Heavy Weight Drill Pipe
Heavy Weight Drill Pipes DefinitionIn the past, the two main components of the Drill String consisted of the Drill Pipe and the Drill Collars. The point where the relatively small OD and flexible Drill Pipe connects to the large stiff Drill Collars is called the Transition Zone. Oilfield studies have shown that almost all of the Drill Pipe fatigue failures are the result of an accumulation of fatigue damage occurring when the Drill Pipe joints were run in the Transition Zone, or were stressed above the endurance limit in crooked holes. Downhole data has indicated that the large change in diameter at the transition zone caused accelerated fatigue damage as a result of the concentration of cyclic bending stress reversals in the bottom joints of the flexible Drill Pipe, since the stiff Drill Collars bend very little from these stress reversals. So HWDP definition will be a drill pipes with higher thickness to withstand the stress in the transition zone.
Heavy Weight Drill Pipes FunctionSo HWDP is used to absorb the stresses being transferred from the stiff Drill Collars to the relatively flexible Drill Pipe. The major HWDP benefits are:
- Increased wall thickness
- Longer tool joints
- Uses more hard facing
- May have a long central upset section
Heavy Weight Drill Pipes Specifications
- Heavy Weight Drill Pipe Quality Standard: API 7 and API 5DP.
- Heavy Weight Drill Pipe Steel Grade: Integral HWDP : AISI 4145HM; Welded HWDP : AISI 1340.
- Heavy Weight Drill Pipe pipe body, AISI 4140HM.
- Heavy Weight Drill Pipe Outer Diameter: 2 7/8” – 6 5/8”.
- Heavy Weight Drill Pipe Weight Range: 6.27#/FT through 27.72#/FT.
- Heavy Weight Drill Pipe Lengths: Range 2 and Range 3.
- Heavy Weight Drill Pipe End Finishes: Internal Upset, External Upset, Plain End Upset, Non Upset.
- Heavy Weight Drill Pipe Tool Joint type：NC38、NC46、NC50、5-1/2 FH.
Heavy Weight Drill Pipe Selection And DesignField tests indicated that fatigue build up in the Drill Pipe in the transition zone is related to the relative stiffness of the Drill Collars and the adjacent Drill Pipe. The stiffness ratio of two sections of pipe in the drill string is expressed by the equation,
The field tests showed that the higher the stiffness ratio at the transition zone the greater the fatigue build up. Criteria for permissible stiffness ratio varies between different operators and areas. The following maximums are typical: For shallow or routine drilling or low failure rate experience, keep stiffness ratio below 5.5. For more severe drilling or for significant failure rate experience, keep SR below 3.5. In order to reduce the stiffness ratio and increase service life of the drill pipe, heavy weight drill pipe having the same OD as the drill pipe and a wall thickness of up to 1 in and weight up to 78 lb/ft is used between the drill pipe and the drill collars. The number of joints used varies between 15 to 21 joints.
Use of Heavy Weight Drill Pipe offers the following advantages
- Reduces drilling cost by eliminating Drill Pipe failures in the transition zone.
- Significantly increases performance of small rigs through the ease of handling.
- Provides substantial savings in directional drilling by replacing most of the Drill Collars string, reducing down hole drilling Torque and Drag.
- Reduces tendency to become Deferentially stuck. This is due to the fact that large diameters are easier to stick than small diameters.