Source of information: JSC Plant «Potential», 2010
The drill ship is an adaptation fo a standard seagoing ship of mono-hull form with the addition of a substructure with a moon pool and/or cantilevers from which the drilling operations may be carried out. These vessels are also equipped with some additional means of positioning the unit over the drill center so that the vessel will maintain a close relationship with the bore hole in the seabed. These vessels may be held in position by either a mooring system or a dynamic positioning system. It is well known that ship type drilling units are very susceptible to wave action and will tend to move in a direct relationship with the sea state encountered. Since the vessel is connected to the seabed by a riser and the drill string is in contact with the bottom of the bore hole, motions of the vessel with respect to the seabed are extremely important to be able to maintain the drilling posture.
Drillships are generally preferred for deepwater drilling in remote locations with moderate weather environments because of their mobility and large load carrying capability. Several marine drilling contractors operate drill ships as well as Semis and Jack-ups. Because of its conventional ship shaped, hull the drill ship is more prone to movement in a seaway than the semi-submersible, and is therefore subject to longer periods of downtime due to wind and wave action. For this reason drill ships are more usually (but not always) found working in the smoother waters of the world, while semi-subs can drill in the most hostile environments. This disadvantage is partially offset by the drill ship's ability to move from one location to the next rapidly and under its own power, with considerable economic advantage.
For oil and gas offshore Exploration and Production (E&P) operations in waters deeper than 300 meters, floating platforms such as Drillships and Semi-Submersible Platforms are used. These vessels must be kept stationary at a desired location (reference) to accomplish their offshore E&P tasks. Therefore, the platform must have means of producing forces and momenta to counterbalance environmental forces (wind, currents and waves) in order to keep it at the desired location. In the most common case, the platform is equipped with anchor lines. A mooring system usually has 8 to 12 anchor lines for each platform.
This problem was overcome with the development of the Dynamic Positioning System (DPS). The DPS controls platform displacements in the three horizontal degrees of freedom: surge, sway and yaw. The DPS is composed of a controller, a sensor system, a thruster system, and a power system. The sensor system feeds to the controller (computer) with information about the platform positioning and environmental parameters – winds, currents and waves. The controller commands the action of thrusters, installed on the bottom of the platform hull, that generate the forces and momenta needed to counteract the environmental forces and thus keep the platform at the reference. The controller keeps the platform within a tolerance radius of about 2% to 6% of the water depth, depending on the surface equipment and the operation to be executed. Furthermore, a DPS can also assist a moored platform as, for example, changing its head to minimize the environmental loads.
The Discoverer Enterprise of Transocean is the first ultra- deepwater drillship with dual activity drilling technology, which aims to reduce the cost of an ultra deepwater development project by up to 40 percent. This massive, multi-purpose vessel can work in the deepest waters being explored in the world today. It can drill, test and complete wells in water depths of up to 10,000 feet - almost two miles. More important, the Discoverer Enterprise's patented dual-activity drilling technology has the unique capability to conduct drilling operations simultaneously, rather than sequentially in series.
A new development in ultra deep water drilling is the use of a surface BOP with a slimline pressurized riser. The objective of this method is obtaining cost reduction in drilling operations by eliminating heavy subsea equipment, which enables the drilling contractor to use a lower day rate rig for ultradeep water depths.