The methods available for the liquefaction of associated gas offshore are reviewed and evaluated against defined criteria. The results of the evaluation lead to the selection of a different type of liquefaction cycle from those published in other papers in this field. The recommended process route consists of a nitrogen expander cycle to provide low temperature refrigeration and a two stage refrigeration unit using refrigerant 22 to provide the high level refrigeration. Budget cost data and power consumption are provided for the recommended route.
Recent history shows that the demand for gaseous and liquid hydrocarbons has increased while at the same time it has become recognised that the untapped supply of these hydrocarbons is finite. It is therefore not unnatural to consider making better use of associated gas. both as a fuel and a chemical feedstock. Large sections of the North Sea oil and gas production are under U.K. control and in recent years the U.K. government has been putting pressure on the operating companies to prevent and restrict the flaring of associated gas.
In 1978 the U.K. Department of Energy commissioned a report on the feasibility and economics of the liquefaction of associated gas from North Sea oil fields. In February 1979 Petrocarbon Developments were asked, as part of this report, to evaluate the possible process cycles and produce preliminary technical and cost information for the selected cycle. This paper is based on the findings of this work.
The design is based on the following specifications and restrictions:
Feed gas composition (see table 1)
Feed gas flowrate - 360 mm SCFD with the capability of turning down to 25% throughput
Feed gas temperature and pressure 5°C and 40 bar
The preferred drivers for process compressors are gas turbines (aero engine derivatives)
20 MW of electricity is available
All process equipment to operate on board a moored barge which will be on site in all water conditions
All process equipment to be located below decks in a fully ventilated space for all weather protection
Height limitations in process area - 32 m l3 decks 8, 12 and 12 m). Approximately 1 m is lost from the periphery of the roof of each deck due to ventilation ducting
Deck size - 80 m × 43 m each
Process to produce only one product - LNG
No flaring, to be allowed from the vessel
The process cycle chosen should permit easy start-up and shut-down up to 10 times per year
Gas pretreatment is required to remove CO2, H2O, H2S and organic sulphur to the appropriate levels prior to liquefaction
Safety - all systems to be intrinsically fail safe
Maintenance - ease of access for maintenance is important when laying out equipment.
An initial inspection of the feed gas revealed that at the conditions given in table 1 it would be transferred from the oil production platform to the liquefaction barge as a two phase fluid and this could give rise to flow instabilities.