Structural Integrity Testing

Structures and equipment are subjected to a variety of loads, pressures and temperatures during operation. These operating conditions can impose complex stresses and strains throughout the structure that can impact performance or cause failure.

We apply combinations of load, pressure and temperature to full-scale structures to reproduce the stresses and deformations that can occur during operations. The sequence that these are applied to the specimen can be critical to the response of the specimen. This is especially true if the stresses exceed the yield strength of the material, resulting in permanent deformations. The test must also simulate the rate at which the operating conditions change since faster or slower changes can result in different responses. Operating at high or low temperature can change the material characteristics, which can also change how the specimen responds.

There are five basic loading scenarios that can be combined to reproduce virtually any operational loading conditions:

  1. Multi-axis loading
  2. Burst
  3. Collapse
  4. Temperature
  5. Fatigue

Burst Capacity of Pressure Containing Equipment

Many industrial processes include pressure piping systems and pressure vessels that can fail due to overpressure. Corrosion, erosion or mechanical damage can reduce the wall thickness of this equipment, reducing their capacity to contain pressure. The distribution pattern of this wall loss can be very complex, making it difficult to predict the burst capacity of the equipment. Material inclusions and weld flaws can also significantly reduce the burst capacity of this equipment.

Tests are typically run with water as “hydrotests” to reduce the amount of energy stored in the specimen. Nitrogen can be used for burst tests, but requires significantly more robust safety systems to protect against sudden failure of the specimen. Heat transfer oil is used in high-temperature applications to prevent the fluid from flashing to a gas when the specimen bursts.

Care must be taken when bursting large structures since the sudden release of pressure can propel the specimen with significant force. Complex or welded structures are particularly dangerous as the location and direction of the release and the resulting movement of the specimen is often difficult to predict.

Instances where burst testing is required include:

  • Line pipe with internal or external corrosion pitting or cracking;
  • Type II vessels – steel vessels with composite overwrap in the hoop direction (FAST-PIPE);
  • Type III vessels – steel vessels with full composite overwrap (gas transportation modules (GTM));
  • Type IV vessels – full composite pressure vessels;
  • Fibreglass reinforced pipe (FRP); and
  • Threaded connections for well casing and tubing.

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Burst Testing

Max. Pressure: 412 MPa (60,000 psi)
Max. Temp: 350 °C (660 °F)
Max. Specimen: 1.3 m (52 in) diameter
Test Media: water, nitrogen, oil

Multi-axis Loading of Full-scale Structures

Most structures are acted on by more than one force acting in different directions, which results in a complex, three-dimensional stress state. In some cases, these loads are imposed by interactions with other structures. In other cases, combinations of load and pressure cause stresses in different directions.

Bending is one of the most common multi-axis loading scenarios. In industrial settings, bending is often combined with internal or external pressure.

Changes in temperature can also impose significant loads if thermal expansion or contraction are constrained. Thermally induced loads can exceed the yield strength of the material. This can result in permanent deformation of the structure.

Some examples of multi-axis loading include:

  • Biaxial tension and biaxial compression in pipelines due to bending loads from ground movement and internal pressure;
  • Torsion plus axial tension in sucker rods operating progressing cavity artificial lift systems in oil wells;
  • Compression plus bending in well casing connections installed in deviated wellbores; and
  • Collapse plus bending in subsea pipelines during installation.

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Multi-axis Loading

Max. Load: 72 MN (16M lb-force) tension
Max. Specimen: 15 m (49 ft)
Max. Stroke Rate: 100 mm/s (3.9 in/s)
Max. Bending Moment: 27 MNm (20,000 kip-ft)

Collapse Resistance of Subsea and Deep Well Systems

Full-body collapse tests of pipes used in subsea pipelines and risers, as well as in oil and gas wells, are often performed because of the complexity of the collapse mechanism. The mechanism of collapse depends on the ratio of the pipe diameter to the pipe wall thickness (D/t). Collapse resistance is also strongly influenced by the ovality, eccentricity and residual stress in the pipe.

Collapse tests typically require that specimen lengths are at least 10 times the outside diameter of the specimen. This ensures that the specimen end caps do not influence the collapse resistance. This means that collapse testing of large diameter pipes can require long pressure vessels.

The testing procedure is also critical in determining the collapse resistance. Analysis of testing procedures has shown that the rate of pressure application can influence the measured collapse resistance.

Bending pipes under collapse conditions can also decrease the collapse resistance. This can be critical in ultra-deepwater pipeline installations and in tubulars installed in deviated wellbores. We perform collapse plus bending tests to assess the impact of the magnitude of bending on the reduction in collapse resistance. These tests have shown that the collapse resistance also depends on whether the collapse pressure or bending is applied first. For welded pipes, the orientation of the longitudinal weld relative to the bending direction can influence the collapse resistance.

We perform collapse plus torsion tests on flexible flow lines and risers. This combination of loads can lead to “bird caging” of the metal windings and damage to the liner or exterior carcass.

We have developed new methods to conduct collapse tests on short rings of thick-wall pipe rather than full-length pipe specimens. A specialized ring testing apparatus and procedure is used to provide repeatable measurements of collapse resistance that are correlated with full-pipe collapse tests. These test procedures could allow collapse tests to be conducted as part of quality control programs for thick-wall pipe in mills.

We have also conducted collapse tests on a variety of other equipment including:

  • Hulls for autonomous underwater vehicles;
  • Submarine hulls with various repair procedures;
  • Flexible subsea flowlines and risers;
  • Subsea valves;
  • Inline inspection tools for pipelines;
  • Downhole tools; and
  • Well liner junctions.

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Collapse Testing

Max. Pressure: 55 MPa (8,000 psi)
Max. Specimen: 1.15 m (47 in) diameter
Bending Moment: 12.2 MNm (9,000 kip-ft)
Torsion: 217 KNm (160 kip-ft)

Temperature Effects on Equipment Performance

C-FER Technologies’ origins are in evaluating the performance of structures in extreme cold environments that are encountered in developing oil and gas resources in Canada’s Arctic. Low operating temperatures cause most materials to become brittle. This can affect the load and fatigue capacity of structures, such as ice breaker hulls and stationary ice resisting structures.

We can test these cold weather structures under representative operating conditions. This requires not only large loading systems, but also high capacity cooling systems. Instrumenting these tests can also be challenging due to condensation and frost accumulations.

Changes in the temperature also cause thermal expansion and contraction of structures. If the structure is unconstrained, the stress in the structure usually remains unchanged. If the structure is constrained from expanding or contracting, stresses can increase in the structure. For extreme temperature changes, these stresses can exceed the yield strength of the material. This can result in permanent deformation of the structure. If the extreme temperature changes are cyclic, incremental deformations can occur with each cycle. Coupon or full-scale tests can be conducted to determine if the incremental damage caused by thermal cycling will lead to low-cycle/plastic fatigue failure.

If equipment operates at high temperatures for prolonged periods, material creep and stress-relaxation can occur. This can lead to permanent deformations. Upon cooling, the deformed equipment may be subjected to large residual stresses and cracking. The challenge with measuring these high-temperature processes is that they can occur over long periods of time. Advanced material testing methods infer long-term material performance while keeping testing times reasonable.

The rate of temperature change can also have an impact on equipment performance. Rapid changes in temperature can cause temperature gradients in large structures. This can lead to differential thermal expansion and non-uniform stresses in structural components. Thermal shock can occur when systems operating at high temperature are suddenly quenched with cold fluid or gas. In subsea or polar regions applications, where the environment is normally cold, thermal shock can occur when warm oil and gas from deep in the earth are brought to surface.

Structural testing scenarios where high or low temperature conditions need to be considered include:

  • Low temperature;
  • Fracture propagation in girth welds in Arctic pipelines;
  • Fill/empty cycling of composite compressed natural gas storage vessels with cooling due to the Joule-Thomson effect;
  • Thermal shock loading of subsea valves;
  • High temperature;
  • Well casing connection performance in high pressure, high temperature (HPHT), thermal and geothermal wells; and
  • Structural performance of downhole tools.

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Temperature Effects

Cooling

  • Nitrogen Expansion: -70 °C
  • Circulating Glycol: -40 °C
  • Ambient (January): -20 °C

Heating

  • Electric Furnace: 900 °C
  • Electric Resistance: +500 °C
  • Electric Induction: +500 °C
  • Steam: 250 °C

Fatigue Induced by Load or Pressure Cycles

Dynamic loads and pressure cycling can cause fatigue failures in structures. Pre-existing cracks in welds and stress corrosion cracks (SCC) in the pipe body are typical features that can grow with each pressure or load cycle.

Low-cycle fatigue or plastic fatigue can occur when the cyclic stresses in the specimen exceed the yield strength of the material. We generally run these tests with the loading system operating in displacement control to simulate field conditions.

Welded fittings and joints on piping systems can also be susceptible to fatigue failure. Traditional weld-o-lets for branch lines can cause stress concentrations that accelerate fatigue. Newer low-stress welded fittings can increase fatigue life for branch lines.

We can also simulate wave loading on flexible risers. These tests assess the performance of the riser in the equivalent of many years of operation under a range of wave height scenarios.

We have conducted fatigue crack growth tests in composite and advanced aluminum alloy wing panels for commercial and military aircraft. These tests can require several hundred strain gauges to monitor crack growth across the panel. Visual image correlation has also been used to measure crack growth. Some tests are further complicated by requiring salt fog to simulate operations in marine environments.

We conduct cyclic pressure tests on corroded pipe specimens to estimate crack growth rates. Rapid pressure cycling allows many years of operation to be simulated in less than one week. Test results are used to estimate the remaining service life of pipe with known crack features.

Similar cyclic pressure tests are run to simulate multiple fill/empty cycles on composite GTM and other vessels for transporting CNG.

Structural Testing Offshore Risers

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Fatigue Testing

Max. Dynamic Load: 5 MN (1,124 kip)
Pressure Cycle Rate:  ~100,000/week
Max. Strain Reading Rate: +1,000 Hz

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