Subsea Production Tree

Overview

A subsea production tree is the valve, choke, and control assembly installed on a wellhead on the seabed to regulate and monitor flow from a subsea oil or gas well. It performs the same role as a surface "Christmas tree" but is engineered to operate unattended in water depths from a few hundred metres to beyond 3,000 m, where ambient pressure alone can exceed 4,500 psi. The tree provides the primary and secondary pressure barriers between the reservoir and the environment, routes produced fluid into the flowline, allows the well to be choked back, and gives access for well intervention. The Tree Body is a single forged steel block that contains the production and annulus bores along with the cavities for the gate valves, so a compact unit can hold a stack of barriers and flow paths.

Trees are rated to API 6A and API 17D. A typical deepwater tree is rated to 10,000 or 15,000 psi working pressure, material class FF or HH for sour (H2S-bearing) service, and product specification level PSL 3G. The wetted surfaces are clad with a corrosion-resistant alloy such as Inconel 625, and elastomer and metal seals are selected for the temperature class and produced-fluid chemistry.

How it works

The tree lands on the wellhead through the Tubing Head Spool, which receives the Tubing Hanger that supports the production tubing and routes the production and annulus bores upward. Produced fluid travels up the production bore, through the open Production Master Valve, past the Production Wing Valve, and into the Production Choke Assembly, where pressure is dropped to the flowline operating range. From the choke the flow passes through the Flow Loop Connection into a jumper that ties the tree to the flowline and then to the host platform or floating production unit.

Every powered valve fails safe. The hydraulic actuators are single-acting: hydraulic pressure from the control system holds the gate open against a spring, and on loss of pressure the spring drives the gate closed. This means any control-system failure, umbilical damage, or emergency shutdown leaves the well isolated rather than open. The Annulus Valve Block gives controlled access to the production annulus for pressure monitoring, gas-lift, or circulation.

Valves and actuation

The barrier philosophy uses two independent valves in series on each flow path. On the production bore these are the production master valve and the production wing valve; on the annulus they are the annulus master and annulus wing valves. Each is a through-conduit slab gate valve: a Slab Gate with a bored hole slides between two floating metal Valve Seat rings, giving a straight, full-bore path when open and metal-to-metal sealing when closed. Gate and seats are faced with tungsten carbide to survive sand-laden flow.

The valves are driven by the Hydraulic Valve Actuator, a fail-safe-close cylinder with a return spring stack and an ROV-readable position indicator. The choke uses a Choke Stepping Actuator that steps the trim open in counted increments, with an LVDT reporting absolute position so the operator can set drawdown precisely. Retrievable choke trim lets the orifice cartridge be replaced by ROV as the well declines, without recovering the whole tree.

Subsea control module

The tree is operated by the Subsea Control Module (SCM), a retrievable pod that lands on a fixed mounting base. Inside the SCM, the Subsea Electronics Module decode multiplexed commands sent down the umbilical, energise the pilot solenoids on the Directional Control Valve Bank, and route hydraulic supply to the correct valve actuator. The same electronics gather data from the tree sensors and return it on the same conductors. Redundant microcontrollers and a local accumulator give the pod the ability to act quickly and to keep functioning if one channel fails.

Sensing is extensive. Pressure and temperature gauges on the production and annulus bores, the Pressure Sensor units on the hydraulic supplies, the Multiphase Flow Meter, and a sand detector all feed the control system so the well can be produced within its sand and erosion limits. The SCM is designed to be pulled and replaced by a light intervention vessel, which is far cheaper than recovering the tree itself.

Materials and ratings

Subsea trees are built for two-decade service with no maintenance access, so material selection is conservative. Bodies and bonnets are forged low-alloy steel (AISI 4130 or F22) qualified to NACE MR0175 hardness limits, with weld-overlaid CRA on all wetted surfaces. Stems and high-load parts are Inconel 718. Seals combine energised metal rings for the primary barrier with elastomer backups rated to the temperature class. The whole assembly is hydrostatically tested to 1.5 times working pressure and gas-tested to PSL 3G before deployment.

Horizontal versus vertical trees

In a vertical tree (VXT) the master and swab valves sit in the vertical bore above the tubing hanger, so the tubing hanger lands in the wellhead before the tree is installed. In a horizontal tree (HXT) the valves are arranged on a horizontal flow loop and the tubing hanger lands inside the tree body. The horizontal layout makes tubing recovery simpler because the tree stays in place while the tubing is pulled through it, which suits wells expecting frequent workovers. Vertical trees give simpler well-barrier verification and are common where intervention is rare. The choice drives the design of the Tubing Hanger and the valve block layout.

Installation and intervention

The tree is run on drill pipe or a dedicated running tool and locked to the wellhead by the Wellhead Collet Connector, a hydraulic collet connector that grips the wellhead profile and energises a metal gasket. Once landed, the flowline jumper is connected and the SCM is installed. The Tree Cap seals the top and excludes debris. Routine operations and contingency overrides are handled through the ROV Intervention Panel, whose hot stabs, torque buckets, and mechanical override stems let an ROV operate any valve or inject chemicals if the primary control path is unavailable.