Dockside hammerhead lowered in one piece

The work was carried out as a joint partnership between Babcock Marine’s Devonport operation, the Ministry of Defence and subcontractor Mammoet.

Constructed in the 1970s, the Stothert and Pitt hammerhead cantilever crane had reached the end of its design life. Various methods were considered to achieve the removal of this colossal crane structure with optimum safety and minimum disruption. The crane’s steel jib structure (120 metres long, 22 metres high by 8.5 metres wide) was supported on a slew ring, mounted on a concrete plinth built on four two-metre-square reinforced concrete legs extending down into the bedrock.

Babcock Marine’s special projects director Malcolm Smith explained that a one-piece removal of the crane jib was chosen as the safest and least disruptive option. “It required one tenth of the hours worked at extreme height, making it safer than the in-situ balanced disassembly methods. It also considerably shortened the facility unavailability period from ten to four months. And it allowed the nearby offices to remain occupied with only short evacuation periods during crane lifts over the buildings, avoiding the need for long-term relocation of several hundred workers.”

Babcock Marine recruited the services of Dutch heavy lifting specialist Mammoet to handle the project. Two of the main gantry beams used in the project were used, extended, in the lifting of the Russian nuclear submarine K-141 Kursk from the Barents Sea in 2001.

The process involved the hydraulic inch-by-inch transfer of the main high-level jib horizontally across temporary beams during good weather to a position directly over a dock while supported on shorter lowering beams. It was then lowered, very slowly onto a floating barge.

Safety was paramount throughout the project, said Smith: “No nuclear submarines were in the vicinity during the lowering, and about 600 staff in the refit complex were moved during the work. This was a huge logistics exercise, in addition to the major engineering feat this project represents.”

The crane will now be disassembled at the dockyard, and the metal towers dismantled. Disassembly will continue through into January 2009.

The 1,450 tonne structure, with a lifting capacity of 80 tonnes, was located in the Submarine Refit Complex serving two adjacent nuclear powered submarine docking facilities to support the refitting of Swiftsure and Trafalgar class submarines, and workshops, training and storage areas.

Its removal makes way for a new world class low level defueling facility (reactor access house and rail system), currently under development as part of the Ministry of Defence £200 million Future Nuclear Facilities investment programme at Devonport, contracted to Babcock Marine and due for completion in about four to five years.

Roger Hardy, Babcock Marine’s submarine managing director added: “The removal of the refuel crane is a major milestone in the upgrade of the nuclear submarine facilities here at Devonport.

“It represents a significant step towards the completion of new defueling facilities which will be constructed to the highest nuclear standards in then world. I have been particularly impressed by the way the joint Babcock Marine and MOD team, along with their sub-contractor Mammoet, handled this complex engineering task with upmost professionalism and to the highest of safety standards.’’