The most extended & significant section of the Drill String Components is the Drill Pipe. Each joint of the drill pipe consists of the tube body and the Tool Joint (connection). Therefore, in this article, we shall discuss API drill pipe specifications (specs), description, classification & drill pipe connection thread types.
In general, D/Ps joints are available in three length ranges:
Range 2 is the most common.
API Drill Pipe Specifications (specs) are generally described as six items. For example, a 5″ drill pipe is 5″, #19.5, E, XH, NC50, Premium. Now let’s understand what are the main items that describe any drill pipe :
- Tube OD
- Nominal weight
- Pipe grade
- Type of tool joint.
- API drill pipe thread connection types.
- Drill pipe classification.
|tube OD||Nominal Weight||Minimum Yield strength||Upset Tool Joint||Connection thread||Drill Pipe Classification (wear)l|
Each description item is explained in more detail below.
OD Specifications (specs)
Which is simply the plain end (tube) outside diameter.
Drill Pipe Nominal Weight Specifications
The D/P nominal weight is the weight per foot, including the weight of an API regular connection. The number serves no real purpose other than identification because the drill pipe does not have an API regular connection.
So, the actual weight depends on the type of connection thread and pipe grade. As the grade (minimum yield strength) increases, the actual weight increases because the upset has more metal.
The actual weight of the 5” D/P in the above example is 20.89 lb/ft. The actual weight for grade G pipe is 21.92 lb/ft.
The Drill Pipe Grade Specifications
Drill Pipe Grade states the minimum yield strength of the metal, which is the tensile stress that will result in 0.5% strain. Grade E D/P has a yield strength of 75,000 psi.
The minimum strength can be converted to a more usable strength in pounds by multiplying the minimum yield in psi by the cross-sectional area of the metal.
|API Drill Pipe Grade||Minimum Yield Strength (psi)||Minimum Tensile Strength (psi)|
Type Of Tool Joint Upset
Tool joint upset type is a part of the drill pipe specifications (specs). Tool joints are mainly screw-type connectors that join the individual joints of D/P. Also, each joint of drill pipe is fitted with a pin (male thread) and box (female threads) tool joints or connectors.
API Drill Pipe Upset Types
- IEU (Internal-External Upset) – The tool joint OD is larger than the OD of the drill pipe and the tool joint ID is less than the ID of the drillpipe. Generally, IEU tool joints are the strongest available. So, the large OD and small ID of the tool joint cause relatively high external and internal pressure losses.
- IF (External Upset) – The tool joint ID is the same as the ID of the D/P to minimize internal pressure losses. The upset is on the OD of the tool joint.
- IU (Internal Upset) – The tool joint ID is less than that of the D/P. So, the small ID causes relatively higher internal pressure losses. The tool joint OD is the same as the OD of the drill pipe. This type is called slim-hole drill pipe because of the small OD.
API & Non-API Connection Thread Types
However, here we shall mention only API drill pipe Specifications only for thread types, but we will give you simple introduction to other thread types.
API Drill Pipe Connection Thread Types
In 1968 “NC” connections were introduced by API to cover a multitude of needs from macaroni tubing to 12 inch drill collars. At the same time, API declared that the “IF” and “Full Hole” (FH) lines were obsolete.
There are 17 NC connections with pitch diameters ranging from 1.1/16 (NC-10) to 7.3/4 inches (NC-77). NC means “Numbered Connection” and the number signifies the Gauge Point Pitch Diameter (GPPD) of the connection. For example,
- the NC-50 has a GPPD of 5.04170 inches,
- the NC-38 a GPPD of 3.80800 inches and so on.
Also at the time, API took the most popular older connections, gave them a new thread and new name and stuck them into the lineup. These included the NC-50 (old 4.1/2 IF) the NC-46 (old 4.1/2 XH) and NC-38 (old 3.1/2 IF).
“Regulars” continue to be the connections for BHA components, even though they are inferior to NC’s of the same bending strength ratio because they have a sharper root radius. The reason for their popularity habit and the belief that changing to NC’s would cost more than it would be worth.
The number preceding “regular” in the name (in, for example, 6-5/8 Regular) refers to the drill pipe size on which the connection was originally to be (but is no longer) used.
Full Hole Connections
All Full Hole connections were made obsolete by API in 1968, to be superseded by NCs. However, the 5 1/2 FH was officially resurrected about 10 years later. Also, the 6-5/8 FH was officially reinstated in 1993, no doubt because of its increasing popularity on 6-5/8 inch D/P.
Non-API Drill Pipe Connection Thread Types
H-90 thread forms are still common on BHA components. They were originally a proprietary product of Hughes Tool Company and are easily recognized by their shallow 90 degree thread angle.
They are about equivalent in performance to Regulars, except that the shallow thread angle can cause high box hoop stress at high makeup torques. There is no apparent reason (except tradition) for their continued use, since superior NC’s exist for most all applications.
PAC connections continue to be very popular for 2-3/8 inch drill pipe as well as small diameter drill collars. They offer good clearance, but often at the price of alarmingly low tensile and torsional capacity.
PAC’s are very poor selections for drill collars. Standard makeup torques in PAC drill collars cause a pin neck tensile stress of 87,500 psi, compares to 62,500 psi in a similar sized NC connection. Since downhole stress can be additive to makeup stress, PAC connections are that much closer to failure even as they pass through the rotary.
PAC’s are also the only connections that still use the fatigue prone V-065 thread form that was discarded by everyone else in 1968. However, this is not as bad as it seems, since in drill collars PAC’s rarely survive tensile and torsional loads long enough to fail by fatigue anyway.
This is a patented design for tool joint pins. It is an NC pin with a rounded-out thread ﬂ ank that gives a root radius of 0.057 inches (compared to the NC’s 0.038 inches). SST boxes are identical to NC boxes of the same size. Tests by the developer show improved fatigue performance for SST’s. Ironically, SST’s are rarely found in BHA components where their larger thread root radius would be most beneﬁ cial.
Hi Torque Connections
This is also a relatively new patented connection. It is simply an NC connection with an additional torque shoulder at the pin tip. This gives torsional capacities 40-60 percent higher than the original NC. This improvement is achieved at higher cost, reduced availability and sacrificed field repairability.
The Classification Based on Wear.
As new drill pipe is rotated in the hole it wears and, therefore, it must be reclassified according to its wear.
Exterior wear includes OD wear, dents and mashes, slip area crushing, cuts, pitting and corrosion. The working strength is reduced because of loss in cross-sectional area.
The D/P is inspected periodically to detect cracks, pits, reduction in wall thickness and other defects. Inspection methods include electromagnetic inspection of pipe body to locate cracks and pits, sonic inspection to measure wall thickness, visual inspection to detect mashes and caliper measurements.
API Drill Pipes Wear Classifications
The API has established guidelines for drill pipe classification classes in API RP7G. The classes are summarized as follows:
|New:||No wear and has never been used.|
|Premium:||Uniform wear and a minimum wall thickness of 80% of original wall thickness.|
|Class 2:||Allows D/P with a minimum wall thickness of 65% with all wear on one side so long as the cross-sectional area is the same as premium class, that is to say, based on not more than 20% uniform wall reduction.|
|Class 3:||Allows drill pipe with a minimum wall thickness of 55% with all wear on one side.|
API Connection Sizes Specifications & Thread Types Table
|DrillPipe Size (in)||Nom. Wt. (lb/ft)||Upset/ Grade||Connection||(W) Adjusted wt (lb/ft)||Standard OD||DrillPipe Size (in) ID|