Offshore drilling for oil and gas was first developed in the Gulf of Mexico as an extension of the highly successful exploration which has taken place onshore in the southern states of the USA The first wells in the late 1950s were in shallow water from simple platforms in an environment where weather and sea conditions, if not always clement, were predictable and manageable from the operational point of view.

Thirty years later, exploration offshore has extended into provinces where, despite advances in meterological forcasting techniques, weather conditions are no longer so well defined Correspondingly, with the move into deeper waters, drilling methods have become more sophisticated, hole depths have increased, and platforms for field developments have shown themselves to be marvels of engineering technology-mini-cities contained within a remarkably small sea bed ‘foot print’.

With the increasing exposure of men and equipment to hostile environmental conditions have come advances to protect those men and structures from ‘hazards’ existing beneath the seafloor The encounter of the unexpected geological condition-either by the initial exploration drill bit probing a completely unknown ‘geomechanical’ environment or by the placing on the seafloor of an immense additional load and the subsequent development of the field by development wells-is now acknowledged to deserve a degree of a study hitherto unknown.

Encouragement for this, for those companies less enlightened with regard to ‘prevention rather than cure’, has been the shallow gas blow-out Shallow gas accumulations have caused conditions hazardous to oil and gas operations right from the start of offshore drilling operations The violent nature of these encounters has caused several fatalities, many injuries, pollution events, and drilling production equipment losses amounting to hundreds of millions of pounds As the search for oil and gas moves further into undeveloped areas, encounters with shallow gas are expected to become increasingly numerous, either because of the greater accumulation of shallow gas in the more recent sediments in basin centres or because of lower levels of knowledge of regional shallow geology

This is undoubtedly highlighted by a comparison of drilling statistics from the Gulf of Mexico and the Norwegian Continental Shelf Between 1979 and 1982, 4449 wells were drilled in the Outer Continental Shelf USA lease area of the Gulf of Mexico Sixteen blowouts occurred, of which eight could be attributed to shallow gas occurrences (ref 1) During the earlier period of 1971–1978, 7553 new wells were started and one blowout occurred for every 250 wells drilled Of these, 30 blowouts were during drilling of either exploratory or development wells (ref 2), and 19 of these were directly attributed to shallow gas

Only 558 wildcat and appraisal wells have been drilled offshore in Norwegian waters since 1966 Of these, 150 have encountered shallow gas, and seven have blown out (ref 3) Statistics for other parts of the North European Continental Shelf were not available to the author at the time of compilation of this chapter

Concern in the Gulf of Mexico was sufficient in the early 1970s for study.

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