Flexible Pipe Sensitivity to Birdcaging and Armor Wire Lateral Buckling

Abstract

Petrobras has been successfully dealing with deep water floating production systems using flexible pipes since 1977. During the completion of Marlim South 3 well in 1977, Petrobras was surprised by the occurrence of two birdcage type failures. At that time, Marlim South 3, in a water depth of 1709 m was the deepest offshore production well in operation. Since then, Petrobras has been testing flexible pipes using a field test known as DIP test. In a DIP test, an empty end capped sample of a flexible pipe, about 150m long, is partially supported by the sea bottom and connected to a lay vessel by a pipe follower or a wire rope. The flexible pipe has to withstand a 4 hour period of cyclic bending due to the motions of the lay vessel. The DIP test has provided Petrobras with information on a new failure mode: lateral buckling in the armor wire. Although a birdcage failure is equally undesirable, lateral buckling of the armor wires implies more danger because it can go unnoticed. In 2001, a research project was set up by the Research and Development Center of Petrobras that was aimed at reproducing the flexible pipe failure modes under laboratory conditions. The purpose was to obtain a better understanding of the failure process, as well as to develop testing alternatives to avoid the significant costs related to DIP tests. In order to assess the effect of cyclic bending as a major factor in degrading the longitudinal compressive strength of flexible pipes 15 destructive tests were performed on 4 inch diameter flexible pipe samples. Two test rigs that accommodated three types of test and a number of test procedures were developed in the project. The number of bending cycles to failure for each sample was determined when subjected to compressive action corresponding to its operational depth. Tests to evaluate the effect of pre-existing damage were also conducted. Special attention was devoted to the effect of layer arrangement on compressive failure. The test results clearly identified the basic failure modes under investigation (i. e. birdcaging and lateral buckling of the armor wires). Suggestions regarding simplified testing procedures and corresponding performance criteria are also presented.