Motion sensing on vessels operating in sea ice: A local monitoring system for transit and stationkeeping operations under the influence of sea ice

Abstract

The diminishing sea ice in the Arctic and Antarctic could lead to a higher number of ship operations in these areas, such as cargo transit, resource exploration, fishing, and tourism. However, despite the sea ice reduction, sea ice remains the predominant risk during ship operations. Due to the remoteness and fragility of the polar regions, accidents are difficult to handle and could have devastating effects on the local ecosystem. Therefore, continuous assessing of the prevailing ice conditions is essential to operating vessels. Individual sensor systems, called ice monitoring systems, provide the necessary information for the ice condition assessment, and by giving early warnings, these systems reduce the risk of accidents.

A reliable ice observer system employs several technologies for ice monitoring such as optical cameras, radar systems, drift buoys, and hull strain measurements. Each additional technology increases the chance of early detection of dangerous ice conditions, and additionally adds redundancies to the overall system.

The aim of this thesis was to present and validate an applicable ice monitoring system that can increase the safety for vessels operating in polar regions. This thesis presents a series of studies, presented as a collection of journal papers, that lead to an on-board motion sensing based ice monitoring system for ships, which bases on distributed measurements of ice-induced vibrations in the ship’s hull. The results of this thesis are based on field data collected during four Arctic cruises performed between 2015 and 2017.

An initial study established a relationship between the prevailing ice conditions and ice-induced vibrations in the ship’s hull. A detailed frequency analysis of ice-induced vibrations concluded that accelerometers in the hull can provide information about the acting ice breaking mechanism, ice conditions around the vessel, and the location of ship-ice interaction along the hull. Two further studies established and validated the application of hull accelerometers as ice monitoring system. A first application study suggested to monitor ice conditions in real time with the help of statistical signal processing and change detection. It was found that the suggested ice monitoring system provides robust real-time information about the local ice conditions and operates independently of weather conditions. A second application study utilised several sensors along the hull, and methods from extreme value statistics, to find a relationship between the ice drift direction and statistical properties of the recorded signals. The proposed methods in this thesis would allow for an early detection of ice drift changes, which is essential for station-keeping operations in sea ice.

The overall research presented in this thesis conclude that motion sensing on vessels operating in sea ice provides a fast and reliable local ice monitoring system for both transit and station-keeping operations. It is further suggested, that motion sensing in the hull of a vessel also provides information about the current sea-state in open water. However, this possible application is outside the scope of this thesis.

Besides the main studies, the thesis additionally offers a contribution in form of a model for distributed motion sensing on ships.