NEW MARINE MONSTERS IN HUNT FOR NORTH SEA OIL WELLS

By LIEUTENANT-COMMAND TREVOR-BLORE, R.N.R.

Issued by British Information Services – The Certificated Engineer June 1965.

A new tribe of sea monsters is growing up in British shipyards six huge, ocean-going oil drilling rigs, which are to spearhead the international hunt for oil and natural-gas fields in the North Sea off the British coast.

'Operation North Sea', in which more than 50 companies of international status are taking part in a multi-million-pound quest, has been. a largely maritime exercise from the outset. It- was initiated by the discovery, in 1959, of an immensely valuable natural-gas field in Northern Holland.

Geologists immediately began to assess the possibility that the rock strata living rise to this reservoir of natural gas associated with petroleum might extend under the North Sea to the British coast. With about half the sea area-100 000 square miles-under British jurisdiction following a recently ratified international convention, the big treasure hunt off the east coast of England was begun.

Several geophysical surveys of the North Sea were conducted, mainly by ships making seismic tests of the sea-bed and below. This is done by releasing explosive charges into the sea at short intervals. The shock-waves hit the sea-bed and penetrate the underlying rocks to be reflected microphones trailing on a cable under the sea surface. The echoes are then recorded and scientifically translated into a geophysical chart of the understructure of the North Sea to indicate formations which might contain oil or gas.

Day after day through the favourable weather seasons of the past two or three years, geophysical survey ships have charted the sub-structure of the bed of the North Sea, seeking particularly the anticlines or hump-like subterranean structures of porous rocks where oil may have collected over millions of years.

These geophysical surveys have depended on precise. hip handling and exact navigation. To help achieve the necessary standards a special Decca navigator chain was installed with stations in Britain, Holland and Denmark giving survey ships a continuous fix within an error-margin of 50 yards (46 metres). When actual drilling begins, the oil experts say they would like to fix their drilling positions within an error of plus or minus five yards (4.6 metres) through the average navigator may wonder how this is to be achieved.

Parallel with the geophysical surveys, another important scientific survey, by meteorologists, hydrographers and oceanographers have been in progress. The North Sea has a reputation for storms, so drillers want to know when to expect the best weather for the maximum activity of their rigs.

What is more, the designers of sea-going drilling platforms need to know all that can be discovered about the wind and wave forces their structures must withstand when on-site or moving from an abandoned dry hole to a new position.

Of course, the North Sea is never likely to produce the full-scale Caribbean-type hurricane which from time to time smites American oil rigs in the Gulf of Mexico. So graphs have been drawn to show the directions and velocities of prevailing winds and the intermittent gales in various parts of the North Sea at various seasons. These have not alarmed the designers and builders of the sea-going rigs.

More important has been the study of wave heights and periodicity in the areas where drilling is contemplated. The North Sea is notorious for its short, awkward seas. A German maritime meteorologist who made over 2 750 observations of waves in the North Sea found that more than half were under five feet high and that only 5 per cent were raised over 15 feet. And the average wave period was measured at between five and six seconds. A sea with wave heights of over 30 feet is rare in these waters.

Another point is that the North Sea is warmer than other waters in the same latitude, and keeps a small temperature range over the area.

All this information helps towards the design of the new sea-going drilling platforms now being built in British yards. Not that the start of drilling will wait on the availability of these. At least two sea-going rigs have been or are being, towed half-way across the world to start work in the North Sea, and a third is being improvised.

First Drilling

First at work is a drilling barge called Mr Cap, which has been towed across the Atlantic from the

The Gulf of Mexico and adapted to North sea conditions in a shipyard in the German Federal Republic. It started to drill on December 26, 1964.

The operator of this rig, Amoseas (American Overseas Petroleum), a company owned jointly by the American oil companies of Texaco and Standard Oil of California, plans to work through the winter in about 85 feet of water on the Dogger Bank, the fishing ground about 150 miles from the east coast of England. The operator points out that Mr Cap has safely weathered 40-feet high waves and winds up to 100 miles per hour in its American operational areas.

This is why the barge has been completely fitted for winter operation for both drilling and accommodating for 46 men-and it will be serviced by two specially built supply boats and helicopters for movement of personnel. It is 176 feet long by 151 feet wide and has three legs, each 175 feet long, to support the drilling platform high above the waves.

Next in the field is expected to be the first drilling rig of the British Petroleum Company (BP) which is due to be towed into position about 40 miles off the Yorkshire coast during the moderating weather of next spring. This will consist of a civil engineering barge, GEM 103, belonging to the British construction undertaking George Wimpey and Company Ltd., on which will be mounted the actual drilling rig with its towering lattice derrick brought from Trinidad by BP. The working platform is raised well above the sea by powerful jacks incorporated in the structure.