Successful Mitigation of Structural Risks for Corroded Conductors in Shallow Water Offshore Wells
Authors: Spuskanyuk O. , Abdulhai W. , Mahmoud A. , Mumtaz S. , Gan C. Lam , Khemakhem A. S. 
Published: November 2017

Ageing wells in mature shallow water fields may develop corrosion of conductor and surface casings, which reduces their capacity to carry well loads. Novel conductor repair techniques were designed and piloted, a comprehensive well maintenance program was developed to mitigate and minimize the probability of structural failures and to eliminate environmental impact through lifetime of the well. Lessons learned during development and implementation of this program and specific experience from conductor repairs performed using several new techniques are invaluable for companies that experience similar corrosion issues of ageing assets.

Qualitative assessment by visual inspection and quantitative detailed ultrasonic testing of several hundred conductors provided an accurate measure of metal thickness of conductor pipes from the splash zone to the wellhead. Physics-based assessment of how the corrosion impacts the well structural integrity was performed in order to establish acceptable, safe limits of corrosion and remaining wall thickness of an integral conductor pipe, which included a simplistic analytical study, detailed structural modeling and 3D finite-element analysis. These analyses allowed the development of safety guidelines and structural failure risk ranking criteria.

After initial assessment through analysis of UT scans, all wells in the field have been risk-ranked and prioritized for maintenance and repairs. Cement top-up campaign took place to ensure cement filled the annulus inside conductor to provide additional structural support and stability and to slow down the internal corrosion of the conductor pipe and external corrosion of the surface casing. Blasting and coating of conductor external surfaces in order to stop any additional external corrosion has been performed. Three cost-effective rigless repair techniques to re-establish full structural integrity of the wells have been selected for pilot operations. Welded sleeves were used for several repairs but required extensive engagement of weather-dependent diving services to construct the cofferdam to create dry working area in the splash zone before welding. Two other techniques with the minimum use of diving operations have been used for the first time in the region: external reinforced cementitious grout and combined clamped-welded sleeves. Before execution of repair operations offshore, the full-scale yard testing of clamps was performed to verify capability of equipment to carry the required loads.

A number of wells have been repaired using novel techniques with varying duration of operation and several technological and operational challenges related to field execution, quality of design and weather dependence. Without doubt, lessons learned from these trials are essential for future execution of repairs with these techniques. Well management program to ensure structural integrity of offshore wells will extend the life of existing wells, optimize costs by prioritizing operations and re-utilizing same conductor pipes through slot recovery for replacement wells, when necessary.