Subsea wellheads can be subjected to extreme external loads when a drilling or production riser remains connected during storm or vessel drive off conditions. In addition, vortex shedding from high currents and wave action on drilling and production risers can subject the wellhead to high cycle fatigue loads. These extreme loads and high cycle fatigue loads can cause failure of the wellhead system when a fully cemented annulus between the 30 inch conductor and 20 inch surface casing is not achieved. This effect of cement shortfall is most damaging when the cement level is just below the wellhead body/conductor housing region. Purposefully setting the cement level far below the mudline can be a cost effective solution to the unintentional cement shortfall problem.
Cementing programs typically call for full cement returns between the 30 inch conductor and 20 inch casing string. This full cement return locks the wellhead body into the 30 inch conductor housing. External loads applied to the wellhead body are transferred through the cement to the conductor housing, causing the total load to be shared between the 30 inch conductor and 20 inch casing. This is the best possible load path for the wellhead assembly.
An unexpected cement shortfall can occur, despite visual conformation of full cement returns at the wellhead. The hydrostatic pressure of cement slurry in the 30 inch conductor/20 inch casing annulus can break down weak surface hole formations, allowing the level of the unset cement to drop. Also, cement near the top of the annulus may not form an adequate bond between the 30 inch conductor and 20 inch casing. This causes the 'effective' cement level to be below the actual cement level. These unexpected conditions may not allow full load transfer between the wellhead body and conductor housing. An external wellhead load would go directly through the wellhead body to the 20 inch casing. This load would be carried by the 20 inch casing down to the effective cement level, where it then becomes shared between the 20 inch casing and 30 inch conductor.
The load path created by unintentional cement shortfall can cause fatigue failures at the transition between the wellhead body and 20 inch casing.1 Analysis has shown that this transition area can also be sensitive to static load overstressing when cement shortfall is present. Some recent wellhead designs address this shortfall problem by not relying on full return cement to transfer bending loads from the wellhead body to the 30 inch conductor housing. These new designs achieve proper load transfer through the use of long tapered contact shoulders, radial interference wedges, or two point lock down mechanisms.
Wellhead systems that do not utilize these enhanced load transfer techniques can be made more resistant to static overstressing and fatigue damage by intentionally setting the cement level far below the mudline. A 20 inch casing string with a deep cement shortfall is more compliant than a 20 inch string with shallow shortfall. Making the 20 inch string more compliant allows two point contact between the wellhead body and conductor housing with low applied loads.