Dual axis chain support with chain pull through

[0001] 1. Field of the Invention

[0002] This invention relates generally to mooring systems for offshore structures such as platforms and vessels and in particular, to a device which supports the mooring chain in such systems.

[0003] 2. Description of the Prior Art

[0004] FIG. 1 illustrates a prior art mooring line connector illustrated in U.S. Pat. No. 6,663,320. This known connector allows the mooring line to rotate in two perpendicular planes relative to the offshore structure to which it is attached. The mooring line is connected using a rod and a latch mechanism. Adjustment of the length of the mooring line, which is often desired before and sometimes after installation of the mooring line, disadvantageously must take place before the rod is connected, most likely on board of another vessel.

[0005] FIG. 2 illustrates a prior art mooring line connector from UK Patent Application GB2351058. The chain stopper provides two axes of rotation, advantageously providing reduced chain wear, but it is not designed to minimize so called out-of-plain bending, which occurs when the chain is under tension and one chain link (the captive link) is fixed by the chain stopper while the adjacent link below the captive link is allowed to rotate relative to the captive link. Also, during chain installation and tensioning, an uplift force is produced in the connector that must be resolved by bending the line of chain or by other means.

[0006] 3. Identification of Objects of the Invention

[0007] A primary object of this invention is to provide a chain support in which fatigue damage due to out-of-plane bending is reduced.

[0008] Another object of this invention is to provide a chain support that allows rotation about two perpendicular axes whereby the maximum total rotation about the first axis is at least 90 degrees and the rotation about the second axis is ±15 degrees.

[0009] Another object of this invention is to provide a chain support that allows the upper end of the mooring line to be pulled up vertically through the assembly while at the same time allowing large vertical angle variations of the lower end of the mooring, without inducing objectionable vertical or lateral misalignment of the assembly with the mooring line.

[0010] Another object of this invention is to provide a chain support that allows adjustment of the mooring line length at any time by pulling in or letting out chain links with the capability of latching into every other chain link.

[0011] Another object of this invention is to provide a chain support with an arrangement of components that minimizes the overall width of the assembly.

[0012] The objects identified above, along with other features and advantages result from providing a chain support with two axes of rotation and an elongated hawse pipe. The longer length of the hawse pipe ensures that small angles between the last captive chain link and the first free chain link result in a rotation of the hawse pipe about either axis of rotation.

[0013] An elongated hawse pipe is pivotally connected to a structure by a hollow trunnion block. The trunnion block is fitted with two pairs of trunnions, providing two axes of rotation that are oriented perpendicularly to each other. By routing the chain through the trunnion block, the hawse pipe remains aligned with the chain below the hawse pipe when the chain is pulled up through the connector. The second pair of trunnions is arranged below the first pair of trunnions so that the overall width of the trunnion block is comparable to that of a conventional single-axis trunnion block. This arrangement provides several advantages including a smaller footprint and interchangeability with existing single-axis chain supports.

[0014] A chain latch for locking off the chain is incorporated into the lower end of the elongated hawse pipe. In most existing chain support designs, the chain latch is mounted on the upper end of the hawse pipe where it is easily accessible during the initial tensioning of the mooring chain. By placing the chain latch at the lower end of the elongated hawse pipe and supporting the free end of the chain that is extending above the chain support, the chain inside the trunnion block is slack and does not impede the rotations of the hawse pipe around the lower pair of trunnions.

[0015] The length of the hawse pipe is chosen such that even very small angles between the captive chain link on the chain latch and the first free chain link below the captive link generate enough torque to rotate the hawse pipe around either one of the axes. This ensures that the bending stresses in the first free link are kept at low levels and the accrual of fatigue damage can be controlled.

[0022] FIG. 3 shows a single point mooring system for a vessel 1 that is moored with nine mooring lines or chains 4. A chaintable 2 is rotatably connected to the vessel 1 and to the mooring lines 4. The vessel is free to weathervane around a vertical axis 7. The upper end of each mooring line includes a section of chain 4 which is connected to the chaintable 2 using a dual-axis chain stopper 3 according to the invention. Each chainstopper 3 allows rotation of the chain around a generally horizontal first axis 5 and a generally vertical second axis 6.

[0023] FIG. 4 shows a perspective view of the chain support 3. The mooring chain 4 is pulled through guide mouth 13 and through the interior of an elongated hollow housing 11. The chain 4 exits the chain support 3 through trunnion block 8. Since the direction of pull is often different from the orientation of the housing 11, the trunnion block 8 is fitted with a guide radius 17. (See FIG. 5) The guide radius 17 includes a groove to distribute the load over the chain links thus preventing unwanted bending stresses in the chain links. The elongated hollow housing 11 is rotatably connected to trunnion block 8 with a pair of lower trunnions 10 and a clevis 12. Low friction, self-lubricating bushings (not shown) are disposed between the pair of lower trunnions 10 and the clevis 12. The trunnion block 8 is connected to the chaintable 2 through a pair of upper trunnions 9. The trunnions 9 are connected to structure 2 with bearing blocks and caps and low friction self-lubricating bushings placed between the upper trunnions 9 and said blocks and caps.

[0024] The chain 4 enters the hollow housing 11 through a lower guide mouth 13. A latch mechanism 14 is mounted inside the housing 11 to latch the chain 4 once the desired amount of chain has been pulled through the chainstopper. If there is a need to let out chain 4, a cable running through pipe 16 and attached to the latch mechanism is provided to keep the latch in the open position so that the chain 4 can be lowered.

[0025] FIG. 5 shows another perspective view of the chain support 3 with part of the hollow housing 11 removed for clarity. As shown in the enlargement of FIG. 6 the latch 14 according to a preferred embodiment includes a hinged flapper 14 mounted atop chain guide 15 which has a cross-shaped opening that rotates the chain so that it aligns with the slot in the flapper 14.

[0026] During operation of the chain support, the chain 4 is pulled upward through the cross-shaped opening of the chain guide. As the chain 4 is pulled up, the flapper 14 rotates upwards when a link of the chain 4 is not aligned with the opening of the flapper 14. The flapper 14 rotates downward and around a link of the chain 4 when the link is aligned with the flapper opening. Thus the flapper 14 fits around every other chain link and acts as a ratchet. When tension on the chain 4 is released, the flapper 14 rotates downward about a link of the chain and rests on the lower end of the housing 11. The chain is now latched off and secured to the structure. By slacking off of the upward pull on the chain 4, the tension on the chain above the flapper 14 is removed and the chain 4 forms no impediment to the rotation of the housing 11 around the pair of lower trunnions 10.

[0027] In a further embodiment of the invention, the chain support includes strain gages 20 (FIG. 5) mounted to the hollow housing 11 for measuring the tension in the chain 4. Alternatively, standard resistance compression load cells (not illustrated) are placed between the flapper of the latch mechanism and the chain guide on which the flapper rests. In another embodiment, a non-contact sensor 22 (illustrated in dotted lines in FIG. 5) is positioned into the interior of the trunnion block to measure the deflection of the upper or lower trunnions. U.S. Pat. No. 6,925,890, incorporated by reference herein, illustrates the placement of a non-contact sensor on a trunnion block.

[0028] As identified in the above identification of objects of the invention, the arrangement of FIGS. 3-6 is a chain support that allows rotation about two perpendicular axes 5, 6 such that the total rotation about axis 5 is at least 90 degrees and about axis 6 is ±15 degrees. The chain support 3 of FIGS. 3-6 allows the upper end of the mooring line to be pulled up vertically through the assembly while simultaneously allowing large vertical angle variations of the lower end of the housing, while not inducting vertical or lateral misalignment of the assembly with the mooring line 4. The arrangement of FIGS. 3-6 advantageously allows adjustment of the mooring line length at any time by pulling in or letting out chain links with the capability of latching into every other chain length. Advantageously, the chain support 3 of FIGS. 3-6 has a minimal overall width.