Evaluating Anchor Impact Damage to the Subsea Canyon Chief Pipeline Using Analysis and Full-Scale Testing Methods

Abstract

This paper presents findings from a study conducted as part of a joint industry effort involving engineers from Williams Midstream, Stress Engineering Services, Inc., GL Noble Denton, and Saipem America. The purpose of this study was to evaluate the severity of damage inflicted to Williams’ subsea 18-inch x 0.875-inch, Grade X60 Canyon Chief Gas Export Pipeline due to an anchor impact at a water depth of 2,300 feet. The phases of work included an initial assessment after the damage to the deepwater pipeline was detected, evaluating localized damage via finite element analysis based using in-line inspection data, full-scale destructive testing including burst tests, and final efforts included the design and evaluation of a subseadeployed repair sleeve. The study included modeling Saipem’s repair sleeve design accompanied by full-scale destructive testing. Strain gages were used to measure strain in the reinforced dent beneath the sleeve, that were then compared to prior results for the unrepaired dent test results. The work associated with this study represents one of the more comprehensive efforts conducted to date in evaluating damage to a subsea pipeline. The results of the analysis and testing work provided Williams with a solid understanding on the behavior on the damage inflected to the pipeline and what level of performance can be expected from the repaired pipeline during future operation. After the engineering analysis and testing phases of this work were completed, the deepwater pipeline was repaired INTRODUCTION The Williams Canyon Chief 18-inch diameter pipeline was hooked by an anchor in late 2005 at a depth of 2,300 feet. The resulting damage pulled the pipeline laterally 1,500 feet from its original path. Inspection efforts using ROVs at the time of the accident indicated that the pipeline was not leaking. However, in the interest of safety, the pipeline pressure was lowered to approximately 800 psi (15% SMYS, pressure, where SMYS is the Minimum Specified Yield Strength of the pipe material) and allowed to continue operation while a remediation method was developed (the repair was made at a reduced pressure). The intent after remediation work was completed that the pipeline would be returned to the full 3,200 psi (55% SMYS) operating pressure. A minimum level of information was available; however, the clearly-defined objective from Williams was to develop a reinforcing solution to restore integrity to the damaged pipeline that involved a dent having material loss in a bent section of pipe. Sources of information included ROV video footage, in-field measurements using ROV-assisted tools, and in-line inspection data that provided the three-dimensional geometry of the dent. A photograph is provided in Figure 1 was taken using an ROV showing the geometry of the dent. As observed in this figure the coating was relatively intact, although the in-line inspection tool did detect metal loss in the vicinity of the dent. Figure 2 includes a sonar image of the bend in the pipeline, which was overlaid with scale circles used to provide an estimate of the radius of bend. As shown, the radius of the bend was between 35 and 80 feet. After the initial inspection efforts were completed, Williams contracted the services of Stress Engineering Services, Inc. to perform an assessment of the pipeline damage. Finite element analysis, along with and full-scale destructive testing were used to evaluate the damage inflicted to the 18-inch x 0.875-inch, Grade X60 Canyon Chief Gas Export Pipeline. At the time of the incident, the operating pressure was 1,450 psi (25% SMYS)., while the maximum allowable operating pressure (MAOP) is 3,600 psi (62% SMYS). Multiple defects were detected in the pipeline and measured during the in-line inspection, some involving dents with combined metal loss. From among the identified defects the most severe dent defect was selected and evaluated for further study. The assessment included detailed modeling, as well as full-scale destructive testing. In a parallel effort, Williams retained the services of Saipem America to assist in the design, assessment, construction, and deployment of a repair sleeve. The main focus of the testing program was to experimentally quantify the severity of damage inflicted to the pipeline by the anchor snag. The limited finite element modeling supported the experimental work, primarily to size the indenter geometry. In-line inspection data provided by Rosen was used to generate a representative dent defect including the associated metal loss. The repair sleeve, designed by Saipem America, was tested as part of the program, with results being compared between the reinforced and unreinforced dent geometries to evaluate the effectiveness of the repair. The sections of this paper that follow provide details on the finite element modeling work, experimental assessment efforts, and design/fabrication/deployment of the sleeve technology to reinforce the damaged Canyon Chief pipeline. The authors of this paper were able to participate in all phases of this project, spanning the initial assessment of the dent in question after its discovery to actually designing and deploying the repair technology.