Pipe Collapse Under Combined Pressure, Bending, and Tension Loads

Abstract

ABSTRACT The paper presents results of a combined experimental and analytical study of the response and stability of pipelines under combined bending, pressure and tension loads. Such loads are encountered during the installation of offshore pipelines. Test facilities for three load combinations consisting of bending and pressure, tension and pressure, and tension and bending are described. Experimental results obtained from relatively thickwalled model tubes (10 < D/t < 40) are presented. These include the response of the tubes to the combined loads and the loads at collapse. For the first two load combinations, collapse envelopes for various loading histories are presented. Preliminary results from an ongoing study of the pipe response under combined tension and bending are also presented. Numerical solution procedures for these combined loads have been developed. They are verified by comparing the predictions to the experimental results. INTRODUCTION Oil and gas produced from offshore wells are gathered and transported primarily through steel pipelines installed onto the sea floor. The ambient external pressure makes the pipeline structures collapse critical. The problem is obviously accentuated as offshore operations move to deeper waters. The most severe loading conditions are usually encountered during installation, when, in addition to the external pressure, the line must sustain tension and bending loads. A typical pipe laying process is shown schematically in Fig. la?. The process is performed off a vessel with a reel onto which long sections of the line are prewound onshore. The vessel travels to the operation site, where the line is unwound and paid into the ocean and the end of the line is pulled through a J-tube to the top of a platform. In the process the line acquires the configuration shown in the figure. The winding and unwinding processes involve severe bending and reverse bending in the presence of tension. The shape of the suspended section is controlled by the tension applied at the vessel. The tension is necessary in order to keep the curvature of the line in the sag bend at a safe level. The process of pulling of a line through a J-tube induces tension and bending in the presence of external pressure. Figures 1b, c, and d show schematically the curvature (II:)-pressure (P), tension (T)-pressure and curvature-tension load histories induced to the line by these operations. In order to design such operations, the limits to which the combined loads can be applied safely must be established [1]. This paper presents results from a combined experimental and analytical study of the main factors which can affect the collapse of a pipeline loaded by combined tensile, bending and external pressure loads. The study concentrated on pipes with low diameter-to-thickness ratios (D/t < 35) which would be suitable for deep water applications. For experimental convenience, the loads are studied in pairs of bending/pressure, tension/pressure and tension/bending. Special testing facilities developed and used in the study of each of the three combined loads will be presented. Typical experimental results and analytical predictions from each case will be shown.