LAYOUT OF AN OIL AND GAS FACILITY - THINGS TO CONSIDER

PLANT LAYOUT CRITERIA AND PHILOSOPHY 

LAY-DOWN AREA
A Lay-down area is a construction term, which refers to a portion of the site, set aside for temporary storage of equipment.

Contrary to popular misconception that a lay-down area is a portion meant for maintenance purposes, a lay-down area is in actual fact, a hard concrete filled area set aside for temporary storage of equipment.

PLANT LAYOUT PHILOSOPHY
So many criteria are considered when determining the requisite and appropriate spacing between equipment on site.

These criteria are usually based on the classification of such equipment within the facility.

Equipment are usually classified into two. They are:-

1. Process Equipment
2. Utility Equipment

Process Equipment are equipment that are directly involved in the handling of the process fluids (crude or gas). They consists of equipment that perform some of the following functions:-

a. Separation
b. Cooling/Refrigeration
c. Heating
d. Fluid transfer (pumps) etc

Some example of Process equipment includes:-

i. Heat Medium Skid ix. Chillers
ii. Surge Vessel x. Booster Compressors
iii. Slug Catcher xi. Pumps etc
iv. Inlet Separators
vii. Distillation Columns
viii. Flare Knock-out drums

Utility Equipment or packages on the other hand are those that are not directly involved in the handling of the process fluid. They however provide support for the smoooth running of the process packages. They consists of equipment that perform the following functions:-

a. Power  Supply
b. Drainage (closed and open)
c. Water Supply
d. Fuel Supply etc

Some examples of Utility packages includes:-

i. Gas Generator
ii. Diesel Storage Tanks
iii. Open Drain System
iv. Closed Drain System
v. Supply Water Tank etc

Plant layout is a crucial factor in the economics and safety of a process plant. Some of the basic factors that engineers consider while determining the requisite spacing between process and utility equipment includes:-

1. Process flow Principle
2. Hazards Assessment - Classification, Ranking and Ratings
3. Containment of Accidents
4. Limitation of Exposure
5. Escape
6. Efficiency and safety in Operation
7. Efficiency ans safety in Maintenance
8. Fire fighting Access & Fire Protection
9. Access for Emergency
10. Security
11. Ventilation

Plant layout can have a large impact on plant economics. Additional space tends to increase safety, but is expensive in terms of land and also in additional pipework and operating costs.
Space therefore needs to be provided where necessary for safety concerns, and not wasted.


HAZARDOUS AREA CLASSIFICATION

When an Electrical Equipment is used in, around or near an atmosphere that has flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers, there is always a possibility or risk that a fire or explosion might occur.



It is a simple method of analyzing and classifying the environment where explosive gas atmospheres may occur. Its main purpose is to facilitate the proper selection and installation of apparatus to be used in that environment, taking into account the properties of flammable materials that will be present.

Those area where the possibility or risk of fire or explosion might occur due to an explosive atmosphere and/or mixture is often called a hazardous (or classified) area/location.

Fig 1. Schematic Of Area Classification on a Gas Handling Facility

Zone System:-

For hazardous gaseous atmosphere there are 3 classification which defines the probability of the hazardous material being present in sufficient quantities to produce explosive or ignitable mixtures.

a. ZONE 0 - Ignitable concentration of flammable gases or vapors which are present continuously or for long periods of time.

b. ZONE 1 - Ignitable concentration of flammable gases or vapors which are likely to occur under normal operating conditions.

c. ZONE 2 - Ignitable concentrations of flammable gases or vapors which are not likely to occur under normal operating conditions and do so only for short period of time.

Fig 2. Legend For Hazardous Area Classification

Asides the general methods employed in the successful Layout of any plant, Experienced engineers design a layout system with some basic factors in mind.
They include:-

1. Minimization of Ignition Potential

A primary method of minimizing ignition potential, is by the adoption of the Hazardous Area Classification as highlighted above.
This forms a basis for the proper selection and protection of electrical equipment in the concerned areas, and for the safe positioning of other potential sources of ignition.

Fig 3. 3D model of a Plant after proper spacing

2. Proper facility and Equipment orientation:-

The prevailing current and wind directions must be considered when establishing spatial arrangements and orientations of facilities.
The orientation of an onshore plant shall be selected so that accommodation blocks, Temporary Refuges (TRs) and administration buildings are upwind or crosswind of the prevailing wind direction of the hydrocarbon facilities.
Orientation shall be such that leaks or spills will tend to drift away from the accommodation block and Temporary refuge under prevailing conditions.

Fig 4. Layout With Respect to Proper Orientation

3. Minimization of Escalation potential :-

In order to prevent the escalation of incidents involving fires, explosions or leakages, sufficient separation generally shall be provided. Those distances are based on three main considerations;
a. The effects of explosions,
  b. Fires
c. Ignition of gas leaks.
The criticality of the various specific facilities shall be considered when determining the acceptable minimum spacing.
This is particularly the case for emergency systems such as firewater pumps, emergency power generation and ESD systems, where their ability to survive the worst credible accident scenario must be carefully considered.

4. Layout as a form of explosion protection:-

The consequences of an explosion shall be minimized and the escalation (as a result of over-pressures damaging equipment or piping or causing progressive collapse of the structure) shall be contained.
Explosion mitigation systems shall be evaluated for areas where the potential for a gas or vapor cloud explosion exists.
The study shall identify the potential for escalation caused by over-pressures which would impair the operation of critical systems and the effects of any subsequent fire.

Explosion effects to be considered includes:-

1. Blast Over pressure
2. Drag Forces
3. Minimize Module Volume
4. Minimize number of ignition sources and keep ignition sources near to the openings as far as it is practicable
5. Design ventilation to minimize the probability of build-up of the most likely types of gas or vapor releases
6. Locate obstacles in the inner part of the module and away from vents
7. Maximize openings (within the constraints of the HAC) particularly in floors or ceilings if possible
8. Do not obstruct the openings in the module boundaries
9. Orientate obstacles so that they present their most aerodynamic profile to the main vent flow direction.
For example, orientate horizontal vessels so that the longest dimension is in the direction of the main vent flow


Plant Layout system consideration should be extensive to ensure the best possible arrangement, positioning, location and spacing to be employed.


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