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Mudline suspension system
From a historic point of view, as jackup drilling vessels drilled in deeper water, the need to transfer the weight of the well to the seabed and provide a disconnect-and-reconnect capability became clearly beneficial. This series of hangers, called mudline suspension equipment, provides landing rings and shoulders to transfer the weight of each casing string to the conductor and the sea bed.
Mudline hanger system
The mudline hanger system (shown in Fig. 1) consists of the following components:
- Butt-weld sub
- Shoulder hangers
- Split-ring hangers
- Mudline hanger running tools
- Temporary abandonment caps and running tool
- Tieback tools
- Cleanout tools.
Each mudline hanger landing shoulder and landing ring centralizes the hanger body, and establishes concentricity around the center line of the well. Concentricity is important when tying the well back to the surface. In addition, each hanger body stacks down relative to the previously installed hanger for washout efficiency. Washout efficiency is necessary to clean the annulus area of the previously run mudline hanger and running tool (Fig. 2). This ensures that cement and debris cannot hinder disconnect and retrieval of each casing riser to the rig floor upon abandonment of the well.
As each hole section is drilled and each casing string and mudline hanger is run, the hanger is positioned in the casing string to land on a landing shoulder inside the mudline hanger that was installed with the previous casing string. Each of the mudline hangers have casing and a mudline hanger running tool made up to it. These running tools are released through right-hand rotation to allow disconnect from the well. The threads on the mudline hanger used by the running tool can be used to install temporary abandonment caps (Fig. 3) into selected hangers to temporarily “suspend” drilling operations at the conclusion of the well.
The main difference between the wellheads used in the land drilling application and the jackup drilling application (with mudline) is the slip-and-seal assembly (Fig. 4). Because the weight of the well now sits at the seabed, a weight-set slip-and-seal assembly is not used. Instead, a mechanical set (energizing the seal by hand) is used, in which cap screws are made up with a wrench against an upper compression plate on the slip-and-seal assembly to energize the elastomeric seal.
Temporarily abandoning the well
The mudline suspension system also allows the well to be temporarily abandoned (disconnected) when total depth (TD) is achieved (when drilling is finished at total depth). When this occurs, the conductor is normally cut approximately 5 to 6 ft above the mudline, and retrieved to the surface. After each casing string is disconnected from the mudline suspension hanger and retrieved to the rig floor in the reverse order of the drilling process, threaded temporary abandonment caps or stab-in temporary abandonment caps (both of which makeup into the threaded running profile of the mudline hanger; see Fig. 3) are installed in selected mudline hangers before the drilling vessel finishes and leaves the location. The temporary abandonment caps can be retrieved with the same tool that installed them.
Reconnecting to the well
A mudline suspension system also incorporates tieback tools to reconnect the mudline hanger to the surface for re-entry and/or completion. These tieback tools can be of two types: threaded and stab-in (see Fig. 5). The tieback tools are different from the running tools in that they makeup into their own dedicated right-hand makeup threaded profile. The stab-in tieback tool offers a simple, weight-set, rotation-lock design that provides an easy way to tie the well back to the surface. A surface wellhead system is installed, and the well is completed similarly to the method used on land drilling operations.
The mudline suspension system has been designed to accommodate tying the well back to the surface for surface completion, and it also can be adapted for a subsea production tree. A tieback tubing head can be installed to the mudline suspension system at the seabed, and a subsea tree can be installed on this tubing head.