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The unitized wellhead is very different from the spool wellhead system, because it incorporates different design characteristics and features.
Characteristics of a unitized wellhead
The unitized wellhead, shown in Fig. 1, is a one-piece body that is typically run on 13 3/8 -in. casing through the BOP, and lands on a landing shoulder located inside the starting head or on top of the conductor itself. The casing hangers used are threaded and preassembled with a pup joint. This way, the threaded connection can be pressure tested before leaving the factory, ensuring that the assembly will have pressure-containing competence. Gate valves are installed on the external outlet connections of the unitized wellhead to enable annulus access to each of the intermediate and the production casing strings.
After the next hole section is drilled, the casing string, topped out with its mandrel hanger, is run and landed on a shoulder located in the ID of the unitized wellhead. A seal assembly is run on a drillpipe tool to complete the casing-hanger and seal-installation process. Each additional intermediate casing string and mandrel hanger is run and landed on top of the previously installed casing hanger without removing the BOP stack. Besides saving valuable rig time, the other advantage of the unitized wellhead system over spool wellhead systems is complete BOP control throughout the entire drilling process.
The unitized wellhead (Fig. 2) consists of the following components:
- Unitized wellhead body.
- Annulus gate valves.
- Mandrel casing hangers.
- Mandrel tubing hangers.
- Metal-to-metal sealing for the annulus seals.
Mandrel casing hangers
The mandrel casing hangers (see Fig. 3) are a one-piece construction, and are manufactured to meet the casing and thread types specified by the customer. The mandrel casing hanger has a 4° tapered sealing area on its OD. The mandrel hanger also incorporates running threads and seal-assembly threads to facilitate installation. The hanger carries a lock ring that locks the hanger down when the seal assembly is installed. The mandrel casing hanger lands on either the shoulder located in the bottom of the unitized wellhead body or on top of the previous casing hanger.
The seal assembly incorporates a metal-to-metal or elastomeric seal (Fig. 4), which is run on a running tool through the BOP stack once the casing has been cemented. The seal assembly seals off the pressure from above and below, and isolates the annulus from the wellbore. The annulus can still be monitored through the outlets on the unitized wellhead body and the gate valves mounted to them.
There is a full range of tools available for the unitized wellhead system:
- Wellhead-housing running tool.
- BOP test tool.
- Casing-hanger running tool.
- Seal-assembly running and retrieving tool.
- Wear-bushing running and retrieving tool.
The unitized wellhead is more often used with platform-development projects than with exploration applications.