Changing thrusters

The choice of lifting equipment on a project in South Africa was key to saving time and money. Machine Moving & Engineering used a range of lifting equipment for its recent work on the offshore oil industry service vessels, Pride Africa and Pride Angola.

Owner of the vessels, Pride Forasol-Foramer, a French-owned company specializing in offshore exploration and drilling, chose to refit the oil drilling ships in dry dock at Cape Town. Competition to host the refit was fierce, explains Brian Stokes, MME managing director. It was fortunate for Cape Town to win the contract as it benefited local industry and service providers, Stokes says.

The 30,000 tonne dwt vessels, considered the most technically advanced afloat, Stokes says, command a daily rate of US$150,000 and are on long term charter in the South Atlantic Ocean off the coast of Angola, west Africa. To minimise downtime it was of primary importance to adhere to tight time schedules and to work closely with the various companies involved in the refit.

Stokes estimates that the approach taken by MME saved at least a week, which translates into a very significant financial saving of just over $1 million in vessel downtime alone, not counting the dry dock charges, shorter equipment rental periods, etc.

Designed to drill at depths of 3,000 m, the vessels are not fitted with conventional propellers but have seven electrically driven Rolls Royce thrusters that are designed to rotate through 360 degrees. The two stern thrusters, which are the largest, each weigh around 50 tonnes. The thrusters are linked to a satellite navigation system and work continuously when the vessels are drilling to maintain the vessel's precise position by compensating for wind and current.

Pride Africa was the first vessel to enter Cape Town docks for its refit. Machine Moving & Engineering was contracted by the vessel's owner to remove and refit the stern thrusters. Brian Stokes explains, “We used our 450 ton hydraulic gantry together with a purpose built cradle to perform this task. The cradle was raised and secured to the thrusters, which were then unbolted, lowered and moved on rails to the side of the dry dock. A mobile crane was used to raise the thrusters from the bottom of the dry dock and to load them onto transport vehicles for relocation to the engineering workshop for refurbishment. Thereafter the thrusters were refitted.”

MME worked closely with the Rolls Royce technicians who were charged with the job to change the thrusters. They had never seen thrusters being handled using a gantry system, Stokes says, and were suitably impressed. On previous occasions they had used chain blocks to do the job – a relatively primitive and time consuming method, Stokes comments.

The lifting cradle was made for MME by a local engineering company and the telescopic hydraulic boom gantry for the lifting was a 500 US ton (454 tonne) J&R Engineering model L1001-4-34 four lift housing from the US manufacturer's 1000 Series Lift-n-Lock range. It has octagonal section booms and maximum lifting height is 34 feet (10.4 m).

Second ship

Pride Angola, sister ship to Pride Africa, arrived in June 2005. “Our brief this time was expanded to remove and replace all the thrusters and to coordinate cranage and site transport for the entire refit,” Stokes says. He continues to explain that his company's co-ordination of all the cranage, from three rental companies, saved time and money and cut down on the number of cranes required. MME, as a central point, knew all the requirements and could schedule the use of the cranes so that, for example, the services of a large crane were not needed in two places at the same time. The lifts for the large crane were staggered so only one was needed on site.

“Once again we used our 450 ton gantry for the stern thrusters and brought in our 65 ton boom truck to handle the rest. The boom truck was also used to transport the thrusters to and from Globe Engineering's workshop, a distance of approximately 1 km, and assisted in handling the thrusters to facilitate refurbishment,” Stokes explains.

The self propelled boom truck is a 65 tonne Riggers Manufacturing unit from the US that lifts to a maximum height of 7.52 m from a minimum closed height of 2.97 m on its four multiple section hydraulic cylinders. Loads can be lifted and carried on top of the platform or heavy duty lifting forks can be used off one end to lift loads from the ground.

The job was done within time and budget and holds promise of future projects of this nature being undertaken in Cape Town, Stokes says. •