Drilling Fluid: Support When a object is submerged in fluid the, fluid will absorbed a part of the weight  we call this a buoyancy factor. The buoyancy factor of any fluid will change if the fluids  weight changes. Fresh water has a specific gravity of 1 or 8.333 pound per gallon. and a buoyancy factor of 0.872. I will not go into the technical data of what causes buoyancy as there really is no need to. As long as you understand there is a factor and why. What this means is about 12 1/2% of the given weight of the material submerged in fresh water is supported by the water and would therefore be deducted from the dry weight The higher the fluid weight the higher the % of the weigh it supports. Unfortunately the higher the fluid weight the slower the penetration rate. To work out the buoyancy factor use the formula: =(65.45-fluid weight in use) / 65.45. As an example a string of 27 pounds per foot casing being run in to the hole to 12890 feet  would weight in air 348.030 lbs. If the drilling fluid had a density of 13.2 ppg the hanging string weight submerged would be some what lighter and give a buoyancy factor of (65.42-13.2)/65.45= 0.7983 . the submerged string weight would be 12890*27*0.7983 =277,832 lbs a difference of 78,197 lbs. Why is it so important  to have an understanding of buoyancy. For starters, the placement of tools in a string such as Jars. weight on bit will need X amount of drill collars. Backing off pipe, Ton mile calculations. The list is long