## 3 Sensor Network Architectures for Monitoring Underwater Pipelines
### Underwater Acoustic Wireless Sensor Networks  
Acoustic wireless sensor nodes can be installed with underwater pipeline infrastructures. Each node has a limited transmission capability in which each node can communicate with few neighboring nodes. Multi-hop communication is used to transfer the sensed information among the underwater pipeline. Wireless networks can solve some of the reliability problems of current wired networks technologies in pipeline systems [20]. For example, wireless sensor networks can still function even when some nodes are disabled. Faults in sensor nodes can be easily tolerated by using other available nodes to cover the faulty ones. Using dense sensor networks with a high number of nodes and/or using wide acoustic transmission range, the network can maintain connectivity and the sensed information can be transported through the network to its destination even with the existence of some node failures. For example, each node in Figure 2 can communicate with two nodes to the left and two nodes to the right. If for example node 3 and 5 are damaged, node 4 can still send its sensed data through nodes 2 or 6. Each sensor node for monitoring pipelines is usually equipped with an acoustic transceiver, a processor, a battery, memory, and small storage in addition to one or more sensor devices. Power consumption is critical to the life span of pipeline communication systems. Pipeline systems are usually installed to be used for many years. Therefore the associated communication systems should also be long lived. Unlike wired networks where the power is not at all a constraint in building the system, network designers have to consider power as one of the main constraints in the wireless system. Power in a node can be consumed when data is sent through the transceiver, when the transceiver is turned on waiting to receive data from other nodes, when sensor devices are turned on, and when the processor is active. Careful scheduling of these resources is needed to optimize power consumption. Figure 2: Reliability in dense sensor networks. Although increasing the range can provide better reliability, more energy will be consumed from the nodes. A dynamic configuration for the wireless transmission range can provide better power management. Example of this configuration is in Figure 3. In this network, nodes 3 and 5 are dead. Therefore, the wireless range for node 4 is increased to reach nodes 2 and 6 while other nodes use a smaller transmission range to reduce the power consumption. In addition, nodes are used to route information from other nodes to the NCC. As a result, nodes close to the NCC will consume more power than other nodes since they will route more packets. All nodes will have the same level of sensing activity; however, closer nodes to the NCC will consume more power due to more packet routings. One of the main issues for wireless/acoustic networks when used to monitor pipelines is the optimal design of a network protocol that balances the power consumption of batteries on the nodes with and without node failures. This balancing is crucial to extend the life of the network. One approach to enhance the reliability of underwater acoustic sensor networks for pipelines is to divide the network into multiple parts. In each part, there is a surface buoy that is linked with one of the underwater sensor nodes through a wire as shown in Figure 4. Each surface buoy is equipped with a radio communication system to transfer collected information to the NCC. The collected information can be transferred either directly through satellite communication links or using radio multi-hop communication among the available surface buoys. All acoustic sensor nodes need to transfer their sensed information to the nearest neighboring node linked to one of the surface buoys. Generally, underwater acoustic networks suffer some major problems [25-27]. First, the available bandwidth is very limited among the nodes. The bandwidth of underwater acoustic channels depends on both the distance and frequency [28].