Best Practice Guidance for Evaluating Knock-Down Factors in Corrosive Environments

The term knock-down factor is commonly used to describe the reduction in fatigue life in a corrosive environment (e.g. sour service) compared to performance in air. However, the mere concept of such a reduction factor is potentially misleading, particularly when comparing different welding procedures that demonstrate different in-air performance. This paper examines the concept and calculation of so-called knockdown factors. To demonstrate the performance of girth welds in a corrosive environment, strip fatigue tests are conducted in air and in a simulated service environment, to determine an appropriate knock-down factor, which is then applied to the base design curve. However, there are a number of ways that such knock-down factors can be calculated, with different degrees of conservatism. For example, two different welding procedures may exhibit a different fatigue performance in air, but a similar performance when tested in a sour environment. The better performing weld (in air) is therefore assigned a greater knock-down factor, and possibly a more stringent sour design curve. In other instances, fatigue performance in air may significantly exceed that required. The determined knock-down factor, between strip tests in air and in a sour environment, can then be very large. Applying this reduction factor to the design curve results in a very stringent sour design curve, and may penalize the use of a girth welding procedure that results in good in-air fatigue performance. There are no explicit, published guidelines for calculating corrosion fatigue knock-down factors. This paper describes an approach, based on experience and considering best practice guidance for the statistical analysis of fatigue data obtained from welded joints. The method is demonstrated using published sour corrosion fatigue endurance data, evaluating both mean and design curves.