The investigation determined that a 6 ¾-inch diameter, 862-pound chain link in the tether chain had fractured and separated near its butt weld. Analysis of the fracture indicated that the chain link had a weld repair and the fracture initiated in the middle of the weld. Three links of the 24-link tether chain were found to have weld repairs. After the chain had been heat treated, the non-US based manufacturer had made weld repairs to the chain by grinding defects and filling the void with weld material. The chain was being built in accordance with Det Norske Veritas (DNV) Offshore Mooring Chain standard. Post heat treat weld repairs are disallowed per DNV’s Offshore Mooring Chain standard. The post heat treat weld repairs made the chain susceptible to hydrogen induced stress cracking due to the extreme hardness of the weld material and the residual stress within the weld.
This explanation seems to be consistent with my understanding of Petrobras' somewhat cavalier attitude regarding cost and schedule versus safety. Either through ignorance, or maybe even intentionally, they allowed a quick repair without studying the implications. Also note that the repair was kept secret from the DNV inspector. My guess is that Petrobras did not have an inspector at the shop and relied upon the DNV to perform inspections. Then the manufacturer, when faced with a financial loss, decided to do the repair without DNV approval.
Engineers know that the strength of steel is dependent upon things at the microscopic level. There have been more than one catastrophic failure because someone decided to strike a welding arc on a specialized steel.
BOEMRE's recommendations are below:
1)Operators should monitor and inspect critical mooring components 100% of the time during the manufacturing process.
2) Operators should ensure that the personnel and companies contracted to perform inspections and quality assurance of critical mooring components are qualified to do so.
3) Operators should treat the area above a buoyancy air can for a free-standing hybrid riser as potentially hazardous. No floating production facility or support vessel should be allowed to pass over a free-standing hybrid riser.
These recommendations seem a little light to me. I would call for an immediate investigation of all other chain on the project. It wouldn't be cheap but I shudder to think about the damage that a buoyancy can rocketing to the surface could do to a vessel. And these recommendations mean nothing unless the inspector, often a contractor whose pay is dependent upon staying in the good graces of his employer, is willing to dig in his heels and reject a very expensive piece of gear.
I also wonder how you are supposed to keep vessels from travelling over the top of the risers.
4 comments:
http://www.gomr.boemre.gov/homepg/offshore/safety/safealt/SA_296.pdf
That's the link from BOEMRE. I missed that rec on the boats passing over the float cans. Totally impractical.
I don't think they care, as long as it establishes that any damage to vessels isn't the oil companies' fault:)
I think the jist of the safet alert is that you should make sure your vessels do not hang out in the vicinity where the free-standing riser is located. I am just impressed that they got it out so soon...
Most operators of ddep water facilities in the GOM are using either direct vertical access wells tensioned by a TLP or steel catenary risers tied off to a floating platform. This is the first instance that I am aware of of free standing risers in the GOM. I'm not sure how you control vessel activity around the platfrom, especially if the boats are not involved with the platform operations. How does the captain know what is beow him? Also, a bouyancy can on free ascent will not track a straight vertical path but will veer one way and anther on the way up.
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