Characterizing long wave agitation in the port of Ngqura using a Boussinesq wave model

[摘要] ENGLISH ABSTRACT: The port of Ngqura is situated on the east coast of South Africa. Since its first operational winterexcessive vessel motions have interrupted container shipping operations and lead to mooring linefailure. A major component contributing to the excessive motions is the presence of seiching in theport, resonating long waves. This study investigates the long wave generation, penetration into theport and subsequent resonance in the vicinity of the problem berths.An extensive literature review identified two predominant types of long waves along the coast ofSouth Africa. Long waves with periods over 12 min generated by resonant air-water coupling andthen shorter long waves between 30 s and 6 min attributed to bound long wave energy and broadlyspeaking, surf beat. A review of the state of the art long wave modelling techniques was includedand contributed to the methodology in this study.Analysis of simultaneous measurements from the outside and inside of the port confirmed thegenerating mechanism of the long waves to be storm systems also responsible for generating shortwaves. Long waves outside the port were found to be on average 8% of the height of the shortwaves. On average 90% of the long wave height outside the port penetrated the port. Themeasurements further identified distinct resonating periods of the long wave energy inside the port.Calibrated Boussinesq wave models allowed for identification of how long waves penetrated the portand subsequently resonated. Both surface elevation measurements and white noise spectra wereused as inputs. The penetration mechanisms were attributed to direct diffraction around the mainbreakwater as well as reflection off the beach south of the port leading to refraction and reflectionoff the lee side of the main breakwater. Tests with both free and bound long waves proved that atleast for some period intervals the long wave energy was indeed bound to short waves.The excessive vessel motions are attributed to berths positioned in line with nodes created by theresonating long waves; nodes are characterized by strong horizontal currents which can induce surgemotions in vessels. Various long waves between the period intervals of 45 s to 125 s resonate in theport to generate nodes at the berths of interest.In conclusion, the port of Ngqura is susceptible to a range of long wave periods resulting insignificant basin oscillations which present nodes at mooring places. As a result of the analyses in thisstudy the mechanisms of interaction between the port, port basins and the long waves penetratinginto the port directly, or via the surf zone as surf beats, have been modelled, documented and betterunderstood. This provides the potential for better prediction of severe long wave events and for theinvestigation of feasible mitigation measures to prevent damage to moored ships in the port.

[发布机构] Stellenbosch University