Civil Engineering May 2021 | Vol 29 No 4

24 May 2021 Civil Engineering AIM & OBJECTIVES The aim of deepening and expanding the Port Victoria basin is to accommodate larger container vessels and tankers within the port than is currently possible, while keeping the environmental impact of the associated construction works within acceptable limits. To achieve this aim, the following design objectives were defined: Q Q Perform a navigation study and dynamic mooring analysis to establish safe depths and horizontal extents of the port basin and berth pockets for the proposed design vessels. Q Q Design dredging of the port basin that limits construction and operational risks. Q Q Assess the potential impact of dredging, reclamation, and spoil disposal activities on the environment through a numerical modelling study. Q Q Compile a DMP that ensures any environmental impacts are kept within acceptable limits. DESIGN APPROACH The design approach closely followed the above design objectives. The design components were strongly inter-dependent and needed to be informed by several other studies (undertaken by WSP and others) as shown in Figure 2. This inter-dependence meant that design decisions needed to be undertaken through an iterative process involving many parties, adding significantly to the complexity of the project. NAUTICAL STUDY General The nautical study was required to estab- lish the extent of the port basin area that will allow for safe navigation and mooring of the design vessels. The study comprised the following: Q Q Determine the required vertical di- mensions of the approach channel and basin and refine by means of a vertical ship motion response study. Q Q Determine the required horizontal extents of the approach channel and basin and refine by means of ship navigation simulations. Q Q Conduct a dynamic mooring analysis to determine the moored ship motions and associated mooring line and fender reaction forces. The design vessels for this study were a Post- Panamax container vessel with a maximum draught of 14 m and a Capesize tanker with an overall length of about 250 m. Tankers of this size typically have laden draughts of 15 m or more. The client indicated that the limiting factor is a draft of 14 m and as such the studies further assumed that the tanker will enter the port only when semi-laden. Prior to the commencement of the nautical study components, the design wave and cur- rent conditions had to be established. These were derived from the output of the wave and hydrodynamic flow modelling studies that WSP conducted using the Deltares modelling suite. Appropriate wind and water level conditions, to be used as input to the models, were derived from available data. Design vertical dimensions The 2014 PIANC Design Guidelines for Harbour Approach Channels were used to determine the required vertical dimensions of the approach channel and port basin. The dimensions were first conceptually estimated and then refined by means of the numerical ship motion model WAVESCAT. WAVESCAT considers the vertical response of vessels to vessel speed (squat) and wave impact (heave, pitch and roll) to determine the response amplitude operators (RAOs) for the six critical keel points on the hull of the design vessels. An example of the RAOs for two of the keel points of the design container vessel, sailing at 6 knots, is shown in Figure 3. The colour-filled contours indicate the amount of vertical response amplitude a keel point will experience, subject to a wave with a specific encounter frequency and direction relative to the heading of the vessel. Taking the static under-keel clearance and the manoeuvrability margin into account, the required water depth was established for the relevant wave conditions along the channel, while keeping within the limiting criteria of the 1% probability of bottom touch guideline of PIANC. Design horizontal dimensions Similar to the design vertical dimensions, the required horizontal dimensions for safe navigation of the design vessel in the port were determined based on the 2014 PIANC Design Guidelines for Harbour Approach Channels. Conceptual dimen- sions were determined and incorporated Water and sediment quality survey Reclamation backfill volumes Bathymetric and geophysical survey Metocean modelling study Disposal site selection Geotechnical investigation Need for safe navigation and mooring of design vessel Nautical study Dredging assessment (modelling study) Tender documentation and approvals for construction Dredge design Dredge management plan Figure 2 The design components for the Port Victoria basin expansion