Well intervention of subsea wells is commonly performed by positioning a rig or intervention vessel and connecting the rig to the subsea wellhead with a marine riser. Once the marine riser is connected to the subsea tree, the crown plugs are removed and normal well intervention services can be performed. Typically, subsea trees have upper and lower crown plugs for dual barrier redundant sealing. Crown plugs can be removed from the subsea tree using different methodologies; however, slickline is often the preferred method because of economics. Conventional slickline with toolstrings that create upward impact have been used successfully in shallow water. However, as development occurs at greater water depths, higher hydrostatic heads have made impact tools less effective. The major concern is that impact loads create only slight upward movement, and high forces attributed to hydrostatic pressure push the crown plugs back into the original sealbore. To overcome this limitation, a battery powered electromechanical pulling tool has been used to successfully pull numerous crown plugs in a variety of different conditions. The original electromechanical pulling tools where simple on/off devices that did not provide any information when the operation was unsuccessful. New technology has been developed that provides data to diagnose cases in which is not possible to pull the crown plug. The new technology has been deployed successfully and the post-job report is used to diagnose cases where pulling of the crown plug was unsuccessful. This paper discusses a job where the new technology provided data that clearly showed the tool supplied maximum rated force without being able to pull the crown plug. With this information the operator understood the situation and was able to move forward quickly with the correct tools to mitigate the circumstances. The understanding provided by the tool data enabled the operator to quickly resolve the situation, helping save substantial rig time in a deepwater environment.
Well intervention of subsea wells requires detailed planning before a rig or intervention vessel travels onsite. One of the first tasks performed during well interventions is pulling the subsea crown plugs in the wellhead. A portion of the planning process involves selecting a method to be used to remove the crown plugs from the subsea tree. Crown plugs can be pulled using a variety of methods, such as a rig, coiled tubing (CT), or possibly slickline. Although all of these methods have been employed, slickline is preferred because of its low cost, quick setup, and speed.
In shallow water, conventional slickline methods using jars have been successfully deployed to pull crown plugs. Slickline jars create very short duration high impact forces used to slowly work the crown plug out of the sealbore. This methodology works well in shallow water because hydrostatic pressure from the water column is typically less than the surface shut-in pressure.
Additionally, the presence of debris in the marine riser can cause additional issues. Debris present in the marine riser can fall and settle on top of the wellhead plugs. An accumulation of debris on top of the crown plug can make pulling more difficult.
In deeper water, hydrostatic pressure created by the water column can be greater than the surface shut-in pressure. With a large cross-sectional area crown plug, there can be a large differential pressure keeping the wellhead plug in place. Conventional slickline has extremely limited pull force because of low wire strength. Jar forces, even substantial ones, cannot work the plug out of the sealbore because, after each jar impact, the line must be relaxed to recock the jars. Every time the line is relaxed to recock the jars, the differential pressure pushes the plug fully back into its seat.