2 pipelines in 1 trench offshore 1

[Durk]

Hi all,

Similar to the previous thread "2 pipelines in one trench", which was based on an onshore basis. I am working on a project where i have two 42" gas lines which i am thinking about running in the same trench over a 200km + distance. I am relatively new to the industry and thought ye might be able to point me in the right direction with this. I am interested to know, is it feasible? What are the problems to be encountered during installation? Is it better/possible to install the two lines together or in two separate campaigns? Can they be kept from crossing, or how do you maintain a min distance, is there a min distance to maintain? (As per DNV standards) Any help is much appreciated. Thanks

 

[BigInch]

I'd say no to two 42" lines in the same trench. These are not possible to be bundled and just the tension required to lay them like that would exceed the capacity of any barge. If you could do it, the route would have to be very straight as making any horizontal turn would be pretty difficult too. Besides actual construction, later movements, washouts, mudslides, unusual temperature expansions, could be catastrophic. Any externally caused damage, such as an anchor drag, would take out two 42" lines and repairs could be complicated by a close proximity.

Besides the above, is construction barge position maintained by anchor handling or dynamics? You don't mention the water depth, so anchor spreads would need to be determined for clearance, or you would have to be very very careful about where you dropped them and hope you don't drag one. From just a risk perspective, I would not put them within a distance that might cause any interference.

I'd, at least initially, plan on building two separate pipelines and keep my fingers crossed for a discount, if its possible for the same contractor to build both pipelines within the scheduled construction time window.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Durk]

Thanks Big Inch very informative as ever. I had thought laying such large pipelines in the same trench would be
close to impossible. It is more for reasons of space that i've had to look at the option. The pipe route itself has
a few large radius bends which will hamper the idea even further. Every option needs to be looked at though given the investigation of a new route would prob end up increasing cost and have unknown risk. Either way though, i think the 2 in one is a dead duck. Again my knowledge of pipelay both practical and theortical is limited. I am curious about your thoughts on anchor location, i believe that the barge is maintained by anchor handling. Once again, another issue i know nothing about(!), especially, when there is such tight space i.e. docks, existing pipelines etc, wich will cause issues with the anchor positions. Can you fill me in any bit on the position of anchors, maybe min max distance from the front of the barge and left and right of the barge, or a given min max radius. There obviously a 101 other things but any thoughts are helpful. The water depth as you mention is pretty constant at 12m. My main aim is to choose a feasible route for one and possibly a second pipeline

 

[BigInch]

In my previous comments I was thinking you might have deeper waters and spacings of up to 1000 feet, or more might be appropriate, but with 12m depths I would imagine you might do better.

Anchor distances can vary from about 4 horizontal to 1 depth to 7 or 8 horizontal to 1 for heavy weather and currents may require more. Watch the exposure to weather conditions.

In shallow water with hurrican or strong storm wave possibilities, wave height increase due to shoaling can be a problem. Pay attention to fetch distance and estimated maximum wave heights for both liquifaction of the bottom material and possible washouts from faster currents.

For 42" at 12 m depth not much relative horizontal tension would be needed in comparison to 150 m+ depths, so tensioner capacity probably isn't an issue, I'm not sure about vertical lift, maybe with 42" on some barges, but I don't think you could get enough horizontal clearance space to do two pipelines on the same barge, even if it could fit two production lines. Just loading the pipe on the barge would be problematic, since production of one string typically requires one whole side of the barge anyway. Two would block loading pipe and anything else onboard and you'd have no storage space, not to mention accomodation of two construction crews.

In tight spaces, dynamic positioning might be required. You'd have to be well away from docks with no blocking of channels by any anchor chains, not to mention the even slight possibility of a collision. Stay well away paralleling channels, as they get dredged once in awhile. Cross at close to right angles, if you must.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Durk]

Thanks BigInch,
Just on a final point, do you believe that the 2 in one is technical viable but in practice risky and unreasonable? And also, is a DP system sufficient for laying pipe, or do you need an anchor system aswell? (Especially with such large pipework)

 

[Durk]

I forgot to include in the case of the 2 in one, is it possible to lay these pipes in two separate campaigns remembering that it is more for space then savings that i would like to attempt this?

 

[BigInch]

I think DP is possible. With shallow water, horizontal components of pipe tensioning are not as much as in deep water.

Its hard to separate technical viability from risk these days. They have technically nothing, but practically everything to do with each other.

First, can you excavate such a wide trench? With a jet sled, will the spoil from one side of the trench fill the other? There is no plow big enough to do a double trench in one pass. Two passes would fill the opposite side of the trench. Seemingly you'd have to dredge the full width. Dredging the full width may be much more expensive than two separate plow/jet sled operations. If one line buckled during placement, whould it impact the other one. Will the wider trench be stable for a long enough to place both lines (assuming two barge passes). If its deep and in soft sands or loose mud, it may not. Can you rely on natural backfill? Will natural backfill take too long with such a wide trench? If one line becomes exposed, will a current vortex quickly excavate the other one. Would crossing one to make a hot tap connection to the other result in an unpractical connection. Would you have to cross both, then u-turn back to connect. Crossing lines offshore is always done by overcrossing, with no lowering of the lines being crossed. Would crossing two lines close together make a long elevated crossing impossible?

I especially don't think its advisable from the standpoint of conducting future maintenance/repairs with lines in close proximity, excavation of one would expose the other, and the possibility of losing both lines in some kind of an accident scenario is very real. If one or both lines were lost and had to be repositioned with a misalignment fittings and reconnected using "hydrolocks", how much clearance between lines would you need? I don't know.

I can see a lot of questions coming before a client would consider it. Just answering those would likely slow the project down enough to make two separate projects the most attractive simply from a maximization of this year's revenue stream, rather than next years the better of the two options.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Durk]

Thanks BigInch,

I see there is more questions then answers with this one, and i certainly am not the man to answer them! The investigation into such an endeavour is probably more trouble (time consuming) than its worth for me anyway. Besides construction, the simple fact of maintenance is a big issue in itself. The questions you have brought up, as you've said will prob be enough to ward the client away from this option. Although i will try and investigate a bit further just to be sure. I'd imagine that something similar has been attempted elsewhere in the world but getting your hands on the appropriate report is nearly always an impossible task (for numerous reasons). Anyway, i really appreciate the advise and hopefully i will put it the good use. No douubt i'll be starting another thread shortly!! Thanks

 

[BigInch]

I'd like to know how this one goes. Post us an update from time to time.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Durk]

Will do