Abstract: An extendable utility boom assembly for assisting fire-fighting operations performed from a semi-submersible service vessel upon an offshore platform is disclosed. The boom assembly is moved to operating position by means of a carriage assembly which is mounted for movement along a track secured to the deck of the service vessel. Orientation of the boom assembly in azimuth and pitch is provided by a supporting frame which is pivotally mounted on the carriage assembly for angular movement through a horizontal plane, and a trunnion which is pivotally mounted on the supporting frame for angular movement through a vertical plane. An elongated utility boom is supported by roller bearing elements for axial movement through the trunnion. A first linear actuator is interconnected between the carriage assembly and supporting frame for displacing the supporting frame through the horizontal plane.