## 1 Introduction  
With the increasing demand for energy and water in the World; petroleum, natural gas, and water resources and facilities have become important assets for most countries. Maintaining the economic progress of most countries is strongly dependant on maintaining and protecting these resources and facilities. One of the main and important facilities for these resources are the pipelines used to transfer water, petroleum, and natural gas. These pipelines are considered one of the main infrastructures between producer and consumer countries. Protecting the pipeline infrastructures is one of the main issues facing these countries. Furthermore, oil and gas industries in the World heavily depend on pipelines for connecting shipping ports, refineries, oil and gas wells, and power plants. For example, there are around 500,000 miles of oil and gas pipelines in the United States that also extend into Canada and Mexico [1]. These pipelines play a critical role in the U.S. economy. This pipeline infrastructure is mainly for providing energy supply to the U.S. [2]. Pipelines can be installed above the ground, under the ground, or underwater. Several long underwater pipeline systems are used for different applications around the World. One of the longest pipelines in use is the Langeled Pipeline that extends for 1,200 km from the Ormen Lange field in Norway to the Easington Gas Terminal in England under the North Sea and used to transfer natural gas to England [3]. This pipeline started operating in October 2007 and can carry 25.5 billion cubic meters per year and supplies around 20% of the natural gas demand in England. Another long pipeline is located between Qatar and UAE under the Arabian Gulf and owned by Dolphin Energy Limited of Abu Dhabi [4]. It is used to transfer processed gas from Qatar's offshore North field to the UAE. It extends for 364 km through the Gulf and transfers a high percentage of UAE's gas needs. In addition, pipelines are intensively used in the Gulf of Mexico to transfer oil. There are around 30,000 miles of underwater pipelines in the Gulf of Mexico [5]. Most existing and planned underwater pipeline projects are considered important infrastructures for economic stability and growth. Having a reliable monitoring and control system for these infrastructures can significantly help in inspecting and saving them. One of main approaches used to monitor different types of pipelines is sensor networks. This paper develops different sensor network architecture designs for monitoring underwater pipeline infrastructures. The developed architectures are underwater wired sensor networks, underwater acoustic wireless sensor networks, RF wireless sensor networks, integrated wired/acoustic wireless sensor networks, and integrated wired/RF wireless sensor networks. The paper compares and discusses the reliability of these proposed architectures. Although the main reason of having reliable sensor networks for monitoring is to protect underwater pipelines, the main focus of this paper is on the reliability of the sensor networks used for underwater monitoring and not on the physical pipeline protection. Three reliability factors are used to compare the architectures in terms of network connectivity, continuity of power supply for the network, and the physical network security. The paper develops an analytical model to evaluate and compare the network connectivity of the developed architectures. In addition, the advantages and disadvantages of the architectures are discussed. The paper also develops and evaluates a hierarchical sensor network framework for underwater pipeline monitoring. The rest of the paper is organized as follows. Section 2 discusses the related work. Section 3 discusses different sensor network architectures for monitoring underwater pipelines. Section 4 develops an analytical model that evaluates the reliability of these architectures. Sections 5 and 6 develop and evaluate a hierarchical sensor network design for monitoring underwater pipelines.