Ageing Offshore Well Structural Integrity Modelling Assessment And Rehabilitation

Zakum Development Company (ZADCO) is operating Upper Zakum (UZ) Field offshore Abu Dhabi consisting several offshore wellhead platform towers (WHPT) which have been in service for over 30 years and evidence of severe corrosion has been found on majority of these well’s conductors and casings.

A major challenge faced by ZADCO is to repair corroded well’s conductor casing to prevent well structural collapse and extend the life further to allow for safe and planned retirement. The challenges faced during the assessment includes the absence of design basis, well age, data accuracy and drilling quality for 30 years, the challenges takes another dimension when factoring the number of wells and possible different configurations. They way forward was to categorise the wells into several groups with close design configurations and then reverse engineer the design basis to a build a structural model for that fits the various wells configuration to establish the minimum thicknesses to assure well’s structural integrity, and the requirement for life extension by suitable repair method. 

This paper outlines the activities undertaken to design and implement an effective process for evaluating the structural integrity of aging well conductors considering over 6000 possible well configuration scenarios. The paper describes the engineering methodology involved in the integrity assessments for well conductor evaluation using analytical methods to assess the present strength and stability states of the conductor/casing assembly based on corrosion inspection data and the knowledge of existing cement elevations and operating conditions of the well. Operability guideline plots are generated to present the available safety margin of the aging conductor/casing system, and to enable well conductors with unacceptable structural integrity to be identified. Where safety margins are poor, repair solutions and mitigation methods are also developed for implementation on the affected conductors and casings. 

A detailed rehabilitation flow-chart is also developed to assist engineers in the repair/replace decision-making process, with comprehensive guidelines for assessing and executing the rehabilitation steps for extended safe operations of ageing conductors, hence prolonging the well operating life.