## 1 Introduction  
In the past few decades, petroleum has always been one of the most important resources in the progressing of human civilization. For petroleum corporations, the periodic inspection of their oil pipelines is vital, where the deployments of these pipelines are everywhere in oil refineries and oil transfer stations (Figure 1). However, the deterioration of these pipelines cannot be avoided as oil is transported over time. Once the oil pipelines get deterioration, it could bring the owners of these corporations huge economic loss during the processes of oil transportation. Furthermore, the damage is not merely confined to the aspect of the economy. For example, the pipeline corrosion and cracks, which are commonly caused by the impurities in the pipelines, could lead to severe soil/ocean pollution or even explosions. In China, the China National Petroleum Corporation (CNPC) is one of the largest energy corporations that manage the refinement, storage, and transportation of oil products. To operate such a huge business, the CNPC is required to conduct careful maintenance, repair, and operations (MRO) on the deployed facilities, especially the oil pipelines which serve as one of the most important components to connect all the facilities. According to the statistical data of the CNPC, the distance of the deployed pipelines achieves 86, 734 (km) in China by the end of 2018 [1]. In particular, about RMB 120 million was invested in the MRO of these pipelines [2]. Therefore, there is a demand for the CNPC to  
Figure 1: Deployment of oil pipelines in unstructured environment. obtain an effective solution for autonomous inspection with robotic technologies. In such a solution, the developed robot is expected to have the capabilities of stably moving inside various standardized oil pipelines that the CNPC is using. In addition, there are multiple sensors mounted on the robot for the environment inspection and robot state monitoring. The contents of the inspection inside the oil pipelines includes the ID of pipelines, the completeness of welding seam lines of pipelines, the existence of water stains, the coverage of anticorrosive layers, etc. Through the fully deployed technology of 5G in the near future, the robot will be able to be operated remotely by skilled workers--or operated autonomously--for a comprehensive inspection of the oil pipelines and upload the inspection results for timely analysis. The CNPC believes that the deployment of this robot will not only guarantee the safety of the on-site inspection workers but also improve the efficiency of the diagnosis on their facilities, enabling an instant alert if faults occur. For the requirements from CNPC, the robot is expected to have a nontowline design that can operate under its self-power and support a cruising range of 60 (km) for every inspection task. Due to the moisture and dust inside the oil pipelines, the IP ratings should be better than IP67. For the mobility criteria of the robot, the robot can move in an incline with no more than \(\pm 45^{\circ}\) and adapt to geometric deformation of 10%, while keeping uniform speed in the standard pipelines of the CNPC (1219 \(\leq\phi\leq\) 1440 mm). With the installed sensors, the robot can achieve the deformation of pipelines with a precision of \(\pm 1\) mm. Besides, the paved road map of the oil pipelines can be mapped simultaneously during the inspection. To design a robot that fulfills the requirements proposed by the CNPC, some literature is investigated. For external inspection of pipelines, magnetic wheels were employed to enable the robot to attach onto the surfaces of pipelines [3] and storage tank [4,5]. For internal inspection of pipelines, while magnetic wheels can work in such a scene [6,7], a robot with an automatic tractive force adjusting function is more reliable as it enables the robot to actively adapt to the pipeline diameters [8]. Except for the use of wheels, caterpillar [9] and shape memory alloy actuators [10] are also a selection for moving inside pipelines.