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Drill String Design Calculations | Complete Guide

Drill String Design Calculations

Drill string design Calculations is made to help you selecting items from a list of ready-made Drill string components, not deciding what an individual component will look like. The objectives of our design efforts should be to:

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  • Keep the maximum stress at any point in the drill string less than yield strength derated by a design factor.
  • Select components and configure assemblies to retard fatigue as much as economically practical.
  • Provide equipment that is resistant to hydrogen sulphide if H2S is expected.

This article covers simple drill string design steps for vertical and directional wells. Loads applied by tension, torsion, combined tension and torsion, burst pressure, collapse pressure, slip crushing and stability forces are considered. Design steps to reduce fatigue damage are also covered.

There are many factors that must be considered in the drill string design such as:

Drill String Design Calculations Steps

Drill Collars Size & Weight Selection

Unless mechanical hole sticking is a problem, the largest diameter drill collars consistent with other needs are generally best. Their increased stiffness means more directional and drill bit stability. Also, they will have fewer connections for a desired weight on bit and therefore a shorter BHA which can lessen the probability of differential sticking.

Larger OD collars in a given hole also means less lateral freedom of movement in the BHA. This decreases buckling stress and the rate of connection fatigue. In practice however, drill collar size is often determined by existing rig inventory. Other factors which come into play are:

  • Fishability considerations.
  • Capabilities of the rig handling equipment.
  • Directional control requirements.
  • Hydraulics
  • Desired exterior features (spiral grooves, elevator grooves, or other features).

Read More about Drill Collar Size selection guide.

One of the drill string functions is to produce the sufficient weight to drill the hole so how many drill collars you will need to deliver the desired WOB. Mainly there are two methods used in drill string design for the Drill collar weight Calculation.

Drill Pipes Selection

The first thing to consider while running the drill string design calculation is to check if the drill pipes you are going to use will be able to sustain the the drill collars and BHA loads in the worst cases which is the stuck. So first thing will be to check your drill pipes.

Maximum Over Pull

Drill string design needs from you to check the maximum over pull you can apply to drill string without drill pipes failure. It is easy to be done just remember don’t exceed 80% of drill pipes yield strength.

Slip Crushing

Slips exert hoop compression on the drill pipe which can deform the pipe if conditions are unfavorable. It will be important in drill string design to check if slip crushing may happen to your drill pipes or not.

Drill Pipes Collapse Pressure

If the drill pipe is subjected to an axial tensile load, the collapse pressure ratings which will be used in drill string design, from the tables must be derated. So you will have to calculate the effective drill pipes collapse pressure.

Drill Pipes Burst Pressure.

The differential pressure acting across the drill pipe wall due to an internal pressure greater than the external pressure is known as the burst load. you will have to check the used drill pipes burst pressure.

Tool joint Torsional Strength 

Assume that the produced torque during drilling the well proceeds the make up torque of drill pipes tool joints or the torsional strength of drill pipes, then what will happen. So checking tool joints torsional strength and drill pipes torsional strength is important during the drill string design calculations.

Heavy Weight Drill pipes

The field tests showed that the higher the stiffness ratio at the transition zone the greater the fatigue build up. In Heavy Weight Drill Pipe Specs & Selection, we shall cover the following subjects:

  1. Heavy Weight Drill Pipes Definition
  2. Heavy Weight Drill Pipe Function
  3. HeavyWeight Drill Pipe Specs
  4. HeavyWeight Drill Pipe Specs Sheets From work strings international
  5. HeavyWeight Drill Pipe Specs Selection And Design
  6. Use of HWDP offers the following advantages

Select BHA Connections And Features

The following points apply not only to drill collars and Heavy Weight Drill Pipe from the rig inventory, but also to the many specialized tools that find their way into the hole. Stabilizers, Mud Motors, Measurement While Drilling and LWD tools, hole openers, under-reamers, Drilling jars and many other tools are all subject to fatigue

Bending Strength Ratio

In Bending Strength Ratio article, you shall discover its definition, ranges and you can download its calculator.

BHA Connection Thread Form  

Thread forms with full root radii should be used in all BHA connections to maximize fatigue resistance.   API Regular, NC and 5.1/2 Full Hole connections meet this requirement, though the API NC thread form (V-038R) is superior to the others. The H90 thread form is also considered acceptable even though it does not have a full root radius.

BHA Connection Thread Form   drill string design

Stress Relief  Features:

Stress relief features as described in API Spec 7 , should be specified on all BHA connections NC-38 and larger. These features include the “stress relief pin” and “boreback box”. They extend connection fatigue life by eliminating on-engaged thread roots which act as stress concentrators.  

Stress relief features are beneficial on HWDP connections.  Pin stress relief grooves are not recommended for connections smaller than NC-38 because they may weaken the connection’s tensile and torsional strength and because fatigue is often less of a problem than static loads on small connections. Boreback boxes could be used on smaller connections without weakening them and should be considered if box fatigue is occurring.

Cold Rolling: 

Cold rolling BHA thread roots and stress relief surfaces increases fatigue life by placing a residual compressive stress in the thread roots.  Cold rolling is also beneficial on HWDP threads, though not on normal weight drill pipe tool joints.   Fatigue is rarely a problem on normal weight drill pipe tool joints owing to the relative stiffness of the tool joint compared to the tube.

BHA Connection Torsional Strength:  

Since torsion is transmitted from the top down, BHA connections are usually subjected to lower torsional loads than the connections above.  However, if “stick/slip” is occurring, or if you are running a tapered or “slim” assembly, especially one using PAC connections, torsional strength should be checked to confirm that it is greater than the expected operating torsion at the BHA.  

Tool joint torsional strength tables cannot be used directly for this purpose because tool joint and drill collar materials have different yield strengths. However, drill collar connection torsional strength can be calculated by the following formula:

Torsional Strength In drill string design

Where:

TS=DC connection torsional strength (ft-lb)
MUT=DC makeup torque (ft-lb) (O.C.T.G. Procter Drill String Design Manual)
f=The factor “f” is simply the decimal fraction of torsion yield strength that forms the basis for the makeup torque value . ( It is different according to Pipe Manufacturer)

For OCTG below table can be used

Thread StyleDrill Collar OD£6.7/8 inchesDrill Collar OD > 6.7/8 inches
PACf = 0.795N/A
H-90f = 0.511f = 0.562
Otherf = 0.568f = 0.625

Stabilizer Placement

In Drilling stabilizer Types, Design & Application article, we shall explain its types and application. The size and placement of stabilizers is often determined by directional considerations ( please visit visit Types Of BHA & Rotary Directional Drilling Bottom Hole Assembly to fully understand directional drilling considerations). However, stabilizers also impact other important design concerns:

In a vertical hole, the lower part of the BHA will be buckled and supported by the sides of the hole. Stabilizers reduce connection stress by restricting the freedom of lateral drill collar movement.   This lengthens connection fatigue life, other things constant.

If mechanical sticking is a concern, more or larger stabilizers may increase the chance of becoming stuck.  On the other hand, stabilizers can reduce the probability of differential sticking by holding drill collars from the side of the hole.

Jar Placement

In Drilling Jar Types & Placement article, we shall explain its types and best practice for its placement in drill string deign. Below the main article headings:

  1. When We Start Thinking About Using Drilling Jar
  2. Positioning & The Drilling Jar Placement In BHA:
  3. Drilling Jars Types
  4. Drilling Jar Operations
  5. Drilling Jar Calculations

Bottom Hole Assembly on Drill String design

The final step in vertical hole drill string design calculations is to select the proper Bottom hole Assembly according to many aspects.

In Bottom Hole Assembly Components, Types, Behavior  article, we shall cover the following subjects:

  1. Main Categories Of Bottom Hole Assembly BHA Used In Oilfield
  2. Bottom Hole Assembly BHA Drilling Components:
  3. How Drilling BHA Act / BHA Behaviour
  4. Rotary BHA Types

Directional Drilling String Design for Deviated wells

In drilling inclined and horizontal wells there are two additional factors, which are not present in vertical wells, that must be considered (1) The frictional forces between the drill string and the hole (2) The ability to use the drill pipe or HWDP to provide weight on the bit without buckling.

The following Subjects Shall Be Covered In Directional Drilling String Design for Deviated wells Article:

Drill String Design for High Angle and Horizontal Wellbores

The objectives of drill string design in horizontal wells are:

  • Provide adequate weight on bit without buckling the drill pipe or heavy weight drill pipe.
  • Ensure that the components in the drilling assembly are not subjected to mechanical loads that exceeds their design limitation

In Drill String Design Of Horizontal Wells Article, We shall cover the following:

  1. How Drill string Design In Horizontal Wells Differs From In Vertical Wells
  2. The objectives of drill string design in horizontal wells are:
  3. The maximum weight on bit without buckling the drill pipe can be calculated in two steps:
    • Calculate the maximum weight that can be applied on the bit without buckling the drill pipe below the tangent point
    • Calculate the maximum weight that can be applied on the bit without buckling the drill pipe above the kickoff point.
  4. Calculation of Axial Mechanical Forces In High Inclination / Horizontal Wells
  5. Summary for high angle wells Drill String Design