Every conventional land drilling rig uses some form of bell nipple flange connected at the top of the blowout preventer ( BOP ) stack. This flange does not hold well pressure, but acts as a fastening device to connect a vertical pipe, referred to as a bell nipple, to the top of the BOP stack. For a typical installation see the illustration below.


Operators have chosen many ways to fabricate this assembly of flange and pipe nipple. Sometimes they make the flange from torch cut plate, thin and flimsy, without a ring groove, using a piece of “soft line” in the BOP ring groove to make a seal. Sometimes they use a flange cut from some piece of wellhead equipment.

Often operators fasten the flange to the tapped upper connection of the annular BOP with only 4 bolts, the remaining tapped stud holes in the BOP upper connection left without the protection of tightly engaged tap end studs. The empty tapped stud holes collect water and waste and rapidly deteriorate and become useless. In some cases, operators attempting to protect the tapped holes with short “screwed-in” all thread plugs; find that only drilling in a machine shop will remove the plugs. These common, less expensive installations, may actually have the greatest cost among the available options.

One of the most practical, and over time least costly, bell nipple and flange assemblies, utilizes a full thickness flange, retained with a full set of tap end studs tightly engaged in all the tapped holes of the BOP.

Because normal operations require unrestricted access to the full bore of the BOPs, a bell nipple flange must have a full bore. The pipe size chosen for the bell nipple must have an I.D. at least equal to the BOP stack I.D. See the illustration below.


Both the flange and pipe chosen to make the assembly should have good field weldability. Either low grade ( J ) casing or 36K line pipe works well to fabricate the bell nipple. A “special” flange with a counter bore sized to accept the chosen pipe, a shoulder for the pipe to rest on prior to welding, a taper to guide the bit to center, and a steel chemistry chosen for field weldability, will allow easy and predictable installation. These factors will also facilitate modification of the bell nipple and flange assembly as necessary for BOP connection changes, or when the drilling rig moves to another location.


7-1/16″8″ Sch. 40, 36 K line pipe,
8-5/8″ 32 Lb. J casing
9″ 10″ Sch. 40, 36 K line pipe
10-3/4″ 40.5 Lb. J casing
11″ 12″ Std. or Sch. 40, 36 K line pipe
13-3/8″ 48 Lb. J casing
13-5/8″ 16″ Sch. 30 or Std, 36 K line pipe
16″ 65 Lb. J casing
16-3/4″ 2M, 3M 18″ Std, 36 K line pipe
16-3/4″ 5M, 10M 18″ Std, 36 K line pipe
20″ Std, 36 K line pipe
20″ 94 Lb. J casing
20-3/4″ 24″ Std, 36 K line pipe
21-1/4″ 24″ Std, 36 K line pipe

Some of the above nominal bore flanges can have counterbores that accept either casing or line pipe when these pipe varieties share the same O.D. 

Typically, fabrication of the bell nipple and flange occurs at the well site to satisfy measurement taken after setting the conductor pipe or surface casing and installing the BOP stack. Each actual bell nipple fabricated atop the bell nipple flange must meet the requirements of the individual situation, and the operator should have the equipment planned for and ready when needed. When the flange dimension and chemistry, and the pipe dimension and grade satisfies the description above, the bell nipple assembly can experience multiple cut and weld changes without frequent replacement.

WOODCO USA manufactures bell nipple flanges as a standard product with specific material to facilitate field welding and dimensions that protect the BOP.

For more information about API Flanges, click here.

Click here to view the API Flange weight reference chart.

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