B777 Captain experiences a pack failure enroute and elects to divert for maintenance prior to crossing the ocean.

Narrative

During cruise flight I observed the left pack EICAS illuminate. The First Officer and I accomplished the appropriate checklist. It was determined to be a left FCV shutdown of the pack and therefore could not be reclaimed. We then contacted Dispatch. Since the flight was ETOPS we were concerned about going over the Ocean at night with a 1 PAC operation. With the concurrence of Dispatch we elected to divert so Maintenance could try to fix the aircraft. They had spare aircraft in order to protect passengers as well. It was also determined that an overweight landing would occur. Maintenance Control was consulted and it was agreed that the overweight landing could be accomplished with minimal risk. Upon contacting Approach Control an emergency was declared due to the diversion and overweight landing scenario. This was accepted by ATC and a normal landing was accomplished. We then taxied to a hardstand for inspection and maintenance. After a delay for approximately 1.5 hrs we were released and continued to original destination.