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LR prepares for next-gen semisubmersibles

Offshore Marine Technology: 2nd Quarter 2016

Whilst industry is downsizing, Lloyd’s Register (LR) is preparing for the next generation of semisubmersibles by publishing lessons learnt from the high oil price projects into rules.

For instance, we have just published a procedure for the strength and fatigue assessments of semisubmersibles and also updates to our Rules for Offshore Units, which cover classification requirements for semisubmersibles. The main driver for these updates is to support industry with knowledge retention at a time when companies are downsizing and there are periods without semisubmersible design and construction projects. This year, double-digit numbers of semisubmersible drilling rigs have been unable to pick up profitable contracts and are being cold stacked. What will become of these units in the future?

The semisubmersible lends itself well to change of use because ship motions are favorable and there is a good amount of deck space to work with. Drilling units can become floating production units, accommodation units and diving support vessels.

In the future we are likely to see more alternative uses, such as floating chemical processing plants, desalination plants and power plants. Who knows if the next generation of semisubmersibles will be the return of the recent cold stacked units into service, perhaps redesigned or modified, some with new owners and some with different roles? What we can be certain of is that, when the growth returns, there will be a new generation of naval architects that will work on these projects. By publishing our best practice we make sure that they have access to our lessons learnt.

Knowledge management
We have also been looking at ways in which we can standardise our design review of semisubmersibles going forward. We have to adapt to delivering these projects in a low oil price climate. This means being more time-efficient. Having more prescriptive requirements will help achieve this.

But it is important not to lose sight of the bigger picture. Every time a new standard comes out, there is the danger that it will reduce the safety review to a ‘tick box’ exercise. There is the possibility that things which fall outside of what the standard was originally intended for get overlooked. There is the tendency to say, ‘well it meets the standard, therefore it must be safe’.

As such, our main goal is actually not to produce standards, but to manage the knowledge necessary to ensure that designs are safe. Producing standards is a great way of depositing knowledge, but they need to be accompanied by training and case studies so that the practitioners understand where requirements came from, what the limits are and how to approach designs from first principles when necessary.

Therefore, we continue to be proactive at sharing case studies with those new to semisubmersibles. For example, I gave a lecture on semisubmersibles at the North East (UK) branch of RINA, hosted by Newcastle University. For every expert in the audience, there was someone else who had not heard about Alexander L. Kielland and Petrobras P-36 before. Talking to people about why we have certain rule requirements is where the real value is created.

The recent fatality on the COSL Pioneer semisubmersible has highlighted a need to bring more attention to designing for wave impacts. We have introduced new rule requirements for model test specifications and reporting in order to bring more rigour to this part of the design stage, where wave impacts can be identified.

Model testing is an area where we saw great opportunity to standardise and offer best practice to industry. Some of the longest reports we receive to review are model test reports, the longest so far being 15,000 pages! Although this was an exception, upwards of 1,000 pages is quite commonplace. Therefore, we wanted to look at a more efficient way of reviewing these reports.

As we investigated further, we found that there was quite a difference in the level of testing and reporting between projects and test facilities. This is not to say that some tests were good and others were not; quite the contrary. In fact, every test had its own merits but it was clear that there was the opportunity to combine the best aspects of several case studies together into standardised rule requirements which not only could be better than any individual effort, but would also allow time savings for reviewing the reports.

Mitigating delays
Looking more closely at our new procedure, it focuses on answering questions that we have been asked by designers on real projects. Every designer is looking to have their design approved first time so we have put a lot of detail into the procedure in order that this is achievable.

What typically causes delays in the assessment phase is the need to revisit how the structure has been modelled, agreeing fatigue factors of safety and adding additional load cases to ensure that the most critical load combinations that could occur in service have been assessed. A lot of detailed requirements have been added to mitigate delays in these areas. Specifically regarding the way structures are modelled, this affects the accuracy of the results, which is something that is often underestimated.

We have found that, for every hour spent designing the hull, another is spent designing the interfaces with the hull. Therefore, the procedure has taken an all-inclusive approach, covering requirements for the interfaces in as much detail as for the hull. By covering both within the same procedure we can ensure that consistent design methods and assumptions are used between the hull and interfaces. This is an area where there is often a disconnect, as a result of the hull and interfaces being designed by different teams, or even different companies, who are not always in communication with each other.

There is also a lot of emphasis on what documentation we need to see. At first glance it is easy to think that it is excessive, but what we have found is that the greater the level of detail, the easier it is to identify errors, which ultimately leads to a safer design.

In addition, what is not always appreciated at the design phase is how highly sought after documentation becomes in later years when the basis of design needs to be revisited for upgrades, modifications or investigations. What we have seen is that sometimes vessels are sold and design documents are not always passed on to the new owners. Similarly, once a vessel has been built, the responsibility for it may change hands within the company from the design team to the operations team. In most cases there is a handover of design documentation but we do not take this for granted. At LR, we, in effect, mitigate the risk of loss of data by retaining very detailed documentation independently.

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