Pipe ramming is one of the most common techniques for HDD. It is a drilling technique which utilizes high-pressure fluid to push the drill string and borehole tools into the ground. This technology helps in minimizing drilling cost and greatly reduces running time. It also creates a stabilized tunnel that allows transporting of extra hazardous materials that cannot be transported on ground.
The major advantages of pipe ramming method are as follows:
- Provides a stable excavation for long legs HDD in hard, weak or fractured rocks which increases the drilling success rate.
- It is used mainly to reduce the stop and start time, thus ultimately minimizing cost for HDD projects.
- The pipe ramming method is applicable for extreme conditions such as those that are hard, soft or fractured rocks and even when the drill bit passes from one earth formation to another.
- Pipe ramming technology makes it possible to pass through water tables, contaminated areas and other environmentally sensitive zones which is impossible with the conventional HDD methods.
This method is categorized into two major parts which are the drilling fluid and the pipe rammer tool.
The first one includes drilling fluids that consist of water or brines (salts), hydrochloric acid (HCl) and polymers/fibers. The most commonly used drilling fluid for horizontal directional drilling equipment is water based gel polymer. This fluid is commonly prepared by mixing water-based polymer with acid and water and then adding chemical additives.
The second part, the pipe ramming tool specifically includes all of the tools required to perform pipe ramming methods in HDD processes.
In certain cases, pipe ramming has been coupled with horizontal directional drilling (HDD) techniques to aid standard HDD operations.
Pusher Tubes are often used where the borehole is close to over-steepened and/or volatile clays.
Pusher Tubes are typically 2–10 times larger than the drill diameter and depending on project conditions pipe rammers may be required in excess of 20 times the drill diameter. The Pusher Tube is designed with a large smooth internal surface for the pipe to slide on. Its function is to gently force the drill against the formation and minimise friction effects owing to excessive strain gradients between the casing and nearby formations.
The Pusher Tube may also be coupled with a water seal or wedge rammers in order to further reduce friction, which helps prevent sticking of the drill string. The water seal may be positioned directly above the drill bit.
In some cases an additional rammer will be used to push on the water seal and further prevent formation damage by reducing friction between the drill string and relatively rough impermeable layer. The wedge rammer is typically positioned around 500–1000mm above the drill bit.
When using two rammers, one on top of the other, an open space between them is necessary to allow for displacement of fluid from around the seal assembly. Otherwise the water may be forced upwards and out of the hole during pumping causing undesirable surging.
These methods are normally used to stabilize a borehole during the drilling process. The target formation has already been penetrated by the drill bit, typically mildly over-steepened clays or silts.
The drill pipe is typically positioned close to the bottom of the drill hole. The rammers are approximately the same diameter as the drill bit, but much longer (typically >5x).
The rammers are positioned within one another so that their weights may be brought in in order to force the drill string against the formation and stabilize it.
Used when the target formation has not yet been penetrated by the drill bit, and is either over-steepened or highly fractured. The objective of Sweep Tube ramming is to exert pressure on the side walls of the hole in order to stabilize it.
The sweep tube consists of a large smooth internal bore, with the pipe sliding on the outside in order to form a seal. The majority of the weight (75 – 95%) is positioned above or behind the drill bit in order to exert force on both side walls, assisting the drill bit achieve penetration above the drill bit, although on occasions it may be coupled with a rammers or dirt pack.
The sweep tube is typically positioned by a top drive in order to provide hydraulic power and depth control.
The objective of rammer heads is to provide a large smooth contact area between the drill string and formation, so as to reduce friction. To do this rammers are often designed with a tapered profile, e.g. a cone shape. This will reduce contact with the formation during drilling and provide a smooth surface for the water seal to ride on during pumping.
Rammer heads are typically positioned behind the drill bit, although occasionally they may be used above or below for example when ramming around the sides of a hole. As with other rammers they are used in conjunction with a pusher tube or sweep tube assembly for example.
It is worth mentioning that the majority of these pipe ramming methods are often used together with horizontal directional drilling equipment to provide various combinations and levels of support, stabilization and penetration force during the drilling process.
This may include the use of different rammers and combinations including:
Sweep Tubes – Rammer Heads – Push Tubes/Assist Pipes – Dirt packs. These various configurations are typically specified by the client or well design engineer in order to meet specific requirements.