Drilling is the first phase of a wellbore, followed by casing, where the wellbore is lined with an outer tube, and lastly, tubing, which involves transporting oil and gas to the surface.
Drilling uses drill pipes to turn the drill bit, enabling it to cut into soil and rock to reach the mineral deposits. Casing makes use of a steel pipe cemented in place which prevents the wellbore from collapsing. In the production phase of the well, a tubing is placed inside the casing to move fluids to the surface.
Oil Country Tubular Goods (OCTG) is the general term given for pipes with connections at the end classified either as casing or tubing.
A drill pipe is made of hollow steel tubes to rotate the drill bit while drilling fluid is injected to aid the drilling process. Each end of a drill pipe is fitted with threaded tool joints used to provide high torque resistance and durability when subject to rotations.
As drilling rigs come in different sizes, there are also different standardized drill pipes suitable for various drilling rig specifications.
Parts of a Drill Pipe
- Pipe Body
- Tool Joints: This can either be a box (female) or pin (male) end connection
API (American Petroleum Institute) and ISO (International Organization for Standardization) are the governing organizations for setting industry standards for oil and gas operations testing procedures.
- API Specification 5B: Threading, Gauging, and Inspection of Casing, Tubing, and Line Pipe Threads
- API Specification 7-2: Threading and Gauging of Rotary Shouldered Connections
- API Specification 5CT: Casing and Tubing
- API Specification 5DP: Specification for Drill Pipe
- ISO 11960: Petroleum and natural gas industries — Steel pipes for use as casing or tubing for wells
Casing and Tubing Grades
Pipe bodies are classified according to the following grades as indicated in API 5CT and the ISO 11960:
Casing and Tubing Grades
1 – Low strength
2 – Pipes suitable for environments with high H2S (hydrogen sulfide)
3 – High strength but lacks corrosion resistance
4 – Higher strength but also lacks corrosion resistance
Steel Grade Definition
Drill pipes are made with different chemical compositions and varying properties and dimensions. One way to classify drill pipes is through steel grade.
Steel grade is computed from required tensile strength and yield strength. Yield strength corresponds to the minimum pressure the drill pipe can take before deformation starts. Tensile strength refers to the maximum stress that the drill pipe can take when tension is applied before breaking.
Types of Drill Pipe Steel Grades:
- E – lowest yield strength per unit area at 75,000 psi
- X – 95,000 psi
- G – 105,000 psi
- S – highest yield strength at 150,000 psi used for deeper wells
The naming convention is the steel grade letter plus the minimum yield strength.
Grades X-95, G-105, and S-135 are considered high strength grades. They exhibit increased yield strength which is required for drilling deeper wells.
API grades (create visual chart here)
|API Drill Pipe Grade||Minimum Yield Strength (psi)|
As the requirements for drilling have become more advanced over the years, manufacturers have created proprietary steel grades to support newer design requirements and harsher drilling conditions. Examples of drill pipes with proprietary steel grades are those used for critical service, sour service, thermal, or low temperature service.
Steel Grade Identification
Markings for the drill pipe grade identification can be found on the tool joint or base pin following the format below:
Manufacturer Symbol / Month Welded / Year Welded / Pipe Manufacturer / Drill Pipe Grade
Sample: AA 12 95 N E
Drill pipe lengths are classified according to the following ranges:
R1 – 18 to 22 ft
R2 – 27-31 ft
R3 – 38 to 45 ft
The length is computed based on the requirement. R3 pipes, while longer, have the disadvantage of increased wear as the load on each pipe will be greater compared to the shorter pipes.
The normal range for drill pipe outer diameter (OD) is from 2 3/8″ to 6 ⅝.
|wdt_ID||DP OD (in)||Wgt (ppf)||ID (in)||DP Cpty (bbl/ft)||Displacement (bbl/ft)||Closed-End (bbl/ft)|
Drill pipes come in different weld types. Deciding on the right weld type depends on the tube billet quality and structure requirements which are considered when designing a rig.
A seamless drill pipe is made when steel is pushed or pulled to forge a pipe over a mandrel, with visibly no seam produced in the product. Technically, it is seamless because there is no welding process when creating this pipe. Seamless pipes are preferred mainly for the uniform structure and high pressure tolerance.
An ERW drill pipe (Electronic Resistance Welded) is the result of cold-formed steel formed into a pipe. The pipe edges are are heated and joined by a high-frequency welder. Because of the manufacturing process, the surface of ERW pipes are easier to maintain and clean.
Submerged Arc Welding is performed by welding bended steel plates. SAW drill pipes are mainly used for pressure vessels because of its uniformity, compactness, and ductility.
The connections at the end of each pipe joins two pipes together to form a string. A connection can be classified as with high tension efficiency, which means that the tension load is higher than the pipe body. If the string breaks, only the pipe will break and not the connection.
If we say less tension efficiency compared to the pipe body, the connection will break if we apply maximum tension on the string.
Drill Pipe Specification with Upset and Tool Joint Chart
|wdt_ID||OD (in)||Nom. Wt (lb/ft)||Upset||Grade||Wall Thkns (in)||ID (in)||TJ Conn Type||TJ OD (in)||TJ ID (in)||TJ Pin Tong Space (in)||TJ Box Tong Space (in)|
Parts of a Thread
- Load flank: the surfaces that supports the string and comes into contact when it is tightly made up
- Stabbing Flank: the surfaces that help guide the pin
- Crest: high points of the thread
- Root: low points of the thread
Different thread profile designs lead to different performance. For example, an ideal gap between stabbing flanks on the pin and box threads will lead to increase load resistance.
Types of Threads
Connections are grouped into two types.
- API: Connections that are compliant to API standards stated in API Spec 5B. These connections are round threaded or buttress. Single shoulder connectors include the following:
- (IF) Internal Flush
- (FH) Full Hole
- (NC) Numeric Connection
- Premium: Connections superior to the API standard requirements, usually made proprietary. Premium connections are manufactured in different sizes and can be made as threaded, coupled, or integral, where the pipe has double shoulders therefore providing more torque.
Sample of Performance Comparison
Drill Pipe Premium Tensile, Grades and Mechanical Specification Chart
|wdt_ID||OD (in)||Connection||Grade||Range||Wall (in)||Nom. (lb/ft)||Adjusted||TJ OD (in)||TJ ID (in)||Tube_ID (in)||TJ Yld (ft/lbs)||Min Mut (ft-lb)||Max Mut (ft-lbs)||Prem Tensile (lbs)|
Round threads feature v-shaped threads where the crest and roots have been rounded. The advantage of this thread is that it has good liquid sealability. However, this thread presents poor tensile resistance and can only support loads lesser than 80% of the pipe body resistance.
Buttress threads look trapezoidal as the stabbing and load angles are designed with different values. Although this presents better tension resistance than a round thread, the liquid seability is poor.
NEW DRILL PIPE — TORSIONAL DATA
|wdt_ID||OD (in)||Nom. Wt T&C||E-75||X-95||G-105||S-135|
Connections are classified according to the following:
- Thread profile: The shape of the thread and how the parts are designed
- Metal seal: Determines the area of high pressure contact on the thread
- Shoulder: Area inside the box where the tip of the pin is in full contact with
- OD: outer diameter
- Taper: rate of the change of diameter varying with length
Drill Pipe Specifications
Product Specification Levels
The advancements in OCTG led to a newer method of drill pipe classification called Product Specification Levels (PSL).
PSL 1 classifies OCTG that follows the API/ISO standards.
PSL 2 and PSL 3 cover the newer requirements that go beyond the API/ISO standards. The pipes classified under PSL 2 and PSL3 differ in chemical composition, mechanical properties, manufacturing process, repair, and certification for traceability.
Drill Pipe Product Designation
In the oil and gas industry, a standard designation is followed to have a common understanding of the mass and dimensions of a drill pipe.
Sample Designation: 9 5/8″ 40# N80 BTC R3
9 5/8″ specified the outside diameter in inches
40 is the weight in lbs/ft
N80 – steel grade
BTC R3- type of connection
When buying a new or used drill pipe, the product designation above allows the buyer to determine if the pipe matches the design requirements.
Drill Pipe specification sheets contain the following data:
- Total Size and Weight
- Tool Joint
- Pipe Body Capacity, Tensile Strength, Wall Thickness
- Tubular Assembly Weight, Length (including the pins)
- Connection Type
Used Pipe Classification
To validate if a drill pipe is suitable for further future use, the following classes have been defined by API.
- Class 1 – New and never been used drill pipes
- Premium – Reused pipe with the thickness of the remaining wall not less than 80% of the specification value
- Class 2 – Thickness of the remaining wall not less than 80% of the specification value
- Class 3 – Thickness of the remaining wall less than 70% of the specification value
Before a drill pipe can be reused, an inspector must check the current condition of the pipe/ This includes taking measurements of the wall thickness and recalculating the new maximum tension capacity. After the inspection, a used pipe can be deemed as qualified or not depending on the purpose.
Drill Pipe HS Codes
When working with customs for importation or exportation of drill pipes, it will be useful to know the the HS (Harmonized System) codes used for drill pipes. The HS Code is a 6-digit identification used globally to classify products.
The first 2 digits classify the chapter, the next 2 are for the heading, and the last two for subheading.
Drill pipes fall under Chapter 73: Articles of Iron or Steel, Heading 04 for Tubes, Pipes And Hollow Profiles, Seamless, Of Iron (other Than Cast Iron) Or Steel.
The following subheadings are examples for line pipes:
10 – Iron
11 – Stainless Steel
21 – Drill Pipe
22 – Casing, tubing and drill pipe, of a kind used in drilling for oil or gas
A complete HS code sample would be 73041120 which corresponds to Stainless Steel blanks for tubes and pipes.
See this link for reference of HS codes.