Rolls-Royce using latest techniques for balanced ship designsVessel & ROV News // October 17, 2006
A strength of Rolls-Royce ship design has always been the ability to design a vessel to suit a customer’s requirements and ensure that the finished vessel delivers on its promised qualities.
Ships are, by their very nature, an interlocking set of compromises. It is the role of the designer to develop a vessel meeting the shipowner’s requirements in the best posssible way within the regulatory framework, reducing fuel consumption and emissions to a minimum, all at an acceptable building cost.
“As part of the process of developing the new generation of supply vessels and anchorhandlers, new design tools and techniques have been perfected,” said Svein Kleven, who is Chief Designer for the UT-Design in Rolls-Royce.
“These allow great confidence to be placed in design calculations, so that at a very early stage in the design process the customer can see with considerable accuracy the parameters of the proposed vessel – bollard pull, motions, fuel consumption in transit and dynamic positioning (DP) mode, DP behaviour, carrying capacity and so on – and can be sure that the goals are achieved in a balanced way, so that one requirement is not being met at the undue expense of others. The customer can see at a very early stage in the design process what he will get. Interaction between customer and Rolls-Royce designers can then produce an optimised design, tailor-made to suit the customer’s needs.”
A key element in this process is computational fluid dynamics (CFD). Rolls-Royce designers, often in in collaboration with universities, have developed their own tools, that give extremely good correspondence between calculated values and results from the tank testing of models.
Rolls-Royce has been focusing on two particular uses of CFD - resistance in a seaway, and ship motions. Both have paid off. The latest UT-Design hullforms show a reduction of resistance in the transit mode of 30-40 per cent compared to reference hulls. This translates directly into a fuel saving. In DP mode savings can be even higher.
At the heart of this is the reduction of wave-making resistance, which recognises that real offshore vessels have to operate in a variety of modes in different sea conditions.
Results based on still water calculations and model tests have only a limited relevance - as do results from considering only a limited range of sea conditions. It is simple to produce a hullform which appears to have an extremely good performance only to find that this advantage has been purchased at a high cost because the design is unbalanced and is bad under other operating conditions.
For example, if a hull is optimised for the head sea condition, which is easy to test in typical model tanks, the vessel may in practice be sub-optimal both in transit and in other modes. The reason is that a typical offshore service vessel operates for only a limited period in head seas, and the wave-making resistance in bow, beam, quartering and following seas plays a major part in determining how much power is required to propel the vessel in real operating conditions.
Unlike most ship types, offshore vessels work cargo and perform anchorhandling and other tasks in open sea conditions, often in bad weather. Motions in dynamic positioning mode are therefore very important both for the success of the operation and for the comfort and safety of the crew.
Rolls-Royce is capturing and analysing data from existing UT-series vessels in DP operation to improve the design process further. Data from vessels’ own motion recording units is cross-checked where possible from reference sources on the platforms being served and with input from wave-rider buoys.
This data is then collated to provide the basis for design improvements, for example location and tuning of roll reduction systems and details of hullform and deck/superstructure arrangement. It also allows offshore simulator performance to be made more realistic; both the desktop type used in the design process and the full scale simulator in Ĺlesund, Norway, used for planning operations and training crew.