillship
- produces much less vibration than any kind of reciprocating engine, especially the low frequencies of really large ones
- a much more sophisticated design, but there are actually fewer components to go wrong
- high operating speed attainable for long periods
- multiple fuel options usually available, which are especially useful on a platform powered normally by associated gas
- easy start up so handy for peak shaving.

On the other hand gas turbine engines (only called 'jets' if the power is taken off as thrust) are noisy, very costly to install in the first place, and to run under idle conditions, require highly specialised servicing which is often purchased as part of the supply package, take longer to reach rated output than conventional diesel sets, and are less responsive to changes in power demand when warmed up. They run happiest at maximum output for long periods and are often remotely monitored.

Skilled manpower

A typical axial-flow gas turbine consists of a multiple stage compressor at the front (air inlet) end, a combustion chamber array right in the middle and a multi-stage turbine at the exhaust which drives one or more shafts, a gearbox then a conventional alternator assembly. Colossal amounts of highly skilled manpower-time have been spent refining the intricate components of these various stages, mostly in the interest of today's ultra-safe and fuel-efficient aviation. The goal has been to match materials and engineering to exceptionally challenging conditions, so that such features as all radial tip clearances are kept to a minimum under all operating conditions; this reduces emissions including noise and minimises power loss through leakage.

Every individual blade, forged out of the finest stainless steel, is usually securely held within the rotor assembly and stator ring by a quality of finely engineered dovetail joints.

Various devices are usually supplied to provide complete flow control throughout the gas turbine's designed operating range. Industrial gas turbine operators – powergeneration packages are usually lightweight derivatives of these – benefit from all this R&D, and many utility operators in distributed off-grid locations now rely on no other form of primary power. Remote oil/gas camps in North Africa and the Middle East are typical.

With thermal efficiencies that can approach 50 per cent today's gas turbines running on clean natural gas are widely available as 'off-theshelf' single units. Typically they operate with turbine speeds up to 10,000rpm or more with an exhaust temperature of around 5000C; various options are available for making good use of this heat.

High-maintenance

However, unless specifically 'aero derived' such gensets can be large high-maintenance devices that may weigh in at 100 tonnes (excluding ancillaries) and invariably require their own sound-attenuated enclosure – not easy on a rig. But to compensate for this they are usually handily mounted on skids for installation and maintenance. So in Gulf of Guinea conditions they are often simply removed for servicing, with a supplier-owned spare slid in to keep up almost uninterrupted power supply. Whether single- or dual-shaft, simple or regenerative cycle, most of these turbine-powered generators actually share a small range of common components.

First the compressor itself, then the first-stage nozzle assembly, and finally an assembly of heavy-duty turbine buckets and wheels manufactured out of high-performance materials to offer maximum thermal resistance. Generators like these from top suppliers like Centrax, GE/Nuovo Pignone, Pratt & Witney Power Systems, Rolls-Royce and Snecma/SAFRAN are specifically designed for fixed power-generation applications.

As just one example, the GE10-1 family from General Electric, available in dual-fuel form, is rated at 11,250kW at the front end and weighs in at just 34t in genset form. The GE compressor itself incorporates 11 stages of axial flow with a 15.5:1 pressure ratio, all driven by a three-stage HP turbine supplied by a single-can type combustion chamber. The whole neat package is supplied on a skid incorporating the gas turbine itself, an 11kV/4-pole generator which also functions as a starter, and a matched epicyclic gearbox. Sound pressure level is kept below 85dBA at 1m and the whole package is designed for outdoor installation.

Whatever family of engine is opted for an optimal balanced approach to distributed power generation is certain if a modern gas turbine is chosen as part of the power supply package.

Alain Charles Publishing, University House, 11-13 Lower Grosvenor Place, London, SW1W 0EX, UK
T: +44 20 7834 7676, F: +44 20 7973 0076, W: www.alaincharles.com

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