A Weight-Set Mechanism for Generation of a Preloaded Structural Connection in a Subsea Wellhead
Authors: Farrell T.J. 
Published: October 2008

ABSTRACT
A novel system has been designed to lock down and preload the high pressure housing of a subsea wellhead system into the structural conductor housing, ensuring efficient transfer of riser-induced loads. The system is entirely passive, preload being generated by string weight during installation, and is simple, cost effective and reversible. This paper discusses the design requirements, principles of operation, analysis and full scale testing of the system, and demonstrates its ability to transfer anticipated loads.

INTRODUCTION
During the past decade, the ability of the conventional subsea wellhead system to survive the extreme static and cyclic loads generated by deep water drilling, TLP tiebacks and other novel production systems has been the subject of significant investigation and development.
Ineffective bending moment transfer from the high pressure housing to the structural conductor housing can cause fatigue failure in the weld between the high pressure housing and 20" casing joint1. The two types of connection between the housings that have found general acceptance for high static or cyclic bending moment applications are the tapered landing shoulder, and full or partial radial support at two elevations2,3. However, neither is ideal for the current and future market's more severe applications, because in both cases radial support and thus bending reaction is lost at the landing shoulder if the housings are separated by flotation, thermal expansion, drilling riser loads, or TLP tieback tension.
Landing shoulder separation may be prevented in the same way as in a flanged connection, by the application of preload. The mating halves of the connection may be held together by the application of a preload force during make-up, the magnitude of the preload being greater than the effective separating force applied to the connection in operation. However, until now the success of this approach has been limited in this application by problems associated with energisation of the preload mechanism, and maintenance of preload after release of the energizing tool.
Many operators have identified the economic benefit of a wellhead system that is suitable for all drilling environments, as well as for later completion or tieback, and an effective connection has thus become a requirement even as a standard feature 4. To satisfy this new market, today's subsea wellhead systems must be designed for optimum load transfer, without a downside of high cost or poor reliability. The intrinsically simple preloaded connection described in this paper is one solution.

PRELOAD ENERGISATION

1. Active System
Most preloaded lockdown systems are "active", i.e. energisation is by external intervention, normally hydraulic pressure. Their use leads to increased rig time and system complexity, compared to a non-preloaded system.
A hydraulic system typically uses a wedge to force a split ring under an internal shoulder at the top of the conductor housing, placing the conductor housing in tension and the high pressure housing in compression, between the internal shoulder and landing shoulder.