ASAP - ADVANCED SAFETY ANALYSIS PACKAGE
ADVANCED SAFETY ANALYSIS PACKAGE (ASAP)
ASAP consists of a process and geometry package as well as physical and statistical models. The main feature of ASAP is the interface model which is a complex but fully consistent connection between the physical/statistical models and the geometry and flow diagram models. Since the interface model cope with any accident scenario it follows that once the geometry and process is modeled, the simulations can start without any further adjustments or evaluations.
The most apparent benefit of the interface model is that the results will change consistently with changes made in the process diagram or in the geometry. If there is a change in the layout of the escape routes, the risk will also change without having to change anything in the program but the escape routes. This is particularly useful in the FEED phase where design developments and changes occur frequently.
Figure 1 - 3-D view of an offshore platform as modeled in ASAP
Modeling of the process
The first step of a process risk analysis is to model the process flow diagram with emergency shutdown valves and blowdown valves. In doing so, the process segments are defined with isolated volumes and blowdown capacities. In the event of a leak the leak profile is then determined. This is the starting point of the transient dispersion, detection and ignition calculations.
The user is allowed to choose whether all ESD and BD valves shall operate successfully or include the possibility of valve failures. Valve failures will normally increase the released volume and thus impact on the leak profile and in the end also the risk.
Figure 2 - PFD (Process Flow Diagram) as modeled in ASAP. The ESD and BD valves are included in the PFD.
The user defines a set of leak rates for the analysis. Combined with the operating conditions for each process unit within a segment (pressure, temperature, fluid composition) as well as the equipment count, the leak frequencies are determined.
The geometry of the installation is modeled in the 3D editor. Modules, escape ways, stairways and evacuation means, construct the platform. The modules consist of several user-defined volume elements that represent a coarse 'grid' by dividing the module into smaller areas. Process units are placed centrally in the horizontal direction in the volume elements and the user sets the height. By placing the process units the user has defined all the leakage points on the installation.
Figure 3 - Foot-print of a platform deck as seen in the editor. Equipment containing HC (hydrocarbons) are located in different volume elements. The escape routes at this level are also shown (filled with green).
Gas and liquid jet direction and dispersion
The jet from a leak could take any direction. The program allows 12 horizontal and 2 vertical (up and down) each with a probability of 1/14. The user may override the default and define number of directions and probabilities.
The gas jet is modeled as a truncated cone with an empirical entrainment parameter that determines the radial growth of the jet. The liquid jet is assumed to have a fixed diameter. Two-phase jets are modeled as gas jets. The amount of gas in the jet is then found by the gas fraction in addition to the flashed liquid.
Figure 4 - Gas jet inside a module (blue color).
Figure 5 - A liquid jet followed by a liquid pool spread on deck.
Obstruction of jets due to equipment and partitions
A gas or liquid jet will probably hit equipment, walls or floors and rarely spread unobstructed in a module. Obstruction of jets alters the dispersion, which is catered for in ASAP. Each volume element has an obstruction factor describing the density of equipment in the element. A high obstruction factor gives a greater radial dispersion of the gas jet leading to shorter gas jet lengths. If the gas jet hits a partition (walls, ceiling or floor) further dispersion is calculated as a wall jet. Liquid jets fall vertically downwards to the nearest floor from the volume element hit by the jet.
Ventilation, gas dispersion, detection and ignition inside and outside modules
The leak rate and the further dispersion, detection and ignition change significantly with time and are therefore treated by transient models. ASAP has in-built models for these calculations and apply dispersion models with analytical solutions. Consequently, the calculation speed is very fast. The models are well-proven and taken from text books. However, when CFD codes like Flacs and Fluent also are used in the QRA for dispersion and ventilation calculations, the results from these are used for tuning of the analytical models such that the benefits of both model applications are gained.
From the point where the gas jet no longer spreads as a turbulent jet, the further spread of gas is determined by the wind conditions inside the module. For a liquid release the conversion from liquid jet to gas dispersion is more comprehensive. First the liquid pool formation is determined. The drainage of the module is an important factor in this calculation. Then the evaporation from the pool is calculated followed by heavy gas dispersion.
Wind conditions in the modules are calculated by ASAP after the user has defined the wind rose (up to 12 wind directions) and mean wind velocities. It is also necessary to define the ventilation openings in each module, stationary heat sources that influence the ventilation in addition to the external pressure coefficients around a module. ASAP has a database for different platform types containing pressure coefficients determined from wind tunnel test and measurements on actual installations. Note that ventilation results from CFD simulations can be imported by ASAP.
In each volume element (grid) the user defines the number of gas detectors and ignition sources (continuous as well as discrete). Based on the gas fill fractions in the volume elements, the probability of gas detection and ignition is calculated for each second in time till the module is free of gas. ASAP makes a distinction between immediate and delayed ignition. Delayed ignition probability is influenced by the calculated gas detection probability whereas immediate ignition is not.
Jet fires and liquid pool fires
Ignition of gas and liquid leaks gives fires and/or explosions. The extension of the fire with corresponding radiation levels and smoke concentrations are calculated and checked for each module, process unit, escape way and evacuation means against threshold values for impairment and fatalities. If threshold values are exceeded, fatalities and impairment is registered.
Figure 6 - Pool fire development adjacent to a solid wall and ceiling.
Flash fires and explosions
A flash fire is modeled as a swift flame with a volume 8 times greater than the ignited gas cloud volume. Explosion pressures are not calculated in ASAP, since this is normally done in separate CFD codes like FLACS. Correlations between cloud volumes and explosion pressures are made from FLACS simulations and entered into ASAP to find the explosion risk.
All personnel onboard and their distribution on various platform areas are input required by the program. ASAP applies a binomial probability distribution of personnel in each area. The distribution is used for calculation of personnel risk, i.e. group risk, potential loss of life (PLL), fatal accidental rate (FAR) and F-N curves (Frequency vs. Number of fatalities).
Investigations of occurred releases indicate that a major fraction of leaks are caused by intervention activities, i.e. when personnel are present. It follows that personnel are more frequently exposed to leaks and fires than what the basic personnel distribution suggests, in particular when the activity level is high. Consequently, one additional personnel distribution can be assigned to the accident area. The benefit of this model is that the personnel activity level may be seen as a significant contributor to personnel risk level. Without such a model, the activity level would have no direct impact on personnel risk.
Escape and evacuation analysis
The escape and evacuation system is modeled in the 3D editor. An escape route consists of escape ways, stairs, modules, exits and evacuation means. For each of these escape segments, the walking speed is defined. After the modeling, ASAP finds the complete escape network with all escape possibilities from any module and the associated evacuation times.
3-dimensional escape routes can be viewed in the 3D viewer, see below figure.
Figure 7 - Illustration of an escape route from one module to the Living quarters and life boat. The escape route is marked with a red line.
In a batch run intermediate and final results are saved to a database for each single scenario. An event tree construction program is available which allows the user to build event trees for his specific purpose. By connecting to the database (Oracle) he may get results at any level of detail. Due to the features of a database, any combination of parameters can be chosen, for instance impairment of escape from a specific module for a single wind condition.
The event tree construction program is compatible with EXCEL so that all data can be exported to a format suitable for the Client’s needs.
Aggregated results can also be viewed in ASAP windows any time during the batch job. Risk values can therefore be “monitored” during the analysis.
How many event trees can be run?
By combining all parameter sets, number of scenarios becomes numerous. A simple example is 12 wind directions, 4 wind speeds and 10 jet directions. This alone gives;
12∙4∙10=480 different scenarios for a leak case.
The following parameter sets determine the number of scenarios;
- Wind direction (max 12)
- Wind speed (any number)
- Jet direction (max 14)
- Leak rates (any number)
- Number of leaking units/leak points (any number)
- Shutdown and blowdown failure cases (determined by number of segment isolation and blowdown valves)
In a QRA, approximately 1000 event trees are run for a potential leak unit (e.g. a compressor). On one processor the calculation time is about ½ hour.
ASAP has a fixed calculation method that ensures high-speed calculation. Low execution time is a necessity because a risk analysis may consist of several million scenarios. The user has a certain degree of flexibility in the analysis by the provision of alternative models. The program is also well modularised such that new models can rather easily be implemented.
All the models may be used for stand-alone calculations or calculated in sequence to be able to evaluate the progress of specific accident scenarios step by step both graphically and numerically. The total risk analysis is executed as a single batch job.
ASAP has a default database containing failure rates, accidental rates, empirical coefficients and other information used in the analysis. The user is free to compose own default data sets representing different standards (ex. the firm standard), and can then easily perform sensitivity analysis.
Professor T.K.Fanneløp developed the fluid dynamic models for use in ASAP through a consultancy agreement with Aker Engineering in 1995-96. The models are documented in the Aker report prepared by Fanneløp in AE-ETYS-IR-96001, rev 01, November 1996: “Analytical solutions to fluid dynamic problems in risk assessment”. The background for contracting this work was the need for effective use of verified models in risk analyses, especially in fast track projects. Quoting the preface of the report;
“The accidental release and spreading of hydrocarbon liquids and gases on offshore platforms are not easily modeled knowing the complex geometry and design conditions that are present. The two most frequent approaches are:
• Use of CFD codes (Computational Fluid dynamics)
• Use of single purpose built models, i.e. models that have a limited area of applicability.
Commercial CFD codes such as FLACS, Fluent and KFX have a wide range of applicability and allow flow interactions with quite complex geometries. The most apparent disadvantage is the long modeling time which is a major drawback in risk assessments where shortage of time is a frequent problem. Consequently, only a small selection of accidental scenarios can be modeled with such codes. Simple and robust models are therefore necessary tools for the risk analyst who often needs to model and compute thousands of scenarios as basis for his risk estimates.
Although handbooks and other literature offer several solutions to fluid dynamic problems, they are usually derived without taking account for flow obstructions. This aspect limits their range of applicability as well as their validity if still used for fluid flow situations in congested areas.”
It should be noted also that the models developed are based on those described in “Fluid dynamics for Industrial Safety and Environmental Protection” by Torstein K. Fanneløp. Industrial Safety Series Volume 3, Elsevier 1994.
As mentioned previously, the use of CFD codes for ventilation and gas dispersion is now normally required by the Client (as well as in NORSOK Z-013) in the FEED and later phases of offshore development projects. Tuning of the analytical models is therefore performed for each module of the platform based on the results from the CFD simulations.
ASAP is installed on a terminal server in Sandvika (Oslo) for current ASAP-users. This facilitates access from any location worldwide. License fees for 1, 3, 6 or 12 months can be arranged for other Clients.
The database is constructed in Oracle and the programming language is Visual Works from Parc Place (Small Talk).
Model development work and preparation of specifications are done by Lilleaker Consulting a.s.
VeloxiT a.s. performs the coding of the program.
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For more information
Lilleaker Consulting a.s.
Phone: +47 67 52 09 50
Mr. T. Gulbrandsen (mobile +47 99 57 64 91)