Buoys – rafts – and aquatic devices – Buoyancy providing attachment for pipe – log – or line
Reexamination Certificate
2001-11-29
2003-10-14
Basinger, Sherman (Department: 3617)
Buoys, rafts, and aquatic devices
Buoyancy providing attachment for pipe, log, or line
C166S367000, C405S224200
Reexamination Certificate
active
06632112
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a buoyancy system for supporting a riser of a deep-water, floating oil platform. More particularly, the present invention relates to a buoyancy system having one or more buoyancy modules including a rigid ecto-skeleton to withstand lateral or bending loads, and a buoyancy vessel to withstand internal pressure.
2. Related Art
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from the less productive oil reserves, or to reach the more distant oil reserves. Such distant oil reserves may be located below the oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves.
In addition, some oil reserves are located farther offshore, and thousands of feet below the surface of the oceans. Certain floating oil platforms, known as spars, or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend the thousands of feet to reach the ocean floor, and the oil reserves beyond.
It will be appreciated that these risers, formed of thousands of feet of steel pipe, have a substantial weight, which must be supported by buoyant elements at the top of the risers. The underlying principal of buoyancy cans is to remove a load-bearing connection between the floating vessel and the risers. Steel buoyancy cans (i.e. air cans) have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture.
In addition, as risers have become longer by going deeper, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the width of the well bays within the platform structure, while the length is limited by the practicality of handling the air cans. For example, the length of the air cans is limited by the ability or height of the crane that must lift and position the air can. Another factor limiting air can length is the distance to interference points with the platform structure below the air can. One disadvantage with more and/or larger air cans is that the additional length and larger diameter air cans adds more and more weight which also be supported by the air cans, decreasing the air can's ability to support the risers. Another disadvantage with merely stringing a number air cans is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
Vast oil reservoirs have recently been discovered in very deep waters around the world, principally in the Gulf of Mexico, Brazil and West Africa. Water depths for these discoveries range from 1500 to nearly 10,000 ft. Conventional offshore oil production methods using a fixed truss type platform are not suitable for these water depths. These platforms become dynamically active (flexible) in these water depths. Stiffening them to avoid excessive and damaging dynamic responses to wave forces is prohibitively expensive.
Deep-water oil and gas production has thus turned to new technologies based on floating production systems. These systems come in several forms, but all of them rely on buoyancy for support and some form of a mooring system for lateral restraint against the environmental forces of wind, waves and current.
These floating production systems (FPS) sometimes are used for drilling as well as production. They are also sometimes used for storing oil for offloading to a tanker. This is most common in Brazil and West Africa, but not in Gulf of Mexico as of yet. In the Gulf of Mexico, oil and gas are exported through pipelines to shore.
Drilling, production, and export of hydrocarbons all require some form of vertical conduit through the water column between the sea floor and the FPS. These conduits are usually in the form of steel pipes called “risers.” Typical risers are either vertical (or nearly vertical) pipes held up at the surface by tensioning devices; flexible pipes which are supported at the top and formed in a modified catenary shape to the sea bed; or steel pipe which is also supported at the top and configured in a catenary to the sea bed (Steel Catenary Risers—commonly known as SCRs).
The flexible and SCR type risers may in most cases be directly attached to the floating vessel. Their catenary shapes allow them to comply with the motions of the FPS due to environmental forces. These motions can be as much as 10-20% of the water depth horizontally, and 10s of ft vertically, depending on the type of vessel, mooring and location.
Top Tensioned risers (TTRs) typically need to have higher tensions than the flexible risers, and the vertical motions of the vessel need to be isolated from the risers. TTRs have significant advantages for production over the other forms of risers, however, because they allow the wells to be drilled directly from the FPS, avoiding an expensive separate floating drilling rig. Also, wellhead control valves placed on board the FPS allow for the wells to be maintained from the FPS. Flexible and SCR type production risers require the wellhead control valves to be placed on the seabed where access and maintenance is expensive. These surface wellhead and subsurface wellhead systems are commonly referred to as “Dry tree” and “Wet Tree” types of production systems, respectively.
Drilling risers must be of the TTR type to allow for drill pipe rotation within the riser. Export risers may be of either type.
TTR tensioning systems are a technical challenge, especially in very deep water where the required top tensions can be 1000 kips or more. Some types of FPS vessels, e.g. ship shaped hulls, have extreme motions which are too large for TTRs. These types of vessels are only suitable for flexible risers. Other, low heave (vertical motion), FPS designs are suitable for TTRs. This includes Tension Leg Platforms TLP), Semi-submersibles and Spars, all of which are in service today.
Of these, only the TLP and Spar platforms use TTR production risers. Semi-submersibles use TTRs for drilling risers, but these must be disconnected in extreme weather. Production risers need to be designed to remain connected to the seabed in extreme events, typically the 100 year return period storm. Only very stable vessels are suitable for this.
Early TTR designs employed on semi-submersibles and TLPs used active hydraulic tensioners to support the risers. As tensions and stroke requirements grow, these active tensioners become prohibitively expensive. They also require large deck area, and the loads have to be carried by the FPS structure.
Spar type platforms recently used in the Gulf of Mexico use a passive means for tensioning the risers. These type platforms have a very deep draft with a central shaft, or centerwell, through which the risers pass. Buoyancy cans inside the centerwell provide the top tension for the risers. These cans are more reliable and less costly than active tensioners.
Types of spars include the Caisson Spar (cylindrical), and the “Truss” spar. There may be as many as 40 production risers passing through a single centerwell. The Buoyancy cans are typically cylindrical, and they are separated from each other by a rectangular grid structure referred
Jones Randy A.
Karayaka Metin
Nish Randall W.
Schoenberg Robert G.
Tallavajhula Sudhakar
Basinger Sherman
EDO Corporation, Fiber Science Division
Thorpe North & Western
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