COSL Innovator, a deepwater drilling rig built in China to operate in the North Sea was in December 2015, struck by a steep wave, leading to the death of one crewmember, several injuries, and extensive damage to the living quarters.

The rig was working for Statoil on the Troll field in the North Sea, offshore Norway, when the horizontal wave smashed the rig’s two lower decks were shattered in the early morning on December 30, 2015.

Initial reports claimed the rig had not been built according to regulations, as it did not have an air gap of 1.5 meters between the underside of the lowest deck and the highest wave crest, and its superstructure was not dimensioned to resist horizontal wave loads.

It would later turn out there were no breaches of regulations, with the Norwegian authorities admitting that the regulations “have been too vague with regard to the applicable calculation methodology for horizontal wave forces on mobile units.”

COSL, the rig’s owner, has said that the practice of “current legislation” did not take into account the horizontal wave loads on external bulkheads, adding that earlier interpretation and practice through the acknowledgment of compliance process also reflects that operators, rig contractors, and rig designers have had the same understanding of these requirements.

Worth noting, the COSL Innovator rig had obtained approvals from the Petroleum Safety Authority and classification body DNV GL, which deemed the rig fit for work on the Norwegian Continental Shelf.

Following the incident and subsequent investigation, DNV GL, has recently issued a new guideline for the rig owners regarding the air gap and horizontal wave loads calculations and requirements, making sure that a similar accident doesn’t happen again.

The Norwegian authorities have demanded from the drillers to comply with the DNV GL guidelines, asking from the companies to deliver proof of compliance by November 1. 2016.

On the back of the release of these new guidelines Offshore Energy Today spoke to Mr. Ernst Meyer, DNV GL director classification, trying to learn what exactly happened, and how the accident and the new guidelines stemming from it will affect the rig owners.

Meyer: When this wave hit the COSL Innovator, it was the penultimate that of last year, and there was a winter storm. The unit had suspended operation and went into a so-called survival mode, according to procedures. And in survival mode, it’s deballasting, so that the rig is sitting a bit higher in the water, than when it is under operation.

That way, it is more protected against waves.

But still, it was struck by a big and steep wave, and several cabins were damaged severely with windows broken and these forces were entering into the cabin, I mean to the living quarter. Two of the cabins were manned and we had one fatality and four INJURY. The injury was not serious in the end, but it was a serious event.

What we didn’t like is that so many cabins were damaged. The accident could’ve been worse if it had happened during the night when people were asleep.

OET: COSL, the rig owner, has said that its rigs had successfully withstood harsher weather than the one in December 2015. Can we talk about the wave? How was this one different?

Meyer: That’s the first question. What kind of wave hit? Because we defined that nothing bad shall happen in storms or waves up to one hundred years return period. So, for the first thing to figure out is, was this wave inside or outside of design criteria. And that’s extremely hard to understand because these are statistical measures and it’s not black and white.

So, this was investigated in a model tank, where it was concluded that it’s possible for a wave like this to return within a 100 years period, but at the same time they could not replicate that wave. And they simulated something like 250 storms.

Waves are like snowflakes…

OET: So the model testing could not come up with a size of the wave that hit the COSL Innovator?

Meyer:No, but it came up with different waves, which could have been severe as well. So this is a little bit like snowflakes. It’s impossible to replicate a snowflake, and it’s the same thing with a wave. So, even though we have 250 hundred-year storms, that particular wave, or those particular forces that hit the COSL Innovator were not possible to replicate.

We’ve concluded that similar types of waves can occur. So, a learning point from this accident is that horizontal forces from waves is something that can occur within a hundred-year storm, and it needs to be taken into consideration for design. And that has not been done before.

So, before, the focus has been on vertical forces, because the waves are coming from underneath and hitting the deck underneath. And that has been critical before and where all the attention has been paid, and all design codes and practices and risk assessments everything has been concerned around that.

The rig had received approvals both from the Norwegian authorities and DNV GL prior to entering service. So, in hindsight, what could’ve done differently with AoC, or your classification procedure?

Meyer: Yes, we should’ve had/we should have clear guidelines somehow to document resilience to horizontal wave loads. That was not included in the rules before, and has not been practice by any regulatory body or any classification society, or any designer, and yard.

This is something that the industry has simply overlooked. This accident was a wake-up call. We cannot say that this is a one off. We don’t dare to say that this cannot happen again, this was specific to the design and the conditions it was operating under.

The accident was too big and too generic for that. So, our conclusion is that we have to learn, we have to take a lesson and make sure that all semi-submersible of this design will be documented for horizontal wave loads; that nothing bad shall happen.

OET: PSA said the wave WAS steep, but it was within the limits for which the unit was designed. However, the rig did not have an air gap of 1.5 meters, and superstructure was not dimensioned to resist horizontal wave loads. Can you explain how this can be?

Meyer: I understand why this may be difficult to comprehend. From the very old days, it was a requirement. I think you will find that requirement in the MODU code, IMO the UN body, so it was a firm requirement that the minimum air gap for any weather condition has to be 1.5 meters, simply because they didn’t want any wave loads to hit the superstructure.

And the negative air gap is in principle something that the maritime industry should deal with. Ships, for example, have a permanent negative air gap so it’s not like this cannot be resolved.

OET: So those rules existed at the time when the COSL Innovator was entering service?

Meyer: Exactly, so in principle, the rule is fine, because the rule is saying that you shall withstand anything, horizontal or vertical. It was just that the risk comprehension in the industry didn’t capture that horizontal wave loads were a risk.

So, nobody paid attention to it and no requirements were put before designers, or owners, or yards to document horizontal strength. All attention was focused on the vertical problem. So what the PSA is saying is that the owner of that unit, which was COSL, had designed the unit according to the common practice.

The rule is fine and will not be changed. Because the rule says that you shall withstand a wave, it doesn’t say how, or what kind of wave. It just says that you shall tolerate that wave. It’s the practice, and the new thing now is that we have come up with a guideline for how to document resilience of horizontal loads.

The guideline will create consistency and it is calibrated and contains what a modern guideline should, in terms of mathematical and statistical formulas and everything that goes into it.

We have sent a letter to all of our customers saying that all rigs should be re-examined according to this guideline.

OET: Can we stick to the air gap a bit more? You’ve recommended the owners to review the air gaps of their rigs. So do we have a number of the rigs “affected”? Is this recommendation only for the DNV GL clients, or for all the floaters out there.

Meyer: We can only control our customers, but we have issued a casualty information to all regulatory bodies, classification societies, and designers all over the world.

OET: PSA has asked all the rig owners to bring in the document proving that they have complied with the DNV GL recommendations. Is this regardless of whether a drilling company is DNV GL’s client?

Meyer: Yes, but that’s only for Norway, as PSA does not have any governance outside of Norway, of course. We recommend other governments such as the UK, Canada, and all the places where a similar weather can occur, to do the same.

OET: So, can we talk about the definition of the air gap? How hard it is to calculate this 1.5-meter gap, as one can hardly predict the size of the next wave? Is it correct to say the bigger the rig, the bigger the air gap?

Meyer: If you think about still water, so no waves at all, the rig is just lying there, normally, the big rig has a big air gap, that’s distance from the water surface, to the lowest point underneath the deck. That’s the air gap. That air gap for some designs can be about twenty meters, and for the smaller designs down to thirteen meters. So it’s a five, six, seven meters difference in the air gap between big and small units.

OET: Do you know how many rigs have this “negative” air gap?

Meyer: Now, that’s one of the things we are trying to figure out because in the past there have been so many different ways of documenting the air gap. So different methodologies can give two, three meters difference or even more different answers.

Wave data

OET: So the air gap depends on the height of the waves? How to compare? Is there a general, massive wave, that one calculates the air gap after?

Meyer: Yes. We have to document the air gap in a hundred-year storm, and a hundred-year storm is picked from different statistics, different statistical data sets, depending on where we operate, so the worst thing you can use, which will secure you a worldwide certification is North Atlantic wave scatter diagrams. That’s the worst you can get. Those storms you can typically find in the Norwegian Sea, or in the Barents Sea – a bit calmer- but up North.

OET: The incident, however, happened in the North Sea?

Meyer: The North Sea is a little more benign, but still quite severe, but you can actually use what you want (to calculate the air gap). You can use the wave stats or weather stats from the area you operate in, and then you get certification for that particular area. But, if you want to get worldwide certification you have to use the North Atlantic weather data.

Then, you have to plug in the 100-year conditions from the North Atlantic wave data sets. You plug that into your design, and then you calculate the air gap.

No more windows?

OET: If proven that the air gap is below the required limits, so to say, how does a rig owner rectify this? Can this be done offshore, or does the rig have to go back to land to undergo needed modifications?

Meyer: Many will document positive air gap, also with the new guidelines. So maybe half, or something like that, will go free.

And then you will find some units which will get negative air gaps. The first thing they have to do is to remove all the windows because windows are the weak points. That was the reason why things went wrong on the COSL Innovator.

So we’re talking about the lower deck, where accommodation cabins are. Not all the rig’s windows?

No, not necessarily. We say that exposed windows according to our calculations, as long as you use the guideline, just have those windows removed. So use some covers, and weld them over the windows, and you’re ready to operate.

OET: So this is a solution? Welding the windows if the air gap on a rig is negative? Previously there have been reports that the modification might be rather costly? What do you say?

Meyer: This is not costly to start with, but this is a short term requirement. So before we go into next winter, it’s a requirement from us, and we have support from the Norwegian government that the windows have to be removed. I think all agree to that. That’s a good measure to do.

And then, every rig has to be analyzed, and that’s not done in a couple of weeks. To do a new structural analysis and check the resilience of a structure to 100-year storms is something that will take months to do for all the rigs.

The outcome from that could be very different. Some may find it difficult to have the rig qualified for North Atlantic operations, and maybe restrict operations to the North Sea, and by that document that everything is fine, and nothing will have to be done.

So, the strategies from each owner can be very different. But if you have a big negative air gap, and you insist on having your rig qualified and ready for North Atlantic operation, it could be that some restructuring is necessary to stiffen up the bulkheads.

Meyer: You have to go ashore to do it. And it is the same with the windows. You have to go ashore to fix the windows too, but it’s a very short yard stay. Also, for the reinforcements – some units have done it already – and it’s typically done in a week or two.

This is based on what we know so far, but we cannot say that this will be the case for every rig. That we don’t know yet, but so far this has been the picture.

Maybe this is more a question for the yards and the owners to figure out how much time it will take.

OET: And the costs? No matter how low they may be, considering the rig owners have previously received all the necessary approvals that their rigs are “good to go,” is it fair now for the owners to bear the whole cost of modifications on their own?

Meyer: We are not a contractual party when it comes to this business. So, the contract is between the rig owner and the yard, or the designer. And then when rig comes into operation, the rig owner has a contract with the charterer, typically an oil company. Those contracts regulate who should pay for this.

Worst case scenario

OET: Going back to the significant negative air gap again. Could you provide what would be the worst case scenario in case a rig – not modified according to your rules – finds itself in a 100-year storm in the North Atlantic.

Meyer: It’s dangerous. If the rig has a significant negative air gap and is not designed to withstand the forces, it can have a similar accident to the one we saw on the COSL Innovator. Ultimately, we have to control the global stability and the global strength of the unit. We have to be certain that the global integrity of the unit is not compromised.

OET: In the worst case scenario, the rig could even sink if hit by a wave big enough?

Meyer: In the extreme, and that goes for everything, if the wave becomes big enough or it’s exceeding the design criteria, it can compromise the integrity of any offshore installation. It has to be within the design criteria. As long as the rig is designed to withstand a 100-year storm in the North Atlantic nothing should happen.

But worst case is that you can get something which is worse than that (100-year storm). But we need to draw the line somewhere. And if we’re talking about things that are more severe, maybe external forces (earthquake, tsunami, other things we are not familiar with), this is very hypothetical.

So, I would say that by having design criteria, and by applying the guidelines, and applying the knowledge we have, the unit will not be compromised.

This must never happen again

OET: So, the owners who don’t make the changes that DNV GL recommends, are they still safe with operating in a milder climate? So, they could go to Brazil, if they can get a contract, and be safe?

Meyer: I think that most of 100 units that we have in our class, they can all continue operations. None has to be scrapped because of this. They will always find some areas in which they will be fine. But, it could be that a few of them will have problems to document resilience to the wave loads that can happen in the worst areas, such as the North Atlantic.

And if they get into such issue, they can either try to rectify it by modifying the structure, or they can put operation restriction on the unit, and keep it in the south, in this case the North Sea. It doesn’t need to be Brazil.

But for Brazil, Australia, and other places more benign than the North Sea, I would think that they will all go clear. But, it remains to be seen and I wouldn’t be totally certain, as we do require all the units classed by DNV GL to be checked, wherever they’re operating.

And that’s also why we want this to be looked at, as there might be plans for a rig to move from Brazil to the North Sea. We want every rig owner to get this under control, to understand their rigs. I think that is not too much to demand and we want those rigs to be checked if they are in our class.

Also, we strongly recommend that all the classification societies do the same. That’s why we have shared our guideline, methodologies. So, ABS, for example, can apply our guideline to keep the industry safe.

OET: To sum it up. Who is to blame for the incident, if anyone, and what has the industry learned from the COSL Innovator incident?

Meyer: We have said that this is new information, new learning. This is something nobody has foreseen. This is an industrial problem, not something you can isolate to a certain design or to a certain area.

We all have to learn from this, and DNV GL is taking the leap, as we’ve come up with a solution that we’ve made available for the industry to follow.

If there is one big message from this incident, is that we, as an industry, we have to learn the lesson, and that this shall never happen again. It will be a huge embarrassment if this happens again.

Source: www.offshoreenergytoday.com