A few thoughts from an Auld hand
I have never used a tethered subsea BOP.
As an engineer I would have a look at the entire system, starting from the guide bases, gimbal and 36”x 30” foundation including the 36” or 30” housing extension positively centralized into the 36” or 30”, even 48”. That is often neglected or underdesigned, as the fit must be snug!
There were studies by Exxon (testing 15K BOPs), Koomey (developing new BOPs) and SINTEF (testing HP BOPs), which addresses the issue. Paul Koomey thoughts were my preferred ones.
There are solutions to reduce weight/height often compounded by the hang-off pipe ram position i.e. the landing pipe ram below the Shear Blind Rams. This forces the use of one higher dual BOP or having the SBR at the top of the bottom dual preventer, taking advantage of the connection between the two ram preventers to create room for the Hang-Off tool. (or on the combination trebble BOP + Single Bop to have the hang off rams on te second position from above).
Obviously, we should refrain from using kill/choke in dedicated spools. We must use the inlets/outlets provided on the preventers.
The type of connection between preventers can be selected /engineered to reduce the height and consequent bending moment. There are techniques such as blind bolts, and my preferred BOP tie down rods (see ssBOP on “Discoverer Seven Seas”).
The same applies for the hydraulic connectors where we recommend larger swallow rather than hub style to resist bending moments.
There is a preference for forged pieces, in general heavier. The casting pieces may be lower in weight, but the inspection of castings to ensure good ones, may be more elaborated (see Stress Coating techniques, used, for instances, by SEDCo).
The riser top tension also plays an important role, as a not too small part of it, helps in keeping the BOP vertical
(I will try to remember – and revert – with more technical details related to this subject, which I dealt with some 30 years ago…)
