Modulift beam lifts tunnel boring machine from underwater

11 December 2017 by Sotiris Kanaris

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Modulift provided a custom lifting beam, slings and shackles as specialist pipeline contractor Stockton Drilling completed multiple lifts of a 28t, 18m-long tunnel boring machine (TBM) that was working subsea at the Beatrice Offshore Windfarm / Direct Pipe Landfalls (BOWL) project at Portgordon, Scotland.

The BOWL project produces 584MW from 84 turbines situated in the outer Moray Firth, powering approximately 450,000 homes at a cost of £2.6bn to its investors SSE, Copenhagen Infrastructure Partners and Red Rock Power.

Stockton’s scope of work included the trenchless installation of two independent 48in x 450m steel landfalls for high-velocity cables by utilising Herrenknecht’s DirectPipe system, a hybrid micro-tunnelling and pipe-jack system that lines the tunnel as it advances, and AVN1000 micro tunnelling machine.

The TBM was recovered from the sea after each drive using a remote disconnect module. Divers were then dispatched to the TBM’s location to attach the 34t capacity, 16.6m-long beam so a 120t capacity crane, placed on a rented jack-up barge, could lift it out of the water and place it on a multi-cat for return to Buckie Harbour.

The TBM is made up of 10 modules that each have their own individual lifting points, all of which were to be utilised to provide equal loading and enable the machine to be lifted in one piece. The beam also had three top lifting points to facilitate rigging to the crane.

Modulift used counterweights so the beam tilted when unloaded in order to have the same inclination as the load on the seabed, ensuring all bottom connections were carrying load. However, the beam had to lift the TBM level. It was also manufactured to withstand the dynamic forces present in the demanding, marine environment.

Patric Ridge, business development manager at Stockton Drilling, said: “The process was further complicated by the low depth of water [less than 10m] in the area.”

Ridge explained that the TBM was submerged for less than 48 hours for each 420m drive, approximately half-a-mile offshore. Surface alluvial comprising pebbles and cobbles, overlaying bedrock of weathered sandstone, prevented the use of more traditional horizontal directional drilling methods. Stockton has retained the beam for future use.