Saturday, 4 November 2017


Drill String Design Calculations

Drilling String Design
A drill string should be designed to deliver sufficient weight to the Drilling Bit and provide sufficient torsional and tensile strength to withstand the vigorous and dynamic conditions of drilling. The drill string 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 the drill string 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

Introduction To Drill String Design: 

To full understand the steps for the drill string design calculations, you will have to get in details on the drill string components including the design of drill pipes specification and drill pipes tool joints selection.

Drill String Design Calculations Steps

Drill Pipes Selection

drill string design - 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 Maximum Overpull 
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

 drill string design 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

drill string design 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.

 drill string design - 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 

drill string design calculations 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.

Drill Collars Size Selection

On which basis you are going to use bigger or smaller Drill collar. Next step to select the proper drill collar size for your drill string design.

Weight On Bit (WOB) Calculations

 drill string design drill collars weight 
One of the drill string functions is to produce the sufficent 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 lengths calculations.

Heavy Weight Drill pipes

 drill string design - heavy weight drill pipes selection 
The field tests showed that the higher the stiffness ratio at the transition zone the greater the fatigue build up. so to complete the drill string design you will have to calculate the stiffness ratio.

The Final step 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. Go in deep details on Bottom Hole Assembly Components, Types, Behavior 

Directional Drilling String Design for Deviated wells

directional drilling drill string design 
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. Go through Directional Drilling String Design for Deviated wells to have more details.

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
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