Breakwater Layout Optimisation Using A Root-Finding Algorithm

Due to the remote and exposed coastal location of many proposed liquefied natural gas (LNG) exportation terminals, a breakwater is often required to reduce the wave energy at the vessel berth. Best practice and esearch techniques for developing the cross-section and sizing armour stone concepts are well established however, little research exists for developing breakwater layouts. This paper describes a methodology which can be used to quickly develop breakwater layouts, optimal for the vessels berthed in the shadow zone. This is achieved on the premise that for any given level of acceptable berth downtime, there exists a corresponding breakwater layout. This allows the relationship to be expressed as vessel /Idowntime=f(length)/N which can be solved through an iterative approach. To achieve this, a random wave transformation model and a dynamic mooring analysis model have been embedded in an iterative root-finding algorithm to transform the waves to, through and around the breakwater. The diffracted and transmitted wave energies are summed using the root mean square (RMS) approach before the `effective wave' is used to determine whether the vessel mooring threshold is exceeded. Performing this for a time series gives an indication of the owntime that will be incurred with this breakwater layout, which is used to iteratively search for the breakwater length which offers the desired level of downtime. This methodology is applied to a rubble-mound breakwater, though could easily be transposed to a berm, caisson or any other kind. A case study is used to demonstrate the effectiveness of this methodology in developing accurate breakwater layouts in a very short time-frame, allowing a greater variety of berth positions and arrangements to be trialled during the conceptual design stage.