Leaving steel body pig in flowline for hydrotest

[kat6787]

Good Morning,

I was wondering if anybody has any experience leaving a steel body pig/pig train in a flowline during hydrotest/leak test?

The pig itself is designed with no enclosed cavities so the pressure should not affect the integrity of the pig itself. But I haven't been able to find any technical papers, conference papers, vendor datasheets, etc. that show if this has been done and how well the pigs held up during the test. I've gotten plenty of verbal answers from pig vendors and pre-commissioning colleagues that this can and has been done before, but nothing in writing.

There have been a couple of projects lately where this option has been presented so I can't give full details of each, but below is some basic information that covers most of the projects:

Pipe size: 8" OD
Test Pressure: Between 8,000-9,000psig (measured at MSL)
Connection Point: Top of riser / topside pig launcher
Test Fluid: Filtered, inhibited seawater
Pig Train: 3-4 pig train of steel body, high sealing disc pigs

Any input/experience would be very much appreciated. I tried searching Google and Eng-Tips with a few different keywords, but didn't find anything that related to this particular issue.

Cheers,
Kat

 

[zdas04]

What is your goal? Why are you using 8-inch mandrel pigs? I understand you have some sort of confidentiality constraints, but I promise that any answer from this anemic information will have to guess a lot of parameters that we are sure to guess wrong. If you want help, we need the info.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[kat6787]

The goal is different for each project.

One goal is to insert a pig train into the flowline that will be left for later dewatering operations by our client. Due to shcedule/vessel restraints, we would like to perform as many operations from the platform instead of mobilizing both a platform equipment spread as well as a subsea testing spread. By inserting the pigs prior to the final system leak test, we can complete the leak test, including the subsea jumper from the platform without requiring a separate subsea test skid. The pigs have to be inserted before the jumper is installed because there is only a single hub to access the flowline and once the jumper is installed, there is no way to access the flowline.

Another goal is to use a pig during flooding operations from subsea to topsides for 2 parallel flowlines. The client has asked to look into the option of not having a subsea pumping unit to push the pig until received in the topsides receiver, but rather to allow the external hydrostatic pressure to push the pig until equilibrium and then top up from the platform side prior to hydrotest. The pig would later be removed after the flowline has been hydrotested and the two flowlines are joined by a jumper and it can be pumped round trip from the topsides.

We chose mandrel pigs due to:
1) The length of the lines
2) The dewatering requirement and previous experience using steel body,high seal disc pigs for dewatering operations for the first project
3) Client requirements to use mandrel pigs for the second project.

My apologies if my original post was not descriptive enough. I realize I didn't provide much project information, but since my main concern was centered around experience with mandrel pigs and not providing an engineered solution for a specific project, I didn't think more detailed project information was necessary. The particular projects have different end games, but the pipe size, pigs and test pressures are similar and both have the same issue of pressure testing with mandrel pigs in the flowlines.

Cheers,
Kat

 

[racookpe1978]

Have a backup plan (if the hydro fails, or if the pig fails, or if the welds have a flaw (if the pig jams where a weld has internal pipe inclusions or drips and blockages) thought out.

Then have an alternative to the backup plan if the backup plan fails.

 

[LittleInch]

Kat,

Taking your OP.

Leaving a steel bodied pig in the test end as part of a pipeline test is quite normal, especially for the type of location you describe (sub sea test ends etc).

I think you answer your own question by stating that there are no items which are affected by pressure. Therefore no one has really looked at this because it's never been a problem or issue. There are many things that can go wrong or to be discussed about testing, but for this type of pig, having it exposed or not to test pressure isn't one of them AFAIK.

Therefore it is kind of difficult to prove a negative, but I dare say as part of a pre-qual or prior to using it in anger, you could prepare a short test piece and pressure test the pig to demonstrate to whoever is worried about it that there is no impact. YOU can then write it up and present the results for future generations....

The key thing is in the detail of the test ends to make sure that during pressurisation the fluid pressure is equal on both sides of the pig to avoid the pig or pig train from getting squashed into the end, but I just cannot see what the concern is here - there are no failure modes for a solid piece of metal and some plastic discs exposed to high water pressure. High hydrocarbon gas pressure might have an impact on the polymers if they absorb gas which then blisters on de-pressurisation, but not seawater.

Testing companies might be able to provide you with sufficient lists of projects which have done this, but that's probably as good as you're going to get.

My motto: Learn something new every day

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

 

[kat6787]

Many thanks LittleInch.

We are planning to do an onshore test with a piece of project pipe. I was just trying to gather as much background information as possible on potential issues that might come up, both at the test and in the field. From responses here it seems that there is very little documentation on this particular question. I'm sure there is a project that has tested with pigs in the line but unfortunately that documentation does not seem to be readily available.

We talked with several testing companies as well as the pig vendor and they provided previous projects but the test pressures were only about half of what we will have for these projects. They all verbally agreed it wouldn't be an issue but nobody was willing to put that in writing.

As you've stated, best bet is to run an quick onshore test to give everybody a warm and fuzzy feeling about it.

Thank you again for your response.

Cheers,
Kat