Page 63 - Hydrocarbon Exploration and Production Second Edition

50 Rig Types and Rig Selection

progression of field development activities. Phased developments using jackets are
common in coastal waters, for example South China Sea and the shelf GoM. Wells
drilled from large production platforms in the North Sea are drilled in a similar
fashion.
Jack-up rigs are either towed to the drilling location (or alongside a jacket) or are
equipped with a propulsion system. The three or four legs of the rig are lowered
onto the seabed. After some penetration the rig will lift itself to a determined
operating height above the sea level. If soft sediment is suspected at seabed, large
mud mats will be placed on the seabed to allow a better distribution of weight. All
drilling and supporting equipment are integrated into the overall structure. Jack-up
rigs are operational in water depths up to about 450 ft and as shallow as 15 ft.
Globally, they are the most common rig type, used for a wide range of environments
and all types of wells.
Semi-submersibles are used for exploration and appraisal in water depths too great
for a jack-up. A semi-submersible rig is a movable offshore vessel consisting of a
large deck area built on columns of steel. Attached to these heavy-duty columns are
at least two barge-shaped hulls called pontoons. Before operation commences on a
specified location, these pontoons are partially filled with water and submersed in
approximately 50 ft of water to give stability. A large-diameter steel pipe (riser) is
connected to the seabed and serves as a conduit for the drill string. The blowout
preventer (BOP) is also located at the seabed (subsea stack).
A combination of several anchors and dynamic positioning (DP) equipment
assists in maintaining position. Relocation of the semi-submersible vessel is made
possible by the utilisation of tugboats and/or propulsion machinery.
Heavy-duty semi-submersibles, for example Deepwater Horizon (rated
15,000 psi), can handle high reservoir pressures and operate in the most severe
metocean conditions in water depths down to 3000 m (Figure 4.3).
Drill ships are used for deep and very deep water work. They can be less stable in
rough seas than semi-submersibles. However, modern high-specification drill ships
such as Discoverer Enterprise can remain stable, and on target during 100 knot
winds using powerful thrusters controlled by a DP system. The thrusters counter
the forces of currents, wind and waves to keep the vessel exactly on target, averaging
less than 2 m off her mark, without an anchor.
Heavy-duty drill ships are capable of operating in water depths up to 3000 m
(Figure 4.4).
In some cases, oil and gas fields are developed from a number of platforms. Some
platforms will accommodate production and processing facilities as well as living
quarters. Alternatively, these functions may be performed on separate platforms,
typically in shallow and calm water. On all offshore structures, however, the
installation of additional weight or space is costly. Drilling is only carried out during
short periods of time if compared to the overall field life span and it is desirable to
have a rig installed only when needed. This is the concept of tender-assisted drilling
operations.
In tender-assisted drilling, a derrick is assembled from a number of segments
transported to the platform by a barge. All the supporting functions such as storage,
mud tanks and living quarters are located on the tender, which is a specially built