Riser-Less Surveillance for Aging Subsea Wellhead with Light Construction Vessel

Abstract

Legacy Unocal have drilled hundreds of exploration wells between 1970's to 1990's in Gulf of Thailand with semi-submersible drilling rigs and drillships. Those exploration wells were plugged and abandoned as per the requirements back in the 1970's which the subsea wellheads were not cut and removed. According to asset retirement obligation, all structures above seabed need to be removed before returning concession area. These subsea wellheads are planned for cut and recovery, however, there are many uncertainties in regard to well conditions which includes trapped gas in casing annulus and casing corrosion. The correct and safest approaches to cut and recovery are to perform wellbore logging runs, gas evaluations, and casing wall thickness investigation. The challenges faced in executing the logging runs in open water environment are to conduct safe, efficient, and reliable operations on Dynamic Positioning (DP) vessel which includes a work-class Remotely Operated Vehicle (ROV). The key factor is to attain as much information and quality logging data to engineer a successful cut and recovery campaign for these complex subsea abandonments. To perform this operation in open water, it can lead to inaccurate logging data due to uncompensated conditions. An innovative solution was collaboratively invented and developed with Business Partners to deploy Logging Tower Subsea Deployment system (LTSDS) to mimic static logging condition. The tower is self-standing design with a capability to rig up on different type of wellhead. The trial was performed but had complications of deployment due to weather impact. The new method, non-compensated real time logging on Electric Line, was introduced to improve time spent per wells. The comparison of logging results from both methods shown similar and reliable log interpretations. The remaining wells were logged with non-compensated real time method, which reduce operational time by 45% as well as lower associated operational risks. Other challenges encountered included logging depths 100 ft below mudline. Since some wells have shallower holdup depths or the tool could not pass due to encountering obstruction at shallow depths, gauge cutter was deployed with slickline unit to remove obstruction, followed by downhole high pressure jetting tool which was effectively helping to achieved logging intervals. Interpretation of logging data were completed on more than 100 wells and results were used to engineer a detailed cut and recovery campaign and utilize new technology that the team had sourced to execute the operation. With innovative designed set up of riser-less operation on light construction vessel, it is proven to meet objective of reliable logging data and cost efficiency and demonstrated flexibility for operation improvement.