Rigging International used the 1,500t capacity Mammoet Twinring crane to swap out four 425 US ton (387t) steam generators at Sequoyah Nuclear Plant north of Chattanooga, Tennessee, USA.
Lifting 387t at maximum radius of 52m and maximum height of 67m is a demanding pick in normal circumstances, but because the load is radioactive, it became even more difficult. The California-based rigging company, working for main contractor Bechtel last year, was forced to use such a huge crane to comply with Nuclear Regulatory Commission guidelines, which up-rated the job in case of an incident such as an earthquake during lifting. The extra load fraction took the crane to 90% of its rated capacity. Boom tip height was about 114m (375ft).
The ring-mounted crane can be set up in a long-reach or a heavy-lift configuration where counterweight rests in a wagon behind the 24.4m diameter ring girder. But because there was not enough space in the set-up area, the crane had to be erected in long-reach configuration. The crane was counterweighted with a total of 1,250t (1,375 US tons) of ballast.
The crane’s first job was to lift the four steam generators, wrapped in a protective covering, out of two kidney-shaped holes cut into the top of the 67m (220ft) high, 40m (130 foot) diameter reactor containment building. As each generator was lifted out in turn, the crane slewed with load about 270° anticlockwise, booming out and down and hoisting down as it went so that the load was only a metre above the ground as it travelled. Keeping the load low was intended to safeguard underground emergency cooling systems of an adjacent operating reactor. The destination was a 32-axle Goldhofer self-propelled platform trailer that carried the generators away in a customised cradle.
The second job was to install four new steam generators. In addition to these eight lifts to replace the generators, the Twinring made a further 14 lifts. It also lifted off and replaced four 120t concrete plugs – sections of metre-thick concrete shell – surrounding the steam generators; lifted and replaced two 80t sections of 12mm-thick steel shell surrounding those; and lifted off two sections of containment building roof.
A 400t Liebherr crawler, two 208tcapacity Manitowoc 4100 crawler cranes and a 100t Grove telescopic were also used on site. The Liebherr lifted 20t-35t steel trusses in place to support the domed roof of the containment chamber during the extraction operation. One of the 4100s, rigged in tower crane configuration, lifted in scaffolding for work platforms around the trusses. The cranes also assisted in the two weeklong job to assemble the Twinring. The Goldhofer trailer ferried counterweight that was placed by the Manitowoc 4100. Two cranes within the containment vessel – a 227t (250 US ton) polar crane and 32t (35 US ton) pedestal crane – also assisted.
The Twinring could not get closer than 24m (80ft) from the edge of the containment vessel because of steam pipes and other underground services. The crane rests on a 2.75m high (9ft) high ring girder, which rests on 48 jacks that were installed over a cast concrete ring foundation. This reininforced concrete pile cap was supported by 80 driven piles augered 8m into bedrock for a total pile length of 12m-18m (40ft-60ft).
With a maximum load moment of 28,500tm, the crane is Mammoet’s fourth-biggest, after the two PTC Platform Twinring Containerised rigs and the MSG Mammoet Sliding Gantry. All these huge machines are built, owned and operated by Mammoet of the Netherlands. It took 100 trucks, including some wide loads, to transport the Twinring crane to the site from its previous job in Canada.
The crane preparation pales in comparison to the work required to replace the generators, which finished in June 2003. The job, which had a 7,000- item activity schedule, cost more than $100m and took two years to organise.
‘These jobs are like a taking apart and putting together a big puzzle,’ says Bechtel project manager Charles Weaver.
To remove the steam generators, ‘we had to dismantle everything inside’ the containment vessel, Weaver says. He also had to consider worker safety and emergency procedures. Although the job went smoothly, lead contractor Bechtel had developed contingency plans for a tornado. Had there been a tornado warning, the team would have to boom down the crane. Weaver says that scenario would have been ‘kind of a nightmare because it could have damaged a couple of temporary construction structures.’
Despite all the work, the operation has become fairly common for Bechtel, which has performed 24 such swaps. These jobs have become necessary because of degradation of metal within steam generators in certain types of containments.
The job is scheduled to be repeated at the Watts Bar nuclear plant in nearby Spring City in 2006, using a similar procedure.
