This invention relates to a novel form of semisubmersible oil and gas production and drilling vessel which comprises an outer production and drilling semi-submersible vessel completely surrounding an independently floating wellhead support buoy.
The first offshore oil wells were simply wells drilled from piers extended out into the water. In later years, the search for oil and gas further away from shore led to the development of freestanding offshore platforms and submersible barges. The drilling operations of these fixed platforms and submersible barges are similar to that of land operations: Wells are completed above the surface and wellheads are located on the platform. Conventional land drilling equipment is commonly used. In deep water, fixed platforms and submersibles become uneconomical with increasing water depth, since the supporting structure on these types of rigs must extend all the way to the ocean floor. In very deep water, construction costs for fixed platforms becomes astronomic. The need for oil and gas production and drilling in deeper waters has led to the development of floating drilling and production vessels such as drilling ships and semisubmersible drilling units. These floating drilling units are towed or moved under their own power to the well location and are positioned and secured by anchors and chains or wire ropes or by means of a dynamic positioning system. A drillship is simply an adaptation of a standard seagoing monohull ship with a moon pool or other means for carrying out drilling operations. A drillship has well-known advantages of mobility and high storage capacity. A drillship can generally travel at a relatively high speed and can fit through narrow passageways such as the Panama Canal so it can travel easily from its construction site to a distant offshore location. Also a drillship has a relatively high storage or payload capacity. The disadvantage of a drillship is that it has a long narrow hull and an extremely large water plane which, as is well known, makes it sensitive to wave action and storm sea conditions and subject to a large degree of pitch, roll and heave. During drilling operations, a floating vessel is connected to the seabed by a riser and the drillstring must be kept in contact with the bottom of the bore hole. Roll, pitch and heave motions make it difficult to maintain a drilling posture as the vessel would always be moving with respect to the ocean floor. Therefore, drillships work best in protected waters and during seasons when the sea is calm and generally are not useful in severe environments such as locations exposed to hurricanes or Arctic storms.
In response to the need for the development of offshore petroleum exploration and development in deep water where hostile sea conditions might be encountered, the semisubmersible drilling vessel was developed. The semisubmersible vessel substantially comprises a submerged base which most commonly consists of a pair of submerged hulls or pontoons. A series of vertical buoyant columns rise from the submerged base and support a horizontal deck or platform. The platform or deck is located far above the water line well above normal expected wave crests of giant ocean waves. This platform holds the living quarters, storage space for machinery and production equipment. Supported by the main deck is the cellar deck which provides for storage of subsea equipment and a substructure and drill floor upon which the draw works, rotary and derrick are mounted. Drilling operations are carried out through a moonpool through the cellar deck and main deck. The moonpool is generally located near the geometric center of the semisubmersible rig to minimize disruptions on the drilling operations caused by roll and pitch of the platform. A semisubmersible vessel is generally outfitted with extensive ballasting systems so that it can be transported to a drilling location in a low draft condition and then ballasted to a high draft condition for carrying out drilling operations. Recent advances in the understanding of hydrodynamics and the relationship between geometric hull configurations and vessel stability have led to the development of semisubmersibles with extremely reduced sensitivity to roll, pitch and heave motions so that drilling operations can be carried out with a minimum of down time due to rough seas and bad weather conditions. An example of such a stable semisubmersible drilling vessel is shown in U.S. Pat. No. 4,646,672.
The major disadvantages of semisubmersible drilling and production rigs as they are now known and used is their limited storage capacity and higher construction costs. The variable deckload capacity and crude oil storage capacity of a semisubmersible is relatively limited due to its open geometric configuration. Extensive ballasting systems capable of shifting ballast rapidly to maintain proper trim and of deballasting or ballasting as cargo is loaded or offloaded is required. Further, because the semisubmersible vessel as currently known and used comprises specially constructed hulls or pontoons held together by massive tubular bracings, fabrication costs of a semisubmersible vessel tend to be higher than standard ship construction.
Concurrent with the development of floating drilling vessels has come the development of equipment, methods and techniques for completing, producing and maintaining wells on the ocean floor. Commonly, complex multiwell systems are installed on the ocean floor with remotely controlled hydraulic and electronic control systems. Installation and maintenance of subsea wellhead equipment and expensive control equipment increases the cost of offshore production and often requires the extensive use of divers and diving operations and more complex remote control equipment to position and set the numerous valves of the wellhead assemblies and to conduct the periodic function and pressure testing of the wellheads. Placing wellhead assemblies on the sea floor makes routine inspection and maintenance more difficult and costly. Moreover, it requires the running of temporary risers for routine workovers. On a floating platform, completion of a well and installation of the wellhead above the surface is not practical because the production riser that connects the subsea wellhead with the above the surface wellhead assembly can be easily damaged. With drilling operations, mechanisms such as a riser slip joint, riser and guideline tensioners and a drillstring motion compensator have been developed to maintain a constant pressure on the drill bit so that drilling is not hampered by vessel motion. With production operations, a rigid connection from the wellhead to the floating vessel subject to even minimal roll, pitch and heave presents many problems not yet solved by proven technology. A production riser must stay in place for several years while oil is being produced from the well and motions of a floating vessel can over time create fatigue in the production riser and possibly lead to failure. Usually what is done therefore, is to install a wellhead assembly on the ocean floor and connect the ocean floor wellhead assembly to a floating vessel or a subsea pipeline with flexible flow lines.
Related art shows the development of floating drilling production and oil storage vessels that comprise extremely deep draft caissons or elongated buoys that minimize wave motion sensitivity and provide a stable platform by locating the bottom of the vessel far below the surface of the water.
U.S. Pat. No. 4,606,673 shows a spar buoy construction for a floating deep water production and oil storage. The vessel includes a riser system whereby risers are connected to a riser float chamber that moves along guides within a vertical passageway within the vessel. Adjustable connections for each riser ensure that all risers are under equal tension. The buoyancy chamber is totally submerged in operation and is held at a selected constant height above the sea bed. This vessel does not have a moon pool and is not designed for drilling or extensive workover operations.
U.S. Pat. No. 4,702,321 shows a deep draft caisson (700 to 800 feet draft) with a center well for conducting drilling operations. Each individual riser is connected to a separate buoyant means within the upper portion of the central well.
A recently developed alternative to the floating drilling and production vessel is the tension leg platform, which comprises a light buoyant platform anchored to the seabed by vertical tendons that are kept taut by excess buoyancy of the hull. The anchor lines consist of parallel or substantially parallel vertical tendons that are under high tension so that the platform is not affected by wave motions and maintains a fixed position relative to the ocean floor. It is thus relatively free from the heaving, rolling and pitching motions that a conventionally anchored floating vessel encounters. Various tension leg platforms are disclosed in U.S. Pat. Nos. 4,468,157, 4,620,820 and 4,664,554. Because the platform maintains a fixed position relative to the ocean floor, it is often possible to install wellhead assemblies on the platform above the water surface so that the numerous valves and gauges of the wellhead can be easily set in position and that periodic function and pressure testing can be carried out easily and so that the wellhead assemblies are close to vital high pressure and control equipment.
A major drawback to the tension leg platform as currently developed is that the vessel's cost increases tremendously as the vessel payload increases. An increased payload directly affects buoyancy requirements which in turn increases tendon and foundation requirements. A heavier platform with complete production, drilling and storage facilities requires a larger vessel to maintain buoyancy and a stronger foundation and tendons to hold the vessel in place. Vessel payload can be a critical factor in designing a vessel for operation in remote waters where there is not an easy access to supply ships for delivering fuel, water and drilling fluids and tankers for removing oil production.