Current Tree System Developments in Support of Anticipated Subsea HP/HT Applications
Authors: Fenton S.P. , Voss R.K. 
Published: October 2008

This paper will primarily review the qualification status of subsea tree hardware for HPIIIT applications, by reference to a recent development program. The purpose of this program was to establish the maximum feasible production bore size and technical credibility of a parallel bore tree system, capable of operation at 15,000 psi and 4OOF, which is beyond the capability of existing subsea systems. Necessarily, this has required detailed structural and thermal analysis of critical pressure and load bearing components, as well as a defined test program, mainly to physically demonstrate the necessary upgrading to sealing capability of critical sealing systems at such extremes. Accordingly, the main objective was to prequalify designs, thereby avoiding project schedule risks, and assuring in service reliability by establishing product standards in advance of market needs.

Introduction

The most immediate application of the developed hardware is most likely to be in the North Sea (probably on Central Graeben), where major operators have expressed support for the program, to the extent that one in particular has provided funding, interest having also been expressed in the Gulf of Mexico and S. China Seas by another operator. The paper will cater for discussion on both detailed component and system wide considerations, such as:the current status of both conventional parallel bore and Horizontal Trees in terms of suitability and related product qualification status, including the impact on, feasibility and availability of suitable installation hardware.work in progress to examine the operational practicalities to enable installation from a suitable JackUp vessel in addition to the traditional semi-submersible used for such operations.rationalization of specific temperature rating (and testing needs) of 'secondary' components (e.g. those which are subjected to significant cooling effects from sea-water).potential need for upgrading to 17,500 psi and 20,000 psi service for SCSSV components, noting that API 17D does not cover equipment exceeding 15, 000 psi.effects on, and issues with interface equipment (wellhead, controls, umbilicals, ROV's, sensors, SCSSV's, BOP's, etc.)Quality level recommendations (given current limitations of API 17D PSL levels)

The main outcome of the work done to date emphasises that the hardware development program is solidly founded on tried and proven equipment, but with particular emphasis on the extension of the capabilities of metal sealing systems to promote in service reliability and low life cycle cost. The limitations of elastomers to provide such capability at elevated temperatures (although previously broadly understood in principle) will be demonstrated by reference to specific laboratory tests conducted. Hence, the outcome of the paper will be to demonstrate that subsea tree hardware solutions now exist to enable the production of hydrocarbons from HPIIIT reservoirs, given that key design elements have either been tested to demonstrate the ability to cope with the extreme pressure and temperatures, or have been justified on the basis of a systematic review of the actual exposure which particular components may experience. Finally, recommendations for further work will be identified.