Load calculation 1

[enlightenme]

I am doing the live load calcs of pipes for an entire refinery, coz a GRE pipe ruptured when crane passed above it. The problem is they now want to find the maximum allowed live load on all the pipe(includes CS pipes n CS sleeves ranging from OD 4" To 100"). I am proceeding as per API 1102, but it only considers HS 20-44 vehicle and its axle loads. How can I calculate Loads for cranes (200 T crawler crane included.)? And how can I generalise a safe load for a particular pipe since the surface area of cantact(wheel) varies with vehicle types. Can anyone pls help? Can anyone suggest a methodology?

Q2. The graphs in API 1102 only provides line size upto 42" OD? How do I get the values of Pipes of above 42"? It is clearly written in the specification Extrapolation is not recommended. If anyone has knowledge about this please help me!

 

[dhengr]

Enlightenme:

I would look for a book that explains the API 1102 methods for arriving at their graphs and charts; the way they calc. their allowable loads and safety factors, etc. There may be several methods of calc’ing. the allowable load as a function of (pipe dia./pipe wall thick.), depth of cover as relates to load distribution on the pipe, etc. Obviously, an 8" pipe with .5" wall acts differently under load at a 2' depth, than a 30" pipe with .25" wall at 2' depth. It’s a pipe (beam) on an elastic foundation problem. Some of the variables: type of pipe material and pipe dimensions; pipe depth from load application surface; road surface characteristics vs. load distrib.; some sort of a min. footprint vs. load magnitude; proper soil (foundation material) bedding and support surrounding the pipe and over the pipe to distribute the loads. Your calcs. or API’s are under ideal conditions, and the specs. to which the pipes were installed will come into play.

The failure that sparked your assignment probably had more to do with improper pipe installation than it did with the exact loading at the moment, unless there was some extraordinary loading which wouldn’t be covered by your allowable load calcs., under ideal conditions. A heavy truck/trailer axle or hard tired heavy fork lift might be the worst loading over a pipe. I wouldn’t think the crane would be, unless one corner of the track was right over the pipe and they were lifting right over that corner. Alternatively, a crane outrigger foot right over a pipe might be a problem too. You can’t or don’t design for those kinds of things, you use signs and supervision to prevent them.

 

[enlightenme]

Dear Dhengr your reply has given me an insight regarding this. Our project deadline is nearing and after a meeting today with the clients they told us to give the following things.
1. The bearing capacities of the paved areas. (Light Duty paving & Heavy duty paving included, 150mm/300mm thickness respctvly.)
2. The bearing capacity when there is an underground facility (pipe) beneath the pavements.
We are to show this in the overall Plot plan of the refinery (show legends) so that the maintainence or other work force can study it and their vehicles can move/ work inside the process area accordingly by avoiding the critical areas and also limit the type of vehicle entering.

Regarding the graphs I m extrapolating it, as I couldnt find anything substantial from the net but I m going to suggest bands or barriers over those higher OD GRE pipelines restricting vehicles.

Hope this way things will pan out better. Pls do suggest an idea if you have one and it ll be of great help.

Thanks a lot Dhengr

 

[dhengr]

Enlightenme:

I think you are now headed in the right direction. Map these pipe trenches out on a facilities plot plan, so that the plant maintenance people and their subcontractors have some idea when they are working in restricted ground loading area. Then you do some special design and engineering at that time, depending upon the equipment to be used, loads to be hauled or lifted, and other pertinent circumstances at the moment. You should probably be able to develop some general rules and loadings for them for everyday maintenance work, but then must also set some limits where extra attention must be brought into play before work starts. This is an ongoing development process as you and your client gather more info. on existing plant conditions. I would want to study the reports from any earlier pipe failures, and their causes.

Regarding the 6" & 12" pavement and its bearing capacity, you need to enlist the help of a Pavement and GeoTech Engineer. This will be a function of the type of pavement, its base and sub-base, and the compaction of the soils below these. This is a double edged sword at pipe trenches; where proper pipe installation involves the pipe bedding, and compaction of the soil around and over the pipe; on the one hand this properly supports the pipe to take max. loads, but it also provides proper support so the pavement can perform in distributing wheel loads. This might involve some testing to get a handle on existing conditions; that GeoTech Eng. needs something to hang his hat on too, in giving you his best advice.

You should probably not be extrapolating or interpolating from graphs and charts, you said API explicitly says this, and that was the point of my first paragraph, in the earlier post. There are methods for doing this pipe loading design, there is literature out there, but it’s been a long time since I’ve done this kind of design problem. You can do a reasonable engineering design for a specific condition, and it may involve added timber mats to distribute loads during some work, for example. At locations which are always getting heavy loading, the fill around pipes could actually be a lean concrete/fly ash mix which supports the pipe better than soils. It is much more difficult to do this problem in reverse, assuming some perfect conditions and arriving at a max. allowable loading, which may never occur, because you are guessing at the variables. Otherwise, all you can say is what the API graphs and their footnotes allow you to say; and that all the pipe installation, backfilling and paving must have been done to industry standards and specs. The pipe manuf’ers. have good literature on their particular pipe. Otherwise, any imperfections can render the whole process meaningless. You shouldn’t be doing this work by yourself if you don’t have the proper experience, and you must learn how to do this engineering problem to render the proper service to your client. How are the allowable loading charts/graphs developed given all the variables, then you can use the charts with confidence, or extend them, or know when another calc. method is required because some variable has exceeded some limit.

 

[enlightenme]

Hii Dhengr,

Yes there have been a slight improvement. Our civil guys have managed to determine the bearing load on the paved and unpaved areas. So that we will be able to determine the maximum allowed wheel load on the pavement. We will take this value at a particular depth( min depth) and give this value as the wheel load in the API Calcultaions. Check if it is allowable and therefore restrict the movement over the pavement. In this way I can show legends in the overall plot plan restricting the vehicle types and movement in particular process areas. And also a legend where an underground facility is passing and there by restricting movement above them. Hereby i think the maintenance workers will be able to navigate by using this.

I m still undecided abt the extrapolation of the graphs, this seems the only problem I m facing, but as you said these lines are buried from a range of 1.4 to 2 m depth, so it ll be able to bear the loads unless it is improperly installed or some unusual loading occurs. I m working in that regard.