Hydrostatic Collapse Research In Support Of The Oman India Gas Pipeline

Abstract

ABSTRACT This paper provides a summary of the collapse test program conducted as part of the technical development for the Ultra Deep Oman to India Pipeline. The paper describes the motivation for conducting the collapse test program, outlines the test objectives and procedures, presents the results obtained, and draws conclusions on the factors affecting collapse resistance. INTRODUCTION Oman Oil Company (OOC) is implementing the 1139 km Oman to India Gas Pipeline Project (OIP). The pipeline will cross the Arabian Sea, initiating at Ra's al Jifan in Oman with landfall at Rapar Gadhwali in India, and will encounter a maximum water depth in the Dalrymple Trough of 3525 m, This is over three times deeper than any pipeline yet laid, Fig, 1 shows the principle features of the route. Likely methods of installation include J-lay and reeling, both of which result in bending strain in the sagbend near the seabed whilst reeling also applies a bend and straightening technique which results in residual ovality after straightening. Tension and bending are known to reduce the collapse resistance of pipe and, during installation, these factors, in combination with a maximum bottom pressure of 35.4 MPa (5135 psi), make the possibility of pipe collapse due to external hydrostatic pressure a major factor in the pipeline design. Gas throughput requirements necessitate a minimum internal pipeline diameter of 24 inches and, based on existing collapse design theory, preliminary engineering selected a 24-inch ID × 1.625-inch WT (max) grade X-70 material for the pipeline. To achieve a realistic production rate, the use of UOE pipe is required, however, little or no data is available on the collapse performance nor on the combined effect ofpressure and bending for UOE pipe of this diameter and D/t ratio (17.2). In order to address these and other issues, and as part of he overall pipeline design verification process, OOC instigated a comprehensive collapse testing program which began in March 1994. MOTIVATION FOR TESTING Although a good body of literature exists on the general theory of pipe collapse, specific tests this project for the following reasons:Existing collapse pressure data on large diameter, low D/t UOE pipe is very limited.Data on collapse under combined bending and pressure does not exist for such pipes.The collapse pressure of pipe manufactured by the UOE process has consistently been found to be lower than that of pipe of the same grade produced by different manufacturing processes.There is little data available regarding buckle arrestor performance for this diameter and D/t pipe and it is therefore necessary to validate any buckle arrestor design experimentally. SCOPE OF TESTING The test program comprised four parts. Part 1 involved the straight collapse testing of 20-inch OD × 1.1 25-inch WT, X65 pipe specimens each approximately 4,5m (15ft) in length. Part 2, 3 and 4 tests used 26-inch OD × 1.625-inch WT, X60 pipe specimens approximately 8.8m (29 ft) in length.