Pipe Stress

[EireChch]

Hi all,

I need to work out the strap stress acting on a PE pipe.

The pipe is 2.5m in diameter and placed on the seabed. The purpose of the strap is stablise the pipe against wave and buoyant forces.

The strap is fixed to a concrete block either side of the pipe. Its an unusual temporary stabilisation solution, that someone else has implemented.

The strap length is fixed and the blocks have been spaced to ensure there is some tension on the pipe.

I want to check if there will be any detrimental effects on the PE pipe from having a 100mm wide cargo strap acting on the PE pipe.

Could someone point me to a good book or reference?

Ta

 

[LittleInch]

Try the vendor or his website first then the design codes.

The issue with PE is that it creeps under constant stress and also really doesn't like external pressure / forces so tends to buckle / collapse. Especially at 2.5m diameter your SDR must be really high so there will be very little capacity for external loading.

A diagram would help here as I'm not quite getting your situation.

Are these basically like a saddle bag arrangement where the blocks are free hanging above the seabed?

But whatever force you start with it won't be that after a while as the pipe will very slowly deform.

You might be able to do an FEA on it and show that the stress levels are below the creep level, but ask the vendor.

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

 

[MJCronin]

PE is not a good choice for piping subject to any kind of waves, external forces or vibration ....


MJCronin
Sr. Process Engineer

 

[EireChch]

Thanks Little Inch - section and plan below. yes a saddle bag to restrain the pipe. Saddle bags spaced along the alignment. This is a very temporary solution for a temporary pipe. Within a few months the pipe will be placed in a trench and backfilled. The bags are sized to resist buoyant and hydrodynamic forces. Severely over designed as its definitely not our preferable solution but its literally all we can do. It has already been installed and then designed afterwards (dont ask).

MJCronin - like I said. Dont ask!!

 

[LittleInch]

The pipe could easily get a bit more oval but this will be limited until the strap gets slack.

Post install trench and backfill a 2.5m pipe. Wow.

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

 

[1503-44]

Any detrimal effects are far more likely to be caused by waves, currents and impact of foreign materials, anchors and trawler boards. Otherwise all you have is buoyancy. I hope you don't have any of those things to consider. Buoyancy of 2.5m PE pipe is way more than enough to worry about. I don't think the straps are going to get s[l]ack. You must have something like a continuous layout of block & strap saddles, right?

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

I hope your straps aren't steel.

There was a project I know where a PE pipe strapped to a steel line was laid with stainless steel straps, but the the fixing clamp was carbon steel. Took about two months but then all of a sudden there was a strange black monster appearing on the surface of the sea....

How far apart are these blocks?

I might have used removable concrete dog houses myself, but start praying for calm weather until you can fix it in place properly.

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

 

[EireChch]

The saddle bags are sized to consider the required weight needed to resist wave and current and buoyant. Again we do not even have met ocean data for this area. So everything is being hugely over designed. We were called in to look at this after a failure, the pipe was meant to be placed in a trench and backfilled but it wasnts. Is very hard to believe but that is what has happened. Now we are trying to stabilise the pipe until we can excavate a trench and place a new pipe (or place the existing pipe in the trench, difficult but an option that is being considered).

5 100mm wide heavy duty polypropylene straps with a SWL of 4 tons are being used to connect saddle bags. The 5 straps (i have drawn 3 in my sketch above) cover a length of 2m. Bags are spaced every 4m. So 2m of strapped length then 2m of unstrapped length.

Existing concrete rings are fitted to the pipe and we are placing saddle bags between these.

Its a passive application of load. The pipe attempts to float or move and the strap force is a reaction.

I have worked out the strap force to be 19kN. This is applied over a 0.1m wide strap.

I need to assess the deformation of the pipe due to the this load (assuming it is applied constantly, which it is not). Any help with that would be very much appreciated.

I have looked at various resources but cant find anything that fits my case. The closest I got was clamp supports on pipes but didnt think it was applicable.