Choke and kill lines: what they do and what they are made of
Choke and kill lines are the plumbing of well control. When a well kicks, everything that comes out goes through them. Here is how they are built and where 4130 pipe and fittings sit.
A choke line carries well fluids from the blowout preventer stack to the choke manifold, where the flow is throttled to control the well; a kill line carries mud from the pumps into the well. Both must hold at least the stack's working pressure. They are built as rigid lines of heavy-wall seamless pipe and fittings, as flexible hoses, or as articulated lines of pipe and swivel joints, to API 16C, and laid out to API Standard 53.
Choke line and kill line compared
| Choke line | Kill line | |
|---|---|---|
| Flow direction | Out of the well, to the choke manifold | Into the well, from the pumps |
| Purpose | Circulate a kick out under control | Pump kill-weight mud into the well |
| Minimum size, API Std 53 (4th ed.) | 2 in for 2K to 5K systems; 3 in for 10K and above | 2 in |
| Erosion | Highest: gas, sand and cuttings at speed | Lower, unless used for flow out |
Rigid, flexible and articulated lines
- Rigid lines are built from heavy-wall seamless pipe, elbows, tees and long sweeps, welded or connected with flanges, clamps or unions. They are strong and simple to inspect, but must be routed and supported with care. This is where 4130 pipe and fittings are used.
- Flexible lines are reinforced hoses that take up movement between the stack and the manifold. They are installed within the maker's minimum bend radius.
- Articulated lines join rigid sections with swivel joints so the line can move, within the maker's allowable movement.
API Specification 16C covers the design and manufacture of choke and kill equipment, and API Standard 53 says choke and kill systems are to be designed, made and installed in accordance with it.
Size and layout rules
| Topic | Rule |
|---|---|
| Routing | As straight as possible |
| Turns | Targets or fluid cushions at 90° block ells, tees and bends under R/d 10 |
| Choke line size | 2 in nominal minimum for 2K, 3K and 5K; 3 in for 10K and above |
| Kill line size | 2 in nominal minimum |
| Pressure | Equipment up to and including the choke rated at least to the lower of the ram BOP and wellhead working pressures |
| Connections at 3,000 psi and above | Flanged, welded or clamped, to API 6A and API 16A |
| Lines downstream of the chokes | Inside diameter at least equal to the choke connection |
| Inspection | Checked for wear, erosion and plugging on the BOP inspection schedule |
From the conformance checklist for the fourth edition of API Standard 53. Check the edition your project is built to.
How designers meet the rule on turns, with targeted tees, cushions or large-radius bends, is set out in long sweep bends vs targeted tees.
Where 4130 fits
| Part | Typical 4130 product |
|---|---|
| Rigid line runs | Seamless pipe, heavy wall, for example 5.5 in OD by 3 in ID for 15,000 psi |
| Turns | Long sweep elbows, or tees used as targets |
| Size changes | Reducers, each end matched to its pipe |
| Manifold headers and branches | Tees and crosses |
| Temporary and test connections | Integral fittings with hammer union ends |
| Manifold junctions | Studded tees and crosses to API 6A |
| Flow control | Adjustable and positive chokes with tungsten carbide trim |
The whole assembly, from studded blocks and chokes to the spools between them, can be built to your drawing as a fabricated choke and kill manifold.
Where the well may produce hydrogen sulfide, every pressure-containing part, and its welds, must meet NACE MR0175 / ISO 15156 limits: for 4130, 22 HRC maximum. See 4130 for sour service.
Frequently asked questions
What is the difference between a choke line and a kill line?
The choke line carries fluid out of the well, from the BOP stack to the choke manifold, so the flow can be throttled while a kick is circulated out. The kill line carries fluid in, from the pumps to the stack, to pump kill-weight mud into the well.
What size must a choke line be?
Under the fourth edition of API Standard 53, at least 2 in nominal for 2,000, 3,000 and 5,000 psi systems and at least 3 in for 10,000 psi and above. A kill line must be at least 2 in.
Are choke and kill lines 4130?
Rigid lines are commonly built from heavy-wall seamless chromium-molybdenum pipe such as 4130, with fittings and long sweeps of the same steel. The materials must suit the service and meet API 16C, including NACE MR0175 limits where the well is sour.
Sources
- IADC, API Standard 53 fourth edition conformance assessment tool
- API Specification 16C, choke and kill equipment
- API Standard 53, blowout prevention equipment systems for drilling wells