Special Session: Advances in Flexible Riser Technology: Flexible Pipe Curved Collapse Behavior Assessment for Ultra Deep Water Developments for the Brazilian Pre-salt Area
Authors: Clevelario Judimar Assis , Pires Fabio , Falcao Gelson , Tan Zhimin , Lu Jing , Sheldrake Terry H. 
Published: July 2010

The collapse strength of flexible pipes is a well known property and predicted by means of numerical or analytical modelling. However, recent oil field discoveries in the Brazilian pre-salt area, in water depths up to 2500m, are pushing the industry to evaluate the effects of flexible pipe curvature on its collapse resistance when the pipe is subjected to extreme levels of hydrostatic pressure such as that expected in the Brazilian pre-salt area. As part of Wellstream's R&D programme, a numerical model was specifically developed to predict flexible pipe collapse resistance when subjected to curvature to assess and quantify the effects of curvature on the flexible pipe collapse resistance. This paper summarizes the numerical modelling process and experimental calibration performed using five different pipe structures specifically designed for the pre-salt cluster. A total of 45 calibration tests were performed using samples taken from the same production run in order to eliminate variance in production machine setup and raw material batches. Regular production equipment, setups and quality control procedures were used to manufacture the test samples so that the collapse resistance of the pipes could be evaluated against conditions identical to commercial manufacturing. Of the five different pipe structures evaluated in this test programme, one of the pipe structures supplied for the TUPI EWT project development is already installed and operating at a water depth of 2140m. The test programme allowed the experimental assessment of the curvature effects on flexible pipe collapse behaviour, demonstrated the suitability of flexible pipe structures and model to predict the effects of curvature on collapse resistance.

Introduction

The industry is focusing on the latest offshore discoveries in the Brazilian pre-salt cluster due to the huge volumes of oil and gas identified in this new exploratory frontier. However, the technological challenges associated with this development are significant and mainly affect subsea equipment suppliers. The ultra deepwater, which can reach up to 2500m, and the presence of contaminants such as CO2 and H2S in the conveyed fluids are major challenges to be addressed. The presence of H2S in the conveyed fluids may make the selection of lower strength metallic materials compulsory to avoid HIC and SSC failures; such failures would be a problem when massive service pressures and loads are expected in service. This scenario requires an innovative design approach to consolidate new technologies and create a pipe structure that can withstand the service requirements.

In 2008, Wellstream established a global R&D programme focused on the development of a new generation of flexible pipes specifically designed for the Brazilian pre-salt cluster. This programme has a diverse scope of work addressing aspects such as metallic and polymeric material development, product evolution, product innovation, model calibration, and alternative design solutions.

This paper focuses on Wellstream's approach in improving the collapse resistance of flexible pipes designed for the Brazilian pre-salt area, summarizing the FEA model development and calibration, and test programme.

Although the industry has established both analytical and numerical modelling techniques to predict flexible pipe collapse resistance, some concerns regarding the suitability of flexible pipes for service in water depths of up to 2500m have been raised; this is mainly due to the bending radius expected for the flexible pipes in the touchdown zone (TDZ) during installation and operation.