Before planning to drill a hole to a certain depth the rating (i.e. capacity) of the rig must be known. To do this, look at the manufacturer’s manual and check the equipment ratings that are listed.
This discussion is intended to help to calculate drill string weights and to make sure that the operating rig stays within its rating.
The four main reasons for knowing the weight of the drill string are to:
- make sure that the load is within the hoist capacity
- help to determine the load on the mast for the mast rating
- determine the pull-back load, which is used to check the pull-back assembly rating
- determine the actual load on the drill bit.
To do this it is necessary to work out the weight of the drill string, including any efect of buoyancy.
Calculations are required to determine the:
- weight of the drill string — this will help to work out the bit pressure and make sure that the ratings of the drill mast, feed cylinders, and hoists are not exceeded.
- buoyancy of the drill string — this is when the drill string weight is reduced because of its immersion in the drill luid. To work out this weight it is necessary to know the volume of the drill string (or how much luid it displaces) and the speciic gravity (SG) of the luid.
- weight on the drill bit — use a combination of drill string weight, buoyancy, and pull-down or holdback force to work this out.