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4 DRILLSHIPS AND SHIP-SHAPED OFFSHORE UNITS
4.1 Drillships
Each type has advantages and disadvantages that help
determine which type vessel is best suited to a particular
application. Generally speaking, drillships are selected for their
rapid mobility, lower day rates, larger storage capacity, and deep-
water capability. The semi may be preferred because of its
stability, particularly for drilling in rough seas. Barges with
drilling rigs are used normally only in sheltered areas but offer
economy of operation.
Drillships date all the way back to 1953 when an ex-naval
patrol craft, the Submarex, was converted to drill core sample
holes to a depth of 3,000 ft off the coast of California. Three years
later, the drillbarge Cuss I became the first full-scale floating
drilling vessel. In 1961, the Cuss I was fitted with four manually
controlled swiveling thruster units for dynamic positioning (DP),
allowing the vessel to drill without a mooring. The first custom-
built DP drillship, Global Marine's Challenger, became fully
operational in 1968.
Station-keeping ability is important in determining the
amount of time a drilling vessel can operate effectively. In recent
years, requirements for the ability of DP drilling vessels to
maintain a given position have increased dramatically, enabling
them to challenge deeper waters and more hostile environments—
thus enhancing oil and gas recovery opportunities. Many of today's
dynamically positioned drillships, utilizing computer-controlled
thrust- ers, are drilling in water over 2,000 ft deep, and some are
drilling in waters of 5,000 to 10,000 ft (fig. 4.1).
Drillships with conventional anchoring systems usually
drill in less than 2,000 ft of water. A typical drillship anchoring
system consists of 8 to 12 anchors, radially spaced from the bow
and the stern of the vessel. Another system, turret mooring, allows
360° rotation of the drillship in relation to the mooring spread,
thereby affording the ability to maintain the ship's heading into the
weather. Drillships use subsea blowout preventers connected to the
subsea wellhead housing.
Typically employed in deep and ultra-deep waters,
drillships work in water depths ranging from 2,000 to more than
10,000 feet (610 to 3,048 meters).

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