Design of an Electric X-mas Tree Gate Valve Actuator

Abstract

The increasing need for more energy in the world is making the oil companies search for hydrocarbons in deeper water and with longer distances between the well and platform(step-out) never experienced before. Current systems are designed to operate in deepwater with relatively short step-outs. Since the search now is starting to exceed this, the operational conditions are changed and the electro-hydraulic system is closing in on its operational limits. Hence, the increasing need for a system that supports the long step-outs and the deeper water. Aker Kvaerner has started looking into the development for an All Electric Subsea Production system. In co-operation with Aker Kvaerner Subsea, a functional design specification for an electric subsea gate valve actuator has been developed and included in this thesis. This functional design specification will include all the requirements that a subsea electric actuator must conform to. This includes the operational requirements, size limitations, failsafe needs etc. Based on this specification, a design for a electric actuator and including motor driver control system layout on the all electric x-mas tree is proposed as a part of the All Electric Subsea Production system. The actuator will be used to operate the gate valves on a x-mas tree which controls the flow of hydrocarbons through the valves into the pipelines. Most of the existing subsea gate valve actuators are currently using hydraulic oil to operate the valves linearly. Here a concept is being suggested in replacement of the hydraulic fluid, while still complying with the necessary design specification. A number of electrical machines for potential use in this application are presented and evaluated. A Permanent Magnet Synchronous Machine is being investigated further. These machines can be custom made, and can also be made with built in redundancy. Calculation of size for the Permanent Magnet Synchronous Machine has been done, to determine suitability within the space limitation of the apparatus. Two layouts for controlling the actuators on the all electric x-mas tree are proposed. These two are also evaluated against the standards made for subsea production systems and the no 'single point failure' requirement made by the industry. The chosen solution proposed for controlling the electrical actuators is an integrated design, where all actuators have their own motor driver inside. This thesis conclude that an electrical actuator will be technically and mechanically possible to build(from an electrical perspective), using the design and control layout proposed.