The dimensions of LTC (Long Round Thread) casing are strictly governed by the API 5B standard (Specification for Threading, Gauging, and Inspection of Casing, Tubing, and Line Pipe Threads).
The "Long" in LTC refers to the thread length ( L4 ), which is longer than the Short Thread (STC) for the same pipe diameter. This extra length provides more thread engagement, increasing the tensile strength of the connection.
Key Technical Parameters
Regardless of the pipe size, all API LTC threads share these fundamental design features:
Threads Per Inch (TPI): 8
Thread Profile: Rounded "V" shape (60-degree included angle).
Taper: 1:16 (0.0625 mm/mm or 0.75 inches per foot on diameter).
Crest and Root: Rounded to prevent stress concentration and improve sealing.
Major Dimensional Components
When looking at an API 5B dimension table for LTC, you will encounter these primary terms:
D : Outside diameter of the pipe.
L4 : Total length from the end of the pipe to the vanish point (Total Thread Length).
L1 : Length of hand-tight engagement.
E1 : Pitch diameter at the hand-tight plane.
E0 : Pitch diameter at the small end (end of the pipe).
API 5CT LTC Thread Dimensions Table (Common Sizes)
Note: Dimensions are in inches. Values are approximate based on API 5B standards.
| Pipe OD (Inches) | Total Thread Length (L4) | Hand-tight Engagement (L1) | Pitch Diameter at E1 |
| 4-1/2" | 2.625 | 1.625 | 4.412 |
| 5-1/2" | 2.875 | 1.875 | 5.412 |
| 7" | 3.250 | 2.250 | 6.912 |
| 8-5/8" | 3.625 | 2.625 | 8.537 |
| 9-5/8" | 3.875 | 2.875 | 9.537 |
| 10-3/4" | 4.125 | 3.125 | 10.662 |
LTC Casing Specifications & Application Comparison Table
| Outside Diameter (OD) | Common Grades | Recommended Depth | Primary Application |
| 4-1/2" - 5-1/2" | J55, N80, P110 | 1,500m - 4,000m | Production Casing: Used for the final string in small-bore wells to directly handle production pressure. |
| 7" (Most Common) | N80, L80, P110 | 2,000m - 4,500m | Standard Production Casing: The mainstream choice globally for medium-depth wells, balancing flow area and connection strength. |
| 8-5/8" - 9-5/8" | J55, K55, N80 | 1,000m - 3,000m | Technical/Intermediate Casing: Used to isolate unstable formations or high-pressure water zones to protect further drilling. |
| 10-3/4" - 13-3/8" | H40, J55, K55 | 500m - 1,500m | Surface Casing: Used to stabilize the wellhead and prevent collapse of shallow formations; LTC provides higher tension resistance than STC in these sizes. |
LTC vs. STC vs. BTC Comparison Table
| Feature | STC (Short Round Thread) | LTC (Long Round Thread) | BTC (Buttress Thread) |
| Full Name | Short Round Thread | Long Round Thread | Buttress Thread |
| Thread Profile | Rounded "V" Thread | Rounded "V" Thread | Trapezoidal / Square-cut |
| Threads Per Inch (TPI) | 8 TPI | 8 TPI | 5 TPI |
| Thread Length | Shortest | Longer (than STC) | Moderate |
| Tensile Strength | Standard (Lowest) | Higher (Better than STC) | Extreme (Highest) |
| Sealing Performance | Good | Excellent (Longer seal path) | Good |
| Resistance to Jump-out | Moderate (Risk of pull-out under heavy loads) | Good | Superior (Prevents pull-out under heavy loads) |
| Makeup/Installation Speed | Fast | Moderate | Fastest (Coarser threads) |
| Manufacturing Cost | Most Economical | Moderate | Slightly Higher |
| Primary Application | Shallow wells, Surface casing, light strings | Medium wells, Production casing, standard operations | Deep wells, Heavy strings, High-pressure wells |
API Thread Factory
FAQ
What is the difference between LTC and BTC threads
The fundamental difference between LTC and BTC lies in their thread profile and tensile strength:
LTC (Long Round Thread): Features a rounded "V" thread profile with 8 threads per inch. It offers excellent sealing performance and serves as the universal standard for medium-depth wells and conventional drilling operations.
BTC (Buttress Thread): Features a trapezoidal (square-cut) thread profile with 5 threads per inch. It possesses exceptionally high tensile strength, effectively preventing thread disengagement (jump-out) under extreme tensile loads, making it the preferred choice for deep wells and heavy-duty tubular strings.
What are the different types of casing threads?
STC (Short Round Thread): Rounded "V" shape, 8 TPI. Best for shallow wells (Economy).
LTC (Long Round Thread): Longer "V" shape, 8 TPI. Standard for medium-depth wells (Reliability).
BTC (Buttress Thread): Trapezoidal/Square shape, 5 TPI. Best for deep wells and heavy loads (Strength).
Premium Connections: Proprietary designs with metal-to-metal seals. For extreme gas or high-pressure projects (Safety).
