1. Field of the Invention:
This invention pertains to systems for deploying and retrieving submersible objects, such as diving bells or the like, from and to floating vessels. More particularly, it pertains to such systems useful to safely perform such operations in rough water where the floating vessel experiences appreciable pitch, heave and roll motions.
2. Review of the Prior Art and the Problems Thereof:
Subsea pipelaying operations require, in the case of long pipelines, that connections be made periodically between sections of pipeline. These connections are made, preferably by welding, on the sea floor. Subsea pipeline welds can be made by divers working directly on the sea floor if water depths are not too great, but even in modest water depths such welds or other activities may take extended time to accomplish. It is not uncommon for subsea pipelaying to involve three days of diving operations for each day of actual pipelaying operation.
The time required to make welded connections between the adjacent ends of subsea pipeline sections can be reduced, and the depth at which such operations are done increased, by the use of subsea habitats installed on the sea floor where the work is to be done. The adjacent ends of the pipeline sections extend into the interior of the habitat where the necessary work can be done at substantially one atmosphere. U.S. Pat. No. 3,641,777 describes various subsea habitats useful in making subsea pipeline welded connections. Workers are moved between a surface pipelaying or other work vessel and the habitat by diving bells and the like.
It is well known that the dynamic effects of surface wave action in an ocean, for example, decrease proceeding downwardly from the ocean surface until, at a depth related to the surface wave length, no dynamic effects are encountered. Above this depth, the orbital motions of water particles increase exponentially to a maximum at the water surface; see FIG. 9.D at page 161, Hydrodynamics in Ship Design, Vol. I, by Harold E. Saunders, Society of Naval Architects and Marine Engineers, 1957. Thus, storms raging over the ocean surface may have no effect on the ocean floor if the water depth is sufficiently great, and pipeline connection activities can be performed safely at the ocean floor even though the vessels above the work location may be experiencing substantial heaving, pitching and rolling motions due to wave action at the surface. Such vessel motions have heretofore restricted the movement of bells and the like between surface vessels and subsea work habitats. These restrictions arise due to the difficulties presented in safely coupling and decoupling bells and the like to the surface vessels while the vessels move in response to wave action. Therefore, diving operations, even by the use of bells and subsea work habitats, in support of subsea pipelaying operations are restricted to periods of relatively calm surface conditions. In some areas, such as the North Sea, such relatively calm surface conditions may be few and far between each other.
it has therefore been proposed to make the connection or disconnection between a surface pipelaying vessel and the like and a diving bell at a sufficient depth below the vessel that the hazardous dynamic effects of surface wave action are avoided or sufficiently reduced to present no unacceptable hazard. Such proposals are illustrated by the systems described in U.S. Pat. Nos. 3,507,241 and 3,641,961. In both of these systems a structure extends downwardly from a floating vessel to a depth where surface wave action is minimal or not present, and the connection between the vessel and a bell (U.S. Pat. No. 3,641,961) or a submersible vehicle (U.S. Pat. No. 3,507,241) is made or unmade. The intent is to make or unmake the connection under conditions where the bell or vehicle is not subjected to displacements by surface wave action. In these systems, however, the structures carried by the floating vessels are effectively rigid to the floating vessels and therefore move with the floating vessels. Another system having below-vessel object deployment and retrieval structures stiffly connected to the vessel is described in U.S. Pat. No. 3,894,640. Thus, while the bells, submersible vehicles or other objects are undisturbed by wave action at the desired connection depths, th structures to which they are to be connected, or from which they are to be released, are not similarly undisturbed. Therefore, these prior arrangements, while enabling submersible object deployment and recovery in other than very calm conditions, are not safely useful in rougher and more adverse surface conditions.
U.S. Pat. No. 1,823,965 describes an arrangement for providing a water-free vertical path between a submerged location and a surface vessel through which a man may move, as by climbing a ladder. In this arrangement, a vertical tubular caisson is provided between the surface vessel and the submerged location. Axially open pumps in the caisson generate a vortex in the water in the caisson, and the ladder extends vertically in the water-free vortex center along the length of the caisson. The upper end of the caisson is described to be gimballed to the surface vessel so that the caisson may be vertical even though the surface vessel may pitch and roll.
In the context of deployment and retrieval of submersible objects, such as diving bells and submersible vehicles, from and to floating vessels, such as pipelaying ships, it is seen that a need exists for improved systems which enable deployment and retrieval operations to be carried out safely under rougher and more adverse surface conditions than has heretofore been possible. The present invention is addressed to this need.