Development of a Failure Detection System for Flexible Risers
a report by
Priscilla Elman and Roberto Alvim
2H Offshore
Flexible risers are responsible for approximately 80% of the oil and gas transportation of Brazilian offshore production and play an outstanding role in deepwater offshore operations worldwide. There are more than 1,200 flexible risers currently in use offshore Brazil, and more than 1,000 flexible risers are operational in the North Sea. The initial cost of a flexible riser is significantly higher than that of more conventional risers, such as steel catenary rsers. Flexible risers are mainly used in highly dynamic loaded environments, where outstanding fatigue performance is essential. Flexible risers are commonly used in today’s harsh ultra-deepwater developments.
A complex engineering process takes place in the design and fabrication of flexible risers, frequently relying on assumptions regarding the environmental loading conditions and other specific requirements of the riser over its lifetime. With the increase of deepwater developments, more advanced flexible riser designs are required and demanded from offshore operators.
A number of in-service flexible risers have failed offshore in the past few years. Each failure presents considerable challenges for the operator in terms of loss of production, additional repair and installation costs and environmental and safety implications. The majority of the reported failures on flexible risers are related to damage at the top section, close to the bend stiffener.1
Investigation
of some of the failed flexible risers reveals significant damage to the outer polymer sheathing. The damage is associated with extensive unintended interference between the flexible riser and the vessel structure. The breach of the outer polymer sheathing exposes the internal armour wires to the corrosive environment. The onset of corrosion along with high fatigue loading aids the rupture of the armoured wires under tensional load and leads to torsional instability, resulting in potential premature loss of the entire riser. These effects may be worse in large-diameter flexible risers since the armour wires are more sensitive to fatigue damage. This is especially valid in high stress concentration regions, such as found in the interior end fittings.
The failure mechanism of ruptured riser armour wires is not well understood as the response during the moment of failure is difficult to measure in-field and data are closely guarded. The critical riser components associated with failures are internal and thus not visible from the outside. This makes it difficult or impossible to detect the damages or onset of damages with traditional inspection methods. It is difficult to know when the riser is under a failure process and thus impossible to prescribe preventative or corrective actions. As a result, flexible risers have failed in service prematurely and exposed the field operator and environment to significant risk and additional cost. Significant efforts have been undertaken by many operators, contractors and professional bodies to better understand and prevent
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damage to or loss of in-service flexible riser systems. Recent developments have shown that a system for failure detection is required by the offshore industry to safeguard flexible risers from harm or even catastrophic failure. A number of technologies are commercially available today. Some key requirements for a failure detection system are:
• cost-effective; • non-intrusive; • easily retrofittable to existing structures; and • compact size.
With these requirements and considerations in mind, 2H Offshore has developed a non-intrusive system for failure detection on flexible risers that records a number of parameters simultaneously, called FLEXASSURE™. The FLEXASSURE system is a result of 2H Offshore’s instrumentation excellence and rigorous system development and testing. This article presents the results of the laboratory tests conducted with an array of sensors installed on a flexible riser test piece under tensile load. The test was conducted in order to detect the failure mode of armour wires using non-invasive sensors. It is designed to help understand the mechanism of in-service flexible riser failures. An additional offshore test was carried out to validate the proposed system under offshore conditions. All conducted tests show that the FLEXASSURE system is capable of detecting the signatures of armour wire failures and can be retrofitted as a means to detect the onset of failure in flexible riser systems.
Flexible Riser Monitoring System
The flexible riser monitoring system FLEXASSURE is a non-intrusive, retrofittable, compact and cost-effective monitoring system that includes monitoring hardware, realtime data acquisition system and data processing. Sensors are used to detect a range of physical properties such as acoustic emission and riser movement. The data logging system captures data in realtime and conducts preliminary processing to detect anomalies. All measured data are saved onto a hard disk for later processing. The measured data are processed in realtime and the monitoring data can be made available onshore or offshore.
The FLEXASSURE system integrates four discrete and proven assurance sub-systems to capture the riser response and failure mechanisms. Each part of the overall system is detailed below and illustrated in
Figures 1 and 2.
Sub-system 1 – Bend-stiffener INTEGRIpod™
A collection of state-of-the-art sensors are simply strapped onto the riser below the bend-stiffener in a low-profile cylindrical housing called INTEGRIpod™ that can be installed easily using divers. Being close to the bend-stiffener, it is installed close to the expected failure location. This
© TOUCH BRIEFINGS 2010
Subsea & Pipelines
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