School of Mechanical Engineering Seminar
Monday, April 20, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206
Liquefaction due to severe storms and pipeline floatation on the Israeli coast-
Rotem Soffer
MSc Student of Prof. Supervisor Eliezer Kit
The goal of this research is to achieve a better understanding of the wave-induced soil liquefaction phenomenon, and its impact on maritime structures and especially on buried coastal pipelines, in the view of discovered in 2006, flotation of the LNG pipeline, that supposedly can be related to soil liquefaction.
The soil liquefaction phenomenon occurs under large cyclic wave loads, usually generated during significant storms that occur under extreme weather conditions, and in loose soils. The research was stimulated by occurrence of such an event comprised by a sequence of two extreme storms, which apparently activated soil liquefaction just after LNG pipelines have been installed at November 2004. A later survey conducted in 2006 revealed that the LNG pipelines, which in 2004 had been laid in a trench below the seabed at water depths varying from 1 to 3 m, changed their vertical position significantly and in some segments were exposed. Another survey from 2009 displayed very similar results, only with 50 centimeters shift that might be related to a wrong benchmark in the 2006 survey since the background bathymetry obtained in this survey differs from other two, 2004 and 2009, which are in a good agreement. It is noteworthy to mention that the dredged in mid-November 2004 sand was left at the trench bank, and was supposed to naturally fill the trench. It was hypothesized that the sequence of storms, occurred in 2004, caused the liquefaction according to the following scenario: the first storm filled the trench with the dredged sand but due to the magnitude of the storm, the fill remained in a "loose" state, and didn't have time to be naturally compacted before the occurrence of the second strong storm, only 36 hours later. Hence, the second storm acted on the loose sand, thus contributing to rapid development of overburden pressure that caused the soil liquefaction and the pipeline flotation.
An algorithm to determine the probability of extreme event comprised of two independent storms in a sequence was developed, in order to assess the return period of such an event. This algorithm was extended to any sequence, given a long enough sample. A numerical model for the pore pressure accumulation based on an analytical solution for wave-soil interaction and empirical source term was adopted and extended to irregular wave loads. Using actual wave time series during the November 2004 event, pore pressure accumulation was calculated by different methods that were developed in the time domain. The approaches included sequences of spectral steps, and construction of randomly occurring wave train groups based on Rayleigh distributions, in order to apply the irregular wave load. The different wave loads were applied to the Palmachim region section and through a "Monte Carlo" simulation was found that the liquefaction can definitely provide a plausible explanation for pipe flotation, however in order to determine more reliable results the soil parameters should be verified in experiment.
This research provides further understanding of the soil liquefaction phenomenon, especially for the Mediterranean coast of Israel, and for irregular wave loads. Additionally, the research emphasizes the importance of soil fill methods in the marine environment.
ברכות חמות לנעם אליעז על הבחרו לאקדמיה הצעירה למדעים.
ברכות חמות לד"ר פבל גינסבורג, אשר זכה במלגת אלון לשנים תשע"ו, תשע"ז ותשע"ח.
