Fitness For Purpose

During the fabrication of a pipeline, recognised and proven quality control (or workmanship) limits will ensure that only innocuous defects remain in the pipeline at the start of its life. These control limits are somewhat arbitrary, but they have been proven over time. However, a pipeline will invariably contain larger defects at some stage during its life, and they will require an engineering assessment to determine whether or not to repair the pipeline. This assessment can be based on ‘fitness for purpose’ (see 4.3.3), i.e. a failure condition will not be reached during the operation life of the pipeline.

Engineers have always used fitness for purpose – in the early days an engineer’s intuition or direct experience could help when a defect was discovered in a structure, and there were many ‘rules of thumb’ developed. We are now better positioned in structural analysis, and we have many tools available that can help us progress from these early days. And remember that ‘rule of thumb’ was derived from a very old English law that stated that you could not beat your wife with anything wider than your thumb….

The fitness for purpose of a pipeline containing a defect may be determined by a variety of methods ranging from previous relevant experience, to model testing, to ‘engineering critical assessments’, where a defect is appraised analytically, taking into account its environment and loadings [Anon., 1999a; Anon., 2000]. It should be noted that fitness for purpose is not intended as a single substitute for good engineering judgement; it is an aid.

Generic FFP

There are various technical procedures available for assessing the significance of defects in a range of structures. These methods use fracture mechanics; for example, the British Standard BS 7910 [Anon., 1999a] contains detailed engineering critical assessment methods, and can be applied to defects in pipelines. Also, there is API 579 [Anon., 2000] which has similar methods, but with a bias towards their use in process plant.

Pipeline-Specific FFP

The above standards (API 579 and BS 7910) are generic; they can be conservative when applied to specific structures such as pipelines. Therefore, the pipeline industry has developed its own fitness for purpose methods over the past 35 years. However, it should be noted that they are usually based on experiments, with limited theoretical validation (i.e. ‘semi-empirical’). This means that the methods may become invalid or unreliable if they are applied outside these empirical limits.

The pipeline industry has used their fitness for purpose methods to produce generic guidelines for the assessment of defects in pipelines. These methods and guidelines are based on pioneering work at Battelle Memorial Institute in the USA on behalf of the Pipeline Research Council International [Anon., 1965; Maxey et al, 1972; Kiefner et al, 1973], with the more recent additions of ad hoc guidelines for the assessment of girth weld defects, mechanical damage and ductile fracture propagation produced by the European Pipeline Research Group [Re et al, 1993; Knauf & Hopkins, 1996; Bood et al, 1999].

‘Best practices’ in structural assessments of defects in pipelines are now emerging (e.g. Hopkins and Cosham, 1997; Cosham and Kirkwood; Cosham and Hopkins, 2001; Cosham and Hopkins, 2002), and a Joint Industry Project sponsored by 14 major oil and gas companies will produce a state-of-the-art Pipeline Defect Assessment Manual in 2002 [Cosham and Hopkins, 2001; Cosham and Hopkins, 2002].

Legal Note

It is important to note that in structural assessments of defective structures, FFP is defined as when a particular structure is considered to be adequate for its purpose, provided the conditions to reach failure are not reached (see BS 7910). This is a technical definition, but ‘fitness for purpose’ may have a legal and contractual meaning in certain countries.

For example, in the UK, a consultant engineer is expected to exercise ‘reasonable skill and care’ in his/her work; however, a contractor carrying out a construction has a fundamentally different obligation – he/she is obliged by law to warrant that the completed works will be fit for their intended purpose. This will be implied in his/her contract – it does not have to be stated explicitly.

Therefore, if a consultant gives a warranty (guarantee) for fitness for purpose (on the completed works) and they are not, he/she will be liable even if he/she has used all reasonable skill and care. The damages awarded following a breach of warranty are different from those of negligence:

1. Warranty – costs of making the works fit for purpose, i.e. the work has to be perfect.
2. Negligence – you pay for anything that could have reasonably been foreseeable, i.e. the work does not have to be perfect.

Engineers should check with their professional indemnity insurance – what is covered

as a company/professional? Usually, professionals/consultants are not covered for

warranties.

Reference

http://www.penspen.com/downloads/papers/documents/thestructuralintegrityofoilandgastransmissionpipelines.pdf