Big fuel savings on first CSOV newbuilds with Ulstein’s TWIN X-STERN® concept

The Norwegian offshore vessel operator has now taken delivery of the second of these unique vessels, Olympic Notos, from compatriot shipbuilder Ulstein after realising these significant fuel savings in dynamic positioning with the previously delivered sister vessel Olympic Boreas during operations for BP off the UK, prior to starting work on an offshore wind project in the UK North Sea.

These are the first CSOVs to employ the TWIN X-STERN system, with four main thrusters fore and aft in a symmetrical dual-stern hull, allowing flexible operability in dynamic positioning (DP2) mode. This is complemented by diesel-electric propulsion with variable speed, hybrid battery power, energy storage and smart energy management to maximise fuel efficiency. 

Based on the ULSTEIN SX222 design by Ulstein Design & Solutions AS, the vessels are intended for work in the offshore wind and oil & gas segments, with a length of 89.6m and beam of 19.2m, and accommodation for 126 people in 91 cabins. They are also equipped with a heave motion-compensated gangway system for efficient transfer of personnel and cargo at variable landing heights.

The Olympic Boreas was nicknamed “the four-wheel drive of the sea” in winning the Maritime Innovation of the Year Award with GCE Blue Maritime Cluster following its delivery from Ulstein Verft this summer. 

Read more and watch film: The TWIN X-STERN concept explained

And this innovative thruster configuration, combined with variable speed capability, is the key factor behind boosting energy efficiency by allowing precise multi-directional positioning control with optimal use of thruster power to drastically cut fuel usage, explains Olympic’s Chief Technical Officer Runar Stave. “Furthermore, high manoeuvrability with the multi-thruster system enables enhanced seakeeping and stability in variable sea states, which contributes to greater operational efficiency and improved safety with gangway crew transfers. Less noise and vibration from reduced thruster usage also gives a more comfortable onboard experience for the crew in accommodation of hotel standard,” he says.

“The ability to run the engines at variable speed means that power production can be optimised based on the vessel’s energy demand. The implementation of several technical measures onboard the vessel has resulted in a power demand of only 250-300 kilowatts under certain conditions. At such low power demands, operating at variable speed enables a more optimal operation of the engine – as opposed to operating at constant speed – and results in significantly lower energy consumption per kWh. Consequently, the ability to operate the engine at variable speed, combined with the technical measures and the overall ship design, has enabled the Olympic Boreas to consume only 2.7 tonnes of fuel per 24 hours during a week of DP operations, which is approximately 50% less than other sailing CSOVs,” according to Stave.

Reduced fuel and maintenance costs

“As well as cutting fuel consumption, reducing engine speed leads to significant reductions in maintenance costs by extending service intervals, contributing to lower operational expenses over the vessel’s lifetime,” he adds.

These vessels have a hybrid battery system that can operate as a spinning reserve, reducing the need for auxiliary generators and further improving fuel efficiency. Additionally, a smart energy management system uses automated digital algorithms to manage power production and consumption in real-time, providing instant power when needed while reducing overall energy usage.

The CSOVs are also equipped with a shore power connection for emission-free port operations and battery recharging. They also have space for extra battery capacity, enabling future full-electric operation once the necessary infrastructure becomes available at sea.

GHG emissions from the vessels have been reduced proportionate to the cut in fuel consumption, and they are also ready to use methanol as fuel, which will further reduce their carbon footprint.

Olympic’s Chief Commercial Officer Glenn Erik Valø points out this is important from both a sustainability and commercial perspective, given the scheduled implementation of the EU Emissions Trading System (EU ETS) for offshore vessels from 2027 will lead to higher fuel-related costs for vessels running on conventional fuels due to the need to compensate for emissions.

“Optimisation of energy efficiency and reduced emissions with these vessels will therefore represent a cost advantage that will be an important competitive differentiator in contract tenders for the vessels,” he explains.

Valø says Olympic’s ambition when starting work with Ulstein on the CSOV concept was to “set a new standard” for such vessels in terms of sustainability, safety and operational efficiency. And this “matched perfectly” with Ulstein’s TWIN X-STERN design, already developed by Ulstein Design & Solutions and applied to other vessels. 

“Olympic has been pursuing sustainable operations since starting with a diesel-electric vessel in 1996 and has consistently been willing to take risks with innovative solutions in our push for greater fuel efficiency. We have a successful track record of commercialising new technologies, including concepts such as dual-fuel LNG and the permanent magnet thruster,” he says.

This approach was earlier demonstrated with its 2009 delivery from Ulstein of the ground-breaking Anchor-Handling Tug Supply (AHTS) vessel Olympic Zeus that was the first such vessel to be equipped with a diesel-electric/diesel-mechanical propulsion system. And, 15 years on, it is still outcompeting most AHTSs due to an average 50% fuel reduction using the hybrid solution.

Valø continues: “The TWIN X-STERN was for us a novel and attractive solution that we believed could be turned into a unique concept for the CSOV segment, based on our accumulated knowledge from operating in the most efficient manner.”

For its part, Ulstein was keen to engage with Olympic’s clearly defined goals in terms of operability and fuel savings to further develop and refine the concept for a new vessel application, according to Ulstein Design & Solutions' System Architect Stein Frode Haugen.

He says performance and safety in dynamic positioning mode were the key considerations when designing the CSOV concept for Olympic with a view to creating a “highly capable ship optimised for its operational purpose”. And this guided design choices in areas such as hull design and selection of propellers, he explains.

"The same considerations are the basis for the design of the power system on board", according to Ulstein Power & Control's senior engineer Geir Haddal. 

"Throughout the entire design phase, energy-efficient solutions have been in focus. The correct setup of dynamic power production and the various power-driven components, including thrusters and propellers so that they interact effectively, has required innovative solutions and major changes in control philosophy. The result has been very energy-efficient solutions", says Haddal, who also praises Olympic's willingness to choose the new power system.

Following a thorough design and engineering phase, the positive collaboration between shipowner and yard was able to realise the ground-breaking CSOV newbuild project within only a two-year timeframe from contract signing in July 2022, with both vessels delivered on budget and according to quality specifications.

“Olympic has once again proven that we can take great ideas and turn them into industry-leading vessels through working together with Ulstein and the wider Norwegian maritime cluster,” Valø says.

Ulstein will now be analysing live data feeds from energy flow monitoring devices on the power and automation solutions of the CSOVs to identify potential for improvements to operations and the power system configuration, with an eye on future advancement of the TWIN X-STERN concept and possible application in other vessel segments including subsea and cable-laying.