Offshore oilfield development has until recently been a process of progressively adapting the traditional onshore technique to an offshore situation. The fairly obvious approach has been to provide a dry-land site offshore to build a platform above water level, and hence continuing a drilling, well completion and production activity with essentially similar equipment and technique This progress started along the oil-rich Texas and Louisiana coastline of the Gulf of Mexico, where the conditions allowed a comparatively simple structure to be built. In the UK it continued with the gas platforms that dotted the Southern basin of the North Sea in the sixties. When major accumulations were discovered in the Central and Northern basins, the size and engineering complexity of fixed platforms grew to the massive and expensive installations known today As further wells were discovered that could be tied back to these installations the economics drove the industry to install the trees directly on the seabed, and so offshore engineering moved towards equipment specifically tailored for subsea operation, and which has become increasingly removed from the original land-based concept.

Over the past two decades the technology of subsea wells has been developed in two main directions They have become more numerous, invariably clustered on subsea templates of up to 2000 tons in weight. Pipeline and riser limitations have led to subsea manifolds with remotely operated chokes and valves and to the need for hydraulic and, more commonly, electrohydraulic control and monitoring technology. This progress is illustrated by the first axis of Fig. 1

In parallel with this progress has been the move to increasing water depth, to a point where diving becomes less economic and equipment is specifically designed to be not only operated but installed, connected and maintained by remote systems operated from the surface. This progress with water depth, which is mainly an exercise in reliability and remote handling for deployment, recovery and maintenance operations, is shown by the second axis of Fig. 1.

This progress however, while impressive, has been confined to elements (Fig. 1 is available in full paper) upstream of the separation train, with all process plant remaining above the surface and derived from basic onshore practice and equipment. The next stage of moving such plant subsea is a step towards offshore oilfield technology that is specifically designed for its environment and does not depend upon massive structural platforms to re-create the land. This is the third direction of progress shown by Fig. 1

The necessity that mothers such invention has only come about as the major fields become developed and the remaining fields become too small to commercially justify the expense of large dry platforms. This inevitable conclusion has been hastened by the 1986 fall in oil prices, but was foreseen before then in late 1984 when the late Marcus Raybould first proposed that Humphreys & Glasgow (H&G) seek EEC support for his concept of a subsea separator By the time funding was confirmed in 1986 the oil price decline made such concepts attractive to the industry.

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