Undergraduate final project · Petroleum engineering
How do you know a drillstring can survive the path it must follow?
An interactive case study in drillstring design, mechanical loads, and validation — by Muhammad Fakhri Helmi.
02 Well path
A directional path changes how the string moves, contacts the wellbore, and carries load.
The study well follows a build-and-hold profile: drilled straight down, turned at a kick-off point, then held at a steady angle to reach a target offset from the rig.
Build. The hole starts to curve. The string is pressed against the wall, so contact and friction begin to add torque and drag.
Conceptual visualization — not operational data
Generic geometry — not the study well's coordinates or survey.
03 The string
Every section contributes weight, strength, and a different response to load.
The longest section. Every joint holds up everything below it and passes rotation down from the rig. In this study, its strength rating was the only thing that differed between the three designs.
Conceptual visualization — not operational data
Illustrative proportions — no real dimensions or grades.
04 Four forces
A viable design has to be checked against more than one force.
The same string, on the same path, is loaded four different ways. Scroll through them, or jump to one.
Tension
Each point holds up everything hanging below it.
Tension is highest near the surface, where the whole string hangs from the rig. Pulling out of the hole adds friction on top of the weight.
CheckWorking load stays below 90 % of the pipe's rated strength — the study's safety threshold.
Torque
Rotation has to overcome friction along the path.
Wherever the string touches the wall, rotation loses some of its torque to friction. The rig must supply the sum of it, so torque builds toward the surface.
CheckPeak torque stays below the make-up torque of the pipe connections.
Drag
Wall contact resists every trip in and out.
Running in, the string's own weight must push through the resistance. Pulling out, the rig must overcome it — within the overpull margin it has available.
CheckRunning in: available weight beats drag. Pulling out: required pull stays within the overpull margin.
Buckling
The lower string is pushed into compression.
Weight on bit puts the bottom of the string in compression. Beyond a critical load, pipe can bend into a sinusoidal or helical shape inside the hole.
CheckCompression stays below the critical buckling load.
Conceptual shape — not a physical simulation.
Four forces become four criteria.
Each design had to pass every check, while drilling and while tripping in and out of the hole.
Tension
Working load stays below 90 % of the pipe's rated strength — the study's safety threshold.
Torque
Peak torque stays below the make-up torque of the pipe connections.
Drag
Running in: available weight beats drag. Pulling out: required pull stays within the overpull margin.
Buckling
Compression stays below the critical buckling load.
Conceptual visualization — not operational data
05 Three designs
Three alternatives. The same well path. A different safety margin.
The three strings share the same length, weight and arrangement. Only the drillpipe's strength rating differs — so the loads stay the same while the limits move.
Held all four loads with margin in both methods. Recommended in the initial study.
| Check | Hand calc. | Simulation |
|---|---|---|
| Tension | Meets study criteria | Meets study criteria |
| Torque | Meets study criteria | Meets study criteria |
| Drag | Meets study criteria | Meets study criteria |
| Buckling | Meets study criteria | Meets study criteria |
Conceptual visualization — not operational data
Statuses summarise the study's conclusions; bar positions are illustrative.
06 · Two methods
Calculation is a starting point. Comparison reveals what needs a closer look.
Every design was checked twice: by hand, with load-mechanics equations applied section by section, and with WellPlan® simulation along the whole path. The trends agreed across all three designs. The numbers did not always.
At this depth — hand calculation 66simulation 61difference 9 %
Conceptual visualization — not operational data
Curves are drawn to explain the two approaches; they are not the study's results or WellPlan® output.
What the comparison showed
- Torque: hand calculation landed close to the simulation.
- Tension: the gap between the two methods was much larger.
For Design B the two methods even disagreed on the verdict — usable but not recommended by hand, not usable in simulation. Both still pointed to the same action: do not run it as-is.
Reason 1
Sections vs. steps
The hand calculation works section by section, using each component's length. The simulation steps through the well in small depth increments.
Reason 2
How the path is read
By hand, angle changes were taken in whole degrees and azimuth was left out. The simulation reads the path more precisely, azimuth included.
Takeaway
What it means
Hand calculation is a sound first read for torque. For tension, the simulation is needed before trusting the number.
So the question became: what should change?
07 Revision
When a design missed the criteria, I changed the arrangement and tested it again.
The limits of B and C could not change, so the load had to. Shortening the heavy-weight drillpipe and extending the drillpipe by the same length keeps the string's reach but hangs less weight below its weakest point, near the surface.
Heavy-weight pipe was the lever, not the collars: collars stiffen the bottom of the string; heavy-weight pipe is flexible weight. Less weight also means less push when running in, so drag and buckling were re-checked after every change.
| Check | Hand calc. | Simulation |
|---|---|---|
| Tension | Meets study criteria | Needs review |
| Torque | Needs review | Exceeds a limit |
| Drag | Meets study criteria | Meets study criteria |
| Buckling | Meets study criteria | Meets study criteria |
Conceptual visualization — not operational data
A conceptual reconstruction of the study's conclusion — the real lengths changed are not shown.
08 Conclusion
The outcome was not a single number. It was a reasoned design decision, checked through two methods.
- ARecommended in the initial study
Met all four checks in both methods.
- BMeets study criteria after revision
Heavy-weight section shortened, drillpipe extended.
- CMeets study criteria after revision
Needed a larger shift than B.
Hand calculation and simulation did not always give the same number — close for torque, far apart for tension. The evaluation therefore never rested on one method alone.
See the methodWhat I would test next
- Compare against actual drilling records from the well.
- Run a sensitivity study on component lengths and operating loads.
- Add checks for other effects such as vibration.
- Cross-check with a second simulation tool.
09 Approach
Test assumptions, compare evidence, and make the reasoning visible.
This research shaped how I approach engineering problems. The same six steps carry over to any design question where the answer has to be defended, not just calculated.
- Step 1Define inputsWell path, rig limits and string arrangement.
- Step 2Test loadsTension, torque, drag and buckling.
- Step 3Compare alternativesThree designs against one set of criteria.
- Step 4Validate resultsHand calculation checked against simulation.
- Step 5Revise designChange the arrangement, then test again.
- Step 6Explain decisionA recommendation with its reasoning visible.