Hydraulic and earth engineering – Marine structure or fabrication thereof
Patent
1982-03-05
1985-06-18
Corbin, David H.
Hydraulic and earth engineering
Marine structure or fabrication thereof
405169, F16L 104
Patent
active
045238774
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention pertains to the use of J-tubes on offshore platforms for installing service lines extending upwardly from the seafloor. More particularly, this invention pertains to the use of J-tubes for installing pipeline risers on compliant offshore platforms which settle in the seafloor.
2. Background Art
In offshore petroleum operations, a bottom-founded platform frequently serves as an operational base and a production terminal for oil and gas wells. In order to connect subsea pipelines to the platform deck, a vertical section of pipeline, called a riser, is often installed along the platform. J-shaped guide tubes, known as J-tubes, have been used to facilitate installation of such risers. This J-tube runs down along the length of the platform, bends near the seafloor and then runs generally parallel to the seafloor a short distance. The lower end of the J-tube frequently includes a flared portion referred to as a bellmouth.
Two basic methods are known for installing the pipeline riser in the J-tube once the J-tube is positioned. In the so-called J-tube method, the riser is inserted in the seafloor end of the tube and pulled through it with a cable or other means. In the so-called reverse J-tube method, disclosed in U.S. Pat. No. 3,595,312 (1971) issued to Matthews, a section of the riser is inserted in the deck end of the tube and is pulled or pushed down through it as additional sections of conduit are joined to the riser at the deck. Both of these methods place tension on the riser as it is forced through the J-tube. Unless resisted, this tension may cause displacements of the riser and J-tube which produce stresses that exceed the limits of the materials and cause damage to the riser of J-tube.
In the past, this problem has been approached by rigidly attaching the J-tube to the platform. The entire vertical length of the tube generally is clamped or welded to the structural members of the platform. In many cases, braces also have been placed across the bend section of the tube to prevent changes in curvature. This approach is not compatible with a compliant tower, such as a guyed tower, which is designed to move in response to environmental forces. Such a tower tilts about a point on its vertical axis some distance below the mudline, displacing the surrounding soil slightly. The tower also twists about its vertical axis. The motion of a compliant, guyed tower is described in Finn, L. D., "A New Deep-Water Platform--The Guyed Tower," Journal of Petroleum Technology (April 1978) pp. 537-544. After the riser is installed and connected to a subsea pipeline, the compliant tower moves relative to the subsea pipeline. If the J-tube is rigidly fixed to the tower, and the relative movement of the tower and subsea pipeline is not accommodated, the riser may fail.
Installation of the pipeliine riser through the J-tube generally requires that the lower end of the J-tube be positioned within approximately five feet of the seafloor. In shallow water, the J-tube can be mounted on the platform with the assistance of divers after placement of the platform offshore. However, in deep water, the use of divers is not feasible. Consequently, in some cases, J-tubes have been fixed at the proper elevation on the platform during land fabrication. In some platform applications, however, the proper elevation of the J-tube on the platform cannot be predetermined during the land fabrication phase. For example, there is a class of platforms supported by weighted bearing surfaces called spud cans which penetrate the seafloor. An example of a spud can-founded platform is described in Finn, L. D. and Young, K. E., "Field Test of a Guyed Tower," OTC 3131, Offshore Technology Conference, May, 1978. Spud can-founded platforms may settle as much as sixty-five feet after placement and the extent of settling cannot be accurately predicted. In deep waters, divers cannot be used to assist in moving the J-tube after submersion. Thus, a means is needed for remotely positioning the J-tube at various elevatio
REFERENCES:
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"A New Deep-Water Platform-The Guyed Tower", Journal of Petroleum Technology, L. D. Finn, Apr. 1978, pp. 537-544.
"Field Test of a Guyed Tower", L. D. Finn and K. E. Young, OTC 3131, Offshore Technology Conference, May 1978.
Brady Michael M.
Finn Lyle D.
Matthews, Jr. Jamie F.
Volkert Bruce C.
Corbin David H.
Exxon Production Research Co.
Johnson Kenneth C.
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