Session: 04-01-02 Flexible Pipes and Umbilicals II

Paper Number: 106952

106952 - Flexible Pipe Fatigue Life Evaluation Based on a Dynamic Full-Scale Test Considering a Preconditioning Corrosion Procedure 

The fatigue design of tensile armour layer is a key parameter during flexible riser design. Usually, the expected design life ranges from 20 to 30 years and the fatigue life must be evaluated considering intact (i.e., dry) and flooded annulus condition to be in accordance with API 17J requirements. In general, the flooded condition drives the design calculations and, nowadays, it can be said that this is a very likely scenario, since external sheath breach is the most common damage found in flexible pipes, according to recent flexible pipe integrity surveys. The corrosion fatigue resistance of tensile armour layer materials is characterized by SN curves obtained in environments which are expected to be representative of flexible pipe annulus during operation and the current industry practice is to use un-corroded armour wires for testing. However, some recent publications have already shown that the armour wires are prone to pitting corrosion if sufficient exposure time is provided.  Therefore, it becomes evident that the fatigue resistance of pitted tensile armour wires becomes very relevant for flexible pipe design and, specially, for Fitness-for-Service evaluations. In this work, a systematic approach was adopted to evaluate the fatigue resistance of a flexible pipe with corroded tensile armour wires. Initially, a baseline SN curve was obtained from un-corroded tensile armour wire samples exposed to a controlled corrosive environment of synthetic seawater and 1 bar CO2 pressure. The SN curve was obtained using a four-point bending test setup. Then, a mid-scale test apparatus was developed to expose a short flexible pipe section (200mm long) to same corrosive environment for 12 months. After the exposure period, additional SN curves were obtained for dry and flooded conditions. In parallel with the mid-scale degradation test execution, a full-scale flexible pipe sample (approximately, 8m long) was preconditioned with the same corrosive environment used for the short sample. After 12 months, a dynamic tension-to-tension test was conducted and the fatigue damage imposed during the test was calculated using the SN curves obtained previously.  It was possible to observe a better correlation between the full-scale test result and the estimation done when the SN curves of corroded armour wires was used. This fact shows that tensile armour wire pitting can play an important role in flexible pipe fatigue assessments.

Presenting Author: Tiago Brun Coser Petrobras

Presenting Author Biography: Engineer working at Petrobras Research Center (CENPES) with experience on research and development projects of SURF products, especially flexible pipes, thermoplastic composite pipes and umbilical cables.

Main skills: research and development project management, full-scale and small-scale testing of SURF products, design of ancillary components for SURF products, design methodology development.

Fields of interest: mechanical behavior of materials, fatigue analysis and structural integrity; numerical methods and computational simulation.

Authors:

Tiago Brun Coser Petrobras
Luiz Antonio Sulino de Negreiros Petrobras
George Carneiro Campello Petrobras
Erik Duarte Radke LAMEF (UFRGS)
Carolina Delwing Rosa LAMEF (UFRGS)
Derek Fonseca De Souza LAMEF (UFRGS)
Luiz Francisco Venturini Rodrigues LAMEF (UFRGS)
Tiago Samuel Renck LAMEF (UFRGS)
Mariana dos Reis Tagliari LAMEF (UFRGS)
Marcelo Favaro Borges LAMEF (UFRGS)