Materias primas complementarias

The causes of leaks shall be identified as basis for leak frequency quantification and identification of possible probability reducing measures. Pipe and equipment (components) where leaks may occur shall be identified for each module/area. The components shall be grouped in component types, e.g.

• pipe (small diameter, large diameter), • flanges (normal steel gasket, greylock), • valves,

• pumps, • compressors,

• vessels (pressure vessels, heat exchangers), • instrument connections,

• pig receiver/launcher, • risers, flowlines, • pipelines.

Within each module/area the number of components (metres of pipe) of the various component types shall be estimated. Frequencies of leaks with given hole sizes shall be determined for each component type based on qualified data and models.

An initial leak rate shall be calculated for each hole size by using discharge calculation models, based on medium, pressure, temperature and density. It will be useful to define leak scenarios to reflect various physical properties, such as:

• the gas leak is not sufficient to give an explosion mixture of a size which give pressure effects, and thus explosion is unlikely and a local fire is a possible scenario,

• the leak gives sufficient time for manual intervention before large explosive mixtures are formed, • the leak is too large for manual intervention to be realistic,

• the leak would quickly fill several areas with explosive gas, • an ignited leak in the area would give a ventilation controlled fire.

Some typical leak scenariosa shall be selected for further studies. Typical quantities/volumes in the various process sections shall be calculated on the basis of the sections the process is divided into by emergency shutdown valves (ESD). One must differentiate between various media (gas or oil). In the ERA the equipment count for an area may be based on previous counting for similar areas (or even the leak frequencies may be quantified based on experiences from a similar area in operation). The number of accident scenaria studied shall be chosen based on the precision of the study, to reflect the risk picture. In the CRA the main components shall be counted, e.g. pumps, compressors, vessels, pig receiver/launcher, pipe segments), and data for similar main components used in the quantification of leaks with given hole sizes. The TRA in the operational phase shall be based on experiences of smaller leaks, and an evaluation of experiences versus the equipment counting for larger leaks.

Discharge amounts as a function of time shall be calculated (using qualified models) for the typical leak rates. Calculations shall also be made to describe concentration development of the discharge medium, i.e. of when/where a flammable mixture is obtained, including ventilation conditions. Often, special discharge assessments have to be made for risers, flowlines and pipelines.

Subsea leaks include a spread in water. The gas and oil concentrations at sea level shall be assessed, and further the possibility of ignitable gas clouds. The assessment shall include possible drift of the hydrocarbons, both in water and air, the vaporisation and the location of possible ignition sources. If there are subsea ESD valves, both the situations with successful closure and failure to isolate shall be evaluated.

B.1.3 Effectiveness of Shutdown Systems

A successful shutdown depends on: • detection of the leak,

• automatic emergency shutdown system (panels, valves),

• operator intervention if automatic emergency shutdown is not achieved.

An effectiveness analysis of the shutdown system shall be conducted, covering the probability that the system operates and the response time. The following types of failures shall be covered:

1. unknown fault (sleeping fault) in the emergency shutdown system, also covering failure to respond on a leak,

2. known faults where emergency shutdown system is not operative and the production continues, e.g. due to:

• detected faults (now under repair) causing unavailability of the safety function,

• function testing of the emergency shutdown system is being done, and the system is out of function during test.

The leak and discharge calculations shall include some typical scenaria with shutdown faults. In the ERA it is normally sufficient to assume a reliability (based on typical results from previous studies) and response time of the safety systems. If a high reliability is required, the possibility to achieve this shall be evaluated.

In the CRA the risk reducing effect of subsea isolation barriers in import and export lines may be subject to evaluation. If found required the best location shall be studied.

B.1.4 Ignition

The probability of ignition depends on: • the availability of a flammable mixture,

• the flammable mixture reaching an ignition source, • the type of ignition source (energy etc).

An ignition probability that takes these into consideration shall be determined for each area (module), each leak category and each medium.

The following ignition sources shall be considered: • hot work,

• faults in electrical equipment, • faults in rotating equipment,

• ignition caused by turbines and combustion engines/hot surfaces, • automatic ignition in the event of a fracture/rupture,

• static electricity, • flare/open fire.

The time of ignition should be taken into consideration by defining "immediate ignition" and "delayed ignition".

In the ERA a less detailed model is sufficient, reflecting the area, leak category and medium. A subsea leak ignited on the installation is in the ERA and CRA counted as an escalated accident. In the TRA the possible consequences are considered for comparison with risk acceptance criteria. The possibility for a fire at sea scenario shall then be assessed.

B.1.5 Explosion

A probability distribution of pressure development for the various areas or modules and leak categories shall be established based on qualified models, taking into account :

• location of leak sources, • gas concentrations (clouds),

• location and energy of ignition sources, • area geometry,

• ventilation areas, • equipment congestion.

Response analyses shall be performed for typical explosion pressures in order to assess the extent of the damage locally in the accident module as well as global for the installation's structural integrity. The analysis shall focus on the areas that have the highest explosion risk and where an explosion is considered to cause the most extensive damage.

In ERA and CRA the explosion calculations are used to establish design loads for the area dividers and equipment in the area, such that the risk meets the relevant acceptance criteria. The design loads from the ERA are mainly used for cost purposes, and thus the pressure consideration may aim at the level rather than a precise figure.