Risk Identification, Assessment And Control For Operation Of Wet Sour Gas Gathering Pipeline System

The dissertation focuses on process risk assessment approach and its application for sour gas gathering pipe system, which is a part of the National Science and Technology Major Project—Research on Gathering Process and Its Safety Control of Sour Gas Field. Plenty of progress has been made on the risk assessment methods at stable or dynamic state, process risk identification and safety control technology of wet sour gas pipe operation by means of theory research, numerical calculation and computer simulation. The main works are summarized as follows:1 Risk Characters about Basic Process Parameters of Sour Gas Gathering Pipe SystemDifferent approaches for calculating the process pressure of wet sour gas pipe are analyzed, two phase flow pressure correlations are utilized to determined pressure gradient and temperature distributions at stable state. Water Gas Ratio (WGR) is research on the influence of pressure and temperature, to clear the risk analysis of hydrate block. Relationship between flow rate and component and nature of sour gas is analyzed. A method of identifying the risk of hydrate block is studied, water of supersaturation is judged in transmission line of wet gas. H2S Concentration impact on Hydrate formation conditions is studied, and the hydrate blocks risk can be determined by the characters of temperature and pressure along the pipe.2 Quantitative Risk Assessments on Wet Sour Gas Gathering ProcessA Hazard and Operation Analysis (HAZOP) is carried to identify the process deviation that influent on the safety of outbound pipe. Combinating with the dynamic simulation technology, a method for quantifying the HAZOP deviations is proposed. Process model is setup to simulate the deviations that are not acceptable for gathering process. Overpressure in a wet gas gathering process with high pressure is studied, and blow down quantities are calculated based on the correspondent forms and causes. Theories about identifying overpressure and safety blow down in sour gas gathering station are posed. The deviation of relief valves capability in fire conditions that identified by HAZOP is studied deeply. Fire characters and forms in high pressure gas gathering equipments are analyzed, and the overpressures caused by fire heat are simulated. A dynamic model for simulating inhibitor injection is established, the dynamic characters of hydrate inhibitor concentration are gained for quantifying the inject index. 3 Risk Characters of Wet Sour Gas Pipe in Unnormal Operation ModeWater in wet nature gas is usually in saturation state, and hydrate formation is favored by low temperature and high pressure, so increasing the temperature and inhibitor injection are the effective measures in stable state of a high pressure gathering pipeline. But, there are wide range changes for the temperature and pressure at the start and shut down state, the risk of form hydrate and chock the pipeline with overpressure is increased greatly. In this paper, a dynamic model of buried pipelines combined with transportation and thermodynamics model is established. The model is used primarily to estimate hydrate potential during startup, shutdown in wet gas pipelines. Examples of application is presented, the results showed the pressure and temperature change characters, and safe time for avoiding hydrate formation is gained. Characters of free water dropout are analyzed at different WGR rate. In this paper, a method for judging the critical flow velocity for avoiding liquid loading is proposed with quasi-mechanics balance equation, and combining with calculating tools for physical property, its fitness is discussed. Relationship between different slug frequency and slug pressure shock is studied for risk control.4 The research of Technological Parameters Influence on the Internal Corrosion of Wet Sour Gas Pipe ProcessInfluence of operation parameters on the internal corrosion of wet sour gas pipe is studied, and risk assessment. Internal Corrosion Directly Assessment (ICDA) technologies and theories are utilized to evaluate the risk for wet sour gas pipe. Three aspects and factors are considered, Technique routes of flow regime determining and corrosion speed calculating are developed, and examples are completed. The relationship between flow parameters and corrosion rate is studied, and other factors correlated with wet sour gas pipe are also analysis deeply.5. Leaking Process of Sour Gas Pipeline and Its Control OptimizationTwo main types of risks—hydrogen sulfide toxicity and gas explosion caused by the leakage of high-sulfur nature gas were studied through HYSYS simulative software. The critical parameters of the high-sulfur nature gas pipeline process, the most potential amount and the worst consequences were estimated, the maximum concentration of H2S and the worse effect of explosions besides the acid gas pipelines were calculated by using Pasquill-Gifford model and TNT equivalence method respectively. The probits based fatality and destroy curve were given through examples, and some improved conclusions based on inherent safety in design were proposed.Most of the existing calculation methods were based on the conditions of steady leaking or dynamic leaking after a simultaneous closure of emergency shut down (ESD) valves. Hence, an equivalent model of gas leakage combined with process of nature gas pipeline is established by using HYSYS simulative software, and the leaking rates in steady and dynamic states are calculated under conditions of closure delays for ESD valves. Based on leak and fire accidents, the shutdown systems were designed to operate in 4 separate priority levels that ranging from a minor process upset (Level 4) to complete field shutdown (Level 1), the scheme also associated with auto and human start, so the scheme can raise the response performance, and prevent the alleviate accident consequence.