Subsea Pipeline Free Span Survey (As Laid & As Built Surveys)

[SDRME]

DNV-OS-F101, Oct 2007 sec 10 calls for As-Laid survey during pipelaying and As-Built survey after all the works and final test (however before operation phase).
During subsea pipeline engineering design, bottom roughness analysis for various phases (i.e. as laid, flooding, hydrotest, opeartion) is performed to identified the spans (length /gap) and then span analysis for various phases is done (DNV-RP-F105, Feb 2006). (the spans that do not meet ULS criteria in each phase shall be rectified ONLY before entering to that phase in reality, but the spans in each phase which do not meet FLS can be rectified before OR after entering to that phase in reality)
However, to compare the calculation with reality (due to uncertainty in calculation), F101 calls for identification of spans during the as laid survey. Question this is why only as laid phase spans are surveyed for identification despite the fact other phases have different spans but normally additional survey for other phases (i.e. flooding, operation) is not done again. (NB, it is considered that As-Built survey covers hydrotest spans as per F101)
I think the main reason this is that 1st in as laid survey the real results are compared with the calculation (bottom roughness analysis) and in case the difference is small for other phases the bottom roughness calculations are reliable and performing other surveys for these phases is not required. 2nd, when the pipeline is penetrated in the seabed it is deformed permanently and also for bottom roughness analysis additional weight for as laid phase (DNV-RP-F105) is accounted for pipelaying movement effect. 3rd the survey is costly operation. To sum up, the results of as laid survey is extended to flooding phase, and the results of as built survey is extended to operation phase too.
Kindly advise / comment on the above. (I should say in the previous project we had only as laid survey and not as built survey)

 

[GregLamberson]

SDRME

I see no one has respond3ed, I'll throw out a few comments. The as-laid survey as you mention is to determine the actual position of the pipeline as it is laid and compare it to the design basis calcs/criteria. Any issues as far as spans & stresses, damage, unusual in-place conditions, etc, must be recitified.

It's goood practice to regularly check the laid position of the pipe a bit past the touchdown point to confirm that the PL has not been moved off the CL due to excessive sagbend tension when maneuvering through changes in the PL route or fro,m cross currents which aren't always noticeable from the barge surface position & heading during installation.

The purpose of the as-built survey is to verify any conditions identified in the as-laid survey have been alleviated and to confirm tthe PL has been installed per the project dwgs & specs. Obvioously should the as-built survey indicate any problem areas, those will need to be dealt with prior to contractor de-mobe.


Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[Eyeoftheneedle]

In my experience it is always good practice to keep checking the situation of the pipeline in all conditions as-laid empty, following flooding, etc. Indeed most clients insist on it as the pipeline is their money-maker so they want to be happy that by the time the installation contractor has left the field and their warranty is up (1-2 years) then the pipeline is still safely remediated from a VIV spanning point of view. The bottom line is always money to the operator and they do not want massive mob/demob costs for rock dumpers once they are on their own.

In terms of timing for rectifying spans, that's upto the installation contractor/design house/client to decide as you may want to look at a phase by phase basis or rectify based on the worst case for all installation and operational phases but rectify just after the as-laid survey (prior to flooding). This can be the best time to do this as you are supporting the pipeline in its lightest (lowest SG) condition. The main thing to be careful of here is that once you dewater the line and the pipeline is lighter again you need to be sure that the pipeline is still in contact with your supports.

So, to cut a long story short - to minimise Operator's OPEX and Installation Contractor's exposure during the warranty period you should do as much surveying as possible as this cost is bound to be less than bringing back in survey teams/rock dumpers a year or 2 later. There are many pipelines in operation around the world that have yearly span rectification plans in place because of insufficient work being performed at installation.

N.B. Remember that DNV-OS-F101 is not mandatory to follow to the letter as it is not a prescriptive Civils Code. It was produced as a guideline to the industry as an approach that could be used, but if you use common sense to 'Engineer' the best solution for your Project then that's always the best option.