Lifetime Estimator

The Lifetime Estimator is used to carry out a statistical analysis on pipe failure data in order to estimate pipe expected lifetime and likelihood of failure. The estimator fits a Weibull curve to the lifetime data of different classes of pipe, using the Maximum Likelihood method, to provide an estimate of failure probability over time. The estimator uses the curve to estimate the likelihood of pipe failure and the network can then be updated from the results.

Likelihood score values can subsequently be used in conjunction with Consequence score values by the Risk Assessment tool to determine a risk score for each pipe.

Using the lifetime estimator

Lifetime Estimator data is viewed and edited in the Lifetime Estimator Editor.

To open the editor independently of a network, drag the Lifetime Estimator into the main window, double click on the Lifetime Estimator or right click on the Lifetime Estimator and select Open from the context menu. Functionality requiring the presence of a network will be disabled.

To open the editor for use with a network, drag the Lifetime Estimator onto the network. Pipe failure data can be collected from the network and network pipe data can be updated from estimator results.

See the Lifetime Estimator topic for further details.

Pipe classes

A separate curve is fitted for each pipe class in the editor. A pipe's class is defined by the Pipe class data field.

Pipe classes may be added from the network by clicking the Refresh button. Pipe classes that are not already present in the editor will be added. Pipe classes can also be added and removed from the list manually by using the Add and Remove buttons.

Getting lifetime data from the network

When the Lifetime Estimator is associated with a network, lifetime data can be collected from the network for all pipes in the currently displayed pipe class by clicking the Get Lifetimes button.

For Collection networks failure times may be calculated by one of the following methods:

• Using incident data - failure times are calculated for pipes that have an associated collapse incident as the difference between pipe Data installed and the incident Date reported
• Using CCTV condition data - failure times are calculated for pipes that have an associated CCTV survey with a structural grade of 5 or more. Failure time is calculated as the difference between CCTV When surveyed date and pipe Date installed

For Distribution networks, failure times are calculated for pipes that have an associated burst incident as the difference between pipe Data installed and the incident Date reported.

Survival times are calculated for all pipes in the current pipe class that have not failed as the difference between the current date and the pipe Date installed.

Fitting curves

Clicking the Fit Curve button will attempt to fit a curve to the data currently displayed in the Failure time and Survival time grids for the currently displayed pipe class.

Clicking the Fit All Classes button will attempt to fit curves to all pipe classes using lifetime data collected from the associated network.

Fitting requires at least two failure time values. A warning will be displayed if less than ten failure times are present. Any errors or warnings will be displayed in an error log. The fitted curves will be displayed in the editor as a cumulative probability graph.

Likelihood of failure estimation

An estimate of likelihood of failure of a pipe can be estimated from the cumulative probability graph.

The graph is split into regions corresponding to different likelihood scores. The regions can be defined either by fixed age boundaries, as a fraction of expected lifetime, or as a percentage probability of failure, by selecting options in the Likelihood of failure estimation group in the editor. Region boundaries are displayed on the graph as red lines.

The Likelihood of failure estimation grid is used to control the regions in the graph. Regions can be added or removed from the grid using the + and - buttons. Likelihood score and percentage values may be edited; age values being automatically calculated from the percentage values and the probability curve.

Updating the network using the Lifetime Estimator

Clicking the Update Pipes button will update the Expected lifetime and Likelihood score fields for pipes in each pipe class in the editor. Only fields that do not already have values will be updated. The flag selected in the editor will be applied to any updated fields.

Expected lifetime fields will be updated with the Expected lifetime displayed in the editor.

Likelihood score fields will be updated by calculating the age of the pipe and then assigning a score according to the Likelihood of failure estimation grid in the editor.