Feature 1 | CONSTRUCTION MATERIALS
which a given hull is subjected can be built up. The structural designer can then use that information to develop a design that is optimised to that environment, selecting scantling layout and hull thicknesses that reflect the anticipated wave loads.
Real-life damage scenarios The ongoing structural analysis programme was given a new challenge after a lifeboat sustained structural damage in high seas in December 2007. The boat experienced a very heavy impact after dropping off a 4-5m breaking wave at speed onto its starboard side. Continuing on its passage the boat sustained further smaller impacts. Once in port, a detailed survey of the hull structure was carried out, along with a survey of secondary structure and fittings. The damage sustained, whilst not compromising the safety of the crew or boat, was extensive. This event, which was well documented,
provided an ideal opportunity to investigate not only the ultimate strength of the hull, but also the variability of hull properties that may be caused. The loads encountered during the drop from the wave were calculated and applied to a detailed FEA model of the hull. Theoretically, the hull structure was sufficiently robust to withstand the event, but it was discovered the small variations in the manufacturing process were sufficient to create a situation where damage was possible. Further detailed studies of the damage
sustained by the frame and stiffeners of the hull revealed that there was the potential for small changes to the design that would significantly improve the hull strength. With the RNLI, Frazer-Nash was able to make recommendations to local design features that would further enhance the lifeboat’s tolerance to impacts like the ones experienced.
Looking to the future The concept of optimising a hull structural design for a given environment is relatively new. Traditionally, the structural integrity of ships and boats is ensured by meeting the requirements of classification societies, and by regular inspection. By their nature, lifeboats operate in environments that are unpredictable, and often go far beyond those anticipated in classification rules. The
22
Creating optimum conditions for the crew – Designing a motion damp seat
To provide additional protection to the crew from wave impact and vibration, the RNLI commissioned the design of a new crew seat specifi cally tailored to the potentially extreme service conditions of its new all-weather lifeboats. Frazer-Nash was selected to work with the RNLI in the design, development and testing of the seat. The fi rst phase of the project involved establishing the nature of the environment to gain an understanding of the in-service pressures of the crew on board. With a specially assembled set of test equipment, capable of withstanding the marine environment and the shock loads, they were able to collect a mass of vibration and shock data. Dynamic models of the boat and the seating system were then created and validated against the
Motion damp seat as designed by Frazer-Nash (credit: Frazer-Nash).
data. The dynamic models, including the HydroDYNA software developed by Frazer-Nash, were then used to predict the motions and accelerations at the crew seat positions resulting from worst case predicted sea states. However, the relative effects of these motions on the human spine were not readily understood. In a subsequent modelling stage a bio-fi delic human spine model was created and subjected to the loads. Using fatigue prediction methods and data for human tissue Frazer-Nash and the RNLI were able to predict the relative rates of injury for different suspension seat concepts. After receiving the results of the test it was agreed that a new seat suspension concept was required in order to ensure that the safety and protection of the crew was maximised. A prototype was then rigorously tested by the RNLI and revealed that the new design was more effective than current seat models in use. The new seat is now in production and installed in new RNLI lifeboats, as well as other rescue craft. The project demonstrates how advanced engineering analysis can effectively lead the development of higher performance systems.
design process described above discards the rule-based approach, in favour of understanding the environmental stresses from first principles, and optimising the hull structure accordingly. The lessons learnt from this study have
already been incorporated into future RNLI designs. Attention must be paid to small design features, such as limber holes and the details of scantling arrangements,
if failures are not to occur in service. Manufacturing processes must also be carefully planned and monitored, so that variations that can occur do not exceed tolerable bounds. As future designs are developed, every hull design is subject to these rigorous assessments, allowing the lifeboat crews to concentrate on their life-saving task, confident in the integrity of their craft. SBI
Ship & Boat International November/December 2008
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72