Identification , Assessment and Control of Major Hazard Sources in the Construction of Liquefied Natural Gas Storage Tank by Air Lift

The aim of study is taking the advantages of the risk assessment of operating conditions (LEC), this study has carried out the risk assessment of the air lift process in the construction of full containment LNG storage tank with volume of 30000 m in one LNG projects. This performance enables the contractor to identify the major hazard sources in the construction and take safety control measures on site and facilitates to greatly reduce the safety risk in construction and effectively guarantee the smooth air lift operation.


INTRODUCTION
LNG storage tank, the main storage facility in the LNG industry chain, is usually constructed by the process of air lift.The air lift process is preferred for its faster speed of construction and lower cost.While, it has also brought challenges including higher construction difficulty, more safety risks and others.In the cases of inadequate preparation in advance or incomplete check in process, the dome may fall after rise due to power failure, lack of air volume or imbalance rise of dome.The safety risk will be increased for the long time of duration in the air lift process.Graham and Kinney (1980) first proposed Risk Assessment of Operating Condition (LEC), Wu et al. (2001), Fan et al. (2010), Wang et al. (2012) and Ji et al. (2017) have applied LEC to various engineering fields.Therefore, it is necessary to carry out the risk assessment for the air lift process in the LNG storage tank construction, to control the hazard sources in construction, reduce the construction risks, prevent the accidents and guarantee the safe operation.et al. (2017) have studied that the process of air lift for LNG storage tank dome roof is in the principle of buoyancy lift by air with micro pressure.The relatively confined space is formed between the dome roof and the concrete tank wall, into which the fans will constantly pump air with low pressure.The steel structure of tank roof will stably rise to the designed height in the predetermined route after getting rid of its gravity and the force of fraction with the tank wall.The process will be finally completed after the connection of the dome roof and the pressure-bearing ring at tank top.Wei et al. (2018) identified, assessed and controlled major hazard sources in the construction engineering, the construction procedure of LNG storage tanks is divided into four step preparation for air lift, commencement of air lift, dome roof in place and close out.

Xiting
The risk assessment of operating conditions (LEC) is applied by Luo et al. (2004) and Zhang et al. (2007) to evaluate the danger and hazard to operators working in the environment with potential risks: The larger the D value is, the higher the risk is.When D value exceed the unacceptable or unallowed risk limit, it will be identified as major hazard source.The values of L, E, C, D and the value range of D for  in the construction of full containment 30000 m 3 LNG storage tank in one LNG projects is shown in Fig. 1.Its hazard has been identified and its risk assessment has been carried out by Job Safety Analysis (JSA) to determine the major hazard sources.

RESULTS AND DISCUSSION
Control principle of the major hazard source in LNG storage tank construction air lift as follow:  After research the response measures for the major hazard source as follow: • Unprepared heavy work: The selected slings for lifting loads should meet the lifting capacity.All the slings should be checked carefully before lifting and the slings shall be changed if any defect is found.The warning shall be set in lifting area and personal custody shall be arranged.Seal plate being blown over: If the case occurs before the commencement of air lift, the pressure increasing shall be immediately stopped, the volume damper shall be slowly turn off.When the tank roof slowly declines to the edge support, the air blowers shall be turn off.If one seal plate is found blown over during the process of air lift, the volume damper shall be adjusted to keep the pressure balance in the tank and the seal device shall be checked after the tank top is stable.
One air blower stalling: If the case occurs during the process of air lift, the air suction control valve shall be immediately throttled down to reduce loss and the spare air blower shall start at the same time.If the case occurs in welding, the control valve shall be shut down and the spare air blower shall start."U" type pressure gage at tank top should be monitored to prevent the over pressure or low pressure.The volume damper shall be controlled.

One generator failing to work:
The backup power shall start to work and the air blower shall be restarted.The emergency shutoff valve of the air blower shall be throttled down.
Tank roof falling caused by the failure of all the draught fans: All the opening shall be closed to reduce tank roof falling speed to the lowest.If the air pressure in the tank still can't support tank roof to raise, the spare draught fan shall start while keeping all draught fans working simultaneously.If both two draught fans fails to work, the grinding machine shall start to cut off the emergent chain block which controls the air channel flashboard, in order to put down the flashboard.All the measures shall be taken to slow down the tank pressure drop.After the tank roof slowly declines to the edge support, the draught fans shall be changed or the electrical circuit shall be inspected and the remedial measure shall be taken.

CONCLUSION
Based on the practical conditions at site, the hazard level has been finally classified after the consultation with specialists, design engineers and construction engineers, assessment of likelihood of accident occurrence and the consequence caused.The details are shown in Table 5.The hazard source shall be identified as major when D value is over 70.Table 5 NO. 3, 5, 6, 7, 10 and 14 are major hazard sources in air lift operation.These major hazard sources should be improved and controlled in the construction to prevent the accidents.Similar to LNG storage tank construction by air lift, large projects with more factors in process, higher construction difficulty and more safety risks, can adopt the risk assessment of operating conditions (LEC) and control the major hazard sources in construction, in order to eliminate and reduce hazard, reduce cost and enhance efficiency.Combined with similar overseas projects, the typical problems in air lift have been analyzed and concluded and the response strategies can be determined by real-time dynamic tracking and critical control.
) = The consequence that may be caused in case of the accident.
Fig. 1: Process flow diagram of air lift

Table 1
Lingcai et al. (2011)er to the study ofLingcai et al. (2011)are shown in the following Table 1 to 4. A LNG Liquefaction Plant Project completed in Shanxi Province, China in 2017 year, the air lift process

Table 4 :
Classification of hazard level

Table 5 :
Identification list of hazard factors in air lift operation