Drill String dynamic behaviour during the oil drilling operation, was a major source for the failure of the Bottom Hole Assembly (BHA). The behaviour produced torsional vibration, which underpins the stick slip phenomena. Besides threatening the safety of the oil drilling process, such failure cause interruptions in the drilling operations and incurred high maintenance cost to the oil drilling company. This issue can be resolved with the implementation of the optimum control mechanism while operating the drill string. In this research, an optimum control mechanism was proposed to suppress the torsional vibration as well as mitigate the risk of stick slip phenomenon from occurring. The mechanism was proposed through a series of rigorous research strategies i.e. updated-mathematical equation modelling, experimentation and simulation. As the first step, a mathematical equation model describing system dynamics was derived to set the parameter of investigation. Representing the freedom torsional of the two degrees - conventional vertical drill string, the model was used to predict the frictional Torque On Bit (TOB) through non-linear friction force, denoting the ground-formation behaviour during drilling activity. Using a velocity feedback system, the drill-string oscillation was reduced while gradually increasing its velocity via gain scheduling method - allowing fast response to load disturbance. To avoid the motor torque from exceeding the maximum threshold, a Weight On Bit (WOB) was introduced. This approach remarks the novel contribution of this research. Next, an experiment on the preliminary test rig within a controlled laboratory set up was conducted. The rotary drill rig was assembled to identify the dynamics (i.e. parameters) of an individual part of the drill string. The results obtained were then applied in the drill string operation experiment, to identify the optimum control mechanism that can avoid the torsional vibration. To enable triangulation of results, a simulation was conducted by applying the same parameters obtained from the test rig experiment in the model- which is the optimum control mechanism that was proposed in this research to minimise torsional vibration, as well as reducing the chance of drill-string failure due to stick-slip phenomenon.

Inom tillverkning är skärande bearbetning en vanligt förekommande process och däribland borrning.Verktygskostnader har stor påverkan på den totala tillverkningskostnaden då till exempel ett heltarbetsstycke i värsta fall kan gå förlorat vid verktygshaveri. Genom sitt arbete med ständigaförbättringar håller Scania sig konkurrenskraftiga med sina produkter av hög kvalitet. I dettaexamensarbete inom masterprogrammet Industriell Produktion på KTH har en specifik typ av borrmed löstagbar borrkrona utvärderats med avsikt att undersöka nya fabrikat.Uppdragsgivaren var motorbearbetningen (DM) på Scania i Södertälje. Arbetet inleddes med attinförskaffa relevant kunskap gällande verktyget som undersöktes. Samtidigt utfördes ennulägesanalys där de använda borrarna på DM kartlades tillsammans med information kring pris,livslängd, m.m. Utifrån kartläggingen sammanställdes statistik som analyserades för att identifierahur stor påverkan en eventuell standardisering skulle kunna ha.Utvärderingen av borrar bestod av praktiska experiment där sju olika verktyg med en diameter på12,2 mm testades under samma förhållanden. Testet utformades i enighet med berörda parter påDM samt efter studier av adekvat litteratur rörande borrexperiment. Fyra parametrar testadesutöver borrarnas livslängd. Dessa var axialkraft, förslitning, håldiameter och spånor. Parametrarnaämnades jämföras och analyseras för att dra slutsatser kring borrarnas prestationer.Nulägesanalysen visade att ett fabrikat utgjorde hela 69% av borrarna på hela DM. Efter slutfördaborrtester hade enbart tre av sju genomfört utan haveri, Borr E, F och G. Samtliga sju borrar visadegodkända spånor och höll sig inom toleranserna för angiven håldiameter. Axialkrafterna varierademellan borrarna där Borr F hade lägst krafter och Borr A högst. Borr E visade bäst motstånd motförslitningar. Utifrån detta valdes borrarna E, F och G att rekommenderas för deras goda prestationermed Borr E på första plats. Utöver det föreslogs att utbudet av fabrikat som kan användas på DMbegränsas för att uppnå kostnadsbesparingar gällande verktygen. / Drilling is a common cutting process within manufacturing. Tool costs have great influence on thetotal manufacturing cost since a tool break down can cause an entire work piece to be scrapped.Through their constant work with improvements, Scania stays competitive with their high qualityproducts. This Master Thesis in Industrial Production and Management at KTH has evaluated aspecific type of drill with indexable heads with the purpose of investigating new brands.The Engine Manufacturing department (DM) at Scania in Södertälje requested this project. The taskwas initiated by obtaining knowledge about the tool that were to be investigated. Simultaneously ananalysis of the current state was performed where the drills at DM were mapped out withinformation regarding price, tool life, etc. The statistics were compiled and analysed to identify theeffect of an eventual standardisation.The evaluation consisted of practical experiments where seven different drills with a diameter of12,2 mm was tested under the same circumstances. The test was design in concurrence with affectedparts at DM and studies of adequate literature regarding drilling experiments. Four parameters weretested in addition to the tool life of the drills. These were axial force, tool wear, hole diameter andchips. The parameters were intended to be used for comparing and analysing in order to makeconclusions regarding the performance of the drills.The current state analysis showed that one brand represented 69% of the drills used at DM. Afterfinished drilling tests three out of seven drills managed to complete the test without tool breakdown. All drills produced chips that were acceptable and the measured diameters stayed within thegiven tolerances. The axial forces varied between the drill where Borr F had the lowest force and BorrA the highest. Borr E showed the best resistance against tool wear. From the results the drills thatwere recommended were Borr E, F and G because of their good performances with Borr E in the firstplace. Ultimately it was suggested to limit the amount of brands used at DM to achieve cost savingsregarding the tools.