Optimizing the Design of Subsea Rigid Pipelines in Ultra-Deep Waters of the Brazilian Pre-Salt, Through Full-Scale Axial Pipe-Soil Interaction Tests

Abstract This paper introduces a novel methodology for addressing the complex behavior of axial pipe-soil interaction in soft marine clays, considering drained, undrained and partially drained soil responses. The results of the proposed methodology are compared with those obtained using the traditional alpha approach for determining the axial friction factor. The novel approach was developed based on extensive full-scale tests conducted at an advanced laboratory facility specifically designed and constructed in IPT (Institute for Technological Research of the State of São Paulo (IPT) with Petrobras support, to replicate geotechnical properties of marine clays. The full-scale tests focus on utilizing Pre-Salt's typical soft marine clay profiles found in southeastern Brazil, and full-scale pipe sections. The methodology encompasses a wide range of parameters, including pipe weight, pipe diameter, soil undrained shear strength profile, pore pressure dissipation ratio at the pipe-soil interface, and over consolidation ratio. Additionally, the test matrix in the laboratory experiments involved imposing axial sweep velocities ranging from 10-3 to 1 mm/s. In recent developments, rigid flowlines have been designed for the subsea systems of the Búzios and Mero assets, located in areas with soil conditions primarily composed of soft marine clays. These conditions introduce new challenges in evaluating axial pipe-soil interaction, especially when compared to traditional methods such as the alpha approach. The axial interaction between pipe and soil plays a crucial role in defining anchoring loads, tolerable and characteristic VAS) induced by thermal buckling, end expansion and pipeline walking. For the design scenarios of production and water-alternating-gas (WAG) rigid flowlines in the Mero and Búzios assets, which operate under high-pressure and high-temperature (HPHT) conditions, the novel proposed method presented in this paper demonstrates an approximately twofold increase in the residual axial friction factor compared to previous methods such as the alpha approach. Furthermore, the proposed method provides a reliable framework for evaluating the residual axial friction factor across varying soil drainage conditions, simplifying design calculations for diverse field scenarios and encompassing the pipe-soil parameters assessed during laboratory testing. The adoption of this novel method significantly enhances the evaluation of residual axial friction factors in pipe-soil interaction, resulting in improved and optimized flowline designs compared to previous methods. These improvements have substantial implications for anchoring loads, end expansion in spools, and axial feed-in within engineered buckles. This paper discusses these findings in detail, with a particular focus on the Búzios and Mero assets, highlighting their impact on advancing flowline design in complex geotechnical environments.