We were asked to assess the implications to personnel, the facility and the environment of a catastrophic failure of one of the fuel tanks of a diesel fuel storage facility supporting the UK Royal Navy’s Submarine build programme. Our analysis methodology uncovered issues that could not be predicted using traditional calculations.
PDL are a preferred engineering analysis supplier to Jacobs UK Ltd, a subsidiary of the global professional services provider headquartered in Dallas, Texas.
Jacobs are striving to be ‘the world’s premier design, engineering, construction and technical services firm’ and we endeavour to be ‘The Engineering Analysis Experts’, so even though we are poles apart in terms of scale (40 v’s 77,000 personnel!) we are aligned in our approach and desire to be the best in what we do.
Jacobs approached PDL to assist with a safety-related assessment of a diesel fuel storage facility consisting of a pair of cylindrical tanks contained within a bund (a suitably sized fluid-tight perimeter wall to contain spillage), not an uncommon facility, but under greater scrutiny in this instance as it supports the UK Royal Navy Submarine build programme.
As part of the safety case assessment, Jacobs’ client had identified a risk posed by a catastrophic failure of one of the fuel tanks and wished to understand the implications to personnel, the facility and the environment.
A typical approach we would take to a problem of this type would be to identify credible and significant failure scenarios. We would use a technique such as Structured What-If Techniques (SWIFT) which is akin to a risk assessment or HAZOP but can usually be completed more quickly by a group of people who are knowledgeable about the asset or facility and who are guided by a SWIFT Leader.
By considering previous incidents and standards while following a pre-determined checklist, the activity will evaluate at a system level and hone in on areas of increased risk, consequences of an incident, safeguards that could mitigate this and recommendations for further evaluation or practical activities.
In this instance, the client was interested in us conducting a first-pass analysis to represent the worst-case diesel loss condition – through an instantaneous loss of the tank skin, however, following this activity it was planned to revert to the SWIFT analysis and subsequently evaluate cases such as un-zipping of welds or pin-hole leaks.
A preliminary assessment of this tank and bund facility had already been carried out using a calculation based upon the methods proposed by the research report ‘HSE RR333 – An experimental investigation of bund wall overtopping and dynamic pressures on the bund wall following catastrophic failure of a storage vessel.’
The Health and Safety Executive (HSE) had commissioned the construction of a laboratory facility and a series of tests simulating sudden tank failure using a ¼ symmetry model and ‘sprung blade’ to remove the tank wall and release the fluid. The concern was that, though failures are rare, history had shown that when they do occur a significant proportion of the liquid is likely to escape over the surrounding bund wall.
Given that this tank and bund facility did not exactly match the test and corresponding calculations, the decision was made to perform a computational fluid dynamics (CFD) analysis of the specific facility geometry using ANSYS Fluent’.
The model build followed the standard PDL process, commencing with an Analysis Plan; it was specified as multi-phase (fluid/air) with a free surface fluid, the adjacent tank and bund walls included as boundaries, and ran transiently to capture the period from instantaneous fluid column release to the point at which fluid level subsided and imposed loads diminished.
We post-processed to investigate bund and adjacent tank pressure loading (for subsequent calculations and failure assessment) and for fraction of fluid lost over the bund versus that contained. The CFD model also allowed us to understand some interesting effects, such as the convergence of two flow fronts leading to a significant wave height in a corner, the timing and location of which calculations would never predict.
- A greater understanding of the system behaviour under a catastrophic event was gained and the outcome and risk to the environment and personnel understood; our client was better placed to plan future investigations and implementation of safety measures.
- Applying transient CFD analysis allowed us to investigate fluid effects that calculations alone could never predict.
- Applying Computer Aided Engineering (CAE) methods allowed us to complete the task in a fraction of the time and cost of even scale model. The virtual instrumentation of the model, in particular, provides a significant benefit to the analysis of the experiment outcomes.
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