1. Field of the Invention
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.
2. Description of the Prior Art
3. Identification of Objects of the Invention
A primary object of this invention is to provide a chain support in which fatigue damage due to out-of-plane bending is reduced.
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.
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.
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.
Another object of this invention is to provide a chain support with an arrangement of components that minimizes the overall width of the assembly.
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.
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.
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.
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.
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.
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.
In a further embodiment of the invention, the chain support includes strain gages 20 (
As identified in the above identification of objects of the invention, the arrangement of
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Number | Date | Country | |
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20100175604 A1 | Jul 2010 | US |