In a few years, there will be a need for performing a considerable number of subsea plug-and-abandonment (P&A) operations on the Norwegian continental shelf. There are certain challenges associated with this. These include more-difficult access to subsea wells compared with ordinary platform wells, high daily rates of semisubmersible rigs, and the fact that these rigs will be allocated for drilling new exploration-and-production wells to sustain the hydrocarbon production. At the moment, there is large focus on finding rigless technologies to reduce P&A cost and use of rig time. A probabilistic approach should be used to assess the cost- and duration-saving potential of such technologies relative to semisubmersible rigs.
In this paper, we will consider rigless technologies for performing parts of the P&A operation. For the first time, a probabilistic approach, including learning curves, correlations, and possible risk events, is used to evaluate subsea batch-operated P&A.
A realistic example that includes the use of a semisubmersible rig for batch-operated P&A of subsea production wells is used to show how a probabilistic methodology can be implemented for obtaining probabilistic estimates of P&A cost and duration. Inclusion of learning curves and correlations makes the application of a probabilistic approach for multiwell campaigns challenging. It is demonstrated how to incorporate learning curves, correlations, and possible risk events to capture a realistic range of cost and duration for multiwell P&A operations. In a second operational example, a light-well-intervention vessel is used to accomplish preparatory work and wellhead cutting and removal for P&A of the subsea production wells in batch campaigns. These two scenarios will be compared from a cost and duration point of view. Moreover, the advantages associated with riserless technologies for multiwell abandonment are discussed.