Well Structure Integrity Threat Due To Near Surface Corrosion of Outer Casing at Offshore Wells

QP Offshore operates in shallow water at Arabian Gulf. The wells are drilled and completed at surface on multi slot wellhead jackets. We encountered well subsidence with some offshore wells. The well subsidence caused stresses to flowlines, gas lift hook up and jacket members. As a result, the flowlines and gas lift hook up were under threat with subsided wells. This meant high risk potential of uncontrolled release of hydrocarbon to atmosphere, with significant potential of asset damage and environmental pollution. The Offshore wellhead jacket structure and conductor casings are protected against corrosion by cathodic protection. These were primarily installed to protect wellhead jacket structure against sea water and its corrosion protection to wells is not known to us. The well subsidence occurred due to structural failure of outer casing caused by the atmospheric corrosion. The corrosion occurred due to outer casing being in contact with sea water and atmospheric air near splash zone. We looked at options of isolating the air and seawater contact from outer casing to prevent further corrosion. It was possible to achieve this by placing bio degradable oil into Conductor. The oil being lighter than water shall stay above the water, thereby isolating air in the oxygen from sea water. We dropped this option as high ambient temperature in summer months shall cause bacterial growth and degrade oil. The degraded oil could result in accelerated corrosion instead of preventing it. The other option considered was to install sealing type cover at the top and replace trapped air inside annular space with nitrogen. This too could not find favor due to lack of assurance in isolating the air inside the Conductor. QP selected to top up the conductor with cement. It shall cut off air between conductor and outer casing. The other major advantage was well structure strengthening by joining the conductor with outer casing to support wells with weakened outer casing. The paper shall illustrate on the findings of corrosion analysis with outer casing recovered during well workover operations for subsided wells.