NICOP - Prediction of broaching-to of ships in an irregular seaway: Classification of motions and calculation of probability based on phase-space information
Abstract
Prediction of broaching-to of ships in an irregular seaway: Classification of motionsand calculation of probability based on phase-space informationAim of the project is to contribute to the establishment of a state-of-art framework forthe probabilistic assessm"ent of the most dynamic of the modes of ship capsize, knownas ~broaching-to~. Records about this type of instability exist in the n""autical literaturefor centuries. The phenomenon manifests itself as a ~loss of heading~ despite steeringeffort, generating a tight"" turn that sometimes ends with capsize. It can occur indisplacement-type vessels operating in steep, stern quartering seas. Whilst"" greatstrides towards understanding its dynamics have been realized over the last thirtyyears, until recently we were lacking the" key tools for studying thoroughly occurrencesof such phenomena in a more naturally occurring seaway. This has been a hindrancetowards the development of consistent methods for the assessment of a vessel~stendency for dynamic instability in high seas.In our pr"evious ONR-funded research, high-run phenomena that have been observedto act as triggers of broaching-to have been studied, for an"" irregular sea environment.These are, basically, involuntary high-speed motions in following/quartering waves thatcan be explained"" on the basis of the nonlinear surge dynamics. Moreover, thecombined surge-sway-yaw dynamics during broaching-to incidents has been"investigated. A few findings can be singled out from these efforts: high-runs were foundassociated with the formation of coherent structures in phase-space. Methods suitablefor their identification have been applied and evaluated. Oscillatory high-run patterns"were found, some actually seemingly being quite widespread. Statistical evaluation ofthe correlation between high-runs and broachi""ng-to, produced concrete evidence thatbroaching-to in irregular seas is, in the great majority of such incidents, preceded bysome" high-run event. Parallel work at NSWC (Carderock) aimed to build capability forthe evaluation of a naval ship~s risk of capsizing in irregular seas. NSWC~s work set itspriority on developing fast probabilistic methods of ship stability assessment. Thisprogres"sed in close interaction with the Strathclyde work. By this collaboration, arigorous and physics-based probabilistic framework for"" the assessment of a ship~stendency for broaching-to has started to emerge, deriving from better understandingof the link between"" nonlinearities, extreme motions and the statistics of instability.A number of further steps, in the direction described above, are"" described in thisproposal. Firstly, classification of nonlinear motion patterns associated with broachingtobehavior will be produ""ced, depending on the frequency content of the excitation.Techniques based on finite-time Lyapounov exponent fields and the cluster"ing ofphase-space particles will be applied. Identified motions will be qualitativelyverified/validated through comparison against" simulations performed with the LAMPship hydrodynamics code. Such a classification would essentially generalize, for multichromatic""waves, the current theory of broaching-to. Another important task isacquiring knowledge about unsafe regions of the phase-space an"d their time variability.This would permit the direct calculation of the probability of broaching-to incidents; acomputationally d"emanding job which, nevertheless, can yield valuable data forbenchmarking the fast practical method of probability calculation bein"g currentlydeveloped at NSWC.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Nov 03, 2017
- Source ID
- N629091712127
Entities
People
- Kostas Spyrou
Organizations
- Office of Naval Research
- United States Navy
- University of Strathclyde