Hoisting device

A method for lowering an object from a floating vessel to a deepwater installation site, the method comprising the step of using a multi purpose hoisting device, the hoisting device comprising:

• a load bearing structure;
• a main hoisting mechanism for raising and lowering an object near the water surface, the main hoisting mechanism comprising:
• i. at least one main hoisting winch;
• ii. a main hoisting cable associated with the at least one main hoisting winch;
• connecting mechanism for releasable connecting an object to the main hoisting cable;
• a main hoist heave compensation mechanism associated with the main hoisting cable for damping the effect of the movement of the vessel, as a result of heave and beating of waves, onto an object supported by the main hoisting cable;
• a deepwater hoisting mechanism for raising and lowering an object to a deepwater installation site, the deepwater hoisting mechanism comprising:
• i. a deepwater hoisting winch;
• ii. a deepwater hoisting cable, the deepwater hoisting cable running along a path from the deepwater hoisting winch to a top pulley supported by the load bearing structure, wherein from the top pulley, the deep water hoisting cable is suspended for supporting a load, and the path of the deepwater hoisting cable is distinct from the main hoist heave compensation mechanism;
• and
• iii. a connecting mechanism for releasable connecting an object to the deepwater hoisting cable;
• a releasable attachment mechanism for interconnecting the main hoisting cable and the deepwater hoisting cable such that the heave compensation mechanism associated with the main hoisting cable is operable in combination with the deepwater hoisting cable, the method further comprising:
• lowering an object from a position near the water surface to an intermediate underwater position near the under water installation site, using the deepwater hoisting winch and the associated deepwater hoisting cable to support the object;
• interconnecting the deepwater hoisting cable and the main hoisting cable such that the path of the deepwater hoisting cable is distinct from the main hoist heave compensation mechanism;
• transferring the load of the object from the deepwater hoisting winch to the main hoisting winch, and using the main hoisting winch, the associated main hoisting cable and the deepwater cable to support the object;
• if the heave compensation mechanism is turned off, switch on the heave compensation mechanism;
• lowering the object from the intermediate underwater position to the under water installation site using the main hoisting winch;
• and
• using the heave compensating mechanism associated with the main hoisting mechanism for damping the effect of the movement of the vessel, as a result of heave and beating of waves, onto the object supported by the deepwater hoisting cable, while lowering the object and landing the object on the deepwater installation site.

A hoisting device for use on a floating vessel, the hoisting device comprising:

• a load bearing structure to be mounted on the vessel;
• hoisting mechanism for raising and lowering an object, comprising:
• i. at least one hoisting winch;
• ii. a hoisting cable associated with the at least one hoisting winch,
• iii. an object connecting device for releasable connecting an object to the hoisting cable;
• a heave compensation mechanism associated with the hoisting cable for damping the effect of the movement of the vessel, as a result of sea-state induced vessel motion onto an object supported by the main hoisting cable;
• wherein the heave compensation mechanism comprises:
• an underload protection cylinder which supports a cable pulley which guides the hoisting cable such that a force is exertable on the hoisting cable, which cylinder is normally positioned in extended position to protect the hoisting mechanism against underload or slack;
• and/or an overload protection cylinder which supports a cable pulley which guides the hoisting cable such that a force is exertable on the hoisting cable, which cylinder is normally positioned in retracted position to protect the hoisting mechanism against overload;
• a control device for controlling the underload protection cylinder and/or the overload protection cylinder, which control device is adapted to switch each of the cylinders between a protection mode in which said cylinder protects the hoisting mechanism against underload or overload respectively, and a heave compensation mode, in which each cylinder is positioned in intermediate position to provide passive heave compensation.