## 3 Sensor Network Architectures for Monitoring Underwater Pipelines
### Underwater Wired Sensor Networks  
Currently, most pipeline sensors are connected using wired networks. Wired networks are either copper or fiber optic cables. The wired networks are usually connected to regular sensor devices that measure specific attributes such as flow rate, pressure, temperature, sound, vibration, motion, and other important attributes, see Figure 1. The wires are not used for communication only but also to transfer electrical power to different parts of the pipeline system to enable the sensors, actors, and communication devices to function. Power for the pipeline resources and networks can be provided by different sources:  
1. _Solar Energy_: arrays of solar cells can be used to generate electric power for the pipeline infrastructure. This power is supplied to the different communication and sensor devices. While this option is suitable for above-ground pipelines, it is not applicable for underwater pipelines.
2. _Pipeline Flow Energy_: electric power can be generated using turbines embedded through the pipeline. These turbines rotate under the pressure of the fluid moving through the pipelines and generate electric power. This can be used for the different sensor and communication devices installed along the pipeline [24].
3. _Other External Energy_: power can be provided from external sources such as external gas-based power generators or third-party power generators. This power is transferred in wires to different communication and sensor devices along the pipeline.  
Wired networks are considered the traditional way for communication in pipeline systems. They are easy to install and provide power supply for through the network wires. However, there are a number of reliability problems related to using wired networks with regular sensors for monitoring pipelines. These problems are:  
* If there is any damage in any part of the wires of the network, the pipeline communication system will be completely or partially damaged. This depends on how the wired network is organized and used. If the communication is done in one direction on the wire, then a single cut on the wire will disconnect all the nodes after the cut from the NCC. If the communication is two-directional then the negative impact on the communication is less as some nodes will use one direction for communication while the nodes after the cut can use the other direction. In this case the NCC needs to be connected to both ends of the network. However, if there are two or more cuts in the network, then all nodes between the cuts will not be able to communicate with either of the NCC. In addition, if there is a power outage, some of the nodes may not be able to operate.
* It is easy for unauthorized people to disable the network simply by cutting some of the network wires.  
These problems make wired networks for pipeline communication systems undependable. Although, wired networks provide an easy solution for underwater pipeline monitoring and controlling, they face a number of major reliability and security problems. The main reason of these problems is the structure and type of networks used for pipelines. If any part of the wired network is disabled for any intentional or natural reason, the monitoring system can be partially or completely affected.  
Figure 1: Underwater wired sensor network.  
One possible solution to enhance the reliability of a wired network is to use multiple networks that expand through the whole area. One of these networks will be used as primary while others are kept as backup. However, unlike other systems, using multiple wired networks and a fault tolerance mechanism among them will not enhance the reliability of the pipelines network. This is due to the fact that all wires will be expanded together along the pipelines and any damage occurring in one of them may also occur in the others. More specifically an accidental (or intentional) break in the line has a very high probability of happening to all wires along the pipelines.