ROV-Based Multi-String Hot Tapping Optimizes Subsea Wellhead Annulus Abandonment

Abstract

Using a remotely operated vehicle (ROV) to access annular spaces within a subsea multi-string wellhead avoids HSE risk and subsequently reduces the cost and time associated with abandonment operations compared to that of typical rig workovers and diver-assisted repairs. This method was selected to access the annuli of plugged and abandoned exploration wells for remediation, prior to wellhead severance. The operation creates a controlled conduit to surface for pressure testing, venting, and cementing operations to each annulus in a multi-string configuration. A set of subsea exploration wells drilled in the late 1980’s, slated for wellhead severance, had been plugged and abandoned. However, a wireline survey found evidence of gas trapped in various annuli, preventing severance until remediated, which could have introduced Health, Safety, and Environment (HSE) risk during wellhead severance operations. These wellheads consisted of a 30 in. wellhead and various combinations of 20 in., 13 3/8 in., and 9 5/8 in. casings. To avoid rig workover or diver-assisted operations, a suite of ROV tooling was developed to gain controlled access to each annulus. The operation involved cutting a window into the wellhead, preparing a sealing surface for an isolation valve, and hot tapping a hole into the annulus. A downline was then connected for remediation operations. The ROV-based subsea multi-string hot tapping method safely accessed a mixture of single and multiple annuli subsea wellheads. Over two campaigns of 42 wells, the typical time to complete a two-annulus well was three and half days and two and half days for a one-annulus well. The annulus remediation had mixed success; some wells pressure tested well after venting, other wells required cementing, and some wells with low injectivity and continuing pressure trends were plugged for remediation by a rig in a later campaign. Overall, the campaign demonstrated a new ROV method for controlled hydraulic access to the annular space of subsea wellheads for intervention and remediation. The ROV-based method described in this paper has been applied for decommissioning the annulus of plugged and abandoned subsea wells prior to wellhead severance but could be applied for other annular access purposes.