A wellhead can be built up one spool at a time as each casing string is set, or it can be a single compact housing installed once at the start. Both suspend the same casing strings and seal the same annuli. The difference shows up in rig time, the number of leak paths and how much flexibility you keep if the well does not go to plan.
Conventional spool wellhead
In a conventional wellhead, a casing head is installed on the surface casing. After the next string is run and hung, the BOP is removed, a casing spool is flanged on top, and the BOP is nippled back up. The process repeats for each casing string, ending with the tubing head spool. See Wellhead Equipment 101 for each component.
- Strengths: simple, widely available and easy to adapt. Slip-type hangers can be set at any depth, so stuck casing is easy to deal with.
- Limits: every spool means nippling the BOP down and up again, which costs rig time and creates a period with reduced well control. Each flanged connection is another ring gasket and another potential leak path.
Compact (multi-bowl) wellhead
A compact wellhead, also called a multi-bowl, unihead or speed head, is one housing with several landing shoulders inside it. It is installed on the surface casing, the BOP is nippled up once, and every later casing string is run and landed through the BOP on a mandrel hanger. Packoffs are set through the BOP with running tools.
- Strengths: one BOP nipple-up for the whole well, which can save many hours of rig time per casing section. Fewer flanged joints mean fewer leak paths, and well control is kept throughout.
- Limits: higher equipment cost, a casing programme that must be fixed in advance, and specialised running tools. If casing gets stuck, an emergency hanger designed for that housing is needed.
Comparison
| Conventional spool | Compact | |
|---|---|---|
| BOP nipple-ups | One per casing section | One for the well |
| Hangers | Slip or mandrel | Mandrel, landed through the BOP |
| Flanged connections | One per spool | Fewer |
| Overall height | Taller | Shorter |
| Equipment cost | Lower | Higher |
| Flexibility for stuck casing | High | Needs emergency hanger |
How to choose
The decision is mostly economic. On wells where rig time is expensive, such as offshore platforms, pad drilling and high-day-rate land rigs, the rig time saved usually outweighs the higher equipment cost. On low-cost land wells with simple casing programmes, a conventional wellhead is still common. Compact designs are also chosen where height is limited, for example on platforms with tight deck spacing.
Well control matters as much as cost. Keeping the BOP on throughout removes the riskiest step in building a conventional wellhead, which is why many operators now default to compact designs on pad wells.
Design points to check
- Load shoulder capacity for the combined weight of all strings hung in one housing.
- Bore protectors and wear bushings to protect sealing areas while drilling through the housing.
- Annulus outlet arrangement for monitoring each annulus.
- Rated working pressure of the housing for the highest-pressure section, since one housing serves the whole well.
Frequently asked questions
Is a compact wellhead covered by API 6A?
Yes. Compact housings, hangers and packoffs are API 6A equipment with the same pressure, temperature and material class requirements as conventional spools.
Can a compact wellhead be used with slip hangers?
Only with emergency slip hangers made for that housing. Normal operation uses mandrel hangers.
Work with RAPID GLOBAL
RAPID GLOBAL designs, analyses and requalifies wellhead, subsea and valve equipment to API standards. If you need help with compact wellhead design, load ratings or requalification, talk to an engineer.
