Subsea production trees have now been used for the control of oil and gas wells drilled under water for some 20 years. The first trees were simple affairs, designed for use in shallow water, being installed and possibly operated by divers. These trees were derived from existing surface equipment using stacked valves with the minimum of integration.
As oil discoveries have moved to deeper water and harsher environments on marginal sized developments, there are greater reasons to consider subsea completed wells as an alternative to fixed platform developments. Two other major considerations influencing the balance in this direction are the use of subsea multiwell manifolds and floating production facilities. Multiwell subsea manifolds can be used to produce either to adjacent fixed platforms or floating production facilities.
There are two basic concepts that describe the modular tree system. The first (Fig. 1) is the construction concept where a manufacturer uses a number of proven design modules to tailor a subsea tree design to suit a customer's particular needs.
The second is the operational requirement to split the tree into individual remotely retrievable modules, so that certain sections of the subsea tree can be retrieved to the surface for replacement or repair without disturbing the other sections of the system. Neither of these concepts are mutually exclusive. (Fig.1 is available in full paper)
The function of a subsea tree can vary depending on the nature of the system to which it is matched.
In its most simple form, the tree is only required to divert flow from the well at the wellhead and to isolate the well in the event of a failure downstream of the tree. In this mode the tree will have a simple control system and will not be operated very frequently; thus it may almost be considered a passive component.
As experience and confidence in subsea systems grow, subsea trees are being required to perform functions in addition to the simple system tests. Trees can be fitted with subsea chokes to actively regulate flow and valve system control, and the injection of wax- or scale-inhibiting chemicals either downhole or into the flowline.
As part of a subsea production system, the tree is no longer a passive component, but may be the only means of isolating one well from another prior to co-mingling at the template for export. Along with this increase in function and status within the system will come an increase in operating cycles and risk of failure.