Drillships
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 exactly as they sound: ships designed to carry out drilling operations. These boats are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or 'moonpool'), extending right through the ship down through the hull, which allow for the drill string to extend through the boat, down into the water. Drillships are often used to drill in very deep water, which can often be quite turbulent. Drillships use what is known as 'dynamic positioning' systems. Drillships are equipped with electric motors on the underside of the ships hull, capable of propelling the ship in any direction. These motors are integrated into the ships computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template, to ensure that the ship is directly above the drill site at all times.
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. However, in water depths deeper than 1000 meters, a mooring system becomes uneconomic or impracticable.
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.
There is always a lag of time between start of exploration drilling and first production in a new area. This has to do with the time to make commercial finds but also, when it comes to the increasing water depths, it has to do with the qualification of new deepwater completion and production technologies. When a field moves into the production phase there is another time lag before well maintenance and intervention is needed.Some oil companies have made three categories of operations on this basis and the service industry have answered by proposing suitable vessels for the three different segments.
Category A: 115 m ships for wireline well intervention and well stimulation.
Category B: 20,000 ton semisubmersibles or 30 to 40,000 ton ships for riser based medium range operations.
Category C: Fullblown all-capacity 40 to 50,000 ton semisubmersibles, 70 to 100,000 ton ships. These vessels are purpose designed for drilling, completion and full workover capabilities with a number of efficiency improvement features. The most advanced are equipped with full dual operations capability.
West Navigator / West Navion was contracted during the boom of 1997, with a displacement of 100,000 tons and an overall length of 253 meters. As of 2002 there were only four competing vessels with basically the same features: the Discoverer Enterprise class vessels and the Saipem 10000. They are all built for maximum efficiency in combining drilling and completion and for full workover of subsea wells. Smedvig took delivery of the new generation drilling units West Navion in February, 2000. West Navion was based on a Navion multipurpose shuttle tanker (MST) already under construction at Samsung, Korea. She is equipped with full capacity dual Ramrig systems, aligned longitudinally. In the past the drilling and completing of subsea wells has been done by standard drilling rigs with all critical path activities being dependant upon the successive availability of the main work string/rotary/drillfloor. There is a huge potential in removing activities from the critical path and have parallel activities done, thus reducing overall time to perform the operations. Many of the rigs contracted during the 1997 building boom are equipped with some sort of efficiency boost tools. The simplest is an onboard make-and-break assembly that builds or breaks out bottom hole assemblies, drillpipe stands and casing stands. The more advanced in this respect is the full capability twin rig systems that are found on less than ten vessels on a world-wide basis. Dual operations from these vessels are now being proven in a gradual step-by-step approach. In a single well application the advantage lies in using the dual system for parallel making and breaking bottomhole assemblies, tool strings and pipestands and in running casing/BOP in parallel with drilling top hole. In a multiwell subsea template situation there is an additional potential in performing simultaneous in-water operations. While the main rig is in a BOP/riser drilling mode, the auxiliary rig can drill tophole and install surface casings in the next well.
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. At 835 feet, the Discoverer Enterprise is almost as long as three U.S. football fields, and it can drill a well more than 6.5 miles beneath its drill floor. 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. Two full-capability, full-capacity drilling stations - in actuality two separate drilling rigs -- carry out these parallel operations under a single, massive derrick. Two full-sized rotary tables are designed into a drill floor more than twice as large as a conventional one.
Global Marine Inc. is a holding company incorporated in Delaware in 1964. The Company, which is headquartered in Houston, Texas, provides offshore oil and gas contract drilling services on a daily-rate ("dayrate") basis and offshore oil and gas drilling management services on a dayrate or completed-project, fixed-price ("turnkey") basis, and participates in oil and gas exploration and production activities. The Glomar C. R. Luigs and Glomar Jack Ryan are two state-of-the-art drillships constructed in Northern Ireland by constructedby Harland and Wolff. The Glomar C. R. Luigs was delivered in the first quarter of 2000, and delivery of the Glomar Jack Ryan took place in the third quarter of 2000. Those delayed delivery dates had no effect on the drillships' operating contracts with customers and only modestly impacted earnings.
The Company's GLOMAR C.R. LUIGS, GLOMAR JACK RYAN, and GLOMAR EXPLORER are dynamically-positioned, ultra-deepwater drillships capable of drilling in water depths up to 9,000 feet, 8,000 feet, and 7,800 feet, respectively, as currently equipped. With modifications, maximum water depth capabilities are 12,000 feet for the GLOMAR C.R. LUIGS and GLOMAR JACK RYAN, and 10,000 feet for the GLOMAR EXPLORER. The GLOMAR ROBERT F. BAUER is a conventionally-moored drillship capable of drilling in maximum water depths of 2,750 feet.
During the 90's IHC Gusto developed a new generation drillship, capable of drilling in maximum 10,000 ft water depth.The design was elaborated to a very detailed level including extensive model testing and approval by DNV.The motion behaviour of this vessel in medium and long periodical seas compares favourable with that of the standard drilling semi-submersibles in the industry. IHC Gusto offers the industry the "Gusto 10,000" design using its "Pelican" drillship experience through to its FPSO technology taking into consideration such important aspects as operational efficiency and HSE. The Gusto 10,000 is able to operate at much higher payloads compared with other deep water rigs, resulting into reduced operating costs. The "Gusto 10,000" is designed for dual redundant, DPS -2, and even triple redundant, DPS-3, dynamic positioning operations (compatible with NMD 2 / NMD 3 notation). In the late 90's two drilling vessels, the "Pride Africa" and the "Pride Angola" were constructed and delivered to their Owner Pride International. Ever since these vessels are in operation in the challenging West Africa environment they proved to be very successful.
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. The longstanding experience of IHC Gusto in the design of drilling vessels has been established with the Pelican Class vessels in the period of 1970 to 1985 and the Gusto10,000 series - Pride Africa, Pride Angola, Glomar C.R. Luigs and Glomar Jack Ryan - more recently. IHC Gusto Engineering B.V. has developed a DP drill ship design in which this technology is incorporated. The vessel is able to drill in water depths up to 12,000 ft using a surface BOP stack and slimline riser, while allowing conventional subsea drilling up to 5,000 ft using a subsea BOP and marine drilling riser.
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