The world’s first two offshore combined power plants will be commissioned in the spring of 2000. Phillips Petroleum Company Norway chose a steam turbine in a combined system for its new water injection platform on the Eldfisk field. Norsk Hydro replaced a gas turbine generator with a steam turbine system on Oseberg Gas. The year 2001 will see the third plant of this kind; Saga Petroleum – now Hydro – chose a combined power plant to generate its power for Snorre B.
The exhaust from gas turbines contains a lot of energy in the form of heat, but is usually just released into the atmosphere. If the gas is conducted through a heat exchanger boiler, however, the heat can be used to produce superheated steam to power a steam turbine. This combination of gas and steam turbines is known as a combined power plant.
Choosing steam instead of extra gas turbines with their associated power need means fewer emissions of CO2 and NOx. Combined power plants are already reality for offshore generation, while other emission-reducing designs are still on the drawing board. The oil companies want the introduction of combined power plants to yield profits in the form of reduced emission taxes. Greater operational reliability and availability of steam turbines as compared to gas or diesel-powered turbines reduces losses due to production stoppages, which is a major economic factor for a production platform. Any increased investment costs are recouped in a few short years.
Combined power plants are a conventional technology, used in gas power stations onshore. This provides a reassuring landmark for the oil companies that have introduced the systems offshore. The reason why such systems have not been installed on platforms before is that the steam plants have been perceived as heavy and bulky. However, new combined power plants have been optimised for power generation and performance in relation to size and weight. The heat exchangers have been compressed and the water buffer tanks integrated into the system as drums/separators. Such compact low-weight systems have enabled steam systems to be installed offshore.
Experience from the operation of steam turbines onshore and in ships – fired boilers – show that they are stable generators of power during load changes and process variations. Continuous operation of the steam turbine systems has documented stability and availability also offshore.
In the steam plants, compressed cold water is pre-heated and sent through a de-aerator drum before being pumped into the boiler. Some of the water passes over into steam, and the two-phase mixture returns to the drum for separation. The liquid re-enters the boiling circuit, while the steam is conducted to the superheater. The superheating is done in two stages, with water injection between. The superheated steam expands through the turbine down towards vacuum pressure. In the condenser the phase is transformed into pure liquid again, which is pumped up to the pre-heater, thus closing the cycle.
Flexible power/thermal plants
The steam systems of Oseberg Gas, Eldfisk and Snorre B show some of the potential for variation as regards power and heat supply. On Oseberg Gas, the steam system receives exhaust heat from two gas turbines driven by compressors, and the load from the gas turbine follows – due to load changes – the fluctuations in energy in the exhaust gases direct. The steam generator is on the network together with two gas turbine generators, which take up the variations in power requirements. Steam can be extracted from the low-pressure part – at one bar – of the steam turbine, enabling heat to be supplied to the onboard thermal system. In full operation and without steam bleeding, the steam turbine yields 16 MW.
On Eldfisk, the steam plant is connected to two of the gas turbines, which power water-injection pumps, and to the gas compressor. Some steam always bypasses the steam turbine for dumping in the condenser, because the steam generator is the main supplier of power, and the surplus steam feeds the steam turbine in case of an upswing in the power requirement. This means that the steam turbine does not react directly to changes in the exhaust gas. Full operation yields MW 10.3 MW, and that is still with 10% surplus steam. An extra heat exchanger is added to the boiler to drive a freshwater generator. This circuit does not influence the steam plant directly, but is a part of an integrated power/thermal system. On Eldfisk, seawater for cooling in the condenser is built into the system for adding to the water injection pumps, so that extra lifting power is avoided.
Yields 17.3 MW
On Snorre B, the steam plant receives exhaust heat from two generator-drive gas turbines, and the steam generator is linked to these in a network. The gas generators absorb load variations on the network, and the steam turbine follows the fluctuations in exhaust gas direct, like on Oseberg Gas. Here, too, there is bleeding off of steam from the turbine for heat for other consumption, but at a higher pressure than on Oseberg, at 6 bars. On Snorre B, full operation with no steam bleeding yields 17.3 MW. The power requirements on a platform can be met by designing the connection between gas turbines and steam turbines on the basis of the most compact and energy-economic solution. Heat requirements can also be integrated into the combined power solution, as on Oseberg and Snorre, or by including a heat exchanger in the low-temperature part of the exhaust gas, as on Eldfisk.
The highest temperature reached in the steam plants is 430 o C, and operational pressure in the area is 20 bars.