Subsea Valve Actuator For Ultra Deepwater
Authors: Ali S.Z. , Skeels H.B. , Montemayor B.K. , Williams M.R. 
Published: October 2008

Abstract
This paper reviews the continuing development of gate valve and actuator technology for subsea completions extending into ultra deep water. The basic technical challenges inherent to subsea valve actuators are reviewed, along with the various factors which affect the design and performance of these devices in deepwater applications. The high external ambient pressures which occur in deep water, coupled with high specific gravity hydraulic control fluids, are shown to have a significant impact on the performance of the actuators. This paper presents design & analysis methods and the verification test procedures which are required to develop and qualify new deep water actuator designs.
Gate valve actuators of the type described in this paper are currently in use on subsea christmas trees on the world's deepest subsea wells offshore Brazil (water depths &gt;3000 feet). New applications of the deepwater actuators are in process for upcoming Gulf of Mexico subsea production systems in water depths approaching 6000 feet. The actuator/valve development method described in this paper has been confirmed by performance verification testing of full scale valves& actuators using a hyperbaric chamber to simulate ultra deepwater operating conditions. Performance of the test valves& actuators correlated very well with analytical predictions. Test result have confirmed that the new valve actuator designs will satisfy API 17Dperformance requirements for water depths up to 7500 feet, well in excess of the upcoming GOM application.

Introduction
As subsea completions have ventured into deeper waters, subsea valve/actuator technology has traditionally wrestled with the adverse effects of increased hydrostatic pressure which, acting on the valve/actuator mechanism, can keep the valve from fully closing or opening under certain conditions. Within the last twenty years, these actuator designs have also had to deal with evolutionary changes in hydraulic control system architecture, control fluid types and specific gravities, external override technology, and the emergence of API Specification 17D. This paper reviews the basic technical challenges inherent to ultra deep water applications, and presents design& analysis methods for developing and qualifying new deepwater actuator designs.

Review of Fail Close Valve And Actuator Design
The artof fail close (or open) gate valve design has been widely used for several decades. An excellent paper on this topic waspublished by Fowler ? and Herd 2in 1976. In principle, the fail close valve'sstem is sized large enough so that pressure inside the valve, acting on the area of the stem, creates a stem thrust force large enough to overcome the friction forces which oppose valve movement (gate/seat drag and stem packing friction). Thus, the stem thrust force will cause the valve to close when hydraulic control pressure in the actuator is released. As pressure inside the valve body increases, the stem thrust force increases in the same proportion as the valve friction forces, so stem thrust alone is usually sufficient to ensure that surface valve actuators will reliably close the valve under all anticipated working pressure conditions.