The offshore oil and gas industry globally, has a moral and indeed, a legal, requirement to ensure the integrity of its offshore steel structures and, in many parts of the world, Gamma Flooded Member Detection (GFMD), has become the core technique for determining the integrity of such structures.
However, there are a number of platform operators, particularly those in relatively moderate climes, whose periodic structural inspection philosophy is founded on ‘Risk Based Inspection’ (RBI). This essentially relies upon computer-aided stress analysis, supported by ROV visual inspection. Such structures are generally claimed to have been designed with a relatively high extent of structural redundancy.
It is our assertion that this approach is fundamentally flawed for the following reasons...
Computer-aided stress analysis is an excellent tool but, by its very nature, is required to work with a number of ‘assumptions’ regarding the status of each individual component of the jacket structure. Any one, or more, of these ‘assumptions’ could be erroneous and will inevitably, to a cumulative extent, affect the result of a stress analysis. This risk-based approach is adequate in respect of a new jacket structure, when all welds and components have been recently certified. But, the structure will have been subjected to considerable mechanical stresses during installation, and more as it ages, with attack from diverse, often severe, environmental factors which significantly increases the probability of structural weld failure.
Irrespective of the calculated structural criticality of any particular component and regardless of the extent of ‘built-in’ redundancy, the fact that any component might be flooded will obviously impact to some extent on the result of any computer-aided analysis. The fact that a component, critical or otherwise, is anomalous, must be entered into the computer ‘model’ to ensure an accurate analysis of structural stress.
Gamma FMD can promptly and unambiguously determine the status of any tubular structural component and should be used to complement the standard risk-based philosophy to ensure that any stress analysis results are as accurate as they can possibly be. This approach allows ‘critical’ nodes to be more accurately determined thus allowing expensive resources, such as inspection divers, to be targeted in a more cost-effective way.
