WHEN IS AN ANCHOR BLOCK NECESSARY ? WE ARE DESIGNING A NATURAL GAS PIPELINE. 1

[tsofa]

Hi all,
we are designing a natural gas pipeline, of 24'' diameter. My question is, when an achor block is required. The pipeline that we are designing is an underground carbon steel pipe, which in some cases has over 40% slope! Is there any rule of thumb or good engineering practice for anchoring the pipeline if its slope is over one specific percentage?

 

[LittleInch]

Steel pipelines are inherently flexible and have an ability to absorb a lot of stress and movement. Introducing anchors just increases stress at critical points. If you design your pipeline correctly you should be able to have no anchor blocks. Even at 40 % slope, soil friction will hold the pipe in place for a buried pipeline. Usually the issue of high slope angles, >10 to 15 %, is washout of the trench during construction or after backfill.

To be honest, the fact you asking these basic type of questions and seem to come from a civil background makes me worried that you don't have the right experience to design such a large diameter gas line. If this pipeline fails in service you can have a very large fire. Please think about using experienced pipeline engineers.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[tsofa]

LittleInch,

thanks for your answer, of course I am not the one who designs the pipe, don't worry, the world will still be a safe place after this one! I am just asking to have a feeling about the project. Do you have any recommendations for 77- 83% slopes? Do you still think that the friction of soil would be able to hold the pipeline? What about using a thrust block?

Thank you in advance

 

[BigInch]

Given that we are not talking about anchor blocks at or near stations and critical equipment, you will need trench breakers to keep the trench from washing out. A wrap or two of geotextile will be a big plus as well.
Anchors, or weights to prevent buoyancy, probably not.

Independent events are seldomly independent.

 

[cvg]

thrust blocks should not be necessary. even temporary anchors may not be necessary if you build from the bottom of the hill up to the top. however, on 83% slopes, design of anchor blocks are only one of your problems. are you actually planning to trench and install an underground pipe on this near vertical slope?

 

[BigInch]

83? more like 38

Independent events are seldomly independent.

 

[LittleInch]

Big inch - that's the difference between our actual ages and how we feel.. .

Tsofa , you're describing a near vertical slope. Where are you routing? Up the andes? Past the normal angle of repose for soil, around 30 degrees, you won't get whatever you dig or blast out to stay put so for most slopes beyond this you need to find another way.

As cvg says you'll have more issues than anchor blocks... Providing that the length of pipeline at that sort of slope is less than 150 to 200 m, you should be ok to build from the bottom up. You may need to make sure that the base is well compacted if you have a formed bend at the base of what sounds like a cliff or escarpment, but the general rule in pipelines is to avoid anchor blocks and hard edges wherever possible as high shear loads and stress concentration around anchors is what breaks pipelines. It would be great to see a vertical profile of these very high slope areas and then you may get more feedback.

Design it then run it through a stress analysis

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[77JQX]

Just had to chime in as a civil. You guys know that an 83% slope is about 40 degrees, right? Darned steep, but not near vertical.

 

[BigInch]

Go read back up to the very top post. 40%
40% slope is 40 y in 100 x
21.8 degrees (from horizontal, for all you civils out there)

Independent events are seldomly independent.

 

[77JQX]

Go back and read the third post. OP is back and asking about 77-83% slopes, to which people were posting about "near vertical."

 

[LittleInch]

77jqx. To be fair you're correct. Pipeliners tend to think in degrees and its easy to forget that 100% slope is actually 45 degrees or 1:1. Either which way it's still a hell of a slope., but if 83% is 40 degrees, you still won't get the backfill to stay there.

You still don't need anchor blocks though.

Tsofa, just confirm you're working in percent and not degrees and post a profile!

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

OK, right. That change in tune got by me.

Angles steeper than the angle of repose of the soil of course are NOT a good idea.
Angles steeper than what would give a reasonable trench breaker center/center distance would also not be reasonable.
Angles steeper than what would be necessary for slope failure to occur during soil staturation conditions.

Each would most likely call for some means of mechanical anchoring into the soil or rock below.
Screw anchors and rock anchors come to mind --- Oh, there's super glue.


Independent events are seldomly independent.

 

[tsofa]

I am working in percentage not in angles!! Of course! a vertical angle would be something out of question!

 

[BigInch]

Vertical angles are possible if you exit the trench. Just need something to bolt it, or rock-anchor it too, such as face of cliff, or vertical mine shaft, etc. You can weld pipes together at the top, then drop the welded-up string down a level and weld on another piece of pipe to the top, kind of like a J-lay construction sequence offshore.

Independent events are seldomly independent.

 

[KernOily]

I assumed by 'anchor block' you mean 'thrust block'. Before you can say whether a thrust block is needed, you need to determine what loadings and design constraints will be presented to the line. Since this is a buried natural gas pipeline, I assume it operates at ambient condition, therefore thermal expansion is probably not a design consideration. Same for seismic loading. If there are any anticipated dynamic loadings, e.g. liquid slugs, fast-moving large chunks of hydrate, a large fast-moving pig, stuff like that, then you may have a huge dynamic load occur at a bend. In that case you may need a thrust block to prevent the pipe from being overstressed at the bend when the slug hits the bend. The soil springs themselves may provide enough restraint for this oondition but it needs to be checked.

You need to first define your loadings and then run the analysis. You can't a priori say whether a thrust block is needeed or not, regardless of slope. An experienced pipeline engineer might be able to provide you with some rules of thumb but he is going to ask you what the loadings are.

 

[BigInch]

If you are, or should ever become interested in anchor blocks, you should read this thread,
thread378-342040

Independent events are seldomly independent.

 

[rconner]

[fyi also

http://eng-tips.com/viewthread.cfm?qid=269533

]

 

[chicopee]

After reading all the responses, one should have asked tsofa the purpose of an anchor block.

 

[chicopee]

Wow, this typing is killing me "...one should have asked what's the purpose...", then you'll have the answer to the OP.