Producing hydrocarbons in a deepwater environment is quite challenging and the goal is always to sustain production for as long as possible. Any shutdown, whether intended or unintended, can prove costly. Such was the case on Oyo-8 when the down-hole safety valve failed to open after a planned production curtailment. The several attempts to open the valve were unsuccessful due to suspected loss of hydraulic communication to the operating mechanism or the malfunction of the mechanism. A well intervention was therefore deemed necessary to re-enter the well.

Well intervention in a deepwater environment is particularly expensive, risky and complex, whether it is rig based or rigless. A vessel based riserless light well intervention (RLWI) operation was determined as the optimal solution for Oyo-8. This entails a through-tubing operation to install a retrofit sub-surface controlled safety valve (velocity valve) assembly across the tubing-retrievable safety valve (TRSV) to permanently hold open its flapper and serve as a sub-surface controlled safety valve.

The notable advantages of a vessel based operation were the reduced personnel and reduced number of third-party service contractor interfaces to manage. The rig-up is significantly simpler than the requirements for rig-based completion/de-completion operations. The fact that it is riserless also eliminates the debris management issues usually associated with the marine risers of mobile drilling units. The key components of RLWI are the subsea well-control package, subsea wireline lubricator, subsea pressure-control head, a compensated winch mounted on a module-handling tower, compensated heavy-duty over-side crane, slickline unit, and two work-class ROVs.

The RLWI operation was completed within the budgeted time with less than 3% nonproductive time. The recovery of the horizontal tree crown plugs was uneventful, as the ROV high-pressure jetting tool was effective in flushing out debris from the top of the upper plug before installing the well control package and carrying out the slickline runs. The velocity valve assembly was installed hanging off the end of a thru-tubing packer set in blank tubing. A carefully planned combined operation with the FPSO ensured that control of Oyo-8 horizontal tree was maintained during well intervention from the production topside.

The intervention operation achieved the set objectives of restoring Oyo-8 to full production within allotted time and budget without any QHSE incidents. The RLWI operation was the first of its kind in Nigerian waters.

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