A Case Study on Setting Up an Auto-Driller to Prevent Instability

Auto-drillers are widely used in the drilling industry to regulate the process by maintaining key drilling parameters such as Weight On Bit (WOB) and Rate Of Penetration (ROP). A lack of understanding of an auto-driller and its control parameters can result in drilling dysfunctions and process instability. This paper is a case study on how to properly setup and apply an auto-driller presented with offshore drilling data. Most common auto-drillers have two main modes namely WOB and ROP. An operator can select the desired mode of operation, or in some auto-drillers, the mode switches automatically based on setpoints or limits. Inappropriate setpoint(s) and/or controller gains for the given formation coupled with the process limits can result in instabilities. This paper highlights the importance of understanding the auto-driller's functionality, setup, and its application for achieving good regulation. In the event of varying formation or the presence of heave, it is difficult to achieve good WOB regulation using an auto-driller in ROP mode. The case study shows that ROP is highly influenced by the rig heave and WOB oscillates with the heave motion when ROP mode is applied. In WOB mode, an auto-driller adjusts block speed to regulate WOB. If the resulting block speed is limited by the operator set process limits, WOB regulation is lost. Therefore, an operator needs to understand the interrelation between WOB setpoint, resulting block speed, process limits, and controller gains. An auto-driller with extreme controller gains against a formation and/or in combination with a heave can also result in irregular WOB regulations. Non optimal controller gains can sometimes be suitable for given conditions and still give satisfactory regulation. The study highlights different scenarios where an auto-driller can be limited due to improper settings resulting in instability, inefficient drilling, increasing probability of bit damage, axial vibrations, and thereby stick-slips. The paper suggests the correct practice of using an auto-driller.