Method for riser string handling and an offshore drilling vessel

A method for riser string handling on an offshore drilling vessel , the offshore drilling vessel comprising:

• a hull having a moonpool extending through the hull;
• a multiple firing line hoist system mounted on the hull at said moonpool, the multiple firing line hoist system comprising:
• a tower having a top side and a base connected to the hull of the drilling vessel;
• a first hoisting device supported by the tower and having a first load attachment device displaceable along a first firing line, which extends essentially parallel to the tower;
• the first hoisting device being adapted to build and lower a riser string in the first firing line;
• a second hoisting device supported by the tower and having a second load attachment device displaceable along a second firing line, which extends essentially parallel to the tower;
• wherein a rotary drilling drive is provided in the second firing line, being adapted to assemble and disassemble a drill string and effect drilling in the second firing line;
• a riser tensioner system arranged in the second firing line, adapted to be connected to a top end of the riser string, in order to suspend the riser string from in the second firing line;
• a suspended riser transfer device, comprising:
• a riser hang-off assembly being actuable between an open configuration in which the riser hang-off assembly is adapted to move around an upper end of a riser string, and a closed configuration in which the riser hang-off assembly engages and supports the riser string, leaving the top end of the riser string exposed to allow for connection to the riser tensioner system;
• a frame with a gimbal device, to which the riser hang-off assembly is mounted;
• rails extending in longitudinal direction along the moonpool between the first firing line and the second firing line, allowing to displace the frame in a suspended riser transfer path in a longitudinal direction of the moonpool while the riser string is supported in a gimballing manner,
• the method comprising the following steps:
• a) building and lowering a riser string in the first firing line, and simultaneously assembling and preparing a riser tensioner system in the second firing line,
• b) suspending the riser string from the first load attachment device,
• c) positioning the frame with the gimbal device to which the riser hang-off assembly is mounted, with the riser hang-off assembly in the open configuration, in the first firing line,
• d) positioning an upper end of the riser string in the riser hang-off assembly and actuating the riser hang-off assembly to the closed configuration,
• e) transferring the weight from the riser string from the first load attachment device to the riser hang-off assembly,
• f) disconnecting the first load attachment device from the riser string,
• g) displacing the riser hang-off assembly in the suspended riser transfer path while it supports the riser string in a gimballing manner from the first firing line to the second firing line,
• h) connecting the top end of the riser string to the riser tensioner system,
• i) transferring the weight from the riser string from the riser hang-off assembly to the riser tensioner system,
• j) actuating the riser hang-off assembly to the open configuration and thus disconnecting the riser hang-off assembly from the riser string,
• k) displacing the frame with the gimbal device to which the riser hang-off assembly is mounted in the suspended riser transfer path from the second firing line to the first firing line.