Review of a technical paper titled 'Prevention of failures of high strength fasteners in use in offshore and subsea applications' by Khlefa A. Esaklul and Tawfik M. Ahmed.

Crucial in the selection of fasteners for subsea use is ensuring their resistance to corrosion and the various forms of environmentally assisted cracking. Oil exploration is progressing into deeper waters and the cost of a fastener/joint failure in financial, environmental and reputational terms can be substantial. This paper reviews the factors associated with the use of fasteners subsea with particular emphasis on the prevention of environmentally assisted cracking under the conditions of cathodic prevention. (Cathodic protection is frequently used in offshore and subsea structures to reduce metal corrosion by making it the cathode in an electrochemical cell.)

The paper discusses the various forms of environmentally assisted cracking (EAC). These include stress corrosion cracking (SCC), hydrogen embrittlement (HE) and sustained load cracking (SLC). In high performance application higher strength fasteners are the norm. A major issue with fasteners used offshore and subsea is that higher strength fasteners are susceptible to stress corrosion cracking and hydrogen embrittlement. In general, higher the strength, the higher the hardness of the material and the greater the susceptibility to these failure mechanisms.

Common bolting materials used for offshore and subsea use are ASTM A 320 Grade L43 and ASTM A193 Grade B7. The paper quotes that such materials have been successfully used offshore but failures have occurred. The use of cathodic protection increases their susceptibility to hydrogen cracking.

One of the conclusions of the paper is that high strength steels with a hardness greater than 34 HRC are highly susceptible to cracking from hydrogen embrittlement induced by cathodic protection. For reference, the maximum hardness of B7 is 35 HRC and hence falls into this range. A further conclusion made by the paper's authors is that cathodic protection does provide sufficient corrosion protection for bolts in subsea applications even if there are dissimilar metals in the joint.

This paper was published in Engineering Failure Analysis and is available from ScienceDirect. Details: EsaklulKA, Ahmed TM, Prevention of failures of high strength fasteners in use in offshore, Eng Fail Anal (2008), doi:10.1016/j.engfailanal.2008.07.012

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