Hydraulic and earth engineering – Marine structure or fabrication thereof
Reexamination Certificate
2001-10-22
2004-05-25
Shackelford, Heather (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
C405S158000, C405S169000, C405S224300, C166S355000
Reexamination Certificate
active
06739804
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an SCR Top Connector Assembly for the articulating connection of a conduit, such as an offshore flowline or pipeline used in the petroleum industry, to a foundation subjected to differential motions of the conduit and structure. More particularly, this invention relates to a subsea pipeline with significant unsupported length, termed a “steel catenary riser” (SCR), which would utilize the SCR Top Connector Assembly to connect the SCR to a fixed or floating structure, including several parts that work together to provide fluid communication from the SCR to the platform piping system, to allow dynamic and relative motions of the SCR and the structure.
BACKGROUND
In the offshore oil and gas sector, recent developments in deeper water depths have demonstrated the need for improved solutions for the economic attachment of a flowline, or pipeline, to a structure, whether fixed or floating. Initial development utilized flexible pipe from the seabed to the floating platform; however, many operators have begun to favor the potential safety and savings offered by the use of steel catenary riser (SCR) configurations, wherein the pipeline is suspended for some distance off the seabed and connected to the structure, or floating platform.
The prior art assembly of SCR flexible joints greatly limit the approach corridors of flowlines due to the degree of dynamic angular movement that can be accommodated. Manufacturing and installation tolerances offshore in deep water leave little dynamic allowance in the prior art once the static offset of floating vessels are included. The angular limits of the prior art assembly pose difficulties for the design and construction engineers of offshore pipelines to ensure that the installation tolerances, fabrication tolerances, and operational conditions will not exceed the limits of the flexible joint and potentially lead to failure of the SCR. If the limits of the flexible joint are exceeded, all flexibility is lost and the SCR is exposed to very large bending moments resulting in dramatically and unpredictably shortened fatigue life, thereby leading to possible failure of the SCR below the flexible joint near the platform. Failure of an SCR in petroleum gas service or an SCR connector component without a viable safety shutdown valve would pose a very high risk of fire and loss of life as the gas in the pipeline (extending frequently 60 miles from the host platform) would be released at the base of the manned structure which contains sources of ignition. Although a gas leak may be more hazardous, failure of a SCR in petroleum liquid service would lead to a fairly large oil spill in open water since much of the oil in the pipeline would be siphoned out of the pipeline by the low pressure wake of the SCR falling to the seabed. Additionally, the oil would be expelled by expanding gases within the oil, as well as normal molecular diffusion.
It is an object of this invention to have features which greatly increase safety over present art by eliminating a risk of gas leakage at an offshore platform by allowing the use of normal and proven safety valves. This reduces the potential for fire, and eliminates potential oil leakage into seawater when used with oil lines. Current SCR flexible joints utilize elastomeric and metal laminations, which provide pressure containment. The same elastomeric materials serving as seals must absorb the full SCR vertical reactions while repeatedly being deflected under high vertical loads. The vertical loads can reach 100 tons due to the suspended riser weight, motions of the SCR subjected to continual environmental loading, and relative platform movements. Under cyclic loading, the elastomeric elements containing the fluids, under various conditions of temperature and pressure, may likely become a path for gas or oil leakage which would result in oil contamination of the surrounding seawater or leakage of gas at the base of the offshore platform. This would cause a gas plume and a risk of sinking the floating vessel or risk of fire to the structure overhead. If a semi-submersible were to sink at a corner, it would likely lose tendons and capsize. The industry considers systems with moving elastomeric parts to have maintenance or replacement requirements at some point in time and therefore leakage considerations are valid considerations with elastomers subjected to high cyclic compressive and shear loads acting as the sole safety barrier under pressure. Although platform piping valves can be closed on the platform side in the present art flexible joint, it is presently not considered possible to provide a safety block valve below the prior art flexible connector due to the high axial loads and high bending moments at the top of the SCR. These loads would be extremely taxing to the integrity of the valve and would not be a reliable safety feature.
A lower block valve, though not practical with present (prior art) equipment, would prevent elastomer leakage from causing the entire pipeline or SCR from back-flowing gas into the platform creating an uncontrollable hazard to life of platform personnel. In fact, the bending moments and related stresses are so high in the top section of the SCR, below the flexible connector, that specially fabricated tapered ‘stress’ joints are required to minimize the stress concentration factors in these installations. The stress intensification values are primarily due to the high bending moments resulting from the flexural high stiffness of the prior art SCR Top Connector to which the SCR is connected. The high bending moments are primarily a result of the high rotational stiffness of the laminated elastomeric elements being deflected laterally in shear while under high compression loads.
The replacement, or leakage failure correction, of the elastomers within the prior art assemblies is essentially impossible by the platform crew or by means that can be flown offshore to the site, or otherwise be effected in a short duration to reduce a prolonged platform hazard. It is a further object of this invention to eliminate elastomers from the multiple role duties of high load absorption, flexural cycling, and high-pressure containment of petroleum liquids and gases.
To correct fluid leakage of the prior art assemblies, it is necessary to remove the SCR and its associated pipeline from service by shutting in the platform and purging the section in order to provide a safe repair environment. It is also necessary to employ the use of costly offshore deepwater service equipment with sufficient lifting capacity to remove and re-attach a new assembly since the elements are not able to be replaced by the platform crew in a timely fashion in the prior art assemblies.
Prior art is described in part by the following patents:
U.S. Pat. No. 3,692,337 Flexible Coupling, Mischel; Howard T., San Diego, Calif. Sep. 19, 1972.
U.S. Pat. No. 3,952,526 Flexible Supportive Joint for Subsea Riser Flotation, Watkins; Bruce J., Rancho Palos Verdes, Calif. Apr. 27, 1976.
U.S. Pat. No. 5,791,695 Flexible Joint for Facilitating Bending of Tubular, Snider; David A., Hurst, Tex. Aug. 11, 1998.
U.S. Pat. No. 5,615,977 Flexible/Rigid Riser System, Moses; Charles J., Alvarado, Tex. Apr. 1, 1997.
U.S. Pat. No. 5,628,586 Elastomeric Riser Tensioner System, Arlt, III; Edward J., Arlington, Tex. May 13, 1997.
U.S. Pat. No. 4,105,266 Laminated Bearing with Plural Modulus Layer, Finney August 1978.
U.S. Pat. No. 4,759,662 TLP Marine Riser Tensioner, Peppel July 1988
It is a further object of the present invention to minimize the potential for fire risk and loss of life; oil spills, uncontrolled and extended leakage, high maintenance costs and pipeline/platform downtime duration.
It is another object of the present invention to provide a system with automatic shut-in safety block valve capability on each side of any non-metallic elements which may be subject to leakage.
It is a further object of the present invention to increase the allowable dynamic displacement angles to reduce the chance of bott
Madan, Mossman & Siram, P.C.
OPE, Inc.
Shackelford Heather
Singh Sunil
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