Basically, if the wells platform is not designed to withstand the load of the wells/wellheads, the Mud line suspension system would have to be used.
Having all the casing loads at the wellhead would be easier, as you would not have the restrictions impossed by using the MLSS.
In general terms, these would be the PROS and CONS of the MLSS
.- Less loads to be handled by the platform, which would represent less cost for the platform construction.
. - If set depth enough, MLSS would be helpful for permanent P&A ops / decommissioning.
. -MLSS could be helpful for batch drilling / pre-drill operations, allowing to tieback and complete the wells later on.
. - Could be helpful to reduce buckling of some casing strings.
.- Difficult to use UNIHEAD-type wellheads, due to the space out WH - MLSS (although, It can be done, and it has been done)
.- MLSS imposes additional internal ID restrictions due to the landing profiles for the ML hangers, which reduce the traditional contingency casing strings.
.- Creates a tight tolerance situation (ID), to run liner hangers and some packers, bridge plugs and cement retainers.
.- Cost: MLSS are not cheap
.- Cost and time required to prepare the MLSS assemblies offline prior to RIH.
.- Could limit to rotate the casing string once the MLSS is M/U into the string (could release the landing sub, if any)
. - Mandates to land the conductor (with MLSS buttweld sub) to be set at the right depth; following casing strings must land on the MLSS, so leaving a casing off bottom would be problematic ansd in some cases catastrophic.
-. Delivery times for MLSS: could be a show stopper.
-. Wellhead: the dry well head to be used with the MLSS has not to be necessarily from the same supplier; it does need to have the proper casing slips to be activated with low casing weight, as most of the casing string would be hanging on the MLSS.
Now, about leaving cement back to surface vs. using the wash ports of the MLSS landing sub it is a tricky question; leaving cement to surface would create issues to abandon / decommissioning the wells, but opening / closing the ports in something some old field guys (MLSS suppliers, operators DS / Co Man) do not like, specially for sizes bigger than 13-3/8”casing, due to the risk of creating a leak path in the casing string.
Hope this helps.