Technical and economical feasibility of zero-emission walk to work vessels

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Urgent action is needed to decrease maritime emissions. The renewable wind energy sector must also decrease its life cycle greenhouse gas (GHG) emissions. Total GHG emissions from offshore wind turbines are approximately 120% more compared to onshore wind turbines. Moreover, it is estimated that Walk to Work (W2W) vessels are responsible for 3.3% of the total life cycle emissions of offshore wind turbines. Especially during operation and maintenance (O&M), there are no substantial emissions other than those caused by marine support. This research investigates the technical and economical feasibility of a zero-emission W2W vessel during its operational life. W2W vessels use dynamic positioning (DP), a motion compensated gangway, and a motion compensated crane to transfer technicians and cargo to offshore structures to enable them to perform installation, commissioning, and O&M.

This research has used literature research to collect data, insights on W2W vessels, insights on the offshore wind market, and information on alternative energy carriers. Furthermore, a parametric model is written to test the technical feasibility and the Robust Decision Making (RDM) method is used to test the economical feasibility.

The building rate of new offshore wind farms (OWF) is expected to quadruple by 2030, wind turbines are increasing in size and height, and OWFs are expected to move further offshore. These three expected trends imply a higher demand for W2W vessels and require adaption of the specifications of the vessel and mission equipment. The operational profile of the W2W vessel is unique due to its high operational mode in DP, which is usually 90% to 98%. Additionally, the fuel tanks of the W2W vessels are excessively large for the required autonomy of 2 to 4 weeks.

Characteristics including life cycle emissions, (future) price development, energy density, and social perspective are used to select hydrotreated vegetable oil as a blend-in fuel, compressed or liquid cryogenic hydrogen, methanol, ammonia, and batteries as potential alternative energy carriers. All of them have a lower contained energy density compared to fossil fuels, leading to increased space requirements. Alternative energy carriers are also more expensive today, but energy prices are expected to decrease, while fossil fuel prices are expected to increase.

A parametric model has been developed and applied to 18 configurations between energy converters, energy carriers, and autonomy duration. The model takes a base case and estimates required energy, volume for storage, length, width, weight, draught, and power for DP operations. It has been concluded that all 18 configurations are technically feasible.

An RDM is performed to test the economical feasibility of the 18 configurations among 13122 different futures based on the formulated uncertainties (energy carrier price, energy carrier availability, annual utilisation of a W2W vessel, day rate, CAPEX, and OWF to port sailing distance) and a potential carbon tax (CT). Results are evaluated for profit (NPV), operational uptime, and emissions per wind turbine connection. First, it has been concluded that batteries and compressed hydrogen are highly unlikely to be feasible options because of their low operational uptime. Secondly, a fuel cell running on either liquefied hydrogen or methanol, is unlikely to make profit. Third, it is highly likely that a single fuel green ammonia ICE configuration will be the most profitable among green alternative energy carriers. Fourth, single fuel green methanol ICE configurations could be profitable but are less likely to be profitable than ammonia under the assumptions made. Lastly, for an average LSMGO price (Nov 2018 - Nov 2021), a high CT is unlikely to make alternative energy carriers more profitable than LSMGO.

The ammonia ICE configuration seems the most robust option for zero-emission W2W vessels, but it requires a nuance. To safely operate W2W vessels running on ammonia, safety needs to be addressed more. Because of its toxicity, the on board systems need to be carefully designed for this, regulations are lacking, and passengers and crew need to be comforted with the safety of ammonia. If a vessel owner wants to invest today, in a single-fuel converter, an ICE with LSMGO leads to most profit, HVO gives a mix between profit and less emissions, or methanol gives no emissions but a risk on the profit side.

The end product of this research is the developed model, which allows for changed input for the variables. In due course, when more accurate information becomes available, the assessment and evaluation can be redone quickly. Hence, decision making can be done easier and faster.