Hydrostatic Collapse Research In Support Of The Oman India Gas Pipeline

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.

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“…The value of the reduction factor for UOE pipes was established based on 22 tests from the conceptual Oman–India study. 12…”

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“…The specification of wall thickness to resist local buckling due to bending, axial tension, and external pressure was developed using limit state design codes and industry recognized experience with the benefits of thermal aging (1,2,3,4) to shed any unnecessary conservatism. Consequently, the wall thickness profile was defined to withstand the loads imposed during its installation and operation.…”

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“…Full-scale testing included 7 tests: 2 collapse tests involving the application of external hydrostatic pressure to collapse and further tested by propagating the buckle; and 5 pressure + bend tests involving the initial application of external hydrostatic pressure to a target value then, while maintaining external pressure, increasing specimen bending until collapse occurs. C-FER's Deepwater Experimental Chamber, shown in Figure 6 and summarized in [2], [3], and [4], was used for all the fullscale collapse and pressure + bend tests. The chamber, designed and built for the Oman-India collapse test program has a tested pressure capacity of 8,500psi, an inside diameter of 48 inches and an overall inside length of 407 inches.…”

“…Figure 17 shows a comparative assessment of various calculation equations for determination of collapse pressures and the results of full-scale tests performed on UOE-process pipes produced by Europipe GmbH. Data points from the qualification for the "Oman -India" project [4], [11] are also plotted in this view, in addition to the test results under examination here, which originate from the qualification for the Mardi Gras project. It is readily apparent that, with the assumption of f o = 0.5% and αfab = 1, the calculation based on API 1111 [10] conforms very well with the calculation in accordance with DNV.…”