API 6A Wellhead Design

Many types of surface wellhead are available based on the different scenarios. Some are simple. Some are complicated. A simple wellhead can consist of one threaded gate valve on the production casing and one threaded flow tee. A complicated wellhead can consist of Base Plate, casing head, Casing Spool, Tubing Spool, Tubing Head Adapter, Lower Master Valves, Upper Master Valves, Cross, Swab Valve, Tree Cap Test Adapter, Casing Slips Hanger, Casing Packoff, Tubing Hanger, Wing Valve, Outlet Gate Valves, Bland Flange, Bull Plugs, and Pressure Gauges. See Wellhead and Christmas Tree illustration in Home page.

Based on lift method, wellhead system can be classified into Flowing Wellhead System and Artificial Lift Wellhead system (Rod Pumping Wellhead system, PCP, ESP, Gas Lift, Frac) in sweet, sour and corrosive well systems. For saving drilling operation time purpose, Unitized Wellhead system is designed with less components performing same function of standard wellhead system.


How to design Casing Head and Tubing Head?

Casing head and Tubing head design must consider the casing/tubing size, service temperature, well fluid properties.

  • Design must meet API 6A requirements (load, temperature, material, NACE)
  • Casing Head and Tubing Head Bore can be straight bore design like C-22, C-29 or tapered design to accept casing slips or thermal wellhead hangers
  • Choose Casing Head Bottom Connections based on well conditions:
  1. SOW (Slip-On Weld): Weld on Casing. Usually used on Nace or high pressure well.
  2. Threaded: Threaded on Casing, usually thread types are buttress or long casing. Usually used on lower pressure or Non-Nace well.
  3. Slip-on-lock: Grip by biting the casing through the teeth. Usually used temporally during drilling phase.

How to design Casing Hanger/Tubing Hanger?

Hangers are mainly used to hanging casing or tubing strings and hold downhole pressure with build-in BPV thread. Minimum design consideration should include casing/tubing size and weight, service temperature, H2S and/or Corrosive fluid.

Design must meet API 6A requirements (load, temperature, material, NACE). There are several types of hanger based on different application:

  • Slip Hanger: Biting on casing through teeth
  • Mandrel Hanger: Landed in regular hanger. Hanging casing/tubing strings by thread connection.
  • Regular hanger: Landed in the shoulder of casing bowl or tubing bowl. Hanger features based on well completion or application:
  1. One hole, dual holes, or triple holes
  2. Instrument ports
  3. Seals for sealing casing or annulus
  4. Features for accepting lockdown screws
  5. Two pieces splitted hanger
  6. Alignment designs when hanger have two or more holes or instrument ports
  7. BPV thread to accommodate back pressure valve
  8. Threads types can be Casing or Tubing thread, ACME thread or Stub ACME for lifting or landing strings

How to Choose or Design a Wellhead Seal?

A variety of wellhead seals has been available to seal casing or tubing string, annulus, flanges, and tooling:

  • O-Rings: O-ring normally used in low pressure application on Hanger, Casing/Tubing Spool, Packoff Flange, Or Tooling. Sometime they can be used in non-sealing applications like ET style lockdown screw. O-ring working temperature is normally below 400F.
  • FS Seal: It is usually used in low / medium pressure wellhead and allowed large clearances between mating parts like Packoff and Casing or Tubing Spool and Casing.
  • S-Seal: A metal garter spring is incorporated into an elastomeric seal element. This elastomer enforcement feature enables it to withstand large extrusion gaps under high pressure. High pressure tubing hangers often use this seal although it is pricey.
  • Graphite Seal: It is usually die-formed and used in high temperature thermal application.
  • Rope Seal: It is a compression packing seal used in wellhead stuffing box, with hangers, or in lockdown screws. The seal can be used in extreme low temperature or extreme high temperature. The application can be static or dynamic. Pay attention on cutting angle, assembly patterns, and appropriate torque in order to achieve designed sealing function.
  • Metal Seal: A transition between wellhead components in severe environment like corrosive, high temperature, or high-pressure environment. Metal seal and seal bore preparation of wellhead component must have tight tolerance and appropriate coating in order to achieve the designed sealing target.