Heave compensated dual hoist crane

A method for lowering height of a vessel provided with a cable comprising:

• a crane structure;
• a boom, the boom having an inner end, a midsection, and an outer end, the boom being pivotable supported by the crane structure such that the boom can be pivoted about a substantially horizontal boom pivot axis;
• a main hoisting assembly, for lifting and lowering a load, the main hoisting assembly comprising a main hoisting winch and an associated main hoisting cable, and a main load suspension device;
• and
• a luffing assembly for pivoting the boom up and down, the luffing assembly comprising:
• a luffing winch and an associated luffing cable;
• and
• a gantry comprising a back stay and a back frame, the gantry supporting a luffing cable sheave for guiding the luffing cable from the luffing winch to the outer end of the boom,
• wherein the back frame is located between the back stay and the boom, and wherein the back stay and back frame are both at a lower end pivotably mounted to the crane structure, for pivoting about a substantially horizontal back stay pivot axis and back frame pivot axis respectively, and are both at an upper end pivotably connected to each other, and
• wherein the gantry is a foldable gantry, the back stay comprising a lower frame section and an upper frame section, the lower frame section and the upper frame section being pivotably connected to each other, and wherein the gantry can be lowered by pivoting the lower frame section relative to the upper frame section,
• the method comprising the steps of:
• lowering the boom in a rest position;
• setting the main hoisting cable under constant tension to thus load the back stay of the gantry;
• initiating pivoting of the lower frame section of the back stay relative to the upper frame section of the back stay;
• and
• paying out the luffing cable to lower the gantry, until the gantry is in a lowered position thereof.

A pedestal mounted dual hoist crane configured to be mounted on a vessel, the crane comprising:

• a pedestal;
• a slew bearing;
• and
• a crane structure which is via the slew bearing rotatable supported on the pedestal;
• a boom, the boom having an inner end, a midsection, and an outer end, the boom being pivotable supported by the crane structure such that the boom can be pivoted about a substantially horizontal boom pivot axis;
• a main hoisting assembly, for lifting and lowering a load, the main hoisting assembly comprising a main hoisting winch and an associated main hoisting cable, and a main load suspension device;
• and
• an auxiliary hoisting assembly for lifting and lowering a load, the auxiliary hoisting assembly comprising an auxiliary hoisting winch and an associated auxiliary hoisting cable, and an auxiliary load suspension device;
• a luffing assembly for pivoting the boom up and down, the luffing assembly comprising:
• a luffing winch and an associated luffing cable;
• and
• a gantry comprising a back stay and a back frame, the gantry supporting a luffing cable sheave for guiding the luffing cable from the luffing winch to the outer end of the boom,
• a heave motion compensation system, having one or more heave compensation cylinders that extend in a vertical direction from the crane structure, via the bearing, into the pedestal of the crane.