Improving Wellhead Performance With Programmed Cement Shortfall
- J.S. Britton (Stress Engineering Services Inc.) | G. Henderson (British Petroleum Co. plc)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1988
- Document Type
- Journal Paper
- 381 - 384
- 1988. Society of Petroleum Engineers
- 1.3.2 Subsea Wellheads, 1.14 Casing and Cementing, 1.10 Drilling Equipment, 1.6 Drilling Operations, 4.2.4 Risers, 1.3 Wellhead design
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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 external and high-cycle fatigue loads can cause failure of the wellhead system when a fully cemented annulus between the 30-in. [76-cm] structural conductor pipe and 20-in. [51-cm) 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-short-fall problem.
Cementing programs typically call for frill cement returns between the 30-in. [76-cm] conductor and 20-in. [51-cm] casing string. This full cement return locks the wellhead body into the 30-in. [76-cm] 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-in. [76-cm] conductor and 20-in. [51-cm] casing. This is the best possible load path for the wellhead assembly.
An unexpected cement shortfall can occur despite visual confirmation of full cement returns at the wellhead. The hydrostatic pressure of cement slurry in the 30-in. [76-cm] conductor/20-in. [51-cm] 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-in. [76-cm] conductor and 20-in. [51-cm] casing. This causes the "effective" cement level to be below the "actual" cement level. These unexpected conditions may not allow frill load transfer between the wellhead body and conductor housing. An external well-head load would go directly through the wellhead body to the 20-in. [51-cm] casing. This load would be carried by the 20-in. [51-cm] casing down to the effective cement level, where it then becomes shared between the 20-in. [51-cm] casing and 30-in. [76-cm] conductor.
The load path created by unintentional cement shortfall can cause fatigue failures at the transition between the wellhead body and 20-in. [51-cm] casing. 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-in. [76-cm] 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. The wellhead system discussed in Ref. 2 takes advantage of some of these design enhancements. Other wellhead systems incorporate some of these design improvements, but have not been described in the literature.
Wellhead systems that do not use 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-in. [51-cm] casing string with a deep cement shortfall is more compliant than a 20-in. [51-cm] string with a shallow shortfall. Making the 20-in. [51-cm] string more compliant allows two-point contact between the wellhead body and conductor housing with low applied loads. This low-load two-point contact can significantly reduce the amount of total load carried by the 20-in. [51-cm] casing string.
|File Size||285 KB||Number of Pages||4|