Lifetime prediction of subsea bolts in natural seawater

Subsea bolt failure is a recurrent and critical issue in the oil & gas, as well as offshore wind, industries: in many cases, 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. To assess accurate lifetime prediction, reliable estimation of hydrogen ingress in subsea/offshore bolts is needed.  

 This is why OCAS and 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 being carried out in natural seawater at ENDURES with different materials: galvanised and uncoated carbon steels, and a Ni-based alloy. Several preloading conditions of the bolts and cathodic protection are applied to better understand their behaviour in natural seawater. The failure and surface analysis of the exposed bolts is observed by electron microscopy. Hydrogen is measured in bolts by thermal desorption to evaluate the diffusion according to different periods of time.  

 The results will be used to improve 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. 

Our new test set-up can identify the causes of subsea bolt failure, reproduce failure under various conditions, and investigate the effectiveness of mitigation strategies

Philippe Legros, Staff Manager, Surface department, OCAS
SIbo Buter, CEO ENDURES, an OCAS sister company