Evaluation of Flow Fields for their Impact on Manoeuvring

When working on the design of a new port layout or other coastal infrastructure, PIANC’s publication “Approach Channels – A Guide for Design” (PIANC-IAPH (1997)) is a valuable tool for dimensioning a navigation channel. However, these guidelines do not apply for situations with a flow gradient. In coastal situations, with a tidal flow running along the coast and across the access channel to a port, a flow gradient near the port entrance is quite common. For this critical part of the channel, the guidelines do not provide an answer. For this reason, the determination of the required width requires manoeuvring simulations, using computed flow fields for the layout and possible alternatives under study. Very often, such flow fields will be available as these are also required for the study of coastal and environmental impacts. In hydraulic advisory, the navigability is only one of the parameters playing a role in the design. Especially, when developing and evaluating layout alternatives or when considering construction phases, the project needs a quick expert opinion on the flow gradient as produced by the flow model. In these situations, setting up and performing manoeuvring simulations (even fast-time manoeuvring simulations) is often conceived as too elaborate and time consuming. However, in order to make his judgement, the expert has to relate the flow pattern to the characteristics of the ships. For such situations it was felt that an evaluation tool was needed to support the requested expert opinion, making use of the flow model results. A situation of a ship trying to maintain a straight track while sailing through a flow gradient is in essence governed by the balance between the rotation in the flow field and the turning ability of the ship. The evaluation tool as developed matches these two elements. The rotation of the flow along the sailing line (the channel centre line) is extracted from the flow model in postprocessing. The turning ability of the ship is, in a linearized form, described by the first order yaw equation of Nomoto (1956). In this equation the ship’s turning abilities are described by two basic manoeuvring coefficients, K and T. Combining the two elements (flow and ship characteristics), the tool calculates the rudder angle as required in order for the ship to stay on the sailing line. This rudder angle is the indicator for the difficulty of the manoeuvre.