Drillers have been using slip and elevator recesses on most spiral drill collars for years. However, after short time in use, the need of lift-subs occurs. The main reason is wear on the connection OD during drilling. Making a switch from the standard API recess to a modified mutual drill pipe recess will reduce Bottom Hole Assembly handling time significantly, and reduce the need of personnel in red zone.
The schematic diagram below shows how the API recesses and the modified slip and elevator recess are designed, and where they differ:
As you can see, there are three main differences between the two:
- The elevator shoulder on the modified version is machined to fit an 18° DP (drill pipe) elevator, compared to the 90° shoulder used on a DC elevator. This gives increased wear tolerances on the connection OD (outer diameter).
- The two different recess OD’s on the API collar, is of cause machined to fit the actual size of the DC elevator & the DC slips. The one mutual DP recess on the modified collar is machined to fit both elevator & slips, preferably in the same size as for the HWDP (heavy-weight drill pipe) used in the string.
- The increased slip length will also give the opportunity for use of power-slips.
Some of the obvious benefits are:
- Less change of elevator / bushings
- Less change of slip / inserts
- No need of lift-subs
- Less crane lifts, handling & logistics
- Increased wear tolerances
- Reduced need of people in red zone
Another benefit is less recut of connections. Traditionally, in any BHA, the two upper DC’s and the bottom HWDP connections take most of the beating (bending stress) during rotation of the string in the wellbore. That is due to the stiffness of the BHA. The modified recess, however, has a smaller OD than the API one. This allows for greater flexibility, which reduces the fatigue on the connections.
Typical recess sizes on OWS drill collars and drilling jars:
|4 ¾”||DC = 3 ½”||DP. Recess|
|6 ½”||DC = 5”||DP. Recess|
|8”||DC = 6 ⅝”||DP. Recess|
|9 ½||DC = 6 ⅝”||DP. Recess|
Improved cost-effectiveness and increased safety ... The benefits of switching from the API recess to the modified slip and elevator recess are too good to be ignored.