Last Updated on Sun, 25 Dec 2022
The primary purpose of non-magnetic drill collars is to reduce the interference of the magnetic fields associated with those sections of the BHA which are both above the below the magnetic compass contained in the survey tool with the earth's magnetic field. The non-magnetic collars reduce this type of interference by moving the BHA sections away from the survey compass.
There are four critical factors in selecting non-magnetic collars: (1) their total length, (2) the location of the survey compass with the non-magnetic collars, (3) the type of material of which the collars are composed, and (4) distinguishing "hot spots".
"Hot spots" are zones of high magnetic field strengths within the material of the collars. These zones are detected with a magnetic permeability probe. The measurements are recorded on a strip chart versus distance into the collar. Hot spots can affect compasses by as much as 4 degrees (2 degrees is common). A "cold" collar will affect a compass by less than 1/4 degree over its entire length.
Non-magnetic collars may be manufactured from many types of material. The selection is based primarily on the corrosion resistance of the material. The most common non-magnetic material is stainless steel while monel, which is 60% nickel and 30% copper, is seldom used.
A practical method of ascertaining the best location for the survey compass and the requisite non-magnetic collar length is to run a BHA into the drill hole and pull a compass through the non-magnetic collars. The portion of the nonmagnetic collars which do not show the affects of the fields of the BHA may be removed from the BHA.
Medium and Large Round Collars
The purposes of large round collars are to provide stiffness next to the drill bit and add weight to the BHA.
The medium collars add weight to the BHA and reduce ever present flexure stresses between large collars and drillpipe or other tools of less rigidity than the large collars. Both may be used for jarring weight.
Heavy weight drillpipe is small drill collars with drillpipe tooljoints. They serve the same purposes as medium weight collars. They may be operated in a buckled mode while drilling or a part of a fishing BHA.
The purpose of keyseat wipers is to ream the "key" section out of the wall of the hole. Keyseat wipers are run in the BHA or the drillpipe. The outside diameter of the cutting structure of the keyseat wipers are commonly selected as 1/8 to 1/2 inch larger than the collars or tooljoints in which they are installed.
Keyseat wipers are either single or double acting; that is, the blade section which slides on a mandrel reams either in the up or down position if single acting but not both.
The double acting type act if the blade section is up or down, but not if the section is floating between up and down.
Because most rigs cannot raise and rotate the drill string simultaneously, the single acting wiper which reams in the down position is required.
Continue reading here: Buoyed String Weights
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