A monohull offshore drilling vessel comprising: - a hull having a moonpool; - a multiple firing line hoist system mounted on the hull at said moonpool, the multiple firing line hoist system comprising: - a mast having a top side and a base connected to the hull of the drilling vessel, wherein the mast has a hollow construction with a first side and an opposed second side, - a first hoisting device supported by the mast and having a load attachment device displaceable along a first firing line, which extends on the outside of and adjacent to the first side of the mast; - a second hoisting device supported by the mast and having a load attachment device displaceable along a second firing line, which extends on the outside of and adjacent to the second side of the mast; wherein the first and second hoisting devices each include one or more cables and one or more associated winches to manipulate the position of each of the load attachment devices relative to the mast, characterized in that a mobile working deck is provided at the first side of the mast, which in an active position covers a portion of the moonpool at said first side of the mast while the first firing line extends through an opening in said mobile working deck, said mobile working deck being provided with a riser string suspension device that allows to suspend a riser string having a weight of at least 200 tonnes, preferably with a BOP attached to the lower end of the riser string, from the mobile working deck in the first firing line, and wherein a BOP storage is present in the hull of the vessel adjacent the moonpool, wherein the first hoisting device is adapted for raising and lowering the BOP to the seabed, and wherein the mobile working deck is movable into a non-active position which allows the BOP to be brought from the BOP storage into the first firing line and manipulated by the first hoisting device. - a hull having a moonpool; - a multiple firing line hoist system mounted on the hull at said moonpool, the multiple firing line hoist system comprising: - a mast having a top side and a base connected to the hull of the drilling vessel, wherein the mast has a hollow construction with a first side and an opposed second side, - a first hoisting device supported by the mast and having a load attachment device displaceable along a first firing line, which extends on the outside of and adjacent to the first side of the mast; - a second hoisting device supported by the mast and having a load attachment device displaceable along a second firing line, which extends on the outside of and adjacent to the second side of the mast; - a mast having a top side and a base connected to the hull of the drilling vessel, wherein the mast has a hollow construction with a first side and an opposed second side, - a first hoisting device supported by the mast and having a load attachment device displaceable along a first firing line, which extends on the outside of and adjacent to the first side of the mast; - a second hoisting device supported by the mast and having a load attachment device displaceable along a second firing line, which extends on the outside of and adjacent to the second side of the mast;