RAMP Simulation Software for Modelling Reliability, Availability and Maintainability (RAM) is a computer software application developed by AtkinsRéalis specifically for the assessment of the reliability, availability, maintainability and productivity characteristics of complex systems that would otherwise prove too difficult, cost too much or take too long to study analytically. The name RAMP is an acronym standing for Reliability, Availability and Maintainability of Process systems. RAMP models reliability using failure probability distributions for system elements, as well as accounting for common mode failures. RAMP models availability using logistic repair delays caused by shortages of spare parts or manpower, and their associated resource conditions defined for system elements. RAMP models maintainability using repair probability distributions for system elements, as well as preventive maintenance data and fixed logistic delays between failure detection and repair commencement. RAMP consists of two parts:
RAMP Model Builder. A front-end interactive graphical user interface (GUI). RAMP Model Processor. A back-end discrete-event simulation that employs the Monte Carlo method.
RAMP Model Builder The RAMP Model Builder enables the user to create a block diagram describing the dependency of the process being modelled on the state of individual elements in the system.
Elements Elements are the basic building blocks of a system modelled in RAMP and can have user-specified failure and repair characteristics in the form probability distributions, typically of Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR) values respectively, chosen from the following:
Weibull: Defined by scale and shape parameters (or optionally 50th and 95th percentiles for repairs). Negative exponential: Defined by mean average. Lognormal: Defined by median average and dispersion (or optionally 50th and 95th percentiles for repairs). Fixed (Uniform): Defined by a maximum time to failure or repair. Empirical (user-defined): Defined by a multiplier. Elements can represent any part of a system from a specific failure mode of a minor component (e.g. isolation valve fails open) to major subsystems (e.g. compressor or power turbine failure) depending on the level and detail of the analysis required.
Deterministic elements RAMP allows the user to define deterministic elements which are failure free and/or are unrepairable. These elements may be used to represent parameters of the process (e.g. purity of feedstock or production demand at a particular time) or where necessary in the modelling logic (e.g. to provide conversion factors).
Q values Each element of the model has a user-defined process 'q value' representing a parameter of interest (e.g. mass flow, generation capacity etc.). Each element is considered to be either operating or not operating and has associated performance values q = Q or q = 0 respectively. The interpretation of each 'q value' in the model depends on the parameter of interest being modelled, which is typically chosen during the system analysis stage of model design.
Groups Elements with interacting functionality can be organised into groups. Groups can be further combined (to any depth) to produce a Process Dependency Diagram (PDD) of the system, which is similar to a normal reliability block diagram (RBD) commonly used in reliability engineering, but also allows complex logical relationships between groups and elements to permit a more accurate representation of the process being modelled. The PDD should not be confused with a flow diagram since it describes dependency, not flow. For example, an element may appear in more than one position in the PDD if this is required to represent the true dependency of the process on that element. Groups may also be shown in full or may be compressed to allow the screen to show other areas to greater resolution.
Group types Each group can be one of eleven group types, each with its own rule for combining 'q values' of elements and/or other groups within it to produce a 'q value' output. Groups thus define how the behaviour of each element affects the reliability, availability, maintainability and productivity of the system. The eleven group types are divided into two classes: Five 'Flow' group types:
Minimum (M): qM = min[q1, q2,...qn] Active Redundant (A): qA = min[Rating, (q1 + q2 + ... + qn)] unless qA < Cut-off, then qA = 0 Standby Redundant (S): qS = as for Active Redundant, but where the first component is always assumed to be duty equipment. Time (T): qT = 0 if component with 'q value' q1 is in a "down" state when time through mission t < t0, otherwise qT = q1 + ... + qm if component with 'q value' q1 is in an "up" state when time t ≥ t0 + (m-1) x Time Delay, where m = 1 to n. Buffer (B): if the buffer is not empty qB = q2 else qB = min[q1,q2], where the buffer empties as output if component with 'q value' q2 is in an "up" state with level at time 0 = Initial Level, otherwise level at time t = level at time (t-1) - (q2 - q1), and the buffer fills as input if component with 'q value' q2 is in a "down" state with level at time 0 = Initial Level, otherwise level at time t = Capacity if level at time (t-1) + q1 > C, otherwise level at time t = level at time (t-1) + (q2 - q1). Buffer input and output may also be limited by buffer constraints. Six 'Logic' group types:
Product (P): qP = q1 x q2 x ... x qn Quotient (Q): pQ = q1 / q2 Conditionally Greater Than (G): if q1 > q2 then qG = q1 else qG = 0 Conditionally Less Than (L): if q1 < q2 then qG = q1 else qG = 0 Difference (D): max[q1 - q2, 0] Equality (E): q1 if q1 lies outside the range PA to PB, q2 if q1 lies inside the range PA to PB Three group types (Active Redundant, Standby Redundant and Time) are displayed in parallel configurations (vertically down the screen). All others are displayed in series configurations (horizontally across the screen). Six group types (Buffer, Quotient, Conditionally Greater Than, Conditionally Less Than, Difference and Equality) contain exactly two components with 'q values' q1 and q2. All others contain two or more components with 'q values' q1, q2 to qn.
Element states An element may be in one of five possible states and its 'q value' is determined by its state:
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