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The offshore lifting sector has been thrown into turmoil by new guidance from certification and standards agency DNV that effectively limits the use of deepwater cranes to just 58% of their rated capacity.
Following a winch drum failure last year, where a flange broke, DNV revised its recommended safety factors for calculating hoop stress imposed on the flanges of winch drums. While the safety factor for drums with conventional helix grooves remains unchanged (1 for one layer of rope, 1.75 for two layers or more), DNV has targeted Lebus-style grooved drums for an increase in the safety factor.
DNV targets Lebus-style grooved drums
When using Lebus-style grooved drums with five or more layers of wire rope – an increasingly common scenario in offshore applications – DNV’s recommended safety factor has been increased from 1.75 to 3. The revised version of DNV’s Standard for Certification No.2.22 Lifting Appliances states that lower factors may only be applied if there is thorough documentation,.
However, Lebus International Engineers says that DNV’s response is too simplistic, lacking any scientific basis. Furthermore, it is causing unnecessary disruption to offshore lifting activities, with many cranes now unable to be used for the purposes for which they were originally specified.
“It is not clear why DNV has targeted Lebus grooving for higher safety factors than conventional helical grooving,” says Cris Seidenather, managing director of Lebus International Engineers and one of the world’s foremost experts in wire rope spooling technology. “The same pressures are exerted on the flanges regardless of groove pattern. The only difference is that the Lebus pattern has been proven over the years – and this is well accepted by engineers and lifting contractors – to ensure much smoother spooling in multi-layer applications. It is an inherently safer design.”
“We understand that DNV needed to respond to concern, and sympathise with the pressures they are under. However, more work is required to refine this guidance. Hoop stresses on the flange are very complicated to calculate. DNV has produced this new formula without a proper scientific basis. Other factors need to be considered, including drum construction – whether the flanges are welded or bolted – and the type and construction of the wire rope being used, in particular its modulus of elasticity.”
Cris Seidenather adds: “Offshore contractors are working in deeper and deeper waters, lifting heavier equipment down to ever deeper depths. Now they are effectively being told they cannot do this anymore. To follow DNV’s new rules would require making the lifting equipment so big that it would simply be uneconomic. No one could afford to buy it.”
Crane and winch deep water operations workgroup
The International Maritime Contractors Association has set up a crane and winch deep water operations workgroup in response to the crisis. Lebus has joined this workgroup to help the industry find a way through this crisis.
“We want to work with contractors to help DNV produce the most appropriate guidance for lifting equipment, and we invite anyone concerned about this issue to contact us directly,” Cris Seidenather concludes.
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