Factory-based Digital Radiographic Inspection: RNLI Lifeboat

For the first time, the quality of a lifeboat hull-to-deck joint in the process of manufacture was non-destructively inspected. Evaluation of the high-quality digital radiographic (DR) images provided information on the composition and construction of the joint. This offers invaluable information on the techniques used during construction of composite-to-composite joins, guiding manufacturing improvements for this respected lifesaving organisation.

Overview

The Royal National Lifeboat Institution’s (RNLI) Shannon-class lifeboat was first introduced into service in 2013. Since 2016 onwards, all of these lifeboats entering service are built solely in the RNLI’s All-weather Lifeboat Centre (ALC) in Poole, Dorset.

The RNLI already apply different non-destructive testing (NDT) techniques on areas of criticality during construction. However, RNLI and TWI found that traditional factory based techniques, such as ultrasonic testing, were unsuitable for evaluation of the hull-to-deck joint as the composite structure was found to be too attenuative to support ultrasonic propagation. Therefore, TWI developed a specific radiography technique for inspection of the hull-to-deck joint and conducted onsite digital radiography at RNLI’s premises through deployment of portable digital radiography kit.

Objectives

Advance an NDT technique that enhances the safety of a lifeboat
Aid the understanding of the manufacturing process of the hull-to-deck joint
Generate a radiographic inspection procedure that is repeatable on other boats.

Figure 1 Shannon-class lifeboat being prepared for digital radiographic inspection

Programme of work

TWI, as a certified holder of HSE consent for industrial radiography practices, had to ensure all necessary safety practices were in place prior to commencement of any inspection using ionising radiation. This included creation of, and adherence to, bespoke Local Rules, Risk Assessment and Method Statement.

Selection of the correct radiographic equipment was crucial to the success of such a large-scale, complex inspection. The X-ray generator needed to be suitably portable to be manoeuvred around the boat

and to allow all shot angles to be accomplished. It also needed to have a suitably low focal spot size so as not to introduce blurring from the inherent geometric magnification in the image. The key factor for the digital X-ray detector was its portability, ability to be positioned independently of cabling and being able to communicate wirelessly. The boat, constructed from relatively low-density composite, also required a sensitive 16-bit detector to distinguish subtle image contrast changes.

The most suitable approach that allowed the inspection of the entire hull-to-deck joint, including bulkheads, required that both the digital detector and the X-ray generator were positioned on the outside of the boat (Figure 1).

Initially, only the port side of the boat was inspected which resulted in the acquisition of 102 individual digital radiographs. The use of digital radiography as opposed to film radiography considerably reduced inspection time as chemical processing of film was not required.

The interpretation of the radiographs revealed some internal features in the joint region. Figure 2 shows a radiographic image demonstrating minor flaws where the adhesive, injected from different points had not quite met, creating two small gaps.

Conclusion

Digital radiography proved to be an appropriate method of inspection for a complex composite hull-to-deck joint of a lifeboat in the process of manufacture. It allowed a speedy and versatile evaluation of the bond-line, providing information regarding its construction that had never been available before.

The process was rapid enough that the entire boat could be inspected in an out-of-hours period. The inspection provided high quality images that could resolve down to small imperfections in the manufacturing process. These could then be evaluated as to their criticality in terms of size and location in relation to the design intent thereby enabling improvements to be made in the future as necessary.

Figure 2 Representative radiographic image showing minor flaws

Georgios Liaptsis Project Leader, TWI Technology Centre (Wales)

Georgios graduated as an Electrical Engineer in 2007 and since then has continued to broaden his knowledge and experience in the field of electrical engineering as well in technology in general. As a result of achieving a Master of Science degree in Non Destructive Testing (NDT), he has is highly experienced and knowledgeable in various advanced NDT methods such as ultrasonic, radiography, electromagnetic and thermography. Giorgos also holds PCN Level 2 Radiographic Testing (Aerospace) and is a NI Certified LabVIEW Associate Developer. In addition, he has expertise in none-standard tomographic reconstruction algorithms as well as advanced scanning geometries as a result of his PhD.

}