Introduction to TPK5170 - RAMS assessment and optimization
Motivating example
Consider a system with 2 pumps in parallel each with 50% capacity wrt to the needed pumping
demand. The following reliability parameters and other quantities of interest are given:
Parameter
Value
Description

10-4
Constant failure rate for each pump
(per hour), i.e., effective failure rate
for given maintenance strategy

1/24
Repair rate (per hour)

5%
Common cause factor
CCM
5 000
Cost of repair upon failure (in NOKs)
CPM
1 000
Cost per PM activity. Current PM
strategy is 2 PM activities per year per
pump
CU
1 000
Cost per hour downtime
CCAPEX,Pu
20 000
Yearly capital cost of a stand-by pump
CCAPEX,Sp
1 000
Yearly capital cost of a critical spare
part?
COPEX,Me
5 000
Extra cost to have an extra
maintenance engineer available to
repair the second pump in case of
double failure
CB
10 000
Capital cost of an extra buffer of 8
hour capacity (16 hours for 50%
demand)

3
Aging parameter
What is RAMS?
RAMS = Reliability, Availability, Maintainability and Safety
Words that follow RAMS
In the literature of RAMS many terms are linked to the RAMS acronym:
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Analysis
Assessment
Estimation
Evaluation
Management
Optimization
Requirements
RAMS analysis: With RAMS analysis we mean systematic use of available information to structure
contributing factrs, model and calculate RAMS performance measures. Several RAMS performance
measures exist such as availability, regularity and risk.
RAMS assessment: The word assessment is used in two ways. Sometimes assess is the systematic
use of information etc to calculate RAMS performance measures. In some references assessment
also includes evaluation in addition to the assessment.
RAMS estimation: In many references the word estimation is used for the process of coming up with
numerical values for performance measures such as risk, availability and reliability. This could be
confusing since the word estimation has a more narrow scope within statistics. In statistics
estimation means to assess numerical values to a model parameter by use of data, or sometimes also
by combining data and statements from experts.
RAMS evaluation: The word evaluation is used to describe the process of judging the tolerability, the
goodness etc of the result from RAMS estimation/analysis. Whereas estimation/analysis point
towards fining numerical values for RAMS performance measures, judging means to compare the
performance measures with the RAMS requirements.
RAMS management: A continuous management process with the objective to (i) define RAMS
requirements, (ii) identify measures that may improve RAMS performance, (iii) monitor RAMS
performance over time, (iv) select the most prominent measures, and (v) ensure that these measures
are implemented.
RAMS requirements: Numerical values for RAMS performance measures that is required to be
fulfilled. Examples could be minimum reliability of a safety instrumented system, availability of a
specific railway line, and maximum lead times for spare parts.
RAMS optimization: RAMS optimization is the selection of a best alternative with regard to some
criteria from some set of available alternatives. In the simplest case, an optimization problem
consists of maximizing or minimizing a real function (object function) by systematically choosing
input values from within an allowed set and computing the value of the function to find the input
value that gives the best result. RAMS optimization essentially deals with finding solutions that give a
reasonable balance between good RAMS performance and low cost.
RAMS related measures and concepts: (From NORSOK Z-0161)
Deliverability: The ratio of deliveries to planned deliveries over a specified period of time, when the
effect of compensating elements such as substitution from other producers and downstream buffer
storage is included.
Production availability: The ratio of production to planned production, or any other reference level,
over a specified period of time.
Regularity: A term used to describe how a system is capable of meeting demand for deliveries or
performance. Production availability, deliverability or other appropriate measures can be used to
express regularity.
Regularity expenditures: The total cost of lost or deferred production due to downtime.
Reliability: The ability of an item to perform a required function under given conditions for a given
time interval. (“Can I rely on the item that it will continue to do the work?”)
Availability: The ability of an item (under combined aspects of its reliability, maintainability, and
maintenance support) to perform its required function at a stated instant of time, or over a stated
period of time (BS 4778) (“Will the item be available for me when I need it?”)
Maintainability: The ability of an item under given conditions of use, to be retained in, or restored
to, a state in which it can perform a required function, when maintenance is performed under given
conditions and using stated procedures and resources.
Maintenance supportability: The ability to provide the resources, services and management
necessary to carry out maintenance Note: Mobilisation times of resources are used to quantify this
ability in a regularity analysis.
Predictive maintenance: Condition based maintenance carried out following a forecast derived from
the analysis and evaluation of significant parameters of the degradation of the item.
Preventive maintenance: Maintenance carried out at predetermined intervals or according to
prescribed criteria and intended to reduce the probability of failure or the degradation of the
functioning of an item.
Corrective maintenance: Maintenance which is carried out after a fault recognition and intended to
put an item into a state in which it can perform a required function.
CAPEX: Capital expenditures
REGEX Regularity expenditures
OPEX: Operational expenditures
1
Replaced by: ISO 20815 Petroleum, petrochemical and natural gas industries - Production assurance and
reliability management. NORSOK Z-016 available at: http://www.standard.no/PageFiles/960/Z-016.pdf
Figure 1 Illustration of relationship between some regularity terms (NORSOK Z-016)
Assessment and optimization questions:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
What is the production availability?
What compensation may be implemented to increase deliverability?
What can be done to increase Uptime and decrease Downtime?
Spare parts may reduce downtime from 24 h to 8 h, does it pay of?
How many spares to keep?
Does it pay of to have another maintenance engineer?
Does it pay of to buy an extra pump as a stand-by pump?
A buffer vessel of 8 hour capacity will increase deliverability, but does it pay of?
Should we increase or decrease the maintenance interval?
What is the optimum preventive maintenance interval?
Does condition monitoring pay of?
A failure mode of the pumps is external leakages of hydro carbons. Propose a SIF (Safety
Instrumented Function) to achieve an acceptable risk level wrt HSE?
What SIL (Safety Integrity Level) level would be reasonable?
What is the proof test interval (interval of function testing) of the SIS = (Safety Instrumented
System)?
How to model the situation when downtimes are not exponentially distributed?
When to conduct preventive maintenance if the pump is located on an installation in the
arctic region where maintenance cannot be conducted in winter time?
How to model cost of failures in that situation?
Methods for RAMS performance calculations
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FTA
RBD
Markov
Monte Carlo Simulation/Discrete Event Simulation
Dedicated formulas and calculation procedures, e.g., PFD = /2
Example system
2
2(1- )


1

2
0