The statement "only the buoyed weight of the BHA should be placed on the bit" is correct. Only using the buoyed weight of the BHA on the bit will prevent the drillpipe from axially buckling and most likely also prevent it from torsionally buckling. It is not true that the drillpipe is in tension just above the BHA. The normal portions of the drill string which are bent, straight, compressed and tensioned are shown in the figure. Note the section which is in compression and is straight.
The location of the cross over from the bent to the straight section is called the neutral point of bending (NPB). This point should be kept within the BHA. The neutral point of tension and compression (NPT&C) is normally within the drillpipe and is of no practical use in the design of BHA's.
If practical, all BHA tools should be placed above the neutral point of bending to prevent their buckling. This would prevent their premature failure.
In vertical holes the equation for ascertaining the maximum allowable weight which may be placed on a drill bit from a BHA is
WOB = Buoyancy Factor * Air Weight of the BHA * Safety Factor
Safety factors range from .5 to .9 and depend on how rough the drilling is. The rougher the drilling the lower the safety factor.
The air weight of the BHA is the sum of all of its parts. Perhaps the most accurate procedure for ascertaining the buoyed weight of the BHA is to weigh it with the rig's weight indicator after running only the BHA in the hole. BHA must be vertical and drag eliminated. The formula for computing the air weights of most components which include the drill collars is
AIR WEIGHT (PPO
9" * 3" * 149' 7.5" * 2.75" * 153"
drilling jar 7.5" * ? * 30' 130.0 7.5" * 2.75" * 90' 130.0
BHA buoyed weight
Weight which may be applied to the bit is
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