testing | Subsea pipelines
Exova argues pipe coating and joint testing programmes should take into account the specifics of the pipe laying process. This artist’s impression shows Heerema Marine’s J-Lay deepwater AEGIR vessel, currently under construction in South Korea
It is important to recognise that damage to both the coating and the field joints can occur at many points during pipeline installation: l Reeling assembles the pipe into long lengths onshore from where it is wound onto a reel on the vessel by yielding the steel. At the field it is then unwound, straightened and laid down to the seabed. Cumulative bending strain experienced by the pipe can lead to reduced fatigue resistance and brittleness, ovalisation of the pipe (which can lead to hydrostatic collapse), and buckling of the pipe during reeling due to large plastic strains and insufficient section stiffness. l S-Lay is typically used for shallow water applications (0-300m), but some vessels are able to use it for deepwater applications (up to 2,000m). During installa- tion, the pipeline will experience high stress at the over- bend and sagbend. A stinger and tensioner is used to maintain bending curvature at the overbend/sagbend. l J-Lay differs from the S-lay method and the overbend stress is eliminated. Multi-joints are possible, depend- ing on the tower height, and in a one station tower all welding, NDT and FJC coatings are performed in one place. However, this technique involves slower produc- tion times than S-lay. l On the way down to the sea-bed the pipe can be exposed to significant thermal gradients. For typical low to mid-latitude waters, sea surface temperatures range from slightly below freezing near the poles to an annual average near 30°C in the tropics. Deep ocean tempera- tures span a much more narrow range, between about 0°C and 4°C. l In the ocean, the downward force of gravity is balanced mostly by an upward pressure gradient force. This upward pressure gradient means that pressure increases with increasing depth. It is also a common
36 PIPELINE COATING | November 2012
misconception that steel does not corrode in very deep water. While it is true that the corrosion rate is less in deep water, it is far from zero. l The seabed is neither flat nor smooth. Expansion of the installed pipeline in service is restricted by the interface friction between the pipe and the soft seabed, so the pipe is subjected to an axial compressive load. When this load reaches a critical value, the pipeline becomes unstable and is displaced in various directions – vertical (upheaval buckling), lateral (lateral buckling) or axial ( pipe walking).
Testing limitations Current testing methods do not relate to the real-life problems that may occur during each of the sections of the pipe laying process. There appears to be more testing related to the reduction in heat transfer and less concentrating on the effects of the application and laying process of these multi-layer and highly complex polymer systems. Damage to the outer layer can expose the more susceptible inner layers and undermine the heat transfer properties of the overall coating. Corrosion can also cause a number of problems.
Leaking can result from corrosion from either damage initiated during the application laying or over a period of time due to the coating not being suitable for the environment encountered during laying or in-situ. Another issue is waxing. This can occur due to incorrect coating selection or damage caused to the coating during installation or use. Either cause results in poor flow. Exova proposes a number of new tests that if
incorporated into the new draft specification (ISO/CD 12736: Petroleum and natural gas industries – wet thermal insulation coatings for pipelines, flow lines, equipment and subsea structures) will minimise the
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