OFFSHORE AND SUBSEA | FEATURE


Offshore lifting operations are more difficult, and more dangerous, than conventional land-based ones. Julian Champkin reports.


100-200m deep. Onshore, such heights are the province of tower cranes, and even they can only lift to 70-80m unless they are tied in along the way to the building they are constructing. And subsea lifting is by no means confined to continental shelves. Shell’s Perdito Spar platform, in the Gulf of Mexico, operates at a water depth of 2,450m. It is fed by a network of pipes 44km long on the ocean bed. Those pipes had to be put into position somehow. So lifting (and lowering) heights can be


O


very great indeed. That means that lift ropes must be very long, and so will need very


ffshore lifting is very different from onshore lifting. For one thing, the lift height is different. The continental shelf is generally


large-diameter drums to hold them. They will also be very heavy: 100m of six-strand 100mm wire rope weighs around 4.4t; 1,000m of it weighs therefore around 40t. That is the weight of the rope in air. It is, though, a load that, at full extension, is not actually borne entirely by the offshore hoist or crane. In water, it weighs rather less, owing, of course, to the upthrust. Synthetic ropes are lighter than steel ones; indeed, many synthetic ropes actually float. For offshore use they can be engineered to have neutral buoyancy, inclined neither to float nor to sink, which makes for one calculation the less during lift planning. Buoyancy affects not only the rope; it


affects the load as well, and generally rather more greatly. When a submerged


Palfinger marine offers active heave compensation on its cranes and winches. www.hoistmagazine.com | April 2025 | 23


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