Page 83 - 4274

3.1 Design and operation
The submersible drilling platform is supported on large
pontoon-like structures. These pontoons provide buoyancy
allowing the unit to be towed from location to location.
Once on the location, the pontoon structure is slowly
flooded until it rests securely on its anchors, of which there are
usually two per corner.
The operating deck is elevated 100 feet above the pontoons
on large steel columns to provide clearance above the waves.
After the well is drilled, the water is pumped out of the
buoyancy tanks and the vessel is re-floated and towed to the next
location.
Submersibles, as they are known informally, operate in
relatively shallow water, since they must rest on the sea floor.
Other floating vessel types are used in deeper water depths. The
term Mobile Offshore Drilling Unit (MODU) is generally used for
all offshore drilling rigs that can be moved from location to
location.
3.2 Evolution
The first offshore mobile drilling platform was the
Hayward-Barnsdall Breton Rig 20, first operated in 1949 [2]. This
rig had evolved from the inland drilling barges which were used to
drill in marshes and protected waters in up to 10 feet of water. The
Breton Rig 20 was 160 feet by 85 feet, and could work in 20 feet
water depth.
By 1958, the number of submersible drilling rigs had
increased to around 30 [2].
In 1961, Shell Oil successfully converted an existing
submersible rig Blue Water Rig No.1 (fig. 3.2) into the first semi-
submersible drilling unit for operation in the Gulf of Mexico when
it was found to have good stability and motions whilst being towed
at a partial draught [3].
Alden J. Laborde designed and constructed the first
purpose-built V-shaped semi-submersible drilling rig, Ocean
Driller, delivered in 1963 [2].

83