## 1 Introduction
### Review of Energy Management Techniques for Extending the Lifespan of WWPM Systems  
In the literature of WWPM, very few studies have tackled the issue of power consumption and sought ways to reduce the energy consumption so as to prolong the monitoring lifetime. However, the issue of energy consumption in WWPM systems is of vital importance since a WWPM system is required to go for long periods of time unattended as a result of the fact the nodes may be difficult to access, especially in the case of buried pipelines. In such cases, the pipelines are required to be monitored throughout their life span, which can extend into years. In this sub-section, we survey some energy conservation management techniques that were discussed in Section 1.2 and how they were applied in previous WWPM studies. In [28], the authors used duty cycling with scheduled wake-up to reduce the power consumption and thus prolong the WWPM lifetime. The power consumption of the sensor nodes was minimized to 2.2 uW based on taking one measurement every six hours, thus given the sensor node a theoretical lifespan of 100 years when powered with two AA batteries. Although the sleep/wake-up method used in this study achieves great energy reduction, the WWPM solution does not provide real-time monitoring. In a more recent study, [PERSON] et al. [29] proposed a leakage-triggered networking method (radio-controlled wake-up) to reduce the wireless sensor network's energy consumption and prolong the lifetime of their proposed WWPM system. The authors proposed the use of three types of control frames (i.e., join frame, active frame, and wave frame) to trigger the network according to the leakage detection results. The coordinator and routing nodes were consistently in the working state, and the terminal nodes within the routing node network were sequentially working and sleeping. The terminal nodes only wake up from sleep when they receive an active frame from the routing nodes. The power consumption of the sensor node was 30 mW, and the proposed energy reduction technique increased the WSN lifetime by a factor of 2. In [30], the authors fused duty cycling and data driven-based schemes for maximizing the information gain about the leak, as well as minimizing the power consumption. In their proposed solution, the duty cycling part provides the sleep/wake-up schedule for the nodes to minimize the sensing, communication and processing-related energies, while the data-driven part implements adaptive sampling where nodes closer to the leak location operate at a higher sampling rate, whereas nodes farther away from the leak operate at a lower sampling rate. The power consumption of the sensor node when there was no leak was 2.5 mW, and they achieved an energy reduction that extended the WSN lifetime by a factor of 2.6. The same author in another study [31] combined duty cycling and data-driven approaches like hierarchical sensing and compression. The scheme relies on implementing hierarchical sensing by using vibration sensors of different sensitivities to detect vibrations due to a leak [32], and on exploiting duty-cycling and wavelet-based signal compression in order to reduce sensing, computation and communication energies. The authors failed to mention the energy consumption of the sensor node and did not provide an overall quantitative measure of the energy reduction of their proposed hybrid technique. Both [30; 31] employed centralized data processing to detect and localize leaks, and the results were validated only via simulations. [PERSON] et al. [33] used clustering and in-network processing to reduce the energy consumed in the network. The authors used wavelength transform and moving average filter to implement in-network processing within the WSN. By integrating the signal processing algorithm in the sensor nodes for distributed event detection and by performing aggregation on data within the cluster, the energy consumed in the network is far less than when all readings are sent to the base station in a centralized network.