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answered Aug 30, 2016 by rbidong (880 points)
The retainer valve should be incorporated to hold the contents of the upper completion string and/or prevent communication of the upper completion string and marine riser contents after an emergency disconnect of the subsea test tree (SSTT) latch has been performed inside the Blowout Preventer (BOP)/marine riser.  In the case where the upper completion string contents are high-pressure gas, the retainer valve prevents the release and expansion of the gas into the marine riser.  This would result in the displacement of the marine riser contents to the rig, which in deep water may result in the collapse of the marine riser due to external hydrostatic pressure.

The retainer valve interfaces with the shear joint sub at the lower end, and either a space-out sub or packoff sub at the upper end, depending on BOP geometry.

Externally, the retainer valve will have a profile suitable to enable it to be installed within the internal profile of the BOP stack.

Internally, the retainer valve should have a bore suitable to enable the passage of the components required to establish the required production barriers in the subsea horizontal tree.  All internal bore transitions are to be tapered so as not to interfere with or snag the wireline tools and tree equipment.

The retainer valve should provide a minimum of one barrier in the production fluid conduit and be capable of retaining pressure from above.
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answered Aug 31, 2016 by rbidong (880 points)
In addition, the retainer valve should not be provided with a pump through facility.  In the fail closed condition, the barrier(s) should not permit the passage of kill fluid.  This is not seen as a disadvantage in the case of the horizontal subsea tree, as the well can be killed via the choke/kill lines and the tree mounted crossover valve, in the event the retainer valve fails to open.  In addition, prior to getting back on the lower SSTT, the landing string can be tripped out of the hole and the retainer valve can be re-configured with pump-through capability if desired, in order to kill the well.

The structural integrity of the retainer valve should be adequate to withstand the combinations of internal and external loading possible under all conditions expected during handling, including both normal and contingency operations.  The material from which the retainer valve is made should be suitable for the duty required, and for the fluids with which it may come into contact.

Note: Attention should be paid to the means by which a secondary release of Tubing Hanger Running Tool (THRT) is to be achieved, to ensure that the desired operation occurs if the retainer cannot be opened.