Ulstein has published a concept study examining a nuclear hybrid retrofit for its SX157 OSV design, with conventional fuel savings estimated at more than 50 per cent in many operating scenarios.
The study, published on September 15, proposes retaining two of the vessel’s four main engines and replacing the other two with electrical machines powered by microreactors. Nuclear-generated electricity would cover lower-demand operations, while the diesel-mechanical engines could provide additional propulsion when required.
Ulstein said the vessel could operate without conventional fuel during transit and light towing. Henrik Høidal, System Architect at Ulstein Design and Solutions and author of the study, said: “The main idea is that you'll have 100 per cent reduction of fuel in transit and a great emission reduction in almost all modes.”
The claim refers to fuel consumed during those operating modes. The study does not include a completed emissions assessment or full lifecycle analysis.
The reference design is an 88.9-metre OSV with a beam of 21 metres, deadweight of about 4,250 tonnes and approximately 300 tonnes of bollard pull. Its intended work includes moving large structures over long distances, including oil rigs and floating production, storage and offloading units.
Ulstein estimated that towing at 45 per cent of maximum continuous propulsion rating would consume 47.87 cubic metres of fuel per day, or 1,436.1 cubic metres over 30 days. At 65 per cent, consumption would reach 68.65 cubic metres per day and 2,059.5 cubic metres over the same period.
About 48 per cent of the design’s heavy fuel oil capacity would be converted to accommodate the reactors and associated equipment. Modifications would be needed to the tanks, structure, piping and compartments, while the existing engine room would not provide enough space for the shielding, power-conversion equipment, thermal storage and reactor system.
Høidal also examined converting the vessel entirely to nuclear-electric power, a process that would involve removing the main engines and auxiliary generator sets. Ulstein identified the hybrid arrangement as the more realistic starting point because it preserves more existing equipment.
The concept uses Emerald Nuclear’s Gem microreactor, a nitrogen gas-cooled design using TRISO fuel. Each unit is intended to produce 5.0 MW, with a stated operating duration of five to 15 years depending on load.
Under Ulstein’s scenario, complete reactors would be replaced at five-year intervals aligned with class surveys. The crew would neither handle nuclear fuel nor refuel the reactor at sea.
The study does not set out a retrofit price, reactor cost or payback period. Assessing the business case would require accounting for time in the yard, periods when the vessel is unavailable, engineering work, approvals, insurance, financing, lifecycle services and decommissioning.
“The primary barrier is that suitable microreactor technology is not yet commercially available for maritime use,” Ulstein said.
Any project would also need classification approval and flag-state approval, acceptance by ports, emergency arrangements, liability rules and provisions for replacing the reactor and handling waste. Ulstein said high-fuel-consumption vessels with predictable power demand and sufficient remaining service life would be the strongest candidates.