Page 241 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

10/218 Service Interruption Risk
Redundant equipmentkupply 25% power generation plant. The most sensitive of the customers is
Points are awarded here when more than one line of defense usually the power generation plant. This is not always the case
exists in preventing customer service interruption. For maxi- because some of the municipalities could only replace about
mum points, there should be no single point of failure that 70% of the loss of gas on service interruption during a cold
would disable the system's ability to prevent an excursion. weather period. Therefore, there are periods when the munici-
Credit can also be given for system configurations that allow palities might be critical customers. This is also the time when
rerouting ofproduct to blend out a high contaminant concen- the supply to the power plant is most critical, so the scenarios
tration or otherwise keep the customer supplied with product are seen as equal.
that meets the specifications. The redundancy must be reli- Notification to customers minimizes the impact of the inter-
ably available in a time frame that will prevent customer ruption because alternate supplies are usually available at short
problems. notice. Early detection is possible for some excursion types, but
for a block valve closure near the customer or for the sweeping
Percentage points, up to a maximum of 25%, should be of liquids into a customer service line, at most only a few min-
awarded when the switching system has utes of advance warning can be assumed. There are no redun-
Human intervention required 0% dant supplies for this pipeline itself. The pipeline has been
Automatic switching 5 ?"o divided into sections for risk assessment. Section A is far
Regular testing of switching to alternative sources 6% enough away from the supplier so that early detection and noti-
Highly reliable switching equipment 10% fication of an excursion are always possible. Section B, how-
Knowledgeable personnel who are involved in 12% ever, includes metering stations very close to the customer
switching operations facilities. These stations contain equipment that could malfimc-
Contingency plans to handle possible problems 2% tion and not allow any time for detection and notification
during switching before the customer is impacted.
Because each section includes conditions found in all
Operator training/procedures 15% upstream sections, many items will score the same for these two
Points are awarded here when operator training plays a role sections. The potential for service interruption for Section A
in preventing or minimizing consequences of service interrup- and Section B is evaluated as follows:
tion episodes. Training to prevent the likelihood of episodes
was already covered earlier in the Operator error section. Product specification deviation (PSDj
Operator training is important in calibration, maintenance, and Product origin 15 pts
servicing of detection and mitigation equipment as well as Only one source, comprising approximately 20% of the gas
monitoring and taking action from a control room. The evalua- stream, is suspect due to the gas arriving from offshore with
tor should look for active procedures and tra;ning programs that entrained water. Onshore water removal facilities have occa-
specifically address service interruption episodes. The avail- sionally failed to remove all liquids.
ability of emergency checklists, the use of procedures (espe- Equipment failure 20 pts
cially when procedures are automatically computer displayed), No gas treating equipment in this system.
and the knowledge of operators are all indicators ofthe strength Pipeline dynamics 11 pts
of this item. Past episodes of sweeping of fluids have occurred when gas
Emergency/practice drills 10% velocity increases appreciably. This is linked to the occa-
Points are awarded here when drills can play a role in prevent- sional introduction of water into the pipeline by the offshore
ing or minimizing service interruptions. While drilling can supplier mentioned previously.
be seen as a part of operator training, it is a critical factor in Other 20 pts
optimizing response time and is considered as a separate No other potential sources identified.
item to be scored here. Maximum points should be awarded
where regular drills indicate a highly reliable system. Delivery Parameter Deviations (DPD)
Especially when human intervention is required and espe- Pipeline failure 242 pts
cially where time is critical (as is usually the case), drilling From previous basic risk assessment model.
should be regular enough that even unusual events will be Blockages 20 pts
handled with a minimum of reaction time. No mechanisms to cause flow stream blockage.
Again, these percentages, up to a maximum of 80%, apply to Equipment 15 pts
the differences between actual and maximum points in the PSD Automatic valves set to close on high rate of change in pressure
and the DPD. This reflects the belief that reliable intervention have caused unintentional closures in the past. Installation of
mechanisms can reduce the change of a customer impact due to redundant instrumentation has theoretically minimized the
an excursion of either type. potential for this event again. However, the evaluator feels
that the potential still exists. Both sections have equivalent
Example 10.2: Service interruption potential equipment failure potential.
Operator error (SectionA) 16 pts
In this example, XYZ natural gas transmission pipeline has Little chance for service interruption due to operator error. No
been sectioned and evaluated using the basic risk assessment automatic valves or rotating equipment. Manual block
model. This pipeline supplies the distribution systems of sev- valves are locked shut. Control room interaction is always
eral municipalities, two industrial complexes, and one electric done.