Deprem Etkisi Altındaki Gömülü Sürekli Boru Hatları

Abstract

Üst yapılarda oluşan ağır hasar ve yıkılmalara bağlı ölümlerin ve yaralıların olması depremin insan hayatı üzerine birincil etkileri olarak tarif edilirse alt yapılarda oluşan hasarlara bağlı ortaya çıkan, depremden sonraki yaşam kalitesini olumsuz etkileyen durumlar da depremin insan hayatı üzerine etkiyen ikincil etkenleri olarak tarif edilebilir. Tarihteki depremler incelendiğinde bu ikincil etkilerin birincil etkiler kadar önemli olduğu açıkça görülmektedir. Burada bahsedilen durumlar dikkate alındığında toplumsal hayatın can damarları olan ve bu işlevleri nedeniyle de “yaşam hattı” olarak adlandırılan altyapı boru hattı sistemlerinin depreme karşı güvenliğinin sağlanmasının kaçınılmaz olduğu açıkça anlaşılmaktadır. Boru hatları sürekli ve parçalı diye iki kısma ayrılırlar. Petrol ve gaz boru hatları genellikle sürekli boru hattı şeklinde, su temini boru hatları ise parçalı boru hattı şeklinde dizayn ve inşa edilirler. Gömülü boru hatları kalıcı zemin hareketi (KZH) ve sismik dalga yayılımı yolu ile hasar görebilirler. Bunun yanı sıra, fay-boru hattı kesişimi durumu da boru hattı güvenliği açısından dikkate alınması gereken bir konudur. Kalıcı zemin hareketi sıvılaşma nedeniyle yanal yayılma, sismik oturma, zemin kayması ve yüzeysel fay hareketi olarak tarif edilmektedir. Genel olarak boru hasarlarına neden olan kalıcı zemin hareketi potansiyeli zemin deplasman miktarı ile ilişkilidir. Zemin- boru etkileşiminin ana konusu olan sıvılaşma nedeniyle oluşan kalıcı zemin hareketi miktarının tahmini zor bir problem olarak karşımıza çıkmaktadır. Zemin kayması nedeniyle oluşan kalıcı zemin hareketi konusuyla sınırlandırılmış olan bu çalışmada zemin kaymasına karşı gömülü sürekli boru hatlarının davranışı incelenmiştir. Newmark kayan blok modeli deprem kaynaklı zemin deplasmanlarının hesaplanması için sıkça kullanılan bir modeldir. Literatürde bu model kullanılarak çeşitli deprem verilerine göre regresyon analizleri yapılmış ve bazı hesap yöntemleri elde edilmiştir. Literatürde var olan yöntemlere ilave olarak regresyon uyumu anlamında daha iyi bir yöntem bu çalışmayla elde edilmiştir. Elde edilen yöntemle gerekli çalışmalar yapılmış ve ulaşılan sonuçlar irdelenmiştir. Bu çalışmada öncelikle İstanbul’un jeolojik ve jeofizik özelliklerine göre zemin formasyonları ele alınmıştır. İkinci aşamada ise gömülü sürekli boru hattı – zemin modellemesi yapılıp bu matematik modellemelerin çözümlemeleri yapılmıştır. Son aşamada ise birinci ve ikinci aşamada elde edilen veriler birleştirilmiş ve elde edilen sonuçlar yorumlanmıştır.

Harm arising from heavy damage in superstructures is defined as primary effects of earthquake on human life. Conditions occurring in consequence of damages in infrastructures and negatively affect life quality after earthquake can be defined as secondary effects of earthquake on human life. When earthquakes in history are analysed, it is clearly seen that secondary effects are as severe as primary effects. Seismic design of buried pipeline has great importance in the field of “lifeline” engineering. The pipelines are often referred to as “lifelines” since carry materials essential to support of life. The pipelines are usually buried below ground for economic, aesthetic, safety and environmental reasons. Pipelines can be categorized as either continuous or segmented. Generally the oil and gas pipelines are designed and consructed as continuous pipeline, while water supply pipelines are designed and consructed as segmented pipelines. The pipelines must be designed and consructed to resist most of the earthquake hazards. Maintaining the safety of lifelines in incidents of earthquakes today constitutes one of the world's most important matters. Buried pipelines can be damaged by an earthquake via either permanent ground deformation (PGD) or seismic wave propagation (SWP). Besides these, fault-pipe intersection is another issue that must be considered in ensuring the safety of pipelines. The current widespread, and growing, consumption of natural gas in Turkey has drawn even more attention to the subject of the safety of gas networks. Added to this is the fact that Turkey has become an important transmission region for international pipelines such as Baku-Ceyhan, NABUCCO, etc., and, because Turkey is situated on very active and major fault lines, such as the North Anatolian Fault Zone (NAFZ), the safety of these gas transmission networks in incidents of earthquakes is a very important concern. The most well known forms of PGD are described as surface faulting, landsliding, seismic settlement and lateral spreading due to soil liquefaction. In general, the potential for PGD to induce pipe damage is related to the amount of ground displacement. Predicting the amount of ground displacement due to soil liquefaction is a challenging problem, and therefore, such estimation is the main subject for soil-pipe interaction. This study has been limited to the permanent ground deformation due to landslide. In this study, behaviour of continuous buried pipelines subject to lanslide is investigated. PGD can be started by a seismic activity. There are several types of PGD. The main two types of PGD are the spatially distributed and localized abrupt PGD. The spatially distributed case commonly occurs in liquefaction and the abrupt case in landslide. Pipelines are generally damaged near the side walls of landslide in the case xx of abrupt PGD, and throughout the PGD zone for a distributed PGD. Localized abrupt PGD is more destructive than one that was distributed. An idealized PGD zone can be characterized by the width (W) and the amount of PGD (δ) and shape. If the pipe axis is perpendicular to the direction of sliding and the width of sliding zone, W, is large enough, the damage of the pipe can be assumed as localized near the side walls. This is true if, and only if, the zone slides as a mass without breaking into pieces. If the zone is narrow, the pipeline may display damage throughout the sliding zone. As the mechanism is vague, no any approach (even an empirical one) has been developed to model the behavior of a sliding mass during breaking into pieces. Abrupt permanent ground deformation has the same properties with a pipe-fault 900 angle crossing case. Calculation or estimation of earthquake-induced ground motions is important both for superstructures and infrastructures. Newmark sliding block model is a model frequently used for calculating earthquake-induced ground displacements. According to Newmark model, ground displacement is calculated based on earthquake acceleration records . Using this model in literature, regression analyses were carried out according to various earthquake data and some calculation methods were obtained. Generally, these approximations are depend on maximum acceleration, critical acceleration and Arias Intensity. Arias Intensity index based on timedependent acceleration of an earthquake. Besides, a regression analysis for Arias Intensity, depends on moment magnitude of an earthquake and distance from the epicentre of the earthquake, has been developed. In this study, a new analysis covering maximum earthquake acceleration has been obtained according to significant earthquake records between 1976 and 2013 in Turkey. Turkey is a country with a number of important and large fault lines. The North Anatolian Fault Zone (NAFZ) ranks as one of the most active fault systems in Turkey. This fault line is an active right-lateral strike-slip fault in northern Anatolia that runs along the transform boundary between the Eurasian Plate and the Anatolian Plate. The fault extends westward from a junction with the East Anatolian Fault at the Karliova Triple Junction in eastern Turkey, across northern Turkey and into the Aegean Sea. It runs south of Istanbul, throughout Marmara Sea. There have been significant earthquake records along the NAFZ since the disastrous 1939 Erzincan earthquake, such as 1939 Erzincan (M=7.9), 1942 Niksar-Erbaa (M=6.9), 1943 Tosya-Ladik (M=7.7), 1944 Bolu-Gerede (M=7.5), 1949 Karlıova (7.1), 1951 Kurşunlu (M=6.9), 1957 Abant (M=6.8), 1966 Varto (M=6.9), 1967 Mudurnu Valley (M=7.1), 1992 Erzincan (M=6.5), 1999 İzmit (M=7.4), 1999 Düzce (M=7.2). The northern branch of the NAFZ in the Marmara Sea is very close to Istanbul's southernmost shore. The distance between the fault line and the center of the city (old Istanbul) is about 15-20 kilometers. The Marmara region has some potential seismic gaps. For example, the middle strand from the Mudurnu Valley region to the Aegean Sea has not experienced a significant earthquake for the last 400 years, excepting the 1737 earthquake in the Biga peninsula. In addition to this, the most westerly portion of the southern strand has not ruptured since 1855. In historical times a number of earthquakes occurred along the northern strand, especially in the Marmara Sea area. Recent seismicity maps indicate a potential seismic gap in the central part of the Marmara Sea. Earthquake records spanning two millennia indicate that, on average, at least one medium intensity earthquake has affected Istanbul every 50 years. The average return xxi period for high intensity events has been 300 years. The Istanbul-Marmara region of northwestern Turkey with a population of more than 15 million faces a high probability of being exposed to an earthquake of magnitude approximately 7.5. One of the aims of this study to analyse the behaviour of ground formations in Istanbul against the expected Istanbul earthquake in terms of its effects on buried pipelines. Through a vast number of geological, geotechnical and settlement evaluation studies the region had been investigated in the past. As the potential for landsliding is a major problem all over the hill sides. Therefore, expected Istanbul earthquake will be able to trigger to landslides in this region. Soil-pipeline interaction is one of the main subjects of this study. In general, surroundings of buried pipe in trench is filled with compressed sand or gravel. While polyethylene (PE) pipes are used for low pressure distribution network, steel pipes (ST), covered by polyethylene, are used for high pressure distribution network. Thus, soil-pipe interface consists of PE. During an earthquake, buried pipelines can be damaged by forces that emerge through soil-pipe interaction at the soil-pipe interface. In such cases the ground moves and causes damage to buried pipelines. Equivalent elasto – plastic soil spring coefficients have been obtained. Then, the straight pipelines under the permanent ground deformation (PGD) and seismic wave propagation (SWP) loads have been examined. To obtain mathemathical models for PGD case, this problem has been examined in two parts as Longitudinal Permanent Ground Deformation and Transverse Permanent Ground Deformation. In the case of SWP, it has been assumed that the ground surface displace sinusoidally. The wave amplitude is much smaller than the wave length. Therefore, it has been assumed that the pipe moves with the soil and the displacement remains in elastic limit. Traffic load, temperature change, soil weight, operating pressure are important effects on the buried pipelines. External pressure due to traffic load can be calculated via Boussineq Method. The result of our study showed that traffic load effect can be negligible. The land surface temperature at certain times of the day increases and decreases markedly, typically after 50 cm depth, daily change of temperature is low enough to be negligible. On the other hand, operational temperature change may cause longitudinal pressure and strain. When a thin-walled pipe is subjected to internal pressure a hoop and longitudinal stress are produced in the wall. In openended cylinder case (such as buried pipelines) only hoop stres occurs, longitudinal stress doesn’t occur, but, because of poisson ratio axial strain can take place. Another external pressure can become due to soil weight. This value may be ignored when comparing with internal pressure. Failure criterias are limit values for a pipeline. Therefore, these criterias have been determined, in this study, to compare with results. As a result, in this study, firstly, geotechnical and geological characteristics of the ground formations of Istanbul have been investigated. Secondly, mathematical models of buried pipeline – soil interaction have been produced and solved. Finally, data obtained from the first step have been applied on the result equations of the second step and outcomes have been evaluated.