There are a variety of types and classifications of drilling pipe that vary in their construction, properties, and applications. In order for an offshore driller to select the most appropriate type of drilling pipe for a given operation, it is important to understand the different types and classifications and their properties.
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In this post, we will discuss the different types of drilling pipes and their uses. We will also highlight some of the factors that you need to consider when choosing a drilling pipe for your project.
Without drills, there cannot be petroleum; without drill pipes, there cannot be drills. Drill pipes, as you know, play a huge, perhaps underestimated role in the drilling industry. These tempered, hollow steel pipes, hardened against accidents and to withstand the pressures inherent to subterranean conditions, pump drill fluid to the drill bit.
Because of the working conditions in which they operate, drill pipes generally have to be carefully checked and rechecked for wear and tear. To that end, they are assessed with spherometers, instruments which can precisely and quickly measure the radius of a sphere.
After a spherometer check, drill pipes are classified in one of three categories:
N-class pipe, or new pipe.
The strongest, newest pipe available. Usually in pristine (or close to it) condition.
P-class pipe, or premium pipe.
A step-down from N-class pipe, premium pipe have endured some wear, but can still function very capably.
C-class pipe
C-class pipe, of which there are three classes (C-1, C-2, and C-3). Depending on their rating, C-class pipes are either well-worn pipes, or nearing the end of their useful lives.
After a pipe has reached the end of its useful service life, they will be graded for scrap, denoted with the application of red bands. Because of the high cost of drill pipes, remember that they are often reused, with many companies selling both new and used pipes (graded according to the criteria mentioned above) often until the very end.
Now that the basics of drill pipes are covered, let's take a look at the several types that are available.
Drill string
The drill string refers to the entire column of tools, essentially the drill pipe, shaft collar, tools, and the drill bit itself. Generally, drill pipe used for the drill string is made from tempered steel. For cost and transport reasons, sections of drill pipe rarely exceed ranges of 30 to 33 feet long.
As a result, given the extreme depths involved in drilling (particularly offshore), which can reach almost 15,000 feet, different sections are usually connected with tool joints; one section of pipe will have a male connection, which is then paired with a female connection.
Note that drill pipes will have different thicknesses, which are specified. Examples include (but are not limited to) 2 ' inch thick drill pipe, 5 inch thick drill pipe, and 6 ' inch drill pipe.
Drill pipes used in the upper parts of the drill string differ in both thickness and construction from heavyweight drill pipe (HWDP), used to transition between drill collars and regular, thinner drill pipe. HWDP will be discussed below
Heavyweight Drill Pipe (HWDP)
Heavyweight Drill Pipes are, as mentioned before, much stronger than regular drill pipes, because they are designed to deal with the added pressures and stresses arising from serving as transitions between drill collars and regular, thinner drill pipe.
Generally, HWDP are preferred for transitions because they are more flexible than alternative, solid connections.
Note that such piping is thicker, ranging in diameter from 3.5 inches to 6 ' inches, weight almost twice as much lb/ft as regular drill pipe, and is often rated with tensile strengths ranging from 200,000 lbs to over 1,000,000 lbs.
Interestingly, drill pipes typically are equipped with a center upset, or thick wear pads that prevent the outside of the drill pipe from grinding against the rough bedrock of the hole walls. Mostly, center upsets are a desirable addition to heavyweight drill pipes, especially since by reducing wear, tear, and friction, they can prolong the lifespan of drill pipes.
With this in mind, remember that drill pipes, while they come in a variety of sizes, diameters, and materials, are divided into two broad types: regular drill pipe, and heavyweight drill pipe (HWDP), used as transitions. Generally, most pipe types can be bought new or used, and are graded according to three broad categories (New, or N; Premium, or P; and C, which has three subdivisions).
Oilfield equipment, such as drill pipes and OCTG, can be subjected to a great amount of stress while drilling. It is important that these items are inspected regularly by Field Service Supervisors to ensure their safety and performance.
Drill pipe is the hollow, thick-walled pipe that transmits drilling fluid and torque through the wellbore to the drill bit on the rig. It is manufactured to withstand severe internal and external pressure, deformation, bending and vibration when rotating and lifting the bottom hole assembly (BHA). Drill pipe is threaded at each end in a length of pipe called a tool joint. It is manufactured, sampled, tested and inspected to standard specifications such as the American Petroleum Institute (API).
type
Standard drill pipe is the long tubular section that makes up the bulk of the drill string. They are usually a 31-foot length of tubular pipe, but can be anywhere from 18 to 45 feet in length.
Heavy Duty Drill Pipe (HWDP) is a tubular tube that adds weight or acts as a transition piece in the drill string. As a transition section of the drill string, it is placed between the drill collar and standard drill pipe to reduce fatigue failure. In other applications, HWDP is used as additional weight to hold down the drill string.
The drill collar is an integral part of the drill string that forms part of the BHA. They are thicker, heavier and stiffer than drill pipe and are primarily used to reduce the weight of the drill bit while dampening vibration and shock forces.
Drill pipe grade
Drill pipe grades include standard API grades (E-75, X-95, G-105 and S-135) as well as proprietary grades.
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API grade drill pipe
API SPEC 5DP specifies three product specification levels (PSL-1, PSL-2 and PSL-3). PS-1 specifies material grade-specific wall thickness, impact strength, and yield strength requirements. Specification levels PL-2 / PL-3 have additional mandatory requirements.
Grade E drill pipe, referred to as \"mild\" steel, exhibits the lowest yield strength per unit area with a yield strength of less than 80,000 psi. It is able to withstand a greater percentage of stretch or \"strain\" prior to fracture than higher strength drill pipe grades. It is also more resistant to corrosion and cracking. Grade E is utilized in medium depth wells from 10,000 to 15,000 feet.
Grades X-95, G-105, and S-135 are considered high strength grades. They exhibit increased yield strength which is required to service deeper wells.
Proprietary Grades
Proprietary grades often exceed the specifications set forth by API SPEC 5DP. Their enhanced performance properties are developed for sour service, critical service, and other user-defined requirements.
Sour service grades resist sulphide stress corrosion (SSC). SSC can occur when hydrogen sulfide is present. Ingress of hydrogen coupled with higher stresses, low temperatures, low pH, and high chloride content decreases the ductility of steel grades leaving them susceptible to crack propagation and failure.
Critical service grades resist corrosion when sweet gas or high concentrations of carbon dioxide are present. They are a cost effective alternative that is used in water injection applications.
Sizing
Drill pipe is manufactured according to standard specifications and is offered in nominal sizes. The two most important dimensional specifications are length and diameter.
Length
The drill string is made up of several sections of drill pipe. The term \"stand\" refers to two or three sections of drill pipe that are fed into the well bore to complete 60 to 90 feet of drilling. Each segment of pipe, referred to as a \"single\", is classified by API into three distinct length ranges; R1, R2, and R3.
Range 1 (R1) is shortest in length, more common for sizing production tubing or casing, and ranges from 18 to 22 ft.
Range 2 (R2) is considered the standard length for drill pipe and ranges from 27 to 31 ft.
Range 3 (R3) is common in casing and also deployed in deep water drilling applications. The increased length decreases the number of tool joints in each stand of drill pipe. The fall back being that the load exerted on each tool joint is greater increasing wear and reducing the expected life of the drill pipe. R3 ranges from 38 to 45 ft.
Diameter
The outer diameter (OD) of the drill pipe is offered in nominal sizes ranging from 2 3/8\" to 6 5/8\". The diameter of pipe is selected based on the borehole diameter. In order to effectively circulate drilling mud and minimize pressure losses, the ratio of drill pipe outside diameter to borehole diameter should be about 0.6.
Tool Joint
Each end of the pipe incorporates a tool joint, and is differentiated by the male threaded connection referred to as the \"pin\" and the female threaded connection which is referred to as the \"box\". They are pre-fabricated and welded onto the pipe. The tool joints provide high-strength, threaded connections. They are heat treated to a higher strength than the steel of the tube body in order to survive the rigors of drilling and numerous cycles of tightening and loosening of threads. Tool joints are specified by upset and thread type.
Upset
The upset (thread-end finish) refers to the wall of the tool joint at the threaded connection. Drill pipe is offered with an internal upset (IU), an external upset (EU), or an internal-external upset (IEU).
IU - In an internal upset increased thickness along the inside walls compensate for the metal removed in threading with a uniform, straight outside wall.
EU - In an external upset the increased thickness along the outside diameter of the tubing compensates for the metal removed in threading with a straight bore.
IEU - In an internal-external upset thickness is increased along both the inside and outside walls of the pipe to compensate for the metal removed in threading.
Threaded Connection
Tool joints incorporate threaded connections. They include standard API threads as well as proprietary threads. Each type of threaded connection is specified by threads per inch (TPI) and it\'s taper. Common API tool joints include regular (REG), full-hole (FH), and internal-flush (IF).
Features
Non-magnetic drill pipe is used to isolate measurement while drilling (MWD) and logging while drilling (LWD) tools from the drill string. This minimizes associated electromagnetic interference and increases the accuracy of the directional surveys.
Hardbanding is incorporated on the tool joints and center wear pad of the drill pipe in order to increase the abrasion resistance.
Spiral grooves on the external surface of the drill pipe reduce differential sticking and improve flow characteristics of the drilling mud.
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