``Slibvaren’’

``Slibvaren’’: Adjustment of the harbour admittance policy by reduction of the minimal required under keel clearance (UKC)

Roukens, G.A.

Vellinga, T. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2016-11-25

All harbours require frequent dredging to cope with sedimentation. In certain cases the deposited sediment/silt forms a weak soil layer referred to as fluid mud. The top layer is often used as nautical bottom, while its presence is not necessarily harmful to the vessel. This could lead to the conclusion that the safety margin underneath a vessel (i.e., Under Keel Clearance (UKC)) is too large/safe and can be reduced. Research indicates that the UKC can be decreased to smaller (<10%) or negative (0 to -10%) values. Reducing the UKC leads to less dredging and allowance of higher draught vessels. This can create economic benefits for the port authority and, depending on the chosen strategy, shipping company. Less dredging is also likely to reduce the amount of sediment that is re-suspended and could thus assist the system in reducing turbidity. Altered vessel behaviour by a smaller UKC and the possible resulting safety impairments create application challenges. These effects are caused by undulations in the water-mud interface (when sailing close to the fluid mud) or a too strong mud layer (when sailing through the mud). A discussion is instigated as the result of different end-user objectives: navigational safety versus optimal port profit. Extensive knowledge from various scientific fields is required to make a sound consideration between safety and benefits. Within this thesis, the alignment between end-users and required scientific topics is studied using a Decision Support Model (DSM). The DSM is based on a Frame of Reference approach by van Koningsveld [2003] which is meant to improve communication and aid decision making. For conceiving the DSM, various objectives from end-users are categorized into three management context: economy, ecology and safety. In reducing the UKC the benefits (economy and ecology) are opposed by the potential downsides (safety). Knowledge from literature led to relevant topics in each management context. Three strategies where used for quantification: draught increase, dredging decrease or draught increase with maintaining the current UKC requirement. The general DSM was applied to the case of Delfzijl where fluid mud and UKC reduction are topics of discussion. Weighing the strategies results in a dredging reduction being optimal. Relative small visiting vessels and high turbidity in the area cause this outcome. Difficulties in quantifying sub-elements are the result of knowledge gaps which might be resolved by further research. After addressing the topics of research, a sound decision on application can be made. The complete DSM and the outcomes where presented to port authority, harbour masters and pilot at Delfzijl. Based on the DSM, these end-users where able to point out topics of concern and provide additional feedback. These topics, mainly regarding safety, where subsequently assessed with a probabilistic tidal window model as proposed by Bouw [2005]. The model copes with the request for extra safety and can be easily extended when new knowledge on survey error and vessel velocity is obtained. From this thesis it was apparent that the main challenge for an UKC reduction is the absence of a means to designate the strength based nautical bottom. Port authorities have no means of assessing the strength of the bottom and corresponding effects on vessel behaviour. This affects the decision whether to start experimentally reducing the UKC for visiting vessels. Resulting in no possibilities to gain experience in the concept. In addition, the port authority is not able to determine the depth contour change induced by an envisioned dredging reduction.

http://resolver.tudelft.nl/uuid:9a846741-a620-4e88-a4b5-ffb9854e7486

Student theses

master thesis

(c) 2016 Roukens, G.A.