Dynamic Positioning (DP) drilling operations have been conducted for several decades, with significant advances in equipment and controls. Yet, some of the major concerns with DP vessels are associated with the consequences following the loss of vessel positioning and failure to disconnect. In this unfortunate event, the rig remains connected to the subsea Blow-Out-Preventer (BOP) and the Well, and as the vessel drifts away, the stressing on the system grows substantially. The moment the tensioners break down, the loads increase rapidly until the weakest link, the riser or the well, fails first.
We were asked by our client, a major oil company, to develop a methodology to assess the ultimate capacity of a cemented wellhead and casing system under the extreme loads transmitted from a deepwater drilling riser. The objective was to ensure that the drilling structure will by no means collapse below the BOP level resulting in potential loss of hydrocarbon containment.
Our methodology involved sophisticated analyses allowing the rigorous simulation of the soil-casing and the cement-casing interface. The importance of other low-probability phenomena such as voids within the cement or imperfect contact between the casing and the cement were also examined and quantified.