Apparatus and a system for accessing one or more of the annuli defined between casing strings in a subsea wellhead that are usually sealed for the production stage of the well. Access to an annulus (30) is made via the wall of the wellhead housing (12) at a location where is communicates with the annulus. The access is a passageway (31) having a first opening in communication with the annulus and a second opening either at the external surface of the wall or at a location on the inner surface of the wall above the casing hangar seal for the inner casing (14), which is not in communication with the annulus apart from the passageway. An isolation valve (32) communicates with the passageway and with apparatus for monitoring or controlling pressure of flow in the annulus. The passageway may be used with an open lower end to the annulus during drilling stages for material transfer and during production stage to monitor/control a sealed annulus.
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