The conventional way of estimating WOB from surface data is to compute the difference between off- and on-bottom hookload. This is a fair estimate, given that the hookload measurements are precise and that the drillstring is rotating, i. e. wall friction is neglectable.
Unfortunately, hookload measurements can have severe errors. For instance, when the hookload is derived from a strain gauge on the deadline, the hysteresis error can be several tons in magnitude and have a sign that is dependent on the direction of the drillstring movement. Surface WOB estimates are therefore only reliable when both the on- and off-bottom hookload measurements are noted when the drillstring is moving downwards.
The extensively tested pattern recognition module in DrillComputer ensures that surface WOB is used only when it is reliable. However, in the case when surface WOB is not reliable we can always rely on the simulated WOB.
In the figure below, we see that the conventional WOB, denoted WOBX, is always positive, despite the fact that the bit is moved on- and off-bottom several times. This can be seen from the bit depth (DBTM) curve. We see that the driller is picking up four times and each time he picks up about half a meter. For a drillstring with this stiffness, this should result in a WOB reduction of about six tons. Notice the counter intuitive behavior of WOBX when picking up. On each pick-up, WOBX actually increases a couple of tons, before it finally reduces but never goes to zero. This is clearly a pattern of hysteresis error. The calculated WOB, denoted WOB_CALC, seems to be more in line with our physical intuition. It reduces exactly when the bit is picked up and it is reduced about six tons as expected. Notice also that the software accurately detects when the bit is on-bottom (dark green) or off-bottom (light green). Finally, notice also the ROP_CALC curve is always smooth and zero when the bit is off-bottom. By comparison, the feeding speed, denoted ROPS, is clearly not a good estimate of instantaneous downhole ROP.