Anchor Protection for Buried Pipeline 2

[GNicholson]

Hello.

I am interested in designing a trench+backfill which will protect a pipeline against anchor drag. I would like to find some resources that give a method to determine the trench shape and backfill profile/material necessary to "deflect" an anchor+line of given specifications.

Your help is greatly appreciated.

 

[BarryEng]

I have looked at several (submarine) pipeline books but I have not found any guidance for pipe cover depth req'd. I suppose that it would depend on literol drift of the sea bed, tide velocities, type of bed material, susceptibility to sediment transport etc. This is not my field of experience, but I can give you a reference on anchors & how you design them - but not a reference on how much cover for the pipeline to ensure that no anchor damage will occur.

If you have no luck in this thread (& you still want the info), I will find the anchor reference for you.

 

[narenr]

Refer:

DNV-RP-F107 Risk Assessment of Pipeline Protection
DNV-RP-F111 Interference between Trawl Gear and Pipelines

You might also want to refer ENV-RP-E301 & 302 which is for design of anchors, these provide guidelines on anchor burial depths and line catenary and seabed lenghts.

Rgds Naren





Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[GNicholson]

Thank you both for the assistance.

However, I still am looking for a comprehensive, one location, resource that will address the problem of trench design for anchor protection.

If I can find any resource, I will post the results here

 

[BarryEng]

narenr
Tks for the info - I have been after some references for some time (not very serious though). I will chase them up. I have designed several outfall pipelines into the ocean for outfalls (sea water desalination & waste water outfalls) & burying the pipes deep enough in the ocean to ensure there is no anchor damage to the pipes.

When the pipes for the North West Shelf gas project (in North West of Western Australia) were constructed, they used the conventional method of laying the pipe on the sea bed & then using a traveller over the pipe & jetting sea bed material away to allow the pipe to settle into its own trench.

GNicholson
Sorry I can't help you on your problem but I can give you the reference on anchors for your future reference.

http://www.vryhof.com

I originally found the reference on this web site (by accident) a few years ago & found it very interesting (for me as a novice in this area). There is a manual on anchors but also a good reference on ropes that may be of interest to you.

 

[narenr]

GNicholson,

Since you have already decided to trench+backfill for protection against anchor damage, I suggested the above references which would help you in designing from basics.

There is UK HS&E guideline for trenching design of pipelines which mandates that pipeline > 16" to be buried. The document OTH561 is avialable in

http://www.open.gov.uk/hse/hsehome.htm

with reference OTH561 or you could also find in libraries with ref ISBN 0-7176-2457-9. This documents provides the design procedure for trench design.




Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[BarryEng]

narenr
I have tried your reference, many times over the last 24 hours but every time the site 'timed out' - can you check the address please?

 

[narenr]

Barry

Try these link:

http://www.hse.gov.uk/research/othhtm/500-599/oth561.htm

http://www.hse.gov.uk/research/othpdf/500-599/oth561.pdf

Rgds Naren

Narendranath R
Pipeline engineering is made easy with state of the art computer software, visit

http://www.narendranath.itgo.com.

 

[BarryEng]

Thank you - that worked OK.

 

[BigInch]

15 ft to top of pipe in the GOM is common. I put one in the Port of Houston-Galveston Anchorage Area at GLV144 in 1982. Still there as far as I know.

http://virtualpipeline.spaces.msn.com

"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[JT100]

biginch, is that trenched and buried to 15ft? i've only been working in pipelines for 18months or so since graduating and i am quite curious as i have mainly been working in the north sea with trenching to 1-2m, and with regards anchor protection it seems to be mainly an avoidance thing, i.e the requirement to avoid rig anchoring and mooring line touchdown locations etc when routing the pipeline. i take it this isnt the case in gom?

 

[BigInch]

Avoidance! Yes that's the key word as you've surmized.
If somebody hangs an anchor on you, your line is going to get pulled no matter what you've put on top, at least if its any kind of a reasonably sized ship. I've seen sidescan images of anchor drags 2000 feet long.

I haven't been working in the US since 1990, but at that time, SOP was to jet sled & bury all pipelines to 3 ft cover in water depths less than 200 ft. Since much of the GOM bottom slope between Port Lavaca and the La Delta is 1 ft/mile, the 200 ft depth line can get pretty far from the beach, so most of the heavy traffic shrimp trawl areas wern't a factor for the buried pipe, only exposed bits at tie-ins were vulnerable. We put trawler cages over valve handwheels at tie-ins.

Shore approach cover depths varied with sediment type but normal practice was 10 ft depth in surf zones and beach approaches. The intracostal canal crossings were buried to 10 ft cover with large lighted NO ANCHORING advisories on both sides.

Over 200 ft depths, no burial was required by the regulations.

Platform placement in designated anchoring areas was not encouraged, but somehow two of them managed to get installed off Galveston to which I had to build a connecting flowline and the gas carrier pipeline to the beach. There was about 10 miles of it in the designated east anchorage area. It is a busy anchorage area, as Houston is probably the highest volume port in the US and there are always ships there. As I recall, the majority of ships transversing the area have anchors with flukes of 10 ft lengths, so COE required 15 ft depth of cover on our permit. We had stiff clay for most of it and they were able to get a good trench profile (they said) to the 15ft depth, but I didn't verify that myself. 15 ft is hard to get if the bottom is sugar sand though, so it also depends on the bottom material as to if its possible to do it. Jetting rock wouldn't work, so you'd probably have to dump rock on top to get some protection, as they do in the North Sea. The profile you would need would depend on current velocity and wave drag velocity at the bottom. Probably the best references for those kind of structures is the COE Shore Protection Manuals. They're available for donwload.

Anyway, with the over 4000 platforms and several thousand miles of pipelines in the GOM, and the high traffic cargo, tankers and fishing fleet, the above practice seems to work well, as there are relatively few incidents. Get out there at night and in a lot of places you can't tell you've left the city. At least there are a lot less incidents than I would expect, but the MMS does report a few platforms out of service from time to time due to anchor drags on connecting pipelines. Most of those apparently from rig service boats that don't know NOT to anchor near platforms .. da!

http://virtualpipeline.spaces.msn.com

"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[jfduroch]

GNicholson, I believe that you have got your response in the previous answers. I won t be adding anything else except that when the pipeline will be built, trenched and backfilled you may need to be able to locate it properly for your as-built survey and subsequent maintenance surveys. Pay attention to the pipetracker that will be used as the deeper the pipe is the more difficult it is to track it.