Flexible Risers in Deepwater Production – Reliability Concerns

New Wave Media

October 28, 2010



Flexible risers are being extensively used worldwide for deepwater O&G extraction, offering many advantages over rigid risers. However concerns still exist about the overall safety of the system, especially concerning failures leading to hydrocarbon leaks in the ocean environment.

The flexible risers being used in deepwater are designed to work for a period of around 20 to 30 years, which is the expected lifetime of an O&G field. A flexible riser failure could happen due to a number of reasons, such as hostile environmental hazards and material fatigue and if the failure is catastrophic, it could lead to high economic and environmental consequences.
Flexible risers comprise multiple layers, and the main layers of concerns are the tensile armor layers which exist to withstand tension loads. The tensile armor layers are of major concern because they are surrounded by other metallic layers and so they are difficult to inspect. Radiography also enables other metallic layers to be inspected including the pressure armor and inner carcass layers.
The layers of steel and polymer that comprise a flex riser have a complex structure, with some of the layers being shielded by others, making the Non Destructive Testing (NDT) particularly difficult. Radiography has long been proven as a versatile method for carrying out NDT industrial radiography. With advances in digital technology, digital radiography has also established itself as reliable and efficient way of conducting radiography in flex risers. Digital advancements have now made underwater radiographic inspection a reality, along with equipment adapted for marine and subsea environments. In deepwater these test are usually done by radiographic testing equipment operated by ROVs.
The versatility of flexible pipe to provide the range of applications called for in deepwater is largely a function of their structure which comprises the helical application of metallic wires and tapes, and extruded thermoplastics to form a composite pipe structure designed to address field specific constrains such as pressure, water depth, temperature, fluid characteristics. Flexible risers are of vital importance for deepwater production and also in some cases for deepwater drilling.
Static jumpers used to connect top tension riser systems to topside floater pipes, usually through a lazy-wave disposition, completes the portfolio, with their ability to absorb vertical movements of the production unit without transmitting dynamic loadings into the topside pipe work providing a significant advantage.
Although there are still concerns over the long term reliability of risers, this problem has been extensively addressed by manufacturers, in such a thorough way that there is little concern over material failures, but failures due to the hazardous environment found in deepwater operations still abound. These can only be tackled by computer simulations, which are not always accurate enough. Regular inspection and maintenance is the procedure most commonly used to curtail problems and failures in the system, including fitting of the risers to flowlines on the bottom and jumpers placed along the way and finally the connection of the flowlines to the FPSO. Risers also need to have their decoupling mechanisms always checked and tested, in case a major accident happen underwater or on the surface that demands a quick decoupling.
As flex risers are intensively used in Brazil´s deepwater plays, the confidence in the system is large and has only increased. However, there remains a constant need for inspections to keep safety standards high.
Claudio Paschoa
Paschoa, Claudio
Claudio Paschoa is Marine Technology Reporter's correspondent in Brazil.
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