Bottom Hole Assembly (BHA) Tools

Bottom Hole Assembly (BHA) Tools covers stabilizers, roller reamers, hole openers, drilling jars, accelerators and shock subs. These tools control hole gauge and direction, free stuck pipe and damp vibration. Gauge surfaces wear and hydraulic tools depend on seals and oil, so a used tool is worth its last measurement or its last service and test sheet.

What each tool does

Stabilizers centre the BHA and set its directional behaviour. A near-bit stabilizer with string stabilizers above it builds, drops or holds angle depending on their spacing. Roller reamers do the same job with rolling cutters, which keep gauge in hard, abrasive formations and cut torque. Hole openers enlarge a pilot hole with fixed arms and cutters.

A drilling jar stores stretch in the drill pipe and releases it as an impact on stuck pipe. An accelerator above the jar holds a compressed gas or spring charge that sharpens the blow and shields the string above from the shock. A shock sub uses springs or elastomers to absorb axial vibration from the bit.

Configurations

Stabilizers with integral blades, replaceable sleeves or welded blades, in straight or spiral blade form, plus non-rotating sleeve types.
Roller reamers in three-point or six-point layouts.
Jars that are hydraulic, mechanical or hydraulic-mechanical, and single or double acting.
Shock subs with a spring stack or an elastomer element.

A hydraulic jar meters oil through a restriction, so the driller sets impact force with the overpull applied. A mechanical jar fires at a preset latch load. Hydraulic-mechanical jars combine a hydraulic delay in one direction with a mechanical latch.

Key specifications

Parameter	Why it matters	Typical classes or units
Hole size and blade gauge	Sets directional response and drift	in, nominal hole size
Body OD, bore and fishing neck	Fit in casing, flow area, room for an overshot	in
End connections	Must match collars and bit sub	API regular or numbered connections
Jar latch or firing loads	Sets the overpull needed to fire	klbf or kN, up and down
Seal temperature rating	Limits the well you can run the tool in	°F or °C
Tensile and torque limits	Must exceed your worst load case	Maker's data sheet

Standards and certification

API Spec 7-1 in its 2023 edition covers stabilizers along with collars and subs, and API Spec 7-2 governs every end connection. No API specification defines a jar or shock sub mechanism, so the maker's design data, service manual and approved parts set the standard for those. DS-1 covers inspection of BHA tool connections and bodies, and many contracts call for it.

Failure modes and inspection

Stabilizer and reamer gauge wear, chipped or lost hardfacing and inserts, and cracking at the blade roots or welds.
Reamer cutter bearing wear, seen as flat spots or play.
Jar seal leaks and oil loss, which change the delay and impact. Detent or latch wear lowers the firing load.
Mandrel and spline wear, and fatigue cracks in the mandrel and housing threads.
Shock sub spring fatigue or elastomer hardening.

Measure stabilizer and reamer gauge with a ring gauge against the nominal hole size. A shop service on a jar strips it, inspects mandrels and housings by magnetic particle or ultrasonic testing, fits new seals and oil and runs it on a jar tester. The test sheet records the firing loads at a stated temperature against the maker's range.

Documentation

Serial number, maker's part number and data sheet for each tool.
For jars and shock subs, the last service date, the jar tester sheet and circulating hours since service.
Connection inspection to API RP 7G-2 or DS-1.
Gauge measurements for stabilizers and reamers, and any re-sleeve or re-hardfacing record.
BHA run reports showing where the tool ran and how many times a jar fired.

Integration with the BHA

Place the jar away from the neutral point and from changes in stiffness, using the maker's placement software or guidance. Size the accelerator to the jar and the drill collar weight above it. Check every stabilizer OD drifts your casing, that fishing necks are long enough for an overshot, and that bores pass the drop balls and survey tools you plan to run.

Refurbishment and when to buy what

Integral blade stabilizers are simple steel and come back into service once the gauge and connections pass. Sleeve stabilizers take a new sleeve. Jars, accelerators and shock subs need a redress with OEM parts and a fresh test after every run or at the maker's hour limit. NOV and SLB are well-known jar makers, and their service centres hold test sheets for tools they redress. Unused surplus jars still need new seals after long storage. For the subs that connect these tools, see Drilling Tools and Accessories.

FAQ

Frequently asked questions

Can I run a used drilling jar without a service?

No, run a used jar only with a recent service and jar tester sheet. The seals and oil decide its delay and impact, and only a strip-down and test show their condition. Plan a full redress with OEM parts if the test sheet is missing or the circulating hours exceed the maker's limit.

How do I check the gauge on a used stabilizer?

Measure the blade OD with a ring gauge or calipers and compare it with the nominal hole size the stabilizer was built for. Then look for chipped or missing hardfacing and inserts and cracks at the blade roots. An undergauge stabilizer changes the BHA's directional response.

What is the difference between a hydraulic and a mechanical drilling jar?

A hydraulic jar delays the impact by metering oil, so the driller controls the blow with the overpull applied. A mechanical jar fires at a preset latch load regardless of how hard you pull. Hydraulic-mechanical jars combine both, with the firing mode differing between the up and down strokes, so check the maker's data for each direction.

Do I need an accelerator with a drilling jar?

An accelerator is recommended in most BHAs and needed in shallow or high-angle holes where the string above cannot stretch enough. It stores energy above the jar, raises the impact at the stuck point and protects the string and top drive from the shock.

What should a jar test sheet show?

A jar test sheet should show the up and down firing or latch loads, the test temperature and the maker's acceptance range. It should also list the serial number, service date and parts replaced. Compare the recorded loads with your planned overpull.

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