Monolithic joint location

[kNdash]

Hi everybody. I have to design the last kilometer of a pipeline but I don't know the order to locate a monolithic joint and an anchor block. Please, someone could tell me if the order is mono - block - receiver or block - mono - receiver. Thanks.

 

[BigInch]

Isolation fittings should not go between pipeline and launcher/receivers at all, unless you like replacing them. That's often the highest stress location you can find on a pipeline. Best place is after the receiver, on the way to plant piping connection. Keeps pipeline isolated from plant piping.

 

[brpillai]

kNdash, Anchor block ( buried) will have to be first and then the monolithic isoaltion joint ) IJ abovegrond, immediately after the buried portion leaving a spece of about a meter, not toching the ground) and then the reciever.

 

[BigInch]

Anchor block? Do you have a welded pipeline, or are you using bell & spigot joints? Or maybe you work in the Mideast, as they are known for their fondness of anchor blocks. If not, chances are you have a nice, big and useless, anchor block. Find a pipe stress engineer that knows how to do an analysis without sticking in anchors just because Caesar needs them to do the math. An anchor block does not remove stress, and in fact it only increases the stress in the pipeline, since elongation totally restrained. We know that total restraint at some point in the pipe always happens, at least in normal pipelines, because we do not see getting up out of the ground and walking off to Riyadh. So that means that an anchor block only localizes the point of full anchorment to a known position. Otherwise it happens at a virtual anchor made up of a long cylinder of soil surrounding the pipeline. What changes then? Only the rate at which the pipeline de-stresses itself as it exits the ground. An isolation joint anywhere in that area isn't the best idea. It either has to cope with high stresses or large rotations and deflections of the pipeline as it goes from anchor point to launcher / receiver. Search this website for numerous comments about removing these useless and costly anchor block accessories.

 

[brpillai]

Biginch
I agree with you that anchor block should be avoided as far as possible, and in all caeses of pipeline where I was involved there was no anchor block. My comment was not on the requirement of anchor block, but on the location, if at all is required, as the query was that the Op does not know the location of "IJ and Anchorblock".

 

[LittleInch]

BI,

You are correct that the "normal" position of IJs , i.e. a few meters after it gets above ground, is often at a high stress location, but once you allow a support or some instrument on the pipeline side, the chance of insulation of the supports and instruments is low and hence the risk of current leakage to ground increases. Also the pipeline is then at a potential relative to earth and there is a spark and touch issue with bare or painted pipe.

Therefore you will fnd most IJs on the first level bit of pipe after it comes above ground as the least worst place. I'm not aware of systemic failure of IJs in these locations, but am open to more information...

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

 

[BigInch]

Failures of isolation joints are probably more common than you think and, if placed between pipeline and launcher/receiver, difficult and costly to repair, often needing complete pipeline shutdown. These are typically not repaired until a leak develops that cannot go ignored. The mfgrs also like that location, because of the largest diameter pipe.

IMO there are far better locations for most instruments requiring connections back to plant, except perhaps for a pigsig. Most other instruments can be moved to station suction and discharge lines, on the station side of isolation joints, thereby being kept off the pipeline entirely.

The isolation joint itself may be responsible for creating the high voltage potentials. Isolation joints, when used, should be used with cathodic isolators and spark gaps, which prevent dangerous voltage level build-ups across isolation joints and flange kits. 2005 NEC had some significant changes to plant grounding requirements. Launchers/receivers should be earthed like this

http://ecmweb.com/design/cathodic-protection-systems-and-nec,

including a cathodic isolator where applicable. More info on that equipment at,

http://www.cathodic.co.uk/files/124280955112186137654.0PipelineEarthingIsolationAccessories.pdf