We have already covered the standard drill collar design in the previous article. Now, let us have a look in two special designs commonly used in rotary drilling, in addition to the standard one.

Spiral Collars

Although most collars are round, engineers design some collars to counteract specific problems that can develop while drilling. For instance, some collars may have spiral grooves machined into the outside surface. Drillers sometimes use spiral drill collars in holes in which the clearance between the drill collar and the wall of the hole is small and in directional drilling, where the collars will likely contact the side of the hole, to minimize differential sticking.

Collars with spiral grooves help prevent differential sticking by reducing the amount of drill collar surface area that contacts the sides of the hole. Differential sticking is a condition in which the drill stem becomes stuck against the wall of the wellbore because of the difference in pressure between the drilling fluid in the wellbore and that of a permeable formation. The pressure is higher in the wellbore. Thus, the differential pressure forces some of the liquid part of the drilling fluid into the permeable formation.

This leaves a layer of concentrated mud solids on the sides of the hole. This layer of mud solids is called filter cake. The lower formation pressure creates suction that holds the collar against the side of the hole. If the pressure differential between the wellbore and the formation is sufficient and the clearance between the drill collar and the hole is small, it is easy for the collar to contact the filter cake and get stuck.

Square Collars

Drillers use square drill collars in an effort to keep the wellbore on as vertical a path as possible. In straight-hole drilling, the driller attempts to keep the wellbore as vertical as possible until the target is reached. One phenomenon that assists the driller is the pendulum effect, or the tendency of the drill stem to hang in a vertical position because of the force of gravity pulling on the weight of the drill collars. The heavier the drill collars, the greater the pendulum effect.

Square drill collars have more steel and are thus stiffer and heavier than round ones. They tend to give, or bend, less than round collars and produce a greater amount of pendulum force. Also, each corner of a square drill collar touches or nearly touches the wall of the hole as the drill stem rotates. This contact (or near contact) acts to stabilize the collar, or keep it stiff and straight. With no wobble, the collar keeps the bit drilling on course.

So these are the main special designs of the drill collars. As it was mentioned earlier, there are several other types of collars produced by the manufacturers, each of them having specific features, and they will be applied by the well planner depending on the characteristics of the particular well.

In the previous article we have provided some very brief introduction to the drill collars and drill string, since these terms and fundamental for proper understanding of the rotary drilling process. Now, let us have a look slightly deeper into the drill collars. As it is the case with nearly every piece of equipment on the drilling rig, they come produced by the different manufacturers and according to the different designs. Below is some information about the standard design of the drill collars most widely used both for the land and offshore drilling.

Manufacturers design drill collars for guiding, stabilizing, and providing weight on the bit. Drill collars are heavy, thick-walled, metal tubes usually made of steel. They range in weight from 16 pounds to 379 pounds per foot (23.81 kilograms to 564.01 kilograms per meter). Most drill collars are round and are 30 or 31 feet (9.14 or 9.45 meters) long. Drilling crews join the required number of collars by stabbing the pin end of one collar into the box end of another and screwing them together.

The well planner determines how many drill collars are needed above the bit by taking into consideration how much weight is needed to drill efficiently and how much each drill collar weighs. Generally, the bigger a drill collar’s outside diameter, the more it weighs. For example, a thirty-foot (9.14-metre) drill collar with an outside diameter (OD) of six inches (152.4 millimeters) and an inside diameter (ID), or bore diameter, as it is also called, of 2 ¼ inches (57.15 millimeters) weighs 2490 pounds (1129 kilograms). On the other hand, a thirty-foot (9.14-meter) drill collar joint with and outside diameter of eleven inches (279.4 millimeters) and an inside diameter of three inches (76.2 millimeters) weighs 8843 pounds (4011 kilograms).

Drill collar inside diameter varies from two inches (50.8 millimeters) to three inches (76.2 millimeters), but the outside diameter ranges from four inches (101.6 millimeters) to eleven inches (279.4 millimeters). The well planner selects the correct outside diameter based on the size of the wellbore, and then determines the number of drill collars needed to provide the proper weight-on-bit to make hole.

An important factor to consider when connecting collars is the relationship between the pin and the box. Engineers have learned to compare the bending strength of the pin near its base, i.e. near the last engaged pin threat referred to as LET, to the bending strength of the box near its bottom. In good design, the bending strength of the box should be 2.5 times greater than the bending strength of the pin.

All above is for the standard design of the drill collars; note that there are also collars of special designs, such as the spiral and square collars, widely used in rotary drilling. Apart from that, there are also other varieties of the drill collars produced by the manufacturers and having some special features, including but not limited to such items as recesses for slips and elevators and hard banding. Each of the designs has its own advantages and disadvantages, so it will be up to the well planner to conduct proper evaluation and choose the one that will better suits for the particular well, providing best efficiency.

In rotary drilling normally employed on the offshore drilling units, a rig crew rotates a bit that, in turn, drills a hole through the earth in search of the hydrocarbons, i.e. oil and gas. The crew attaches the bit to a hollow length of pipe that serves two purposes: it provides the weight to make the bit dig into the earth’s formations, and it provides a passageway to circulate a fluid – drilling fluid – to the bit as it rotates. This drilling mud cools and lubricates the bit and carries the rock cuttings from the bottom of the hole to the surface. Surface equipment removes the cuttings and recirculates the clean mud back down the pipe. As the crew drills deeper into the earth, crew members add more pipe to that which is connected to the bit. Before a well is completed, the drill stem may be thousands of feet long.

Because the drill stem serves two purposes, it consists of, among other things, two basic types of pipe: the drill string and the drill collars. These two types of pipe are similar in that they are hollow lengths joined together to make one long conduit from the surface to the bottom of the hole. They are constructed somewhat differently, however, because they fulfill different functions.

Drill Collars

Drill collars are heavy-walled metal tubes, usually made of steel, that the crew positions directly above the bit in order to guide, stabilize, and put weight on it (commonly referred to as WOB – weight on bit) and to make hole. Manufacturers cut threads into each end of a drill collar so that it can be joined with another collar. The end with the threads cut on the inside, the female end, is called the box, and the end with the threads cut on the outside, the male end, is called the pin. The number of drill collars screwed together and placed on top of the bit depends on the weight needed to make hole efficiently, the type of formations to be drilled through, the weight of each drill collar, and other variables. After the crew attached the required number of drill collars to the bit, crew members attach additional pipe to continue the conduit from the top of the drill collars to the surface; they use the drill string for this purpose.

Drill String

Drilling crews use drill string to transmit the rotation of the rotary table or top drive to the bit and to serve as a conduit for the drilling fluid. Though the drill string completes the connection between the drill collars and the surface, the crew does not use it to put weight on the bit. Since the drill string is not used to put WOB (unless drilling horizontally, where it is used in compression to put the bit horizontally through the formation), the pipe body is lighter and much thinner than the drill collars. The metal wall of the drill pipe is usually less than half an inch (12.7 millimeters) thick and too thin for threads to be cut directly into it. Manufacturers, therefore, produce short, thick, heavy piece of pipe called tool joints with threads cut on either the outside or the inside. These thicker-walled joints are welded to each end of the drill pipe so that the crew can join the pipes together.

The term “drill string” refers to the column of drill pipe with attached tool joints. The “drill stem” refers to both the drill string and the drill collars as well as the swivel, Kelly, stabilizers, and various specialty items used for rotary drilling. The drill stem is one of the most expensive, integral pieces of rotary drilling equipment. Consequently, handling it properly and maintaining it in good condition are priorities for any crew.