Comprising four fields - Hamilton and Hamilton North (gas), Douglas (oil), and Lennox (oil and gas) - the development's recoverable reserves are estimated at 1.2 trillion cubic feet of gas and around 150 million barrels of light crude oil. With peak gas supply rates likely to reach approximately 300 million cubic feet a day, contract sales to PowerGen's new power station at Connah's Quay are due to start in mid-1996. Oil production is expected to peak at 70,000 barrels per day with the anticipation that oil production wells at Lennox will be converted to gas around six years into the estimated 20-year life of the development.

Fast-track

This fast-track project, which has continued to run on, if not ahead, of schedule, has called for a �1 billion plus investment to provide the complex facilities required for this near and onshore development. As production starts up, BHP and its partners LASMO, Monument and PowerGen are set to reap the rewards of a challenging project which has entailed both technical excellence and a firm grasp of the principles of sound environmental management.

Speaking at a recent SPE meeting in London, BHP's production manager for the Irish Sea, John Methven, set out to discuss some of the operational challenges posed by the development in more detail.

But first he set the scene by explaining that a hub concept was adopted to make the development economically viable. This approach involves a central gathering and processing platform for all hydrocarbons produced in Liverpool Bay located at Douglas. The platform is bridge-linked to an accommodation and utility support platform and also the Douglas wellhead tower. Three minimum facilities, not normally manned satellite platforms at Hamilton, Hamilton North and Lennox are connected to Douglas by approximately 250 kilometres of subsea pipelines. In total, the six platforms have a total topsides weight of some 13,700 tonnes with a sub-structure weight of around 9,400 tonnes.

Following initial processing and compression, gas produced in Liverpool Bay will be further processed at a new custom-built gas processing terminal at Point of Ayr, North Wales. This location was specifically chosen out of a total of 44 potential sites because of its proximity to existing industrial facilities and the advantages of the shortest pipeline and cable route to the offshore platforms. Additionally, the area has suitable road, rail, sea and air transportation links.

The development is an important new industry for the region, with around 250 jobs expected to be created - 70 based onshore and 180 offshore. In addition, annual expenditure during the operational phase is estimated at �40 million and is expected to generate economic benefit in the Liverpool Bay area.

John chose to concentrate his presentation on three topics to give an idea of the complex and diverse issues surrounding Liverpool Bay operations. Namely, the environment, sulphur management and integrated operations.

Environment

The Liverpool Bay environment - with its 31 Sites of Special Scientific Interest - comprises a complex blend of natural features and human activities. There are diverse bird populations and conservation sites along the Dee and Mersey Estuaries which contrast but co-exist with high levels of tourist activity, shipping and industry. Aligned with BHP's philosophy of giving environmental issues top priority, all these factors were taken into consideration in the planning and design of the facilities.

'The Liverpool Bay development was the first to require offshore environmental impact assessments (EIAs) for all facilities,' John said. 'As part of the consultation process with local and national statutory and non-statutory groups, we sent out documents covering the area's meteorology, oceanography, geology, and ecology as well as our environmental management strategy.' Testament to the success of this work, UKOOA's guidelines for exploration operations in nearshore and sensitive areas published last year were based closely on BHP's environmental experience and practices in Liverpool Bay.

Central gathering and processing facilities at Douglas - located some 24 kilometres off the coast of North Wales - allowed the three satellite platforms to be designed to minimise visual impact and eliminate discharges and emissions during normal operations. Thus, Lennox which lies just eight kilometres off the tourist beaches of Southport became a feasible option.

Added John: 'Onshore too, a major challenge for the environmental mitigation work revolved around implementing the recommendations of EIAs carried out for the terminal site, the landfill and onshore pipelines.'

Only one third of BHP's onshore site will be used for the initial gas terminal facilities - the majority of the land owned by the company having been developed into an environmental amenity area to balance the new development.

One of the key issues to be addressed was the loss of a winter roosting area for up to 100,000 wading birds. Historically the Dee Estuary, and particularly Point of Ayr, attracts waders and BHP has ensured that the environ-mental mitigation work carried out immediately adjacent to the site and the creation of a wetland at Warren Farm across the road offers an attractive alternative for local wildfowl.

BHP continues to work closely with the Royal Society for the Protection of Birds and the Countryside Council for Wales on all its environmental activities and is proud of its achievements so far. John referred to the report of the influential European monitoring group on estuaries, RAMSAR, which has called BHP's environmental safeguards and standards exemplary.

Restoration Already it is difficult to trace the route of the underground onshore pipeline which will take processed gas from the Point of Ayr terminal to PowerGen's new power station at Connah's Quay. With an emphasis on minimal disruption to the surrounding area and quick restoration of the landscape, BHP undertook lengthy consultation before construction work commenced. The 17-mile pipeline route involved dealing with 70 different landowners and crossing roads, railway lines, rivers and sites of archaeological importance.

High levels of hydrogen sulphide (H2S) and mercaptan sulphur in the oil and gas produced from three of the Liverpool Bay fields - excluding the sweet gas from Hamilton North - have led to a number of technical and people challenges, John said. 'On the technical side we had to consider corrosion mitigation and process blending, while taking account of how sulphur impacts on the way people can operate.'

Levels of H2S range from 1100 ppm in Hamilton gas to around 50,000 ppm in Douglas crude, which in turn contains some 1200 ppm mercaptan sulphur compared with 40,000 ppm for Hamilton condensate production. The sulphur levels for Lennox crude fall in between - at 19,000 ppm H2S and 900 ppm mercaptan sulphur.

Material selection and chemical control have been key considerations in corrosion mitigation. To avoid the formation of brittle fractures, caused by hydrogen penetrating the steel, a high proportion of exotic materials have been used in the metalwork. 'Some 80 per cent of our pipework, which includes major valves, is made from exotic corrosion-resistant materials such as Inconel, Alloy 28 and Super Duplex steel,' John noted.

The main elements of chemical control depend on a criticality-based inspection regime and careful fluid analysis. A dedicated contractor has been employed to handle all chemical selection to ensure an ongoing focus in this area.

Crude oil and condensate will be processed offshore for export to the oil storage installation some 17 kilometres to the north. By the time it leaves Douglas, the H2S levels in the liquids will have been reduced to less than 5 ppm, well below accepted levels.

The gas will be partially treated offshore to ensure the H2S content is below 1600 ppm by the time it reaches the Point of Ayr terminal. A proprietary tail gas unit will strip 99.8 per cent of the H2S from the gas for conversion to molten sulphur, which will then be sold as a commercial by-product to local markets.

Added John: 'Training of our people both on and offshore has been of paramount importance, and our training programmes have been tailor-made for Liverpool Bay operations. Protection for individuals is maximised by the use of saver sets for those working in production and process areas, and for certain tasks breathing apparatus will be worn as routine and a 'buddy' system will apply. As an extra precaution, close circuit TV has been installed at strategic positions around the terminal and the platforms, which is monitored in the respective control rooms.'

While integrated operations have provided a third challenge for Liverpool Bay, they have also led to BHP finding innovative solutions for certain activities.

A new-build offshore support vessel (OSV) will be used for electrical submersible pump maintenance at the Douglas wellhead tower and for well servicing and maintenance at all four fields. Although widely used in the Gulf of Mexico, this is the first example of the concept being used in the UKCS. Known as the Irish Sea Pioneer, the OSV is a self-propelled jack-up vessel ideal to cope with the shallow water depth of Liverpool Bay, which varies between seven and thirty metres. 'The OSV is very versatile and helped keep the size of our satellite platforms to a minimum,' John commented.

The nearshore proximity of the oil platforms called for particular attention to the provision of rapid oil spill response. Under existing regulations, BHP was faced with providing five vessels in Liverpool Bay for standby and pollution response duties. However, industry lobbying and detailed consultation with the relevant authorities resulted in the revision of a Merchant Shipping Notice to allow vessels to be multi-functional - thus dispensing with the requirement for BHP to provide two dedicated pollution control vessels. The three vessels in the field will serve as standby and pollution response vessels, storing oil in their hulls if necessary.

Environmental factors also came to the fore in the decision to adopt the concept of an oil storage installation (OSI) some 17 kilometres north of the Douglas facility. Located in a position to avoid busy shipping routes, the double-sided vessel has a total capacity of around 870,000 barrels and is designed for full oil field life. By modifying the traditional CALM buoy with shuttle tanker system used in the North Sea to a catenary anchor leg rigid arm mooring (CALRAM) system, continuous production can be sustained - thereby mini-mising the potential risks arising from frequent production shutdown and start-up. An uptime of over 97 per cent is expected. Tankers loading at the OSI will ship the oil to international markets.

It was BHP, as Hamilton, who produced the first oil offshore the UK from the Argyll field in 1975. The risks at that time were pioneering ones - into the unknown from an engineering perspective. Some 20 years later, BHP will deliver the first oil from the Irish Sea. Concluded John: 'The engineering risks are now well understood but the pioneering spirit lives on. The Liverpool Bay development has taken our industry into an area where the challenge is not only more complex but is also more visible than it was back in 1975.'