One of the objectives of a Torque & Drag calculation is to ensure that the tension in the drill string never exceeds the maximum allowable tension. This means that the tension line for all operational modes should be to the left of the tension limit line at all times.
When the Torque & Drag model is calculated, the tension limit line is constructed from the tension limit specified for each element in the drill string. The limit line is adjusted by applying the input safety factor and margin of overpull (MOP). As such, the input tension limit should therefore always be entered at 100% to avoid double correction.
The only "exception" to this is the drill pipe class used (e.g. New, Premium, Class 2 or Class 3). Each of these classes include a predefined wear (wall thickness reduction) of the pipe. This is not included in the tensile limit calculation so that the "100% tensile limit" for the correct pipe class must be used. When selecting the correct pipe class from the Torque & Drag lookup tables, the predefined wall thickness reduction is taken into account (i.e. a reduced tensile limit will be used as the 100% value).
Take care with input of the tension limit of a drill string element. If the connection of the element has a lower tension limit than the pipe body (e.g. for some casing connectors), ensure the lowest of the two is used.
The safety factor and margin of overpull are applied as shown in the sample chart shown below.
1.The Input tension limit is reduced by the input safety factor (1). E.g. if a safety factor of 80% is specified, the new tension limit is calculated as Input tension limit x 0.80.
2.The resulting tension limit is then further reduced by the margin of overpull (2). E.g. if a margin of overpull of 100,000 is specified, the resulting tension limit (for each drill string element) will be reduced by subtracting 100,000.
3.The applied Safety Factor ensures that there is always a margin when applying (tensile) loads to a drill string element. The purpose of the margin of overpull is to ensure that if the calculated tension in the drill string is approaching the tensile limit line (i.e. when pulling out of hole), there will still be sufficient margin (i.e. at least the MOP) to apply additional overpull on the drill string in case it becomes stuck in hole. Of course, if the calculated tension is below the (adjusted) tensile limit line, then the available overpull (before reaching the safety factor limit) will be the total of the margin to the limit line (3) and the MOP.
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