Pipeline On-Bottom Stability Calculations: Comparison of Two State-of-the-Art Methods and Pipe-Soil Model Verification

Abstract

ABSTRACT This paper presents three stability related work tasks conducted by the American Gas Association (A.G.A.) during 1989. These tasks are:Comparison of the A.G.A.'s pipeline stability design methodologies detailed in the Design Guidelines and Software1, with those presented in Veritec's Recommended Practice for On-Bottom Stability Design of Submarine Pipelines (RP-E305)2.Comparison of weight coating designs using the two "design level" analysis tools described in the above documents.Verification that the pipe/soil interaction model used in the A.G.A. software can predict pipe/soil behavior. INTRODUCTION Discussions for each of the above tasks are presented in separate sections below. Some familiarity with both the A.G.A.2,4,5,6 and PIPESTAB7,8,9,10,11 efforts is assumed, and is necessary to appreciate fully the comparisons. Section 1 describes the A.G.A. and Veritec design methodologies. It then points out the similarities in research, as well as some fundamental differences in design philosophies and finally, the design tools produced from each research effort. Section 2 compares approximately 200 pipeline designs based on the A.G.A. and Veritec level 2 design procedures. Section 3 presents results of numerical simulations of pipe/soil model tests simulating random seas. The results of this exercise support the implementation of the soil model, and its predictions of both embedment and soil resistance. SECTION 1.0 - COMPARISON OF DESIGN METHODOLOGIES The following compares design methodologies detailed in; a) the recently developed A.G.A, design guidelines and software; and, b) that presented in Veritec's recommended practice (RP E305, 1988). Veritec's RP-E305 is based on the PIPESTAB project. Basic research details, concepts, and principles used in the A.G.A. work are compared with those used in the PIPESTAB project. Table 1 provides an overview of the design procedures and summarizes differences between the A.G.A. design method and the Veritec design method. The following paragraphs briefly describe the design methods presented in the A.G.A. Guidelines, and in Veritec's RP E305. A.G.A. Design Methodology Three design procedures (Levels 1, 2, and 3) are presented in the A.G.A. design guidelines and software1. The first procedure (Level 1) is based on traditional stability analysis methods (Morison type hydrodynamic forces and frictional soil resistance). It is intended only as a reference to the type of static analysis which has been done in the past. The most detailed procedure uses finite element time domain simulation software (Level 3). The software provides detailed information regarding pipe movement and stresses during design events. Pipeline safety is then assessed based on these results. The Level 2 procedure assumes no net movement of the pipe, and is based on a quasi-static calculation which simulates the embedment process modeled in the Level 3 software. (See Figure 1) The process modeled is that of a pipe embedding itself into the soil during small amplitude displacement oscillations caused by wave loadings. The resulting lateral soil resistance is calculated and compared to the expected hydrodynamic forces to determine pipe stability. The Level 2 and Level 3 analyses differ from the traditional method (Level 1) in several important areas: