This paper describes part of a comprehensive joint-industry project on upheaval buckling. It develops a semi empirical simplified design method and detailed design methods based on a new numerical analysis, and illustrates their application by examples. It assesses alternative design strategies, and the implications of strain-based design.
When a pipeline is operated at a temperature and pressure higher than ambient, it will try to expand. If the line is not free to expand, the pipe will develop an axial compressive force. I£ the force exerted by the pipe on the soil exceeds the vertical restraint against uplift movement created by the pipe's submerged weight, its bending stiffness, and the resistance of the soil cover, the pipe will tend to move upward, and considerable vertical displacements may occur. The pipeline response might then be unacceptable because of excessive vertical displacement or excessive plastic yield deformation. Upheaval buckling is hence a failure mode that has to be taken into account in the design of trenched and buried pipelines.
The possibility of upheaval buckling is of increasing concern to the operators of flow lines in the North Sea and elsewhere. Their concern has been heightened by upheaval buckling incidents in the Danish and Norwegian sectors, and by the cost of protective measures such as rock dumping. Shell International Petroleum Maatschappij (SIPM) initiated a comprehensive upheaval buckling research program in 1987. In its final form, the research program included six phases:
review of available models;
cover response, testing and modeling;
numerical upheaval buckling model;
analysis of alternative concepts;
development of an upheaval buckling guideline.
The paper is primarily concerned with the design aspects of upward buckling of buried pipelines. Unburied pipelines can buckle in a related but different mode, in which they snake sideways across the seabed. This mode too is important in practice and can play a major part in relieving the effects of axial force, but it is outside the scope of the present paper.
The paper first briefly describes the phenomenon, and puts forward a simplified method for preliminary design. It goes on to describe a conceptual design method based on the application of the UPBUCK program, and applies the methods to a design example. The paper then considers alternative strategies to resolve upheaval problems, and examines in more detail one of these strategies, a reduction in wall thickness made possible by the replacement of stress criteria on longitudinal stress with strain-based design criteria.