Validation of a Computer Model for Flexible Pipe Crushing Resistance Calculations
Several types of laying equipments are used to install flexible pipelines for offshore field developments. These pipelines installation tools apply generally compressive radial forces to hold the pipe suspended weight. The deeper the pipeline has to be laid, the higher is its suspended weight and therefore the higher these radial loads need to be. As each flexible pipe construction is optimized for each field application, a design methodology is necessary to be able to evaluate the radial load acceptable by the flexible pipe. The failure mode associated with this type of loads is an instability thought to be similar to the hydrostatic collapse mode. Therefore an adequate design safety factor has to be considered. The water depth of offshore field developments becoming ever deeper, the resistance of the flexible pipe to installation loads becomes often a driving design criterion. A finite element model to address this type of loading has been developed and improved over the past years. To avoid over-sizing the flexible pipe with current design approach, this finite element model needed to be improved for the latest materials used and for higher crushing loads. To this effect, a new powerful test bench with a crushing load capacity of 1200 tons over one meter has been designed, procured and is now operational. It can handle pipes from 4" to 19" internal diameter. Many types of flexible pipe samples have been tested up to permanent deformation using bi-, tri- and quadri-caterpillar tensioners. The results of these tests have been used to validate a new finite element model using in particular non-linear elasto-plastic material laws. In this paper, several test results will be presented and compared with the calculations. The relevance of different possible design criteria depending on the type of loading regime, the slenderness of the pipe and the number of radially resistant layers, will also be discussed. This new model is operational and allows to optimize the flexible pipe design in particular for ultra-deep water applications down to 2500m or more.