Research On Integrity Technology For Gathering&Transportation System Of High Sulfur Gas Field

At present, there are many high sulfur natural gas fields under development in the world.In order to ensure safe, stable and economic development of high sulfur natural gas fields,Integrity management has to be conducted in the high sulfur natural gas, and the technologyof integrity is also researched. This paper is funded by sub-topic high sulfur gas fieldgathering and transportation process and security control technology of the national oil andgas major science and technology project large oil and gas fields and coal bed methanedevelopment. Some achievements have been obtained in risk assessment of gas gatheringstation, gas field gathering and transportation system maintenance period, gathering andtransportation system maintenance risk, operational integrity of gathering and transportationsystem, dynamic risk analysis of gas gathering station and its key equipments etc. The mainresearch content and research achievements are summarized as follows.1. Risk assessment of high-sulfur gas gathering stationBoth qualitative and quantitative risk assessment are implemented for gas gatheringstation. In the qualitative risk assessment, based on Dow chemical company, fire andexplosion index and API581, two methods are applied to evaluate and classify risk of keyequipments such as metering manifold, metering separator and methanol storage tank in gasgathering station. Then the results are compared and the advantage and disadvantage of eachmethod are analyzed. For quantitative risk assessment of gas gathering station, the genericfailure frequency of key equipments obtained from DNV Leak database, and then specificfailure frequency is obtained by modifying generic failure frequency according to API581.Fire&explosion and toxic effect regions of typical release hole sizes in equipments arecalculated by applying PHAST software to determine the risk of fire&explosion and toxicity.The results show that risk of wellhead separator is the largest and methanol storage tank’s isthe least. In addition, the acceptable risk value is suggested.2. High sulfur gas field gathering and transportation system maintenance periodresearchFormation conditions, influence factors and prediction of natural gas hydrate areanalyzed and summarized to prevent natural gas hydrate blockage frequently occurring inhigh-sulfur gas field gathering and transportation system. And the methods to prevent hydrateformation are proposed. Two methods to determine the gas hydrate pigging period arepresented. One is based on the gas supply efficiency, pigging is determined according to the relationship between the downstream pressure and upstream pressure in the pipeline, needingto strengthen the pipeline pressure monitoring. The other determines pigging period based onthe experience feedback. The pigging period is adapted according to the analysis of thenumber of gas hydrate blockage and gas hydrate blockage precursor in the last pigging period.Adjusting the integrity evaluation period is an important measure to reduce the risk. Thiswork presents two methods to determine inspection period based on pipeline risk level andremaining life, respectively. The inspection period based on pipeline risk level include twoways based on API581and API570, respectively. When calculating remaining life of theequipment, one way is to determine the actual minimum wall thickness based on the corrosionrate considering the statistical characteristics of corrosion depth. The other way use MonteCarlo method to assess remaining life of the equipment, fully considering the impact of thecomplexity of the pipeline corrosion factors.3. High sulfur gas field gathering and transportation system maintenance risk analysisThis work present pigging process and related devices usually used in wet gas gatheringand transportation for high sulfur natural gas field. Detailed analysis is performed on thehazards of pigging which mainly include pig selection risk, equipment risk, operation risk ofpig and blind plate, personal injury, pig stuck/blocked and blowby risk, poison and fire&explosion risk. The combination of LEC method and Job safety analysis is applied to conductsemi-quantitative risk analysis to determine the step with the highest risk. For receiving pigwith the highest risk in pigging, the OMT technology is used to establish the model for leakrisk analysis. The model fully considers the impact of risk influence factors. The riskinfluence factors include personal characteristics, task characteristics, characteristics of thetechnical system, administrative control, organizational factor, etc. finally, an example isgiven.4. Operational integrity study on gathering and transportation system of high-sulfur gasfieldThis paper evaluates process safety and identifies the opportunity to improve processsafety for gathering and transportation system of high sulfur gas field. Firstly, process securitysensitive output and input variables are identified, then zone boundaries of process securitysensitive output variable of each pipeline are determined according to the physical limits,regulations, engineering practice etc, and the boundary of operation for the equipment failureis set. The most interesting part is the zone boundaries identification for security sensitiveinput variables. Dynamic modeling and simulation methods are used for full dynamicsimulation of high-sulfur gas field gathering and transportation system. By simulating processstates in a variety of scenarios, the effect of each input variable on the whole gathering andtransportation system is carefully analyzed. Then the operation space is classified in order toprevent equipment degradation and accidents. This work performs quantitative evaluation ofprocess state and predicts the possible failure to obtain the operating guidelines for the safeoperation of equipment. 5. Risk analysis of the key equipment of the gas gathering stationRisk analysis of the separator is an important part of safety management in high-sulfurgas gathering station. Traditional evaluation method cause-consequence diagram is simpleand intuitive, clear logic with some limitations. Bayesian network is a relatively new riskanalysis method that can better express the uncertainties between variables and has a two-wayuncertainty reasoning ability, but it is less visually. Low liquid level of the separator isanalyzed by cause-consequence diagram and Bayesian network, and then results arecompared. Both methods’ advantages are made the best use of. In addition, to ensure the safeand reliable operation of wellhead separator in high-sulfur gas gathering station, dynamic riskanalysis is performed according to its operational status. Traditional methods of quantitativerisk analysis can’t effectively take advantage of operational status data of wellhead separatorfor dynamic risk analysis, and then easily make assessment result deviate from the actualsituation. In this paper, the event tree model is constructed to model the progress of abnormalevents of wellhead separator, and then safety barrier failure probability of wellhead separatorand abnormal event consequence likelihood are dynamically analyzed using accidentprecursor data based on Bayesian method. And the fuzzy loss rate method is employed toquantify the consequence of the accident loss. Then dynamic risk analysis of wellheadseparator is performed. All of these provide references for risk analysis and control ofhigh-sulfur wellhead gas-liquid separator and other related equipments.6. Fuzzy dynamic risk assessment of high-sulfur natural gas gathering stationDynamic accident modeling for a gas gathering station is implemented to preventhigh-sulfur natural gas leakage and develop equipment inspection strategy. The progress ofabnormal event occurring in the gas gathering station is modeled by the combination of faulttree and event sequence diagram, based on accident causal chain theory, i.e. the progress isdepicted as sequential failure of safety barriers, then, the occurrence probability of theconsequence of abnormal event is predicted. Consequences of abnormal events are dividedinto accidents and accident precursors which include incidents, near misses and so on. TheBayesian theory updates failure probability of safety barrier when a new observation (i.e.accident precursors or accidents data) arrives. Bayesian network then correspondingly updatesfailure probabilities of basic events of the safety barriers with the ability of abductivereasoning. Consequence occurrence probability is also updated. The results show thatoccurrence probability trend of different consequences and failure probability trend of safetybarriers and basic events of the safety barriers can be obtained using this method. The criticalbasic events which play an important role in accidents occurrence are also identified. Inaddition, taking the uncertainty of consequence severity into account, this work employsfuzzy loss rate to quantify accident loss of gas gathering station. The risk is the product of theprobability of failure and the consequence of failure. Risk level is determined as a function ofrisk value. The overall risk of the gas gathering station is determined as a function of riskvalue of different types of accidents and their importance degree. The overall risk level isdetermined based on the membership functions. All of these provide useful information for the maintenance and inspection of the gas gathering station.