Residual Strength Test- Yes or No?
I attended the 2nd ENOC Middle East Regional Marine Forum and OCIMF conference in 2018. This conference focused on the 4th edition of the Mooring Equipment Guideline(MEG 4). Andy Dogherty – MEG4, who led the study group that wrote the 4th edition and Howard Flegg, Inspector of Marine Accidents (MAIB) who led the investigation of the unfortunate event on the Zarga were on the panel. I was fortunate to listen to them, among others.It has been a while since I attended the conference, but the conversations around the MEG4 still happen. There are many questions that come up in these conversations. Here, I try to reason why should we do residual strength test.
It was after the Zarga that the industry realised that we cannot determine the path a rope takes when it snaps. And so all those sectors we used to draw on the deck were done away with. The only way to protect the users is by design – human centred design and relying on their own judgement to decide where a snap back hazard might be. MEG 4 aligned terminology so that everyone now has the same language to speak in (we still haven't gotten used to saying LDBF instead of rope MBL). Everything related to mooring is now based only on the Ship Design MBL.
The MEG 4 recommends that the ropes should be fit for purpose. This would mean that the operator and the rope manufacturer get involved from the design of the ship. And continue being involved through out the lifetime of the rope where both of them keep learning and making better ropes. The recommendation that the operator and the rope manufacturer be involved in DESIGN, SELECTION, OPERATION and LEARNING (DSOL) is, to me, the beating heart of the MEG 4. This calls upon everyone to keep talking to each other and to share information. Learn and improve.
While it has its spirit the right place, the industry isn't yet geared to follow the Guidelines. The Guidelines are ahead of its time. While the MEG4 allows existing ships provisions to ease into compliance, the way it is understood by the market leaves it open to various interpretations. These interpretations are making a uniform understanding difficult. The reasoning by the operators are driven more by Intellectual laziness rather than thorough research. We borrow an already existing plan to make ours. We react to observations given by SIRE inspectors or TMSA auditors and modify procedures. And we remain conveniently ignorant so long as everyone else is doing it the same way and SIRE inspectors or TMSA auditors don’t give an observation.
“We never had a problem in a SIRE” “TMSA auditor was happy with our answers”
Even 2 years after its publication, the industry is not ready for the recommendations. A crucial component in this cycle of DSOL is residual strength testing. It is difficult to find testing facilities that are able to test to the OCIMF standards. Even mooring rope manufactures have been struggling to meet these stringent criteria and get their Base Design Certificates. For many of them it involves capital expenditure. Some of them wonder why does OCIMF want them to test for such stringent criteria when the rope may not experience these various forces during its lifetime. They are calling for reasonable test criteria.
The Guideline is categorical in their retirement criteria: retire the ropes when they reach 75% of their strength. How do you determine a rope has reached this threshold? The Guideline magnanimously leaves the method to determine the residual strength on the industry. We have not yet mastered the skill to visually examine a rope and say whether its strength is now 75% or it is still at 78%. I doubt if there are visual indicators which will correlate to residual strength. The only way to determine the residual strength, it appears, is by destructive testing.
For destructive testing, the operator needs to send a section of the rope ashore. Operators traditionally had 220m of rope which if you cut 20m would become 200m. What if the SIRE inspector now gives an observation? What if the terminal finds the ropes not long enough? Well, you could order 250m ropes. You could. But if were to continue testing, even the 250m is at some time going to be shortened to a length that is not sufficient. A rope need never be retired if its strength never falls below the threshold. This would mean a rope could last a vessel’s lifetime. But then we need to regularly test, so we know when to discard or retire it. As we keep cutting sections of this rope to test it, there will come a time when the rope will not be fit for purpose. Or if fit for purpose, may not be accepted by SIRE inspectors or terminals.
Well, we could do a representative testing, maybe 1 rope in a year. This sounds reasonable. But how could we extrapolate the test result of that one rope to decide on the strength of the other 15? While it will be impossible to test all the ropes regularly to determine when that rope falls to the threshold, we can have a reasonable extrapolation.
To make a reasonably reliable retirement criterion, we need data points. If we have sufficient data points on rope usage (and hopefully reliable test results), we should be able to correlate them. We should then be able to say, for example, that the Type X rope manufactured by A reaches 75% residual strength after 10000 hours of usage, when used in weather conditions of BF5 or less and always led directly out.
To begin with, let us set a reasonable retirement criterion based on our historical experience – say 5 years or 10000 hours. Then we bring in a policy to learn from our rope usage which will help us push or pull the set retirement criterion. If we had 100 ships and we tested one rope every year, we would have 200 data points by 2 years and 500 data points after 5 years. If we were using 4 rope manufacturers consistently across the fleet, we could assume we have about 100+ data points for each manufacturer. And now, we could sit down and say that the rope from manufacturer A has its residual strength come down to 80% after 2000hrs of usage. And so, it would not be reasonable to expect it to continue until the original criterion: 10000 hours. We will have to shorten its retirement criteria. Or we find that the rope type Y made by B has 85% strength even after 10000 hours of use, we could then extend its retirement criteria.
A retirement criterion cannot be set in stone. It has to be continually reviewed based on our learning. Service life should be extended or shortened to ensure that the strength of the ropes in use is always more than 75%. There is no other way I can see where the industry can comply with the very categorical retirement criteria recommended by the Guideline. By consistently collecting data and analyzing them, we will be able to decide what discard criteria is reasonable for a particular type of rope used in a particular trade. While this may not be fool proof, this could be the only reasonable way until we find some non-destructive method to determine the strength of a rope. Collect as many data points as possible to cover uncertainties.
Now, having said that, there is a very serious doubt on the quality of the data we are collecting. We have no way to establish the standards that these testing facilities are carrying out the test to. With even rope manufacturers struggling to meet the exacting test standards, independent laboratories will struggle even more. So even if I were to test these ropes, I can't be sure that two test results from two laboratories can be compared.
Why not get it tested by the rope manufacturers themselves? Not considering the obvious conflict of interest, many of the rope manufacturers are just setting up these facilities. Some of them have given up willing to bide time. What if we used a different manufacturer to test another manufacturer’s ropes? Well, again a conflict of interest. Also it rides against the spirit of the MEG 4 which expects the operators and rope manufacturers to learn and design.
So, while the Guideline has its heart set right and expects continual learning, there is no reliable (and affordable) method to test and thereby learn. The industry isn’t ready for this and is struggling to make sense of the guidelines. These recommendations are way ahead of its times even for new ships. The investigation report: https://www.gov.uk/maib-reports/failure-of-mooring-line-on-board-lng-carrier-zarga-with-1-person-injured Snapback zone photograph: https://www.nautinst.org/resources-page/200933-preventing-mooring-accidents.html Newsletter on the conference=: https://www.ocimf.org/news/newsletter/archive/2018-archive/issue-69-november-2018#toc_3 Intellectual Laziness: this is a phrase I borrowed from Capt. Nippin Anand who used it during his session on Accident Investigations.