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Piloted Hydraulic

In the piloted hydraulic control system, hydraulic power to operate tree functions is supplied, via the umbilical, to a control pod and accumulator on the tree (see Figure 3). Umbilicals also contain individual hydraulic lines transmitting control signals to pilot- operated, spring-returned hydraulic valves mounted on the tree. Hydraulic valves direct fluid from the accumulator to actuators of production valves and chokes. Use of pilot-operated valves improves response of the system as only a control hydraulic signal is transmitted from the topside to the tree. In addition, as the supply of fluid to actuators is from the accumulator rather than from a remote topside location, the response time of tree valves is further improved. However, the response time of the system is still dependent on the volume of pilot lines and thus application of a piloted hydraulic control system is limited to distances between the topside and the tree of up to 10km.

Figure 3: Piloted Hydraulic System

The umbilical termination for a piloted system can be identical to that of a direct hydraulic system. Its design depends on the method of umbilical installation and hook-up. The accumulators are usually mounted on the tree and piped into the system. In some cases, the accumulators are part of the control pod, which allows retrievability. A separate, remotely retrievable accumulator package is also used. The size of the accumulator depends on the response time requirements, umbilical hose type/size and the distance between topside and tree location (offset distance). All subsea hydraulic-piloted control valves are located in a control pod that is usually mounted on the tree frame. Depending on water depth and diver access, etc., various types of pod are used, generally falling into two categories: remote retrievable and diver retrievable. A remote retrievable pod is locked to a pod receiver plate, usually using stab connections that interface control lines on the tree with the pod. Locking the pod to the receiver plate is carried out either using a remote operation vessel or a hydraulically operated connector.

A piloted hydraulic system, like a direct hydraulic system, is highly reliable and easily accessible for servicing. Although the system performance is faster and allows for larger offset distance, the piloted system requires more complex hydraulic control umbilicals and inclusion of tree-mounted components, which adds to the cost of installation. As in a direct hydraulic system, a piloted system does not have direct feedback information about performance of subsea functions, but, again, some information can be obtained by monitoring pressures and flow at the topside.

Sequential Piloted Hydraulic

The basic set-up of a sequenced piloted hydraulic system is similar to a pilot-operated system; however, pilot-operated, two-position hydraulic valves are operated in a predetermined sequence (see Figure 4). An independent operation of individual valves is not possible in this system. Hydraulic control valves are connected in parallel to hydraulic power supply lines from the topside. The sequencing is obtained by changing the pilot pressure, which shifts the valves into an open position. The actuators of production valves are then moved in an order dictated by the magnitude of pressure. The system is relatively simple and requires fewer hydraulic lines (see Figure 4) with sequential piloted hydraulic system umbilicals in comparison with a piloted hydraulic system. An operating sequence must be determined in advance, however, which provides less operating flexibility than either direct or piloted hydraulic systems. The overall response of the system is similar to the previous system and, again, the only monitoring of system operation is obtained at the topside by measuring fluid flow rate and pressure.

Figure 4: Sequential Piloted Hydraulic System

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