Distribution system for this lifeline consists of several critical components including pipelines, valves, stations, pumps,distribution lines and finally the risers. Riser as the last element at costumer location has the most uncertainty in seismic vulnerability because its performance under earthquake load highly depends on characteristics of the customer building and its seismic behavior which cannot easily have considered in seismic design of the riser. This paper presents the results of an extensive experimental and numerical investigation on seismic behavior and retrofit of riser system. Several tests have been conducted on full scale risers with real conditions to assess the performance of existing risers in different load scenarios. After finding the weak points and failure modes, several retrofit countermeasures have been implemented to find the best solutions for upgrading the seismic performance of the system considering the assessment and modification on all minor and major components of the connections. The results have been developed numerically to obtain the details of stress and deformation distribution using calibrated models. Finally, the best solutions as innovative countermeasures have been presented.
Gas riser system has been used in many large cities including the capital and several other seismic prone areas. The damage histories show that the existing system has a high vulnerability and requires appropriate countermeasures (Gross, 1985). In the capital city of Tehran itself there are more than one million risers with different sizes and more than ten million residents. Similar condition exists in other large cities in Iran. This fact raises the importance of finding an appropriate retrofit solution for the issue. The riser steel pipe and its welded joints have shown ductile behavior during past earthquakes. The threatened steel joints have also shown good performance but with less ductility compared to welds. This joints are located under the Mitter stop valve, under and top of the regulator and at the meter location. However, the weakest point of the system is the Insulator unit on top of the Mitter stop valve which connect it to the regulator with a short steel pipe. This insulatorunit is made from plastic to provide electric current insulation from corrosion protection system (MGL, 2010).