Large sea-going vessels in narrow waterways: experts investigate their interaction
What impact do ship dynamics have during navigation of the estuaries? Simulation provides insights.
The coastal waterways of the North Sea and the Baltic Sea are the gateway to the world for foreign trade and navigation. A criterion for a smooth flow of traffic is the existence of coastal waterways which fulfil the necessary standards in a safe manner. Whether in development or maintenance issues, or economical dimensioning and navigability in the region of the tidal estuaries and the German Baltic coast - the Hamburg Office of the BAW provides reliable advisory services to the German Federal Waterways and Shipping Administration (WSV). This includes the recording and prognosis of waves or currents caused by shipping, and the prognosis of ship dynamics for the dimensioning of future fairway depths. Furthermore, the BAW is also developing various modules for ship handling simulation, particularly for economic use of the coastal waterways by large container vessels - at present with a maximum capacity of approximately 14,000 TEU1. So these literally are large-scale topics.
Under the magnifying glass: ship waves and currents
What types of wave are produced by ships of varying sizes and draught - and what impact do they have? This specific question is a matter for the professional field of study ''interaction sea-going vessel / coastal waterway''. Not least because coastal waterways such as the Lower Elbe have an increasing significance for tourism and leisure activities alongside their role as traffic arteries. For the cost-effective maintenance of revetments in the waterways, for the assessment of load on tidal gate structures - and ultimately, also, with regard to the load caused to natural banks, beaches and adjacent marinas, the prognosis for and analysis of ship-induced waves, and their impact, is of central importance.
Extensive model tests were therefore carried out to the scale of 1:40 in the ship wave basin of the BAW in Hamburg regarding the adaptation of fairways to modified fleet structures. The dimensions of the basin, at around 100 metres in length and 35 metres in width (max. water depth 0.7 m), permit, for example, the installation, true to scale, of planned reaches where vessels meet with adjacent port basins.
The physical mechanisms of action determined also with hydraulic scale models are added to the expertise acquired by the BAW over many years on the topic ship/waterway in the form of prognosis values in BAW expert reports for approval procedures subject to public law. A further task of this specialist field is the prediction by the BAW of extreme bank load caused by shipping, according to state of the art technology and science on the basis of model tests - and, as far as possible, prevention of this load at the actual sites by means of well substantiated recommendations.
Navigational dynamics of large vessels in the estuaries
Accurate model research: on one hand, the predictions are secured according to the current state of technology and science by carrying out test series on hydraulic scale models. On the other hand, e.g. potential theoretical numerical models are also being refined and validated. This concerns not only the ship-induced load, dependent on speed and cross-section and generated by waves and flow in wide, inhomogeneous coastal waterways, but also, for example, the navigational dynamics with squat2 during navigation through the tidal estuaries. The parameter squat is included in the depth dimensioning of the fairway with its maximum value at bow or stern because it leads to a reduction in under keel clearance.
Fleet structures are constantly changing, and the lengths and widths as well as the draught depths of ships using the approaches to the seaports are increasing. For this reason the hitherto used formulas for the maximum squat must be constantly checked regarding their validity. One of the most important tasks of the BAW is, therefore, to determine tendencies in the squat behaviour of the newest container ships using systematic experiments, to secure the results with measurements taken on container ships navigating the estuaries - and, by this means, to enable a validation of tidal schedule programmes of the Federal Waterways and Shipping Administration (WSV).
Ship handling simulation: also a tool for dimensioning and navigability analysis of waterways
Whether it is a matter of single transit or situations where unusually large vessels meet: ship handling simulation is growing in significance for the coastal directorates of the WSV for the dimensioning of new fairway widths and for the navigability analysis of coastal waterways. Since the existing simulators are primarily intended for the training and development of nautical personnel, and are certified accordingly, it has for some years been a task of the BAW to represent the increased demands on ship handling simulation for the requirements of the WSV in relation to the placing of orders with the operators of simulation systems.
The BAW's quality assurance is based on short and medium term test procedures. Thus, the yaw moment (change of heading) that affects a ship as it passes close to a slope can be calculated from measured data from hydrodynamic scale models. By means of project-related sensitivity studies, a plausibilisation of the simulation results - and subsequently, a detailed evaluation of the project-related simulation result - can be made.
Perspectives for the perfect simulation: in the medium and long term the further development and optimisation of the simulation software using its in-house ship handling simulator in Hamburg is, for the time being, the BAW's first priority. The high standards demanded of the ship handling simulation in dimensioning tasks and navigability analyses of the coastal waterways for the requirements of the coastal offices of the WSV will therefore continue to be assured.
1 TEU = twenty-foot equivalent unit = storage slot size of a conventional commercial container (20 feet)
2 Squat = Speed-dependent sinkage of a sailing ship in the water, together with the primary wave system generated by the ship itself (backwater at the bow - drawdown - stern wave)