Wellhead Definition / Meaning
The wellhead is the surface termination of a wellbore that provides a structural and pressure-containing interface for drilling, completion, and production operations. In offshore and subsea environments, the wellhead is situated on the seabed (mudline) and serves as the critical connection point between the wellbore and subsea equipment such as blowout preventers (BOPs), subsea trees, and flowlines. Its primary functions include suspending casing strings, sealing annular spaces, enabling pressure control, and supporting the weight of the wellhead assembly itself.
Core Components
A typical subsea wellhead consists of the following key components:
- Casing head – The lowermost section that supports the conductor and surface casing strings. It includes a mandrel or thread profile for attaching the BOP or subsea tree.
- Tubing head – Located above the casing head, it provides a seal around the production tubing and houses outlets for annulus monitoring and injection.
- Hangers – Mechanical devices (slips or mandrel types) that support the weight of casing and tubing strings within the wellhead.
- Valves – Manual or actuated valves (e.g., master valve, wing valve) that isolate flow paths for pressure control.
- Connectors – High-pressure locking mechanisms (collet, clamp, or thread profiles) that attach the BOP stack or tree to the wellhead.
- Seal assemblies – Metal-to-metal or elastomeric seals that prevent fluid migration between annuli.
Subsea Wellhead vs Surface Wellhead
The design and operation of a subsea wellhead differ significantly from onshore or platform (surface) wellheads, as shown in the comparison below:
| Feature | Surface Wellhead | Subsea Wellhead |
|---|---|---|
| Location | At ground or platform deck level | On the seabed, often thousands of feet underwater |
| Pressure rating | Typically 2,000-5,000 psi | Can exceed 15,000 psi (high-pressure/high-temperature HPHT applications) |
| Installation method | Installed with casing and then above ground | Run on drill pipe and locked into a subsea template or mudline |
| Remote operation | Manual or local hydraulic control | Remotely operated via ROV or subsea control modules |
| Maintenance | Accessible for intervention | Requires ROV or vessel-based operations |
Installation and Operation
During drilling, the wellhead is installed in stages. First, the conductor casing is driven or drilled to a set depth and cemented. A casing head housing is then attached to the top of the conductor. As each subsequent casing string is run and cemented, a casing hanger lands in the housing, and a seal assembly is energized. After all casing strings are in place, the tubing head is installed and the well is completed. The BOP stack is connected to the wellhead before drilling proceeds, and after completion, a subsea tree is installed for production or injection.
In subsea fields, wellheads often include additional features such as corrosion-resistant alloys (CRAs) for sour service, stress joints to handle bending loads from risers, and subsea control pods for actuation. The wellhead must also withstand ocean currents, fatigue from riser motion, and thermal cycling during production.
Usage Example
“A typical deepwater drilling scenario begins with a 36-inch conductor pipe cemented into the seabed, then a 18-3/4” (476.3 mm) 15