The new offshore energy support vessel Sea1 Diamond has reached another important construction milestone at the COSCO shipyard in China.

A 250-tonne Huisman offshore crane and a helideck have now been installed on the vessel as construction moves into the final outfitting and commissioning stages.

Sea1 Diamond will be the first of four Offshore Energy Support Vessels — OESVs ordered by Norwegian offshore vessel operator Sea1 Offshore.

The vessel is being built at COSCO Nantong Shipyard in China.

Sea1 Diamond was launched in July 2026 and is currently undergoing outfitting and commissioning ahead of its scheduled delivery in January 2027.

One of the vessel’s most important features is its 250-tonne Huisman offshore crane.

The crane has been designed for subsea lifting operations in water depths of up to 3,000 metres.

This capability will allow Sea1 Diamond to perform a wide range of operations, from the installation of subsea equipment to maintenance work on offshore oil and gas infrastructure and renewable energy projects.

The vessel has also been fitted with a helideck.

It is designed to accommodate helicopters in the Sikorsky S-92 class, which are widely used to transport personnel to remote offshore installations.

Having a helideck significantly expands the vessel’s capabilities during long offshore campaigns, allowing personnel changes and specialist transfers without requiring the vessel to return to port.

Sea1 Diamond is based on the ST-245 design developed by Norwegian naval architecture company Skipsteknisk.

The vessel is approximately 120 metres long.

It has around 1,400 square metres of open deck space.

Accommodation is provided for up to 120 people, including crew and offshore specialists.

Subsea and ROV operations were a major focus during the design process.

Sea1 Diamond will feature two ROV hangars, allowing remotely operated underwater vehicles to be permanently deployed from the vessel.

The ship is also equipped with a moonpool, enabling equipment and subsea vehicles to be launched and recovered through the central part of the hull.

The combination of a 250-tonne crane, ROV facilities and a moonpool makes the vessel suitable for complex deepwater projects.

Sea1 Offshore intends to deploy the new-generation OESVs across both the traditional oil and gas sector and the expanding offshore renewable energy industry.

This means the same vessel will be capable of performing subsea construction, inspection, maintenance, installation and other operations on oil and gas fields as well as offshore wind projects.

Energy efficiency has also been an important part of the vessel concept.

Sea1 Diamond will be methanol-ready, allowing its energy system to be adapted for the use of methanol in the future.

Its installed generators will also be capable of operating on 100% biofuel.

The energy concept includes modern generators, battery systems and an optimised thruster configuration.

This is intended to reduce fuel consumption and emissions during different operating modes, particularly when the vessel is working under dynamic positioning.

Kongsberg Maritime is supplying the vessel’s integrated energy and propulsion package.

Sea1 Offshore’s new series has been designed for global offshore operations and with a long service life in mind as the industry gradually transitions towards alternative fuels.

Sea1 Offshore initially ordered two OESVs from COSCO Shipping in November 2024.

In March 2025, the company expanded the programme by ordering another two vessels, bringing the total series to four ships.

Sea1 Diamond is the first vessel in the series.

The second vessel has been named Sea1 Citrine.

The remaining vessels will also be based on the same concept and are intended for both offshore oil and gas and renewable energy markets.

Sea1 Diamond currently does not have an announced long-term contract.

Sea1 Offshore has said the vessel remains available to clients while the company evaluates projects and long-term employment opportunities following delivery.

Sea1 Diamond is expected to be delivered to its owner in January 2027.

With a 250-tonne crane capable of operating in water depths of up to 3,000 metres, two ROV hangars, a moonpool, helideck and methanol-ready energy system, Sea1 Diamond is being developed as a versatile new-generation offshore platform for both oil and gas and offshore renewable energy operations.