Plug and Abandonment (P&A) can easily contribute with 25% of the total costs of drilling exploration wells offshore Norway. Cost efficient P&A technology is therefore necessary to reduce cost of exploration drilling. In this paper, qualified technology for cutting and retrieval of wellheads using a separate vessel is described in detail. It is shown how to use this technology to significantly reduce the total costs of exploration drilling. The technology has now been used on several abandonment operations on the Norwegian continental shelf.
In the paper it is presented through examples how efficient P&A operations are run using a dedicated vessel to perform parts of the wellhead cutting and retrieval operation earlier conducted with the drilling rig. Examples illustrate how the different wells are permanently plugged back to maintain all barrier requirements before the drilling rigs leave the wells with wellheads in place. During a later wellhead removal campaign a dedicated vessel arrives cutting the casings underneath the sea bed and finally removes the wellheads. It is shown that removal of more than two wellheads in a campaign is necessary to make this type of operation cost efficient.
By transferring activities like Plug and Abandonment (P&A) and anchor handling from rig to dedicated vessels, the cost of drilling operations will be reduced and drilling production will be increased (Sørheim et al., 2011). The objective of transferring these activities to the dedicated vessels is to maintain the drilling rig activity at their core functions, which are drilling and completing wells. For example, in offshore drilling operations it is generally more cost effective to pre-set anchors prior to arrival of drilling rigs than to let the rig be an active part in anchor handling (Saasen et al., 2010). Similarly it can be efficient to pre-set conductors prior to rig arrival. As mentioned, permanent P&A is also an operation where activities can successfully be moved from rigs to dedicated vessels. In addition to the economical benefits of moving activities to dedicated vessels, there is a significant Health, Safety and Environment (HSE) benefit. These transferred activities are now conducted by specialised personnel on dedicated vessels. On the rigs these activities will be parallel to other rig activities and thereby represent a slightly higher HSE risk.
Going and Haughton (2001) presented tools for casing string recovery including casing cutter, a hydraulic casing spear and a combined marine swivel/hanger seal extractor have been presented earlier. This system has successfully been used on drilling rigs to remove casing strings and wellheads. P&A without the use of drilling rigs is currently a routine operation on land wells. See for example Tettero et al. (2004). These techniques however, are not straightforward in offshore operations.
P&A of offshore exploration wells represents a significant part of the drilling cost especially for production wells. Normally these operations are conducted by removing completion equipment followed by placement of a series of cement plugs. This application of cement plugs is described by for example Liversidge et al. (2006). Also, while drilling exploration wells, the P&A operation is a significant cost. Therefore, a method using a concentrated sand slurry for P&A of the reservoir has been applied to minimise the time to wait for the cement to cure (Saasen et al., 2011). In the following it is shown how parts of the P&A operation successfully have been transferred from the rig to a dedicated vessel and thereby reduced rig time on non-drilling activities.