Deploying a riser-less light well intervention (RLWI) system from a mobile offshore drilling unit (MODU) for subsea well plugging and abandonment activities

Project Summary

The reservoir decommissioning phase is traditionally undertaken by either:

1. In part by a light well intervention vessel (LWIV) then a mobile offshore drilling unit (MODU)
2. Mobilising an MODU and associated support services from the outset

These options can be costly; thus, a key industry challenge is reducing the cost of decommissioning, whilst increasing safety and considering the environmental impact.

We worked with Well-Safe Solutions on a feasibility study, evaluating the options available to deploy a RLWI system from a MODU for subsea well plugging and abandonment activities. This combined system could enable low-cost access to existing subsea wells by providing a fully integrated package of well control barriers and fluid circulation/injection facilities into a single package.

A single integrated delivery solution provides continuity, flexibility, reduced multi-operation interfaces, and technical options when re-entering subsea wells for the first time. An integrated MODU would provide a total well decommissioning solution. The project looked at developing the concept of combining a RLWI and MODU.

Industry value:
The integrated solution could provide cost savings between £25,000 – £35,000 per day.

A combined system enables low-cost access to existing subsea wells. A single package integrates well control barriers and fluid circulation/injection facilities, resulting in continuity, flexibility, reduced multi-operation interfaces and technical options when re-entering subsea wells.

Key results:
The FEED study covered the operational and functional requirements of an STL 5-1/8” 10ksi riser-less light well intervention (RLWI) system deployed from a mobile offshore drilling unit (MODU).

Lessons learned:
The system is designed to enable the user to interface with a suitable subsea xmas tree running tool or interface tool, and in conjunction with that, to install the system on a live subsea oil, gas or water injection well. The RWLI system provides a means for the entry and exit of subsea slickline and electric line toolstrings into and out of the well while the well remains at full working pressure. Both the RWLI and MODU are configured in such a way that deployment and recovery can be done without the need for over-the-side work, eliminating the need for standby cover. The system is also configured in such a way that it can be used for the injection of fluids into the well for stimulation, well kill purposes and cementing operations.