Global demand for “ green and clean energy” is growing strongly because of the high oil price. It is estimated by the international authority institute that natural gas will be the most demanding natural source, with an increasing proportion of primary energy consumption. Abundant natural gas reserves provide the fundamental conditions for the steady development of the natural gas industry. However, most of the large natural gas resources are located in the desert areas that are far away from the places where the gases are consumed; meanwhile, there are often large areas of oceans and complex landforms in between. Long distance pipelines and gas transportation across the oceans are often restricted to cost and technical issues. The development of liquid natural gas (LNG) provides an economical and feasible method for the long-distance transportation of natural gas.
Since the first LNG receiving station technology was introduced by CNOOC at the end of Last century, CNPC and Sinopec have also begun to build the LNG receiving stations with high attention, and develop their own teams. Petro China`s Dalian LNG terminal project and Petro China’s Jiangsu project, as the pilot projects have entered the peak period of construction. Due to the domestic LNG receiving station engineering technology and application is still in the process of exploration and construction, project management has many known and unknown risks. The issue that how to reduce the quality and safety risks in the process of construction and production operation has raised high attention.
Ⅰ. Dangerous conditions of LNG receiving station leakage and explosion:
(1), Combustible materials: LNG is a cryogenic liquid that can be easily heated to NG by the surrounding air. When LNG is released in large quantities, the vapor cloud density will be less dense than air; will flow along the ground or spread to lower level, and will ignite or even reignite, causing explosions.
There are Normally three types of LNG leakages or spills:
- ① LNG will react violently with some materials( fluorine, chlorine,etc), becoming brittle, delicate, cold shrinkage, resulting in the damaging of equipment and leakage of LNG, then cause fire.
- ② LNG itself is not very corrosive, while, due to the existing defects of the storage tanks( such as stress corrosion, intergranular corrosion, crevice corrosion , etc. ), and electrolysis may happen due to the accumulation of the rain, the metal shells storage tank and the contacting of soil, which then lead to the spot corrosion in large areas, resulting in LNG leakage.
- ③ Problems happen in the design and construction of production or storage and transportation devices, which can not meet the requirements of normal production or cannot find leakage in time.
(2) Comburent. The main comburent is air. The temperature of the low-temperature circulating working liquid is lower than that of the air liquefaction, and the air on its surface will condense and form oxygen atmosphere, forming combustible auxiliary.
（3）Ignition sources: In the presence of air and LNG, any hest source can be a trigger to an explosion. And these potential heat sources and materials that may trigger the explosion mainly include:
① Fire and sparks;
②Electrostatic spark;
③Lightning protection facilities or grounding damage or failure, lightning strikes.
④Electric leakage may caused by improper installation and operation against the electrical circuit or electrical equipment, poor maintenance and grounding, zero damage or failure.
Ⅱ,Leak and explosion Analysis on the LNG Terminal
1, Material hazardous analysis
The main ingredient of the LNG is methane, a flammable and explosive gas, which is prone to combust and explode in case of open fire and high temperature. NG can form explosive mixture when it is mixed with air. The LEL of the gas is 3.6~6.5%, and upper limit is 13~17%, and will act a violent chemical reaction with fluorine and chlorine. At low temperatures, LNG vapor is heavier than air and can spread far at low places and will lead to back burning when exposed to open flame. At room temperatures, NG density is smaller than air, and will spread rapidly in the air.
2, Technique hazardous analysis
（1）(1), The raw materials and products are combustible substances, most of the areas inside the station are explosion hazardous zone 2. During the operation process, the damage of LNG storage tank, BOG pump, pipelines, flanges and other facilities will cause NG leakage, which then lead to fire and explosion accidents.
(2), The application of the high speed operating equipments, (such as pump etc), may cause sparks, and then become the ignition source, and a potential explosion.
(3), There are a large number of loading and unloading tanks in the LNG transportation process, loading and unloading terminals and loading islands in the station. During the loading and unloading process, wrong operation of storage and the aging of facilities may lead to loading and unloading failures, causing LNG leakage and then becomes combustible sources.
(4), If the operators didn`t maintain or do the inspections following the related rules or operating procedures, it`s easy to form a spark and other ignition sources.
Ⅲ. LNG Receiving station leakage and explosion early warning proposals
1.Hidden Leakage Detection
Detect the suspended pipelines by using the laser methane remote detector, scanning the pipeline within 80m range; detect the places that are hard to reach by using the laser methane remote detector. During the detection, the detector will send out a beam laser and scanning the places that with a hidden leakage, by reading the data displayed on the screen, please will know if leakage happens. The laser methane remote detector is found with high accuracy(ppm level, one part per million concentration), fast response(0.01s),long distance detection(80m),easy, convenient and accurate etc. advantages. It solved the issue that the some part of the pipelines and natural gas leakage is hard to be detected by using traditional detectors. Through the comparisons at different locations, the laser methane remote detector was proved with high advantages in early warning of potential gas leakage, and is the foundation of improving operations and an effective instrument to supplement the disadvantages of traditional detectors.
2.Open path methane detection system---makes early warning possible.
In the process of LNG loading, unloading, storage and gasification and transportation and other production process, there are slight changes in parameters ( temperature, pressure, gas concentration etc.) Through the detecting of slight changes against the facilities` temperature and other factors, and also the detection of tiny methane leakage, the open path detection system realized the comprehensive monitoring of tanks area, loading and unloading port, loading island, BOG pump room, SCV,ORV facilities etc., thus make early warning possible at the very beginning; make detailed assessment and give related proposals, thus found a comprehensive monitoring system for the receiving station. Early warning, early issue solving. And through the analysis of the accumulated large data to establish a on-site security warning model, find issues at the very beginning of the improper operation status of the system happens. Through the analysis of the data and diagram, people get to know the estimated leaked concentration on site, and prevent the explosions and accidents from happening during the operation on site. Generally, the open path methane detector can effectively and precisely detect methane leakage, thus make early warning possible to protect people from danger and ensure the safe operation of gas storage.
