Abstract
This paper examines the statistical properties and the quality of the speed through water (STW) measurement based on data extracted from almost 200 container ships of Maersk Line’s fleet for 3 years of operation. The analysis uses high-frequency sensor data along with additional data sources derived from external providers. The interest of the study has its background in the accuracy of STW measurement as the most important parameter in the assessment of a ship’s performance analysis. The paper contains a thorough analysis of the measurements assumed to be related with the STW error, along with a descriptive decomposition of the main variables by sea region including sea state, vessel class, vessel IMO number and manufacturer of the speed-log installed in each ship. The paper suggests a semi-empirical method using a threshold to identify potential error in a ship’s STW measurement. The study revealed that the sea region is the most influential factor for the STW accuracy and that 26% of the ships of the dataset’s fleet warrant further investigation.
Highlights
When the Udev boxplot of one ship’s median is close to 0, the inter-quantile range (IQR) is narrow enough to span from −1 to 1 knot, and the whiskers span between −2 and 2 knots, and the UCOP distribution conveys the same story it is indicative that the Doppler velocity log (DVL) measurements can be trusted
The vessel class and the DVL manufacturers info are some of the features out of a range of valuable categorical variables that has helped classify the data and identify if they relate to the speed through water (STW) accuracy
Given the fact that UCOP is calculated by spatially interpolating into the two-dimensions grid (1.5 km ×1.5 km) and later temporally interpolating within whole hours, when the region is highly volatile in sea currents magnitude and direction, the calculation might generate some additional error and more outliers than usual
Summary
The shipping industry has a strong wish to reduce the environmental effects of their operations and become sustainable. Maritime speed logging devices use one of the following measurement principles to obtain the STW: water pressure, electromagnetic induction, or the transmission of low frequency radio waves [5] The latter refers to as the Doppler velocity log (DVL), which is the speed-log used by the ships dealt with in this paper, and used by the majority of today’s merchant ships. Reported examples from daily operations include sudden jumps and drifts in the STW signal with measurements constantly decreasing/increasing for relatively long periods (hours), despite a maintained constant speed as secured by crew. Such incidents, apart from bringing mistrust to the crew navigating the ship, they shift the outcome of any dynamic voyage planning and they cause headaches to performance analysts. Individual cases are not enough to hold assumptions and generalize for a larger extent
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