A renaissance of wind-powered ships

What is the right solution?

Today, an increasing number of shipowners are contacting SSPA to find out whether wind propulsion could be a solution for them. In shipping there are no general answers – each case is unique. A shipowners’ way towards wind propulsion often starts with a broad review of available technologies. The first thing to decide is whether the target is wind assistance or wind propulsion?

Wind assistance technology is typically a device that provides a forward thrust resulting in about 5–10 percent fuel reduction. Apart from the wind technology device, the ship is a conventional ship. Recently, several wind assistance technologies have been installed on commercial vessels, for example Flettner rotors on Viking Grace and Maersk Pelican.

Wind propulsion technology denotes a ship concept almost completely propelled by wind. Typical solutions are large rigid wings, Dynarig, or other soft sail rigs. Today very few large vessels are built for wind propulsion, apart from a few super yachts. However, several projects are in the concept stage, for example the ongoing Swedish research project Wind-Powered Car Carrier (wPCC) with the project partners SSPA, Wallenius Marine and the Royal Institute of Technology (KTH). A wind propulsion technology requires that the ship is really designed for sailing, as opposed to a wind assistance solution that can be retrofitted to conventional ships (and removed again).

The number of available solutions may seem large, but practical considerations often limit the options. The ship may have to pass under certain bridges, restricting the rig height. A container ship has limited space on deck for wind propulsion devices. Dry bulk carriers need to allow space for cargo loaders or cranes to access the cargo holds. Most shipowners do not favour a solution that requires a larger crew and absolutely not a solution that puts the crew at risk when handling the device. Bridge visibility may also be a limiting factor on smaller vessels. A customised technology review makes a solid basis for the decision process.

Ranking the options

The main question when ranking technologies is often the amount of reduction in CO₂ emissions provided by the wind technologies under consideration. The efficiency can, however, differ considerably from case to case depending on the ship size, route, speed and other substantial issues, and therefore published numbers from other vessels with the same technology can be misleading. Ranking of wind propulsion technology options at this preliminary stage should be based on a numerical model of the ship in question and reliable data on the aerodynamic performance of the wind devices.

It is also important to consider the weather on the intended route. Some types of sail work excellently in a limited range of wind angles: kites, for instance, operate best in a following wind. The performance in all wind directions needs to be considered.

SSPA’s route simulation software, SEAMAN, together with our ship hydrodynamics database, is an efficient tool to predict the CO₂ reduction. The largest uncertainty is the data of the wind propulsion device. Published numbers must be combined with engineering judgements and our experience of similar devices. The power consumption from operating the device is, of course, also considered as well as the installation costs.

Concept design and 3rd party assessment

When a supplier is selected, the concept can be evaluated in detail. Computational Fluid Dynamics (CFD) is an invaluable tool at this stage. The studies are often done in cooperation with the supplier to improve and customise the design for the given vessel.

The example illustrated shows a detailed flow analysis of a generic Flettner rotor. The flow is complex, with unsteady vortices which may cause vibrations. It should be noted that the flow is highly affected by the presence of the vessel freeboard and superstructure and by the atmospheric boundary layer. If there is more than one device, there may be strong interactions between them. These effects have a large impact on the total performance and must not be neglected.

The dynamic behaviour of the vortical flow behind a Flettner rotor can be categorised in three regimes depending on the spin ratio SR (i.e. the ratio of the peripheral velocity of the rotor to the freestream velocity). 

In Regime 1, where the SR is low, a pair of bound vortices is shedding alternately from the pressure and the suction side of the rotor. The tip vortices are weak and stable.

Regime 2 is in a quasi-steady state, in which the bound vortex is not shedding and a pair of tip vortices are created at the end disc. A trailing vortex is also formed near the root. These vortices are strong and persistent.

In Regime 3, the tip vortices generated at the end disc are fluctuating. There is one or more trailing vortices started at the root and wandering along the rotor between the root and the tip.

It may seem obvious to focus the work on the airflow around the wind propulsion devices, but the hydrodynamics should not be forgotten. The lateral force from the wind propulsion device makes the ship slide sideways, which affects the resistance, creates an asymmetric flow into the propeller and requires additional rudder actions. If the thrust force from the wind propulsion device is large, the propeller as well as the main engine may operate at a non-optimal load point. These effects are all combined into a prediction of the CO₂ savings for the given vessel and route.

How to ensure an accurate safety level?

Current ship design regulations and classification rules are generally only address conventional ship types, and a dedicated risk assessment study is required to identify regulative gaps and to find solutions demonstrating equivalent safety levels.

Safe ship operation in adverse weather conditions imposes tough requirements on rig foundation and intact stability, and extreme wind loads of 50 m/s or more should be considered. The forces from wind propulsion devices may impair the manoeuvring performance, especially in waves. Smart integration of the rig control system, rudder control and propellers may, however, offer additional manoeuvring and deceleration capabilities. The installation of a wind propulsion system may permit the use of a less powerful mechanical propulsion engine, although the minimum engine power must also be addressed from a risk perspective.

Wind propulsion devices will add to the windage of the vessel and correspondingly also increase the mooring loads, possibly calling for enhancement of fender and bollard equipment on the quayside.

Risks related to technical or operational failures of wind propulsors or control system components should also be addressed by the risk assessment study, e.g. by the use of Failure Modes and Effects Analysis (FMEA). Model testing may be necessary to confirm compliance with manoeuvring criteria and possible counteractions.

How does a wind-powered ship influence the logistics performance?

The logistics performance of a wind-powered ship is crucial for shipowners to consider in order to attract new and keep current customers. Applying wind propulsion technologies may influence the logistics performance, especially if the aim is a high degree of wind propulsion. Major logistics aspects to consider are the cost of the operations, emissions, service level (such as transport time, frequency, reliability), route and risk of damage of goods.

A ship with a high degree of wind propulsion will most likely have a slower speed than a conventional ship, and, for it to be successful in its implementation, the customers’ sensitivity to transport time needs to be assessed. Large emission savings might be an order winner for new customers.

Ongoing research projects

Wind-Powered Car Carrier (wPCC)

Towards a wind-powered car carrier vessel, from concept to a technical and financially feasible design. This project will develop a design ready to be built within 3–5 years. SSPA’s experts will contribute with extensive research, for example regarding unconventional experimental methods, aerodynamic and hydrodynamic simulation methods, risk simulation and risk mitigation and new logistics solutions.

• Project period: 2019–2022
• Partners: 3
• Financed by: the Swedish Transport Administration

Wind-Assisted Ship Propulsion (WASP)

An EU–Interreg joint development project for commercially attractive wind solutions. This project will help accelerate the decarbonisation transition by giving the market and policy makers clear indicators on operational parameters, fuel savings, business models and a collection of additional demonstrator vessels to highlight the wind-assisting propulsion potential.

• Project period: 2019–2021
• Partners: 15
• Financed by: the Interreg North Sea Europe programme, as part of the European Regional Development Fund (ERDF).

SSPA is a full member of the International Windship Association (IWSA) www.wind-ship.org

Illustrations

Wings, rotors or kites? Selecting and ranking the concepts at the early stage needs the combined knowledge of wind propulsion devices interacting with the ship hydrodynamics and taking risk and logistics into account. The process goes from a broad technology review to detailed assessment of fluid dynamics, risk, cost and logistics.

Influence of ship hull on flow around a rotor sail. The flow is highly affected by the presence of the vessel freeboard, superstructure and by the atmospheric boundary layer. If there is more than one rotor, there may be strong interactions between them. These effects have a large impact on the total performance and must not be neglected. The figure shows a generic design.