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Subsea wellhead systems
The subsea wellhead system (Fig. 1) is a pressure-containing vessel that provides a means to hang off and seal off casing used in drilling the well. The wellhead also provides a profile to latch the subsea blowout preventer (BOP) stack and drilling riser back to the floating drilling rig. In this way, access to the wellbore is secure in a pressure-controlled environment. The subsea wellhead system is located on the ocean floor, and must be installed remotely with running tools and drillpipe.
Subsea wellhead system
The subsea wellhead inside diameter (ID) is designed with a landing shoulder located in the bottom section of the wellhead body. Subsequent casing hangers land on the previous casing hanger installed. Casing is suspended from each casing-hanger top, and accumulates on the primary landing shoulder located in the ID of the subsea wellhead. Each casing hanger is sealed off against the ID of the wellhead housing and the outside diameter (OD) of the hanger itself with a seal assembly that incorporates a true metal-to-metal seal. This seal assembly provides a pressure barrier between casing strings, which are suspended in the 18¾-in. wellhead.
Once drilling is complete, the wellhead will provide an interface for the production tubing string and the subsea production tree, or, if required, a point to tie back to a platform. The design objective of the subsea wellhead system is twofold:
- To provide the operator with the latest equipment technology, incorporating reliable solutions for the well conditions to be encountered, as well as maximum strength and capacities.
- To provide a system that is easy to install, and requires a minimal amount of handling and rig time.
A standard subsea wellhead system will typically consist of the following:
- Drilling guide base.
- Low-pressure housing.
- High-pressure wellhead housing (typically 18¾ in.).
- Casing hangers (various sizes, depending on casing program).
- Metal-to-metal annulus sealing assembly.
- Bore protectors and wear bushings.
- Running and test tools.
Drilling guide base
The drilling guide base (Fig. 2) provides a means for guiding and aligning the BOP onto the wellhead. Guide wires from the rig are attached to the guideposts of the base, and the wires are run subsea with the base to provide guidance from the rig down to the wellhead system.
The low-pressure housing (typically 30 or 36 in.; see Fig. 3) provides a location point for the drilling guide base, and provides an interface for the 18¾-in. high-pressure housing. It is important for this first string to be jetted or cemented in place correctly, because this string is the foundation for the rest of the well.
The subsea high-pressure wellhead housing (typically 18¾ in.) is, effectively, a unitized wellhead with no annulus access. It provides an interface between the subsea BOP stack and the subsea well. The subsea wellhead is the male member to a large-bore connection, as shown in Fig. 4 (the female counterpart is the wellhead connector on the bottom of the BOP stack), that will be made up in a remote subsea, ocean-floor environment. The 18¾-in. wellhead will house and support each casing string by way of a mandrel-type casing hanger. The ID of the 18¾-in. wellhead provides a metal-to-metal sealing surface for the seal assembly, when it is energized around the casing hanger. The wellhead provides a primary landing shoulder in the bottom ID area to support the combined casing loads, and will typically accommodate two or three casing hangers and a tubing hanger. The minimum ID of the wellhead is designed to let a 17½-in. drilling bit pass through.
Fig. 4—18¾-in. wellheads are manufactured with several different locking profiles to mate with the wellhead connector located on the bottom of the BOP stack or subsea production tree. The wellhead systems are usually rated for 10,000 or 15,000 psi and can be installed with a standard lock ring or a rigid lockdown mechanism, which is the preferred choice for deepwater operations.
All subsea casing hangers are mandrel type, as shown in Fig. 5. The casing hanger provides a metal-to-metal sealing area for a seal assembly to seal off the annulus between the casing hanger and the wellhead. The casing weight is transferred into the wellhead by means of the casing hanger/wellhead landing shoulder. Each casing hanger stacks on top of another, and all casing loads are transferred through each hanger to the landing shoulder at the bottom of the subsea wellhead. Each casing hanger incorporates flow-by slots to facilitate the passage of fluid while running through the drilling riser and BOP stack, and during the cementing operation.
Metal-to metal annulus seal assembly
The seal assembly (Fig. 6) isolates the annulus between the casing hanger and the high-pressure wellhead housing. The seal incorporates a metal-to-metal sealing system that today is typically weight-set (torque-set seal assemblies were available in earlier subsea wellhead systems). During the installation process, the seal is locked to the casing hanger to keep it in place. If the well is placed into production, then an option to lock down the seal to the high-pressure wellhead is available. This is to prevent the casing hanger and seal assembly from being lifted because of thermal expansion of the casing down hole.
Bore protectors and wear bushings
Once the high-pressure wellhead housing and the BOP stack are installed, all drilling operations will take place through the wellhead housing. The risk of mechanical damage during drilling operations is relatively high, and the critical landing and sealing areas in the wellhead system need to be protected with a removable bore protector and wear bushings, as shown in Fig. 7.
Running and test tools
The standard subsea wellhead system will include typical running, retrieving, testing, and reinstallation tools (see Fig. 8). These tools include:
Conductor wellhead running tool
The conductor wellhead running tool runs the conductor casing, conductor wellhead, and guide base. This tool can be used for jetting in the conductor, or cementing the conductor into a predrilled hole. The tool is a cam-actuated tool that minimizes any high torque that may be encountered during operations.
High-pressure wellhead running tool
The high-pressure wellhead running tool operates just like the conductor wellhead running tool, but it runs the high-pressure wellhead and 20-in. casing. It is a cam-actuated tool that minimizes any high torque that may be encountered during operations.
Casing-hanger seal-assembly running tool
The casing-hanger seal-assembly running tool runs the casing, casing hanger, and seal assembly in one trip. It also allows testing of the seal assembly (after installation) and the BOP stack, and it has the additional benefit of bringing back the seal assembly, if debris is in the way and the seal assembly cannot be installed.
Multipurpose tool and accessories
The multipurpose tool runs and retrieves the nominal bore protector and all wear bushings. A jet sub and/or jet sub extension can be attached to the multipurpose tool so that wellhead washout can occur during the retrieval process. The multipurpose tool also retrieves the seal assembly, and becomes a mill-and-flush tool by attaching the mill-and-flush adapter.
BOP isolation test tool
The BOP isolation test tool allows testing of the BOP stack without allowing pressure to be applied against the casing-hanger seal assembly. The BOP isolation test tool can land on the casing hangers or wear bushings.
Seal-assembly running tool
The seal-assembly running tool is used in the event that a second seal assembly needs to be run. The seal-assembly running tool is a weight-set tool, and, like the casing-hanger seal-assembly running tool, it allows testing of the BOP stack and recovers the seal assembly if it cannot be installed (because of debris in the sealing area of the annulus).
Big bore subsea wellhead systems
As the offshore oil and gas industry has continued to explore in progressively deeper waters, the requirements for well components have changed, as a result of the challenges associated with deepwater drilling. Ocean-floor conditions in deep and ultradeep water can be extremely mushy and unconsolidated, which creates well-foundation problems that require development of new well designs to overcome the conditions. Second, underground aquifers in deep water have been observed in far greater frequency than in shallower waters, and it quickly became clear that these zones would have to be isolated with a casing string. Cementing requirements changed, and wellhead equipment designs would also have to change to accommodate the additional requirements.
With subsea wellhead systems, conductor and intermediate casing strings can be reconfigured to strengthen and stiffen the upper section of the well (for higher bending capacities), and overcome the challenges of an unconsolidated ocean floor at the well site. Each “water flow” zone encountered while drilling requires isolation with casing and, at the same time, consumes a casing-hanger position in the wellhead. It became obvious that more casing strings and hangers were required to reach the targeted depth than the existing wellhead-system designs would accommodate.
The 18¾-in. Big Bore Subsea Wellhead System (Fig. 9) was designed for wells that will be installed in unconsolidated ocean-floor conditions and penetrate shallow water-flow zones. These well conditions require additional casing strings. The wellhead system incorporates an 18¾-in. high-pressure wellhead housing designed for 15,000 psi and 7 million pounds end-load carrying capacity. Unlike conventional subsea wellhead systems, the big-bore high-pressure wellhead housing (Fig. 10) is run atop 22-in. pipe (as opposed to 20-in. pipe), and has a large minimum ID bore to pass 18-in. casing. The wellhead system incorporates a rigid lockdown mechanism to preload the connection between the high-pressure wellhead and the conductor wellhead. A supplemental hanger adapter is installed in the 22-in. casing to provide a landing shoulder and seal area for the 18-in. and 16-in. supplemental hangers and their testable, retrievable seal assemblies.
Optional 28-in., 26-in., and 24-in. supplemental casing-hanger systems can be incorporated into the design to:
- Accommodate a secondary conductor string.
- Increase the overall bending capacity of the upper section of the well.
- Provide an additional barrier for a water-flow zone.
All casing hangers and seal assemblies are run, set, and tested on drillpipe in a single trip. These subsea wellhead systems can easily accommodate alternative casing programs, and can be configured to address any deepwater (and shallow-water) drilling application.