MOORJNGS
9
examples include ships converted to floating production Systems. In the late
1960s, companies initiated research and design for these semisubmersible, multihull tubular structures and ships that would remain relatively stable in rough
seas. In the late 1990s, the first three draft caisson vessels, or spars, were
installed for use in 180 m water depths. Spars are floating vertical cylinders that
support production decks above storm waves. These structures are controlled
to remain essentially still in stormy seas. Some need to be towed from place to
place; others are self-propelled. During drilling and production operations, these
structures are kept in place with mooring lines and thrusters. The computercontrolled thrusters monitor the mooring line forces and accurately position
the structure over the wellhead. One of the first semisubmersible structures
was the Sedco 709 with a water depth rating of 1800 m. By the year 2000,
semisubmersibles using dynamic positioning were designed for 3000 m water
depths.
1.3 MOORINGS
Temporary Anchor Moorings
A classical example of temporary offshore mooring is the spread mooring
configuration for a ship in relatively shallow water. Six to eight cables of wire
rope or chain are unreeled from onboard winches symmetrically placed around
the perimeter of the ship. Tug boats aid in spread mooring installations. In
place, each cable hangs as a catenary curve and is attached either directly to
a drag embedment anchor in the seabed or to a buoy that is anchored. An
example of a spread mooring configuration is shown in Figure 1.6. Particular
mooring configurations were reported by Baar et al. (2000) and O’Brian and
Muga (1964).
Figure 1.6 A spread mooring configuration.
In a typical spread mooring operation for a semisubmersible in deep water,
a work vessel transports each anchor while pulling out its cable attached to the
semisubmersible. The vessel lowers the anchor and installs a locating surface
buoy just above it. At présent, temporary Systems for semisubmersible drilling
9
examples include ships converted to floating production Systems. In the late
1960s, companies initiated research and design for these semisubmersible, multihull tubular structures and ships that would remain relatively stable in rough
seas. In the late 1990s, the first three draft caisson vessels, or spars, were
installed for use in 180 m water depths. Spars are floating vertical cylinders that
support production decks above storm waves. These structures are controlled
to remain essentially still in stormy seas. Some need to be towed from place to
place; others are self-propelled. During drilling and production operations, these
structures are kept in place with mooring lines and thrusters. The computercontrolled thrusters monitor the mooring line forces and accurately position
the structure over the wellhead. One of the first semisubmersible structures
was the Sedco 709 with a water depth rating of 1800 m. By the year 2000,
semisubmersibles using dynamic positioning were designed for 3000 m water
depths.
1.3 MOORINGS
Temporary Anchor Moorings
A classical example of temporary offshore mooring is the spread mooring
configuration for a ship in relatively shallow water. Six to eight cables of wire
rope or chain are unreeled from onboard winches symmetrically placed around
the perimeter of the ship. Tug boats aid in spread mooring installations. In
place, each cable hangs as a catenary curve and is attached either directly to
a drag embedment anchor in the seabed or to a buoy that is anchored. An
example of a spread mooring configuration is shown in Figure 1.6. Particular
mooring configurations were reported by Baar et al. (2000) and O’Brian and
Muga (1964).
Figure 1.6 A spread mooring configuration.
In a typical spread mooring operation for a semisubmersible in deep water,
a work vessel transports each anchor while pulling out its cable attached to the
semisubmersible. The vessel lowers the anchor and installs a locating surface
buoy just above it. At présent, temporary Systems for semisubmersible drilling