SS: Subsea Well Intervention: Riserless Subsea Well Intervention: Technology and Practice
Authors: Crawford Neil C. , Still Ian R. 
Published: July 2010

The need for cost effective subsea well intervention has been discussed and documented over the last few years. Both the increasing number of installed subsea wells combined with the increasing age of subsea fields continues to drive demand for more efficient subsea well intervention. The team at Blue Ocean Technologies has been providing innovative solutions for various types of subsea well intervention for the past four years.

This paper discusses some of the technology challenges Blue Ocean Technologies has overcome and some of the best practices we have developed in order to provide safe, reliable and efficient riserless subsea well intervention.

Background

Riserless subsea well intervention is becoming a more widely used and familiar technique with various wells around the world being worked on without a riser over the last years. The pressure to work without a riser has brought riserless intervention into the mainstream, driven by deepwater rig costs and availability although the work to remediate wells downed by hurricanes in the last five years, from Ivan to Ike, has also acted as a catalyst in getting operators interested in, and the necessary investment to develop the technique.

What is new in riserless well intervention is that we are now working in deeper water and facing and solving the associated challenges. With these advances in technology, depth, capability, and complexity of operations is increasing. As operators have developed deep water prospects going through the 5000' depth barrier in 1997 and through 9000' in 2006, the need for cost effective ways to intervene has grown. Deep water well completion technology enables wells to be completed with a view to less intervention but as the fields mature and energy prices fluctuate, increasing recovery, and reducing abandonment costs have become more important.

Several operators have shown interest in investing in the necessary technology but most have been driven by the need to complete the fields with the lowest capital expenditure and hand over to their production groups to do the rest. Accepting that fact, the work done to remediate downed wells has served to increase operator interest and although we would make the case that no real breakthrough technology was developed in these shelf/shallow water projects, the application of technology to the problem and the experience gained in that work increased operator comfort levels in use of riserless intervention. Blue Ocean also grew through experience gained in remediation of downed wells evolving over the last few years to provide deeper water subsea intervention capabilities. The initial push was to plug and abandon wells less expensively by using DP vessels, freeing up rigs to drill exploration and production wells. This push developed into production intervention projects to improve recovery of hydrocarbons from these deeper water subsea wells. Riserless intervention has " come of age?? with routine interventions at 3000' water depth and deeper depths planned.

Challenges

Amongst challenges in riserless intervention is maintaining an effective and reliable seal against the wellbore pressure.

In shallower water depths, approximately eight hundred feet or less, this can be done with slightly modified land systems, with elastomer sealing elements if slick line is to be deployed through the lubricator. This was predominantly the method used for the downed wells, the majority of which were in water depths under eight hundred feet, and for which slickline could provide the necessary wire line capability.