July 15, 2026 · Drillstrings Team
The bottom hole assembly (BHA) is the lower portion of the drill string — everything from the bit up to the drill pipe — that delivers weight to the bit, controls hole direction, and houses the measurement and downhole tools. A well-built BHA drills a smoother, straighter (or precisely curved) hole with fewer trips and fewer failures. A poorly built one causes vibration, deviation, washouts, and twist-offs. Getting the components and their order right is fundamental.
Core BHA components
From the bit upward, a conventional BHA typically includes:
- Bit — cuts the formation.
- Bit sub / near-bit stabilizer — centralizes just above the bit.
- Mud motor or RSS (directional work) — provides bit rotation or steering.
- Measurement/logging tools — MWD/LWD collars carrying telemetry.
- Drill collars — thick-walled tubulars that supply weight on bit and keep the assembly in compression.
- Stabilizers — blade tools that control OD contact and hole trajectory.
- HWDP — transition weight above the collars.
- Jars and accelerators — for freeing stuck pipe.
- Drill pipe — carries the assembly to the surface.
The specific BHA components change with the objective — see our directional and horizontal BHA essentials for the steerable variants.
Getting the order right
Order is not arbitrary; each position controls a mechanical behavior.
Keep the neutral point in the collars
The neutral point is where the string transitions from compression (below) to tension (above). It must sit within the drill collars or HWDP — never in the drill pipe. Drill pipe run in compression buckles and fatigues fast. This is why you carry enough collars and HWDP to supply the full weight on bit plus a safety factor, so the neutral point stays comfortably below the drill pipe.
Stabilizer placement sets the trajectory
Stabilizer spacing is the primary directional control in a rotary (non-steerable) BHA:
- Packed / stiff assembly (near-bit + two string stabilizers close together): holds angle, drills straight. Good for tangent sections.
- Fulcrum assembly (near-bit stabilizer, then a gap): builds angle.
- Pendulum assembly (first stabilizer set back from the bit): drops angle.
Small changes in stabilizer position shift build/drop tendency significantly, so model it before you make up the string.
Place jars where they can work
Drilling jars should sit in the string where the expected free-point and available impact energy line up — usually within the HWDP or just above the collars, in tension during normal drilling. Placing a jar in compression or too deep in a heavy BHA reduces its effectiveness when you need it.
Common mistakes
- Neutral point in the drill pipe. The single most common and most damaging error. It leads to buckling, connection fatigue, and twist-offs.
- Too little weight capacity. Running to target weight-on-bit with the neutral point riding up into the pipe because the BHA is under-weighted.
- Ignoring connection compatibility. Mixed connections (NC46 vs NC50, etc.) or worn box/pin shoulders concentrate stress. Verify every crossover.
- Wrong stabilizer gauge or spacing. Undergauge or misplaced stabilizers cause the well to wander and induce vibration.
- Skipping the vibration check. Stick-slip and bit whirl destroy bits and tools. Model the BHA’s dynamic response, not just its statics.
- No jar or a mispositioned jar. When the string sticks, the wrong jar placement can cost days.
- Uninspected or fatigued components. Reusing collars and HWDP without current DS-1 inspection invites fatigue cracks at the connections.
Design it before you build it
Every reliable BHA starts as a torque-and-drag, buckling, and vibration model for the actual well — hole size, mud weight, angle profile, and formation. The model tells you how many collars, how much HWDP, where the stabilizers go, and where the neutral point lands. Building by habit instead of by model is how good crews still get bad holes.
Rent or buy
BHA needs shift with every well — a stiffer packed assembly here, a build assembly there, a different collar OD next hole. Renting lets you assemble exactly the right configuration without owning every size of collar, stabilizer, and jar. Where a configuration runs continuously, buying inspected components can be cheaper long term. We supply both, backed by our quality and global network.
Send us the well plan and we will help you spec a BHA that stays in one piece. Contact us to rent or buy your drilling BHA components.