ABSTRACT

Subsea flowlines and risers experience fatigue cycles due to thermal and/or pressure transients or vortex induced vibration (VIV). Flowlines and risers also experience extended period of constant hold (i.e., static load) during operation. Low alloy bolts on subsea equipment also experiences constant load in addition to the fatigue loading during start up and shut down. Although there has been work on the fatigue damage of flowlines and risers, little work has been done to understand the crack growth rate (CGR) resulting from the static loading during operations in production environment. Tests were performed in this work under constant stress intensity factor (K) levels to measure the sub critical CGR associated with the constant load periods mentioned above. Tests were performed in a mildly sour environment and in seawater under cathodic protection (CP) to understand the difference in the CGR. The applicability of a crack tip strain rate based model in rationalizing the CGR vs K.

INTRODUCTION

Subsea flowlines and risers experience fatigue cycles due to thermal and/or pressure transients or vortex induced vibration (VIV). Flowlines and risers also experience extended period of constant hold (i.e., static load) during operation. Low alloy bolts on subsea equipment also experiences constant load in addition to the fatigue loading during start up and shut down. Currently, the industrial practice pays more attention to the damage of the subsea pipelines and bolts during fatigue cycles largely because the structure and component life is based on Engineering Critical Assessment (ECA), which mainly considers the fatigue damage accumulation and the fracture tolerance of the structure/component. Therefore, little attention has been paid to the CGR resulting from the static loading during operations in production environment and in seawater under cathodic protection.

Typically fatigue crack growth rate (FCGR) in sour environments and seawater under CP increases with decreasing frequency and reaches a plateau at low frequency1-6. Recently work performed in highly inhibited sour environments, indicated no evidence of a plateau in FCGR down to 0.1 mHz7. This was attributed to the effect stable static crack growth at Kmax associated with the fatigue cycle7.

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