Trenchless laying and restoration of pipelines

Recommendation points

• Trenchless technologies
• Horizontal directional drilling
• Vibration drilling
• Hydraulic breakdown
• At the end

In this article: what is the reason for the need for trenchless technologies; types of trenchless technologies; horizontal directional drilling technology; vibration drilling technology; hydraulic fracture technology.

The laying and repair of underground utilities, in the minds of many of us, is associated with a serious amount of work – digging trenches, laying pipes in them, followed by blockage with soil, filling and compaction of a gravel pad, asphalting or concreting. In the summer and spring, dust and mud and slush are integral attributes of trench-laying work, not to mention the complete impossibility of using the roadways in the area of ​​the work site. However, trench chaos with all its attributes can be completely avoided by using HDD drilling (horizontal directional).

Trenchless technologies

The era of industrialization has changed the cities of the Earth – their expansion was accompanied by the development of a complex system of aboveground and underground communications. And if above-ground overpasses could be repaired and replaced with minimal damage, then underground work on replacing pipes could be performed in any other way than by trenching. 70 years ago, the problem of repairing and restoring leaky metal and cast-iron pipes that act as overpasses for hot and cold water, natural gas, sewage, etc., was less acute than today.

Since the problem of massive critical wear of underground utilities could be predicted, in 1976 the National Association of Sewerage Services (NASSCO) was created in the United States, the goal of whose members was to find and develop technical solutions in the field of trenchless repair of underground utilities. Over time, similar associations were created in a number of countries and in 1986 were united into the International Society for Trenchless Technologies (ISTT), headquartered in London (UK). Among the members of ISST there is also a Russian representative office – NPO “ROBT”, formed in 1996 and reformed in 2003.

As a rule, trenchless laying of communications is carried out by the following methods:

• vibration (pneumatic punches). It is used both for the construction of new pipelines and for reconstruction works (destruction of old pipes and their replacement with new ones). The condition for using this equipment is the admissibility of vibration loads on the ground;
• horizontal directional drilling. Equipment of this type is used only for laying new pipelines;
• hydraulic destruction. Most often, this technology is used to repair worn out communications.

Next, let’s take a closer look at the production methods and equipment for trenchless work..

Horizontal directional drilling

This method allows you to create underground pipelines with a length of several meters to many kilometers, the permissible maximum pipe diameter is over 1200 mm, they can be made of both steel and low-pressure polyethylene (HDPE).

The horizontal directional drilling machine consists of a metal frame and body, a wheeled or crawler chassis. Its body contains a hydroelectric power station, a diesel engine, a drill carriage, a rod feed system and a machine control panel. HDD machines differ among themselves in the highest pulling force of the rod string (measured in tons), the bend of the rod string (the radius is given) and the rate of flow of bentonite solution (measured in l / min).

HDD drilling includes the following stages: drilling preparation; drilling a pilot well; increase in the diameter of the well; pulling the pipeline through the well; final works.

Drilling preparation. Studies of the composition and characteristics of soil, a diagram of existing underground utilities are being carried out, the necessary documents are being drawn up. In order to select the most optimal trajectory for the well passage, the work area is probed in several places, in the case of the proximity of underground pipelines, pits are laid.

Drilling a pilot well. The drill head, equipped with a cutting tool, is installed on the HDD machine. The locating system probe, located inside the head, allows tracking its position, and the bevel of the cutting tool – to carry out controlled drilling. During the work, signals from the probe sensor are received by the locating unit and displayed on the monitor of the operator controlling the horizontal directional drilling machine – data on the depth and slope of the drill head are displayed with reference to time. In addition, the position of the drill head is monitored by an operator with a hand-held locating device, following the drill through the area. If the drill deviates from the calculated trajectory, the rig operator stops rods rotation and corrects the bevel angle of the drill head.

A flexible shock absorber rod, rod strings with a drill head, performs two tasks – reduces the load on the rods and helps control the drill string. The nozzles, which the drill head is equipped with, are designed to supply bentonite solution of a special composition – during drilling, it is fed into the well through hollow rods under pressure. The main tasks of the bentonite solution are: removal of rock from the wellbore; cooling and lubrication of the drill string and head; bringing and maintaining the rock in suspension; soil stabilization around the rod string; soil erosion (hydro-monitoring).

Drilling of the pilot well ends with the drill head coming out at the design point.

Increasing the borehole diameter. At the exit point, the drill head is disconnected, in its place is attached a reamer head with a larger diameter, also equipped with nozzles for the output of bentonite solution, which is supplied continuously during the expansion of the well. By rotation and pulling force, the reamer is pulled along the borehole in the opposite direction, while increasing its diameter to the required one – the final diameter of the wellbore should be 30% larger than the diameter of the pipeline that will be inserted into it. The stages of drilling and pulling the reamer are repeated several times, with each new stage the diameter of the heads increases.

Pulling the pipeline. The pipes intended for the pipeline are welded together in advance. After the finishing pass of the drill head (the borehole is expanded to the required diameter), instead of it, an expansion head, a swivel (a device that allows you to avoid transfer of rotation from the string to the pipe), a shackle (connects the swivel with a gripper) and a gripper to the pipe are sequentially installed on the rod string. When ready, the HDD unit is started up and it enters the pipeline into the prepared wellbore.

At the final stage, the contractor prepares and transfers to the customer the documentation containing the plan-diagram of the pipeline position in several planes with reference to landmarks on the work site.

Benefits of horizontal directional drilling:

• laying and repairing pipelines under any terrain, in any type of soil (including floaters and rocks), in various security zones, in difficult urban conditions;
• a significant reduction in the number of permits and the terms for obtaining it, since there is no need to stop traffic on highways for the duration of the work;
• the use of modern drilling facilities allows to reduce the terms of work;
• does not require the involvement of heavy equipment and a large number of workers required for trenching;
• autonomy of HDD machines, i.e. they do not need external sources of energy;
• the groundwater level does not affect the completion time;
• minimal impact on the ecological balance and landscape at the work site.

Disadvantages of HDD work:

• if the distance of the pipeline is less than 2 m, then the use of this drilling method will be expensive;
• control of the HDD machine and the drilling process can only be carried out by professionals – any mistakes dramatically increase the cost of work;
• the process of trenchless work using HDD technology cannot be accelerated – the work can be performed only at the estimated time.

Vibration drilling

To carry out vibration drilling, a special tool is used – a pneumatic punch, which allows punching both horizontal and inclined wells of a given diameter in the soil. Vibration drilling is used when laying wells at distances of up to 15 m and a borehole diameter of no more than 203 mm – for example, they are used to create punctures under roads, without opening the roadway and without stopping traffic.

The pneumatic punch consists of a cone-shaped metal body, inside it there are mechanisms for impact, reversal and air distribution. Compressed air is supplied to the pneumatic punch from the compressor through a flexible air hose with a diameter of 25 mm. The length of the hose corresponds to the distance to which the wellbore is pierced, and the diameter of the punch body corresponds to the diameter of the well. Pneumatic punches are produced with capsule diameter from 44 to 203 mm.

Stages of work during vibration drilling: preparation of pits; well drilling; placement of communications in a finished well; completion of work.

Preparation of foundation pits. Before you start punching a hole with a pneumatic punch, you need to dig the starting and receiving pit – in the first, the capsule of the pneumatic punch will be installed, in the second it will come out after punching the hole. If the required length of the well exceeds 15 m, then it will be necessary to open intermediate pits every 15 m – most of the models of pneumatic punches work within this distance. The depth of the pits should be ten times the diameter of the hole to be punched, i.e. with a required diameter of 44 mm, the depth of each pit should be 440 mm or more.

Drilling a well (uncontrollable puncture). A guide is laid at the bottom of the starting pit towards the receiving (intermediate) pit, a pneumatic punch is installed on it, air hoses are connected to its body, connecting the capsule to the compressor. After setting the direction, the compressor starts up and the pneumatic punch, under the pressure of compressed air, starts impulsive movement to the receiving pit at a speed of about 300 mm / min, moving parallel to the ground surface. The punch capsule cannot deviate in any direction from the set course – its design prevents it. After passing the well, the pneumatic punch comes out into the receiving pit, after which the compressor stops, the compressed air is vented and the hoses are disconnected from the capsule, damped and pulled through the well in the direction of the starting pit.

Communication lines are brought into the prepared well, for which it was intended – cables or pipes, the diameter of which should be 25-30% less than the diameter of the well.

At the final stage a well with communications is plotted on the plan, with reference to landmarks in this area.

The advantages of vibration drilling:

• provides the creation of an underground horizontal well (puncture) without damaging any surface coatings and communication routes;
• the cost of work is much lower than with the trench method;
• significantly lower labor costs and complete absence of the need to attract heavy equipment;
• no support wall is required for the operation of the equipment;
• the small size of the equipment makes it possible to use it in the basements of buildings;
• use in soft soils and floaters is allowed.
• during the operation of the pneumatic punch, the direction of its movement cannot be changed;
• limited borehole length (no more than 15 m) and its borehole diameter (203 mm);
• the need for starting, intermediate and receiving pits.

In addition to creating new wells, pneumatic punches are used to replace pipes in existing urban communications. The capsule of the pneumatic punch is installed in the starting well, the anchor device is installed in the receiving well, a steel cable is pulled from it through the pipe of communications and fixed on the nose of the capsule of the pneumatic punch. Under the action of compressed air and the tension of the cable, the capsule moves along the worn-out pipe, destroying it and expanding the well, while simultaneously tightening new polyethylene pipes, which are built up to each other with a threaded connection in the starting well. With the help of a pneumatic punch, you can start a new pipe made of polyethylene inside a worn-out pipe – of course, the diameter of the new pipe must be less than the diameter of the existing one.

Hydraulic breakdown

Using hydraulic devices, two types of work are carried out – forcing steel cases and breaking pipes with replacement with new ones. The first type of work involves cyclic punching of pipes using a system of hydraulic jacks. The headband of the first pipe is equipped with a conical knife that cuts through the soil, which is excavated through the hollow barrel formed by the rod pipes. This method allows you to puncture in the ground for pipes with a diameter of 1-1.4 m and a length of up to 50 m, regardless of the obstacles located above the drilling site.

The second type of work – hydraulic destruction – will be considered in more detail. Unlike the capsules of pneumatic punches used in vibration drilling, the destructive punch (“mole”) is equipped with special knives and is driven from a hydraulic pumping station. The largest diameter of the punch is 1200 mm, the maximum length of the passage is 50 m.

Stages of work: preparation of foundation pits; equipment installation; destruction of an existing worn-out pipe; installation of a new pipe made of low pressure polyethylene.

Preparation of foundation pits. The starting and receiving pits are torn off to the depth of the communications that must be replaced. The overall dimensions of the receiving pit should be sufficient to ensure the free movement of the installed pipe, the dimensions of the starting pit – to ensure the free installation of the hydraulic breaker and the entry of the rods. The walls and the bottom of the starting pit must be carefully aligned – the centering of the punch in relation to the pipe to be destroyed must be as accurate as possible. At the bottom of the starting pit, a gravel pad is poured or a boardwalk is laid – a measure to avoid the deviation of the destructive head in case of flooding of the pit.

Equipment installation. The demolition punch, installed on a metal frame, is put into the pit and centered inside it using a crane. In order for the punch frame not to move towards the pipe to be destroyed, a vertical stop made of a steel plate 1200 mm by 2500 mm, at least 15 mm thick, is required – the back thrust force of the device is more than 50 tons and in the absence of a strong stop it will inevitably pull itself into the ground … The base plate is fitted on one side with a narrow notch in front of the pipe to be destroyed and replaced. Upon completion of the installation, hoses are connected to the punch from a hydraulic station located outside the pit..

Destruction of an old pipe. At this stage, the cutting head is not installed, only rods are inserted into the channel of the old pipe, which are extended with new sections until their end appears in the receiving pit. The flexibility of the rods allows a 20 ° bending angle of the pipeline channel, but no more.

Installing a new pipe. After the rods exit in the receiving pit, a cutting head is attached to their end, the diameter of which corresponds to the outer diameter of the new pipe; using a collet grip, a pipe replacing the old one is attached to the head. The operating mode of the punch is switched to the opposite one, a steel stop is installed. In the process of movement, the head destroys the old pipe, pressing its fragments into the walls of the channel. The process of replacing pipes continues until the end of the new pipe comes out in the starting pit, after which the stop panel is removed, the rods are dismantled and the hoses of the hydraulic system are disconnected, then the punch is removed from the pit. It remains only to connect the new pipe to the communication network and the integrity of the pipeline will be restored.

Benefits of trenchless hydraulic tools:

• produced without damaging the roadway;
• laying of a new pipe is carried out in the old channel;
• one-time replacement of pipes of significant diameter (up to 1200 mm) on a section of more than 50 m;
• allows an increase in the diameter of the pipeline in relation to the diameter of the old channel;
• in comparison with trenching works, which require the involvement of a large number of equipment and a significant labor force, works using hydraulic destruction technology are performed with less effort and in a much shorter time;
• – does not require preliminary flushing of the old pipe channel;
• – there is no vibration during work;
• – works are not accompanied by any environmental damage.
• – preparation of pits is necessary;
• – higher cost of work compared to trenching.

At the end

Taking into account almost 90% of the deterioration of existing utilities in Russia and the CIS countries, trenchless technologies are the only way out of this situation. The endless patching of leaking pipelines with the trenching method commonly practiced by utilities has long outlived its usefulness..