This paper summarizes the analysis procedures that Esso Resources Canada used to develop a drill string inspection frequency for drilling the Norman Wells Expansion Project (NWEP) The NWEP required directional wells to be drilled with programmed inclinations up to 70 ° and build rates up to 8 ° /30 m. By calculating fatigue crack propagation rates at operating conditions, the time for failure was determined for each critical component. From these fatigue lifes, inspections could be scheduled to detect cracks or damage prior to a major downhole separation.


In July 1982 ERCL launched the NWEP to produce oil from the oil bearing reef located at Norman Wells. The project called for approximately 165 wells to be drilled with over 130 of these to be directional. Because of the shallow depth of the reservoir and presence of the MacKenzie River over most of the pool. high angle and high rate of build directional wells were required. (True vertical depth from surface locations to targets in the reservoir ranged from 400 m to 700 m). Horizontal displacements on these directional wells range from 20 m to 840 m. For ease of planning the directional wells were grouped into three classifications as indicated in Figure 1. The criteria for directional type was maximum angle and total displacement of the well.

At the time the project began there was little information available an what would be an appropriate inspection schedule for a stiff Bottom Hole Assembly (BHA) operating in an environment as severe as Norman Wells. We were concerned about how the stiff BHA would resist fatigue at 110–120 rpm in wellbore curvatures of 8 ° /30 m or greater.

Expectations at the outset of the project were that drill string failures would occur occasionally. The impact an drilling operations of a separated drill string is substantial in bath problem time and additional drilling costs. Original programming, based on conventional drilling practices, required long sections of the well to be drilled with a rotating string.


Before any fatigue crack inspection schedule could be developed for drill string components, we had to identify the life expectancy of a particular drill string in the downhole environment. For this, it was necessary to determine the length of time for crack initiation to occur and subsequently crack propagation rates to failure in BHA components. Esso's field experience has been that drill collar connections, pipe connections and drill pipe upsets were most susceptible to stress concentrations. The critical sections in these components are:

  1. Stress relief grooves in the drill collar connections.

  2. Last engaged threads in the pin and box connection of the drill collars.

  3. Drill Pipe Upsets.

Figure 2 shows a drill collar connection with the critical areas labelled as indicated below:

  • P1 and P2 - stress relief groove in pin

  • P3 and P4 - last engaged threads in pin

  • B2 - last engaged thread in box

  • 83 and 84 - stress relief groove in box

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