Stick-slip is a dysfunction of rotary drilling, characterized by large cyclic variations of the drive torque and the rotational bit speed. It is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. This paper presents a new system for curing and preventing stick-slip motion. Like other competitive systems it fights the stick-slip oscillations by smart control of the drive. But in contrast to other active systems it does not use any kind of torque feed-back, not even the motor current. Fundamentally, the system is a PI-type speed controller that is tuned to effectively dampen torsional vibrations at the observed stick-slip frequency. In addition to automatic tuning of the speed controller the system includes a suite of support functions, such as automatic determination of the stick-slip frequency, estimation of the instantaneous bit rotation speed and calculation of the stick-slip severity, defined as the normalized downhole speed amplitude. All software is implemented in a standard programmable logic controller (PLC). The paper also includes results from a field test and from Hardware-In-the-Loop (HIL) simulation tests. In the latter tests utilizing the HIL, the software runs on a PLC communicating with an advanced real-time computer model for the drive and the drill string. HIL testing has proved to be cost efficient because the PLC software can easily be tested over a wide range of different downhole conditions that rarely occur in the field. Introduction The stick-slip phenomenon has been studied for more than two decades and it is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. The problems are closely related to the high peak speeds occurring during in the slip phase. The high rotation speeds in turn lead to extreme accelerations and forces, both in axial and lateral directions. A large number of papers and articles have addressed the stick-slip problem and a selection is included in the reference list. Many authors (Kyllingstad and Halsey 1988; Dufeyte and Henneuse 1991; Brett 1992, Pavove and Deplans 1994; Fear et al. 1997; Shuttleworth et al. 1998, Robnet 1999) focus on detecting stick-slip motion and on controlling the oscillations by operational means, such as adding friction reducers to the mud, increasing the rotation speed or reducing the weight on bit. Even though these remedies sometimes help, they are either insufficient or they represent a significant increase cost. Some papers also recommend applying smart control of the drive to dampen and prevent stick-slip oscillations. Halsey et al. (1988) demonstrated that torque feed-back from a dedicated string torque sensor could effectively cure stick-slip oscillations by adjusting the drive speed in response to the measured torque variations. As pointed out by Jansen and Steen (1995) the drawback of this approach is the need for a new and direct measurement of the string torque. They show that an alternative feedback based on available motor current and speed can be used. Their method has been patented (Warrall et al 1992) and the system, which is called Soft Torque Rotary System, has been commercially available for many years. Sananikone et al. (1992) and Pavone and Desplans (1994) also mention the use of a feed-back system for controlling torsional vibrations, but they give little or no details on how to do it.

Stick-slip oscillations are self-sustained and periodic twist and torque oscillations of a rotating drill string, characterized by large and harmful variations of the downhole rotation speed. This paper is a field evaluation of an active stick-slip prevention system.The evaluated system is active with smart control of the top drive, meaning the top drive speed is varied in a way that dampens stick-slip oscillations. It is software based, in many cases it requires no extra instrumentation and can be implemented on virtually any types and brands of top drives.The paper includes field test results, both from ordinary tests with surface data on top drive speed and torque, and from special tests including downhole measurements. The field data verified existence of the expected 2 nd mode stick-slip in longer strings and proved that the system is able to reduce the downhole speed variations. Therefore a general conclusion is that the active stick-slip prevention system significantly lowers the critical rotation speed below which stick-slip oscillations persist. Simultaneous surface and downhole measurements indicate that the reduction of stick-slip oscillations improved drilling performance and the rate of penetration (ROP).The positive field test results are good news to the drilling industry that has struggled for a long time with harmful stickslip oscillations, causing premature tool failures, excessive bit wear and poor drilling rate. Installation and use of an active stick-slip prevention system is therefore a very cost effective solution to a long outstanding problem.

Hardware-In-the-Loop (HIL) simulation is a technique where machine control software runs virtual machines, which are mathematical models emulating the physical machines. The technique has become increasingly popular in recent years, because of its ability to shorten product development time for complex dynamic systems, through quick and cost efficient testing over a wide range of conditions. This paper focuses on the lessons learned from HIL simulation testing of an advanced and patent applied method for curing and preventing stick-slip oscillations in drilling. In this project the HIL simulations have provided several advantages, such as a high repeatability in test conditions, the ability to test under challenging conditions rarely occurring in the field, thus avoiding time consuming field tests. The HIL simulations have also proven to be an efficient tool for debugging the control software, because programming errors are discovered long before the commissioning and field test phases. The HIL simulation of stick-slip oscillations uses an advanced dynamic model for the entire rotary system, including the top drive electronics, the motors, and the drillstring. The drillstring is modeled as a series of lumped inertia and torsional spring elements where the friction torque for each element is a non-linear function of the wellbore contact force and instant rotation speed. The lowest element also includes the bit torque as a function of weight on bit. The first version of a stick-slip prevention system, which has been described in an earlier IADC/SPE paper, was HIL tested over a wide range of drillstring and well geometries. Simulations with very long strings (typically 5000 m or longer) revealed that higher mode stick-slip oscillations tend to appear when the normal stick-slip oscillations are cured. The second mode frequency is roughly three times the frequency of the fundamental mode so it falls outside the effective absorption band for the first version prevention software. However, after adding an inertia compensating term in the tuned speed controller, the stick-slip prevention software was able to prevent both the fundamental and the higher modes stick-slip oscillations at the same time. In summary, the HIL simulations improved the stick-slip prevention system so it is more robust, easier to operate and has improved performance. The HIL simulations also helped to find and eliminate code bugs.

This paper presents a new and patent applied method for detecting and localizing valve leaks in reciprocating mud pumps. The detection method is based on the observed fact that a leak flow through a closed defective valve generates strong and high frequency vibrations in the fluid module housing the valve. These vibrations are picked up by accelerometers and continuously processed and monitored by a computer. A timer signal is used for constructing phase based windows that selectively pick the acceleration signal during the valve closing phases. The vibration strength is then calculated for each valve closing phase and normalized through division by the median value. A leak alarm is triggered if one or more valves have an excessive value. The presented system, called the Leak Detection System, was originally developed for Hex pumps having six pistons driven by a rotating asymmetric cam. Later on the method was also tested on shaft driven piston pumps, such as triplex pumps and quintuplex pumps. These tests revealed that these pumps require a modified and slightly different method to reliably detecting and localizing valve leaks. The reason is that these pumps have a much higher vibration transfer from the leak source to another valve. This is especially true for quintuplex pumps having only one integrated valve block. The Leak Detection System basically consists of a proximity sensor, accelerometers and suitable hardware and software for reading and analyzing the sensor signals. The main advantages of the new system are 1) high sensitivity and early leak detection, 2) reliable leak localization, 3) remote diagnosis with no human exposure to the hazardous environment near running pumps, 4) easy retrofit to existing pumps. The paper includes examples from lab and field tests where the new system has been successfully tested.