INTRODUCTION
The continuing growth of commercial vessel sizes is putting increasing pressure on the world’s port authorities to adopt effective expansion strategies to ensure that their asset is able to meet growing capacity demands. Channel capacity expansion projects usually involve the consideration of extensive dredging which introduces considerable constraints with respect to cost and environmental impacts.
This paper presents the full scale validation of an improved integrated approach for optimizing shipping channel capacity utilizing DHI’s new state-of-the-art 3D under keel clearance (UKC) model Nonlinear Channel Optimization Simulator (NCOS). Measurements used for validation consisted of high resolution time series of UKC, roll and pitch obtained during vessel inbound and outbound transits through the Port of Brisbane. Vessels included a mix of large bulk carries, tankers and container vessels.
The aim of this validation exercise is to demonstrate the accuracy envelope of a 3D method for UKC prediction through various approaches for treatment of key input forcing parameters, wave frequency response, dynamic heel and squat.
METHOD
Serving as the bases for this validation are measurements taken during vessel transits through the Port of Brisbane. Differential Global Positioning Systems (DGPS) were located at the bow and on both the port and starboard bridge wings of the vessels to measure trajectory and the vertical position at each location. From these measurements roll, pitch and vertical excursion of the vessels throughout the transits were calculated and compared to the NCOS results.
NCOS belongs to a new breed of UKC models that converge towards the same level of sophistication and realism as Full Bridge Simulators. The NCOS model uses the numerical 3D engine in the Full Bridge Simulator SIMFLEX4 by FORCE TECHNOLOY for predicting wave-induced UKC allowance, which greatly improves the potential for using it in close integration with detailed maneuverability studies. The model uses a 2nd Order 3D panel method for evaluating vessel frequency response incorporating implicitly the effect of vessel forward speed and varying water depths. Adopting a Rayleigh distributed sea state; the probabilistic vessel execution is evaluated in each time step for various return periods.
Calculating accurate predictions of UKC relies on temporally and spatially varying environmental inputs such as wind, wave and hydrodynamic data to serve as forcing inputs to the model.
For operational UKC prediction purposes, there are often several practical implications associated with adopting a high level of sophistication and volume of environmental data that needs to feed the UKC model. In addition most ports have long term experience with the use of empirical squat formulas when estimating basic UKC, which raises a demand for assessing how these can be incorporated into a sophisticated 3D UKC framework.
As a result we assess the performance impacts of incorporating an array of well-known squat formulations and we also investigate the effect of representing waves using either synthetic wave spectra from integral parameters to using full directional spectra as modelled by a spectral wave model.
RESULTS
In order to validate the model, results from NCOS have been compared to the measurements taken. The comparison includes timeseries of roll, pitch and vertical excursion. Comparisons to date show that NCOS has very accurately reproduced the measurements numerically which gives confidence that it can be used by ports to achieve target levels of channel operability, while potentially reducing required dredge volumes significantly compared to conventional estimates.