Buckling like a snake
“We must let the pipeline buckle like a snake on the seabed, although in a predicted and controlled manner. This will avoid future environmental accidents and reduce intervention costs,” says Roberto Bruschi of Snamprogetti.
Although subsea pipelines are the safest way of transporting oil and gas, accidents do happen. Increased pressure and temperature in the pipeline causes it to move and deflect in uncontrolled directions. Luckily, none of these events has yet caused a release of hydrocarbons, but the serviceability of the pipeline has been threatened.
“To explain what happens with subsea pipelines we can draw a comparison with old railway tracks. Here the solution was to leave a little gap between each piece, allowing it to expand in a controlled fashion on hot summer days. Nowadays, railway tracks are fully welded together. This is possible because the tracks are fixed onto concrete sleepers, which in turn are embedded in gravel,” explains Roberto Bruschi, who is known as one of the world’s foremost pipeline scientists.
Uncontrolled pipeline buckling
Similarly, pipelines may be fixed by trenching and burying them below sand or dumping gravel on top of them. This has been the common practice until today. However, several of these pipelines have buckled in spite of this, mainly due to new field developments experiencing increases in pipeline temperature and pressure, especially since the gas being put directly into the pipelines without being cooled. The temperature difference between the gas inside and the external environment may be up to 150°C, which naturally causes thermal expansion and buckling. Further, there is no public design criterion against which to rationally consider the associated uncertainties.
“Due to insufficient measures to mitigate thermal expansion, pipelines are expanding and buckling downwards, upwards and laterally in an uncontrolled fashion, just like a snake moving in random directions. This can lead to reduced transport capacity in addition to permanent damage. Such transformations may in the worst case trigger accidents that have a dramatic impact on the environment,” explains Roberto Bruschi.
Will allow buckling
Because of the increase in unpredicted pipeline global buckling, there has for many years been a need for a reassessment of the current design practice. Due to this, a new DNV Recommended Practice has been developed as a joint venture between Snamprogetti, DNV and several industrial sponsors. In addition to stipulating criteria for fixing the pipeline, this also includes a design procedure to allow it to buckle in a controlled manner, thus minimizing intervention cost.
“This has been a four-year-long project which has led to the development of a new recommended practice. Several pipeline projects have already been designed in different areas of the world according to this practice,” says Roberto Bruschi.
Safety first. Roberto Bruschi emphasises that the need to reassess the design standard was merely related to safety aspects, although it is clear that the costs may also be reduced dramatically. First of all, enormous amounts of gravel and digging efforts may no longer be required and the time frame of these operations will decrease significantly. Secondly, by preventing accidents, the environment will be safeguarded and the future cost of and efforts put into correcting damage to the pipeline will be reduced.
There is only one way of achieving safer and more controlled pipeline behaviour. Much more effort must be put into the technical design compared to earlier projects. We must predict and control the pipeline’s behaviour like a ‘snake charmer’.
“New knowledge and technology now give us an opportunity to control the expansion of pipelines in locations where this is anticipated. In other words, we can now predict where a pipeline will expand and how it will expand. We can also calculate where gravel dumping and burying is still necessary to keep the pipeline in place. Whatever solution chosen, the pipeline’s integrity must be documented for the sake of safety,” concludes Roberto Bruschi.
Snamprogetti
Snamprogetti is the technology contractor of ENI and was established in 1956 to provide services to the petroleum industry encompassing all phases of engineering and development work related to producing, processing and transporting hydrocarbons from wellhead to consumer, placing special emphasis on minimizing environmental impact and pollution.
For the past 30 years, Snamprogetti has developed unique expertise in the offshore pipeline industry and has carried out a large number of successful projects, particularly in deep-sea and difficult environmental conditions, involving more than 19,000 km of pipelines.
Author: Svein Inge Leirgulen
日付: 20 December 2005
