Reverse s-lay / Pipeline Decommissioning

[PPN_LCP]

Hi All,

I'm currently in a project where a client chooses to decommission pipeline by reverse s-lay method.
I personally have exp in conventional rigid pipelay using s-lay method where a lay barge equipped with tensioner(s) and stinger is utilized.
Nothing fancy for start up/normal lay/laydown operation.

Normally during pipelay, the pipeline needs to be applied tension to ensure sound structural integrity of the suspended s-shape section. This too applies when we do reverse s-lay. Given that both ends of the pipeline are cut(normally by diamond wire), when we recover the first pipe end, there is not much an issue. We can use PRT (Pipe Recovery Tool or equivalent) to insert into the pipe cut end. The PRT is connected to A&R winch on the barge for providing tension as required. However when we approach the other end, I still question of how to apply tension to the suspended recovered pipeline since the pipeline end is already cut thus there is no mean of restraint.

Another thing that I can think of is to stop the barge at X distance (approx 1000+ metre) to the pipeline end. This is to ensure we have enough friction between the section of pipeline on seabed against the tension that we need to apply. The remaining "X" length of the pipeline are to be recovered by Cut and Lift method.

Anybody who have done reverse s-lay before please feel free in shim in and advise. All recommendations are welcome.

Thank you.

 

[LittleInch]

Depth of water?
Size of pipe?

But what's the problem in just pulling it back up and then when you get close enough the end will simply start dragging itself along the seabed and your tension will fall until you end up pulling it vertically.

The stinger will need to be gradually lowered until its vertical or you'll snap it, but can't see why you need to maintain a true catenary.

Or attach a bunch of floats and lift it off the seabed and then you'll have a similar thing going on.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Are you trying to salvage still round pipe for reuse? Why pull it up on a stinger?

 

[PPN_LCP]

Guys - Appreciate your replies. Here are more details and responses.

@LittleInch


WD - 75m-80m
OD - 12" & 16". Wall thickness is 0.625"
Grade - API5L X42 & X52
Service life - 25-30 years


To me, a point of concern is at overbend region of the pipe string i.e. last barge roller and/or stinger rollers.
The worst case scenario is where the pipe end tip is just recovered break off seabed. The entire dead weight of suspended section is now rest on the roller supports.
The stinger can be lowered to alleviate bending stress at its roller/tip but the barge could not trim that much. We will have to play with roller height set up to keep the stress within acceptable ranges (might go beyond elastic region since they will be scrapped afterward). Might be workable. I guess we have to wait and see the analysis result to verify.

Attaching float is surely one of viable options. To be noted on more cost due to time and might have to introduce another small DSV for prep work (the main barge only have ROV).


@ 1503-44
No resue. Just recover, decontaminate and scrap them onshore.
It is requirement from the local government as far as I know.
My client (oils & gas operator) has end concession after 40 years and has to decommission some of pipelines and platforms which they have installed.
I believe they've tried to convince the gov to just leave the asset in-situ (with preservation) but failed to. So now they are left with total remove option.

 

[1503-44]

I'd try lifting straight up without putting it on a stringer.
I guess you have a few inches of concrete coating too, right?

Do you care if you damage the pipe or coating?
Pull it up as a "reverse J-lay".
No lateral tension needed.
You'll be doing that when you get near the end anyway.

 

[PPN_LCP]

The external coating is only FBE. no concrete.
I would reckon damage to pipe is acceptable to some levels as long as no major rapture that would pose hazard risk to the working equipment or personnel.

Reverse J-lay or reel lay would operationally work. ​
The only issue is the installation contractor only have s-lay laybarges in their fleet.
And to mobilize a suitable vessel from other regions or to build/set up J-lay tower on a barge would generate complications in term of commercial/contractual.
I'll see what I can do with the project team. I'm newly added into the team so trying to get ahead of the game here.

Thank you for your input.

 

[1503-44]

A pipeline barge generally can configure enough winches to port or starboard to lift a pipeline without involving a stinger. There is no reason I see that you need to stop local buckling, so S lay with tension is not required. You just don't want to lift with more tension than would reach pipe ultimate strength. You can probably pull it over or close enough to the deck enough to cut off joint by joint.