THE BATHTUB CURVE EXPLAINED

The bathtub curve is a graphical representation of the failure rate of a system over time, typically used to model the life cycle of equipment, components, or systems in various industries, including oil and gas. 

The curve is called the "bathtub curve" because of its shape: it has a high initial failure rate (infant mortality), a period of low and relatively constant failure rates (useful life), and then an increasing failure rate as the system approaches the end of its useful life (wear-out phase).

1. EARLY FAILURE OR INFANT MORTALITY (Decreasing failure rate)
2. STEADY STAE FAILURE (Constant failure rate)
3. Wear Out(Increasing failure rate)

Early failure zone: We have decreasing rate of failure with time; may be product defects, bad design

But as the products goes on it gets decrease.

Constant Failure: when machines are up and running all the time but have occasional failures. It is not increasing or decrasing.

 Wear Out: Most of us experience face this kind of failure; bougth juicer machine; running for last 10 years; motor wear out. Better to replace and buy new one.

At some point our projduct start to fail more conssistenlty; more related to electronic components.

Better to upgrade.

Failure rate of system = ∑λi x Ni 

λ: failure rate of a part; 

N: number of parts used

To minimize the fall in plant operation rate (business loss) caused by aging, costly spare parts and maintenance checks are necessary.

Recently, the technologies behind the core components of DCSs, such as CPUs, ICs, and hard disk drives, are advancing at an exceptionally rapid pace. As a result, component manufacturers are discontinuing older products more quickly than before, while adapting to technological innovations, reassessing sales strategies and production processes, and focusing their efforts on producing their most essential products.

Example in Oil and Gas Industry:

DCS/PLC Component Examples in Oil and Gas Processing:

1. DCS Controller (e.g., Emerson DeltaV, Honeywell Experion):

   • Infant Mortality: Issues during installation, such as incorrect controller configuration or failure of initial components.
   • Normal Life: The controller operates reliably, but components such as power supplies may need periodic replacements.
   • Wear-Out: Controllers experience system failures or software malfunctions due to outdated firmware or hardware degradation.

2. PLC (e.g., Siemens S7, Allen-Bradley ControlLogix):

   • Infant Mortality: Failures in the initial commissioning phase due to programming errors or faulty I/O modules.
   • Normal Life: PLC system runs without issues, with occasional maintenance like software updates, I/O card replacement, or sensor recalibration.
   • Wear-Out: As the PLC ages, it may experience component failure such as a crashed CPU, I/O board degradation, or loss of communication due to physical wear from vibration and temperature extremes.

3. Field Devices (e.g., Pressure Transmitters, Temperature Sensors):

   • Infant Mortality: Failures in the first few months after installation due to incorrect calibration or defective sensors.
   • Normal Life: Field devices provide reliable readings for process control, with periodic recalibration and maintenance.
   • Wear-Out: As sensors degrade over time, their accuracy or response time deteriorates, increasing the likelihood of failures.

4. Communication Networks (e.g., Fieldbus, Modbus):

   • Infant Mortality: Early failures might occur due to installation problems or communication protocol misconfigurations.
   • Normal Life: Communication between the DCS/PLC and field devices operates smoothly.
   • Wear-Out: Over time, issues like network latency, signal degradation, or electrical surges can cause communication disruptions between control systems and field devices.

---

Managing the Bathtub Curve for DCS and PLC in Oil and Gas:

• Early Failures: Minimize early failures by ensuring proper testing, commissioning, and training during the installation phase. Ensure systems are installed correctly, and perform thorough factory acceptance tests (FAT) before deployment.

• Normal Life: Perform regular maintenance to ensure that all devices (sensors, controllers, I/O) are functioning well. Maintain a routine for software updates, calibration, and diagnostic testing.

• Wear-Out Failures: Proactively replace components as they approach their end of life. Use predictive maintenance tools to analyze trends in system performance and plan for upgrades or replacements of aging systems and components.

By applying the bathtub curve to DCS and PLC systems, oil and gas facilities can ensure the reliability, safety, and efficiency of their operations, minimizing unexpected downtime and maximizing the useful life of their control systems.

• Instrumentation in Oil & Gas:

  • Consider the installation of level transmitters in tanks storing crude oil or natural gas. The infant mortality phase may involve failures due to faulty installation or incorrect calibration.
  • In the useful life phase, the level transmitters work reliably for months or years, with periodic maintenance like recalibration, sensor checks, or cleaning to keep them functioning well.
  • Finally, as the transmitter ages, internal components may begin to degrade, leading to failure in the wear-out phase. For example, the electronic components inside the transmitter might fail due to prolonged exposure to extreme temperatures, vibration, and corrosion.