The static load is determined from the maximum drilling depth and load that the rig will carry to that depth. Typically the heaviest load is 9 5/8” 43.5lbs/ft. casing as this is the largest conventional casing size to be used for completions.
The other load added to this is the full complement of drill pipe that is racked back in the derrick a good average for this is 5” 19.5lb/ft. drill pipe.
The last load added is to account for drilling operations that require the derrick to withstand the grip of the formation on the drill string. This is the over pull the amount of force above the other combined loads the derrick can with stand. This is set at 100,000lbs as a guideline.
1.2. Pump Sizing
This requires some work to be done before hand to calculate the maximum pressure losses that will be encountered by the rig. In addition the optimum flow rates for the hole needs to be known. Typically the hydraulic calculations are done for the longest hole section to be drilled. Generally for an 8.5” hole flow rates vary between 200 – 425 gpm
Draw Works Power
This is the power required to loft the loads both the drill pipe and the casing. This involves the weight of tubulars being moves and the vertical or trip seed, typically 50 fpm for casing and 80 fpm for drill pipe.
Rotary/Top Drive Power
This is the power required to rotate the drill string. Done in two stages, the torque to move the drill string is calculated, then this is multiplied by the drill string rotational rpm, typically 110-120rpm, is a conservative estimate, however 200+rpm could be applied. Be warned this could lead to an over estimation.
This is divided into two sections the drilling power systems and the fluid power systems which isolate the systems critical to the drilling operations:
- Draw Works
The rig engines handle the rotary and the draw works so there power requirements are summed. The pumps engines are considered separate.
Additional to this is the electrical system required to run the other rig systems (eg. accommodation, housing and offices). This is typically a given for any rig and not specifically sized.
In a modern rig with electrical systems the values listed above are converted to kW to facilitate comparison with actual rigs.
Notes on Sizing
It must be noted that with all design/sizing situations, especially using rule of thumb calculations, you design for worst case scenario and a safety factor is also added typically between 1.10-1.25.
These calculations are guidelines for rig selection, they are not meant to be used to specifically build a rig to these said specifications. They are meant to be used to SELECT the rig that best approximates the values calculated. The actual rig values should be higher.