Pipeline - Line and Grade

[spd60]

Trying to satisfy my curiosity.

Has anyone designed a petroleum pipeline where the client has required that it be designed and built to have specific grade with predetermined low points?

 

[GregLamberson]

I've never seen a client specify a line pipe grade without a study or some sort, FEED.

The only exception I can think of, and this was on a small lines I remember one example where the client had a stockpile of surplus 6" pipe that was X-60 and wanted to use it for a flow line. Then it's just a matter of assuring it is applicable to the system requirements - which in that case it was overkill, but the client had already written off the capital cost of the pipe and it ended up being a good deal for him.

That would be the only time or type of situation where a client will prescribe a pipe grade. On a mainline transmission line - never. They may look at various options, grades, wall thicknesses, etc, but only as a "look back" evaluation of available options.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[GregLamberson]

spd60

Pre-determined low points? That's an interesting one - I guess my question would be - what would the drivers be for wanting a pre-determined low point?

Normally it is just a matter of depth of burial versus the natural grade of the terrain and a survey will determine that, then it becomes part of the design and the system low point is determined from that and considered in the hydraulics.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[vzeos]

spd60,
As I see it, grades and low points are important for hydro-test water removal. Usually we take advantage of the natural grade of the terrain but in areas where the terrain is flat I can see where it would be necessary to specify a grade and specify low points. My guess is that your client intends to hydro-test the pipeline in segments. In this case, the high end and the low end of the pipeline segment would be fitted with test heads (end caps with test connections). The hydro-test water would be introduced at the high end and drained at the low end. After all segments are tested, the test heads would be removed and pre-tested tie-in pieces would be installed to tie the pipeline together.

 

[GregLamberson]

vzeos

When you hydrotest multiple pipeline sections, you leave an overlap so that when you cut off the test headers on each section, you have a tie-in (single weld) and do not have to install a pre-tested section (2 welds).

For hydrotesting, low points simply help determine the pressure at the test pump/dead weights.

And when you hydrotest pipeline sections you fill & test from the low point, not the high point, that assists the pigs in evacuating all air from the pipeline and you can get a better line pack.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[dcasto]

spd60, as you can see we are perplexed as for the reason and it appears we have never seen this type request. It may be the right time to quiz your client mentioning that industry peers have not seen this type request and could they clarify.

 

[vzeos]

GregLamberson,
Thank you for the alternative point of view. I’ve directed dozens of large diameter (upto 36”) transmission main projects including the design, construction, hydro testing, and tying-in operations. It’s always good to learn a new way of doing things. I agree with some of what you said but not everything.

I agree that you can leave an overlap during the hydro-test, this is where the pre-tested pipe comes from. Sometimes it is useful to make pre tested pipe by welding test heads on a straight length of pipe outside of the trench and connecting it to the main test run by means of a high pressure braided hose.

Anyway, even the most experienced welder would have a problem cutting the overlapped pipe and ending up with an alignment and gap that would be suitable for a pipeline quality weld and meet the DOT spec for maximum miter. For example, suppose you needed to tie-in two 2,000 feet sections of 36” steel pipe. By your method , one of the 36” pipes would have to deviate from the lane so that you can make the overlap. After the hydro you would have to cut the overlapping pipe to the exact length and then lift and realign the pipes and wind up with a 1/16” gap and aligned within 1.5 degrees. I guess if the gap is too large you could always drag 2,000 feet of 36” pipe to shorten the gap.

I would not do it that way. I would construct both pipe sections in their proper alignment leaving about 3 feet between them with the test heads installed. After the hydro-test is over, the welder would cut off the test heads and take an actual measurement of the resulting gap between pipe segments say, 9’ - 7.5”, the welder would then cut a tie-in piece of pre-tested pipe about 9’ – 7.3” (allowing for the welding gap). The tie-in piece would then be lowered into the trench aligned and welded.

I agree with you that your method would save a weld but the logistics of your overlapping method would be a waste of time and money when applied to a large bore steel transmission main.

On your next point you assert that “low points simply help determine the pressure at the test pump/dead weights. “ The pressure at the pump and the dead weight tester is determined by the MAOP for which you are certifying the main.

I agree that the test connections should be taken at a low point. But this is not a hard and fast rule. Your assertions about filling are also not hard and fast. My test heads typically have a vent and isolation valve at the 12 o’clock position and a drain and isolation valve at the 6 o’clock position. On a long test run, I would start filling (via drains) from both ends because it’s faster and the displaced air would vent from both ends. (By the way, you don’t use pigs to evacuat air from the pipeline for a hydro test. You are probably thinking of purging operations.) When you begin to get liquid at your low point vent you would close the isolation valve. You should still be getting air at your high point vent. When you begin to get liquid at your high point vent the pipe is filled with liquid.

These methods are somewhat over simplified but basically describe how I’ve done my projects in the past.

 

[GregLamberson]

vzeos

With all due respect I've been building pipelines for over 25 years and have to say I do disagree with all of your statements above with regards to mainline pipelines.

If you don't use pigs to evacuate air, how do you evacuate the water? We always use bi-directional poly pigs, pre-loaded in the pig trap and run them with water to evacuate the air. Once the test is complete, we run them with air to evacuate the water. The only time we don't use pigs for filling & dewatering is in plant piping. If you fill from both ends without pigs what do you do about air pockets?

In your world of pipeline construction, what do you do about you normal every day ties ins? What about loose ends for NDT crawlers, road crossings, river crossings, do you not leave a lap at these locations so you can make a tie-in? Do you use pup pieces for every tie-in you make? If you cut a length of pipe "to fit", and keep in mind this pipe will be exposed, unless the ambient temperature has zero degrees of change, by the time you make a 36" weld, thermal expansion (or contraction if it get's colder) is going render your "cut to length" pup piece useless.

I honestly do not understand your methodology, but as you say, there's always alternative points of view.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[spd60]

Hi all,

Thanks for your responses.

Yes, we've had many conversations with the client on this matter and have explained that this is not the standard way mainlines are designed (admittedly this is a fairly short piping run which is why this is feasible). The client wants to have predetermined low points with drains so that any water that precipitates out from the product over time collects in known points where it can be pumped out from.

We also have a unique situation in which the client does not want to allow field bending because the pipeline is required to be internally coated. The client is concerned that the field bending could cause the internal coating to debond from the piping and cause downstream problems. So we're planning to use hot bends in both the horizontal and vertical planes (hot bends because we have a requirement that the pipeline be designed to be piggable).

It's an interesting project.

 

[vzeos]

GregLamberson,
I don’t want to belabor this, but let me try one more time. When you fill a pipe with water, the water displaces the air that is in the pipe. Since the water is heavier than air the air will tend to be pushed up by the water and the air will tend to be forced out of the test head vent(s). When you want to drain the pipe, you would open the vent(s) and open the drain(s). The water will flow under gravity. That’s why you might want to design the pipe with a grade. If your grade is insufficient to produce sufficient flow, you can always apply compressed air to the vent(s) to force the water out of the drain(s). You don’t need a pig to remove air from a pipe when filling with water or to drain it. Pigging has its place in pipeline construction. Upon completion of the pipeline, after all segments have been tied-in, you should set-up a pig launcher and receiver at opposite ends of the pipe to run a squeegee pig as part of the pipeline drying operation to remove residual water from the pipeline. You may also use a pig to purge and inert a pipeline prior to start-up. But, you would not install a pig launcher and receiver on every test segment to run the hydro-test.

 

[vzeos]

spd60,
In the gas distribution industry they would call these drip pot locations or just drips used to collect condensate. They would install a small tank (drip pot) connected to the pipe via an isolation valve so that the tank could be isolated and pumped every so often.

Is you're internally coated pipe going to be welded construction?

 

[GregLamberson]

spd60

Drips/drains always go at low points when required (as vents always go at the high points when required). However, normally a client does not designate the location, it is found via a survey, and it is where it is. Unless for some reason the client needs a drain at a particular XY coordinate for some reason.

I have to agree, the client is taking an odd approach.

As far as bending, hot bends will work, they're more expensive and depending on the grade, size and wall thickness of your pipe, you'll have some free stress to work with, so unless it's really tight, shouldn't be an issue.

What code has jurisdiction? Is it above ground or below? Does "piggable" mean intelligent pigging, maintenance pigging or batching?

vzeos

As far as hydrostatic testing, for plant piping, you are 100% correct, but for mainline pipeline testing, call or email any reputable hydrostatic testing company, (TD WIlliamson, Milbar, BJ Services, etc.) and ask them for a quote on hydrotesting a mainline pipeline without using pigs. You'll be met with silence.

Let me give you an example, you have a mainline pipeline that is 36" dia and 60 miles long. You have a test section with the following profile over a 5 mile test section:

P1 = 0'
P2 = 100'
P3 = 30'
P4 = 100'
P5 = 30'

The client does not want drains or drips (which most do not and in fact disallow them on their transmission pipelines). How would you fill that pipeline, evacuate the air, not isolate the water columns? And how would you dewater without pigs?

Let's complicate it (as is generally the case), there is no intermediate water sources, so you must transfer water from one test section to the next. How do you do that without pigs and pig traps?


Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[GregLamberson]

Cut & pasted below is from the International Pipeline Contractors Association recommended practices for "Construction of Welded Steel Cross Country Pipelines", Volume 2, Part 1, Section 13, Hydrostatic Testing:

13.4 TEST EQUIPMENT AND FACILITIES
The Contractor shall provide all testing equipment including a mobile test cabin, filling and pressurising pumps, compressors, test end manifolds, pipeline pigs, instrumentation and recorders and all equipment and materials which may be necessary for water treatment such as
strainers, settling tanks, chemical dosing pumps, corrosion inhibitors and other chemicals.

Test end manifolds shall incorporate end closure and bypass valves and piping to enable cleaning and gauging pigs to be launched and received in a continuous series. A pig alert shall be fitted on the receiving end. Test end manifolds and other test pipe and fittings shall be designed for an internal design pressure equal to the test pressure and a design factor of 0.72. All welds shall be subject to
radiography and such other NDT as may be required to prove the integrity of the welds. Manifolds and other assemblies shall themselves be pressure tested at 1.25 times the pipeline test pressure.

Prior to each re-use, test ends shall be cut back to remove all weld metal and the heat affected zone. Test end manifolds and other assemblies shall be inspected prior to each re-use. Suspected damage shall be subject to such further inspection and testing as considered necessary.
Flexible hoses shall be armoured and shall have a safe working pressure at least equal to the test pressure of the test end manifolds. They shall be used only in short lengths for connecting to the test end and for instrumentation and the size shall not exceed ~“ nominal diameter.

13.8 TEST SECTIONS
Test sections shall be as long as practicable. Test lengths and locations of test points shall be set out in the Test Schedule and shall take account of the location of suitable water supplies and disposal facilities and allowable elevation difference.

On sections with significant elevation change the preferred test point and equipment location is at the section end having the lower elevation.

Adjacent test sections shall be constructed so that they overlap, and can be joined by a single tiein weld.

13.10 INTERNAL CLEANING AND GAUGING
Prior to testing, a single swabbing pig shall be driven through the section to be tested. This shall be followed by successive swabbing pigs until any water and debris have been cleared and the cleanliness of the section established.
Cleaning pigs shall be driven by compressed air and received in a suitable trap.

After cleaning, an approved gauging pig shall be driven through the test section. The gauging plate shall be aluminium, with a diameter of 95% of the minimum linepipe internal diameter. It shall be inspected before insertion and after passing through the test section. If any deformation or damage is unacceptable, or if the gauging pig becomes stuck, the Contractor shall determine the
cause and rectify it. The gauging pig shall be re-run if necessary.

Removal of pigs shall not commence until the pressure has fallen to atmospheric. Pigs should preferably be trackable.

13.11 WATER FILLING
After cleaning, the receiving test end manifold shall be welded to the test sections. (Where permanent launchers or receivers have already been installed, they may be used for filling and testing).

The test section shall then be filled with water. At least two swabbing pigs shall be used __ during filling. Washwater shall precede the first pig and shall be injected between pigs. The pigs shall be the correct fit, with new rubbers, and shall be separated in their travel to ensure that test water does not become aerated. The Contractor shall ensure that no air is drawn into the filling pump
suction. All fill pumps shall be fitted with 50 mesh suction filters.

On downhill sections the backpressure shall be adjusted to prevent runaway of the pigs. After all water-filling pigs have been received, water shall be flushed through the test section.

13.17 WATER REMOVAL AND DISPOSAL
Requirements with regard to water removal and disposal for a specific project should be addressed in the Particular Specifications. However the following general requirements shall apply. Uninhibited test water shall not remain in the pipeline for more than two weeks.

Before emptying a test section, the pressure shall be reduced to the hydrostatic head at the test point.
Wherever possible water from a test section shall be transferred into the next section. Water shall be
displaced using a minimum of two dewatering pigs. Water shall not be transferred directly to the next
pipeline section, any transfer shall carried out using a break tank Water disposal arrangements shall be set out in the Test Schedule and shall meet the requirements
of the relevant authorities. Particular attention shall be paid to the safe disposal of water, which has
been treated with inhibitors, bactericide or other chemicals.

Prior to transfer of water into adjacent sections of the pipeline, the new test section shall be cleaned in
accordance with Clause 13.10. Interconnecting pipework and test end manifolds shall be bled of air before filling commences. Filling and gauging of the new test section shall be in accordance with Clause 13.11. If test end manifolds of tested and emptied sections are cut off without immediate tie-in to adjacent sections, the pipe ends shall be sealed with secure end closures.

I rest my case.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[dcasto]

The client needs to review the design of these low points. Evedently they are expecting some bad stuff if they are doing internal coating. Whatever connection on the bottom of the line is going to be a point where the coating will disbond and the water will attack. pigging is a resonable solution or designing the line with velocities high enough to keep the line swept.

 

[BigInch]

I'm glad I find this thread sooner. I think I'll stay out of it entirely.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

BigInch, thats not like you......

Have you ever seen this before?

 

[GregLamberson]

I agree, it's very un-characteristic of you BigInch.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com