Drilling Semi-Submersible
Semisubmersible rigs are floating offshore drilling units with pontoons and columns that, when flooded with water, cause the unit to partially submerge to a predetermined depth. Most semisubmersibles are anchored to the sea bottom with mooring chains, but some use dynamic positioning ("DP"), which allows the vessels to be held in position by computer-controlled propellers, known as thrusters. Semisubmersibles are classified into five generations, distinguished mainly by their age, environmental rating, variable deck load, and water-depth capability. Third-generation, conventionally-moored semisubmersibles rigs are suitable for drilling in water depths ranging from 1,200 to 3,400 feet. The fourth-generation semisubmersible are capable of drilling in water depths of up to 5,750 feet.
The expansion of the drilling industry to offshore locations has led to the development of several types of drilling units. These consist of submersible drilling platforms, jack-up drilling platforms and two major classes of floating vessels. The two major classes of floating drilling vessels are those of conventional hull form or drill ships and those of semi-submersible or column stabilized units.
A semi-submersible drilling vessel for offshore drilling of the catamaran type has two substantially horizontal, parallel and spaced, elongated submersible hulls, a deck supported above said hulls and connected thereto by a mounting structure, the deck having a central opening overhanging the space between the two hulls and being surmounted by a derrick. Said mounting structure has vertical columns, whereof at least two pairs connect the ends of the hulls to the deck.
In response to the need for development of offshore petroleum exploration and development in increasingly more hostile and deeper water, the significance of a vessel with improved motions with respect to the seabed became apparent. For this purpose, industry has adopted and used a series of semi-submerged or semi-submersible drilling units. In essence, all of these semi-submersible drilling units or vessels comprise a wide base and totally submerged pontoons or mats. A series of vertical buoyant columns rise from this submerged base supporting a horizontal deck or platform which is maintained well above the normal expected wave crests. Upon this platform or deck, the living quarters, machinery spaces and drilling package are located. The drilling center usually consists of a cellar deck or storage area for the subsea equipment, a moon pool through which the drilling operation is carried out which is usually located in the cellar deck and a substructure which is mounted above the cellar deck area upon which the draw works, rotary and derrick are mounted. Adjacent to this area is a pipe rack area for the storage of the marine riser, drill pipe, drill collars, casing and other tubular products. The semi-submersible drilling unit is also maintained in position against the forces of the environment by use of either a fixed mooring system or an active propulsion system (dynamic positioning) a combination of the two (thruster-assisted mooring). In either case, the semi-submersible is still supported on the ocean surface by its own buoyant effect and is also suspectible to wave induced motion.
There have been recent developments in rotatable and swivable drilling units of the ship type to reduce the sensitivity of the unit to the roll and pitch motions and thereby improve their motion characteristics, thus reducing their down time for weather. These developments, however, have done little or nothing for the improvement of the heave or vertical motions of the platform with respect to the ocean floor. Many new devices have been introduced, such as motion compensators, riser tensioners and guideline tensioners which are active devices and greatly reduce the effect of the heave motion of the unit for the drilling operation. These items, however, are of a mechanical nature and do result in down time due to maintenance and also have their limits with respect to the range of sea states which they can effectively dampen.
The generally recognized design of semi-submersible platforms for minimizing the sensitivity of the unit to wave induced motions is known to consist of a lower hull or a group of pontoons upon which are deployed any number of buoyant columns arranged such that their collective water plane areas are spread significantly to provide a stable platform. The buoyancy for the unit is provided by the displacement of the lower hull or hulls and the vertical columns of the unit below the waterline. The water plane area of these vertical columns, the effective cross-sectional area of the columns at the lever of the waterline, is known to be a significant design factor for both minimizing the wave motion sensitivity and providing a stable platform with significant load carrying-capability to allow the vessel to perform its intended function. It is a trade-off between these requirements for improved motion characteristics for better drilling operations and required water plane area for a stable platform that is normally the prime concern of a naval architect with respect to the design of a semi-submersible drilling unit.
The geometric configuration of the columns of a semi-submersible unit are somewhat determined by the vessel's intended service as well as by its designer's philosophy. With increased needs for semi-submersibles on a worldwide market, it became essential to have units which were more highly mobile thus the current gereration of twin hull semi-submersibles having four, six, and eight columns evolved. The lower hulls of these units are generally of a ship shape form and during transit the unit performs similar to a catamaran type vessel.
Various equipments are provided on such drilling vessels, particularly cranes and means for the storage and handling of pipes. The term pipes is understood to mean two basic types of long cylindrical elements used in large quantities on such a drilling vessel and namely drill pipes having a length of e.g. about 10 meters and a diameter of about 12 centimeters and risers having a length of about 15 meters and a diameter of a little more than one meter. Usually these pipes are stored horizontally. It is therefore necessary to provide a handling means, which is designed for lifting the pipes and for placing them in their vertical use position.
To obviate this needless change of position of the pipes, means have already been proposed for the vertical storage of individual drill pipes. Thus the drill pipes are stored in juxtaposed manner over the entire surface of a storage area located beneath the deck of the drilling vessel, so as to lower the centre of gravity thereof. Storage particularly suffers from the disadvantage of occupying a considerable floor or ground space and of only permitting a successive access to the different drill rods, unless a device is provided for the upward extraction thereof and which is able to move above the entire ground surface, which increases the overall dimensions.
An article in the journal "Ocean Industry", August 1985 reports on a catamaran-type drilling vessel project, in which the risers are stored vertically within a parallelepipedic median caisson connecting the two hulls of the catamaran. However, this median caisson suffers from serious disadvantages. On the one hand its presence between the hulls is prejudicial to the handling of the templates, cf. U.S. Pat. No. 4,435,108 and whose overall dimensions are about 10.times.40 m. Moreover, this median caisson produces a considerable draught of water and is highly exposed to a swell. There is nothing which outweighs the advantages of a catamaran more than adding a cross-wall thereto, which makes it possible to significantly increase the number of anchoring means.
According to another drilling vessel project of Friede & Goldman (New Orleans, La., USA) and called the "Trendsetter", below the derrick is provided an annular, central, cylindrical caisson, in whose peripheral thickness are vertically stored the risers. However, as in the case of the previous project, the presence of a central caisson is prejudicial to the handling of the templates. In addition, the volume of this central caisson must be severely limited otherwise it would offer an excessive surface area to the waves. Furthermore, the drill pipes are still horizontally stored. This caisson is also exposed to very high forces and stresses as a result of hydrostatic force, which involves a considerable bracing. However, braces are generally the weak point of drilling vessels due to the relative weakness of the welds. It is therefore best not to have an excessive increase in the number of braces. Finally, there is a risk of the water rising within the open volume of the caisson during ramming movements of the drilling vessel, which would have the effect of producing a prejudicial swabbing effect.
In February 2001 Maersk Contractors ordered a new deepwater semi-submersible of the type DSS-20-CAS-M for use in the Caspian Sea.This is one of the DSS-20 types of Drilling Semi-Submersibles which has been developed jointly by Keppel FELS and MSC.The DSS-20-CAS-M, named Maersk Explorer, and also called Lider, has just been delivered to Maersk Contractors and is the most modern and powerful drilling unit in the Caspian Sea. The unit is rated for a drilling depth of 9,140 m and a maximum water depth of 1,000 m with a variable load capacity of 4,000 m.t.
The DSS-20-CAS-M design is a moored deepwater drilling semi-submersible. Main advantages of the semi concept are excellent seakeeping characteristics in combination with large working deck spaces and a very large moonpool area. The hull shape comprises two pontoons, four columns and a large enclosed twin-level deckbox structure. At operational draft the two catamaran pontoons are well below the water surface and waves for optimum seakeeping characteristics, while for the transit condition the unit will be deballasted to floater draft so as to minimize drag when the unit is towed to a new location. The pontoons are fitted with four pump rooms, one below each column, storage winch spaces, and various tanks for consumables and water ballast tanks.
Capable of operating in water depths of up to 1000 meters and drill depths of up to 9140 meters, it has a variable deck load of 4000 tons and is able to accommodate 130 people. It is specifically designed to handle large water depth and high formation pressures, which are anticipated at the first well in the Zafar Mashal structure.
The unit is equipped with a 180 ft drilling derrick rated at 2,000 kips, fitted with a crown mounted compensator, a topdrive and set back with star racker system. A 49.5" rotary table, control cabin, racker system, roughneck, riser tensioners, drawworks and manifolds are fitted at drillfloor level. Access to the drillfloor is from two sides; a catwalk equipped with a conveyor belt is fitted on the aft side for the supply of pipes and casings to the drillfloor. On the forward side of the drillfloor risers are supplied from the vertical riser storage. A vertical riser storage is fitted to speed up riser handling and to provide an efficient storage area for the risers. The drillfloor is centrally mounted above the moonpool. The large transversely oriented moonpool opening size is 7.5 x 28 m. The size of the total moonpool area (clear area between bulkheads, opening plus cellar deck) is 13.5 x 35.5 m. The moonpool is fitted with a BOP transporter/elevator and a Xmas tree trolley. Access to the moonpool is provided from both starboard and port side. An overhead crane has been fitted on the port side in order to move the BOP from its storage area on the main deck to the BOP transporter in the moonpool. On the starboard side, Xmas trees can be moved from their storage position on the main deck to the Xmas tree trolley in the moonpool.
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