This paper describes and highlights, via two case studies, the benefits of conducting rigless subsea well intervention from specialist monohull vessels. The case studies provide an overview of the operations on subsea wells, from intervening through the production tree (XT), to XT change-out and well P&A / wellhead removal operations.
A brief overview of rigless intervention techniques and equipment is also described.
The operations described in the case studies were completed in two very different operating regions. The United Kingdom North Sea (UKCS) and West Africa (Equatorial Guinea).
In the first case study, the paper describes how over a 68 day period a Light Well Intervention Vessel (LWIV) successfully and safely performed mechanical repair & well maintenance (and well integrity surveillance); data acquisition (production logging), well suspension (for a XT recovery); scale-squeeze pumping operations (production enhancement) and finally well plug and abandonment (decommissioning) on a variety of wells across several subsea Fields.
The second case study summarises a 105 day campaign in which a LWIV was contracted to undertake subsea well interventions (for both well maintenance and production enhancement needs) and a temporary well suspension (for a XT change-out) on multiple subsea wells within a Field. The vessel optimisation, through performance, coupled with its ease and speed of transit and set up (DP3 rated) were key to this particular campaign as the 7 wells identified to be worked on by the Operator were a number of kilometres apart precluding the efficient use of a moored unit.
Rigless and Riserless Light Well Intervention (RLWI) activities have been executed for over 25 years across the North Sea. The purpose of these case studies is to demonstrate that with effective project management (planning, developing, execution and close-out) the use of RLWI is both a flexible and extremely cost effective method which Operators can utilize for a single well, or across a number (a campaign) of subsea wells, irrespective of subsea tree type or close proximity.
Case study #1 outlined below summarizes the flexibility & efficiency of multi-service LWIV in the North Sea with the ability to perform intervention/subsea operations in a cost effective and efficient manner, allowing remedial operations to be conducted in order to maintain well integrity, and other remedial well servicing techniques in order to maintain and often enhance hydrocarbon production (manage life of well productivity) before eventual Cessation of Production.