Drilling Vibration Measurements on a BOP Stack

Abstract

ABSTRACT Measurements of drilling vibration on a BOP stack indicate that significant vibrations may occur during floating drilling operations. Results of these measurements will be useful in developing quantitative vibration specifications for electronic packages to be mounted on BOP stacks. Triaxial vibrations were measured both in normal and abnormal conditions during a floating drilling operation in the Gulf of Mexico. Under normal conditions, the maximum peak-to-peak acce1erationwas less than 3g (g = 32.2 ft/sec2) and the dominant frequency ranged from 250 to 600 hz. Under abnormal conditions (loose rotary table bolts), however, the maximum acceleration increased to 14g and significant energy was found at frequencies up to 10,000 hz. Drilling one 8000-ft well in a soft formation is about equal to a relatively easy-to-meet military vibration specification for a 1000-mi1e overland truck trip. INTRODUCTION As it is impractical to structurally analyze numerous components to assess possibly damaging effects of environmental vibrations, it is common practice to conduct accelerated vibration tests on the packages to check their adequacy over an expected service life. This vibration test requires measurements of the magnitude and frequency contents of environmental vibrations. To define the drilling-induced vibrations on a BOP stack, three accelerometers were, used to measure the triaxial accelerations resulting from a floating drilling operation. Figure 1 shows the orientation of the three perpendicular axes relative to the ship and Appendix A gives details of the measurement system. The measurements were tape-recorded and subsequently processed to give statistics and power spectra for drilling vibrations. Nature of Drilling Vibrations In the field test program, drilling vibrations occurred as bursts as a result of the drill pipe slamming against the casing. A burst starts out large and decays exponentially before the next burst starts. Figure 2 shows a typical vibration on the acceleration axes during normal drilling operation while rotating the drill string at about 150 rpm. To characterize such vibrations, the mean rectified acceleration, A, over 125 milliseconds was chosen as an intensity parameter. (See Appendix B for details of data processing.) The maximum acceleration amplitude, Amax inside the burst is related to A, and vibration spectra show frequency contents. The overall drilling vibration environment is characterized by a cumulative distribution of A. RESULTS OF DATA INTERPRETATION Normal Drilling Figure 1 shows an expanded vibration burst in the transverse axis while drilling under normal conditions. At the onset of the vibration, there are some high-frequency components that decay very fast, leaving only lower frequency components. The average zero-crossing frequency of this burst is 460 cycles. The maximum peak-to-peak acceleration within the burst is about 3g. Figure 4 shows the corresponding power spectrum for the burst and the dominant frequency of 275 cycles. This dominant frequency lies within the range of 100 to 1000 hz; the resonance frequencies of typical electronic components soldered on a printed circuit board occur in this range. Accelerations in the other two axes are similar to Figures 3 and 4, with dominant frequencies from 250 to 600 hz.