Subsea development is becoming commonplace in the UK sector of the North Sea. Over 50% of all fields to be developed in the next ten years on the UK Continental Shelf will be developed subsea and tied back to existing facilities or floating production facilities. In order to facilitate the development of these subsea fields a number of semisubmersibles have been converted from exploration to development mode. These rig modifications have included top drive installation, improved solids control equipment and oil base mud handling facilities, special subsea tree handling equipment, semi-permanent production test facilities and addition of saturation diving equipment. These modifications have enhanced project economics by accelerating the drilling and completion of subsea wells. Each modification is discussed in detail.