In 2013-2014 in Brescia (Italy) a project related to the study and realisation of a Dual Fuel (LNG-Diesel) system for heavy-duty trucks was developed. This project was sponsored and funded by AIB (Associazione Industriale Bresciana) and CCIAA (Camera di Commercio Industria Artigianato e Agricoltura di Brescia). We performed a qualitative risk assessment of this system in order to understand if it is safe in comparison to existing diesel vehicles.

The use of LNG (acronym for Liquefied Natural Gas) as fuel allows to obtain several advantages in terms of system weight, installation space, energy density, and fuel efficiency. Furthermore, LNG is able to reduce the environmental impacts because of the decrease of NOx, particulate, CO2, and noise.

Also recently, as underlined by Directive 2014/94/EU, natural gas has been identified as currently one of the principal alternative fuels with a potential for long-term oil substitution, and LNG might also offer a cost-efficient technology allowing heavy-duty vehicles to meet the stringent pollutant emission limits of Euro VI standards as referred to in Regulation (EC) No 595/2009 of the European Parliament and of the Council.

In such a context, the installation of a Dual Fuel system with LNG and diesel on a heavy-duty truck represents a solution for achieving environmental benefits for existing diesel vehicles through a retrofit action. Into the system (Figure 1), LNG is stored into a cryogenic tank (Figure 2) and delivered as gas to the engine (thanks to a heat exchanger located between the tank and the engine). HVM High Vacuum Maintenance S.r.l. (Livorno, Italy) was the manufacturer of the cryogenic tank, Landi Renzo S.p.A. (Corte Tegge – Cavriago, Italy) was the system’s designer, and Ecomotive Solutions S.r.l. (Serralunga di Crea, Italy) realised a prototype of this system into a Euro 5 heavy-duty truck.

Intrinsically, LNG is characterised by some properties: it is a cryogenic substance, it is able to generate large volumes of gas from small amounts of liquid, and it is flammable when its concentration is about 5 ÷ 15 % by volume in air. From these properties, some potential risks and consequences of LNG can arise: fire (pool fire, jet fire, flash fire), explosion (Vapour Cloud Explosion, Boiling Liquid Expanding Vapour Explosion, Rapid Phase Transition), exposure to cold (hypothermia, frostbite or cold burn, effect of cold on the lungs), and exposure to gas (asphyxiation). In our study about risk assessment of the Dual Fuel system, we were interested to analyse these consequences in terms of harm to people, and of damage to  the environment and structures starting from some adverse initiating events that can occur in usual modes of operation of a truck.

In order to perform a precise risk assessment, we have exploited some methods and techniques available in the literature that we have combined into a methodological framework that is able to identify, analyse, and evaluate the risks. An important source of information to conduct this risk assessment has been represented by a historical analysis of major LNG accidents and near-misses. Indeed, this analysis has allowed us to understand the different scenarios that may arise from real events, focusing on the causes and consequences that occurred more frequently.

Generally, the LNG industry, transportation and storage have a good safety record. Although, over the years some incidents have occurred and, in particular, until February 2014 we recorded 25 accidents and 23 near-misses (Figure 3). Note that only 33 of them are described in the literature in detail in order to reconstruct the accident scenario: for example, the causes of these 33 incidents are depicted in Figure 4.

One of the most serious incidents related to LNG occurred in Cleveland, Ohio (US), in 1944: a mechanical and/or electric fault in a storage facility caused flash fire, RPT and pool fire and then over 120 deaths and more than 200 people injured.

Moreover, LNG trucks were involved in 4 accidents: the cause was the collision and the consequences were the same, i.e. an immediate fire, a BLEVE, and a fireball. In particular, these accidents occurred in Tivissa (Spain) in 2002, Mexico City (Mexico) in 2013, Murcia (Spain) in 2011, and Hunan (China) in 2012. Finally, at the date of our analysis (February 2014) there was no information about accidents of vehicles with a Dual Fuel system (LNG-Diesel).

Regarding the Dual Fuel system (LNG-Diesel) under consideration, our study reveals that this system is characterised by a similar risk in comparison to a traditional diesel system. Indeed, although LNG presents some risks of fire and explosion, and some peculiar risks, the combination of fuel properties, Dual Fuel system design, and adoption of safety measures makes the risks of this new technology comparable to traditional diesel systems. Some examples of prevention, mitigation and recovery controls are reported in Figure 5. Note that these safety measures shall be adopted during the entire system’s life cycle. Fundamental technical documents and regulations for identifying a complete set of measures are: Regulation No. 110 by UNECE, EN 1251-1, EN 1251-2, and EN 1251-3 European standards, ISO 12991 and ISO 12614 (all parts) International standards, NFPA 57, NFPA 52, and NFPA 59A American standards, Operations manuals by NexGen Fueling and by HVM, and LNG Material Safety Data Sheet.

A similar level of risk between the Dual Fuel system and a traditional diesel system was obtained thanks to the application of methods and techniques (e.g. bow-tie analysis, Fault Tree Analysis, Failure Mode and Effects Analysis, and risk matrix) that compose our methodological framework. In particular, risk identification allowed to recognise releases of LNG, LNG vapours, and natural gas as critical events in the several modes of operation of the vehicle (the use of the truck on public roads, its parking outside or inside). These events can originate from some negative causes (e.g. collision, radiation from fire, or tampering with the vehicle) and, thus, can produce severe consequences (such as fires and explosions). The possible pathways that can provoke a consequence are quite articulated, and for the occurrence of a consequence a critical event, the “failure to detect the release”, and the “failure to detect the fault” are required.

Risk analysis highlighted failure modes of components, sub-systems, and connectors of the Dual Fuel system arising from the negative initiating events (previously identified). In these conditions, the Dual Fuel system can fail its function because of separation, fracture, or embrittlment that provoke some local and final effects, such as releases, fires, explosions, exposures to cold or to gas. If these effects happen, the following aspects should be taken into account: (1) LNG evaporation can extend at a distance the range of consequences and damage stemming from vapours ignition, (2) effects of radiation arising from LNG ignition and pool fire are more severe than those produced by diesel spills, (3) unlike diesel, LNG could produce individual risks related to the exposure to cold, and (4) in general, LNG is odourless and colourless and, therefore, also a large release can not be detected by people. Moreover, indoor parking can confine vapours, producing asphyxiation and hypothermia. However, an analysis of the likelihood of damage pointed out that the Dual Fuel system has a lower probability of leakage in comparison to a diesel system because of the presence of a double-walled tank and superinsulation, and components are designed in order to ensure proper protection against heat, and the fuel tank is designed to withstand severe temperatures and environmental exposure. In addition, a diesel spill has some additional risks due to persistence of the liquid pool resulting from the spill, and, therefore, duration of risks and dangerous situations resulting from the diesel spill can be potentially extended.

As a consequence, we deemed the likelihood of the consequences for the Dual Fuel system lower than the likelihood of a diesel system, while the severity of consequences for the Dual Fuel system is higher than the severity of a diesel system. This consideration leads directly to the result that the risk between the Dual Fuel system and a diesel one is similar. Finally, we assessed the risk as acceptable: existing preventive and mitigation controls are sufficient for managing the potential risks of the Dual Fuel system, and the Dual Fuel system is adequately safe.

Details on methodological framework and results of this study can be found in our paper: Stefana, E., Marciano, F., Alberti, M., 2016. Qualitative risk assessment of a Dual Fuel (LNG-Diesel) system for heavy-duty trucks. Journal of Loss Prevention in the Process Industries, 39, pp. 39-58.