1. Field of the Invention:
This invention relates to methods of erecting marine structures at an offshore site of a body of water, such as an ocean, by controlled submerging of the hollow tubular members associated with the structure. The lower portion of each of the members is closed by a watertight removable plug. Each plug is provided with a vent which is normally closed but which vent can be opened after the members are in contact with the bottom to substantially equalize the pressure on both sides of each of the removable plugs before the plugs are removed from their respective legs. Once the plugs are removed, piling are driven through the hollow legs into the bottom of the body of water to secure the structure in place.
2. Description of the Prior Art:
The increasing demand for oil and natural gas has resulted in a rapid increase in the drilling for oil and gas at offshore sites in bodies of water such as oceans, seas, lakes, straits, etc. and at steadily increasing depths of water. It is known to fabricate marine structures having hollow cylindrical members such as the main legs of the structure, skirt piles, conductors, etc. The bottom ends, or feet, of the main legs are adapted to contact the bottom of the body of water at the offshore site. The upper portions of the legs of the structure have, or are adapted to have, a platform secured to them. The lengths of the legs are chosen so that when the bottoms of the legs firmly engage the bottom of the body of water, the platform is above the highest waves likely to be encountered at the site.
Such structures are generally fabricated in shipbuilding facilities because of their size and weight, for example, the tubular members can be several hundred feet long. The structures are generally transported to the offshore site by barge, pontoons, or by towing the floating structure, normally in a horizontal position. To provide buoyancy when the structure is towed to its site, the tops and bottoms of the tubular members can be sealed to make them watertight.
At the site the structure is caused to float upright by selective flooding of the tubular members, by cranes lifting on the structure, or both. Then the structure is submerged until the bottoms of the legs contact the bottom surface of the body of water. Once the marine structure is in position with its legs firmly in contact with the bottom of the body of water, it is customary to drive skirt piling and/or piling through the legs into the earth to firmly secure the structure in place.
Removal of the plugs closing the bottom of the legs has presented problems. However, it is desirable to seal the bottom ends of the legs by plugs that can be readily removed and when the plugs are removed it is desirable that the interiors of the leg in which they were placed be left substantially clear of obstructions. Conductors and/or skirt piles may be built into the structure in the shipyard for convenience and to provide buoyancy to the erection site--plugs are useful in these tubular members. A zip-out plug is one such type of removable water-tight plug. A zip-out plug has a pressure vessel which is held in place in a tubular member of a marine structure by an elastomeric material, such as rubber, or a synthetic rubber, with a coil of wire rope embedded in the elastomeric material. The coils of the plug rope are substantially uniformly spaced apart so that a reasonable force applied to one end of the plug wire rope will cause the elastomeric material to fail progressively until the pressure vessel is no longer secured or attached to the inner wall of the tubular member. The other end of the plug wire rope used to disconnect the pressure vessel from the tubular member is secured to the pressure vessel to lift it out of the tubular member.
A zip-out plug solves many problems encountered in erecting marine structures such as providing a means for buoyancy that can be conveniently regulated, preventing silt and debris from entering into a tubular member while the plug is in position and reducing the remnants of the elastomeric material of the plug adhering to the inner wall of the tube after the plug is removed to a small, or negligible, amount which does not interfere with driving piling through the member. One set of problems not solved by such plugs is caused by the pressure differential which can, and generally does, exist across a zip-out plug at the time it is disconnected. Such a pressure differential can force, or drive, the pressure vessel of the plug up through the interior of the hollow, tubular member and possibly blow it out the top with a risk of damaging the marine structure and the men who may be working on or in its vicinity. If the pressure within the tubular member is greater, then the pressure vessel will be forced downwardly and could be forced out of the tubular member so that it would be difficult, if not impossible, to recover the pressure vessel particularly if the plug wire rope fails. A pressure vessel embedded in the earth could interfere with driving piling through the tubular member. A difference of pressure across the pressure vessel when the zip-out plug is disconnected from the tubular member can also cause the pressure vessel to become wedged, damaging the member, the pressure vessel and effectively blocking the tubular member to prevent piling from being driven through it.