An increasingly common problem faced by subsea well operators is plugging and abandonment (P&A) of wells using modern drilling vessels. This challenge is due to older wellhead and conductor designs which are not fatigue resistant and which have less bending load capacity. Combined with the current trend of using larger and heavier BOP stacks provided by 5th and 6th generation drilling vessels, this results in large motions and loads transferred to the wellhead, and hence higher risk of fatigue failure or component capacities being exceeded during P&A. This paper discusses the methodology for assessing the feasibility of performing P&A on wells with modern vessels and evaluates mitigation options available.

Firstly, analysis is performed to determine historical fatigue accumulation in the wellhead and conductor system from drilling and completion operations. Historical fatigue accumulation can be refined by incorporating details from the operational history including vessel heading, metocean conditions, and riser configuration. Fatigue accumulation from planned P&A operations with modern day drilling rigs is then assessed to determine if there is sufficient remaining fatigue margin for the planned P&A operations. If required, mitigation measures such as optimizing riser tension and vessel heading, reducing BOP stack size, calibrating analysis models using monitoring data and using a BOP tether system are evaluated. 

Strength assessment for P&A operations is also performed to determine vessel station keeping requirements. 

Examples from a number of case studies of recent subsea well P&A in Australia are presented. 

The effectiveness of different fatigue mitigation measures are compared. Findings presented in the paper allow operators to efficiently evaluate and plan safe P&A operations on older wells with modern drilling vessels.