Functionally, the packoff sub is similar to the slick joint. The primary purpose of the packoff sub is to enable an annulus seal to be established by the closure of the BOP annular preventer around the external profile of the packoff sub, while allowing control fluid to be conveyed to and from the functions of the tool(s) below.
A secondary purpose of the packoff sub is to establish an upper seal on the annulus between the BOP and the subsea test tree (SSTT), which will allow a secondary release of the SSTT latch and lower ball valve functions.
Externally, the packoff sub will have a profile suitable to enable a seal of sufficient integrity to be developed between the packoff sub and the annular BOP to maintain pressure control in the annular space below the preventer.
Internally, the packoff sub should have a bore suitable to enable the passage of wireline tools and tree equipment. All internal bore transitions are to be tapered so as not to interfere with or snag the wireline tools and equipment.
The wall of the packoff sub should accommodate the conduits necessary for the conveyance of control fluid required to effect discrete functions of tool(s) below the slick joint. Alternatively, the packoff sub may be constructed of discrete inner and outer shells, with control tubing between the two to convey hydraulic fluid to the various SSTT functions.
The wall thickness of the packoff sub 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, in conjunction with BOP Annular closure.
The material from which the packoff sub is made should be suitable for the duty required, and for the fluids with which it may come into contact.