VISIR-I.b: waves and ocean currents for energy efficient navigation

Abstract

VISIR-I.b, the latest development of the ship routing model published in Mannarini et al. (2016a), is here presented.The new model version targets large ocean-going vessels by accounting for both waves and ocean currents. In order to effectively use currents in a graph-search method, new equations are derived and validated versus analytical benchmarks.A case study is computed in the Atlantic Ocean, on a route from the Chesapeake Bay to the Mediterranean Sea and vice versa.Ocean analysis fields from data-assimilative models (for both ocean state and hydrodynamics) are employed. The impact of 5 waves and ocean currents on transatlantic crossings is assessed through mapping of the spatial variability of the routes, analysis of their kinematics, distribution of the optimal voyage duration vs. its length, and impact on the Energy Efficiency Operational Indicator of the International Maritime Organization. It is distinguished between sailing with or against the main ocean current.The seasonal dependence of the savings is evaluated, indicating, for the featured case study, larger savings during the summer crossings and larger intra-monthly variability in winter. The monthly-mean savings sum up to values between 3 and 12%, 10 while the contribution of ocean currents is between 1 and 4%. Also, several other ocean routes are considered, providing a pan-Atlantic scenario assessment of the potential gains in energy efficiency from optimal tracks and linking them to regional meteo-oceanographic features.

Preliminary Inter-comparison of AIS Data and Optimal Ship Tracks

“…VISIR (an acronym for "discoVerIng Safe and effIcient Routes") is the ship routing model resulting from the prototype first published in Mannarini et al (2013). The model eventually evolved to compute least-time tracks in presence of time-dependent fields from wave models (Mannarini et al, 2016), and has been recently extended to deal also with ocean currents in Mannarini et al (2019).…”

“…Conversely, disabling them allows reducing the computer RAM allocation of the computations and, thus, increasing the maximum number of time steps considered for VISIR paths. In fact, the vessel type considered in this experiment is (both dry and wet) bulk carrier, which top speeds (Table 2) are generally lower than container ships considered instead in Mannarini et al (2019). A lower speed implies a longer sailing time, which is represented in VISIR through a larger number of time steps and, thus, requires higher RAM allocation.…”

“…Safety constraints and intentional speed reduction are also disabled as, according to Mannarini et al (2019), for this actual route, they do not to significantly impact the results. Conversely, disabling them allows reducing the computer RAM allocation of the computations and, thus, increasing the maximum number of time steps considered for VISIR paths.…”

“…Finally, CMEMS 3 three-hourly wave fields are averaged into daily fields before being employed by VISIR. Again, as discussed in (Mannarini et al, 2019), the reason for this approximation is the reduction of RAM allocation.…”

“…This paper is part of a broader strategy for a comprehensive verification and evaluation of the VISIR ship routing model. The verification (i.e., checking if its equations are solved correctly) included comparison to both static (Mannarini et al, 2016) and time-dependent (Mannarini & Carelli, 2019) analytical solutions as well as comparison to the outcomes of another path planning model (Mannarini et al, 2018). The first step in evaluation (i.e., assessing whether the model fairly represents reality) is presented in this work.…”