Drillstring and bha designs

9 Drillstring & BHA Design Habiburrohman abdullah Drill String Design • • • • Drill Pipe Pressure Control Equipment Drill String Loads Monitoring Equipment Bottom-Hole Assembly (BHA) Design • Purpose • Components • Assemblies: - Slick, Packed, Pendulum, Directional • Properties: - Weight, Stiffness Introduction • The drillstring design is the mechanical linkage connecting the drillbit at the bottom of the hole to the rotary drive system on the surface • The drillstring has several functions: - transmit rotation to the drillbit - exerts weight on bits (WOB) - guides & controls trajectory of the bit - allows fluid circulation Drillstring Components • The components of drillsting: Drill Pipe Drill Collar Accessories including: - HWDP - Stabilizers - Reamer - Directional control equipment Figure 1: Drillstring Components Drill Pipe Selection Table 1: DP grade and yield strength Grade • • Minimum Yield Strength, psi Letter Designation Alternate Designation D D-55 55,000 E E-75 75,000 X X-95 95,000 G G-105 105,000 S S-135 135,000 Only grade E, G and S are actually used in oilwell drilling API RP7G established guidelines for Drill Pipe as follows: - New = no wear, never been used - Premium = uniform wear, 80% wall thickness of new pipe - Class = 65% wall thickness of new pipe - Class = 55% wall thickness of new pipe Tool Joints • Tool joints are screw-type connectors that join the individual joints of drillpipe • All API tool joints have minimum a yield strength of 120,000 psi Washout in Drillstrings • Tool joint failure is one of the main causes of fishing jobs in drilling industry This failure is due entirely to the joint threads not holding or not being made properly Figure 2: Make Up Torque Washout in Drillstrings • Washout can also develop due to cracks develop within drill pipe due to severe drilling vibrations • Washout are usually detected by a decrease in the standpipe pressure, between 100 – 300 psi over – 15 minutes • The life of tool joints can be tripled if the joints if hardfaced with composites of steel and tungsteen carbide Approximate Weight of DP and Tool Joint • Nominal weight of DP is always less than the actual weight of DP and tool joint because of the extra weight added by tool joint and due to extra metal added at the pipe ends to increase the pipe thickness Figure 3: Tool joint dimension 10