Foinaven first application of Expro's dual-bore subsea test tree
BP Exploration gained fast track approval from the British government for the development of the Foinaven field, the first to be developed in the Atlantic frontier. The area consists of several field fields, some not fully explored.
However, Foinaven and Schielhallion reserves are proven, to the extent that subsea development is underway. The total amount of recoverable reserves in this area is still somewhat unknown and well costs are very high due to the nature of the environment. These factors have led BP to adopt an aggressive and cost effective program. Some of the key drivers for this project are:
- Fast track to first oil.
- Appraisal wells to be re-used as future producers.
- Reservoir protection is paramount.
- Data is critical and wireline testing will be extensive.
Cut lead times
The 5-in. by 2-in. dual bore subsea test tree was developed by Expro specifically to enable operators to achieve a fast track development. The tree enables fields to be developed independent of long lead items (subsea tree), without compromising key drivers or putting the well at risk, and allows extended flow periods without utilizing a drilling blowout preventer (BOP) system as the annulus barrier.
Located within the drilling BOP stack, the dual bore subsea tree with its 5-in. production bore and 2-in. annulus bore interfaces directly to the tubing hanger via the tubing hanger running tool. The dual bore tree orients and locates to the wellhead, insuring perfect alignment for all completion equipment. Both annulus and production bores are protected by dual ball isolation maintaining full pressure integrity to the well, independent of the BOP stack.
In order to clean up the well with a permanent completion installed, the dual bore subsea test tree design consists of
the following components:
- Latch/unlatch: This provides the primary disconnect function which allows the landing string to be disconnected from the valve sections in the event of rig positioning loss, severe weather, or any other operational problem. The latch mobile also allows positive re-orientation and are-latch.
- Annular isolation: Two 2-in. fail-safe ball valves can be closed to isolate the annular side. The dart profile used for the secondary unlatch is made in the annular bore of the tool.
- Outer orientation sleeve: This forms the outer housing for the assembly. The outside diameter is identical to the tubing hanger and also provides rotational guidance during re-latching.
The complete assembly replaces the conventional joint found in normal completion systems and is located on to a normal tubing hanger running tool. A major safety factor provided by the tree is that in both the latch and unlatch conditions, the shear rams can close above the tree.
In addition to the dual bore subsea test tree, Expro has developed retainer and lubricator valves, which can be run within the 5-in. and 2-in. riser. These valves have an extremely good cutting ability and may be used for cutting instead of the 5-in. isolation valves.
Foinaven applicationsFirst site: The first application for the dual bore subsea test tree was the installation of the first permanent completion on Foinaven (UK204/24a-A (02). This field will be developed in one of the most difficult environments and in severe weather conditions. The water depth is over 500 meters and there are very strong two-way currents in excess of three knots. Heaves up to eight meters have been recorded. The currents in this area result in rig and riser movements far greater than in other operating areas.
To establish confidence in the system, the dual bore completion tree was introduced on a limited application where it successfully completed a full cycle of operations, including unlatching and relatching. Second site: The second application for the tree was on well UK204/24a-A(03). The completion was installed by the tree and landed successfully. The well was then cleaned and suspended without incident. The completion tree performed all of the necessary functions and the well was subsequently suspended with hydrocarbons across the production interval.
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