Devonport Dockyard is the UK's only naval base that refits nuclear submarines and the Navy estimates that the Dockyard generates about 10% of Plymouth's income. Plymouth has the largest cluster of marine and maritime businesses in the south west with 270 firms operating within the sector. Other substantial employers include the university with almost 3,000 staff, as well as the Tamar Science Park employing 500 people in 50 companies. Several employers have chosen to locate their headquarters in Plymouth, including Hemsley Fraser.