Home » Drill String » Drill String Definition, Components Complete Guide

Drill String Definition, Components Complete Guide

Drill String Definition

We can define the drill string as it is a group of pipes and tools. These components allow us to deliver the rotation, the weight, and the fluids from the surface to the bottom of the drilled hole. We can say that it is an important part of the rotary drilling process in oil and gas wells. To understand more about it we will have to go through two subjects :

  1. Its Components
  2. Its Design  

Drill String Functions

Drill string components serve several purposes and functions which include the following:

  • The drill string Provides a fluid conduit from the drilling rig (drilling rigs types) to the Drilling Bits.
  • Also, it had another function which is to impart rotary motion to the Drilling Bits.
  • Because of the existence of drill collars, it will provide the Weight On Bit (WOB) necessary to drill the hole.
  • Lower and raise the Drilling Bits in the well.
  • Drill String functions are a lot more but it depends on what you need to do like it can allow formation evaluation and testing but this will be done by special tools for such application.

Drill string components

drill string components diagram

Its components falls under three main topics:

  1. Bottom Hole Assembly
  2. Transition pipe, which is often heavy weight drill pipe (HWDP)
  3. Drill Pipe

Bottom Hole Assembly:

The Bottom Hole Assembly (BHA) is one of the drill string components which consist of a Drilling Bit, which is used to break up the rock formations; Drill Collars, which are heavy enough to apply weight to the drill bit; drilling Stabilizers, which centralize the drill string in the hole; Reamers, Which is used to enlarge the drilled hole for certain reasons; Jars, which used to free any stuck may happen during drilling; Shock subs, which used to reduce the vertical oscillation (bouncing) of the drill string. The Bottom Hole Assembly may also contain other components such as a downhole mud motor and rotary steerable system (RSS), measurement while drilling (MWD), and logging while drilling (LWD) tools. The components are joined together using rugged threaded connections. Short “subs” are used to connect items with dissimilar threads.

Transition Pipes:


This component is mainly heavy weight drill pipe (HWDP). In the past, the two main components of the Drill String consisted of the Pipes and the Collars. The point where the relatively small OD and flexible Drill Pipe connects to the large stiff Drill Collars is called the Transition Zone.

Oilfield studies have shown that almost all the Drill Pipe fatigue failures are the result of an accumulation of fatigue damage occurring when the Drill Pipe joints were run in the Transition Zone or were stressed above the endurance limit in crooked holes. Downhole data has indicated that the large change in diameter at the transition zone caused accelerated fatigue damage as a result of the concentration of cyclic bending stress reversals in the bottom joints of the flexible Drill Pipe since the stiff Drill Collars bend very little from these stress reversals.

Drill String Component | Drill Pipes:

The longest component of the Drill String is the D/Ps. Each joint of the drill pipe consists of the tube body and the Tool Joint (connection). Drill pipe joints are available in three length ranges:

  • The length of rang 1: 18-22 ft
  • The length of rang 2: 27-30 ft
  • The length of rang 3: 38-45 ft

Drill Pipes Range 2 is the most common.

Drill pipe component is generally described by tube OD, nominal weight, pipe grade, type of tool joint, thread connection, and classification. For example, a 5″€ drill pipe can be described as,

5″19.5Grade EXHNC50Premium Tube
ODNominal WeightMinimum Yield StrengthTool Joint UpsetConnection ThreadWear Classification

Each description item is explained in API DRILL PIPES DESCRIPTION & SPECS Article.

Drilling String Design

The drill string components should be designed to deliver sufficient weight to the Bit and provide sufficient torsional and tensile strength to withstand the vigorous and dynamic conditions of drilling. The components should also withstand burst and collapse pressure loads and be designed to minimize hole stability problems. There are many factors that must be considered in its design such as:

  • Total depth
  • Hole size
  • Mud weight
  • Over pull
  • Bottom Hole Assembly
  • Hole angle
  • Pipe weights and grades
  • Corrosive environment
  • Ability to fish tools out of hole

If you would like to have more details about the design you will find it in the DRILL STRING DESIGN Article.