Deep and ultradeep water

In deeper, more open waters up to 5,000 feet deep over continental shelves, drilling is done from free-floating platforms or from platforms made to rest on the bottom. Floating rigs are most often used for exploratory drilling and drilling in waters deeper than 3,000 feet, while bottom-resting platforms are usually associated with the drilling of wells in an established field or in waters shallower than 3,000 feet.

One type of floating rig is the drill ship, which is used almost exclusively for exploration drilling before commitments to offshore drilling and production are made. This is an oceangoing vessel with a derrick mounted in the middle, over an opening for the drilling operation. Such ships were originally held in position by six or more anchors, although some vessels were capable of precise manoeuvring with directional thrust propellers. Even so, these drillship roll and pitch from wave action, making the drilling difficult. Additional production is established by a direct connection with the production platform or by connecting risers between the subsea wellheads and the production platform.

The Seastar floating system operates in waters up to 3,500 feet deep. It is essentially a small-scale tension-leg platform system that allows for side-to-side movement but minimizes up-and-down movement. Given the vertical tension, production is tied back to ‘dry’ wellhead (on the surface) or to ‘trees’ on the platform that are similar to those of the fixed systems. Drilling platforms capable of ultra-deepwater production line that is, beyond 1,830–2,130 meters deep, include tensionleg systems and floating production systems (fps), which can move up and down in response to ocean conditions as semisubmersibles perform.

Without such infrastructure, wet trees are used and petroleum is exported to a nearby FPS. A more versatile ultra deepwater system is the spar type, which can perform in waters nearly 3,700 meters (approximately 12,000 feet) deep. Spar systems are moored to the seabed and designed in three configurations: (1) a conventional one-piece cylindrical hull, (2) a truss spar configuration, where the midsection is composed of truss elements connecting an upper, buoyant hull with a bottom element (soft tank) containing permanent ballast, and (3) a cell spar, which is built from multiple vertical cylinders. In the cell spar configuration, none of the cylinders reach the seabed, but all are tethered to the seabed by mooring lines.

Author: Mohsen Fatahi, Directional Drilling Project Manager.