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.