Riser Allows Live-Well Intervention With Coiled Tubing From Monohull Vessel
Authors: Carpenter Chris 
Published: July 2023

_
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 212939, “A Proven and Versatile Coiled Tubing Live-Well Intervention Solution From a Monohull Vessel in the Norwegian Continental Shelf,” by Kjetil Austbø and Stein Kristian Andersen, Equinor, and John Stuker, SPE, SLB, et al. The paper has not been peer reviewed.

_
The complete paper describes the use of a riser for coiled tubing (CT) operations from a monohull vessel already performing riserless well interventions (RLWI). The project was developed by the operator, an intervention-vessel company, and several service providers. The solution covers 200- to 500-m water depths and converts back to riserless operation after CT operations.

Introduction
The operator has more than 600 subsea wells globally in multiple countries and more than 550 subsea wells on the Norwegian Continental Shelf (NCS), which accounts for more than 83% of all subsea wells in the area. Successful interventions here using RLWI vessels encouraged the operator to investigate the possibilities of also performing CT operations from a vessel. The best option was found to be a strategy that included a monohull vessel that could perform both RLWI and CT operations.
The analysis of operations in rough-weather conditions in the NCS showed that riser-based CT operations from such a vessel could be performed in the summer season only. Thus, the operator required a flexible solution such that the same RLWI vessel could cover both year-round RLWI and summertime riser-based CT interventions efficiently. The solution materialized when a larger vessel was engaged to expand the usable weather season and allow year-round RLWI operations.

Description and Application of Equipment and Processes
To enable riser-based CT intervention in the NCS, the chosen option required more equipment and interfaces than were required when performing RLWI. To conduct riser-based CT interventions in live wells, a way to handle, deploy, and tension the high-pressure riser was required, as was an interface for both wireline and CT equipment in the vessel. A system to handle live-well returns from the well also was necessary.
After many engineering iterations, the RLWI vessel implemented a modular riser-handling package that could be installed and then removed when not needed.
On top of the surface flow tree, a custom CT tension frame (CTTF) was developed for the CT and wireline stacks and pressure control equipment (PCE), each inside separate support frames within the CTTF. With the CTTF connected to the riser, each support frame could be skidded to the well center, eliminating the need for crane lifts while changing between wireline and CT.

CT Planning
Well Control and Barriers. Because of limited space on the RLWI vessel, only one dual-ram blowout preventer (BOP) would fit in the CT support-frame cartridge within the CTTF. In addition to the dual strippers, the rest of the PCE requirement considered the subsea emergency disconnect package (EDP) and lower riser package (LRP) to complete the well-control equipment. The EDP and LRP included a safety head, a production isolation valve, and a retainer valve, all capable of cutting wireline and CT.