The Sailing Tug

Abstract

The international maritime sector accounts for about 2% of all global carbon dioxide (CO2) emissions, the main greenhouse gas that causes global warming. Nations meeting at the United Nations International Maritime Organization (IMO) in London have proposed an initial strategy for the reduction of greenhouse gas (GHG) emissions from ships, setting out to reduce GHG emissions from international shipping by phasing them out as soon as possible with the aim to achieve zero emission by the end of this century. Heerema Marine Contractors (HMC) is a world leading marine contractor in the international offshore energy industry and aims to be a role model in environmental responsibility through carbon emission reductions.

As a result of HMC’s global activities, their fleet covers a considerable distance through transit across the globe. Their large semi-submersible crane vessels are conventionally transported through towing by a tug. The great amounts of fuel required for these transits provides a significant opportunity for the reduction of HMC’s carbon footprint. One such initiative proposed within HMC is the application of wind-assisted ship propulsion on the tow configuration.

This report presents an initial investigation of the feasibility of wind-assisted towing of the HMC’s Thialf, a semi-submersible crane vessel. Previous internal research at HMC showed the feasibility of using a discarded Panamax vessel as a floating breakwater. While operationally the discarded Panamax was found to be feasible, economically this was not the case. In this research, using the Panamax as a wind-assisted tug for towing the Thialf is investigated. As such the Panamax vessel can be employed for multiple purposes; for wind-assisted towing and as a floating breakwater, improving the financial feasibility.

To test the performance of a wind-assisted tow operation, a comprehensive 2D model is developed in this research to be able to check configuration variations in a wind-assisted tow setup. A conceptual design of a Panamax vessel converted into a sailing tug is implemented in a 2D model simulation. The performance in combination with the Thialf is assessed under the common environmental conditions experienced by the Thialf for various transit routes.

Results showed that the use of a wind-assisted tow configuration based on a Panamax, without using the Thialf propulsion is not feasible. The main point of failure is the required force balance transverse to the sailing direction. The Panamax basis used for the preliminary wind- assisted tug design proved to be not the optimal base case due to the limited leeward force generation under a drift angle and the large sensitivity to environmental loading. Although implemented measures improved the systems performance, it is debatable whether a wind- assisted tow configuration with the associated uncertainties is the most promising area to accomplish significant CO2 reductions.