Until recently the motion of moored vessels was a technically challenging and computationally intensive task to assess using numerical modeling techniques. In studies where vessel motions are induced by complex non-uniform flow-fields, such as wave-agitation in a harbor basin, or where motions are caused by vessels passing in close proximity in confined channels, the most common option was to carry out expensive and time consuming physical model tests. Recent advances in numerical modelling, however, have now made it possible to carry out these types of studies efficiently using numerical modeling tools to provide a cost effective and less time consuming alternative to conducting physical model tests. The current numerical modelling tools have been shown to accurately reproduce observed vessel motions.
During recent years, long period moored vessels motion induced by nearby passing vessels, has been of growing concern to harbour masters and port officials around the world. The phenomenon becomes particularly problematic for large vessels moving in constrained areas such as rivers and narrow estuaries, which can induce a surge motion in nearby moored vessels of several metres causing hazardous conditions at the berth. Growing ship sizes and increased port traffic calls for regulations dictating minimum vessel passing distances and speed limits, as well as optimization of the mooring system of the affected vessels in order to increase operational time.
The Port of Brisbane Corporation needed to investigate the potential for building berthing facilities for car carriers at the Port West Estate on the Quarantine Flats reach of the Brisbane River. As part of the planning studies, DHI Water & Environment was commissioned to investigate moored vessel movements at the conceptual berths as a result of passing vessels in the navigation channel and to recommend measures to reduce the risks of adverse motions and allow for safe mooring. DHI had undertaken previous studies of moored vessel motions at the Port of Brisbane that identified displacement waves generated by passing vessels as the cause of the observed severe motions. Subsequently, an extensive physical modeling study was conducted to assess the factors causing the motions and to define mitigation measures to reduce the motions. Building upon the results of the previous studies, the current study was able to use numerical modeling techniques in place of physical modeling to simulate moored vessel motions at the proposed car carrier berths for a range of wharf designs and operational conditions.
Figure 1 – Utilizing an unstructured mesh to provide the spatial representation of the domain allows for an efficient resolution of the governing hydrodynamic processes.
A numerical model was set up consisting of a coupling of the hydrodynamic model, MIKE21 HD FM, and the vessel response model, WAMSIM1. The MIKE 21 HD FM model was used to model the displacement wave produced by the moving vessel and calculate the resulting hydrodynamic flow field around the moored vessel. From the modelled hydrodynamic flow field, the WAMSIM model simulated the motions of the moored vessel for all six degrees of freedom. The numerical models were successfully calibrated against the results of physical model testing and field measurements of moored vessel motions from the previous vessel motion studies at the Port of Brisbane (DHI, 2004). A very good match was obtained between the numerical model predictions and the data from the previous studies and it was concluded the models could be used to simulate moored vessel motions at the proposed car carrier berths with a high level of confidence.
Figure 2 – Comparison of surge, sway and yaw motion between the numerical model and the physical model for a passing speed of 8 knots at a separation distance of 130 m and a moored vessel line pre-tension of 10 t.
Following calibration and validation, the models were used to simulate a range of scenarios, including options for the mooring arrangement, distance between the moored and passing vessels, passing vessel speed, mooring line types and pre-tensions and winds applied to the moored vessel. Criteria for assessment of the predicted motions were identified from published values for ro/ro vessels and used in the assessment of the model results. The assessment of the results identified key features of the wharf design that should be implemented and defined operational constraints to minimize the risk of adverse motions and allow for safe operation of the berths.
Overall, it was concluded that with the appropriate operational and design constraints there was a low risk of adverse vessel motions at the proposed car carrier berths as a result of displacement waves generated by passing vessels under normal operating conditions. In the event of an out of control vessel along the edge of the channel the modelling predicted that, whilst vessel motions are severe, the vessel will remain at the berth and there is a low risk of damage to the vessel.
The response from the Port of Brisbane Corporation about their experience with choosing numerical modeling to carry out their vessel motion study was overwhelmingly positive:
“The results were faster, more accurate and allowed us to test more scenarios than physical modeling would allow. The project allowed the Corporation to critically assess a key strategic direction for the port “.
Robert Hancock , Manager Infrastructure Planning and Sustainable Engineering, Port of Brisbane
The success of the Port of Brisbane project demonstrates a promising future for the application of numerical modeling in studies of moored vessel motions. These tools provide accurate and cost effective answers for decision-makers and allow for a dynamic and flexible project execution.
References:
DHI (2004). Port of Brisbane, Vessel Interaction Study, Phase 2 Vessel Monitoring and Physical Modelling. DHI Water and Environment, July 2004.
DHI (2008). Port of Brisbane, Vessel Interaction Study, Quarantine Flats Reach, Stage 2 – Final Report. DHI Water and Environment, October 2008.
1 both developed by DHI Water & Environment