Production wells have been constructed in an area offshore which is known for having localised deposits of very soft soil close to mudline. During well planning, it was identified that two other operators had attempted to drill in the region but had to suspend or abandon the well due to excessive degradation of the soil supporting the well foundation. The objective of this work was to provide real-time wellhead integrity data to support the well management team during operations.
The real-time monitoring system was deployed on a subsea BOP stack and riser during drilling and completion operations to monitor motions. From the recorded signals, wellhead integrity was inferred by determining the BOP stack characteristic frequency and effective fixity depth. These integrity parameters were relayed to the vessel via a hydroacoustic link and then delivered onshore. An automatic alarm system was setup based on pre-defined limits which would be triggered during operations if the integrity of the well foundation became at risk of permanent damage. Additionally, the BOP stack motion data allowed the accumulated fatigue damage to be tracked.
The alarm system provided a regular 15-minute check on the integrity of the wellhead and conductor system. If advisory limits were triggered, the well management team were able to assess weather forecasts and make an informed decision to continue drilling or to disconnect from the wellhead to avoid a period of rough weather. This would prevent excessive degradation of the soil and protect the integrity of the well. When increased BOP motions were observed by the drilling team, operations were able to continue with confidence because the integrity data remained within acceptable limits. This avoided unnecessary downtime, providing significant savings in both cost and rig-time as well as improving the overall safety profile of the operation.