Tendon-based floating structure

Details Tendon-based floating structure Tendon-based floating structure

1999-04-15

2002-08-13

Morano, S. Joseph (Department: 3617)

Ships

Floating platform

C405S224000

Reexamination Certificate

active

06431107

ABSTRACT:

BACKGROUND
The invention relates generally to floating structures. More specifically, the invention is directed to a floating structure for supporting a deck structure or other superstructure above a water surface.
Offshore petroleum operations, such as exploration, drilling production, and storage, generally require a deck structure or other superstructure supported above the water surface with sufficient air gap to remain clear of the waves. A superstructure may comprise a diverse array of equipment and structures depending upon the type of offshore operation to be performed. For example, a superstructure for drilling a well and producing hydrocarbons may include equipment for drilling and producing hydrocarbons, living quarters for a crew, equipment storage, and a myriad of other structures, systems, and equipment. During operation, additional payload of drill pipes, drill mud, hydrocarbons, helicopters, and other items may be added. The combined weight of such superstructures and payload is typically measured in thousands of tons. The superstructure may be supported on a generally rigid structure fixed to the seafloor or on a floating structure. Fixed structures are typically viable in shallow waters, typically waters with depths less than 1,000 feet. Floating structures are generally viable in both shallow and deep waters.
There are several basic requirements for a floating structure employed to support a superstructure. The floating structure must provide sufficient buoyancy to support the weight of the superstructure and any payload. The floating structure must be stable in any condition while supporting the weight of the superstructure and payload above the water surface. The floating structure must be able to “keep station” about a fixed position within a limited range of lateral excursions throughout the duration of a given operation. The floating structure must have acceptable “seakeeping” characteristics relating to the oscillatory motions, velocities, and accelerations of the floating structure. The station keeping and seakeeping characteristic requirements are generally determined by operational concerns, such as crew comfort, equipment operability, riser safety, and station keeping system fatigue.
Floating structures generally provide buoyancy through means of a submerged hull employing Archimedes principle. Typically, a void portion of a hull extends below the water surface, displacing a volume of water to provide an uplifting force. Hull construction is typically reinforced steel plating, but other materials, most notably concrete, are also employed. The submerged portion of the hull is most commonly placed directly adjacent to the water surface, such as for a typical ship. Unlike a ship, however, placement of buoyancy is variable.
Floating structures are generally stabilized by one or more of several methods. The first and most common method provides stability through placement of buoyancy directly adjacent to the water surface to create waterplane area. Many configurations of waterplane area are utilized to stabilize the floating structure. Ships are one example wherein a single large waterplane area provides the required stability. A semi-submersible provides an example wherein multiple waterplane areas, spaced widely apart, are employed to reduce the size of the waterplane area required to provide stability. In both examples, as the floating structure pitches and rolls, the center of buoyancy of the submerged hull moves as the waterplane changes to provide a righting moment. While the center of gravity for the floating structure may be located above the center of buoyancy, the floating structure can nonetheless remain stable. Increasing the waterplane area or using multiple, widely spaced waterplanes is generally the cheapest and simplest method for providing stability. The seakeeping consequences of a large waterplane, however, are generally undesirable.
The second method provides stability by placement of the center of gravity of the floating structure below the center of buoyancy. The combined weight of the superstructure, hull, payload, ballast and other elements may be arranged to be below the center of buoyancy. The floating structure will pitch about the center of rotation with the reversed pendulum effect of the weight providing a righting force. Arrangement of the center of gravity below the center of buoyancy may be a difficult task. One method employed to lower the center of gravity requires the addition of fixed ballast below the center of buoyancy to counterbalance the weight of superstructure and payload. Fixed ballast, generally is a negatively buoyant hull structure or material added to the floating structure to lower the center of gravity. There are two main types of fixed ballast, structural weight and non-structural solid ballast. Examples of structural fixed ballast include permanent ballast tanks, flooded truss portions, and concrete oil storage tanks. Examples of solid ballast include metal filings, pig iron, iron ore, and concrete placed within or attached to the hull structure. The advantage of the weight arrangement is that it may be achieved such that seakeeping performance is unaffected while stability is increased. Another method is to move the center of buoyancy higher, generally by placing buoyancy adjacent to or near the water surface. The disadvantage of buoyancy rearrangement is that it may require an increasing waterplane area and a hull structure near the water surface, both generally having negative seakeeping consequences.
The third method provides stability by arrangement of station keeping elements attached between the seafloor and the floating structure. Typically, marine tendon systems are composed of sections of steel pipe arranged vertically. The tendons are attached in a widely dispersed pattern about the center of rotation of the floating structure. Pitching of the floating structure induces elongation in the tendons on one side of the center of rotation and contraction on the other side to produce a righting moment. The pretension on the tendons also act in a manner similar to solid ballast. The pretension functions as ballast weight lowering the effective center of gravity for the floating structure. Tendon-based platforms have heretofore generally been costly floating structures. This result is due to the large tendons required to provide adequate vertical stiffness and pretension along with complications associated with the installation of rigid tendons. The cost of tendon-based floating structures also tends to increase significantly with water depth, due to a reduction in tendon stiffness that occurs as tendon length increases. Tendon size must be increased to maintain the required vertical stiffness, resulting in costs which may geometrically increase with water depth. The advantage is that seakeeping performance for tendon-based structures is generally superior due to the extreme stiffness of a marine tendon system in the vertical, or heave, direction. Floating structures whose vertical stiffness is primarily controlled by the stiffness of attached station keeping elements, rather than the vertical stiffness of the waterplane, shall be referred to as tendon-based floating structures.
Floating structures may employ the aforementioned methods of stabilization, either alone or in combination. Those floating structures whose stability is satisfied upon an arrangement of waterplane area or placement of the centers of gravity and buoyancy may be referred to as self-stabilizing floating structures. Such floating structures have the advantage of being stable independent of the function of an external station keeping system. The seakeeping characteristics of self-stabilizing floating structures not employing tendons, however, is generally inferior to that of tendon-based floating structures employing station keeping elements to provide or augment stability. Marine tendon systems, however, have heretofore generally been seen as unfeasible for ultra deep water operations due to increasing costs and installation di

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