6.6 Methodology for calculating time to failure of engineering systems

6.6.1 Integral diagnostic method

Goal of the integral method

The goal of the integral method is to ensure the reliability of operation at the necessary technological modes and increase the efficiency of costs for maintaining the proper technical condition of engineering systems.
Sub-goals of applying the integral method:
1. determination of the actual technical condition of engineering systems;
2. search and determination of malfunctions in engineering systems;
3. development of recommendations for ensuring safe operation of engineering systems under actual conditions.

Tasks of the integral method

Determination of strength, durability (residual life), and reliability (safe operation periods and deadline for eliminating malfunctions) of engineering systems and their elements.
Identification of malfunctions (deviations, defects, damage) at early stages of development, prevention of their escalation, and establishment of causes.
Reduction of operation costs by transitioning from emergency repairs to preventive ones.
Reduction of the number of failures (accidents/leaks) through their prevention.
Supplementing and clarifying the assessment of the technical condition taking into account the results of diagnostic examinations conducted by other methods.
Optimization of diagnostic examination programs by reducing the scope of measures while simultaneously increasing efficiency.
Reduction of repair and restoration work volumes and increase of cost efficiency through the use of selective measures to ensure operation reliability at established modes and safe operation.

6.6.2 Main stages (general scheme) of work within the integral method (using a pipeline as an example)

Stage 1. Data analysis
Analysis of project, as-built, and operational documentation, engineering survey materials, results of certifications, diagnostic examinations, and inspections.
The list of studied technical documentation for the selected pipeline section includes:
1. Project documentation:
- general data;
- plan, profile;
- specifications of pipes, fittings, shut-off, and control valves.
2. As-built documentation:
- working drawings;
- certificates for installed pipes, fittings, shut-off, and control valves.
3. Operational documentation:
3.1. data on technological modes of pipelines:
- pressure (inlet/outlet),
- temperature (inlet/outlet),
- data on failures (accidents/leaks) and repairs for the entire period since the beginning of operation.

Stage 2. Building a mathematical model
Based on the results of Stage 1, a spatial model (digital model) of the pipeline is built.

Stage 3. Additional instrumental infrasonic control of dynamic characteristics and parameters of the pipeline, actual loads, and external influences.
At this stage, infrasonic control of above-ground sections of technological pipelines is carried out using infrasonic control devices.
Using infrasonic control devices, linear accelerations and angular velocities in three directions are recorded. Based on the recorded data, amplitude-frequency oscillation characteristics are determined: displacements and rotation angles of the pipeline at the device installation points.

Stage 4. Solving a complex of dynamic problems
At this stage, a complex of dynamic problems is solved regarding the distribution of displacements and forces occurring in pipeline elements; calculation of the dynamic stress-strain state, including in zones of increased stress concentration caused by both design features (pipeline turns and branches, supports, welded joints, etc.) and defects (corrosive damage, cracks, weld defects, etc.) identified during previous non-destructive testing inspections.
1. Determination of dynamic loads acting on the pipeline:
1.1. calculation of pipeline axis bends, axial and transverse forces, bending and twisting moments, stress-strain state (in all sections of the pipeline) caused by a combination of all loads and influences:
- constant and variable components of internal pressure;
- weight of the pipe and the transported product;
- thermal deformations;
- wind, snow, and other transverse loads;
- movement of support nodes.
1.2. identification of pipeline zones where the stress-strain state exceeds the regulatory limit;
1.3. determination of the causes of the non-regulatory stress-strain state:
- high cyclicity of internal pressure;
- level of actual external loads and influences exceeding the level provided by the project;
- deviations from project decisions during construction in terms of ensuring the stable position of the pipeline;
- violations of the planned-altitude position and conditions of support/fixation of the pipeline during operation.
2. Identification of pipeline sections with reduced stiffness:
2.1. geometry defects:
- transverse wrinkles;
- longitudinal folds;
- dents.
2.2. weld defects.
2.3. pipe wall defects:
- general corrosion metal loss;
- erosion metal loss;
- cracks.

Stage 5. Calculation of residual life and determination of safe operation period of the structure as a whole and its individual elements
At this stage, malfunctions (deviations, defects, damage) leading to the limitation of the safe operation period and the causes of their occurrence are identified. The residual life calculation and determination of safe operation periods for the pipeline as a whole and its elements (pipes, fittings, welded joints) are performed, including:
1. calculation of residual life and determination of safe operation periods for pipeline elements taking into account:
- design parameters of the pipeline;
- characteristics of pipe metal and welded joints determining their resistance to constant and variable loads;
(A database formed on the characteristics of materials used in the construction and operation of pipelines is part of the described technology).
- dynamic stress-strain state of the pipeline (point 1, Stage 4);
- information on parameters of defects identified by previous diagnostic examinations, inspections, tests (hydraulic tests, geodetic measurements, in-line diagnostics, corrosion state diagnostics, etc.). (In the absence of information on defect parameters, the maximum defect not detected by diagnostic examination is included in the calculation).
2. identification of malfunctions (deviations, damage, defects, etc.) leading to residual life limitation and assessment of their impact on durability reduction;
3. development of recommendations for compensatory measures (additional diagnostic and repair/restoration work):
- elimination of pipe defects;
- elimination of pipeline fixation and support malfunctions;
- elimination of corrosion protection malfunctions;
- engineering protection of the pipeline;
- a combination of the above measures.
4. ranking pipeline elements by safe operation periods into 3 groups:
- further operation at project modes is associated with unacceptable risk. The malfunction must be eliminated as soon as possible; until elimination, modes must be limited;
- safe operation period is less than the term of the next diagnostic examination minus repair time; corresponding repair and restoration work must be completed before the safe operation period expires;
- safe operation period exceeds the term of the next diagnostic examination taking into account repair time; performing repairs before the next diagnostic examination is inappropriate; safe operation periods are clarified based on the results of subsequent diagnostic examination(s);
5. formation of proposals for draft plans for repair and restoration work and diagnostic examinations, taking into account safe operation periods, the composition, volume, and methods of compensatory measures;
6. exclusion of pipeline elements from short-term repair plans whose safe operation period exceeds the term of the next diagnostic examination, allowing clarification of the rate of defect development and residual life.

Stage 6. Development of recommendations.
At this stage, recommendations are developed regarding the timing and methods for eliminating malfunctions (defects and damage), engineering protection, and ensuring standard pipeline operation conditions.