On February 9, 2001, the USS Greenville was conducting submarine exercises 13 miles off the coast of Oahu, Hawaii. While attempting an emergency main ballast tank blow procedure, the submarine struck and sank the Japanese fishing and high school training vessel Ehime Maru in 2,000 feet (600m) of water.

The collision, which took the lives of nine personnel aboard, created tension and at times strained relations between the Japanese and United States governments. As a sign of sorrow and goodwill, the US Navy promised to locate and recover the missing remains if it were technically possible. What followed was one of the most ambitious and technically challenging salvage efforts of its kind.

In the days immediately following the accident, search and recovery crews scoured the ocean surface for survivors. As search efforts showed no signs of any of the missing crewmembers, attention turned to locating the sunken vessel on the seafloor and searching it for trapped crewmembers. Due to the extreme depth, divers would not be able to be used. The Commander in Chief, Pacific Fleet (CINCPACFLT) called on the Supervisor of Salvage and Diving (SUPSALV), and Submarine Development Squadron 5 (SUBDEVRON 5) from San Diego to mobilize their ROVs and accomplish a subsurface search. Both quickly mobilized and flew their equipment to Honolulu.

SUPSALV, using ROV Deep Drone and the Shallow Water Intermediate Search System (SWISS) side scanning sonar system, and SUBDEVRON 5, using the ROV Scorpio, quickly located the vessel and visually searched the exterior and surrounding seafloor. After working around the clock for over two weeks in adverse weather, the subsurface search was called off. None of the missing crewmembers were located. With relations between the US and Japanese governments strained because of the incident, the US Navy made the commitment to the surviving family members that if it were technically feasible to raise the Ehime Maru, the US Navy would do so. Under direction from CINCPACFLT, Captain Bert Marsh, Richard Asher, and Tom Salmon from NAVSEA 00C assembled a feasibility study team composed of NAVSEA 00C, NAVSEA 05, Japanese technical, and various contractor personnel. The salvage firms Smit International from Singapore and Smit Tak from Rotterdam functioned as the prime contractors, based on pre-existing salvage contracts with NAVSEA 00C.

Due to the depth of the Ehime Maru, the team was limited in the techniques they could employ for the recovery. Since saturation divers were out of the question and other means of gaining entry into the vessel at depth had very low probabilities of success, the team had to look at moving the vessel to an area in which personnel could safely search the interior. Before being able to say this was feasible, a structural analysis of the damaged ship had to be performed.

Structural Analysis
With information gained from the initial ROV search, Tom Packard and Ed Kadala led a team of engineers and naval architects from NAVSEA 05P in developing a structural model of the roughly 850-ton ship. Using a hull model based on a beam type finite element of over 200 nodes and the shipbuilder's lightship weight distribution, the group generated numerous bending moment curves for a number of different lifting arrangements. Incorporated into each analysis were reduced section moduli in way of the damaged areas which assumed a worst case damage scenario.

The result of the analyses was that the Ehime Maru could withstand a static steady state lift, but that the dynamic effect imparted by ocean swells could be a problem. Concurrent with this effort, Smit engineers conducted an independent analysis and came to the same conclusion.

Based on the detailed structural analyses and a concept of recovery operations from Smit, Captain Marsh concluded that that it was technically feasible to raise the Ehime Maru and estimated that there was an 80 percent chance of success. The results of the feasibility study and the odds of success were sufficient to convince CINCPACFLT to pursue recovering the fishing vessel.

The concept of operations called for placing two large lifting straps beneath the ship and carrying it into shallow water where divers could safely enter and search the vessel. This required a site where the ship could be relocated without causing damage to the environment.

To prove this, CINCPACFLT had to accomplish an Environmental Assessment before the operation could begin. CINCPACFLT N45 (Environmental Division) was chosen to head up this effort. Drawing on assistance from Edaw Inc., from Irvine, California, numerous subcontractors, NAVSEA 00C25 pollution engineers, and SUPSALV Emergency Ship Salvage Material (ESSM) contractor personnel, CINCPACFLT N45 conducted the assessment in less than 13 weeks at a cost of nearly $2 million. The Environmental Assessment culminated with (CINCPACFLT) signing a "Finding of No Significant Impact" for moving the Ehime Maru from its current location to about a mile off the Honolulu airport.

Commencing Lift Operations
While the environmental assessment progressed, mobilization for the recovery operation began in earnest. To accomplish the rigging and lifting operations, specialized offshore equipment was mobilized from Singapore, Europe, Philippines, Texas, Louisiana, and California.

Additionally, Haliburton's oil field drilling and diving support vessel Rockwater 2 was contracted as the host work platform. Since the Rockwater 2 was already operating in eastern Asia, it was outfitted for the operation in Batangas, Philippines, and was used to transport some of the lifting equipment to Hawaii. Three separate ROV systems, a coiled tube drilling unit, and rigging equipment were mobilized from the Gulf coast to Port Hueneme, California, and barged to Honolulu on the Crowley Marine 250-6 barge.

Once the Rockwater 2 and remaining support equipment arrived on-station, Pacific Shipyard was subcontracted to complete vessel outfitting. This entailed welding down the ROV equipment, installation of portable generators for the deck equipment, and welding of additional rigging equipment for the deep ocean lift.

While the Rockwater 2 was in the process of being mobilized and outfitted, a separate ROV support vessel was contracted to conduct preliminary support operations on the Ehime Maru. The cable laying ship Ocean Hercules was brought in to remove the center mast of the Ehime Maru since it was determined to be an obstruction to the lifting hardware. For the mast removal, Jet Research Corporation from Texas was subcontracted to cut the mast off using underwater explosives. The Ocean Hercules was also used to accomplish dredging of bottom sediment in the way of the lift straps and to place a constellation of underwater ROV navigational transponders around the sunken vessel.

After outfitting was complete, the Rockwater 2 immediately transited to the Ehime Maru site to begin recovery operations. The first order of business was to calibrate the navigational transponder constellation so that the ROV pilots could maneuver in near-zero visibility conditions.

Once calibration was complete, the recovery team went to work on putting two large lifting straps beneath the hull. The first attempt at doing so incorporated a technology called coiled tube drilling, routinely used for drilling into oil wells. It had never before been adapted to deep ocean salvage work nor operated by ROVs. The general principle of coiled tube drilling is to pump high pressure water through two-inch steel pipe outfitted with a drilling nozzle on the end to essentially drill with the high pressure water through the soil. While the modified technology was successful in drilling at 2,000 feet (600m), it was not successful in placing the lift strap messengers under the vessel.

The next attempt at placing the lift straps beneath the Ehime Maru incorporated lifting the vessel via the stern and slipping the straps beneath it while suspended. The first try at lifting the stern resulted in the temporary lift strap breaking when it slipped onto and around the rudder post. On the second try, the team used a cable reinforced lifting strap. This time, the team was successful. With the stern lifted, the ROV pilots pulled a messenger wire underneath the after area of the hull. The team was unable to install the forward lifting strap messenger.

After the stern of the vessel was set back down, the ROVs were then used to rig one end of the aft messenger wire to the lifting strap and the other end to a heave-compensated crane aboard the Rockwater 2. With this setup, the crane was successfully used to pull the aft lifting strap underneath the hull.

The forward strap posed a new challenge. With two forward lift strap installation procedures having been attempted and unsuccessful, the recovery team developed a third technique. With the aft strap installed, they rigged a temporary lift wire through the anchor chain hawse pipes and lifted the bow over and onto the forward lift strap. This turned out to be a difficult procedure, since the bow had buried deep into the sediment when the stern was lifted. The solution to the problem was to dredge the material from around the anchors. While the procedure was effective, the time it took to conduct the dredging was considerable.

Once the temporary lift wire was rigged through the hawse pipes, the recovery team then placed the lower spreader bar assembly above the Ehime Maru. The spreader assembly was designed to be buoyant so that when it was placed above the vessel and attached to the lifting straps, it would keep the lifting straps tensioned against the ship.

Since they had deviated from the original plan, they had to reconfigure the rigging. Instead of attaching both the forward and aft straps to the spreader assembly, only the aft lift wire was attached. In order to balance the spreader assembly, one of the heave compensated cranes aboard Rockwater 2 was attached to the other end of the assembly. The temporary lift wire through the hawse pipes was then rigged directly to the linear winch. With this configuration, they could now lift the entire vessel and move it laterally onto the originally designed forward lift strap. This procedure was a success, as the team now had both the forward and aft lift straps attached to the lift assembly as originally designed.

The next step was to lower the upper lift assembly down from the Rockwater 2 with the hydraulic linear winches. The two 500-ton linear winches were installed such that their lift wires and associated sheaves went over the side of the Rockwater 2 and down to the lower spreader assembly. Once the sheave was at depth (approximately 1,900 feet, or 576m) it was then stabbed and connected into the lower lift assembly. The entire connection process was accomplished using the ROVs, the two heave-compensated cranes on the Rockwater 2, and the two linear winches. With all the rigging hardware connected, the Ehime Maru was ready to be lifted as soon as favorable weather conditions appeared.

After closely monitoring the weather forecasts, an acceptable window finally presented itself. With seas less than six feet, the team commenced the relocation process by raising the Ehime Maru approximately 25 feet above the seafloor. With the vessel lifted, the ROVs maneuvered beneath the ship so that the area damaged by the submarine rudder could be inspected. The inspection confirmed that the structural damage was well within the estimates made in the feasibility study. All conditions were "go" for relocating the Ehime Maru to shallow water.

Transferring the Vessel
Transiting with an armada of other support vessels, the Rockwater 2 began its slow trek to the shallow water site. Ensuring there were no undiscovered problems with the lift, the initial transit speed was set at 0.2 to 0.3 knots. Since the Rockwater 2 was outfitted with a dynamic positioning system for navigation and station keeping, this low speed was achievable.

Once the team was comfortable with the lift arrangement, the speed was slowly increased to 0.5 to 0.7 knots. Speeds were kept under one knot so that the dynamic loading on the lift system were kept to a minimum. The speeds were also kept low so that the Ehime Maru could be slowly raised with the winches as shallower water was approached. After completing the 13 mile journey, the Ehime Maru was set down at the 110-foot (33m) shallow water site without incident.

The next phase of the operation called for the Mobile Diving and Salvage Unit ONE (MDSU ONE) divers to penetrate the sunken vessel and search for the missing crewmembers. After monitoring the ship for 48 hours to ensure that it was stable, the Navy divers began their difficult task. They dived with surface-supplied equipment that was staged aboard a 400-foot work and accommodations barge that SUPSALV hired from Crowley Marine Services. Using knuckle cranes with stages attached, the divers were lowered to the vessel. Once on the bottom, the MDSU ONE divers climbed a ladder to gain entry into the ship. Working under low- to no-visibility conditions, they searched every compartment of the ship for the missing crew members. After 29 days and 534 dives, MDSU ONE successfully recovered eight of the nine missing crewmembers, as well as a significant number of personal effects.

The last phase of the operation was to place the ship back in deep water. Due to the cost of leasing the Rockwater 2, it was decided that the diving support barge provided by Crowley Marine would be used as the hoist platform. Instead of using linear winches to lift the Ehime Maru from the shallow water sea floor, SUPSALV and Crowley opted to use ballast water to accomplish the lift. This was done by ballasting the barge down to a draft of 20 feet at the stern and connecting the lower lift frame assembly to four chains that were suspended from the rear of the barge. Once connected, the barge was then deballasted to a draft of 13 feet. The lift went without incident and the Ehime Maru was successfully towed back to sea and laid to rest in 8,500 feet (2,578m) of water on November 25, 2001.