An offshore vessel capable of installing and removing a subsea well control device and a riser string includes a hoisting device having a travelling block for connecting a load. The travelling block is displaceable along a firing line which extends through a moonpool and a heave compensation system. The vessel further includes a working deck supported by the hull of the vessel which covers at least a portion of the moonpool to allow the assembly of a riser string. The working deck is provided with a riser string suspension device that allows to suspend a top end of a string of risers. A heave compensation connection system is adapted to connect the working deck to the travelling block, such that the hoisting device can move the working deck when the working deck is connected to the travelling block between a lowered riser assembly position and a raised heave compensated position.