As the global well base continues to increase, it is becoming apparent that under-investment in maintenance has helped contribute to a decreasing average production per subsea well. To combat this trend, more focused and cost-effective intervention techniques have emerged such as open-water hydraulic access (OWHA) using coiled-tubing (CT).

OWHA using CT provides operators with a fit-for-purpose solution to deliver fluids to a subsea well without the requirement for mechanical intervention. In scenarios which meet the requirements outlined within this paper, OWHA using CT offers an attractive alternative to conventional intervention approaches.

Three predominant configurations of OWHA CT systems are apparent, all developed around differing drivers: a conventional system using a CT injector head (IH), a specialized system using a CT IH, and a powered reeler with deployment sheave. Both the benefits and limitations of each are considered to guide the reader in selecting the correct configuration for their application.

The primary cost driver for hydraulic intervention is vessel selection. Factors including regional availability, mobilization requirements, and specification are key to the selection process. An advantage of OWHA using CT is the ability to use a vessel of opportunity due to the decreased equipment footprint and specification; this directly enhances service flexibility and cost management.

When compared to alternative oil sources, the comparative cost per barrel of oil unlocked by OHWA is at least 43% less than the closest alternative. When compared to alternative intervention approaches, the simplified equipment and requirements result in a reduced total cost thus enhanced return on investment with accelerated payback.

Using average global vessel rates and generalized assumptions, for a single well intervention the total costs can be in the order of the following magnitudes: floating rig USD 9-16M, light well USD 6-10M, OWHA USD 2-5M. Using an average well production gain of 1,950 BOPD this gives a typical payback time period of ~9 months for a rig and ~3 months for OWHA.

In scenarios which meet the requirements outlined within this paper, OWHA provides the most economical solution with the ability to provide positive returns in the same financial year.