CaCO3 scale build-up may cause severe production restrictions. A common resolution is electric line (e-line) clean-out with bailers built into the toolstring to capture the debris while milling. This paper will provide details of an innovative tweak that allowed an offshore operation to continue despite the presence of CaCO3scale. The paper will also discuss the equipment and Riserless Light Well Intervention (RLWI) method to derive valuable lessons learned for future operations.


The operation took place in the British shelf of the North Sea. A subsea well was facing severe scale issues that were threatening to stop production. From the beginning, it was the plan to perform the operation as a RLWI from a vessel. During the initial drift run, it was discovered that the CaCO3 scale had become so severe that it would not be possible to run the e-line milling solution as originally planned. The team needed to remove ~15 ft. of scale in order to accomodate the clean-out toolstring below the subsea tree before the operation could continue.

Standard dump bailers were not available on the vessel and, as weather prevented the flying of equipment to the vessel, the team worked together to quickly contrive a readily available solution that would allow Cal-Acid scale dissolver to be spotted precisely and safely at the holdup depth.


The solution became to modify the clean-out tool by removing the drive shaft and mill bit, introduce check valves, and pre-load the bailers with Cal-Acid. This enabled the tool to act as a dump bailer with the Cal-Acid being displaced out of the pump ports and onto the scale.

The Subsea Intervention Lubricator (SIL) had been pre-filled with Cal-Acid and, once the milling tool was activated in the SIL, the acid circulated into the bailers, which eliminated the need for any surface handling. The tools were then run to the holdup depth and the Cal-Acid unloaded. This was performed 16 times resulting in ~ 150 L of Cal-Acid being spotted around the scale build-up.

A subsequent run with gauge cutters confirmed that the Cal-Acid spotting had been successful and access for the remainder of the operation had been achieved.

Novel info

The innovative use of readily available tools, modified to a bespoke application, allowed the operation to continue, resulting in significant time and cost savings, and the concept of filling the bailers in the SIL made the operation inherently safer through reduced manual handling and recovery of subsea equipment.

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