Novel Defect Detection Technique to Be Used on T-Class Submarine

(May 17, 2010) -- An innovative engineering solution developed to address a costly and time-consuming issue in submarine maintenance programmes is to be applied next month for the first time since being qualified for use, on Trafalgar class submarine HMS Trenchant during the current Revalidation and Maintenance Period (RAMP) being undertaken by Babcock at Devonport Royal Dockyard.

The new development is a technique deploying leading edge Non Destructive Testing (NDT) technology to enable the propulsion tailshaft on HMS Trenchant to be inspected for defects and revalidated in-situ, with significant benefits over conventional removal methods.


A rigorous inspection regime is required to ensure safety and operational effectiveness. (The tailshaft operates in a high fatigue salt water environment which can cause problems if seawater has been able to seep beneath the protective glass reinforced plastic (GRP) layer). Traditionally this has involved removal of the tailshaft (which may be up to 20 metres long and weighs some 20-25 tonnes), and machining off the GRP layer, to inspect and revalidate it. This is a costly and time-consuming process, and will often find no problems. The new method saves considerable time and money, as well as avoiding potential delays during docking periods, thereby contributing to improved submarine availability.

The novel solution was developed by a Babcock-led team including the MoD Maritime Equipment Services team (who had approached Babcock to find a solution), Frazer-Nash Consultancy, and NDT industry partners Imes and Sonovation.

Instead of removal for external inspection, the new technique enables the tailshaft to be inspected from inside the tailshaft bore, using advanced NDT technology to penetrate the wall thickness and identify defects on the external surface from within. This is extremely technically challenging, as the sensors have to be able to identify small defects (typically 1.5 mm) through a comparatively thick wall (50-200mm) at up to some 20 metres distance, and must distinguish between corrosion pits, cracks, and welded machining repairs.

Specially developed equipment is fed into the hollow tailshaft through a four inch aperture at the outboard end of the shaft, featuring a section of track held between two centralising spider assemblies. A transducer carrier with ultrasonic probes moves up and down the track and sweeps round, indexing 1-2 degrees at a time around the full 360 degrees. Extensive trials and testing during a three year development programme have demonstrated that defects can be accurately located, sized and categorised using this new system. This data is then analysed to determine the implications on the life of the tailshaft and confirm that it is fit for purpose, or identify the need for repair or replacement.

"Previous attempts to address the issue had not been successful, due to the significant challenges involved either in using external NDT inspection techniques or in deploying equipment through the centre of the tailshaft," Babcock Integrated Technology Project Manager Steve Cann explains. "The solution our team has developed is right on the cusp of the technologically possible, in order to meet the considerable demands successfully."

Commenting on the use of the new technique on HMS Trenchant, Babcock Submarine Operations Director Devonport, Gavin Leckie, comments: "The technique allows the condition of a tailshaft to be assessed in about a month. When you consider that removing and replacing a tailshaft normally takes in the order of 18 weeks and 'burns' over 9000 man hours, the cost and time benefit to this RAMP and future programmes is enormous. "

The new tailshaft inspection technique is now fully operational following full qualification last year. MoD Maritime Equipment Services Head of Shafting Jon Nicholson states: "We now have a robust, cost-effective alternative for the management of tailshaft safety, with the potential to save considerable time and money during docking periods. Moreover, the ability to inspect and revalidate tailshafts in-situ now means that inspections can be carried out on a three yearly basis, at both Devonport and Faslane, with potential to provide added safety and submarine availability benefits."

Source : Babcock International