Optical Sensors Monitor Vulnerable Top Sections of Flexible Risers
- Adam Wilson (JPT Special Publications Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 2018
- Document Type
- Journal Paper
- 57 - 58
- 2017. Offshore Technology Conference
- 1 in the last 30 days
- 55 since 2007
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This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 28080, “Monitoring Flexible Risers With Optical Sensors—Operational Experience and Future Perspectives,” by B.M. Santiago, S.R. Morikawa, W. Carrara, A. Kravetz, and J.A. Martins, Petrobras, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.
The integrity management of flexible pipes used in subsea production plays a major role in maximizing the availability of the production systems while minimizing safety and environmental risks. One of the key areas for integrity management is the riser top section, where high tensions and curvatures result in high stresses and may lead to fatigue issues. A monitoring technique known as Monitoring Based on Optical Fiber Attached Directly on Armor Wires, or MODA, has been developed to assess the integrity of the tensile armor at the top section of flexible risers.
A flexible pipe is composed of several layers, each having a specific function (Fig. 1). The carcass is a metallic layer designed to withstand external compression forces in the radial direction. The internal pressure sheath is a polymeric layer used to contain the fluid inside the pipe. The pressure armor is a metallic layer designed to withstand the internal pressure. The tensile armors are made of metallic wires designed to resist axial forces acting on the pipe. In risers, the tensile armors are responsible for keeping the flexible pipe suspended from the floating production unit (FPU). The outer sheath wraps the other layers, protecting them from the external environment. The region inside the internal pressure sheath is called the bore, and the region between the pressure barrier and the outer sheath is known as the annulus space.
The riser top section is subjected to high tensions because of the weight of the riser and to cyclic tensions and curvatures because of FPU movement, which results in high constant stresses and significant cyclic stresses in the tensile armors. This combination may lead to fatigue issues, especially in a flooded-annulus condition, which severely hinders the pipe capacity to withstand cyclic loads.
The MODA monitoring technology has been developed to better assess the integrity of the tensile armor at the top section of flexible risers.
MODA Development History. At its beginning, the MODA concept was only retrofitted to risers already in service. To install the MODA sensors, it was necessary to remove a portion of the outer sheath, attach the optic-fiber sensors to the wires, repair the outer sheath, and then provide an optical path to an equipment room at the FPU where the MODA equipment was to be installed. Substantial resources were needed in each MODA installation, and only the most critical risers were considered for monitoring.
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