Pushing the Limits of a Subsea Wellhead System's 8.4MM lb. Compressive and 17.5 KSI Internal Pressure Capacity With All-Metal Seals
Authors: Radi A. , Landriault S. 
Published: October 2008

The operating requirements for subsea wellhead systems suitable for use in high-pressure, high-temperature (HP/HT) applications are rapidly approaching the limits for existing subsea wellhead systems. Limiting factors include axial capacity, combined bending moment and axial capacity, and pressure containment. This paper describes an all-metal seal wellhead system for use in critical service environments where the total casing loads are expected toexceed4,000,000 Ibf, The design loads are assumed to act in combination with extreme environmental loads, and with multiple cycles of BOP testing to 17,500 psi.

The increase in load carrying capacity is accomplished by separating the load path along which the bending moment is reacted from the load path carrying the axial compressive load. In the design described in this paper, the bendingmoment is reacted using a field proven double-socketed interface, while axial loads are communicated to the conductor housing through a compressive load shoulder. The design also incorporates a 7-5/8" casing hanger that engages asecond load path into the high-pressure housing. This load path is in turn used to communicate the casing load and pressure end load to the high-pressure housing.

Also required is the use of metal-to-metal seals throughout the wellhead system for pressure containment at high temperatures. Elastomeric seal components may degrade with exposure to corrosive fluids, or at elevated temperatures, The metal-to-metal seal selected for this wellhead system has been in use for the past five years, and has an excellent record in service.

INTRODUCTION

Figure 1 shows a 10 ksi subsea wellhead system. This system, first introduced in 1974, offer features such as:10 ksi service ratingSingle stack configuration for internal pressure ContainmentTorque set, single trip packoff installationFlexible casing program

Figure 2 shows the current generation of a 15K subsea wellhead system. Designed in late 80's, this system made extensive use of computer-aided engineering and analytical techniques to reduce the size of the wellhead system while increasing its load carrying capacity. This system offersadditional features, such as:15 ksi service ratingThree-hanger wellhead system, each capable of carrying one million lb. of casingAll-metal sealsSingle trip casing hanger running tool with weight set metal-to-metal seal

The remainder of this paper focuses on a wellhead system, based on the design shown in Figure 2, developed for use in HP/HT applications. This wellhead system can be used in extreme service conditions with a service pressure of 15,000 psi and at 350 degrees Fahrenheit, and with BOP test requirements up to 17,500 psi. The system accommodates internal casing loads of nearly four million lb., excluding weight of the 20" casing. With the environmental loads over two million ft.-lb., the total structural loads communicated to the conductor housing, are then in excess of 8.4 million lb. of equivalent compression.

Also featured in this wellhead system is all-metal sealing. The concept and features of a true all-metal seal wellhead system were explored by Danner and Henderson [1]. Metal sealing system considerations and principles were further explored.