Classifications for Drill Pipe, HWDP, and Drill Collars

In oil and gas drilling, the drill string is an important assembly made up of drill pipe, heavy-weight drill pipe (HWDP), and drill collars. Each part has a unique purpose, and the American Petroleum Institute (API) sets the standards for these components to ensure they are safe, reliable, and effective.

1. API Classification (Spec 5DP & Spec 7-1)                                                                                                     ✓

The API provides standardized specifications to ensure the quality and performance of drill string components:

  • Drill Pipe: API Spec 5DP specifies technical delivery conditions for steel drill pipes with upset pipe-body ends and weld-on tool joints. This standard is relevant to pipes used in oil and gas drilling and production operations. It includes three product specification levels (PSL-1, PSL-2, PSL-3), with increasing quality, testing, and performance requirements. PSL-1 sets basic standards for wall thickness, impact strength, and yield strength, while PSL-2 and PSL-3 impose additional mandatory requirements.

  • Heavy Weight Drill Pipe (HWDP): HWDPs are governed by API Spec 7-1, which covers rotary drill stem elements. HWDP is manufactured in compliance with API Spec 7-1 guidelines, ensuring durability in high-stress environments.

  • Drill Collars: Drill collars are also covered under API Spec 7-1, which outlines requirements for drill collars, including both standard steel and non-magnetic types. This standard ensures that drill collars meet mechanical property, dimensional, and material quality standards for demanding drilling conditions.

These standards ensure that each component can withstand the internal and external pressures, bending, and vibrations encountered during drilling.

2. Grade, Wall Thickness, Yield Strength, and Size Variations                                                  ✓

Drill Pipe: E, X, G, S Grades

Drill Pipe is classified into standard API grades based on its minimum yield strength, as specified in API Spec 5DP:

Grade                     Minimum Yield Strength (psi)                              Tensile Strength (psi)                                      Application

E-75                                 75,000                                                                       100,000                                                         Shallow wells, less demanding conditions

X-95                                 95,000                                                                      105,000                                                         Moderate to high-stress applications

G-105                              105,000                                                                   115,000                                                          Deeper wells, more demanding operations

S-135                               135,000                                                                    145,000                                                         Deep, extreme drilling conditions

The choice of grade depends on well depth, pressure, temperature, and formation characteristics. For example, E-75, known as "mild" steel, offers the lowest yield strength but is more resistant to corrosion and cracking, making it suitable for shallow wells. X-95, G-105, and S-135 are high-strength grades designed for deeper wells with increased yield strength requirements.

The Wall Thickness of drill pipes varies with nominal size and weight, impacting the pipe’s ability to withstand pressure and bending. For a 3 1/2" OD Drill Pipe:

  • Nominal weight 9.50 lb/ft: 0.156"

  • Nominal weight 10.40 lb/ft: 0.179"

  • Nominal weight 13.00 lb/ft: 0.219"

The outer diameter (OD) of drill pipes ranges from 2 3/8" to 6 5/8", with lengths in:

  • Range 1: 18–22 ft

  • Range 2: 27–30 ft (most common)

  • Range 3: 38–45 ft

HWDP: Intermediate-strength Grades

Unlike Drill Pipe, HWDP doesn’t have distinct API grades. Instead, it’s categorized by its material and role in drilling operations. Typically, HWDP is made from AISI 4145H modified alloy steel or other proprietary alloys, offering an intermediate strength between Drill Pipe and Drill Collars. While the material needs to meet the mechanical properties specified in API Spec 7-1, it isn’t classified by yield strength like Drill Pipe. Some manufacturers even offer proprietary grades, such as DPM-65MS or DPM-HW90SS, which are designed for conditions, such as environments with H2S.

Outside diameter of HWDPs range from 2 7/8" to 6 5/8", with lengths similar to Drill Pipe (Range 2 or 3).  They are up to three times thicker than drill pipe of similar size, enhancing strength and weight.

Drill Collars: Standard vs. Non-magnetic Grades

Drill Collars are not classified by specific strength grades but by their material type:

  • Standard Drill Collars: These are made from AISI 4145H modified alloy steel, heat-treated to achieve a Brinell hardness range of 255 to 341, as specified in API Spec 7-1. They provide high strength and durability for general drilling applications.

  • Non-magnetic Drill Collars: These are constructed from materials with low magnetic permeability, such as Monel or other specialized alloys, to prevent interference with measurement while drilling (MWD) tools. They are essential for operations requiring precise magnetic measurements.

The outside diameter of drill collars ranges from 2 7/8" to 11", with some manufacturers offering up to 14". Lengths are typically 20–30 ft.

3. Weight Distribution Considerations                                                                                                             ✓

The drill string’s weight distribution is crucial for efficient drilling, stability, and fatigue prevention:

  • Drill Pipe: This comprises most of the drill string length and is lighter and more flexible. It transmits torque and drilling fluid while returning cuttings to the surface.

  • HWDP: Positioned between Drill Pipe and Drill Collars, HWDP provides intermediate weight and stiffness. It reduces stress concentrations and fatigue, particularly in directional or horizontal drilling.

Drill Collars: The heaviest components are placed at the bottom to provide weight on bit (WOB) and stabilize the bottom hole assembly (BHA). The number and size of Drill Collars are selected based on the required WOB and hole size.