Curved pipe pullback for directional drill

[LearnerN]

Has anyone designed a curved pullback area for a directional drill, like when there are large site constraints that have to be worked around for the pullback? Is there a pipeline rule of thumb related to a minimum welded-pipe stringing radius? Obviously the pipe won't be bent on this curved pullback so field-bend details in ASME B31.4 don't really apply to this situation. I was thinking of maybe using the 100D rule of thumb used for the vertical profile of the drill, since that should be conservative (since that's to account for stresses during installation).

 

[LearnerN]

The Pipeline Rules of Thumb book also has a minimum radius of curvature for a cold bend on pipe per the yield strength of the pipe, so that could possibly be useful.

 

[LittleInch]

I have done it, but for a horizontal bend you really need to think about practicality. I would initially go for something like 1000 to 1500D bend radius (did you mean 1000 instead of 100 in your post?). The issue is how do you get the pipe to bend and stay bent elastically without needing to build large resistance into each roller, which then needs to be at an angle to maintain the bend radius. Your pull force will always be acting to shorten the curve which your rollers need to resist

The bigger the angle the greater the increase in pull resistance is so you really want to reduce this as much as possible. There are also pulling devices similar to pullback control on lay barges which can lessen this effect and split some of the thrust force between the rig and this device on the entrance to the hole.

Whilst pipe over a long distance can be come fairly flexible, you will still need a few pipe joints of straight before you try and manoeuvre the pipe into your desired horizontal bend radius.

Kind of depends on your pipe material (steel?), diameter and wall thickness so a few more details would help frame the responses better.

Most HDD companies have seen it before and can provide some guidance, but they can be a little bit boastful in order to get the contract so bear that in mind when talking to them.

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

 

[LearnerN]

LittleInch,

Thank you for the response. The pipe is steel, 10.75", 0.365" WT, X52 Grade.

 

[LearnerN]

And the length of drill is around 800ft, so not a very long distance of pipe in which the pipe would otherwise become fairly flexible. That makes sense that the challenge would be getting the pipe to stay in the curved orientation in pullback, in terms of the resistance the rollers would need. Sure is a tricky situation.

 

[LittleInch]

10" rel thin wall should be pretty whippy pipe. 1000D = a radius of about 250-300m. You might get down to 200m but will need quite a few rollers providing guidance to the bend so they would need good anchoring / resistance to the bending force.

Do you have a sketch or plan you could share? Remember also you can lift this pipe over things with a couple of cranes.

If you have room can you build it straight then feed it into a curve to get round your obstacle?

At 250m length, you could also build it in two or more lengths then stop and weld the sections together. Usually to be avoided if you can, but at that sort of pull length a big rig should have plenty in reserve to re-start the pull after you've welded it into a single string.

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

 

[LearnerN]

LittleInch,

Those are some really great ideas. We should have room to build the pullback in straight sections and then feed it into a curve, like you suggested...very good point. Don't have a plan/sketch yet, but the situation is basically a drill exit with the new replacement pipe (offset 10' south of the existing line), and then ~200ft west of that is the valve site on the existing pipeline (that won't be replaced), and then ~100ft west of that is the terminal. So the plan would be to curve the pipe pullback a little going north of both the valve site and the terminal. And there are also power poles to work around and such.

 

[BigInch]

I agree with LittleInch. The object is to keep it as elastic and flexible as possible. 1000 D radius. The more flexibility the better. Reduces friction and pulling force.

 

[rconner]

I think the "100D" rule of thumb is intended to give you a minimum radius in feet but only when you enter "D" in inches (don't ask me why, as the 1000D of LI and BI to me is less confusing, with only the units of "D" to worry about!) However, both of course give near at least similar magnitude values.

While actually a whole lot of factors go in to wanting to keep the radius no tighter than this, as others have hinted (e.g. you don't want to oval the pipe too much, result in pipe collapse along with mud pressure, fulcrum the pipe too much against the drill path, create too much capstan resistance, and/or wear out in fatigue heavy/expensive drill rods trying to drill/rotate them also bent etc) you generally would not be permanently "bending" the pipe at that minimum radius , as per a similar goofy rule the pure bending stress alone is about ED/(24R), in psi only if you enter E in psi, D in inches, and R in feet. For your case at minimum R = 1,000 ft this relationship would thus give a pure bending stress of 12,500 psi (but before adding effects of any other stressors like pulling load, thermal etc) per this "rule". [Then again, the pipe bending stress vs vs minimum HDD radius rule can also be expressed as ED/(2R), when all of the terms are expressed arguably more consistently in inches or psi's and will give the same value!]

 

[bimr]

Agree with rconnor, the rules of thumb are misquoted:

The radius of the arc of the drill path should consider the diameter of pipe to be installed. The minimum radius for most drilling applications is:

• the diameter of the pipe to be installed in metres multiplied by 1200; or
• the pipe diameter in inches multiplied by 100 to obtain a radius of curvature in feet.

These formula gives you 1,075 feet or 327 meters.

This formula is used to ensure a conservative radius of curvature that will allow for the easy installation of the pipe and minimize the bending stresses on the pipe. In most applications, the radius of curvature will not be lower than 250 m.

You might find the planning guideline useful.

 

[Compositepro]

This video of steel tubing blowing out of an oil well gives one perspective of the tubing flexabilty.

https://www.youtube.com/watch?v=lkqpEXy0frE

 

[BigInch]

Nice video, but that's not all that bad. All the tubing landed within sight of the rig.

 

[racookpe1978]

Think about the "pull" of the pipe, or the "push" of the pipe around that curve.

If you are pulling the pipe into the final bore, from the other end of the bore (within the final bore ditch)m, then the pull will act to require the rollers be on the inside of the bend, and before that bend on the outside of the straight layout area.

If you are pushing the pipe into the final bend before the bore, then the rollers need to be on the outside of the bend and the inside and outside (above AND below) the straight layout area in the assembly area.