SAFETY ASSESSMENT OF NATURAL GAS COMPRESSOR STATIONS

A. Pijnacker Hordijk

N.V. Nederlandse Gasunie, The Netherlands

J.F. Sauer Gaz De France, France

G. Linke

Ruhrgas, Germany

A. Cappanera Snam, Italy

ABSTRACT

Gas installations are designed to ensure a maximum level of safety. This safety level has been reached through improvement of knowledge over more than fifty years of activity. Nevertheless, gas companies have a concern to constantly improve the safety associated with the operation of gas installations. Four companies started a research project for the development of a common and validated methodology for the safety assessment of their natural gas compressor stations, which resulted in a methodology and a software tool. It was necessary to assess the significance of undesired events in terms of likelihood and consequences and moreover to compare threshold values given in the various regulations. The combination of both parts led to an objective evaluation of the plant safety being used to support management decisions regarding design, construction and operation. The poster illustrates the structure of the fault tree, the functionality and reliability of safety devices resulting in various outputs such as heat radiation graphs.

RSUME

Cinquante ans damlioration des pratiques et des connaissances du mtier gazier assurent une conception des quipements gaz au plus haut niveau de scurit. Malgr cela, les compagnies gazires restent toujours soucieuses daugmenter encore la scurit lie au fonctionnement de leurs installations. Cest dans ce but que quatre dentre elles se sont lances dans un projet de recherche ayant pour but le dveloppement dune mthodologie commune dtude de scurit des stations de compression et pour rsultat un guide de savoir faire et un logiciel. Ce projet a notamment ncessit dvaluer les consquences et les probabilits doccurrence des vnements indsirables et de comparer les diffrents seuils rglementaires deffets. Cela a permis daboutir une mthode dvaluation objective de la scurit des installations, pouvant servir de support dans les phases de conception, de construction et dopration. Le poster met en avant quelques rsultats du projet, dont la structure de larbre des causes, le rle et la fiabilit des systmes de scurit et des rsultats de calcul tels des graphes disoflux.

INTRODUCTION For three years, four major European Gas Companies, GAZ de FRANCE, GASUNIE, RUHRGAS and SNAM, have worked together in order to build a common methodology for performing safety studies on onshore natural gas compressor stations. Their aim was to list all the potential hazards on such a plant, to assess their consequences and likelihood and to obtain the different safety zones inside and around the plant. Still, mandatory differences and specific requirements by local authorities were constantly taken into account so that, at the end, this common methodology would fulfil the specific purposes of each participating company. The project eventually leads to a know-how guidebook on one hand and prototype software on the other hand This document explains both the elaboration of the project and its results. For the elaboration part, four steps are described, that are the definition of a Compressor Station common to all four companies, a description of the fault trees and its construction, a draft of the hazard analysis methods used to find the potential hazards and top events, and the importance analysis conducted on the fault tree basic events. For the results, beside the hazard analysis and the fault tree itself, the project involved one task dedicated to the construction of a safety/reliability database which contains all the figures linked to fault tree basic events likelihood, and another task which dealt with the consequence calculation methods, tools and software that could be used when performing a Compressor Station safety study. Then the combination of the likelihood and the consequences of each accident scenario lead to a safety evaluation (in terms of risks) described in the guidebook. A description is also given of the prototype software that has consequently been developed to gather all those information (map of the station, database, fault tree, consequences calculations...) and provide to the companies an integrated tool for assessing the safety of its compressor station. THE COMMON RESEARCH PROJECT In order to carry out the safety assessment both the failure frequencies of the relevant incident scenarios and the connected physical consequences have been calculated following some different steps:

1. definition of a typical compressor station; 2. the approaches CFT and HCS (Hazard Categorisation Scale) 3. definition of a global compressor station fault tree (CFT); 4. developing of a data base; 5. the importance analysis of the incident scenarios; 6. consequence analysis; 7. safety evaluation.

All these steps have been defined and described in a common Methodology Manual that defines the principles and the different approaches to be followed in a compressor safety analysis, giving details about the techniques and the tools available in the Companies participating to the project, and in a prototype software tool, named CAT (Compressor station Assessment Tool), that can allow a faster application of the methodology principles. In the following paragraphs a schematic description of each developed step is reported.

The Definition Of A Typical Compressor Station The first step of the activity was to define the common terminology and the identification of a typical compressor station, in order to have a unique and clear definition of the kind of plant to be studied. The typical plant considered is a transmission natural gas compressor station and is composed of:

one or more compressor units; gas piping (including filters and coolers); electric power generation; control and safety devices; station auxiliary equipment and facilities.

In order to have a more complete identification of the typical compressor station a matrix description of the compressor station components with the relevant links, plausible failures, possible consequences and installed safety and protection systems has been produced. Also a specific TAG-numbering system has been applied to all equipment so that they are easily identifiable throughout the project, meaning in the fault tree, database and software. In this common definition any information about design, operational criteria and preventive safety measures applied by each Company have been utilised. The Approaches CFT And HCS In order to apply a safety assessment of the Compressor Stations two possible approaches have been defined by the WGCS group (see also ref. [1-2]): The Hazard Categorisation Scale and the Compressor Fault Tree. These two approaches can be used separately for getting a final safety evaluation or can be combined. The way to be followed depends on the available data of the plant analysed, on the requirements of national legislation or the Company goals of the performed analysis. The two approaches differ mainly for the different path followed to select the relevant scenarios to be analysed in a safety evaluation. In the HCS approach a consequence evaluation on people and/or structures filter, the relevant incident scenarios thats the failure frequency has then to be calculated. In the CFT approach the filter action is performed directly on the failure ratio. Figure 1 shows, in a schematic way, the different steps of each approach and the connections existing.

HAZARD ANALYSIS: Toolbox, HAZOP, FMEA, PHA

List of Potential Hazards - Undesired Scenarios

LIST OF TOP EVENTS / UNDESIRED EVENTS

FAULT TREE ANALYSIS - FREQUENCIES EVALUATION

CONSEQUENCES EVALUATION

SAFETY EVALUATION

HCS APPROACH CFT APPROACH

HCS application to Potential Hazards

List of Top Events Construction of

the Fault Trees for Undesired Scenarios

CFT construction

CFT analysis

List of Undesired Events - CFT modified

Construction of Fault Trees for Top Events

if a CFT is available

if a CFT is not available

Figure 1.

Figure 2: Compressor Fault Tree

It was eventually decided to combine CFT and HCS approaches together. HCS is used for its ability to identify hazards as a qualitative approach and CFT for the quantitative approach. The previous step of the methodology for the safety assessment on a natural gas compressor station led to the characterisation of a list of Undesired Events (the so called Top Events) starting from a list of Undesired Scenarios and Potential Hazards. These were determined on the basis of the results of the matrix descriptions analysis, experience of associated companies, the historical data collection and the application of hazard identification techniques. An integrated fault tree has been built, named the CFT (Compressor station Fault Tree), to identify the incidents and to obtain the causal chain that allows estimating the probabilities, through a qualitative failure analysis. The CFT has been carried out including the following seven undesired events:

1. explosion in enclosure; 2. explosion in hall; 3. explosion in gas path; 4. fire in enclosure; 5. fire in hall; 6. fire outside hall; 7. fire at vent stack.

A fault tree has been constructed consisting of 200 basic and the causal chain of the selected incidents. With this fault tree it is possible to calculate the relevant probabilities of incidents leading to an undesired loss of containment. Figure 3 gives an example of one of the 50 branches of the fault tree. Developing Of A Reliability Database A dedicated reliability database has been developed with support from Det Norske Veritas (DNV), for the probability assessment of incidents (the Undesired Scenarios considered in the CFT). In this database the equipment of the typical compressor station has been considered (see ref.. [3-5]); however the figures of the database can be customised, during the analysis of a compressor station, so the database is very flexible and fits the equipment of the plants of the four Companies. The reliability database is composed of several types of data: - leak frequency data: a mathematical model, depending on the diameter and the leak

equivalent diameter, for piping, valves and flanges expresses the leak frequency. For equipment (such as filters, pumps, heat exchangers) the data come out mainly from offshore equipment databases.

- Safety systems failure data: the PDS method has been used to quantify failure probabilities for safety systems. The PDS method (PDS is the Norwegian acronym for reliability of computer based safety systems) was developed in the late eighties/early nineties in a joint industry project by SINTEF, Norway sponsored by oil companies and vendors of safety equipment.

- Ignition probabilities: ignition probabilities have been estimated using an ignition model developed for offshore QRA, and describes ignition probabilities for releases within offshore modules as a function of time.

The ignition probability modelling is divided into three main parts:

Dispersion modelling Ignition sources Ignition probability model

The final result is an ignition probability value that depends on the outflow rate of the flammable/explosive fluid. In this study the estimation of human failure probabilities has been based on the TESEO (Tecnica Empirica Stima Errori Operatori), see ref. [6]) method. The human failure probability is defined as the probability that an operator does not complete a job successfully when requested by the system, within a certain maximum time limit permitted by the system. The TESEO model calculates the failure probability as the product of five factors:

Failure probability = K1 K2 K3 K4 K5 The factors Ki are assigned a number as described below. K1: Type of activity (routine, requiring attention) K2: Time available, routine activities K2: Time available, non-routine activities K3: Operators skills (expert, average) K4: State of anxiety (emergency, normal) K5: Environmental ergonomic factor (excellent microclimate, normal)

The result is the probability that the operator fails the requested operation. Estimation of rare events were considered as the problem of estimation of frequencies or probabilities in situations where the available information is scarce has been treated by standard Bayesian technique. All the collected data are the basis for the calculation of failure frequencies of basic events and utilising the fault tree of top event probabilities. The Importance Analysis Of The Incident Scenarios The Component Value Analyses is used, in order to prioritise the contributions of the different basic events and global assessments. The Compressor Fault Tree (CFT) consists of more than 200 so called basic events. To calculate the risk of a Compressor Station one would not only need the consequences of the relative events, but also the chances of occurrence of the given events. In order to be able to minimise the events from the fault tree it is necessary to identify the most important ones. In the standard Fault Tree software programme used by the WGCS members four methodologies are mentioned and implemented to calculate the component importance. These methodologies are: - Vesely Fussel - Birnbaum - Criticality Importance - Smallest cut set The importance analysis is also included and automatically applicable by the prototype CAT software and can give a very fast indication to the user on the contribution of a given component to the overall calculated consequence or risk value. It is possible in this way to analyse different possible technical solutions focusing the attention to the relevant unsafe contribution of each component in the compressor station.

Consequence Analysis The consequence evaluation activity was expressed in terms of severity of the damage on

people and structures based both on threshold of the physical effects (heat radiation, overpressure, and so on). This activity aimed at three points.

1. the objective was to list the physical phenomena to be studied and to give theoretical

models to simulate them. 2. to give indications on some software models, available among the Companies, that can be

used for performing partly or totally the consequence calculations connected to the undesired events that can be identified as hazardous through the compressor station safety study.

3. the goal was to give an indication on how to interpret the results given by the software models, in terms of physical consequences.

The effects considered were heat radiation, either from a jet fire or a pool fire, overpressure from gas plume ignition (unconfined release) or from explosion (confined and semi confined release, see ref. [8-11]).

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Figure 3: heat radiatian field

Therefore, it was necessary to implement modules for: - leak size dependent un-steady mass flow calculation - calculation of LEL shape or spill area - flame length estimation - calculation of time-dependent heat field - conversion of thermal radiation into dosage fields - estimation of overpressure effects - assessment of thermal threat and overpressure effects on people - et al. (see ref. [12]) Safety Evaluation

The main objective of the safety study is to achieve information about the safety level connected to the considered activities inside and outside the Compressor station areas.The safety assessment goes through the combination of the expected frequencies of the undesired scenarios in terms of occurrences per year and the estimation of the expected damages per one event. The

calculation results can be managed in order to provide safety indications through some matrixes, maps and indexes to be used for ranking different compressor stations. It is possible to : - look at the results (like heat radiation or dosage) at a single location, along a line or inside a

defined area - draw iso-risk contours of iso thermal curves - generate dosage maps or risk profiles. Single gas or oil release sources can be analysed in depth as well as sets of sources (e.g. inside the compressor hall or in the enclosure) or as a complete station. The next figures give an example of the output of such evaluations. Of course it is possible to generate this as an table of figure or an overall integrated risk figure for the entire station.

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Figure 4: 3D heat fields Figure 5: Dosage fields THE PROTOTYPE SOFTWARE TOOL CAT

A software package for the safety assessment of the compressor stations has been developed according to the CFT approach described in the Methodology Manual. The software allows the application of the methodology in a quite simplified way, in order to have a fast assessment of the level of safety of a compressor station and compare it with other similar plants and/or identify the safer areas inside the compressor station itself. CAT is based on :

a schematic representation of the compressor station area and facilities; the reliability database for the estimation of the frequency of the Top Events of the CFT; simplified mathematical models for the calculation of consequences (heat radiation in

case of fire, overpressure in case of explosion, Probit equation for the evaluation of the damages).

Since the prototype software is based upon simplified models it should rather be used as a first approach during design stage or as an analyses tool to improve the safety of the station. The application of the methodology can then be performed to analyse the compressor station in a deeper way. However the two methods are independent and can be used also separately.

D E V ELO P M E N T O F a C O M P R ES S O R A S S E S S M E N T TO O L

Fa ilu re frequencyconvertion

CAT - Com presso r Assessm ent Too l

SAFETY EVALUATION

C onseq uence

M ode l

Data handling- Input / resu lts /output

R ISK

FF - fo rm u las

CS-S tation DA TA

failurefrequency

exposure [yr's]length [meters]diameter [inches]pressure [bar]# stations [-]# equipm ent [-]

[ ]F d d

fo r m m d D

d

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A pplicab le fo r :enclosurehalloutside

y valvesy process pipingy flexiblesy buried pipesy flangesy filtersy heat exchangers (tube)y heaters (e lectrica l)y press. vesselsy storage vesselsy pum psy com pressors

A pplicab le fo r :com ponent failuresleak size d istributionhum an errorsfailure on demand

station lay-outenvironm entconditions

Figure 6: Compressor Assessment Tool

VALIDATION OF THE METHODOLOGY AND TOOLS The methodology and the software tool CAT have been validated by the Gas Companies on their own installations. The task was, mainly, to test the methodology and the software, and have a feedback on the carried out work, in terms of improvement of the method, applying guidance. The methodology was applied to real plants in order to verify the completeness of the methodology itself, the availability of all the tools and the consistency of the results with those obtained using other methods. The validation of the methodology and the prototype software itself be presented in a separate paper at another occasion. CONCLUSIONS In this paper an overview has been given of the three-year work carried by four companies (i.e. Gasunie, Gaz de France, Ruhrgas and Snam). The project has produced the following deliveries:

a list of unintended modes of operation (top events) the relevant parameters (so-called basic events) influencing the occurrence of such top

events a complete integrated Compressor Fault Tree connecting all relevant basic events in a

logical manner the importance of each parameter (based on suitable and well-known criteria)

reliable data on the likelihood of an basic event such as failure frequencies of safe guarding equipment, relevant components or probabilities for loss of containment, the presence of ignition sources or the occurrence of human error.

The results of this common research project can be used to optimise aspects from a design, maintenance, operational or safety point of view. Possible features will be : sensitivity of components in relation to the overall safety of the plant; investigation of the influence of different design criteria for the same component; contribution of the test interval of safe guarding components or system to the overall

failure frequency; relative ranking of the safety level of several compressor station in order to prioritise the

weakest spots in a installation; demonstration of safety contours to the authorities; quantitative assessment to be used in official safety reports;

With help of the Hazard Categorization Scale, the Compressor Fault Tree together with the Compressor Assessment Tool which are all laid down in the so-called methodology manual one is able to assess the risk level Future research will focus on the enhancements of the software. ACKNOWLEDGEMENT The authors of this paper like to acknowledge the work done by their colleagues in the three task groups within the participation agreement and also their colleagues of the research, safety and operational departments of the associated companies. DNV will be acknowledged for their work carried out to assess failure frequencies of the basic components leading to an undesired loss of containment of the so called typical compressor station and the conversion of the failure frequencies into more comprehensive formulas, which have applied into the prototype software CAT. REFERENCES [1] Taylor, J.R.; Risk Analysis for Process Plant, Pipelines and Transport, E&FN SPON -

1994 [2] Lees, F.P.; Loss Prevention in the Process Industries, Butterworth-Heinemann 1983 [3] DNV (1999): ARF Technical Note 14 Failure Frequencies for Process Equipment [4] OREDA92 - Offshore Reliability Data, 2nd Edition (1992) [5] OREDA97 - Offshore Reliability Data, 3rd Edition (1997) [6] G.C.Bello & V.Colombari: The Human Factors in Risk Analyses of Process Plants:

The Control Room Operator Model TESEO , Reliability Engineering, Volume 1, 1980

[7] H.E.Martz & R.A.Waller: Bayesian Reliability Analysis, John Wiley & Sons, 1982 [8] J. Cook et al.: "A comprehensive program for calculation of flame radiation levels", J.

Loss Prevention in the Process Industry, 1990, Vol 3, January [9] EFFECTS Version 2.1 Manual - TNO Dpt of Industrial Safety 1996 - The Netherlands [10] KAMELEON FireEx 99 User Manual - SINTEF Energy Research - Norway [11] FLACS User Manual - GEXCON AS Norway [12] Methods for the determination of possible damage: CPR 16 E The Green book TNO

1992

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