Abstract
Subsea bolt failure is a recurrent and critical issue in the oil & gas and offshore wind industries: For most of the recent subsea bolt failures, the likely failure mechanism is environmentally assisted cracking. Cathodic (over)protection of subsea bolts leads to hydrogen production and ingress, which subsequently embrittles the bolt alloy.
There is a market pull from the offshore industry to learn more about the effects of cathodic (over)protection on hydrogen uptake in steels, the fundamental mechanisms that govern hydrogen embrittlement and how to improve material selection for large size bolted connections.
There is a need for reliable estimation of hydrogen ingress in subsea/offshore bolts to enable a more accurate lifetime prediction.
This is why OCAS & ENDURES have implemented a test set-up in seawater to identify the causes of subsea bolt failures, to reproduce failures under various conditions and to prove the effectiveness of mitigation strategies.
Tests are carried out in real seawater at ENDURES with different materials: galvanized and non-coated carbon steels, Ni-based alloy. Several preloading conditions of the bolts and cathodic protection are applied to better understand the behavior in natural seawater.
The failure and surface analysis of the exposed bolts is observed through electron microscope. The measurement of hydrogen in bolt is done by thermal desorption to follow the diffusion according to different periods of time.
The results will be used to improve the material selection for large size bolts and will enable a more accurate lifetime prediction.
This methodology and test bench are available for any project aiming to test specific bolts in seawater.