Ships – Mooring device – Boom type
Reexamination Certificate
2000-05-25
2001-05-08
Avila, Stephen (Department: 3617)
Ships
Mooring device
Boom type
C114S230190
Reexamination Certificate
active
06227135
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of mooring systems for offshore tanker loading/offloading facilities. In particular, the invention relates to soft yoke mooring systems which provide a resilient restoring force for vessels moored to a fixed tower.
2. Description of the Prior Art
Soft yoke mooring systems which use heavy counterweights to provide a restoring force against vessel offset, perform well under moderate environmental conditions. Such systems also perform satisfactorily under fairly severe environmental conditions as long as wind, wave and current directions are nearly collinear.
However, under strong cross current conditions (called “crossed sea” conditions), the moored vessel will be pushed into a quartering or near broadside orientation with respect to the wave direction. The resulting yaw and sway motions of the vessel will, in turn, cause lateral oscillations of the heavy counterweights of the prior soft yoke mooring systems. For pendant lengths between 10 m and 20 m, the natural lateral oscillation period of the yoke and counterweight will be in the order of 6 to 9 seconds. This will often coincide with prevailing wave periods, causing very large counterweight oscillations due to resonance amplification. In many cases, this resonance problem may be unacceptable.
For a submerged yoke system, the fluid drag resistance of the seawater may dampen the lateral oscillations significantly. On the other hand, the submerged counterweights will also be subject to direct excitation by the wave action.
3. Identification of Objects of the Invention
It is a primary object of this invention to provide an improved soft yoke mooring system which overcomes the disadvantage of prior suspended counterweight systems.
Another object of this invention is to provide a soft yoke mooring system which provides superior performance under “crossed sea” conditions as compared to suspended counterweight systems.
Still another object of the invention is to provide a new soft yoke mooring system, which is cost effective, and competitive with prior suspended counterweight systems, especially under severe environmental conditions.
SUMMARY OF THE INVENTION
Rather than using heavy suspended counterweights to provide a resilient restoring force for keeping a moored vessel on station, the soft yoke mooring system of this invention uses torsional spring energy to provide the required restoring force.
Torsional spring energy is provided in two ways in several embodiments of the invention. The first way uses multiple, high strength steel tubular shafts to provide the required torsional spring energy. The tubular shafts are assembled in a nested coaxial array and interconnected to provide effective torsion shaft lengths of up to 100 m or more.
The second way uses torsional spring energy provided by an elastomeric torque spring arrangement. The torque spring arrangement is built up from standard elastomeric shear fender units arranged in a circular or circular arch pattern.
Both sources of torsional spring energy are relatively simple mechanical arrangements. The structural arrangements of the soft yoke system present a “clean-cut” appearance and are functional. The mechanical hinge and U-joint components of the torque arm mooring system are similar in design to that for existing soft yoke systems.
REFERENCES:
patent: 3064615 (1962-11-01), Waltman
patent: 4392447 (1983-07-01), Kaps
patent: 4494475 (1985-01-01), Eriksen
patent: 4534740 (1985-08-01), Poldervaart
patent: 4665856 (1987-05-01), Pedersen
patent: 5036787 (1991-08-01), Rogers
patent: 0798206 A1 (1997-03-01), None
Avila Stephen
Bush Gary L.
FMC Corporation
Mayor Day Caldwell & Keeton, LLP
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