High pressure product swivel

Details High pressure product swivel High pressure product swivel

2001-07-25

2002-09-17

Nicholson, Eric K. (Department: 3679)

Pipe joints or couplings

With fluid pressure seal

C285S190000, C285S900000, C285S121100, C285S121600

Reexamination Certificate

active

06450546

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to equipment for transferring fluids. In particular, the invention relates to a fluid swivel joint and a swivel stack assembly adapted for transferring fluids between tankers, storage vessels and the like and one or more conduits beneath the ocean surface. The fluid of the swivel may be product such as hydrocarbons to be transferred from the seabed to a vessel or may be water or gas to be transferred from the vessel to the seabed for well stimulation.
Still more particularly, the invention relates to a sealing arrangement for a fluid swivel joint which uses the pressure of the fluid flowing through the joint to substantially prevent seal glands, and extrusion gaps in which dynamic seals are placed, from enlarging as a function of high pressure of the fluid commonly encountered on offshore loading terminals for oil and gas tankers. The invention also relates to an arrangement by which a spool is coupled between a swivel joint and a swivel stack base. The invention also concerns a procedure for disassembling and reassembling a swivel so that seals of individual swivel assemblies in a swivel stack can be replaced without the need of an overhead crane and without removing an assembly from the stack.
2. Description of the Prior Art
The offshore search for oil and gas has greatly expanded in recent years and progressed into deep rough waters such as the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex mooring systems for offshore loading terminals which serve as centralized production sites for the entire field have been developed. Flexible fluid lines called risers extend from a subsea location to the mooring site to permit the transfer of fluids between a moored vessel and a subsea location. For example, certain fluid lines may be used to convey oil and gas into the floating vessel while other fluid lines may be used to inject liquids or gases back from the vessel into subsea wells for purpose of control, well stimulation or storage.
Floating vessels can be moored to a single point mooring system, which permits the vessel to weathervane and rotate 360° about a single mooring point. To permit the vessel to rotate and move freely without causing twisting or entanglement of the various risers to which the vessel is attached, it is necessary to provide a swivel mechanism to connect the fluid lines to the mooring site. Furthermore, since a plurality of risers are involved, it is necessary that swivels be stacked in order to have the capability of accommodating multiple fluid lines or risers.
Separate swivel assemblies are stacked on top of each other with a swivel stack base fixed to a stationary frame anchored to the sea floor.
Prior high pressure product swivels have provided an inner housing and an outer housing which is rotatively supported on the inner housing by a bearing so that the outer housing is free to rotate about the inner housing. A toroidally shaped conduit chamber is formed between the two housings when the two housings are placed in registration with each other. An inlet from the inner housing communicates with the chamber, and an outlet in the outer housing communicates with the chamber. Upper and lower dynamic seals in the form of face seals or radial seals are placed in grooves or gaps between axially opposed or radially opposed surfaces of the inner and outer housings to prevent fluid from leaking past the two facing surfaces while the high pressure fluid is present in the chamber.
When high pressure is present in the inlet and passes through the toroidal chamber and out the outlet, the pressure in the chamber acts to separate the inner housing and the outer housing from each other. In other words, the inner housing is forced to contract radially inward as a consequence of the force generated by the fluid pressure acting on an effective area between the two dynamic seals; the outer housing is forced to expand radially outward by the force of the fluid pressure acting on an effective area between the upper and lower dynamic seals. Separation occurs between the facing surfaces as a result of high fluid pressure in the chamber. High pressure as used herein is meant to be at the level of 2,000 psi and above.
As the pressure of flowing product increases, the separation between the facing surfaces in which the seals are placed increases. Such separation, can be large enough due to the high product pressures, so as to prevent leak-free operation of the product swivel at the high pressures by seal extrusion failure.
Swivel component deformation has been the subject of much effort by prior developers. The prior art has considered the idea of adding more material to the swivel components so that deformation as a function of pressure—especially high pressure in the 5,000 to 10,000 psi range—will resist deflection. With high pressures, however, the swivel components, i.e., the inner and outer housings, become so large and heavy that they are disadvantageous from weight, cost, handling and size standpoints and without necessarily achieving the desired gap control.
The prior art has disclosed swivels which use exterior pressure sources to apply balancing or “buffer” fluid pressure at the dynamic seal interface. Examples of such “active” pressure compensation for dynamic seal gap control are shown in U.S. Pat. No. 4,602,806 to Saliger; U.S. Pat. No. 4,669,758 to Feller et al., U.S. Pat. No. 5,411,298 to Pollack; U.S. Pat. No. 6,053,787 to Erstad et al., and U.S. Pat. No. 4,662,657 to Harvey et al. All of these patents disclose separate anti-extrusion rings above and below the annular fluid manifold in combination with active pressure compensation.
U.S. Pat. No. 4,555,118 to Salinger discloses at FIG. 4 a free floating anti-extrusion ring placed above and below a toroidal passage between inner and outer rings. The free floating antiextrusion ring is initially displaced (i.e., at zero pressure) from the inner joint ring by a small seal extrusion gap. In operation, the internal pressure of the pressurized fluid in the toroidal passage is transmitted to the outer side of the anti-extrusion ring such that the pressure differential across the seal presses the anti-extrusion ring against the outer surface of the inner ring. In other words, the seal extrusion gap width varies as a function of internal pressure. Metal to metal contact of the anti-extrusion ring with the annular surface of the inner ring can cause friction and scoring problems during operation.
U.S. Pat. No. 4,819,966 to Gibb at FIGS. 2, 3 and 4 shows an annular ring having an annular groove which registers with the inlet of an inner housing. An annular chamber is formed outwardly in the annular ring such that upper and lower lips are created in the annular ring which faces the exterior surface of the inner housing. The lips carry dynamic seals and are forced into sealing engagement about the cylindrical surface of the inner housing above and below the inlet when pressure is in the chamber. A constant seal gap is maintained as a function of pressure by proper shaping of the chamber and the ring and the lip. A lubricating system may also be provided for injecting a controlled fluid.
Another problem inherent in high pressure production swivels is that at extremely high pressures, e.g., 5000 psi and above, a fixed connection of the inlet spool to the inner housing can cause pipe loads and a seal stab connection can cause forces to be applied to the inner housing as a result of the pressure acting at the connection. For example, U.S. Pat. No. 6,053,787 at FIG. 2B shows a spool connected to an inner housing, and although sealed with respect to the inner housing, high pressure in the spool causes forces to be transmitted to the housing as a result of the pressure. U.S. Pat. No. 4,662,657 shows at FIG. 1 a pressure balanced connection of a spool at a swivel stack base. Such connection acts as an expansion joint in that the connection is pressure balanced and does not transmit force. This allow

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