Aging Offshore well Conductors Structural Integrity Issues and Challenges in their Life Extension

Abstract

ADNOC Offshore operates more than 1300+ numbers of oil, gas and water offshore wells of age 59 years on wards, at 300+ well head platforms located in shallow to intermediate water depth of 7m-34m for various fields. Well conductors being outer pipes of the well construction, its functionality is to resist various combined axial and bending forces acting on it. Also, conductors to be effective in transferring loads through the cement bond / skin friction to the surrounding soil/rock layers with sufficient factor of safety. Offshore well conductors were originally installed as bare steel without coating. Therefore, splash zone and atmospheric zone of the conductor is directly exposed corrosion. Fully submerged and buried part of conductor the is protected from external corrosion from jacket cathodic protection system. Some conductor's annuli are without proper cement top-up or poor cement (above sea-bed to top X-mas tree flange). Seventy percent of them have exceeded the original design life of 30 years and require life extension for extra 50+ years, since oil and gas reservoirs will be having active production potential. Methods, Procedures, Process: Above water baseline visual inspection of all well conductors were conducted to compile as-built data, coating, corrosion, inspection / breathing windows status by setting up the anomaly acceptance criteria. After the assessment of baseline inspection data, scheduled the detailed inspection plans were prioritized using NDT- ultrasonic testing (UT) or Advanced NDT technique. Structural assessment of the well conductor is carried out for axial load, internal bending moment due to internal casing and external bending moment due to environmental load. Minimum Required Thickness (MRT) is worked out to resist the combined axial and bending moment. Using MRT, Current Average Thickness (CAT) and Corrosion Rate (CR), current risk, the remaining life and mitigation plan were communicated through conductor passport Results, Observations, Conclusions: Systematic inspection criteria and strategy was set up to prioritize the inspection of all well conductors. Sixty percent of well conductors are found with open inspection windows on the conductor's surface or on the conductor flange, to monitor the internal cement level during drilling of the wells. The resulting moisture and air entry into the conductor annulus caused internal pitted corrosion in the atmospheric zone of the conductor. The local area thinning of the conductor reduced its axial, bending, buckling strength and will lead to collapse of the conductor. From HSE considerations, the affected well conductor require immediate intervention and this will involve major repair cost, production loss due to unplanned shut down of the well head tower. Novel/Additive Information: Detailed inspections of critical 126 well conductors were carried out using magnetic crawler mounted Saturated Low Frequency Eddy Current (SLOFEC) and Magnetic Eddy Current (MEC) method. The inspection data was assessed for the current average thickness (CAT) and calculated the minimum required thickness (MRT) to conclude the remaining life of the well conductor. There are 11 well conductors identified with remaining life zero (RL=0). Well conductor passports were generated to evaluate the current risk of the conductor against collapse. Short term, long term remedial actions are being implemented to assure the structural integrity of zero remaining life well conductors for the life extension.