Submarine tailshaft integrity case study

THE CHALLENGE

A combination of rotating-bending stress cycles and seawater exposure can cause corrosion pitting and fatigue crack growth in submarine tailshafts. As tailshafts are a safety-critical feature, cracking or damage can lead to catastrophic loss of the submarine.

With this in mind, Frazer-Nash developed an Asset Integrity Management plan which would counteract the threat to existing submarines in service, and propose design improvements to prevent this happening in future.

OUR SOLUTION

We began by developing a Life Prediction Model to represent the through-life corrosion fatigue characteristics of a typical tailshaft, using operational loading and lifecycle data. This demonstrated that to ensure safety of the submarine, early warning detection of sub-critical cracks must be carried out to inform decision makers of immediate potential risk. Also, detecting smaller, less-critical flaws can inform the through-life decision-making process, providing maximum flexibility through costed options.

We then devised and managed a successful development programme including:

• Cost Effectiveness Analysis to understand the options for managing tailshaft safety, and their associated through life costs
• Requirements Capture to set acceptance criteria and provide a description of the System Attributes. This has been used to assess the technical maturity of the system.
• Market Research to investigate existing commercial technologies
• A Trials and Design Review programme which introduced planned pits and cracks into the tailshaft specimens, in order to develop the Qualification Process and Procedures.
• Asset Integrity Management Software (AIMS) which holds historic, manufacture, refurbishment and operational data to provide a complete evaluation of the risk associated with tailshafts following the analysis of inspection results. This also included a tailshaft planning and risk ranking facility.
• Ongoing design improvements to address future tailshaft design

THE BENEFITS

A comprehensive strategy of robust inspection, structural analysis and operational risk evaluation offers the most cost effective through-life maintenance strategy. It provides:

• Increased confidence in structural integrity and component safety
  A greater understanding of the susceptibility of the component to damage
• Significant reduction in through-life costs
• Reduction in the impact on planned maintenance activities