Good structure

Will doubling up prevail on future tanker designs? The situation already exists where new tankers require a double hull in order to improve their safety margins. But the majority of tankers still only have a single propulsion system. In the event of a failure anywhere in the propulsion train, they have no back-up. Machinery manufacturers argue the reliability of their systems is at a higher level than ever. But the latest statistics from the International Tanker Owners? Pollution Federation (ITOPF) and Intertanko show that grounding is still a significant cause of damage to tankers and that machinery failure is still the main reason for grounding. As a result, we have recently seen a couple of large tankers delivered that feature dual propulsion: Polar Endeavour and Stena Vision. But these are the exception rather than the norm. And dual propulsion in smaller classes of tanker (Aframax and below) is all but unheard of. That is until now. French classification society Bureau Veritas has worked with French shipowner Services et Transport to develop an Aframax tanker design similar in concept to the larger Polar Tankers vessels. They have presented their concept to a number of Japanese shipyards and are now hopeful they will secure a charter (and therefore the incentive to build the vessel) from an oil major, probably Total Fina, in the near future. “It is the same concept [to the Polar vessels], with dual propulsion, but with this design we have put particular emphasis on the structure and accessibility inside the tanks,” reveals Jean-Fran?ois Segretain, head of the development department at Bureau Veritas. In developing the design the companies looked at the four major causes of oil spills. They addressed each cause in turn and implemented measures in the tanker design to minimise the risk of such a failure. While redundancy of propulsion is their answer to minimising the risk of grounding through machinery failure, their answer to oil spills resulting from navigational error is to specify the latest in navigation technology. Fire and explosion risk is largely addressed through the implementation of a tank inerting and ventilation system, based on a concept developed by Dr Michael Kennedy of Hellespont Corp. This concept acknowledges the failings in membrane-type gas carriers built in the 1970s and 1980s, which are similar in structure to double hull tankers. These failings, says Segretain, are cracks at the level of the lower hopper. “We know these ships are damaged so we prefer a solution with efficient scavenging of the double hull space,” says Segretain. He adds that this involves permanent piping in the double hull and double bottom installed in such a way so as to allow ventilation. Minimising the risk of hull failure is addressed through implementing the findings of extensive research into the ultimate strength of hull girders. This research looked at all sorts of potential scenarios ? normal working conditions; flooding of one double hull; bending moment increase etc. It has identified shear forces in longitudinal bulkheads and buckling of bottom plating and webframes as critical areas of potential structural weakness. Some proposed solutions, such as a corrugated transverse bulkhead have been rejected (in this case because of its excessive weight penalty), but others, such as the use of mild steel for the top of the double bottom, have been selected. In this case the weight penalty of 4% was considered an acceptable compromise between current tanker design norms and construction using mild steel throughout (which would have an unacceptable weight penalty of 15%). Shear stresses on the connection between longitudinal and frame are addressed without increasing scantlings through changing the detail of the connection. Best tank coating practices are specified and permanent staging allows easy inspection by surveyors. Such attention to safety inevitably comes at a cost. Segretain says the quotes it has received from Japanese shipbuilders put a price premium of about 25% on the tanker. However he makes the point that the bulk of the increase is in machinery costs. “By adopting better [structural] design, the cost is only marginally higher,” he says.