Wellheads and Christmas Tree are not the same.
Wellhead systems are required to control the pressure during drilling and production, and serve as a point to suspend casing & tubing strings, as well as a connection to the surface pressure-control equipment, i.e. blow out preventer (BOP) ( during drilling stage) and christmas tree ( during production stage). It also allows access to a live well (under pressure), in a safe and controlled manner. A wellhead consists of heads, hangers, spools and a series of seals.
Christmas Tree consists of chokes, valves and spools. The main purpose of the christmas tree is to control the flow of well fluids or gas during production. Additional functions of christmas tree include pressure relief, chemical injection, a conduit for control systems to monitor and control safety valves in the down-hole.
Both, wellhead systems and christmas tree are manufactured in different sizes and configuration, with various pressure and temperature rating, material class requirements and completion capacity, i.e. single or multiple zones. Standard working pressure rating classification (in psi) is 2K, 3K, 5K, 10K, 15K and 20K. Wellhead systems and christmas tree (BOP during drilling) are the most critical well component, as they act as a “gate” between subsurface and surface.
Wellheads are divided by application and types.
- Land / Onshore
- Surface ( jack-up or platform)
Christmas Trees are divided by application and type, as well.
Subsea Trees far more complex than Surface Trees, since the Subsea Trees must be designed to allow tie-ins and umbilical connections subsea. Christmas tree could be made from a single block of steel ( i.e. machined from a big size steel bar) and called a Solid Tree, or a Composite Tree, which is an assembly of valves around a central steel body. Composite Trees are mainly used for low risk and low-pressure wells, as the composite nature of the tree may be prone to leaks. API 6A and ISO 10423 are the standards used in wellhead and christmas tree equipment manufacturing. Below is a quick reference guide for material and temperature classification ( note, API 6A some several qualifications and reference to other standards)