New concepts, such as that developed by Salt Ship Design and Scana, can enhance operability and safety and reduce costs


capex, with financial people taking decisions based only on figures.” Reducing those operational costs is one of the motivations behind a proposal to fit OSVs with contra-rotating propellers (CRPs), explained Tor Henning Vestbøstad, sales director at Salt Ship Design, in a joint presentation with Henning Brautaset, research and development manager for Scana Volda. Together they argued that CRP propulsion would improve efficiency and reduce off- hire risk. Their background notes described the option as “an alternative


to state-of-


the-art azimuth propulsion or conventional propulsion where redundancy is required in combination with high efficiency at transit and/or high pull efficiency.” Mr Vestbøstad told delegates that some operators have reported drawbacks with azimuth propulsion units,


including high


maintenance costs and “when something goes wrong you have to go to a dock”. As a result, he said, there is a trend back towards conventional propellers in an effort to reduce off-hire even though “they don’t achieve much on propulsive efficiency and the investment cost might be higher, but it’s this robustness they are looking for”. Those extra costs are recovered through operational savings, he said.


In the concept developed by Salt Ship Design and Scana, two propellers are each driven independently by permanent magnet motors with no gearbox, so there is a high level of redundancy. Model tests indicate that this arrangement will reduce fuel consumption by about 5 per cent when the ship is in transit compared with thruster operation, thanks to the energy recovered by the contra-rotating screws. His co-developer, Mr Brautaset, described a 4,000kW project that is in production for this concept and due for delivery this summer while two twin screw versions – 2 x 3,000kW and 2 x 8,000kW – have been developed.


www.osjonline.com That larger size is aimed at anchor handlers,


a market that has attracted attention from MacGregor, which spoke about its new electric anchor-handling/towing winches. They can pull at up to 600 tonnes with initial speeds of up to 25m per minute and a brake holding capacity of up to 800 tonnes. According to Francis Wong, the company’s offshore segment sales and marketing manager, demand for such machines is driven by exploration moving further offshore and into deeper water, boosting business for OSVs.


But those two


factors also push vessel performance, reliability and low maintenance up the agenda for operators and charterers, he said, together with quality of life on board “so safe deck operation in heavy seas is very important”. Bringing


those


aspirations together with a need for efficient OSV vessels has led the market to demand an optimum solution when it comes to


deepwater


anchor handling. “This is the driver that pushed us to develop further this electrical variable frequency drive,” he said. Its advantages, he outlined, include environmental benefits such


as reduced


hydraulic oil use and quieter operations, together with energy savings because of its low starting current and


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standby mode. Safety gains include peak torque protection and high speed payout in the event of an emergency release, while efficiency is enhanced by its minimum power requirements during lowering and its more precise controls than alternatives. In service, Mr Wong predicted that the unit’s components will have long lifetimes and that it will be simple and inexpensive to maintain. As exploration moves into deeper water, it is not just the vessels and their machinery that come under scrutiny: the kind of rope that the cranes on vessels use must also be reviewed. Robin Collett, technical sales manager for Samson Rope, said that fibre ropes have many advantages over conventional steel wire. Not only is it lighter – about a seventh of the weight for the same strength and buoyant – but it can be spliced and repaired on board, is easy to inspect and does not need lubricating.


It costs about two-and-a-half times as much as its steel equivalent, but it will last about three times as long, since it is not affected by the environment. But its advantages outweigh that, Mr Collett suggested. Indeed, once savings on winch costs and shipping costs are factored in, even the capital expenditure for a synthetic cable installation is similar, he said. OSJ


Annual Offshore Support Journal Conference and Awards 2013 I 17


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