Cradle of comfort

by | 10th August 2018 | News

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The Naval Architect: September 2018Craddle Hull

Whether it’s smaller offshore workboats or lifeboats, launch and recovery of daughter craft is one of the more hazardous operations to take place on board a vessel. Around two years ago, Rolls-Royce Marine was challenged by an operator to see if it could find an alternative solution.


The result was the Boat Transfer System (BTS). Martijn de Jongh, chief designer, Rolls-Royce Ship Design and Systems, explains: “It’s basically a cradle-based system with the cradle being continuously attached to the vessel, meaning there’s no free-swinging boat. We have been targeting it towards windfarm support, but it’s suitable for any maritime operation where you are launching or recovering larger boats (of 9-20 metres), such as naval vessels and also standby rescue ships. Recovery of lifeboats is one area where we see big potential.”


One of the biggest challenges with traditional davit systems is that they are reliant on a single lifting point. In difficult conditions this can lead to what is known as the ‘pendulum effect’, when the boat swings due to the long wire length. Another common hazard in boat recovery occurs when the boat is being attached to the painter line at the bow, for positioning purposes, leaving the boat crew responsible at risk of being struck by the free-swinging recovery hook.


Larger boats not only demand bigger hooks, but in the case of dual point lifting systems the risk from swinging hooks is effectively doubled. Moreover, it is not uncommon for one of the hooks to become detached and need re-securing during lifting. A further problem can arise if the vessel is unevenly loaded, which can upset the balance of the davit system.


By contrast, the BTS dispenses with the need for hooks altogether. The cradle is secured to rails and lifting wires which lower the boat out of the water, while the boat is essentially nestled within the cradle and held in place with friction by dampers at the bottom. A further advantage over conventional davit systems is that there is effectively an ‘unlimited’ safe vertical travelling distance, allowing the boat’s crew to step off at deck level. The rails end before the waterline of the hull so wouldn’t affect resistance.


De Jongh says: “We were a bit surprised that nobody had done it before. There is a patent pending on it as there are no similar systems.”


Because testing of the system required realistic wave conditions working at model scale wasn’t a practical option, so instead Rolls-Royce worked with the Offshore Simulation Centre in Ålesund to develop a full 3D simulation. Rolls-Royce identified the burgeoning requirements for windfarm support and maintenance – where technicians need to be transported from the main ship to be deployed at the turbines for service and maintenance work – as a prime target market. Consequently, the sea state for the simulated tests focused on a wave height of 2m, based on typical wind farm conditions.


But de Jongh also notes the potential application of the technology onboard expedition cruise ships where, for example, parties of passengers may need to be transported to shore for excursions and then returned safely to the deck of the mother ship afterwards. It is also envisaged as a solution for lifeboat and life raft recovery, where it may be necessary to retrieve elderly passengers with limited mobility.


However, de Jongh concedes the BTS does have some drawbacks which may rule against widespread application on commercial vessels. The cradle and rails occupy more space than a traditional davit system and are also significantly heavier, which he explains is partly due to safety factors.


Rolls-Royce has developed two versions of the system. The ‘heavy duty’ version has a work load limit (WLL) of 20tonnes, a safe working load (SWL) of 40tonnes, 4.9m beam width and can accommodate a boat of 20m. The ‘light’ version has a WLL of 10tonnes, SWL of 20tonnes, 3.8m beam width and is suitable for a boat up to 13m in length.


Both versions are adaptive to the boat’s shape and can be fitted with either a knuckle boom or overhead crane handling unit, depending on the application and arrangement of the vessel. De Jongh adds that additional sizers are also available and that in terms of cost it will be comparable with a high-end davit system.


Rolls-Royce is currently seeking partners to allow them to build and test the system in real life, probably initially as a retrofit.

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