Heave plate on floating offshore structure

Abstract

A floating offshore structure having a buoyant hull with an open truss frame having heave plates vertically spaced along the truss frame. One or more of the heave plates include a skirt plate that is attached around the outer perimeter of the heave plate and increases the effectiveness of the heave plate at trapping water mass between the heave plates for reducing heave motions of the floating offshore structure. A skirt plate is attached to one or more heave plates at essentially a right angle to the heave plate. The skirt plate may extend beyond the upper and lower surfaces of the heave plate or be positioned so as to be flush with the one surface of the heave plate and extend only beyond the opposite surface of the heave plate.

Description

FIELD AND BACKGROUND OF INVENTION[0001]The present invention is generally related to floating offshore structures and, more particularly, to floating offshore structures with heave plates.[0002]In the offshore oil and gas industry some floating structures such as the spar structure described in U.S. Pat. No. 5,558,467 use an open truss frame below the buoyant main hull to reduce the cross sectional area subject to environmental forces of waves and currents. This reduces drag and the effect of these forces on the structure. These structures also incorporate the use of heave plates spaced vertically in the truss frame to function as water mass entrapment plates during vertical motion of the structure. During vertical motion of the structure, the effect of the water mass trapped between the plates serves to reduce the heave motions of the structure caused by waves. The reduction of heave motions is caused by the heave natural period of the structure to be longer than the period of the ...

AI Technical Summary

Benefits of technology

[0009]The present invention is drawn to a floating offshore structure having a buoyant hull with an open truss frame having horizontally oriented heave plates vertically spaced apart along the truss frame. The heave plates include a skirt plate that is attached around the outer perimeter of the heave plate and increases the effectiveness of the heave plate at trapping water mass between the heave plates for reducing heave motion of the floating offshore structure. A skirt plate is attached to one or more heave plates at essentially a right angle to the heave plate. The skirt plate may extend beyond the upper and lower surfaces of the heave plate or be positioned so as to be flush with one surface of the heave plate and extend only beyond the opposite surface of the heave plate.

Problems solved by technology

However, there are situations that may require the use of larger or smaller heave plates that are not easily or efficiently dealt with through simple modification of the floating structure or the heave plates themselves.

Increasing the span of the heave plates outside the perimeter of the offshore structure and the truss frame presents additional difficulties in construction and transport of the structure.

Thus, the excessive extension of the heave plates outside the perimeter of the truss frame presents difficulties during construction since the extended heave plates may not be permitted because of the dimension constraints of the support structure.

The ineffective solution is to add the extended portion of the heave plates once the structure is floating at the offshore site where it is to be installed for its normal operation.

This is not acceptable due to the increased difficulty and expense required for such fabrication while offshore in conditions where the structure is unstable due to the effect of waves, wind, and currents.

As mentioned above, the over-extended heave plate beyond the truss frame may not fit the supports while lying on its side either in a fabrication yard or on a heavy lift vessel.

There are also situations that may require modification of the designed effectiveness of the heave plate after a structure has been built.

The same issues faced with spar type structures also apply to semi-submersibles.

While the heave plate has proven itself to be useful in reducing heave motions of floating offshore structures, it can be seen that there are physical and operation constraints on the maximum size of heave plates.

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