Predicting the failure of damaged oil and gas pipelines is essential for the determination of design tolerances, post integrity assessment and effective maintenance action. Both offshore and onshore pipelines can experience significant corrosion damage which will reduce the pressure strength and degrade resistance to fatigue cracking, local buckling, leakage and bursting.
The existing criterion used in the assessment of corrosion damage, ANSI/ASME B31G (1991), was developed on an empirical basis over 20 years ago. The code was developed based on a fracture mechanism calibrated by extensive testing of vessels with narrow machined slots in the external pipe wall. It has been recognised that the B3 1 G code can be over-conservative mainly due to the simplification embodied in the method and through its application to a complex variety of defect shapes which have an inherently different failure mechanism.
High resolution on-line inspection tools, developed over the past two decades, have enabled the accurate location and sizing of pipe wall corrosion. Modem numerical analysis methods have enabled the modelling of realistic defect simulations and with experimental validation have proven a useful tool in establishing the pressure strength for corroded pipe.
In order to reduce or negate unnecessary repair and replacement actions and to optimise pipeline design, British Gas have embarked on a three year programme of group sponsored research aimed at developing a fitness-for-purpose assessment procedure for corrosion in pressurised pipe.
This paper describes the current programme of full scale hydrostatic pressure testing of vessels with simulated internal and external corrosion defects, small scale pressure testing of rings with machined defects and non-linear finite element analysis which all combined will provide the basis for the development of a rigorous fitness-for-purpose procedure. The paper also describes some of the findings to date and provides a comparison between the new guidance in relation to existing methods.
The existing criterion, ANSI/ASME B3lG (1991) has been used for the assessment of corrosion in pipelines for many years. This code was developed on an empirical basis using extensive full scale testing of pipe vessels with narrow slots in the external wall of the pipe. The testing database was limited to large diameter (24″-30″), thin wall (0.25″- 0.5″), low grade (Grade B - X52) pipe (h4axey et al., 1972 and Kiefner et al., 1973) with defects of a relatively short length. Furthermore, the experiments were dominated by materials whose actual yield stress exceeds the specified minimum yield stress (SMYS) by a large margin. This margin does not occur in higher grade material. Consequently, there are uncertainties in applying the B31G code (Coulson and Worthingham, 1990) mainly due to the application of a single equation to a variety of complex defect shapes which will have inherently different failure mechanisms.
The original B3lG code was derived fiom a comprehensive database of flawed pipe burst tests (axial through wall and part wall, crack like defects) carried out during the 1970's at Battelle (Maxey et al., 1972 and Kiefner et al., 1973).
