A self-propelled semi-submersible column stabilized service vessel for tending offshore production and drilling operations is disclosed. The vessel includes a pair of submersible hulls having ballast compartments for controlling the buoyancy of the hulls. A rectangular service deck or platform which includes fire fighting equipment, rig inspection equipment, repair facilities, and load lifting equipment is supported by means of vertical stability columns which are compartmented and include ballast chambers for controlling the buoyancy of the columns. The fire fighting equipment includes an extendable fire boom for clearing debris and for positioning explosives on the deck of a burning production or drilling platform. An array of monitors is also provided for establishing a curtain of water for thermally shielding the service vessel and for extinguishing a fire on the platform. The rig inspection equipment includes a diving bell and constant tension hoist apparatus for accurately positioning the diving bell along the sloping underwater structure of an offshore production or drilling platform. The load lifting equipment includes a revolving crane which is strategically located on the bow center line of the service platform. The submersible hulls are provided with fixed in-line propulsion units on each hull and variable heading azimuth thrusters disposed near the bow end of each hull. The main propulsion units and variable azimuth thrusters cooperate with anchor lines under constant tension winch control for accurately positioning the service vessel adjacent an offshore production or drilling platform for transferring supplies and equipment or for fire fighting purposes in moderately heavy seas. The buoyancy of the stability columns and hulls is differentially controlled for maintaining stability and trim during heavy load lifting operations. The buoyancy control features are also utilized in combination with davits and constant tension winch assemblies for lifting submerged pipelines for inspection and repair.