Subsea Wellhead Systems

The foundation of any subsea well is the subsea wellhead. The function of the subseawellhead system is to support and seal the casing string in addition to supporting a BOP stack during drilling and also the subsea tree under normal operation. Installation of equipment to the seabed is enerally done by one of two methods:

1. With the use of tensioned guidelines attached to guide sleeves on the subsea structure orienting and guiding equipment into position;

2. With a guideline-less method that uses a dynamic positioning reference system to move the surface vessel until the equipment is positioned over the landing point, after which the equipment is lowered into place.

Regardless of the guidance system, the procedure in which the wellhead system is installed is as follows:

1. The first component installed is the temporary guide base. The temporary guide base serves as a reference point for the installation of subsequent well components compensating for any irregularities of the seabed. For guideline systems the temporary guide base also acts as the anchor point for guidelines.

2. The conductor housing is essentially the top of the casing conductor. The casing conductor and housing are installed through the temporary guide base, either by piling or drilling, and provides an installation point for the permanent guide base and a landing area for the wellhead housing.

3. The permanent guide base, which is installed on the conductor housing, establishes structural support and final alignment for the wellhead system. The permanent guide base provides guidance and support for running the BOP stack or the subsea tree.

4. The wellhead housing (or high-pressure housing), which is installed into the conductor housing, provides pressure integrity for the well, supports subsequent casing hangers, and serves as an attachment point for either the BOP stack or subsea tree by using a wellhead connector.

5. To carry each casing string, a casing hanger is installed on top of each string and the casing hangers are supported in the wellhead housing, which thus supports the loads deriving from the casing. To seal the inside annuli, an annulus seal assembly is mounted between each casing hanger and the well housing.


The marine riser is used as a conduit to return the drilling fluids and cuttings back to the rig and to guide the drill and casing strings and other tools into the hole. Geiger and Norton [4] have provided a short and relevant description on floating drilling. The well is begun by setting the first casing string known as the conductor or structural casingda large-diameter, heavy-walled piped to a depth that is dependent on soil conditions and strength/fatigue design requirements. Its primary functions are to:

  • Prevent the soft soil near the surface from caving in;
  • Conduct the drilling fluid to the surface when drilling ahead;
  • Support the BOP stack and subsequent casing strings;
  • Support the Christmas tree after the well is completed.

The depth and size of each drilling string is determined by the geologist and drilling engineer before drilling begins. When drilling from a emisubmersible or drill ship, thewellhead and BOP must be located on the seabed. The BOP stack is used to contain abnormal pressures in the wellbore while drilling the well. The primary function of the BOP stack is to preserve the fluid column or to confine well fluids or gas to the borehole until an effective fluid column can be restored. At the lower end of the riser is the lower flexjoint. After the hole has been drilled to its final depth, ectric logs are run to determine the probable producing zones. Once it has been determined that sufficient quantities of oil and gas exist, the roduction tubing is then run to the zone determined to contain that oil or gas. Only after this takes place is the well “completed” by removing the BOP stack and installing the fittings used to control the flow of oil and gas from the wellhead to the processing facility.

Tree and Tubing Hanger System

To complete the well for production a tubing string is installed in and supported by a tubing hanger. The tubing hanger system carries the tubing and seals of the annulus between casing and tubing. To regulate flow through the tubing and annulus, a subsea tree is installed on the wellhead. The subsea tree is an arrangement of remotely operated valves, which controls the flow direction, amount, and interruption.


[1] World Oil, Composite Catalog of Oilfield Equipment & Services, forty fifth ed., Gulf Publishing Company, Houston, 2002/03.

[2] International Standards Organization, Petroleum and Natural Gas Industries – Design and Operation of Subsea Production Systems – Part 7: ompletion/Workover/Riser System, ISO 13628-7, 2005.

[3] T. Clausen, R. D’Souza, Dynamic Risers Key Component for Deepwater Drilling, Floating Production, Offshore Magazine, vol. 61, (2001), May.

[4] P.R. Geiger, C.V. Norton, Offshore Vessels, Their Unique and Applications for the Systems Designer, Marine Technology, vol. 32 (No. 1) (1995) 43–76.

[5] American Petroleum Institute, Recommended Practice for Design, Selection, Operation and Maintenance of Marine Drilling Riser Systems, API-RP- 16Q (1993).

[6] American Petroleum Institute, Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platform (TLPs), API-RP- 2RD (1998).

[7] H. Matlock, Correlations for Design of Laterally Load Piles in Soft Clay, OTC, 2312, Offshore Technology Conference, Houston, Texas, 1975.

[8] K. Vandiver, L. Lee, User Guide for Shear7 Version 4.1, Massachusetts Institute of Technology, Cambridge, 2001, March 25.

[9] J.F. Archard, Contact and Rubbing of Flat Surfaces, Journal of Applied Physics, vol. 24 (No. 8) (1953) 981.