Stability Analysis Of Front Open - Out Derrick Of Deep - Water Semi - Submersible Platform Drilling Rig

Compared to the land based derrick, the derrick of drilling rig in floating platform not only subjects to normal operating load, but also subjects to the inertial load caused by floating platform’s motion acceleration, the drill string’s additional dynamic load caused by platform heaving, the transverse component of gravity due to the tilt of platform, the ocean wind load and so on, which worsens the stress state of the derrick. At present, the design standards of floating platform drilling rig were not drawn out at home and abroad, designing and manufacturing enterprises only choose one level higher component parts than the land rig to support floating platform drilling rig, such designing considerations lack scientificity. Here, basing on the motion response of a deep-water semi-submersible platform in the South China Sea, stability of the matching K-type dynamic offshore derrick is investigated in various sea conditions and working conditions of its experience. The findings would provide theoretical bases for design of the drilling equipment in floating platform.1) Considering the platform’s motion response, the ocean environment load and operating load of drilling rig, stress distribution of the derrick of drilling rig in floating platform in various sea conditions and working conditions is systematically analysed, which provides the load condition for the research on the stability of derrick.2) The derrick is a latticed beam column with variable cross section, through equivalent to the critical load of columns under axial pressure, the component with variable cross section is transformed into the same height of members with uniform cross section by defining equivalent moment of inertia; on this basis, including lacing shear deformation, the derrick is transformed into the same height of piecewise solid cantilever beam with uniform cross section, which is used to the theoretical analysis model of stability.3) In-plane stability model of derrick which subjects to complex loads is established by means of energy method. Then the most dangerous condition of instability of derrick is determined through comparing the deformation of the derrick under various sea conditions and working conditions; In this condition global stability of the derrick are simulated to study stability reserve and to get the critical load of instability. At the same time in-plane deformation of the derrick are calculated by using finite element software to verify theoretical calculation method.4) The strength of derrick under various sea conditions and working conditions is calculated by finite element method, its results of internal force are regarded as the load condition of local stability of derrick and the front-limb stability; The stability model of each bar and front-limb is established to study the stability reserve by buckling analysis; The buckling critical load and corresponding buckling mode of the derrick are determined through the overall buckling analysis; The optimization approach of derrick is put forward from the angle of stability.