With much of the world’s “easy” oil already extracted from onshore and shallow-water reserves, deepwater E&P, in locations such as the Gulf of Mexico and Brazil, are now a focal point for many operators. However, increased development of these deepwater prospects is presenting fresh challenges in terms of the provision of suitable technology, including the requirement for risers to perform effectively at ever greater depths.

Risers are a critical part of field architecture. A failure in operation could result in a significant loss of revenue and production and have a negative environmental impact. Risers are subject to a number of loading issues, including internal and external pressure, vessel motions, and current and wave actions. As a result, risers, and in particular the flexible type, endure significant strain levels that can impact on their integrity and functionality.

The changing times

The traditional industry method for combating these issues has been extensive onshore testing on small sections of the riser, allowing the operator build up a bank of fatigue and reliability data that is used to statistically forecast the strains and stresses the riser will encounter. This data takes into account expected changes throughout the lifecycle of the riser such as material degradation and environmental issues, including storms and hurricanes.

While such testing was an accepted monitoring method in the past, technology advances mean that nowadays a suite of real-time monitoring tools can provide a far more accurate picture of a riser’s condition during operation. This improves decision-making by allowing structural, pressure, and temperature issues to be detected at the earliest possible stage and rectified in the most efficient manner. This ensures that risers satisfy safety and regulatory requirements, and it helps maximize oilfield productivity.

Technology at work

Distributed strain sensing can monitor minute strains at 3-ft (1-m) intervals along the length of a riser. Sensors can provide data on a riser’s tensile load and potential failure points. If, for example, wave conditions change quickly and a particular flex is under stress, an operator can almost immediately determine whether a riser should be shut down.

The latest dynamic strain measurement (DSM) technology allows measurements to be taken 10 times per second along the entire riser length. This compares with previous distributed strain sensing systems that provided measurements every five to 10 minutes at certain pipe sections, meaning a significant level of averaging had to be carried out to source any meaningful data. The level of monitoring with the DSM system is crucial, as a fast technique is required to monitor dynamic loads.

Strain sensing of this type also could extend a riser’s field life, as providing such clear information on a riser’s performance could allow an operator to see that a riser is under less strain than what was assumed. This could in turn create a welcome reduction in capex and opex for a project. With more accurate data, the safe operational lifetime of the riser can be maximized.

Identifying temperature changes along a riser is also critically important. If a riser gets too hot or cold, it can change the state of waxes, hydrates, and condensates being transferred through it, resulting in production issues.

For example, waxes can form blockages if the temperature falls below a critical level. If this happens, a riser may have to be taken offline to rectify the problem. Another serious issue that needs to be monitored is a breach, which could result in saltwater breakthrough and corrosion of the riser. A distributed temperature sensing (DTS) system can provide such data because it can measure temperature changes of less than 32ºF (0.01°C) at all points along the riser. DTS would also be crucial in identifying riser leaks, which can have disastrous environmental effects and can significantly impact production.

Real-time riser monitoring is set up to play an increasingly important role over the next five years as operators start to insist on the adoption of this technology in the risers delivered to them. As oil production reaches into deeper and deeper water depths, the real-time understanding of the integrity of the risers will be of paramount concern.

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