The nonlinear response of an ocean system subjected to random excitations can exhibit very complex dynamic behaviors including jump phenomena and coexistence of attractors. In this study, the stochastic system response behavior of a simple (Duffing) oscillator under narrow-band random excitations is first examined in the subharmonic resonance region. A semi-analytical procedure based on the nonlinear response characteristics of the corresponding deterministic system is developed to derive the response transition probabilities within individual attraction domains and among finite attraction domains under the assumptions of stationarity and Markov process. Overall response amplitude probability distributions are obtained by applying the Bayes formula to the two different types of response transition probability distributions. To validate the prediction capability of the semi-analytical method, numerical simulation of the responses of the Duffing system are generated and statistical characteristics of the response behavior are compared with prediction results. It is shown that the semi-analytical procedure provides more accurate predictions than other approximate methods available in the literature. A parametric study on the effects of variations in excitation intensity and degree of narrow-bandedness is conducted. Results confirmed that the nonlinear response characteristics including jump phenomenon and co-existence of attraction domains are preserved under narrow-band random excitations. The semi-analytical prediction method developed above is then applied to analyze the stochastic response behavior of a nonlinear mooring system subjected to random ocean waves. For modeling of the structural system, a nonlinear�� structure, nonlinearly-damped (NSND) model is employed and a reverse multiple�� input/single-output technique is applied to identify the system coefficients. To verify the accuracy and capability of the semi-analytical method in predicting the complex behaviors of the nonlinear mooring system, analytical predictions are compared with experimental results and numerical simulations. System response amplitude probability distributions predicted by the semi-analytical procedure are shown to be in good agreement with experimental and simulation results. / Graduation date: 2001

Thesis (Masters Diploma (Civil Engineering)--Cape Technikon, Cape Town, 1987 / Between December 1982 and January 1984 the berthing impacts o~ 50 ore carriers were monitored at the Saldanha Bay ore-jetty. The actual displacement o~ the monitored vessels ranged between 60 and 263 kilotonnes. Only 507. o~ the monitoring runs yielded complete sets o~ data. Approach velocities recorded were high and the design limits were exceeded several times resulting in ~enders being de~lated on ~our occasions. The added mass coe~~icient ~or each impact was calculated. The actual values o~ C" varied between 1 and 7. This agrees with values ~ound in literature. Some o~ the\high values could be attributed to inaccuracies in the measurement techniques. From the values obtained ~or added mass it seems that the value used in the design was very low and that a unitary value ~or CH is not very satisfactory.

Seagrass ecosystems are some of the most productive in the world and provide a variety of ecosystem services but are facing global decline chiefly due to anthropogenic disturbance. Mechanical disturbances to seagrass meadows from anchoring, propeller scars, and mooring scars result in losses or damage to both shoots and the underlying rhizome. I conducted a literature synthesis on the extent of, recovery from, and ecological impacts of these mechanical disturbances to seagrass meadows. The literature suggests that anchoring damage tends to be worst in deeper water where larger vessels anchor and can cause large (>100m2) loss per anchoring and recovery may take decades to over a century. Propeller damage is of largest concern in shallow (<2m) areas experiencing heavy boat traffic because propeller damage can only occur where the propeller can come close to the seagrass. Mooring damage is highly variable depending on the type of mooring used (>10m2 to >1000m2 scoured per mooring). Seagrass patches experiencing these mechanical disturbances have, in some studies, been found to have lower seagrass percent cover and shoot density than reference areas. This indicates scars can have "halos" of impacted seagrass meadow. Some seagrass systems cannot recover within a century (e.g. Posidonia oceanica meadows at the extent of their depth tolerance) while others recover annually from some disturbances (e.g., anchor scars <1m2). Systems face altered species composition when scars are preferentially recolonized by certain species and patterns of recovery are affected by altered biogeochemical conditions following disturbances. Additionally, mooring, anchoring, and propeller scarring frequently alter meadow density, cover, patch size, patch shape, patch isolation, edge area, and ratios of edge to interior meadow leading to changes in faunal community structure. Correlations between these disturbances and faunal abundances, densities, and richness in seagrass ecosystems are complex, vary temporally (sometimes on the scale of days), and may result in species showing positive, negative, or no responses to a wide range of disturbance regimes. To explore the connections between mooring scarring, the surrounding seagrass meadow condition and epifaunal community, in the second part of this thesis I measured 30 mooring scars to determine average scar size. To explore any potential "halo" effect around mooring scars for seagrass or epifauna and to seek any difference in epifaunal community between mooring and reference sites I also sampled paired sites at eight locations in Nantucket Harbor, Massachusetts three times each in the summer of 2015. Each location consisted of a meadow site actively experiencing mooring scarring and a reference site without moorings. My conservative sampling methods of the 30 sampled mooring scars found scars to average 21.1m2. Across my paired sites, seagrass was found to have lower cover and lower canopy height in mooring versus reference sites. Seagrass cover and canopy height were lower in the first few meters (typically 2-3m) surrounding each scar in comparison to paired reference quadrats indicating a "halo" effect of each scar. I did not detect a difference in epifaunal community composition or density per blade between mooring and reference sites; however, the relatively constant per blade concentration of epifauna combined with the differences in seagrass biomass between the mooring versus reference sites indicate an overall increase in the total amount of epifauna in areas with less disturbance. Epifaunal community composition was different and between locations and sampling dates indicating these factors are more important than proximity to mooring scarring in determining epifaunal abundance and richness. When considering scar area and the "halo" of each scar I estimate that at least 32ha (2%) of Nantucket Harbor was impacted by mooring scarring. Given that the estimates of seagrass do not include areas previously denuded of this plant and that my measurements were conservative, likely a larger portion of this harbor's potential seagrass habitat is impacted. Combined with the findings of loss due to these direct boat-related physical disturbances of seagrass meadows worldwide across regions, this halo effect is likely to also be found for moorings globally. I encourage management of the issue by employing and fine-tuning mooring methods (such as deploying moorings with anchor connections that do not drag on the sea floor) to minimize these unintentional but strong effects of mooring on the recipient habitat.

Thesis (Ph. D.)--Rutgers University, 2007. / "Graduate Program in Civil and Environmental Engineering." Includes bibliographical references (p. 194-200).

Thesis (MScEng)-- Stellenbosch University, 2013. / 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.

Um projeto portuário adequado deve garantir a segurança dos navios atracados e a eficiência das atividades de carga e descarga. Para isso, é preciso boa combinação entre condições de abrigo (natural ou artificial), e sistemas de amarração e defensas. Em geral, os problemas mais sérios de atracação estão relacionados a efeitos de ressonância. Nesse contexto, as obras de abrigo são pouco eficazes porque elas não alteram significativamente os períodos dos fenômenos hidrodinâmicos, e a restrição dos movimentos dos navios atracados depende principalmente dos sistemas de linhas de amarração e de defensas. Analisar um sistema de amarrações e defensas implica estudar a dinâmica do sistema de um navio atracado (navio/amarração/defensas), as ações aplicadas (principalmente as ações ambientais), e as características dos componentes desse sistema. Propõe-se, dessa forma, um estudo para elaboração de modelos que ajudem na compreensão do comportamento dinâmico desse sistema e uma comparação dos resultados obtidos nos modelos com as exigências de documentos técnicos. Partido de análises de modelos estáticos simples até a introdução de não linearidades geométricas e de perda de contato, esta dissertação discute, inicialmente, métodos convencionais de obtenção de esforços de amarração. Em seguida, o sistema é avaliado por meio de modelos dinâmicos que permitem uma compreensão maior sobre o comportamento de um navio amarrado. / An appropriate harbour project should ensure the safety of moored ships and the efficiency of loading and unloading activities. For these reasons, a good combination of (natural or artificial) shelter conditions, mooring systems and fenders is necessary. In general, the most serious mooring problems are related to resonance effects. In this context, shelter structures are not very effective because they do not significantly alter the periods of hydrodynamic phenomena, and the restriction of the moored ships\' movements depends mainly on mooring lines and fenders. Analysing a system of mooring lines and fenders involves studying the dynamics of a moored ship\'s system (ship / mooring / fenders), the actions applied (mainly environmental actions) and the characteristics of the components of that mooring system. Therefore, this study carries out models that help in the understanding of the dynamic behaviour of this system; and makes a comparison of the results obtained in the models with the requirements of technical documents. From the analysis of simple static models to an introduction of geometric nonlinearities and contact loss, this dissertation discusses, initially, conventional methods of obtaining mooring efforts. Then, the system is evaluated by dynamic models that allow a greater understanding of the behaviour of a moored ship.