A Mineral Storehause
109
stand on the sea bed, are in continual danger of being swept off—
station. The flexibility of the drilling pipe allows some leeway but
the movement this permits is limited—generally to about 5 per cent
of the length of pipe in the water in any one direction. Obviously,
the problem is most serious with oating and semi—submersible rigs,
and ever since these rigs were thought of, experiments have been
carried out to nd the best way of holding them on—station. The
Shell Oil Company, for example, have devised a mooring system
whereby the rig is held by eight lines connected via spring buoys to
anchors on the sea bed. The tension in the mooring lines is monitored
continuously and can be adjusted using electrically operated winches.
Another system does away with the anchor lines and instead uses a
propeller, which can swing in any direction to keep the rig on—
station. Any change in position is recorded by instruments which
feed the information direct to a computer in the control room. The
computer then calculates the necessary correction and instructs the
propeller controls accordingly.
One of the greatest searches for oil and natural gas involving these
islands of steel centres on the continental shelf of Europe in the
shallow North Sea. At the time of writing, by far the most important
nds have occurred in the British sector of the shelf. The search
started o‘iciaHy in 1964 when the British Government invited
applications from oil companies to explore and exploit this area of
the North Sea. The area had been divided up into concessions or
blocks each of approximately 100 square miles. In all, about 34,000
square miles of the North Sea have been allocated to twenty—ve
groups representing more than a hundred different concerns. By
1968, at least four major gas elds had been located and between
them they could probably supply some 85 million cubic metres of
gas a day—the equivalent of three to four times the present output
of town gas in the United Kingdom—for twenty to thirty years. Oil
is also being sought in the northern area of the sea off the coast of
Scotland.
The gas generally occurs in the layers of porous rock known as the
Rotliegendcs sandstone. Since this particular bed of rock seems to
have been deposited originally under desert conditions, the gas is
unlikely to have originated there. However, the Rotliegendes lies
immediately above coal measures; the gas has probably migrated
gradually out of the coal into the porous
sandstone. Above the
Rotliegendcs is a formation called the Zechstein which consists of
109
stand on the sea bed, are in continual danger of being swept off—
station. The flexibility of the drilling pipe allows some leeway but
the movement this permits is limited—generally to about 5 per cent
of the length of pipe in the water in any one direction. Obviously,
the problem is most serious with oating and semi—submersible rigs,
and ever since these rigs were thought of, experiments have been
carried out to nd the best way of holding them on—station. The
Shell Oil Company, for example, have devised a mooring system
whereby the rig is held by eight lines connected via spring buoys to
anchors on the sea bed. The tension in the mooring lines is monitored
continuously and can be adjusted using electrically operated winches.
Another system does away with the anchor lines and instead uses a
propeller, which can swing in any direction to keep the rig on—
station. Any change in position is recorded by instruments which
feed the information direct to a computer in the control room. The
computer then calculates the necessary correction and instructs the
propeller controls accordingly.
One of the greatest searches for oil and natural gas involving these
islands of steel centres on the continental shelf of Europe in the
shallow North Sea. At the time of writing, by far the most important
nds have occurred in the British sector of the shelf. The search
started o‘iciaHy in 1964 when the British Government invited
applications from oil companies to explore and exploit this area of
the North Sea. The area had been divided up into concessions or
blocks each of approximately 100 square miles. In all, about 34,000
square miles of the North Sea have been allocated to twenty—ve
groups representing more than a hundred different concerns. By
1968, at least four major gas elds had been located and between
them they could probably supply some 85 million cubic metres of
gas a day—the equivalent of three to four times the present output
of town gas in the United Kingdom—for twenty to thirty years. Oil
is also being sought in the northern area of the sea off the coast of
Scotland.
The gas generally occurs in the layers of porous rock known as the
Rotliegendcs sandstone. Since this particular bed of rock seems to
have been deposited originally under desert conditions, the gas is
unlikely to have originated there. However, the Rotliegendes lies
immediately above coal measures; the gas has probably migrated
gradually out of the coal into the porous
sandstone. Above the
Rotliegendcs is a formation called the Zechstein which consists of