Seals help ensure smooth lifting

Paul Boughton
Double-acting seals on a crane hook are the answer for the world's largest crane vessel.

Trelleborg seals are helping ensure smooth lifting operations for the world's largest crane vessel. The crane vessel is Heerema Marine Contractors' Thialf, a huge semi-submersible barge equipped with two super-cranes. Since it entered service in 1986, the Thialf has installed many of the production platforms in the North Sea and elsewhere around the world.

As the industry increasingly develops oil and gas fields in waters too deep for bottom-founded platforms, the vessel is called on to install equipment such as subsea production facilities or moorings for floating platforms directly onto the seabed. Lifting operations can now take place in water depths well in excess of 2,000 metres.

Underwater operations mean that the hook assemblies, the business ends of the cranes, are exposed to seawater. Measuring more than six meters in height, each four-prong hook assembly rotates on a massive roller bearing packed with lubricating grease.[Page Break]

Protection from seawater

Trelleborg's seals are required to protect the bearing from seawater and resulting corrosion and wear to the metal parts, while allowing the smooth rotation of the hook shaft. Any interruption of normal operations can be costly for Heerema Marine Contractors (HMC).

Previous seals proved unsatisfactory for the job, allowing seawater to get in when the hook assemblies were used underwater, according to Jurgen de Jong, HMC's Senior Technical Superintendent for cranes. De Jong's responsibilities include arranging repairs, overhaul and certification of the crane blocks.

"In 1999 HMC and Trelleborg worked together to come up with a better solution," says Ralph van de Grijp, Business Unit Manager, Offshore & Energy, at the Trelleborg Sealing Solutions marketing company in the Netherlands.

"Armed with specifications provided by HMC, we proposed using a double-acting seal to prevent fluid passing in either direction, to stop seawater going into the bearing or grease from the bearing leaking into the external environment."[Page Break]

1.8m ring diameter

The solution chosen was Trelleborg's Turcon Roto Glyd Ring, which is widely used in other industries. An unusually large size was required, with a diameter of 1.8 metres.

Over the next decade the Thialf continued to perform lifts, and the bearings in the hook assemblies functioned satisfactorily, with no sign of problems. When the vessel returned to its home port of Rotterdam for maintenance in 2010, a proper evaluation of the seals' performance was possible. Trelleborg Sealing Solutions was called in to inspect and replace the seals, a job requiring three days for each seal. The results were surprising.

"My colleagues had never seen seals in such good condition after 10 years in a harsh working environment," says van de Grijp. "There was very little deterioration."

De Jong agrees. "The bearing and the internals of the block were in very good condition, which shows that the seals worked effectively," he says.

Still, for HMC it made sense to install new seals.

"Normally when a block is overhauled we renew all seals preventively, so we do not have to take everything apart again for a leaking seal shortly afterward," de Jong says.

But van de Grijp and his colleagues at Trelleborg Sealing Solutions were left with the tantalizing question: How much longer might the originals have continued to function satisfactorily?

The Turcon Roto Glyd Ring PTFE seal is energised by an elastomer O-Ring.

As pressure from liquids such as seawater or oil increases, the pressure with which the O-Ring pushes the PTFE seal onto the counter surface also increases, thus ensuring that the seal remains tight.[Page Break]

PTFE material

The Turcon Roto Glyd Ring is a seal made of Trelleborg's proprietary PTFE (polytetrafluoroethylene) based material. The characteristics of PTFE have won it wide use in industrial applications, especially in environments where movement is involved. It has a low friction coefficient and is stick-slip-free, thus allowing smooth transition from a stationary state to movement.

In addition, it is a long-lasting material that displays excellent chemical resistance and fluid compatibility.

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