Fragility functions for pipeline in liquefiable sand : a case study on the Groningen gas-network

Abstract

Any pipeline network is of strategic importance for the country in which it is located, both because its correct functioning is of vital importance for today’s functioning of the community and because any disruption, malfunctioning or rupture represents a hazard for the community. In this paper the fragility function for a simple segment of a high pressure pipeline installed in loose, saturated sand is investigated with respect to shallow earthquakes. These earthquakes can cause local permanent ground deformation due to liquefaction effects. Since 1986, a low intensity seismic activity is present in the Groningen gas-field area (Netherlands), due to the tremors following the compaction of the gas reservoir due to stress decrease. An extensive study performed by the Dutch Meteorological Insititute (KNMI), see Dorst et al. (2013), shows that in the last decades (2003-2013), the seismic activity changed from low intensity activity with constant events rate per year to a higher rate with slightly increasing magnitude. The depth of the earthquakes is at 3 km, being the depth of the gas reservoir. On 16 August 2012 an earthquake with a local magnitude of M=3.6 occurred near Huizinge and it is the largest event that occurred until now. A large pipeline network is present in the area affected by the induced earthquakes and it is a strategically important network for the Netherlands and Europe, representing an important node from which the natural gas is transported to several European countries. The importance of this network rises the need to investigate the fragility of this transportation network with respect to both seismic action and the effect of possible ground failure (liquefaction) due to the characteristics of the soft soil. Fragility functions for pipelines are available in literature (O’Rourke et al. (2007) or Pitilakis et al (2010)) based on observational analysis of the performance of lifelines subjected to earthquakes of large magnitude. However, in this paper we want to investigate the performance of a pipeline segment subject to a seismic activity that is not of tectonical nature and is characterized by short duration of the signal, a local amplification and possible ground failure. Therefore, the effect of soil-pipe interaction in presence of loose saturated sand that is typical for delta-regions is considered. Furthermore, additional uncertainty is related to the definition of the fragility function in the ranges of magnitude above the maximum measured event (M3.6). This uncertainty is all of epistemic nature, since no observation of M>3.6 is available. Our study aims to the definition of fragility curves for a high pressure pipeline, in absence of available data, and is based on the results of a fully probabilistic model that takes into account 12 th International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12 Vancouver, Canada, July 12-15, 2015 2 the performance of a gas-pipeline element with respect to seismic shake and the local response within the pipe-soil interaction. 1. SEISMIC ACTION AND GMPE The Dutch Metereological Institute (Dorst et al. 2013), performed the data analysis and a Probabilistic Seismic Hazard Analysis (PSHA) with all data available until 2013. The Ground Motion Prediction Equation (GMPE) used for the prediction of the ground motion characteristics (peak ground acceleration and peak ground velocity), as function of the magnitude Mw and source-site distance R, is the GMPE of Akkar et al. (2013) recently derived on a large dataset that includes shallow and low magnitude events and a correction factor to take into account the faults typology and amplification for local seismic response in soft soil that makes it more suitable for the typology of the events in the Groningen area. The general expression of the GMPE of Akkar et al. (2013) is reported in Eq.(1), while we refer to Dorst et al. (2013) for the specific expressions and the coefficients to be used. In Eq.(1) the ground motion characteristics X (peak ground acceleration or peak ground velocity) is a function of the ground motion parameter Xref at bedrock (that depends on magnitude, fault geometry and source-site distance), of the parameter S function of velocity of propagation of shear waves at the considered site, of σ , the standard deviation of the lognormal distribution of X and e a standard normal error. The standard normal error between peak ground acceleration and peak ground velocity is herein considered strongly correlated. ln X = ln(Xref) + ln(S) + eσ (1) 2. SOIL-PIPE INTERACTION Generally, the behavior of a pipeline segment during an earthquake event, depends on the earthquake intensity and on the material and geometry characteristics of the pipe but also on pipe placement technique and soil property of the more superficial layers. Buried pipelines are subject to deformations due to the effect of the shear waves (s-waves) generating mainly horizontal oscillation with a certain period and amplitude. The soil deformation is transferred to the pipe to a degree that depends on the soil-pipe interaction and interface. However, the dynamic effect of the swaves is not so severe for a large pipe section of high steel grade, while more severe effects can be generated by permanent ground deformations due to soil liquefaction. The term “liquefaction” indicates a phenomenon for which a saturated and zero cohesion soil loses its shear resistance due to the accumulation of plastic deformations caused by transient and cyclic force actions in un-drained conditions. Indeed, the development of excess pore water -pressure reduces the effect of in situ confinement of the soil. Sand boils, cracks and lateral spread phenomena are a sign of liquefaction. When liquefaction occurs the strength of the soil has nearly vanished. Liquefaction is a phenomenon that arises only when a seismic event has an intensity that can induce such deformations in the soil that can generate a significant increase of the neutral pressure and when the soil shows significant degradation of resistance properties under cyclic load. Therefore, liquefaction occurs only for earthquake events of certain magnitude and durations. In addition, it can occur only in saturated non-cohesive soils (sand), with low plasticity index and low relative density.