TWI Industrial Member Report Summary 670/1999
A T Smith
Pressure vessels used in the offshore petroleum industry can be up to 30 years old and in many cases are approaching the end of their effective lives. In some of these vessels, corrosion has already reduced the remaining wall thickness below the limits prescribed in the initial design; even so, general experience has shown that such vessels can continue to operate safely. For vessels with only a few years remaining design life, the cost involved with replacement is prohibitive. There is therefore a need to extend the useful operational period of corroded vessel life to maximise the profitability of the plant. However, the continued operation of existing pressure vessels must not compromise the safety of the plant, and a procedure for the assessment of corroded pressure vessel components is therefore required to ensure that the vessel is fit-for-purpose.
This document examines alternative procedures for the structural assessment of the integrity of corroded nozzles. Corrosion can lead to the failure of nozzles through the general plastic collapse of the nozzle ( ie global collapse), or collapse of the corroded ligament (local collapse). The aim of the procedures is to assess the possibility of either collapse mechanism taking place so that remedial action can be instigated where necessary.
The effect of non-uniform corrosion on the local stress distribution at a nozzle is complex. Consequently, no standard method of corrosion assessment has been generally accepted. However, a corrosion allowance is incorporated into the design of pressure vessels. This is done by locally increasing the thickness of the vessel including the nozzle, so as to allow for anticipated corrosion. This review will investigate available design methods to see how these can be adapted to assess corrosion damage.
- Review the basis for nozzle design methods, corrosion assessment procedures and inspection procedures described in the following codes: BS 1515; BS 5500 (1994); ASME VIII Division I (1995); TBK-1-2 1994; Issue 6 of the draft API RP579 (1997) and API RP 510 (1992).
- Develop a procedure to evaluate existing corrosion damage based on the above standards and codes for the design and assessment of nozzles. In particular, to address the following issues:
- where the boundary of the nozzle lies, ie at what distance from the nozzle can corrosion be no longer considered to influence the stress behaviour in the nozzle.
- possible assessment strategies for corrosion inside the nozzle.
- to extend the procedure to consider manways and openings.
- to discriminate between where the procedure is acceptable and where most detailed stress analysis procedures are needed.