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

Stuart, Duncan Charles Alistair (2013-12)

Thesis (MScEng)-- Stellenbosch University, 2013.

Thesis

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

AFRIKAANSE OPSOMMING: Die Ngqura hawe is aan die ooskus van Suid-Afrika geleë. Sedert die hawe se eerste operasionele winterseisoen, het oormatige skeepsbewegings operasies van behoueringskepe onderbreek en gelei tot faling van vasmeertoue. Die teenwoordigheid van langgolf resonansie is ‘n groot bydraende faktor tot die oormatige skeepsbewegings. Hierdie studie ondersoek die opwekking, penetrasie en gevolglike resonansie van langgolwe in die areas aangrensend tot die problematiese kaaie. ‘n Uitgebreide literatuurstudie het twee tipes langgolwe aan die Suid-Afrikaanse kus geïdentifiseer, naamlik langgolwe met periodes langer as 12 minute wat deur resonante lug-water koppeling opgewek word en korter langgolwe met periodes tussen 30 s en 6 min wat aan gebonde langgolfenergie of, meer algemeen, surf beat toegeskryf word. Verder is ‘n studie rakende die jongste langgolfmodelleringstegnieke ook uitgevoer waaruit die metodiek van hierdie studie bepaal is. ‘n Analise van gelyktydige opmetings binne en buite die hawe het bevestig dat kortgolwe wat deur stormsisteme gegenereer word, die opwekkingsmeganisme van lang golwe is. Daar is bevind dat langgolwe buite die hawe gemiddeld 8% so hoog soos kort golwe is. ‘n Gemiddeld van 90% van die langgolfhoogte het die hawe penetreer. Die opmetings het ook verder duidelike resonansieperiodes van langgolfenergie binne die hawe aangedui. Gekalibreerde Boussinsq-golfmodelle is gebruik om te indentifiseer hoe langgolwe die hawe binnedring en gevolglik resoneer. Beide oppervlakmetings en wit geraas spektra is as invoerwaardes vir die model gebruik. Die penetrasiemeganismes is toegeskryf aan diffraksie rondom die hoof hawemuur asook refleksie vanaf die strand, suid van die hawe, wat lei tot refraksie en refleksie teen die lykant van die hoof hawemuur. Toetse met vry langgolwe het bewys dat die langgolfenergie, vir ten minste sommige periode intervalle, aan die kort golwe vebonde is. Die oormatige skeepsbewegings is toegeskryf aan die kaaie wat in lyn met nodes van die langgolfresonansie geposisioneer is. Nodes word gekarakteriseer deur sterk horisontale strome wat surge bewegings in skepe kan veroorsaak. Verskeie langgolwe met periode intervalle tussen 45 s tot 125 s resoneer in die hawe en vorm nodes by die kaaie van belang. Ten slotte, die Ngqura hawe is vatbaar vir ‘n reeks langgolfperiodes wat ossilasies in die bekkens van die hawe veroorsaak en nodes naby kaaie vorm. As gevolg van die analises in hierdie studie is die meganismes van interaksie tussen die hawe, sy bekkens en langgolwe wat die hawe direk of via die brandersone binnedring gemodelleer, gedokumenteer en beter verstaan. Hierdeur is die potensiaal vir beter voorspelling van ernstige langgolftoestande verhoog en is dit moontlik gemaak om lewensvatbare oplossings vir skade aan vasgemeerde skepe te ondersoek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/85567
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