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Steering as if by magic

Author: John English

John English of Houston-based Horizontal Technology explains why adopting magnetic steering tools in HDD can be an asset.

Many HDD contractors avoid using wireline magnetic steering tools – often due to uncertainty and misinformation. It requires a different mindset, but with proper planning and application, any contractor can be comfortable using a magnetic wireline system.

Understanding magnetic-steering tools will allow contractors to undertake a wider range of work. Some projects, or a portion of some projects, will require a magnetic steering tool. Some pipeline owners have already eliminated the use of walk-over systems altogether, adjusting their HDD specifications to suit.

As far as the HDD sector is concerned, wireline magnetic steering systems (WMS) are a fact of life. There are three basic questions most HDD contractors ask:

  1. When do I use it?
  2. What additional equipment do I need?
  3. How much will it cost?

When to use WMS

Factors to consider include the obstacle being crossed, the formation, depths and signal interference. Installation and product line specifications may exceed your system’s capability. A large diameter pipeline cannot be placed in a crooked hole easily.

The long-term liability of forcing a high pressure gas line, of any diameter, in a bore not within the stress specifications is not worth the risk. You know the limitations of your guidance systems as well as the personnel on your crew.

If it is possible that a walk-over system might be inadequate, it could be money well spent to plan ahead and include a wireline magnetic steering system in the project’s budget. Steering tool service companies are often asked to step in mid-project and that is never a good situation. Sometimes it cannot be helped, but a few days wasted trying to complete a bore using a walk-over system before making the switch to a WMS that was not budgeted for is money and time that cannot be recovered.

Should you wait for the project owner to tell you that a magnetic steering system is required? It has been my experience that most engineering, design and inspection firms are reluctant to specify when a contractor must convert to a wireline steering system. I have repeatedly quizzed them about this topic and I am repeatedly told that it is not their job.

Although making operational decisions is not their job, telling contractors what they should have done after the fact certainly is. According to one of the HDD industry’s most prestigious engineering firms, when it comes to steering and line placement, over 95% of the lawsuits involve walk-over systems.

When it comes to legal battles, these expert witnesses usually have no problem testifying as to what the contractor should have done. It is obvious there are no specific criteria to signify the point a contractor should convert to a wireline system.

Additional equipment

If you do need a wireline magnetic steering system, what additional equipment will be needed? Let us start with the changes (additions) to your current down-hole tooling. A magnetic steering tool must be housed inside non-magnetic pipe.

This is commonly called the ‘non-mag’ collar. In almost every situation, you will need a non-magnetic orientation sub that has a stainless steel flow-through stinger. The steering tool itself will thread first onto a centraliser and then onto the flow-through stinger and be secured tightly in the orientations sub.

The non-mag collar will slide over the steering tool and thread onto the orientation sub. The drill pipe will then be threaded onto the non-mag collar, sometimes directly and sometimes with a cross-over sub to match the thread pattern. There are variations but this is the basic set-up of the equipment that will be used below the surface. The idea of this assembly is to keep the steel elements of your drill string as far from the steering tool itself as possible.

Your drill pipe will attach to the rig side of this non-mag assembly and your drill bit to the other. Many contractors will simply attach the sonde housing and drilling head that their driller is most comfortable using. Sometimes the contractor will attach a bent sub with a roller cone bit. Other times it is necessary to utilise a drilling motor. The decision as to which drilling assembly to use is dictated by the formation, not the use of a magnetic steering system.

There are a number of surface components that come with the steering system and these are set up on the surface near the driller. The surface gear must be connected to the down-hole steering tool by wire.

It only takes a minute or two for an experienced crew to make a wireline connection and add a section of drill pipe. The benefit of the real-time information provided through the wire far exceeds the minimal time it takes to install the wire.

Using a fish tape, a segment wire is pulled through the next few stems of pipe, well ahead of time. The most common wire is coated, ten-gauge, 16-strand. Specially manufactured HDD wire is available to minimise ‘shorts’. As each stem is added, the wire is quickly stripped and the exposed wire placed into a connecting crimp designed to permanently connect the wire.

A crimping tool secures the attachment and a short segment of heat shrink is placed over the connection and heated with a small hand-held propane torch until the connected section of wire is moisture proof.

After each segment is drilled, the guidance technician takes a survey and provides the driller with information, such as the location of the drill in reference to the proposed path and the direction the drill is headed, for example, closer to the running line or further away. This process is repeated until the bore is completed.

To utilise a wireline steering system, a rig must be equipped with a side-entry sub or a rear-entry mechanism. This mechanism allows the wire to be pulled tight, connecting the down-hole steering tool to the surface gear.

The wire can be constantly connected to the surface gear using a collector ring. Otherwise, the wire is disconnected while rotating and reconnected at the appropriate time using an alligator clip.

The biggest benefit of a magnetic wireline steering system is the ability to verify the location using a secondary surface tracking method. Magnetic steering tools are affected by magnetic interference. This includes your rig, drill pipe, drill bit and other structures in the area, such as bridges.

Each drill is unique and the extent you go to set up the artificial magnetic field on the surface will determine the accuracy of the system. More accurate fields require more precise and time consuming set up. If you are only concerned with a general verification that the drill bit is headed where the magnetic steering tool data indicates, a quicker, less time consuming set-up will do.

Knowledge of how the system operates will not only give you more confidence, but will save time and money. The better your understanding, the quicker and more comfortable you will be with the decisions you make concerning when to stick with your walk-over system and when to adjust. A magnetic wireline system will provide the user with information such as azimuth (right, left) and inclination (up, down).

This information will allow the user to determine the tool’s location after each stem is drilled. This is called a survey and is why some refer to the steering technician as a surveyor. The azimuth is based on the earth’s magnetic field. An azimuth of zero/360° would be north and 180° would be south. The azimuth will be affected by magnetic interference and there are natural magnetic anomalies as well. The inclination is provided in degrees, with 90° being level or flat. 100° would be headed down and 80° would be headed up.

The inclinometers in the steering tool are not affected by interference. In a magnetically clean environment, a bore can be accurately completed using the directional data provided by the steering tool alone. There is very little construction taking place in magnetically clean environments.

The biggest advantage of a magnetic steering system is the ability to use a secondary system to verify the steering tool’s location. This secondary tracking system allows the user to create an artificial magnetic field. Simply placing a wire on the surface, telling the computer the location of the wire then inducing a current (most often using a DC welder), results in a magnetic field and the steering tool can determine precisely where it is located in reference to the field. The system will allow the use to verify the location of the steering tool with the accuracy depending on the detail of the set up.

There are many ways to set up a magnetic steering project. The contractor should ask questions to be sure the steering tool operator understands the project requirements. Bear in mind the steering service company is not a pipeline design or engineering firm. The guidance person or steering tool operator’s duty is to tell your driller the current location of the bore at all times. All the set up work he does is to that end.

He will most likely use land survey tools, data collectors and computers to help speed the project and to enhance the accuracy of his guidance work. He will not and cannot engineer the project. The guidance company must assume that licenced pipeline engineering firms will have planned the line long before his arrival.

All the information must be provided to him: entry point, exit point, depth of cover, radius requirement, tolerances, the location of existing utilities and any other pertinent information.

He will combine all this information as he understands it into a drawing and provide it back to the contractor for review and approval.

Any advice he gives is simply a good faith suggestion based on experience and can be taken or declined. Everyone involved should carefully check all aspects of the drawing for accuracy and omissions before signing that they have checked and approved the steering person’s understanding of the project.

Costs

How much will all this cost? The costs here are for rental. Some contractors will find it beneficial to purchase either various components or a complete steering tool system.

Contractors must make a profit and if your project requires you to provide a wireline service you should make a profit on that additional expense as well.

The cost can be broken down into four different categories: the steering tool and operator cost; the extra non-mag rental tool costs; the consumable items (wire, crimps, etc.); and the third-party costs (freight, airfare, lodging).

Freight charges will be a high percentage of the overall costs. Most of the non-mag rental equipment and the guidance supervisor will be charged at a rate per day.

There will be inspection and possible repair charges upon return of the non-mag rental tool equipment. The steering-tool service company has no influence on the experience of the driller, the crew, or the condition of the rig and drill pipe. The risk of losing equipment/down-hole items is real and the issue of insurance or financial protection will come up.

Costs will vary from job to job, with the largest factor being the length of the drill and the formation. Hard rock will drill slower than soft, extending the bore duration. Changing formations will present directional problems, as will tight tolerances.

Gravel, cobble and extremely soft formations will delay progress and add cost. The longer the drill takes, the higher the cost. Some wireline magnetic steering systems will charge an additional fee per foot. All other charges are basically the same with the footage fee tacked on, as if everyone charges it. They do not.

In fact, some systems that charge footage fees are less accurate than ones that do not. If you are asked or required to use a magnetic wireline system that charges additional footage fees, ask why and consider where the additional money ends up.

In summary, contractors should use their walk-over systems whenever feasible. By becoming familiar with magnetic steering systems, the contractor will not only utilise his walk-over system more efficiently, he will also increase the number of projects available to him. Increased options should produce increased profits. 

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