Fatigue Crack Growth Performance of Sour Deepwater Riser Welds in the Near-Threshold Regime
Authors: Holtam Colum , Baxter David 
Published: May 2011

Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments. SCRs can be subject to low-stress high-cycle fatigue loading (e.g. VIV) and corrosive environments (internal and external). When the production fluids are sour, higher fatigue crack growth rates (FCGRs) are expected and shorter overall life compared to performance in air.

Published data from tests carried out on C-Mn pipeline steel have shown that sour FCGRs can be 100 times faster than in air. Shallow cracks have been shown to grow up to an order of magnitude faster than deep cracks in a sour environment at the same (low) value of applied stress intensity factor range (?K). In many cases it is material behaviour in this low ?K regime that dominates fatigue life.

The possible influence of crack depth on material behaviour should be considered when conducting tests to determine FCGRs for use in fracture mechanics calculations. Low ?K data are usually determined under decreasing ?K conditions, by which time the crack is deep. In reality a flaw is likely to grow under increasing ?K conditions, such that the flaw is shallow at the beginning of life (when ?K is low). At low values of ?K (i.e. approaching threshold) FCGRs in C-Mn pipeline steel determined from decreasing ?K tests have been shown to be substantially lower than those determined from increasing ?K tests. This demonstrates a potential non-conservatism if the low ?K data generated under decreasing ?K conditions are used to predict the behaviour of a much shallower flaw.

This paper presents the results of recent sour FCGR tests on C-Mn pipeline steel. Increasing ?K tests were performed on specimens containing shallow initial flaws at very high stress ratios, to obtain data at lower ?K. This paper provides valuable insight into current best practice methods for generating sour FCGR data when qualifying girth welds for sour service.

Introduction and Background

Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments to transfer production fluids from the seabed to surface facilities. SCRs can be subject to both fatigue loading, for example from wave and tidal motion, vortex induced vibration (VIV) and operating loads, and corrosive environments (internal and external). When the production fluids are sour (i.e. contain water and H2S) higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared to performance in air, as a result of the interaction between fatigue crack growth and sulphide stress cracking (SSC).

Successful design is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced and rates of fatigue crack growth are increased. C-Mn steel is generally the most economic material for the construction of offshore pipelines and risers and there is an industry need to understand better and to quantify its fatigue crack growth behaviour in sour service environments. If carbon steel can provide adequate performance under (mildly) sour conditions then the use of more expensive corrosion resistant alloys (CRAs) and clad materials might be avoided.