Market changes pressure development of rules for specialized offshore well intervention vessels
Alexander Wardwell
DNV
Driven by global demand for energy, subsea exploration and well construction has boomed in the past decade. According to consultants Infield Systems and Douglas Westwood, the total number of subsea wells will balloon to more than 5,500 by the end of this year. While some projects may be delayed due to financing issues related to the global economic downturn, the rapid construction of new wells is likely to continue, if not accelerate, in the years to come.
This creates challenges for energy companies and suppliers alike. Increased demand has forced many energy companies to:
- Re-evaluate stranded or marginal fields
- Work in deeper waters
- Use more complex tieback solutions
- Improve recovery rates for aging wells, which are about 10-20% below platform wells.
These demands push well intervention operators to develop new technologies to improve access to subsea wells, creating a demand for more efficient subsea well intervention systems, including Riserless Light Well intervention (RLWI) units. While not appropriate for deepwater, RLWI units are optimal for repair, scale removal, installation, and manipulation of some equipment (such as valves, plugs, screens, etc.), re-perforation, zone isolation, fluid sampling, PLT, chemical treatment, and well abandonment, among other services.
In the past, this work was performed by semisubmersible drilling rigs. However, developments in dynamic positioning systems, ROVs, and other specialized onboard systems allow well intervention equipment to be placed on monohull units that can move quickly from one well to the next to help reduce chartering costs and to improve well recovery rates by up to 50%. Riser well intervention units still are preferred for some work and in water depths up to 500 m (1,640 ft), but new composites in development for wire lines may soon allow RLWI units to work in deeper waters.
Vessel or offshore unit?
The first monohull well intervention unit (Seawell) was built in 1986 by WellOps. The concept succeeded, and over the next 10 years, demand for LWI units grew. However, because these units are often similar in design to offshore supply, support, or multi-purpose vessels, there was uncertainty on how to class them: Are they vessels or mobile offshore units?
Based on its experience in the North Sea (home to about 40% of the world’s subsea wells) and other regions, DNV moved to manage these issues. According to Per Jahre Nilsen, DNV’s business development manager for well intervention, the development project, which began in 2007, created some challenges.
Marine Subsea’s Sarah, an Ulstein SX121 designed OCV/IMR type vessel, is designed for light well intervention and subsea construction. Its hull-form is designed to give low noise and reduces fuel consumption.
“At the time, there was not a lot of useful data out there to help us develop the right approach,” he says. “But based on our experience, technical research, and feedback from the industry, we concluded that if the unit is capable of taking control of subsea equipment, such as opening or closing valves on a producing well, it would be classed as offshore, not maritime.”
Nilsen says these criteria are consistent with the way many national authorities differentiate between offshore operation and maritime ships/vessels operation and notes that Mobile Offshore Development Units (MODUs) code compliance applies to offshore. Once developed, the new rules were submitted to external hearings for review and additional comments were solicited from owners and operators.
Class notation
Today, DNV is the only class society offering the Well Intervention Unit class notation. Nilsen says that defining the parameters for a mandatory class notation for well intervention units required an exhaustive review of different technical elements and a broad range of safety principles, including ventilation, areas classification, shut down and gas detection, escape, evacuation, and communication. The organization used in-house expertise on structural design to account for substructure and foundations for well intervention equipment and drill floors, when applicable. Other issues include fire protection, dynamic positioning, and a number of supplementary requirements, ranging from gas treatment in the event of a leakage to rescue ladders in the moonpool. Based on experience gained by developing these new rules, DNV released a new, optional notation known as WELL Intervention this past October. Nilsen explains that the scope the WELL class notation includes design verification of the well intervention equipment and systems, and survey and follow-up during fabrication. Once completed or certified, the equipment follows traditional classification principles and is inspected regularly.
“By introducing the new, voluntary WELL class notation together with the revised and mandatory Well Intervention Unit notation, DNV was able to offer owners and operators of well intervention units the same options that owners of drilling units have had in the past,” says Nilsen.
To date DNV has issued certification for six well intervention vessels, including four optional WELL notations, for a number of subsea service companies.
Early adopter
One early adopter of the new DNV rules was Aker Oilfield Services. The company has a newbuild program to provide subsea intervention, light drilling, and riser and riserless well intervention services.
Alf Kristensen, manager engineering projects at Aker Oilfield Services, says the company has one well intervention unit classed by DNV – the newbuildSkandi Aker.
“While there are many components to winning a contract, we felt the DNV notations gave us an advantage over competing oilfield services companies,” says Kristensen.