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Ensuring the structural integrity of deepwater risers

TWI carried out a series of very detailed engineering critical assessments (ECAs) for a set of deepwater steel catenary risers prior to their installation in an oil and gas field.

The risers had the following characteristics:

  • Constructed from C-Mn steel, incorporating alloy 625 cladding in critical locations (touch-down point and top of riser)
  • A variety of pipe geometries, in view of the different service requirements (production, water injection, gas export, oil export)
  • A requirement for sour service.

The objective of the ECAs was to define maximum initial allowable flaw sizes that would ensure the integrity of the risers through a complex cycle of installation and operation, taking into account the possibility of fatigue loading, sour service and internal corrosion during the operational period.

Figure 1 Example of a steel catenary riser
Figure 1 Example of a steel catenary riser

Designed to withstand the deep

As illustrated in Figure 1, a deepwater riser is a type of vertically oriented pipeline connecting subsea hydrocarbon facilities to a drilling/production unit. In one direction, they transport produced hydrocarbons from the well, whilst in the other they may carry control/injection fluids from the platform to the seabed. In view of their contents (highly flammable fluid under high pressure) and their proximity to the platform (which may be manned) and to supply shipping, ensuring the structural integrity of risers is of paramount importance.

As oil and gas reserves from ever-deeper water (so-called ‘ultradeep’ fields can have water depths exceeding 1500m) are exploited, so the requirements for the structural integrity of the risers have become increasingly rigorous.

Steel catenary risers are subjected to a variety of loads, both static and cyclic, including internal pressure, self-weight, fatigue loading and loading from ocean currents, and to a range of environmental degradation mechanisms, including external and internal corrosion, and environmentally assisted cracking (particularly in the presence of sour fluids, ie those containing hydrogen sulphide).

Minimising the risk of catastrophic failure

Risers are made by welding multiple lengths (approximately 12m long) of linepipe together, either in the field or long before installation (in the latter case, they are reeled onto a spool and reeled off at the point of use). Whichever installation method is used, the structural integrity of the girth (circumferential) weld is of particular significance, since girth welds are subjected to cyclic (fatigue) loading and are typically uninspectable once installed. Leakage or failure of a riser could, of course have disastrous consequences for human safety, for the environment, and for the 
cost-effectiveness of the field.

Although the weld (an example of which is shown in Figure 2) is made by an automated process and to a very high quality, it should be recognised that any welding process has the potential to introduce flaws, and the ability of non-destructive testing (NDT) to detect, size and, if necessary, reject such flaws needs to be critically considered. In practice, failure of the girth welds is avoided by careful attention to the quality of materials, welding and inspection, backed up by an ECA, which is a quantitative analysis of the relationship between flaw size, materials properties and the loading on the riser. In this way, it is possible to ensure that flaws that might escape detection during NDT will not grow large enough to lead to failure of the riser during its design life, as illustrated in Figure 3.

Working in accordance with the relevant standard

The calculations for this case were carried out using BS 7910 (‘Guide to methods for assessing the acceptability of flaws in metallic structures’). This standard, backed up by substantial validation data and recognised by both industry and safety authorities, provides the means to carry out complex ECA calculations in a reproducible manner. The document contains a range of information, including stress intensity and reference stress solutions for cracked bodies (including pipes), fatigue crack growth data, residual stress profiles and information on inspection reliability. This allows the analyst to carry out an assessment with minimal reference to sources outside it, although information such as applied stresses and materials properties (fracture toughness and tensile properties) are, of course, generated specifically for the project.

If you would like to find out more about TWI’s work with the oil and gas sector, or about its ECA services, please email contactus@twi.co.uk

Figure 2 Example of girth weld subjected to a transverse stress, ie along the pipe
Figure 2 Example of girth weld subjected to a transverse stress, ie along the pipe

For more information please email:


contactus@twi.co.uk