Department of Engineering - Annual Report 1996/97

Energy And Fluid Mechanics

Pipeline Buckling
Impact Damage
Ice Mechanics
Global Warning
References

Research activities have initially centred on pipelines and ice mechanics (which is a central factor in offshore petroleum development in the Arctic)(H1). It is intended to move into other areas, particularly drilling mechanics and reservoir simulation and characterisation.

Pipeline Buckling

Professor A.C. Palmer

A major current concern to the constructors of underwater pipelines is thermal buckling, which can make a pipeline arch upwards into the sea or sideways across the bottom, in response to the axial compressive forces induced by the operating temperature and pressure. The problem has caused severe cost overruns on several North Sea projects. Research into both lateral and upheaval buckling has continued. David Miles(H2) has focused on lateral buckling, and has developed understanding of the factors which determine whether many small buckles form (which is comparatively harmless) or whether one large buckle forms (which can have severe consequences).

One way of preventing buckling of a trenched pipeline has been to bury the pipeline, most economically by backfilling the seabed soil previously ploughed out to make the trench. However, it has been found that the processes of ploughing and backfill so heavily deform the soil that its mechanical properties are severely degraded, and it no longer has enough strength to stop the pipeline moving. Two students, Amy Faranski and Andrew Barefoot, have investigated the deformations that occur during backfilling, and have measured the reduction in strength. This work is now being continued by another student, who is working with a major contractor who is supporting the research. In parallel, research into the striking speed effect that occurs during ploughing has continued.

Another manifestation of pipeline buckling is instability on hills. A serious accident occurred when a pipeline under construction in South America suddenly buckled and slid downwards. Under a contract with the oil company for which the pipeline was being constructed, the group investigated the cause of the accident, recommended tests, and developed guidelines to prevent a recurrence.

Impact Damage

Professor A.C. Palmer

A pipe can be damaged by fast-moving debris from an explosion, and that was a factor in the escalation of the Piper Alpha disaster. Some previous work suggests that liquid-filled pipes are more vulnerable than empty pipes, an unexpected result. Suj Sivadasan carried out an experimental study of pipes filled with water and molten Wood's metal (a heavy alloy which melts at the temperature of hot water, and avoids the safety problems of mercury), and confirmed that the effect is real and that it correlates with density. Further work is being carried out to examine the effect of liquid fill on the detailed mechanisms of deformation around the impact point.

Ice Mechanics

Professor A.C. Palmer

Sea ice forces on offshore structures show a scale effect: the observed forces on the scale of an offshore production platform are much lower than would be expected from a simple interpretation of model tests at small scale. It is known that this is a consequence of a changeover from a mode dominated by plastic deformation to one dominated by brittle fracture, an important result with wider implications for geotechnical modelling. Research continues on the detailed mechanisms of repeated fracture, in collaboration with work in the geotechnics group on the implications of particle fracture. On 1 August, work started on the LOLEIF (low-level ice forces) program, a major research program supported by the EU and led by the Hamburg Ship Research Institute, in which the Department joins with other institutions in Finland, Norway, Sweden and France. Cambridge theoretical work on fracture modelling will complete field work on an instrumented lighthouse in the Baltic, and will incorporate measurements made in the Russian and Canadian Arctic.

Global Warming

Professor A.C. Palmer

Petroleum contributes to the huge additions of carbon dioxide to the atmosphere, which are generally agreed to be a primary contributor to the greenhouse effect. One way of reducing emissions is to capture carbon dioxide at sources such as power stations and cement works, and then to dispose of it in the deep oceans. It is agreed that controlled dumping will have much smaller environmental impacts that uncontrolled dumping by absorption from the air, which is what happens now. If the carbon dioxide can be injected as a liquid at a depth of 3000 m, it will sink and form a stable lake of liquid carbon dioxide in a hollow on the bottom. There are a number of interesting engineering problems(H3), some of them related to existing subsea engineering technology.

H1. Palmer, A.C. Proyektirovanie i prokladka morskikh truboprovodov v Arktikye: opyt zapadnovo Yamala i Alyaski. (Design and construction of marine pipelines in the Arctic: experience from western Yamal' and Alaska) Proceedings, Conference on Oil And Gas Pipeline Projects in Russia and the CIS, Moscow, Russia (February 1997).

H2. Palmer, A.C., Calladine, C.R., Miles, D., Kaye, D. Lateral buckling of submarine pipelines. Proceedings, Offshore Pipeline Technology Conference, Amsterdam, the Netherlands (February 1997).

H3. Palmer, A.C., Ormerod, B. Global warming: the ocean solution. Science and Public Affairs, 49-51 (Autumn 1997).

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