Development of the 18-3/4" - 20,000 psi Subsea Wellhead System for High Pressure and High Temperature Applications
Authors: Singeetham Shiva P. , McCoy Bernard 
Published: October 2008

Abstract
There is an increasing need for higher pressure and higher temperature rated equipment as wells are being drilled in such reservoirs in deep water subsea fields. This paper presents the development of the first 18-3/4" - 20,000 psi subsea wellhead system rated for 0°F to 350°F temperature applications utilizing high capacity running and specialty running tools. Included are details of the three year development project including component design, reliability analysis, and prototype testing of the equipment in the laboratory with simulated field conditions. With this technology, operators can explore fields while further completion technology is developed. This 20ksi subsea wellhead technology will enable operators to drill wells using 15ksi BOP stacks while the 20ksi BOP stack is under development. The wellhead system includes special provisions to enable annulus seal assemblies to be tested to 20ksi while isolating the BOP stack. Drilling into high pressure zones for logging or testing can be performed through a 13-5/8"-20ksi BOP stack which latches directly to the subsea wellhead. In synopsis, this paper highlights the wellhead system details utilized for extreme environments up to 350°F and up to 20,000psi working pressure. The goal of this paper is to open the industry's horizons to production fields and reservoirs that operators previously could not develop due to technological constraints such as the higher bottom hole pressures or greater well temperatures with existing subsea mudline wellhead equipment.

Introduction
Operator requirements for subsea drilling and completions systems continue to push the boundary of known, proven, and delivered technology and equipment. As these boundaries get pushed further and further, the need for a new higher pressure, higher temperature, and higher casing capacity subsea wellhead system grows as modifications to existing technology can only be used in fewer and fewer applications. Additionally, as drilling in the Gulf of Mexico continues to push into deeper and higher pressure formations, the need for increased casing suspension capacity is of equal importance. The development of this system was performed to eliminate barriers for operators that current industry developed systems could not handle, including pressure ratings in excess of 15ksi working pressure, casing loads in excess of 1.5million lbs per casing string, and producing temperatures in excess of 300°F.

System Requirements
The 18-3/4?? - 20ksi wellhead system is designed around the maximum logical requirements of the industry for the next 10 years. Those requirements include producing temperature ranges of 0°F to 350°F and a working bore pressures of 20ksi. Additionally, as required by API, the high pressure wellhead was designed to be tested to 30ksi for an API factory acceptance shell test and is preloaded to the LP Housing for increased fatigue resistance. The casing string weights for both the intermediate and production casing strings are 2.0 million lbs each, while the associated running tools have capacities that are 10% higher than those figures (2.2 million lbs) to include enough overpull allowance for emergency operations should casing get stuck. The production casing hanger and wear bushing (2nd hanger position) can both support a full 20ksi BOP test and the intermediate casing hanger and wear bushing (1st hanger position) can support a 15ksi BOP Test with the same tool in both cases. The 18-3/4?? - 20ksi metal-to-metal annulus seal assembly was designed for the full temperature range of 0°F to 350°F with 20ksi bore pressure, despite the temperature effects from the wellhead being subsea (showing that the seal region is much cooler than 350°F). Finally, the production casing hanger could see potential thermal growth in the field approaching 3.2 million lbs, so a Lockdown Bushing is provided to positively prevent the casing hanger from moving back and forth during start-up/shut-in operations. Figure 1 shows the system details.