해양시추선용 수밀댐퍼 시스템 개발에 관한 연구

Abstract

Semi-submersible drilling rigs are offshore plants that perform function such as ocean exploration for oil and gas acquisition, drilling and production, storage and unloading of crude oil and gas. In recent years, drilling and production operations for the development of offshore oil and gas have been radically shifting to the deep sea. Semi-submersible drilling rigs consist of pontoon, which is installed at the lowest part to create buoyancy and column which connects upper deck and pontoon. Because the column consists of several legs, semi-submersible drilling rigs can receive less waves and ocean currents, so that they have high stability characteristic. Nevertheless, the drilling rigs’s outer hull is damaged due to an irresistible accident semi-submersible drilling rigs often sink. Therefore, in this study, Emergency Safety Shut-off Damper(ESSD) has been developed and installed in pontoon to column area ventilation ducts of semi-submersible drilling rigs. In other words, It is intended to ensure buoyancy in the column during an emergency so that the drilling rigs do not sink any more. In chapter two, this study has designed and evaluated the safety of watertight damper for offshore rigs. The watertight damper is an improved design for conventional DN 350 butterfly valve. Numerical analysis has been performed to investigate the safety factor and seat leakage of the designed 3-type disk dampers. The structure analysis results present the deformations, the equivalent stresses and the safety factor. It was confirmed that the designed disk, rib-disk and streamlined disk of watertight damper are safe enough in maximum operating pressure of 0.98MPa. The results show that the disk damper had the smallest maximum deformations and stress among 3-type disk dampers and the safety factor was 4.3. Therefore it is confirmed that the disk damper has the most excellent strength. Also linear static structural analysis was individually conducted for disk and body. The results show that the maximum disk deformation was larger than the body deformation, Therefore the disk damper is causes no seat leakage.

In chapter three, the FSI(Fluid-Structure Interaction) analysis has performed to investigate the safety factor for the watertight damper. When watertight damper of disk was closed, the disk of pressure value is constant. However depending on the opening angle of disk, the flow velocity and pressure are changed. The maximum velocity was appeared at the end of disk on the small outlet area of duct. When the opening angel of disk is 90°, the maximum velocity was appeared at the center of ending disk. So we were found the opening angle of disk is bigger, the flow rate is increased and velocity is also increased from the result of FSI analysis. We can find the least deformation and stress when the opening angel of damper is 90°. When the 45° opening angle of disk, the largest deformation and stress was found and the minimum safety factor 1.3 was calculated. As a result, we found that the structure of watertight damper is safe enough irrespective of opening angel.

In chapter four, semi-submersible drilling rigs use watertight dampers as emergency buoyancy holders. Since the watertight damper is an emergency shutoff device, it is mainly driven by a pneumatic driving system that can operate without power supply. The pneumatic driving system has highly non-linear characteristics due to compressibility of air and external disturbance such as static and coulomb friction. In this paper, a new control algorithm is proposed for a watertight damper driving system based on the sliding mode control with disturbance observer. To evaluate control performance and robust stability of the designed controller, the control results were compared with the results obtained using the state feedback controller. As a result, it was confirmed that the pneumatic driving system for driving the watertight damper using the sliding mode controller can obtain excellent control performance against the parameter changes and non-linear characteristics. The effectiveness of the sliding mode controller is confirmed through simulation and experiment.