Keel joint centralizer

A riser centralizer for transferring lateral loads from the riser to a platform hull includes a keel centralizer mounted on a keel joint. The keel centralizer is received within a keel guide sleeve secured in a support mounted at the lower end of the platform hull. The keel centralizer includes a nonmetallic composite bearing ring having a radiused peripheral profile for minimizing contact stresses between the keel centralizer and the keel guide sleeve in extremes of riser and platform motion. The internal surface of the keel guide sleeve is clad with a corrosion resistant alloy and coated with a wear resistant ceramic rich coating.

BACKGROUND OF THE DISCLOSURE

The present invention relates to keel joint centralizers for a tension leg platform (TLP) for testing and producing hydrocarbon formations in offshore waters.

Traditional TLPs having a four-column construction, include a four column semi-submersible floating substructure, multiple vertical tendons attached at each corner, tendon anchors to the seabed, and production risers. The TLP production deck is supported above the water surface by four columns that pierce the water plane. These types of TLPs typically bring a well(s) to the surface for completion and are meant to support from 20 to 60 wells at a single surface location. The production risers are restrained at the production deck and at the seabed. Restraint of the production risers in this manner allows environmental loading to move the risers considerable distances and requires large spacing between risers at the production deck to prevent riser interference.

Traditional solutions to guiding risers have utilized elastomeric joints, ball joints, and steel centralizers. These solutions have been used on Spars that are restrained to the seabed using mooring lines. TLPs, however, are connected to the ocean floor by rigid tendons, so the motions are smaller and a TLP hull is not typically as deep as a Spar hull. Spar hulls do not typically allow the use of external tieback connectors, which require an opening of at least 50 inches diameter. The present invention allows full passage of external tieback connectors, and is still compatible with internal tieback connectors having a smaller outside diameter.

In a mono-column TLP it is desirable to keep well bay spacing to a minimum, and to keep the hull diameter to a minimum. Therefore the production risers must be restrained at the lower end of the hull. Applying restraint to the production risers at the lower end of the hull produces an increase in bending stresses at the point of restraint. A common practice on subsea risers for controlling bending stresses has been the use of tapered riser keel joints to distribute the load over a sufficiently long section of the riser joint.

Some problems associated with previous keel joint riser centralizers include high cost and excessive friction forces applied to the TLP's hull. In addition, use of elastomeric concepts is very difficult to analyze and quantify their useful life. Previously used concepts on Spars have relied on a steel-to-steel interface, which is subject to corrosion, galling, high friction forces and requires a large size.

It is therefore an object of the present invention to provide a riser keel joint centralizer for transferring lateral loads from the riser to the TLP hull.

It is another object of the present invention to provide a riser keel joint centralizer having a radiused peripheral profile for preventing binding of the keel joint centralizer during riser and TLP motions.

It is yet another object of the present invention to provide a riser keel joint centralizer utilizing a non-metallic composite bearing material for minimizing contact stresses at the working surfaces of the keel centralizer.

It is still another object of the present invention to provide a riser keel joint centralizer including corrosion resistant properties.

It is still another object of the present invention to provide a riser keel joint centralizer for accommodating angular offset of a riser relative to a keel guide sleeve.

It is still another object of the present invention to provide a riser keel joint centralizer generating low friction without stick-slip characteristics at the riser to platform hull interface.

SUMMARY OF THE INVENTION

In accordance with the present invention, a riser centralizer for transferring lateral loads from the riser to a platform hull includes a keel centralizer mounted on a keel riser joint. The keel centralizer is received within a keel guide secured in a guide structure mounted at the lower end of the platform hull. A radiused peripheral profile enables the keel centralizer to avoid binding in extremes of riser and platform motions. The keel centralizer includes a non-metallic composite bearing ring having a modulus of elasticity sufficiently low to allow deflection of the bearing ring to spread environmental loads applied to the platform hull over a larger area thereby minimizing contact stresses between the keel centralizer and the keel guide. The keel guide is clad with a corrosion resistant material and coated with a wear resistant ceramic rich coating.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first toFIG. 1, a mono-column TLP platform, generally identified by the reference numeral1, is shown. The platform1includes a column or hull3projecting above the water surface5supporting one or more platform decks thereon. Pontoons7extend radially outward from the base of the hull3. The floating platform1is anchored to the seabed9by tendons11. The hull3includes an axial passage or central moonpool13extending therethrough, which moonpool13is open at the lower and upper ends thereof.

Production risers15extend from a wellhead17at the seabed9to the production deck of the platform1. The production risers15are tubular members connected end to end providing a protective barrier for production and/or injection tubing extending therethrough. The production and injection tubing provide passageways for hydrocarbons, such as gas and oil, or injection fluids to flow between the wellhead17and the production deck of the platform1, and then to storage facilities. The production risers15may be thousand of feet in length and are typically restrained at the production deck of the platform1and at the seabed9. The production risers15are therefore affected by environmental loading, such as ocean currents, and may be moved considerable distances laterally. To prevent riser interference at the production deck of the platform1, large spacing between risers15is typically required.

For mono-column TLP platforms, illustrated inFIG. 1, it is desirable to keep the well bay spacing and platform hull diameter to a minimum. Therefore, in a preferred embodiment of the present invention, the production risers15extend through the moonpool13in the platform hull3. Only one production riser is shown in the drawings for purposes of illustration, however, it is understood that multiple production risers15may extend through the moonpool13of the platform hull3. Lateral movement of the production risers15is restrained at the lower end of the platform hull3by the keel centralizer of the present invention. The upper ends of the production risers15are connected to hydraulic tensioners (not shown in the drawings) mounted on the production deck of the platform1for providing vertical tension to the production risers15in a known manner.

Referring now toFIG. 2, a riser keel joint10, in accordance with the present invention, is shown. A keel centralizer12is mounted on the riser keel joint10. The riser keel joint10is one of the tubular members of the production risers15. The riser keel joint10is located in the production risers15so that it is received in a guide sleeve of the keel centralizer assembly of the invention described in greater detail below.

Referring now toFIG. 4, the keel centralizer12of the invention is received in a wear sleeve20mounted in a support frame14. The wear sleeve20comprises a tubular body22that extends above and below the support frame14. The wear sleeve body22is preferably fabricated of rolled steel plate. A guide flange24extending radially outward at about a 45° angle is welded to the upper end of the wear sleeve body22. The guide flange forms a funnel-like entrance to the wear sleeve20and aids in guiding the keel centralizer12into the wear sleeve20. A ring26is welded or otherwise secured to the lower end of the wear sleeve body22. Downwardly opening lock brackets28are mounted to opposite sides of the ring26and are aligned for releasably receiving a riser guide (not shown in the drawings) for guiding the production risers15to the wellhead17.

The internal surface of the sleeve body22is clad with a corrosion resistant alloy which is ground or machined to a final size to form a smooth bearing surface. Further enhancement of the wear and friction characteristics of the keel centralizer of the invention is obtained by applying a coating containing ceramic particles on the internal surface of the sleeve body22. The coating may include marine fouling resistance to facilitate the removal of marine growth on the sleeve body22.

In a preferred embodiment of the invention, properties of the coating applied to the body22of the wear sleeve20preferably include adhesion strength from 25.5 Mpa to 27.98 Mpa and a wear resistant average loss of 10 mg or less per 1000 cycles per ASTM D4060 Tabor abrasion using a load of 1000 g on CS 17 wheels. The wear resistant average loss of the coating would more preferably be 5 mg or less per 1000 cycles per ASTM D4060 Tabor abrasion using a load of 1000 g on CS 17 wheels. The flexibility percent elongation average of the coating is in the range of 10% to 20%. More preferably the flexibility percent elongation average of the coating is 15%. The low static friction value of the coating is preferably from 0.133 to 0.153 per ASTM D4518–90. The water permeability coefficient of the coating is preferably in a range from 0.0019 to 0.0021 (g/Pa*s*m) and the impact resistance range of the coating is preferably 89–91 inch-pounds per ASTM D2794 Intrusion Direct Impact. In addition, the ceramic rich coating applied to the sleeve20preferably exceeds 2000 hours of exposure to Salt Fog Test per ISO 7253, and more preferably exceeds 6000 hours.

The wear sleeve20is mounted in a keel support frame14extending across the moonpool13of the platform hull3. The support frame14is oriented substantially perpendicular to the axial axis of the hull3. The frame14, best shown inFIG. 8, supports one or more wear sleeves20spaced substantially equidistant from each other across the support frame14. The support frame14is welded or otherwise secured at the lower end of the platform hull3in the moonpool13, as shown inFIG. 1. The support frame14may include wear sleeve guides16mounted thereon for aiding in guiding the wear sleeve20onto the support frame14, which wear sleeve20is affixed to the support frame14by welding or the like. Openings18formed in the frame14adjacent to the wear sleeve20provide passageways for guidelines or the like which may be required to guide the production risers15to the wellhead17. Additional openings19formed in the frame14permit fluid, such as seawater, to pass through the frame14.

Referring now toFIGS. 3A and 3B, the keel centralizer12comprises a substantially flat body29defined by first and second planar surfaces30. The planar surfaces30are generally opposed and define the thickness of the body29of the keel centralizer12. A centrally located bore32defines the rotational axis of the keel centralizer12and is adapted to receive a mounting member, such as the keel joint10. The bore32is further defined by integrally formed collars34that circumscribe the bore32and project outwardly from the surfaces30of the keel centralizer body29. The collars34are oriented perpendicular to the flat body29of the keel centralizer12and provide axial length to the bore32. It is understood that the diameter of the bore32may vary to accommodate the diameter of the keel joint10received through the bore32. Angular brace members33welded between the planar surfaces30and the outer surface of the collars34provide additional structural strength to the keel centralizer12.

Referring still toFIGS. 3A and 3B, the opposed planar surfaces30of the keel centralizer12terminate at the outer periphery of the body29of the keel centralizer12which is defined by a continuous end surface that extends between the keel centralizer surfaces30, thereby defining the thickness of the keel centralizer body29. The keel centralizer12further includes a circumferential flange member36, which may be welded on or integrally formed with the keel centralizer body29, as best shown inFIG. 5. The flange member36includes an integrally formed radially outwardly projecting circumferential shoulder40forming the lower end thereof. One or more apertures42formed in the keel centralizer body29provide passageways for fluid to pass through the keel centrtalizer12.

The keel centralizer12transfers loads from the production risers15to the platform hull3. The keel centralizer12is received in the keel sleeve22, as shown inFIG. 4, and is free to move with respect to the keel sleeve22. Contact stresses that may damage the working surfaces of the keel centralizer12and the keel sleeve22are minimized by a nonmetallic bearing ring44secured on the flange member36about the outer periphery of the keel centralizer body29. The bearing ring44is fabricated of composite material having a modulus of elasticity that is lower than that of steel. The modulus of elasticity of the bearing ring44is in the range of 0.3×10^6 to 3.0×10^6, and more preferably is 0.5×10^6, compared to 30×10^6 for steel. The lower modulus of elasticity allows sufficient deflection of the bearing ring44to spread the load of the production risers15over a larger area. The bearing ring44characteristics further include dimensional stability in water of 0 to 0.5% and impact resistance of 5 to 20 ft-lb/in. More preferably, the bearing ring44dimensional stability in water is <0.1% and its impact resistance is >10 ft-lb/in IZOD. The compressive strength normal to laminate of the bearing ring44is in the range of 20,000 psi to 50,000 psi and its coefficient of friction is in the range of 0.01 to 0.15 in water and 0.1 to 0.2 dry. It is preferred that the bearing ring44have compressive strength normal to laminate >40,0000 psi and a coefficient of friction as low as 0.01 in water and 0.13 to 0.2 dry. The static coefficient of friction of the bearing ring44is preferably in the range of 0.13 to 0.15. The bearing ring44additionally includes a radiused profile for minimizing binding of the keel centralizer12within the keel sleeve22in all extremes of production riser and platform motions. Preferably the profile of the bearing ring44defines a spherical profile formed by radiused surfaces45and47on the bearing ring44. The contact stresses of the keel centralizer12are sufficiently minimized by the bearing ring44to avoid galling of the keel sleeve22and enable the over all profile of the keel centralizer12to be maintained at a small compact size.

The bearing ring44is secured about the keel centralizer body29by expanding it sufficiently with heat to slide over the flange member36so that the lower edge of the bearing ring44abuts against the retaining shoulder40on the flange member36. A capture ring46, which may comprise a single ring or multiple ring segments, is secured to the top of the flange member36by bolts48. The bearing ring44is thereby securely retained on the keel centralizer12between the capture ring46and the shoulder40of the flange member36.

Referring now toFIG. 5, the keel centralizer12of the invention is mounted about a tapered portion50of the keel joint10. The tapered portion50is a back to back tapered section formed on the keel joint10for controlling the bending stresses of the keel joint10. The tapered portion50of the keel joint10includes an enlarged portion52defined between spaced and opposed transition shoulders54and56and machined to match the internal dimensions of the bore32extending through the keel centralizer body29. The external diameter of the enlarged portion52is slightly larger than the internal diameter of the bore32of the keel centralizer12. An interference fit is established by heating the keel centralizer12to expand the bore32so that it will slide over the enlarged portion52of the keel joint10. An internal circumferential shoulder58formed adjacent the upper end of the collar34is machined to match the profile of the transition shoulder56of the enlarged portion52of the keel joint10. The keel centralizer12is slid over the enlarged portion52until the shoulder58on the collar34engages the transition shoulder56. A capture ring60machined to match the profile of the lower transition shoulder54of the keel joint10is positioned in facing contact therewith and welded to the lower end of the collar34. As the heated keel centralizer12cools, an interference fit is formed about the enlarged portion52on the keel joint10securely locking it thereon.

In a preferred configuration of the present invention, a nonmetallic bearing ring44having a radiused peripheral profile mounted on the keel centralizer12and a corrosion resistant clad keel guide sleeve22painted with a wear resistant ceramic rich coating cooperate to minimize corrosion, galling and friction forces between the keel centralizer12and the keel guide sleeve22. The radiused profile of the composite bearing ring44minimizes binding of the keel centralizer12as it slides freely within the keel guide sleeve22in response to the motions of production risers15and the platform1. The dimensions of the keel guide sleeve22are designed to accommodate the extremes in environmental conditions for the offshore location of the offshore platform1and production risers15so that the keel centralizer12is not in danger of sliding out of the keel guide sleeve22in extreme environmental conditions. InFIGS. 6A–6CandFIG. 7, movement of the keel centralizer12within the keel guide sleeve22is depicted. During any up/down stroke, the keel centralizer12is free to move vertically and angularly without binding within the keel guide sleeve22.

While a preferred embodiment of the invention has been shown and described, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims which follow.