## 1 Introduction  
Natural Gas (NG) has a central role, in the global energy scene, in the shift to an economy with lower pollutant emissions. It is currently in second place, behind oil and together with coal, when it comes to global energy consumption, with a percentage close to 24% [1]. Globally, it has seen from 2010 the second highest demand growth rate, after renewable sources. According to [1], in 2040, NG should exceed coal by about six percentage points, in order to satisfy global energy needs. The traditional systems for the supplying of NG are mainly two: the transportation of Liquefied Natural Gas (LNG) by ships and the conveyance into pipelines. An alternative system is the transportation using pressure cylinders. This method is often referred to as Compressed Natural Gas (CNG). The advantages of CNG, with respect, in particular, to LNG, can be summarized as follows:* The exploitation of stranded gas fields or flare and associated gas can be economically viable [2; 3];
* It is an economic way of transporting small quantities of gas and exploiting small reserves [3];
* It can be an economic way of transporting ng over short distances, between 800 and 3000 km, or 250-5000 km, making also use of cooling [2];
* The containment of costs due to the lack of liquefaction and regasification plants;
* The return time of the investment can be shorter;
* The largest investment (85%) for cng concerns the ship, which is a mobile value that can be used in other places, while for an Ing project, 60% of the cost falls within liquefaction and regasification plants [4], which cannot be used in other places;
* CNG does not have the problem of the loss of load due to the boil off. CNG technology consists of compressing the gas at high pressure, from 200 up to 300 bar. The volume reduction is 200-300 times compared to those in standard conditions [2; 3]. However, the density of CNG is at most equal to about half of that of the LNG (218 kg/m\({}^{3}\) for methane at 300 bar and 20 \({}^{\circ}\)C versus 415 kg/m\({}^{3}\) for LNG at 1 bar and 162 \({}^{\circ}\)C), but the energy consumption of the compression process is about half of that required by liquefaction and regasification. Once compressed, the gas is stored in tanks placed on the ship and transported near a gas pipeline, where it is discharged at high pressure. Therefore, the most important phases of this transport technology are the following:  
* Gas treatment;
* Compression and possible cooling;
* Loading and transport with ships;
* Reception and unloading by decompression. The safety of this technology can be a problem, but since the discharge can take place offshore, some kilometers away from the coast, the risks remain confined and do not concern entire populated areas, as is the case for the LNG technology regasification terminals. In any case, the risk related to the storage of a flammable substance at high pressure was one of the problems that in the past did not allow an important development of this technique. Technological development with regard to storage tanks has allowed this transport system to become more efficient and safer. In particular, the introduction of composite materials has brought greater safety and, at the same time, a considerable weight saving [2]. The first CNG transport project using ships dates back to the 1960s and involved the use of a series of vertical cylinders [2]. This project was abandoned due to the very high cost of pressure vessels. Subsequently, the renewed interest in the technology has led to the development of some alternatives. In the last decade of the last century, [PERSON] and Stenning designed a new type of pressure tank called Coselle [2; 3] (from the English \"coil\", winding, and \"carousel\", bobbin).