Pullback design and planning is a fundamental component of horizontal directional drilling projects. It is the procedure through which critical information about all geotechnical, structural, environmental and operational aspects pertaining to a project are evaluated in order to develop appropriate strategies for constructing underground works and achieving successful completion of the project.
In essence, pullback design and planning is all about carrying out a risk analysis for the project and implementing measures to minimize those risks.
Pullback Planning
The design and approval of suggested pipe specifications, as well as the computation of forces and resistance, are all part of the pullback planning process. It should take into account scenarios for both installation/temporary and long-term/permanent situations.
The designer should keep an eye out for any issues that arise during the installation process that could have long-term consequences. The pullback should be built in such a way that the suggested pipe does not distort permanently.
Resolving issues pertaining to pullback design and planning early on in the hdd crossing
construction process helps considerably lower the overall cost of projects.
For example, if a contractor operates equipment such as HDD systems over long distances or in extreme terrain, it will have to provide for additional cost components such as fuel, materials transportation and labour costs.
To minimize these costs, a contractor needs to ensure that the equipment used is appropriate for the tasks at hand. In addition, it must also assess in advance how best to avoid potential difficulties that could cause delays or costly problems down the line; pullback design and planning can help the contractor to do that.
Pullbacks are used for directional drilling to:
- Tie into a continuous string of casing or liner.
- Stop and hold in a hole that has encountered obstructions such as boulders, lost circulation zones, etc. By using the pullback technology, they can drill hdd crossing right over these obstructions and move on to planned completion with less time and money.
- Bypass the lower portion of the hole by leaving behind either a small liner (for large diameter holes) or, more normally, some form of casing in which they drill through to travel through an obstruction zone while continuing to advance the drill bit above it.
When should it be done?
During the design stage, planning is usually done as fast as possible, especially prior to the project’s bidding/contracting. Background information such as geotechnical research results, needed pipe size (hydraulic requirement), and site constraints are preferably collected before beginning the pullback planning.
Here are some basic pullback aspects that should be part of your planning:
Pullback Case Selection: Are you using a standard or special pullback?
If it is a standard case, what size will work best in this particular hole? A common mistake made by those new to pullbacks is the use of the wrong size case for certain things such as flares and pulling back through obstructions. This will cause problems, potentially serious ones.
Pullback Length: How long a pullback should you use?
This depends on the following:
- The type of hole (vertical or directional),
- The soil properties (rock and grain size),
- If casing is being used in the hole (top-drive, hammer, or pullback),
- If you are planning on beefing up the hole in order to handle a large pullback and flow volume (larger casing or liner, etc.),
- What kind of stand-off needs to be maintained between the bit and the wall of the hole.
Pullback Stand-Off: What is your stand-off distance going to be?
Stand off is the space between the wall of the hole and where you want to place your pullback. This depends mainly on how hard it is for the person at the hammer (or top drive) to hit that exact spot with force. A lot of planning goes into an effective stand off, doing this correctly is what makes or breaks the success of a project.
Pullback Stand-Off Safety
You must ensure that everyone has enough room to work while still maintaining the required stand-off distances for safety purposes.
Pullback Hole Flow Through Testing – Top Drive/Hammer
Test your pullbacks with flow through tubing on top of your liner or casing. This will give you a live feedback of how much flow through volume and pressure is necessary to maintain the stand-off distance and the ability to pull back without any issues with sticking, etc.
Make sure that this information is known by all those involved in drilling before they enter the field for actual drilling operations.
Pullback Equipment Needs: How much water and flow volume are you going to need for pulling back?
This is mainly important if you are planning on using a top drive or hammer. It’s best to have a fluid testing pump at the site so that the flow volume needed can be determined before you begin drilling as opposed to waiting until it is too late to correct the flow volume or pressure.
Pullback Hole Flow Figure: What will be the maximum flow through volume that you need to pull back from over an obstruction?
This must be known before drilling because once it is drilled, there is no going back if you cannot get the necessary flow. Also, how much water are you going to need in order to pull back?
Pullback Case Orientation: What angle should your pullback be placed in the hole?
This is extremely important, as it plays a major role in the success of pulling back. This also depends on whether you are going to be pulling back at depth or on grade or shallower. Most cases, there will be a diagram of the pullback orientation included with it.
Pullback Selector Application
If you are using one, this app is very important for determining set up and pulling out information about your particular case. It’s important to use this correct information when planning your hole as well as knowing how much flow back pressure you will need.
Pullback Hole Deflection Testing
Test the hole’s deflections in order to determine if you will run into any issues with hitting an obstruction when pulling back at a stand off distance. Deflection testing is difficult because of the type or configuration of your gun and pullbacks, so this testing needs to be done before drilling begins to ensure that you are not going to run into any problems later on in the project.
Pullback Hole Information
Make sure you have all the information regarding your pullback hole that is needed in order to make a decision on how to go about pulling back. Before drilling, know what methods are going to be used for pulling back and what equipment will be needed for this operation.
Will you need a surface pump or should there be one installed at the site so that the pullback can be done on grade? Will you need a mud dump set up or is this going to be done without one? Will your pullback require that the casing and liner be pulled back with it, along with the cement behind that?
All of these things should be discussed before drilling begins so there are no major surprises.
Takeaway
Pullback design and planning are absolutely essential throughout any horizontal directional drilling project because they are the key determining factors when it comes to deciding whether the drilling operation will be successful.
Pullback systems ensure that the operator can control the horizontal advancement of a drill string during directional drilling, and they are also essential for gaining access to damaged materials when an HDD is being used in waterway construction or other similar projects.
