Fathom Spotlight: Will Voyage Optimisation Supersede Traditional Weather Routing?

Dr Henry Chen, Chief Naval Architect at Jeppesen Marine

The advent of supercomputers and numerical models has significantly improved the accuracy of weather forecasts over the past decade. However, the accuracy of each numerical model used to forecast the weather can vary highly due to model resolutions and how the physics are implemented, along with many other factors.

In this week's spotlight Fathom spoke to Dr Henry Chen, Chief Naval Architect at Jeppesen Marine to find out more about how voyage optimisation is helping to drive the maritime industry forward.

Ahead With Algorithms

Most weather routing software solutions use variations of Dijkstra's algorithm, in which the program simulates a vessel departing with full power toward the arrival port with different headings. After each time interval (e.g. six hours), the ship's dead-reckoned position forms a so-called isochrone until it arrives at the destination. In other words a line on the map connecting points relating to the various times and locations.

Unfortunately, the problem with such an approach is that the algorithm ignores one important option: speed management. As storms move across the ocean, it is possible for the ship to slow down and let them pass and then catch up, instead of sailing a longer distance to go around, or "hove-to" in bad weather. Such a strategy not only significantly reduces fuel consumption for a given arrival time, it also reduces the risk of heavy weather damage when fully implemented with ship response and engine overload.

If speed and heading are both considered in the route optimisation algorithm, the computation will be more accurate because it solves for a multi-dimensional problem. Without the fundamental principle of modelling the ship's performance in various loading and environmental conditions, it is not possible to minimise the fuel consumption for a given arrival time without exceeding the safe operating limits.

Ship Response and Engine Overload

Cost-cutting trends in the shipbuilding industry and marine classification societies have resulted in reduced design safety margins in ship structures. Shipyards use sophisticated finite element models and high tensile steels to reduce steel weight and production costs in order to be competitive. Similarly, the propulsion systems are often optimised for calm weather trial conditions in order to satisfy the recent IMO requirement on Energy Efficiency Design Index (EEDI).

One such design consequence is the coupling of slow-speed diesel engines with direct-drive high-pitch propellers and low acceptable sea margin. In calm weather conditions, a lightly loaded vessel with a clean hull easily maintains the contracted speed in accordance with the EEDI requirements.

Unfortunately, such practice will lead to frequent engine overloading when the ship encounters high wind or seas, or when there is higher resistance caused by propeller and hull fouling.

Improved level of detail

The use of ensemble forecasting allows providers to quantify the uncertainties in the prediction. It is now possible to estimate the probability of exceeding a given threshold, e.g. seven metres of wave height under a nominal forecast of five metres. The threshold can be established based on motions and seakeeping events which define the risk of heavy weather damage.

Figure 1 Histograms displaying passage fuel consumption and ETA of alternative routes using 22 members of ensemble forecast.

While the southern route in figure 1 yields less uncertainties for on-time arrival, it would also consume considerably more fuel than the recommended northern route. This type of simulation offers the user the ability to trade off fuel consumption versus ETA and to estimate the schedule reliability for planning port/terminal operations.

The Importance of Route Planning Tools

A ship slows down either involuntarily due to increased resistance from the wind and waves, or voluntarily due to navigation hazards or fear of heavy weather damage from excessive ship motion, propeller racing, slamming, or boarding seas.

The optimised route solution must take both involuntary and voluntary speed reductions into account when estimating dead-reckoned ship positions in relation to the movement of weather systems. Otherwise, the recommended route could lead the ship into a dangerous situation.

Furthermore, if weather routing tools cannot predict such events, they can lead to over-predicted ship speed and wrong diversion decisions when facing heavy weather, not to mention inaccurate estimates of fuel consumption and time of arrival.

The capabilities of weather routing have evolved into the science of voyage optimisation in order to bring added benefits in ship design and operational logistics.

Today's technology enables accurate ship seakeeping performance predictions and intelligent, informed operational decisions that can help ship masters save fuel, reduce GHG emissions, and avoid heavy weather damage.