TWI Technology Briefing 707 - 2000
A T Smith
Background
PD6493 (1991) provides guidance for the assessment for fracture and fatigue of flawed welded and non-welded structures. This code has gained widespread acceptance within industry. Corrosion defects can be assessed by assuming the defect is analogous to a planar defect. However, this assumption will often result in unnecessary conservative estimates of the fitness-for-purpose of the corroded structure and requires additional information not strictly relevant to the assessment of corrosion damage. There is a need to review existing procedures, in order to develop a more realistic model for the assessment of corroded pressurised components.
Corrosion in pressurised components is an area of considerable economic importance and a number of procedures exist for the assessment for fitness-for-service of corroded pipes in particular. Because of their considerable economic importance and the volume of literature available, special attention has been paid to line pipe.
The aim of this review is to examine published procedures for fitness-for-service assessment of corroded pressurised components.
Objectives
This review describes existing methods for assessing the structural integrity of corroded components. The principal aim of this review is to see how currently available procedures can best be adapted for the consideration of corrosion in pressurised components. Specifically, the objectives are:
- To compare various candidate assessment procedures against commonly available burst test data.
- To recommend a procedure for the assessment of axially aligned corroded regions in pressure vessels and line pipes.
- To provide a summary of simplified assessment strategies for corrosion in commonly available pressurised components.
Approach
Principal features of the more commonly available simple assessment procedures are described. The procedures are critically examined and where possible, assessed against available test data. Recommendations are made as to the safe use of the procedures.
Discussion
The principal methods for assessing axial corrosion in pipes are similar in approach. However, the procedures differ in detail, notably in how they define the effective flow strength and the assumed shape of the corroded profile. Of the various methods reviewed the draft API RP 579 document was found to be extremely flexible in that it considered a wide range of pressurised components, but as yet it is not fully validated. The BG Technology procedure presented in BS 7910 Annex G, though less conservative, is recommended for the structural assessment of axially aligned corrosion in line pipes provided that adequate safeguards are taken to ensure that the toughness of the pipe is sufficient and the corroded profile is such that brittle fracture can be disregarded. The application of the BG Technology/BS 7910 Annex G procedure to pressure vessels is also discussed. Other corrosion assessment methods such as Shell 92, RSTRENG and DnV RP F101 are discussed.
Conclusions and recommendations
Several standardised procedures and proprietary methods for the assessment of corroded regions at pipes have been reviewed and compared. Predicted burst pressures have been compared with data from the AGA database for corroded pipes.
The following conclusions were drawn:
- The preferred method of assessing axially aligned corroded regions is the BG Technology/BS 7910 Annex G single defect procedure (with the interaction criteria if appropriate) because this method generally has less inherentconservatism than the other methods considered. The proviso is given that the parent plate or pipe must be produced to a standard higher than API Grade B, unless it can be demonstrated that the transition temperature is below theoperating temperature. The draft API RP 579 procedure for the assessment of locally thinned areas (level 1) can be used for materials produced to API Grade B or below.
- Circumferentially aligned corroded regions should be assessed using the DnV guidance, though further validation of this procedure is required.
- The procedures recommended for the assessment of axial and circumferential corrosion are applicable to corrosion at welds provided that crack-like defects are not present, the fracture toughness properties of the weld metal andheat affected zone are adequate to prevent brittle fracture, and the weld metal is not significantly under or overmatched.
- More complex corrosion profiles should be assessed using the RSTRENG procedure unless adequate validation data can be provided for an alternative method which is both safe and conservative.
