Wellhead

A wellhead is a general term used to describe the pressure containing component at the surface of an oil well that provides the interface for drilling and production equipment.

The main purpose of a wellhead is to provide a pressure barrier connecting the casing strings that run from the bottom of the hole sections to the surface pressure control equipment.

Whilst drilling the oil well the surface pressure control is provided by a Blowout preventer or 'BOP'. If the pressure is not contained during drilling operations by the casings, wellhead and BOP, then a well blowout can occur.

Once the well has been drilled, a completion is placed in the well to provide the conduit for the well fluids. The surface pressure control is provided by a christmas tree which is installed on top of the wellhead, and has isolation valves and choke equipment to control the well fluids.

Wellheads can be located on the facility oil platforms/onshore called a surface wellhead, or subsea subsea wellhead/mudline wellhead.

A wellhead system provides the following basic functionality:

* connection for a blowout preventer or bop
* support of the casing and tubing strings (the tubing string may also be suspended in the Christmas tree;
* providing a seal between the different strings;
* allowing for access to annuli between the different casing/tubing strings.

Components

The primary components of a wellhead system are:

* casing head
* casing spools
* casing hangers
* packoffs and isolation seals
* bowl protectors
* test plugs
* mudline suspension systems
* tubing heads
* tubing hangers
* tubing head adapters

Specification

The basic requirements for materials, dimensions, test procedures and pressure ratings for wellheads and wellhead equipment are defined on API Spec 6A: Specification for Wellhead and Christmas Tree Equipment. Wellheads are cemented in place and are generally permanently kept in place, although in exploration wells they may be recovered for use again.

Design factors

Wellheads are manufactured for numerous different purposes.

The main oil industry specifications are :

1. API 6A Specification for Wellhead and Christmas Tree Equipment
2. ISO 10423 Wellhead and Christmas Tree Equipment

Functions

A wellhead serves numerous functions. Some of these are:

1. Means of casing suspension. (Casing is the permanently installed pipe used to line the well hole for pressure containment, collapse prevention, etc.)
2. Means of casing pressure isolation when multiple casing strings are used
3. Means of attaching a blowout preventer during drilling
4. Means of attaching a tree for well control during production, injection, or other operations
5. Means of well access
6. Means of pump attachment
7. Means of tubing suspension (Tubing is removable pipe installed in the well)

From http://en.wikipedia.org/