ABSTRACT:

The paper presents an FEM analysis of the collapse behaviour of thick tubes (10<dο/t<40). The loads and load combinations considered in this paper are: external pressure, combined pressure and tension loads, combined pressure and bending loads. The effects of initial ovality, residual stress, strain-hardening, yield anisotropy and loading paths are included in the analysis. The analysis procedure is validated through a systematic comparison with excellent experiments published. Sensitivity studies on the factors affecting tube collapse have been carried out since it is easy to control parametric variables in the FEM procedure. Collapse envelopes from the FEM analysis are presented for the load combinations. Equations to predict collapse strength and collapse envelopes are discussed.

INTRODUCTION

Thick tubes are widely used in deepwater pipelines, oil casing, tendons of tension leg platforms and risers in offshore engineering. The high ambient pressure in deep water requires use of tubular with lower diameter-to-thickness ratios, Do/t, normally between 10 to 40. Practically, when a pipeline is pulled through a J-tube, some tension load exists due to pulling of pipelines, while large plastic curvature is applied together with high external pressure. This type of load combination also occurs in the sag bend of tubes in laying pipe. In this situation, all three loads are moderate. The combination of external pressure, tension and bending loads is also encountered in tendons of tension leg platforms (TLPs). During large storms such as hurricanes or in ship collision accidents, the tendons of TLPs are subjected to large pressure induced by deep water as well as steady and time-varying tension and bending loads due to the storms or the impacts. The present study is devoted to an FEM analysis of the collapse behaviour of the thick tubes subjected to external pressure, tension and bending loads as well as their combinations.

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