A huge length of chain, weighing in at over 11 tonnes and measuring 225 feet in length, has been designed and manufactured in the UK by Renold. It is used to drive a 3,000 tonne capacity carousel for the laying of undersea pipe and umbilical cable for the offshore oil industry. The carousel, dubbed Colossus, is 24m in diameter and weighs, when empty, some 450 tonnes. When fully wound it is capable of storing up to 120km of cable for a variety of applications, including connecting land bases to offshore rigs and wellheads.
Designed and built by The Engineering Business for Subsea 7, the giant carousel is positioned on its side on an offshore support vessel for the winding and subsequent laying of cable. The design team at The Engineering Business considered various drive options for turning the carousel but eventually opted for a chain drive system with four ac motors positioned around it, like the cardinal points of a compass.
The chain has an expected life span of more than 20 years despite the harsh environments in which it will be operating. It is secured around the circumference of the carousel where it engages with sprockets attached to the electric motors. As the sprockets turn, the teeth (featuring a special rack tooth form) drive the chain, which subsequently turns the carousel.
Another special feature of the chain is its extended pins that allow for up to 30mm of lateral movement. The whole reel structure sits atop a series of sprung roller units that are depressed when the reel is fully loaded. As the carousel is unwound, it is gradually elevated by the rollers and the chain's extended pins allow for this lateral movement.
The equipment was commissioned just 36 weeks after the initial kick off meeting and, according to Subsea 7's equipment manager, Brian O'Neill, it's working very well.