Annulus shut off with rigid lockdown vs SOVs

Hi All,

We're drilling deepwater exploration wells where we can't be sure of the level of axial support provided by the jetted conductor-formation bond and therefore the amount of axial force going through the surface casing to support the weight of the well.

We are currently using a wellhead system that has integral annulus shutoff when the rigid lockdown is activated. One of the slumping risk points identified is when the rigid lockdown is activated directly after the surface casing cement job. In order to activate the rigid lockdown we need to lay off all the surface casing weight onto the conductor string and this is typically done immediately after cementing back to seabed. 

At this point we have a long, heavy, unsupported (wet cement) surface casing string so there is a risk of the conductor slumping with the casing weight. If it does start to slump we would be unable to lay off the casing weight until the cement has gone hard and we would wait for the lead to set up to avoid the risk of buckling the surface casing string (have had simple calcs performed in the past for critical buckling force which suggested casing could reach buckling force if uncemented over a few hundred feet, let alone >2000ft).

Once cement goes hard in the ports of the wellhead system, there's a good chance it'll stop the annulus shutoff and rigid lockdown mechanism from moving and rigidizing the HPWHH and LPWHH together.

There's a view (which seems to be shared by riser analysis companies) that if we have cement back to the seabed and wellhead that we have effectively rigidized the HPWHH and LPWHH together and they will move together and provide a similar resistance to bending and fatigue (transfer of bending to conductor without lift-off of the HPWHH). If adopting this view, the rigid lockdown becomes an insurance of sorts, incase you don't get cement back to seabed. However, there's a bit of scepticism around whether cyclic movement at the wellhead could cause the cement there to fail, crack and crumble, so not sure if cement to surface does provide the rigidisation required to stop movement between the HP and LPWHH.

On previous projects with a big bore system, Side outlet valves allowed us to close the cement ports immediately after cementing, but gave us the option to WOC before laying off the surface casing weight and rigidizing the wellhead. I understand it's most common in the GOM to use wellhead systems with SOVs for the surface casing cement returns, however the annulus shut off mechanism like ours is also very common in deepwater Africa. One of the advantages of removing the SOVs is removing a potential clash point with a BOP or tree.

Has anyone given consideration to potentially not being able to rigidly lockdown the wellhead due to cement in the ports? What conclusions did you reach? Were you satisfied that cement to seabed gives adequate rigidisation (making the extra lockdown mechanism effectively redundant)? Did you find any compelling reasons to choose a wellhead system with side outlet valves over an annulus shut off system or vice versa?

Thanks in advance,

Arun

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