NEW SYSTEM RESTORES HEAVY MARINE CHAIN

A new hydraulic inspection system repairs and restores worn marine anchor chains in days, thereby extending the life of the chains and eliminating expensive replacement costs, according to HER Group of Aberdeen. Offshore oil exploration vessels and rigs typically use eight lengths of K-4 grade chain, each about 4,500-ft long and 180 tons (Fig. 1) . Typically, the chains use 35,000 links to secure the vessel to the ocean floor.

Jan. 9, 1995

2 min read

Offshore oil exploration vessels and rigs typically use eight lengths of K-4 grade chain, each about 4,500-ft long and 180 tons (Fig. 1). Typically, the chains use 35,000 links to secure the vessel to the ocean floor.

Chains are usually inspected once every 4 years. If the chains do not pass inspection and must be replaced, a new set of custom-made chains may take months to receive and costs about $3.25 million.

The custom-made K-4 chain links are reinforced with a center stud, a cross member inside the link, to increase durability. The chain's manufacturing process prohibits exposure to excessive heat. Thus, the studs cannot be welded in place; rather, they are held in place by the compressive force of each link's side bars.

Over time, weather and wear stretch the links and loosen the stud's position beyond the required dimensions. Often a vessel will remain in dock for awhile until new chains are installed. According to Stuart Gibb, managing director of HER, expensive chains are sometimes discarded after only 5 years of use. Thus, a system of restoring chains, instead of replacing them, could save the industry millions of dollars.

The chain restoration units are rectangular steel containers, 20-40 ft long, with openings on both ends for the chain (Fig. 2). A massive diesel-powered hydraulic chain winching device pulls the chain through the container. Inside the first unit, an inspection section blasts the chain clean and prepares it for link testing. Next, the chain is pulled into the repair unit, which is fitted with high-capacity Power Team pumps and rams with special tooling. After the links are positioned, pressure is applied to the stud, forcing a compressive fit between the stud and link sides, restoring the links to acceptable dimensions.

The chain restoration system can repair 40-50 links per hour. A typical 4,500-ft length of chain requires about 5 days of round-the-clock work for restoration.

The chain restoration units can operate from land or barge. To minimize downtime for rigs, HER typically rents one chain length to an operating rig and repairs each of the eight rig chains one at a time.