Upheaval Buckling Failures of Insulated Buried Pipelines: A Case Story

Abstract

ABSTRACT The first recognized upheaval buckling of a subsea, buried pipeline took place in 1986 in one of Maersk Olie og Gas AS' interfield pipelines in the Danish Sector of the North Sea. It is the objective of this paper to present the case story of this pipeline upheaval giving a chronological description of all actions taken, in order to re-establish the pipeline integrity. Based on the upheaval buckling experience within Maersk Olie og Gas AS, a recommendation for the design and installation of buried, hot pipelines is presented. INTRODUCTION During the annual pipeline inspection survey in July 1986 along the buried Rolf A/Gorm E two phase pipeline in the Danish Sector of the North Sea, a pipeline section was discovered to have protruded the seabottom, and was standing in an arch. The 17 km long pipeline is an O.D. 8.625" × WT 14.3 mm carbon steel line (API 5L grade X 52). The pipe is insulated with a 2" thick polyethane foam (PUF) of min. density 96 kg/m3 encased in a high density polyethylene (PE) jacket. The exposed PUF at the ends of the PE jacket is sealed with water tight end cap sleeves. A 2" thick concrete weight coating is applied on top of the PE jacket resulting in an overall pipe diameter of 0.45 m. A 3" gas lift line is "piggy backed" to the 8 " pipeline. The pipeline build-up is shown in Figure 1, and a detailed description of the pipeline is given by Pallesen et.al. (1985) in ref. [1]. The pipeline was laid in the summer of 1985 in 40 m water depth utilizing conventional lay barge techniques. Trenching of the line was carried out using water jetting equipment. The pipeline was brought into service January 1986 for transport of unstabilized hydrocarbons with a temperature of up to 180°F (82°C) from the Rolf satellite field to the central process facility at Gorm. Upheaval buckling analysis of the 8" line was carried out as part of the design documentation resulting in a lowering requirement of 1.15 m to top of the concrete coated pipe. The certified upheaval buckling calculations were performed according to the state-of-art at that time, i.e. using the "classical" upheaval buckling analysis, where the design criteria against upheaval snap buckling is based upon post buckling equilibrium curves assuming a pipe of uniform weight on a rigid foundation. Following the detection of the exposure an upheaval buckling research programme was immediately initiated at the Technical University of Denmark. The results from the study showed that the "classical" design approach did not always yield conservative results. Thus, a new and improved theoretical model was developed and made available late 1986, and formed the basis for the repair work. The new design approach has been published in ref. [2], [3] and [4].