Patent Application
20070224000
The invention is generally related to floating offshore structures and more particularly to semi-submersible floating offshore structures.
Offshore structures include a topside structure that houses a large variety of equipment including production and control equipment, piping, production trees, crew living quarters, electrical equipment, storage, etc. When it is known that the size and/or weight of the topside will exceed the lifting capacity of vessels, the topside must either be installed in two or more separate pieces on the supporting structure or be floated over the structure in a single piece. Installing the topside in several pieces greatly adds to the complexity and expense of work offshore as a result of all the connections, such as piping, that must be made and tested once the separate pieces have been installed and joined together. Also, derrick barges, which are very expensive, tend to be required at the offshore site while the connection work is accomplished. A single piece topside eliminates the need for the connection work offshore and allows the derrick barge to be released for other work as soon as the topside has received its first welding connection to the supporting structure. The potential disadvantage to a single piece topside is that some floating offshore structures do not readily lend themselves to a float over installation if the lifting capacity of available vessels is exceeded. Also, while there are a very limited number of vessels with high lifting capacities, these vessels are extremely expensive on a daily basis.
The semi-submersible is a type of floating structure that has vertical columns supporting topsides and supported on large pontoons. The structure is held in position by the use of spread mooring lines that are anchored to the seafloor. The semi-submersible has a number of unique characteristics compared with other floating structures such as a spar and TLP (tension leg platform). These advantages include: The semi-submersible has good stability because of a large footprint and low center of gravity for the topsides. The hull requires lower steel tonnage. The hull can be a new build or converted from an existing drilling semi. The semi-submersible may include drilling capability. The semi-submersible can support a large number of flexible risers or SCR's (steel catenary risers) because of the space available on the pontoons. The topsides can be integrated at quayside and thus reduce cost and save scheduling time. The semi-submersible has a relatively short to medium development schedule. The initial investment is relatively low.
The semi-submersible also has several deficiencies when compared with the spar and TLP. The most significant is the large heave motion because of the shallower draft and large pontoons. As a result, it has not been suitable for a dry tree riser arrangement. The dry tree riser arrangement puts the tree above the water using top tensioning devices such as mechanical tensioners or supporting buoyancy cans and has significant economic benefit for well completion, work-over, and intervention during the life of the offshore production facility. Risers supported by top tensioners are known in the industry as top tensioned risers (TTR) and vertical risers. Another problem from the large motion of the semi-submersible is that it causes fatigue in the SCR's more easily, which requires more stringent fatigue design for the SCR's and thus costs more. For a platform in ultra deepwater with large diameter SCR's, the solutions to this problem could become technically or economically unfeasible.
The ideas that have been explored by the industry to overcome the semi-submersible motion problem generally fall into the two categories below.
The first is a semi-submersible wherein the concept is to increase the draft from the normal range from sixty to eighty feet to one hundred to one hundred ten feet so that the wave action at the keel is reduced and, thus, the structure will have less motion. This makes the semi-submersible option feasible in some locations where the conventional semi-submersible would not be chosen because of the difficulties in dealing with the SCR riser fatigue issues. However, the heave motion is still relatively large compared with spars and TLP's. Also, the dry tree arrangement is still not feasible. Also, the SCR's deployed on these semi-submersibles usually still need to be strengthened to meet the fatigue life requirement.
The second is a semi-submersible that adds damping and added mass to the platform which will reduce its heave motion under wave conditions.
Most concepts based on the added damping have a heave plate or pontoon as an extendable part at the bottom of the semi-submersible hull. The heave plate or pontoon is retracted at the fabrication yard and during transportation. After the hull is located on site, the heave plate or pontoon is then extended or lowered to a deeper elevation and locked at that position.
The known designs suffer several deficiencies. The extendable columns take too much deck space and require extensive and expensive deployment equipment. In some cases it could be as much as thirty percent of the total deck space, which is impractical from a topsides equipment layout point of view. The column-to-deck connections are complicated. Extending and movable parts complicate structural integrity and construction. They are hard to build, risky during installation, and difficult to maintain. The advantage of a large pontoon area for riser supports from the conventional semi-submersible hull is compromised.
The present invention addresses the deficiencies in the known art. The hull includes columns that are supported by pontoons. The columns support the topsides. The columns extend deeper below the water surface than previous semi-submersible concepts. Heave plates within the columns minimize the vertical motions (heave) of the structure. The structure is held in place by a spread mooring array in the same manner as a conventional semi-submersible structure. The topsides and the pontoons/columns are constructed separately and assembled together at the offshore site where the structure is used for drilling and/or production.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated.
In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same:
The invention is generally indicated by numeral 10 in
The pontoons 12 are comprised of interconnected buoyant pontoons. The pontoons 12 may include hard and soft tanks for adjusting the buoyancy of the offshore structure 10 as required during tow out, installation, and normal operations. The pontoons 12 define a center space or center well between them.
The columns 14 are rigidly attached to the pontoons 12. The columns may be circular or rectangular in cross section. The cross sectional shape of the columns will depend upon the normal water conditions and the need to reduce drag, wave run-up, and/or vortex induced vibrations caused by waves and current. The combination of the pontoons 12 and columns 14 define a buoyant hull.
The topside 16 is comprised of standard topside equipment normally provided on an offshore structure.
As seen in
An alternate embodiment of the invention may also be constructed that eliminates the second heave plate 30 and has only one heave plate 18 between the pontoons.
In a preferred embodiment of the invention, the dimensions of the offshore structure 10 are as follows. The columns are each one hundred twenty feet in diameter and two hundred thirty feet tall. The column spacing, on center, is two hundred thirty-five feet, which makes the outer dimension at the pontoons approximately two hundred ninety-five feet. The heave plate is one hundred seventy-three feet across. The pontoons are twenty feet tall. The preferred operational draft is approximately two hundred feet, with a freeboard of fifty feet to the upper end of the columns. The second heave plate is preferably positioned at least seventy feet above the pontoons. It should be understood that these dimensions are provided as an example of one suitable structure and that various other dimensions and ratios may be used.
The preferred two hundred foot draft places the offshore structure 10 at approximately the same draft as the buoyant hull of a truss spar structure as described in U.S. Pat. No. 5,558,467. This provides the advantage of positioning the pontoons below the normal major effects of waves and currents and allows the heave plate(s) to efficiently act as a damping mechanism against wave induced heave motions. The general arrangement of the offshore structure 10 also reduces the cross sectional area exposed to wave and current forces in the major wave and current zone at shallower depths.
The invention provides several advantages over the known art. The pontoons act as a hull for transport of the erected columns. The structure provides float over capability as well as conventional lift for installing the topsides. The structure allows the use of existing barges for transport and installation of the topsides. Float over capability allows the installation of single piece heavier topsides than current lifting capacity. The structure is construction friendly. The shallow draft is sufficient for transport of the pontoons and columns. The structure is accessible to water depth restricted construction facilities. The structure has flexibility of piece fabrication location. Low motions of the structure also will allow the use of dry trees.
The key global performance advantages are motion characteristics that are similar to or better than a spar and better than a semi-submersible based FPS. The structure has reduced vortex induced motions compared to a spar. The reduced motion at the keel is more riser friendly than conventional semi-submersibles. Conventional semi-submersibles typically allow only catenary risers because of the motions. The invention allows catenary as well as vertical risers (TTR) with a dry tree. The invention also has low vertical motion, a solid structure, no movable parts, fixed heave plates, and it is easy to build.
While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.
