## 2 Background and Related Work  
Two types of threats may occur in pipeline infrastructures: intentional and non-intentional. Intentional threats can be for reasons like terrorism or illegal tapping. Pipelines in the Middle East for example are principally at risk of terrorist attacks. This is one of the main ongoing security problems in Iraq. In another example, in 2002 there were over 900 attacks on the Cano Limon oil pipeline that caused losses of around 2.5 million barrels of crude oil [6]. In addition, the pipeline was out of service for 266 days due to the fact that part of the pipeline were blown up some 170 times in 2001. The Cano Limon oil pipeline is owned by Occidental Petroleum Corp and the Colombian state oil company Ecopetrol. It transports around 110,000 barrels of crude oil a day from the Cano Limon field to the Caribbean coastal town of Covenas. Oil pipelines have also been repeatedly attacked in Nigeria. In some cases the attacks caused major damages and death of some people. The problem of illegal tapping is known in the South East Asia region. In one case a company was losing about $4m worth of oil a year through illegal tapping from an underwater pipeline [6]. Non-intentional threats may occur due to accidents such ships crashing into a pipeline, human mistakes in the pipeline operation or maintenance, or natural disasters such as volcanoes and earthquakes. For example, several underwater pipelines in the Gulf of Mexico were damaged by hurricanes in 2005 [5]. It was a very difficult and time consuming process to inspect the pipelines and find the locations and types of damages inflected by the hurricanes. Non-intentional threats can also happen due to defects in the pipeline systems. These defects can be leakage or high pressure in the pipelines. Any defect or damage in underwater pipelines may result in major environmental and economic consequences. To reduce the impact of these consequences, underwater monitoring systems can be used. These systems can provide effective and fast detection mechanisms to discover defects and respond to them in a timely and more effective manner. There are a number of technologies to monitor, maintain and protect pipelines. Examples of these technologies are sensors, mobile robots, algorithms [7-10]. Most of these technologies are designed specifically for detecting and locating pipeline leakage [11,12], corrosion, and damage. These technologies were designed to provide a remote facility to detect and to report the positions of any defect. Some of these available solutions rely on the availability of a network to transfer the information and report the defections or any important sensed information [13]. These networks are usually wired using copper or fiber optic cables [14,15]. These wired networks are usually connected to regular sensor devices that measure specific attributes such as flow rate, pressure, temperature, _etc_. There were some efforts to develop algorithms and methods for detecting defects such as leakages in pipelines. These algorithms and methods are based on the availability of networks along the pipelines. All these efforts are not to develop reliable and fault tolerant networks that monitor pipelines as we discussed in this paper, but rely on the existence of reliable networks. One example is PipeNet [16]. PipeNet is a wireless sensor network for monitoring large diameter bulk-water transmission pipelines. The network collects hydraulic and acoustic/vibration data at high-sampling rates. Algorithms for analyzing the collected data to detect and locate leaks were developed. In [17], a method was developed to detect faults for oil pipelines. In this method, Rough Set was used to reduce the parameters of a pipeline system. Artificial Neural Network (ANN) with three levels is used to form a detection model. In addition, a general framework using acoustic sensor networks to providecontinuous monitoring and inspection of pipeline defects was developed [18].