Hydrogen Embrittlement of Subsea Structures. David Jones Minton, Treharne & Davies Ltd.

Transcription

1 Hydrogen Embrittlement of Subsea Structures David Jones Minton, Treharne & Davies Ltd.

2 Overview Hydrogen Embrittlement is a failure mechanism typically linked to corrosion and corrosion control processes, It is often encountered in the marine environment particularly in the offshore energy sector, It can affect a variety of metals but particularly high strength steels.

3 Effects of Hydrogen Embrittlement Hydrogen Embrittlement can result in:- Reduced load bearing capacity of components, Cracking of components, Failure at unexpectedly low loads often below the Yield Stress of the material, Catastrophic brittle failure.

4 Why is Brittle Failure Dangerous? In brittle failure a material cracks or collapses without plastic or elastic deformation, Brittle failures are typically sudden and occur with little or no warning, Examples of brittle failures range from shattered glasses to faults formed in the crust of the earth.

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6 How Does Hydrogen Embrittlement Occur? The hydrogen atom is the smallest atom in existence, At ambient temperatures hydrogen can diffuse into steel, It accumulates at defects in the microstructure of the steel, It makes the steel brittle.

7 What Hydrogen Embrittlement Looks Like

8 What Hydrogen Embrittlement Looks Like

9 Causes of Hydrogen Embrittlement The embrittlement is caused by the introduction of hydrogen gas into the component, Hydrogen can be introduced by a variety of mechanisms including: - As a by-product of a chemical reaction From the use of Cathodic Protection By the action of Sulphate Reducing Bacteria and microbial communities 1 Hydrogen

10 Chemical Reactions Hydrogen can be released by mechanisms that include: Any industries handling hydrogen gas Electroplated components Environments containing Sour Gas (hydrogen sulphide) Metallurgical investigation, in particular hardness testing, can help determine the potential susceptibility of materials to this cracking mechanism

11 Cathodic Protection Cathodic Protection is used to prevent metal structures from corroding by using sacrificial anodes. The anode is attached to the metal structure (cathode) so in corrosive environments it preferentially corrodes and protects the structure. This reaction creates an electrical potential which causes hydrogen ions (H + ) to be pulled into the metal structure, leading to hydrogen embrittlement.

12 Cathodic Protection The larger the difference in electric potential, the more hydrogen that can be pulled into the cathode.

13 Sulphate Reducing Bacteria Sulphate Reducing Bacteria (SRB) consume organic debris on the sea bed which leads to the release of hydrogen sulphide. The hydrogen sulphide can react with steel to release hydrogen which can be absorbed into the steel. This can result in cracking, known as Sulphide Stress Cracking or SSC.

14 Sulphide Stress (Corrosion?) Cracking SSC is NOT a form of stress corrosion cracking, it is a hydrogen cracking mechanism Hydrogen sulphide (H 2 S) reacts with the iron in steel creating iron sulphide (FeS) and liberating hydrogen (H 2 ) In the presence of H 2 S the hydrogen diffuses into the steel If the steel is hard (>248HV) and is under stress then SSC can develop

15 Microbial Communities There are many varieties of organism that form on subsea structures that have a direct effect on hydrogen uptake. Microbial films can form directly on subsea surfaces and results in anaerobic areas in which bacteria can produce hydrogen. When in contact with steel the hydrogen can be absorbed which leads to hydrogen embrittlement.

16 Microbial Community

17 Microbial Community

18 What SRB Look Like 0.5 micron

19 Failure due to Hydrogen Embrittlement Once hydrogen has entered a component it can cause failure of that component by Hydrogen Induced Stress Cracking (HISC) This results from the application of a stress to an embrittled material

20 Hydrogen Induced Stress Cracking (HISC) In order for Hydrogen Induced Stress Cracking to occur an applied stress and susceptible microstructure and hydrogen need to be present. Applied stresses can occur as stress from service conditions, or from residual stresses resulting from during production (e.g. welding) The microstructure of the steel will influence it s susceptibility to HISC. Ferritic steel microstructures and heat affected zones caused during welding are typical most susceptible.

21 Hydrogen Induced Stress Cracks

22 Hydrogen Induced Stress Cracks

23 Hydrogen Induced Stress Cracks

24 Case Study Brittle Failure in a Process Plant 1 Welds in a Sour Gas processing plant failed shortly after plant start up Release of process gases posed the risk of fire, explosion and poisoning

25 Case Study Brittle Failure in Process Plant 2 Dye Penetrant inspection revealed branching and meandering cracks Microstructural Inspection revealed fine cracks typical of brittle failure Hardness testing discovered hardness levels in the welds and heat affected zones above 300HV SSC was determined to be the cause of the cracks

26 Case Study Brittle Failure in Process Plant 3 Incorrectly designed/performed welding process created changes to the previously sound parent metal adjacent to the weld This was a physical change rendering the parts unfit for service, a clear diminution of value Does this describe damage as envisioned by the policy?

27 Summary For failure by hydrogen embrittlement to occur an applied stress, susceptible microstructure and hydrogen are required. Applied stress can be difficult to avoid due service conditions and residual stresses. Different microstructures are more susceptible to hydrogen embrittlement and HISC Cathodic corrosion protectors, sulphur reducing bacteria and microbial communities cause uptake of hydrogen which can lead to HISC.