An installation vessel for installing an offshore structure, the installation vessel comprising a hull and a plurality of legs, each leg comprising vertical chords and cross braces, the hull having a plurality of leg openings each configured for passage of a leg therethrough, each leg opening having a number of jack-cases for supporting pinions thereon, each pinion being configured for rotatably engaging a rack disposed on a chord of the leg to move the leg relative to the hull, each jack-case comprising a number of guides for guiding passage of the leg through the leg opening, each guide comprising a plurality of angled surfaces configured for sliding engagement with a corresponding plurality of surfaces of a rack, each of the plurality of angled surfaces comprising a wear plate.