2014 IR&D Annual Report

Crack-Size Effect in Corrosion-Fatigue Crack Growth, 18-R8357

Principal Investigators
Baotong Lu
Stephen J. Hudak Jr.
W. Fassett Hickey
Guadalupe B. Robledo
Carl F. Popelar

Inclusive Dates: 01/01/13 – 06/30/14

Figure 1.  Newly developed small-crack corrosion-fatigue crack growth facility
Figure 1. Newly developed small-crack corrosion-fatigue crack growth facility.
Figure 2.  Comparison of the model prediction
Figure 2. Comparison of the model prediction; da/dN ( ∆Keff) obtained in this study and the CFCG rates reported in literature (by Holtam).

Background — Premature failure of steel risers and flowlines caused by corrosion-fatigue in sour environments is a major concern in offshore oil and gas production. In 2010 SwRI developed a joint industry project (JIP) to develop a corrosion-fatigue crack growth (CFCG) model to predict the service life of critical riser and flowline components exposed to sour brine environments. Although the CFCG model is capable of predicting CFCG rates in the intermediate and high growth rate regime, this model results in overly conservative predictions in the low stress regime because it lacks the ability to capture the small-crack effect in which small cracks existing in real structures can grow faster than larger cracks commonly adopted in laboratory tests for CFCG rate measurements. The crack-size effect is of great practical significance in structural integrity assessments because a major fraction of the service life of structures is consumed in the small crack growth stage.

Approach — The objective of this project was to develop the experimental capabilities and corrosion-fatigue crack growth model capable of capturing the crack-size effect in sour environments. The approach was to 1) develop an experimental capability to measure crack growth rate and crack closure from small cracks in sour environments; 2) conduct critical analyses to identify the mechanisms of the crack-size effect; and 3) build a corrosion-fatigue crack growth (CFCG) model based on the fundamental understandings on the crack-size effect.

Accomplishments — Results of this project demonstrated the ability to quantify the small-crack effects in sour brine environments. A new experimental facility was successfully developed to measure CFCG rates and crack closure of small cracks in sour environments (see Figure 1). Results obtained with the newly developed experimental facility demonstrated that crack closure is the most important mechanism for the crack-size-dependent CFCG behavior and Δ𝐾𝑒𝑓𝑓 is the proper mechanical parameter to characterize the driving force for CFCG in the near-threshold regime (see Figure 2). A new CFCG model was developed based on the fundamental understandings and experimental findings. This model was capable of predicting the fatigue life of offshore structure steel exposed to sour environments (see Figure 3). Based on fundamental understandings achieved in this project, a technical approach was developed for a second phase of the JIP, which is expected to commence in the first quarter of CY2015.

Figure 2.  Comparison of the model prediction
Figure 3. Comparison between predicted fatigue lives with new CFCG model and experimental data. The fatigue lives in sour brine measured with different frequencies are represented by the color of the points: blue - 0.01 Hz, yellow - 0.33 Hz, red - 1 Hz).
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