Damage Mechanism Of The Casing String And Applied Research Of Full Casing Drilling Method

This paper is based on the Special R&D Program for Institutes by MOST—Research on Ф2000casingrotator, drilling tools and construction technology(2011-2013). Subject investigated is the damage andearly failure phenomenon of the casing and casing joint in casing drilling process, it is a weak link formanufacturers no matter domestics or abroad, and it is also the key to the question why Full casingconstruction technology unable to be spread.After deeply mechanical analysis of the key points such ascasing, casing joint and casing clamping device, the following conclusions can be drawn.1. Establishing the mechanic model of the casing string. It has concluded the calculation method offriction resistance for large diameter casing string by referring and pertinently correcting that of thesteel-pipe pile.2. Figuring out the position of the maximum deformation energy by fitting the function curve of theequivalent stress which is calculated by means of the plastic-elastic mechanics. This positionlocates at the1/3interval near the upper part of the casing string, it basically accords with theactual fracture position.3. Conducting finite element simulation of the Φ1200casing string, casing joint and locking taper pinwhich had went wrong in Shenzhen construction site. The simulation of the preload inequalitycondition proves that the joint and the taper pin are destroyed, the conclusion basically accordswith the actual situation. According to the results of simulation, it suggests that improving thematerial and doing heat treatment will ensure the service life.4. Comparing the clamping device of the casing oscillator with that of the casing rotator. It provesthat the clamping device of the casing oscillator will deform the casing; that of the casing rotator isreasonable for its uniform clamping force.5. Conducting finite element simulation of the Φ1500casing joint under the condition of radicalclamping. It proves that the taper pins occur severely stress concentration when the radicalclamping force is near the joint. This phenomenon especially obvious when parts of the taper pinsloosen.6. Research on systems of the constant bit pressure and torque and automatic chassis leveling play animportant role in avoiding casing shoes overloading and improving the verticality of the casingstring.The main innovation points:1. Making rectify of the friction resistance mechanic model of the steel-pipe pile for calculating thatof the casing string. It has rectify the principal of dividing parts, reduction coefficients of the soilcore in open-ended pipe and the vacuum effect in pulling casing according to the particularity ofcasing drilling. It the first time to propose the calculation method of friction resistance for largediameter casing string in complex stratums in domestic.2. The data for analysis of the casing string was collected in Shenzhen construction site. With actualdata as the basis, the mechanic model has been verified to be suitable, it has laid the foundation ofthe system for evaluating the casing friction resistance in complex stratums.3. The finite element simulation verified the effect that the radical clamping exerts to the casing joint.Mechanical analysis of the casing string in full casing drilling fills the gap of research on mechanic oflarge diameter casing, research on damage mechanism enrichs the theory for full casing drilling, hasthe very vital significance for R&D of casing rig and casing drilling tools.