TThese are the two main motivations for the Northern Lights project, which is part of the Norwegian initiative to achieve large-scale CCS. The project involves capturing CO2 from cement production and a waste-to-energy plant in the Oslofjord region and shipping liquid CO2 from these sites to a land terminal on the Norwegian west coast. From there, the liquefied CO2 will be transported by pipeline to an underwater storage site in the North Sea for permanent storage.
“As pipelines cannot always provide the complete solution for CO2 transportation, we have to learn how to make CO2 transportation work,” says Frank Ollerhead, marine transportation manager in the Northern Lights Project. “The objective of this initiative is to provide viable and necessary solutions for the maritime transport of CO2.
Viable vessels for CCS
Northern Lights is a cooperative project between Equinor, Shell and Total, while DNV GL provided technical support.
“The Northern Lights ships are relatively small and the schedule is tight,” explains Ollerhead. “For this reason, we have decided to use proven technology rather than relying on R&D for our solutions. Most of the technology is already there. Now we assemble it into a convenient solution.
The project seeks to design vessels as close as possible to Phase 3 of EEDI, says Ollerhead. “We want ships to be as green as possible, but we are not developing new green technology for ships.” He reports that LNG and dual fuel are being considered fuel options, with batteries added to maneuver for up to 45 minutes. “They are basically standard ships with new containment tanks and batteries added. The resulting reduction in CO2 emissions may seem small, but the introduction of batteries to improve efficiency is a welcome feature nonetheless. “
Design cost-effective solutions
The introduction of a high pressure and low temperature containment system in standard designs has reduced costs, improving the profitability of ship-based solutions over pipelines, says Magnus Lindgren, Senior Senior Inspector, Ship Type Expert Gas Carriers at DNV GL Maritime.
“Standard size is a prerequisite for a vessel to be economical, but the tanks in this case are designed for very high pressure. This requires high strength materials. The process with project partners focused on approving tank materials using a design compatible with standard vessels, Lindgren reports. When using high strength materials in tanks, fatigue properties can become critical. The design team took these properties into account to maintain the security levels defined in the codes.
The decision to incorporate a single large-diameter Type C cylindrical cargo tank into the Northern Lights project improved the economics of the liquefied CO2 transporter design, Lindgren notes, adding that the tank design offers efficiency of relatively high volume when applied to ships generally used for LPG transport.
“The tank containment system is new, but the vessel is still based on a proven vessel design,” explains Ollerhead. “We have selected a design solution that will be economical. It’s a little outside the norm, which is traditionally a cost factor, but ships will still have a good economy. With funding from project partners, DNV GL completed the pre-class assessment of innovative Type C containment tanks for liquefied CO2 for the Northern Lights project in October 2019.
Key changes in tank design to accommodate CO2
In order to liquefy carbon dioxide, which sublimates directly between the gaseous and solid state (dry ice) at atmospheric pressure, the temperature must be maintained around -50 ° C, with a pressure of 7 bar, in line with LPG containment systems. The DNV GL approved CO2 containment system reduces refrigeration requirements by achieving an operating design temperature in the order of -30 ° C. This required the Type C tank to withstand higher pressures of 19 bar.
The combination of high density cargo, high design pressures and large tank diameter required innovative solutions to meet the strength requirements of cargo tanks. The design changes included strengthening the design of the Type C tank in various places, but also softening the design where larger deflections were used to reduce dynamic stresses in the tank.
The vessels have been designed to withstand the build-up of pressure during laden voyages and the corrosive effects of liquid CO2. The chosen design balances the competing demands of economical tank manufacturing while meeting tank safety requirements by replacing the standard low temperature steel for pressure vessels with a better quality steel product.
Robust solutions ensure security
The tank solution has been designed to adapt to the physical characteristics of liquid CO2 in the loading and unloading phases. Particular care has been taken in controlling pressure drops, which can cause the formation of dry ice, with the possibility of abrasive effects on the cargo pumps. The tank has also been designed to withstand the variations in tank pressure encountered between laden and ballast trips, in addition to the acceleration loads of liquids when fully charged.
The design and material selection used high tensile strength properties to withstand higher extreme loads and provide crack resistance. “Documenting the safety of the tank design was a key criterion before the design could gain pre-class approval. This required extensive testing to ensure that the materials could meet safety requirements against the development of fatigue cracks, ”Lindgren reports.
The design was concluded to be able to withstand dynamic loading without exceeding stress levels that would allow fatigue cracks to propagate. Equinor received the GASA statement confirming the DNV GL design approval in November 2019.
Northern Lights: Part of a larger CCS picture
Industrial scale CCS was defined as part of the solution to achieve the COP21 1.5 ° C target. “Besides helping meet UN goals, the biggest effect of using ships to move CO2 is to enable stakeholders to deal with businesses outside the energy chain,” says Ollerhead .
“CO2 is already used in other industrial applications, and in a transportation context it is similar to the liquid gas trade. Tank technology has been used in food industry vessels. We just apply thicker tanks and lower temperatures.
Starting the CO2 trade using ships is a great value proposition in the Northern Lights Project, confirms Frank Ollerhead. “For example, the potential for producing carbon neutral cement will guide investment decisions. With the solutions developed in this project, shipowners should see potential new markets that will open the door to other industries. Overall, the timing is good to introduce this technology.
Source: DNV GL, Maritime impact (https://www.dnvgl.com/expert-story/maritime-impact/Northern-Lights-shows-the-way-to-seaborne-CCS-solutions.html)