In situ turret bearing remediation and assembly

In a three row, roller bearing assembly coupling a vessel to a turret, the bearing assembly having a support row assembly disposed between an inner ring connected to the turret and outer rings connected to the vessel, a method and arrangement for in situ remediation of a damaged support row assembly. Couplers are secured to existing inner ring stud bolts. A continuous bearing ring below the couplers is assembled and a support bearing arrangement is installed between the couplers and the bearing ring. Reaction plates are mounted to the vessel. Each reaction plate has a jack screw which is positioned directly below the bearing ring. The jack screws are turned to elevate the bearing ring and form a flat surface for support of the support bearing arrangement. The turret axial load is transferred from the damaged support row assembly to the support bearing arrangement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a turret structure for vessels such as offshore drilling or production vessels, and more particularly to a turret bearing assembly and bearing repair procedure that can be performed in situ.

2. Description of the Related Art

Vessel mooring systems are known in the art in which a vessel may weathervane about a turret that is moored to the sea floor. The turret extends through a moon pool or cylindrical opening that extends through the hull of the vessel. The vessel is supported for rotation with respect to the turret by turret bearing structures placed between the turret and the vessel. Horizontal and vertical bearings transfer horizontal and vertical loads between the turret and the vessel.

U.S. Pat. No. 8,671,864, issued on Mar. 18, 2014, in the names of Lindblade et al., discloses such a turret mooring system. FIG. 1 of the '864 patent discloses a bow of a vessel having a moon pool extending through the vessel hull. Mounted within the moon pool is a turret about which the vessel weather vanes. The moon pool is normally of a circular cross-section, and the turret is typically of a cylindrical shape to fit within the moon pool. Anchor legs are connected to the turret and secured to the sea bed by suitable anchors to restrict rotation of the turret. Risers extend from subsea wellheads or distribution facilities on the sea floor and are connected to the turret. A manifold deck is supported on the upper end of the turret and includes valves connected to the risers. A swivel stack extends upwardly from the turret and manifold deck, and allows fluids to be transferred from the turret to the vessel. The turret is supported on the vessel by a turret bearing assembly.

One type of turret bearing assembly commonly used is an integrated three-row roller bearing assembly as shown and described in U.S. Pat. No. 5,893,784, issued on Apr. 13, 1999, to Boatman. The three-row roller bearing assembly includes a circular array of support rollers radially aligned about the turret for supporting the turret weight. Any uplifting turret force is resisted by a second set of radially aligned rollers. A third set of rollers, coaxially aligned about the turret, serves to transfer radial loads between the vessel and the turret. The three bearing rows of the three-row roller bearing assembly are preferably lubricated and sealed within a common volume by seals to provide protection from the elements and prevent corrosion. The integrated three-row roller bearing assembly is a precise assembly that requires a high degree of flatness for proper load distribution and is somewhat intolerant of distortions and deflections, which cause high point loading stresses on select rollers.

Most three-row roller bearing assemblies on turret systems located offshore are not replaceable or repairable in situ due to factors such as the size, weight and access to the various components. Although these bearing assemblies are designed for the life of the system, if they fail or exhibit problems, no design or method exists to correct major problems on location. Since large vertical loads are always on the bearing support row, this is where wear problems are most likely to occur.

U.S. Pat. No. 8,197,293, assigned to Bluewater Energy Services B.V., discloses initially installing a secondary bearing assembly in place and loading the secondary bearing assembly when the primary bearing assembly no longer functions. One drawback to this is that including two bearing assemblies (one as a spare) is cost prohibitive and adversely affects capital expense.

It would be desirable to be able to replace or repair a damaged turret three-row roller bearing assembly in situ. It would also be desirable to be able to replace or repair a damaged turret three-row roller bearing assembly while the vessel stays on station. It would be desirable to be able to remove the large vertical load from the existing bearing support row and allow the vessel to stay on station. It would be desirable to be able to provide a new vertical load path in situ while allowing the vessel to weather vane about the turret.

SUMMARY OF THE INVENTION

The invention provides a repair to a turret bearing assembly with damaged support row rollers and/or support race. The repair can be performed and assembled in situ offshore and transfers the load off the main turret bearing's support race and onto a new replacement race in a single lift sequence, thereby simplifying the operation and minimizing the timeframe to perform the load transfer. Additionally, the arrangement removes the need to uninstall the vessel from its offshore location to perform remedial work on the damaged bearing assembly, thereby relieving the vessel's owner of considerable risk and cost implications.

An aspect of the invention is removing axial load from an in place damaged three-row roller bearing assembly and allowing the vessel to continue to weather vane in either a free-state or with assistance. The invention removes load from the existing bearing support row and transfers it to a remedial bearing support row. Radial load will continue to be transferred through the main bearing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1depicts a cross-sectional view of a typical three race roller bearing assembly10presently found in use on many turret systems. In this system, the turret has a single three race roller bearing assembly10installed to react all loading acting between the turret and turret supporting structure14of a floating vessel. It is to be understood that the turret supporting structure14is part of the floating vessel and has no movement relative to the floating vessel. The turret, represented by turret structure12, is geostationary and the floating vessel (including the turret supporting structure14) is allowed to weather vane about the geostationary turret. The bearing assembly10is the only means of load transfer across this rotational interface for an external turret, or an internal turret without a lower bearing.

The three-row roller bearing assembly10includes a circular array of support rollers with support race and cage, all generally designated as support row assembly20. The circular array of support rollers are radially aligned about the turret for supporting the turret weight. Any uplifting turret force is resisted by a second set of radially aligned rollers with uplift race and cage, all generally designated as uplift row assembly40. A third set of rollers with radial race and cage or spacers, all generally designated as radial row assembly60, coaxially aligned about the turret, serve to transfer radial loads between the vessel and the turret. The third set of rollers for transferring the radial load typically have spacers between the rollers. However, sometimes the bearings are designed full complement, meaning there are no radial cages or spacers and just rollers.

The lower support roll assembly20is the most loaded when a downwardly acting axial force is applied. Of the three rows of roller bearings, the bearing support rollers and races of the support roll assembly20typically receive the most load and are the most likely to sustain damage in medium to deep water systems. For shallow water systems, the radial load can be quite high and the vertical load nominal. Thus, in shallow water systems, the bearing radial rollers and races60may be the most loaded and the most likely to sustain damage. The preferred embodiments primarily address remediation or replacement of the support race and rollers20.

The bearing race provides a path on which the rollers roll. To realize the longest service life, the roller path of the bearing race needs to be as flat and circular as possible allowing for a smooth load distribution upon the rollers themselves.

Referring toFIG. 1, a bearing inner ring30is secured to the turret structure12via a plurality of circumferentially-spaced inner ring fasteners, preferably threaded stud bolts32, washers36, and nuts34. A bearing outer upper ring42and outer lower ring22are secured to the turret supporting structure14of the floating vessel via a plurality of circumferentially-spaced outer ring fasteners, preferably threaded stud bolts46and nuts44.

The preferred embodiments of the present invention address the in situ repair or remediation of a three-row roller bearing assembly10in which the support row assembly20is damaged. The preferred embodiments have a goal of in situ repair or remediation while allowing the vessel to continue to weather vane in either a free-state or with assistance. The preferred embodiments employ either rolling elements or self-lubricating sliding bearing pads (i.e., plain bearings) to remove the axial load from the main support row assembly20. In the preferred embodiments, the axial load is taken underneath the existing bearing assembly10from the bearing inner ring30to the outer turret supporting structure14. The axial load is preferably transferred or re-routed from the damaged support row assembly20of the existing three row roller bearing assembly10to a new support bearing assembly (i.e., either a plain bearing assembly100as shown inFIGS. 2-4or a support roller bearing assembly200as shown inFIGS. 5-7) with jack screws or hydraulic jacks as discussed below. Radial load will continue to be transferred through the existing main bearing assembly10.

In one preferred embodiment, the plain bearing assembly100is installed beneath the existing main bearing assembly10as shown inFIGS. 2-4. Upon installation of the plain bearing assembly100, the plain bearing assembly100removes the vertical and moment load off the existing support row assembly20while the radial load path remains through the radial row assembly60of the existing main bearing assembly10. Uplift rollers in the uplift row assembly40of the existing main bearing assembly10remain available for potential uplift or overturning moments.

This embodiment reacts vertical load from the turret structure12of the turret into the turret supporting structure14of the vessel through the plurality of existing inner ring stud bolts32of the bearing inner ring30as shown inFIGS. 2-4. A spherical washer118and a special nut112, preferably externally and internally threaded, replaces the existing washer36and lower nut34on the inner ring stud bolts32. A coupler110is threaded onto the special nut112at the lower end of the existing inner ring stud bolt32. A bearing disk116is inserted into the lower portion of each coupler110as shown inFIGS. 2 and 4. Preferably, the bearing disks116are made of a self-lubricated, composite material. A set screw114locks each coupler110to the existing inner ring stud bolt32. The height of the coupler110with bearing disk116is set to form a flat planar lower bearing surface for even load distribution.

A continuous bearing ring120is preferably formed by a plurality of bearing ring segments. A sliding surface122, preferably of stainless steel, is placed on an upper portion of the bearing ring120and positioned between the coupler bearing disks116and the bearing ring120. The sliding surface122may be fastened securely to the bearing ring120, as for example, by bolts124as shown inFIG. 4. Preferably, the stainless sliding surface122is segmented and forms a continuous ring that is alternatingly segmented with the segmented bearing ring120. The bearing disks116of the couplers110are arranged and designed to slide on the stainless sliding surface122on the bearing ring120upon completion of the turret bearing remediation process.

Preferably, a plurality of reaction plates126and a plurality of support plates130are fastened to the turret head supporting structure14of the vessel. Typically, the number of reaction plates126and the number of support plates130matches the number of radial stiffeners16, as for example twenty-four (24). A plurality of holes18A are drilled in a bottom plate18below the turret head supporting structure14. Preferably, the holes18A are axially aligned with the holes for the outer ring stud bolts46. The holes18A allow the mounting of the support plates130to an upper surface of the bottom plate18and the reaction plates126to a lower surface of the bottom plate18with new large stud bolts132extending through aligned holes in the support plates130, reaction plates126and bottom plate18. After installing nuts134, the new large stud bolts132are tensioned, preferably hydraulically with hydraulic tensioner144(FIG. 4).

A plurality of spreader bars136, preferably the same in number as the plurality of support plates130, may be installed to further support the axial load of the turret structure12. Each spreader bar136may be positioned directly above a support plate130. Preferably, the spreader bar136is connected with a nut138to an upper end of the two outermost new large stud bolts132connecting the support plate130to the bottom plate18as shown inFIGS. 2 and 3. The spreader bar136is attached to the turret head support structure14by replacing the existing outer ring studs46, located above and between the axially aligned two outermost new large stud bolts132, with new longer outer ring studs46′ (FIG. 2) threaded into the spreader bar136. Referring toFIG. 3, spacers140allow the new longer outer ring studs46′ to be tensioned without over loading the structure. This arrangement forms a stiff load reacting assembly attached to the existing structure.

A plurality of jack screws142are installed in the reaction plates126below and in contact with the continuous bearing ring120as shown inFIGS. 3 and 4. The plain bearing assembly100is loaded by turning a plurality of jack screws142with a hydraulic torque wrench to a specified height to lift the turret structure12and form a flat surface for bearing support.

A preferred method of installing the plain bearing assembly100in situ will now be described. The plurality of holes18A are drilled through the bottom plate18of the turret supporting structure14around its circumference. The holes18A are preferably substantially aligned with the outer ring stud bolts46. The inner ring stud bolts32are de-tensioned one at a time and the washers36and lower nuts34replaced with the spherical washers118and new special nuts112. The special nuts112are preferably internally and externally threaded. The inner ring stud bolts32are re-tensioned such that a predetermined length protrudes below the lower special nuts112. For example, 10 millimeters of the inner ring stud bolt32may protrude below the special nut112. Referring toFIG. 4, an upper portion110A of each coupler110is internally threaded, allowing the couplers110to be threadably installed onto the lower special nuts112. The couplers110are all positioned to the same elevation, preferably by laser measurement. A set screw114threadably engaging a threaded lower portion110B of the coupler110is tightened against the bottom of the inner ring stud bolt32to maintain the proper elevation of the couplers110. The self-lubricated, composite material bearing disks116are installed in the lower end of the couplers110.

In the preferred embodiment, the reaction plates126, support plates130and spreader bars136are in sections sized to be aligned and cooperate with five adjacent outer ring stud bolts46. For each section, the existing inner three outer ring stud bolts46are removed. The plurality of support plates130are positioned onto the upper surface of the bottom plate18with five new large stud bolts132received in holes130A extending through each support plate130. Nuts134are attached to the upper end of the stud bolts132.

The segments of the continuous bearing ring120and the continuous stainless sliding surface122are assembled together forming a continuous ring around the circumference of the turret. Preferably, the continuous bearing ring120with sliding surface122are supported, temporarily on a stand, below the couplers110.

Individual reaction plates126, preferably with the jack screws142installed, are positioned below the bottom plate18in alignment with a corresponding support plate130. The five new large stud bolts132in the corresponding support plate130are lowered through the bottom plate holes18A and through corresponding holes126A in the reaction plate126. Lower nuts134are installed on the lower end of the large stud bolts132and the stud bolts132are tensioned. Preferably, a predetermined length of the large stud bolts132protrudes below the lower nuts134. Once enough reaction plates126are installed to support the continuous bearing ring120, the support stands can be removed. The remaining reaction plates126may then be installed.

The spreader bars136are installed by lowering the spreader bar136over the upper ends of the five large stud bolts132such that the outer large stud bolt132at each end extends through an outer lower hole136A at each end of the spreader bar136. A nut138is threaded onto the protruding upper end of the outer large stud bolt132and tensioned to secure the spreader bar136to the lower reaction plate126. The three new longer outer ring studs46′ are inserted through the outer upper ring42, outer lower ring22, turret supporting structure14, and a spacer140before being threaded into a threaded opening of the spreader bar136. Referring toFIG. 3, spacers140allow the new longer outer ring studs46′ to be tensioned without over loading the structure. This arrangement forms a stiff load reacting assembly attached to the existing structure. The jacking screws142are then used to align the continuous stainless steel sliding surface122with the bearing disks116. The plain bearing assembly100is loaded by turning the plurality of jack screws142with a hydraulic torque wrench to a specified height to lift the turret structure12and form a flat surface for bearing support. The turret structure12is raised to remove the vertical load from the damaged original support row assembly20.

The above-described embodiment of the plain bearing assembly100installed below the existing bearing assembly10provides the following benefits and characteristics:uses existing inner ring bolting32to react load below existing bearing assembly10;minimizes hot work (e.g., welding or flame cutting);uses short lead time materials;utilizes self-lubricating composite bearing material used on many applications;deflections and tolerances are less of an issue with compliant bearing material;higher levels of breakout torque required to rotate the chaintable (i.e., turret structure12);uses jacking screws142to lift the chaintable12to original position; andradial load path remains through the radial row assembly60of the existing bearing assembly10.

A second preferred embodiment comprises installing a support roller bearing assembly200in situ underneath the existing main bearing10as shown inFIGS. 5-7. The support roller bearing assembly200solution shares many of the same components as the plain bearing assembly100solution so that the planning of both embodiments is available to maximize uptime of the floating vessel, typically a floating production storage and offloading unit (FPSO). The support roller bearing assembly200solution also removes the vertical and moment load off the existing main bearing10while the radial load path remains through the radial row assembly60of the existing main bearing10. The uplift row assembly40in the existing main bearing10remains available for potential uplift or overturning moments. With the use of rollers, the second preferred embodiment remains a low torque solution similar to the original design.

As with the plain bearing assembly100, this embodiment reacts vertical load from the turret structure12into the existing inner ring stud bolt32assembly and into the turret supporting structure14of the vessel. A spherical washer118and a special nut112, preferably externally and internally threaded, replaces the existing washer36and lower nut34on the inner ring stud bolts32. A coupler210is threaded onto the special nut112at the lower end of the existing inner ring stud bolt32. A set screw214locks each coupler210to the existing inner ring stud bolt32. The height of the coupler210is set to form a flat planar bearing surface for even load distribution.

An upper bearing ring250is preferably formed by a plurality of upper bearing ring segments. A self-lubricated composite bearing material ring252is placed into an upper portion of the upper bearing ring250to absorb minute deviations in the height of the couplers210. The upper bearing ring250is segmented to form a continuous ring. Preferably, the bearing material ring252is segmented and forms a continuous ring that is alternatingly segmented with the segmented upper bearing ring250.

A continuous lower bearing ring260is preferably formed by a plurality of lower bearing ring segments. Hardened and ground steel races262are placed in recesses of the upper bearing ring250and the lower bearing ring260to provide the rolling element surface for new rollers264separated by cages266. The lower bearing ring260is supported by a plurality of jack screws242mounted in the reaction plates126.

On the turret supporting structure14of the vessel side, all of the assembly is identical to the plain bearing assembly100embodiment described above with respect toFIGS. 2-4, with the exception of the jack screws242which differ slightly. As shown inFIG. 8, preferably a port244in the jack screws242is in fluid communication with ports268in the lower bearing ring260to provide lubrication to the steel races262, rollers264and cages266of the support roller bearing assembly200. The stiffening assembly comprising the reaction plates126, support plates130and spreader bars136fastened to the existing turret supporting structure14provides ample support for the rolling elements and minimizes deflections under load.

This new support roller bearing assembly200is also loaded by turning the plurality of jack screws242with a hydraulic torque wrench to a specified height to lift the turret structure12and form a flat surface for bearing support.

The preferred method of installing the support roller bearing assembly200in situ is performed in a similar manner as the plain bearing assembly100. The plurality of holes18A are drilled through the bottom plate18of the turret supporting structure14around its circumference in the same manner as above. The inner ring stud bolts32are de-tensioned one at a time and the washers36and lower nuts34replaced with the spherical washers118and special nuts112. The inner ring stud bolts32are re-tensioned such that a predetermined length protrudes below the lower special nuts112. For example, 10 millimeters of the inner ring stud bolt32may protrude below the special nut112. An upper portion210A of each coupler210is internally threaded, allowing the couplers210to be threadably installed onto the lower special nuts112. The couplers210are all positioned to the same elevation, preferably by laser measurement. A set screw214threadably engaging a threaded lower portion210B of the coupler210is tightened against the bottom of the inner ring stud bolt32to maintain the proper elevation of the couplers210. The height of the coupler210is set to form a flat planar bearing surface for even load distribution.

In the preferred embodiment, the reaction plates126, support plates130and spreader bars136are in sections sized to be aligned and cooperate with five adjacent outer ring stud bolts46. For each section, the existing inner three outer ring stud bolts46are removed. The plurality of support plates130are each positioned onto the upper surface of the bottom plate18with five new large stud bolts132received in holes130A extending through each support plate130. Nuts134are attached to the upper end of the stud bolts132.

The segments of the continuous lower bearing ring260are joined together and the hardened and ground steel race segments262are installed in the upper recess of the continuous lower bearing ring260. The steel race segments262are alternatingly segmented with the lower bearing ring segments260. The continuous lower bearing ring260with continuous steel race262are assembled forming a continuous ring around the circumference of the turret.

The segments of the continuous upper bearing ring250are joined together and the self-lubricated composite bearing material ring segments252are placed into the upper portion of the upper bearing ring250. The bearing material ring segments252are alternatingly segmented with the upper bearing ring segments250. The hardened and ground steel race segments262are installed in the lower recess of the continuous upper bearing ring250. The steel race segments262are alternatingly segmented with the upper bearing ring segments250. In a preferred embodiment, the upper bearing ring250comprises top upper bearing segments250A and bottom upper bearing ring segments250B which are alternatingly segmented and fastened together to form the continuous upper bearing ring250.

Referring toFIG. 8, the new support rollers264and cages266are installed between the upper and lower support races262in the lower and upper recesses of the upper bearing ring250and the lower bearing ring260. Preferably, the assembled upper and lower bearing rings250and260with the support rollers264, cage266and races262are supported, temporarily on a stand, below the couplers210.

Individual reaction plates126, preferably with the jack screws242installed, are positioned below the bottom plate18in alignment with a corresponding support plate130. The five new large stud bolts132in the corresponding support plate130are lowered through the bottom plate holes18A and through corresponding holes126A in the reaction plate126. Lower nuts134are installed on the lower end of the large stud bolts132and the stud bolts132are tensioned. Preferably, a predetermined length of the large stud bolts132protrudes below the lower nuts134. Once enough reaction plates126are installed to support the assembled upper and lower bearing rings250and260with the support rollers264, cage266and races262, the support stands can be removed. The remaining reaction plates126may then be installed.

The spreader bars136are installed by lowering the spreader bar136over the upper ends of the five large stud bolts132such that the outer large stud bolt132at each end extends through an outer lower hole136A at each end of the spreader bar136. A nut138is threaded onto the protruding upper end of the outer large stud bolt132and tensioned to secure the spreader bar136to the lower reaction plate126. The three new longer outer ring studs46′ are inserted through the outer upper ring42, outer lower ring22, turret supporting structure14, and a spacer140before being threaded into a threaded opening of the spreader bar136. The spacers140allow the new longer outer ring studs46′ to be tensioned without over loading the structure. This arrangement forms a stiff load reacting assembly attached to the existing structure. The jacking screws242are then used to align the upper bearing material ring252of the upper bearing ring250with the couplers210. The support roller bearing assembly200is loaded by turning the plurality of jack screws242with a hydraulic torque wrench to a specified height to lift the turret structure12and form a flat surface for bearing support. The turret structure12is raised to remove the vertical load from the damaged original support row assembly20.

The embodiment of the support roller bearing assembly200installed below the existing bearing assembly10provides the following benefits and characteristics:uses existing inner race bolting32to react load below existing bearing assembly10;minimizes hot work;uses short lead time materials with the exception of possibly the rollers264and hardened races262;utilizes rollers264riding on hardened races262similar to those used in segmented bearings;deflections and tolerances which are more of an issue for roller loading is aided by the bearing material ring252;low level of breakout torque required to rotate the chaintable (i.e., turret structure12);uses jacking screws242to lift the chaintable12to original position; andradial load path remains through the radial row assembly60of the existing bearing assembly10.

While the invention has been described in detail above with reference to specific embodiments, it will be understood that modifications and alterations in the embodiments disclosed may be made by those practiced in the art without departing from the spirit and scope of the invention. All such modifications and alterations are intended to be covered. In addition, all publications cited herein are indicative of the level of skill in the art and are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth.