Exposure of offshore wells to marine environment may lead to significant corrosion of a conductor pipe and deterioration of its ability to carry well loads up to the point of well collapse. In order to ensure a safe operation of the well throughout its expected lifetime, a comprehensive well assessment, surveillance and maintenance program should be put in place to ensure long-term structural integrity of the wells.
Several steps in assessing structural integrity of conductors have been developed and implemented: qualitative assessment to determine a general structural condition for wells; quantitative assessment by detailed Ultrasonic Testing to measure metal thickness from splash zone to wellhead; evaluation of corrosion effect on structural integrity using physics-based assessment involving detailed structural modelling and 3-dimensional finite-element analysis. It was demonstrated that the minimum mean wall thickness of the corroded pipe can be used as a measure to determine critical loads, which then allowed defining operational criteria for the conductor based on its measured metal thickness and typical expected well loads.
The limits of acceptable conductor corrosion have been established for all offshore wells in the company. Several hundreds of wells were UT-scanned and a number of them needed to be repaired. All wells were scheduled for blasting and painting to prevent or slow down the external corrosion of the conductors. Annuli between the conductor and the surface casing were topped up with cement to take advantage of the additional strength and lateral stability provided by the cement and to slow down the internal corrosion of conductors and the external corrosion of surface casing. Structural integrity risks for all wells in the company were assessed and wells were categorized based on the risk matrix. Two types of risk categories included the higher risk wells to be repaired immediately and high-medium risk wells with repair decisions depending on business impact. Several innovative cost-effective rigless repair solutions were selected to re-establish full structural integrity of wells, three of which were successfully completed. Most repairs were performed without the need of a well shut-in, thus preserving significant production and injection volumes. The repairs were largely executed above the splash zone with a remote access to sub-sea areas of conductors, which eliminated or minimized diving operations and weather dependency.
As a result, structural risks have been eliminated or significantly reduced by completing 100% repairs of all higher risk and several high-medium risk conductors using novel cost-effective rigless and diverless repair techniques. In addition, the life of existing wells was extended, and plug and abandonment operations were deferred. Future plans include enhancements in metal thickness measurements, improvements in surveillance, testing and further consideration of additional repair techniques and development of new methods in structural integrity assessment.