Abstract. 1. Introduction

Transcription

1 Source and dispersion simulation of sour gas in an offshore complex of the sonda de Campeche R. Melo-Gonzalez, M.A. Resales-Torres, J. Martinez- Fernandez, J. Villanueva-Gonzaalez & J. Lopez-Ledesma Instituto Mexicano del Petroleo, R.I.Z.M., Av. Universidad # 72, Co/, f afro/era, MWw/o 2, 247&0, CW. Campeche, Mexico rmelog@mpsnet.com.mx Abstract In this paper, fire, toxicity and explosion consequences from source and dispersion of sour gas leakage/rupture in a process line or equipment are evaluated. Thermodynamics and transport phenomena models were employed by using the specialized software PHAST Professional version 5.1, licensed by DNV Technica. The input parameters were predominant weather conditions, physical and chemical properties of materials, scenario equipment layout, working conditions and surface characteristics. It could be noted that the worst consequences involve high pressure leakage whose leak diameter was bigger than % in; 1. Introduction Offshore marine complexes are considered high risk facilities because of the high inventories, humidity and saline environment that favors corrosion, high working pressure and temperature, so it is necessary to do a risk analysis every five years to detect potential process hazards. A

2 30 Risk Analysis marine complex is a facility located at about 50 miles from the coast and is composed of different platforms (drilling, production and compression) connected through bridges. The main objective of a complex is to take out crude oil from wells and split it into gas and oil (by separators and rectifiers), as well as to provide enough kinetic energy to them to be sent via separated lines for further treatment in onshore facilities. Toxicity, fire and explosion consequences from source and dispersion of sour gas leakage/rupture (typical scenarios) are evaluated by using Thermodynamics and transport phenomena through the specialized software PHAST Professional version 5.1 (DNV*), licensed by DNV Technica. Source Models are based on mass balance that results in the Bernoulli equation and dispersion models are based on Pasquill-Gifford theory, which clumps into atmospheric stability classes as a function of wind speed and solar radiation. There are some necessary input parameters such as: predominant weather conditions, physical and chemical properties of material, scenario equipment layout, working conditions and surface characteristics of the spilt occurs. The analyzed scenarios involve leakage diameter that are more likely to occur and diameters that have been presented during facility lifetime. The fire effects are evaluated through monitoring some radiation levels. Similarly, shock waves at different levels and H^S toxicity are assessed. 2. Methodology The evaluated scenarios come from the HAZOP study results (Hazard and Operability study) (Melo^'^), where deviations from normal operation can result in line/equipment leakage or rupture. The first step to assess a scenario is its description, which consists in determining the phase of the substance to be released, and the scenarios considered for this paper are gas leakages among the studied in (Melo**) are the following three more representative points of the process:

3 Risk Analysis 31 Scenario (1) Sour gas leakage fromfirststage rectifier Scenario (2) Sour gas leakage from second stage rectifier Scenario (3) Sour gas leakage from compression platform outlet pipeline. The next step is the inlet parameter definition, which means to set the physical and chemical source properties and in our study, was assumed a 15 minutes emission because this time is long enough to detect the leakage and to start the process section shutdown. To put the thermodynamic variables into PHAST 5.1, the RDIPPR v.1.40 (DNV^) database was used. The pressure and temperature of the emission was taken as the averaging operating ones. The reference ground level for the emission was the sea surface and the equipment layout was also considered by means a roughness parameter of The point of the leakage and its diameter was established taking into account their likelihood of occurrence and frequency. The toxicity effects on the personnel are evaluated by the receptor properties and the concentration levels regulated by the government, such as the IDLH (Immediately Dangerous to Life and Health) established by NIOSH (National Institute for Occupational Safety and Health) was considered. The IDLH H2S concentration of 300 ppm was monitored because this component shows the highest toxicity. To set environmental properties, some temperature, pressure and humidity data reported by the nearest meteorological stations were employed. The sampling time was 10 months (from July 1995 to April 1996) (Melo"'^) and their statistical means were used for simulation. The vapor cloud dispersion profiles were calculated through the Unified Dispersion Model (MDU) and TNT Model for explosion effects, which are employed by PHAST. This unified model considers form the regimen dominated by the emission pressure to the dispersion dominated by atmospheric turbulence. In this paper, only the three radiation and explosion levels of interest as a function of distance from source point (radio) are reported.

4 32 Risk Analysis 3. Results and Discussion In the table 1 are showed the study results, which can be interpreted by using table 2 and it can be stated that the worst consequences belong to the scenario (3) with diameters bigger than % in, because the release rate is the highest too. It's worth noting that this scenario has the highest working pressure and implies the highest release rate among the three showed. The simulation results for a 3/4-in are showed in the shady row of table 1. In the leakage at 71.6 kg/cm^, an operator must be at meters from %-in source point so that his exposition to H^S doesn't cause irreversible injury to his health, and at a distance further than meters so that shock wave doesn't cause any damage (at 1 psig of overpressure). Other way, at meters a 5 psig shock wave is presented, which cause damage to surrounding lines and equipment to the source point. It's interesting to point out that thermal radiation consequences aren't presented in this diameter leakage and it must be because the release rate isn't high enough and the vapor cloud is diluted with the air entrainment. The distance of 8.66 m. for LFL corresponds to the furthest extent where vapor cloud-air mixture has a combustible concentration. It can be seen that for % in diameter leakage and greater, there are likely affected areas by 4, 12.5 and 37.5 kw/rn^ radiation levels. This implies that for a big leakage or line rupture (the diameter of the high-pressure line head is about 24-in), the consequences are catastrophic.

5 TABLE 1. Matrix of causes and explosion, fire and toxicity consequences. Operating Conditions P (Kg/Cm') 1.7 T CO 68.0 CAUSES Scenario 0) 4> (in) % % 1 '/2 2 EMISSION Release Rate (Kg/Cm') L G TOX. 300 ppm H%S (m) CONSEQUENCES EXPLOSION Shock Wave (psig) as a function of Radio (m) FIRE Thermal Radiation (Kw/m^) as a function of Radio (m) LFL as a function of Radio (m) (2) % % 1 '/ (3) % % 1 '/ , on"

6 TABLE 2. Consequence Evaluation Criterion Shock Wave (psig) EXPLOSION Effects Severe damage to vessels and lines. Light damage to facility and process equipment. Partial damage to corrugate steel or aluminum panels. CONSEQUENCE EVALUATION CRITERION Radiation Level (Kw/nf) FIRE Effects Light damage to process equipment 100% lethality in 1 minute. Minimal energy required to develop a controllable fire 1% lethality in 1 minute. 2. Degree burns after 20 seconds. 0% lethality. TOXICITY Concentration of Effects Interest (IDLH en ppm) Maximum allowable H2S 300 Concentration that doesn't cause irreversible health injury, during an exposing time of 30 minutes.

7 Risk Analysis Conclusions The simulation results showed that a big diameter emission of sour gas represents a risk for the personnel, equipment and the environment. To compute these consequences help us to take the necessary actions for mitigating the risks. In the study done (Melo**), these consequences and the reliability results are the base for deciding where the improvements mustfirstdone. It's worth noting that these simulations are helpful to do contingency planes and locate or relocate equipment,fireand gas detectors. References [1] AIChE/CCPS, Guidelines for Chemical Process Quantitative Risk Analysis, ISBN : , New York. pp., [2] AIChE/CCPs, Guidelines for Evaluating the Characteristics of Vapor Cloud Explosions, Flash Fires, and Sieves, ISBN: x, New York, pp., [3] API RP 14G, Recommended Practice for Fire Prevention and Control Open Type Offshore Production Platforms, 3th edition, Washington D.C., December, [4] API RP 75, Recommended Practice for Development of a Safety and Enviromental Management Program for Outer Continental Shelf (OCS) Operations and Facilities, 1st edition, Washington D.C., May 15, 1993.

8 36 Risk Analysis [5] Crowl, A. D. & Louvar, F. J., Chemical Process Safety Fundamentals with Applications, Prentice Hall, Englewood Cliffs, New Yersey, pp., [6] DNV Technica, Inc., PHAST (Process Hazard Analysis Softwar Tools), Software solutions for Risk, Safety and Reliability, Release 5.1,Temecula, CA, [7] DNV Technica, Inc., RDIPPR v.1.40, Temecula, CA. (1997) [8] Hanna, S. R. and Drivas, P. J., CCPs, Guidelines for use of Vapor Cloud Dispersion Models, AIChE, First edition, ISBN : , New York, pp., [9] Hanna, S. R. and Chang, J. J., CCPS, Guidelines for use of Vapor Cloud Dispersion Models, AIChE, Second edition, ISBN : , New York, pp.,1996. [10] Melo G., R., Munoz P., A., Rosales T., M., Lopez L., J., Ceron B., J., Incremento de produccion en el Complejo Marino KU-A, GOFGBC- 1702, I.M.P., Representacion Institucional, Z.M., Cd. del Carmen, Camp., Enero, [11] Melo G., R., Garcia M., L., Martinez F., J., Rosales T., M., Estudio de Analisis de Riesgo del Complejo Marino POL-A y sus Plataformas Satelites, GOFGCO-1206 I.M.P., Representacion Institucional, Z.M., Cd. del Carmen, Camp., Diciembre, [12] Melo G., R., Garcia M., L., Villanueva G., J., Lopez L., J., Estudio de Analisis de Riesgo del Complejo Marino AKAL-J y Plataformas Satelites, GOFGCO-1207 I.M.P., Representacion Institucional, Z.M., Cd. del Carmen, Camp., Diciembre, 1997.