Design and Modelling of Linear Permanent Magnet Actuator with Gas Springs for Offshore Applications
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- Institutt for elkraftteknikk 
Although rotary steerable systems have been evolving rapidly and delivering unprecedented rates of drilling penetration, drillers nevertheless seek technologies that further accelerate the drilling process. Currently, most down-hole motors with rotating drill bits are driven by positive displacement motors and down-hole turbine motors. These motors are classified depending on the type of power sections that are used to convert the hydraulic energy of high pressure fluids to mechanical energy in the form of torque output. In these designs, the power is transmitted from the power section to the drill bit through different mechanical arrangements, such as drive shafts, coupling assemblies, bearings, etc. During the transmission process, part of the developed torque is wasted, which reduces the efficiency. In addition, bearing stresses limit the transfer of possible forces to the drill bit. We proposed a novel concept to overcome these difficulties. The proposed concept is unique because of the combination of a linear electrical machine and gas springs. These gas springs allow the piston of the linear electrical machine to oscillate at different frequencies depending on the required load conditions. The characteristics of the high energy gas springs provide the opportunity to oscillate heavier piston with longer stroke lengths. A large electromagnetic force from permanent magnets and coils, in combination with gas springs, creates a high power linear electric actuator. The oscillating heavy piston causes a considerable vibration in the housing, which is utilized in the drilling application. The load can then be attached directly to the housing and the machine can be made hermitically sealed, which eliminates leakage problems. These features make the actuator suitable as a hammer in oil drilling applications. A linear permanent magnet actuator (oscillomotor) is an energy conversion device that integrates a linear permanent magnet (PM) machine and gas springs into a single unit. My research work mainly addresses the design criteria, analytical and finite element method (FEM) design calculations and dynamic analysis of the linear permanent magnet actuator with gas springs. The selection of a suitable linear PM machine configuration is very important to achieve the required demands, such as a compact diameter size, easy integration of gas springs, reduced losses to reduce heat generation, robustness, less maintenance and higher force from the starting position. Different linear PM machine configurations, such as flat, double flat, tubular longitudinal and transverse flux, are discussed for drilling applications. The tubular longitudinal flux configuration is selected as the most promising for drilling applications. This study analyses the selection of different mover configurations and different magnet patterns. Design parameters such as pole pitch, magnet width, stator inner diameter and air gap size are optimized based on the analytical analysis of force characteristics. The complete design process of the linear machine is discussed. Dynamic models of the linear actuator are developed. Three prototypes of the linear actuator are built to verify the novel concept and validate the force characteristics and dynamic modelling. The Force characteristics of the prototype I is verified with the FEM simulated results. Impact tests are carried out using prototype III. The test results shows the power input required at resonance is very low compared to the other frequencies. The impact force per amp at resonance is 9.26kN. The possibility of producing these higher power impacts at resonance, suitable for hard rock drilling. Drill test is carried out using prototype II. The Effect of feed force on the dynamic behaviour is discussed. The rate of penetration with different resonant frequencies and different weight on bits are discussed. This study also focuses on the suitability of this novel concept in other offshore applications, such as wireline jar applications and ocean wave power generation. Henceforth, this thesis describes the concept of the conversion of the movement of a float in a wave power plant, which is characterized by high force and low speed energy, to a high speed and low force energy in a linear generator by using gas springs. The simulation model shows that an oscillating piston that is supported by gas springs and is compressed and decompressed by a wave can convert more kinetic energy per volume from the float to a secondary piston compared to a secondary piston that is simply attached to the float. This model has not yet been validated with experimental work. According to the simulation results, the size of the generator can be reduced by a factor of 12 for the same power output compared to a buoy-connected piston.