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8.3) Wellheads Subsea

Following with the drilling operations the next piece to be landed is the Wellhead this is a strong cylindrical piece of Low Alloy Steel that has to comply with the strong forces acting up and down it we are talking in million of pounds and also has to be able to support the casing strings attached to it and suspended by means of casing hangers, and the final tubing hanger string, the basic designs are for pressures ratings to 10,000 PSI and 15,000 PSI WP. The nominal and most common used size for Subsea Wellhead Housing is the 18-3/4". Figure of the left shows a generic Subsea Wellhead  the top of it has a special arrangement that make contact with the BOP Stack adapter for continuing the drilling operations.







After have landed with the Running Tool for installing the Wellhead is time now to set and land secure the BOP Stack like shown in the figure of the right


The BOP Stack is mandatory in this operation and allows a safety device that can take quick action on the event of a well's kick down allowing immediately shutdown, and rapid control of the well; other vital functions are close the rams no matter what is in the middle of it, which means Shear pipes, Tools Connections and Secure the well. Now with this device installed the bore under control is in the order of the 20" ID which allows the drilling and installation of the Casings of 13-3/8" OD;10-3/4" OD ;9-5/8" OD; 8-5/8" up to 4-1/2" OD Production Tubing , and also any mix of tube Programs in the rang mentioned.

Typical tube Programs can be: 30" OD Casing Conductor x 20" OD Casing String Connected to the Wellheads Housing x 13-3/8" OD Casing x 9-5/8" OD Casing x 7" OD Casing x 4-1/2" OD Tubing


Ahead is the final configuration of the Subsea Wellhead with the Casing Hangers and the tube program, this is a typical configuration for a Depth Well also is the Tubing hanger in the inner of the figures


The BOP attached to the top connector of the Wellhead makes possible the control of the Well's Fluids, as well as the process of drilling because is a closed container that goes from bottom of the wellhead up to the Drill Floor (Diverter System) on the ship via a casing string of 20"OD Casing, allowing close control of all the tubes being runned. There is also he good to mention that exist a variety of running tools for each mandrel hanger, packoff; as well as bowl protectors, test plugs, cup testers, and each Wellhead manufacturer has its own proven design, preferred design for top connection to the BOP is perhaps the Vetco H4 profile, but other competitions has their own design also. There is many details in this Subsea Wellhead Completions like the holes distributed in the mayor OD of any Casing Hanger allowing the passage of Control Fluid and Cement Returning, this holes are critical and has to be calculated accurately in order to give a consistent flow by area of fluids that makes possible the Well's Control as well a rapid Cementing Injection Process.

The time consumed for the Well's Completion until this stage is several months and depend on factors like the good process of drilling and running the Casing Downhole, also the weather is critical, and the good response of the formation too, many casing strings like 16" OD,13-3/8" OD where lost in the process of drilling because the formation didn't support them, and they were swallowed literally, other cases show problems of severe leakage to the ocean in the Casing Housing of 20" OD attached to the BOP so the cases are tons and in this business is always a new threat and challenge.


Updated on:02/21/05Copyright @2003 Daniele Molaro Consulting

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