Integrated surveillance technology optimizes subsea production

The path from the pore to the pipeline is most critical in deepwater and ultra-deepwater wells. Drilling and completion pose significant challenges; however, the phases can be finalized within a few weeks. Production can continue for 20 years or more, so the industry must identify ways to sustain production with minimal intervention. To accomplish this, a system is required to predict and detect potential events that could adversely affect production; ensure and enhance safety of personnel, environment, and equipment; allow durable and efficient complexity management; expedite the decision process related to HSE issues; and optimize operator assets.

Commonsense approach
A comprehensive asset integrity management system can be achieved by adopting an integrated approach from the outset. Integrity management is an important part of technical strategy – it is essential to invest today to have protection in the future, if needed. In addition, the operator can anticipate production changes and take action to make efficient decisions.

A production system needs to be monitored and any potential problems detected before system integrity is threatened. A comprehensive system incorporates:
1. Implementing a proactive design that anticipates future issues;
2. Using a comprehensive open architecture that encourages standardization but accepts customization for special cases;
3. Providing sufficient bandwidth in the communications network to accommodate both production and system integrity data, taking into account future needs as the reservoir matures;
4. Ensuring sensors are robust, reliable, accurate, and qualified; and
5. Developing system software with simulation capability.

A conceptual subsea chain illustrates productivity from pore to pipeline. (Illustrations courtesy of Schlumberger)

A holistic approach is needed during the system design. Each node of the system should be supported with a business case that details the financial implications of its implementation or the financial consequences if the node is omitted.

Individual elements of the subsea production system will not solve the problem regardless of the system’s sophistication; a fully integrated network solution is necessary.

Integrated surveillance system
The ability to collect continuous data to provide a complete in-service operational history is an important element of the integrated surveillance system. Continuous data give operators real-time operational feedback to manage the subsea production system.

Starting with the well, surveillance is applied through downhole gauges and sensors supported by high-bandwidth telemetry that transmits continuous well performance and flow parameters to the surface and allows control signals to be transmitted downhole to operate adjustable flow control media in the well. Among others, these components provide an early warning of sanding, water encroachment, or changes in reservoir drainage patterns.

At the seabed, or perhaps in the well itself, flow boosting equipment such as electrical submersible or multiphase pumps help move the production to surface. They also provide the opportunity for surveillance of their performance and any significant changes in the fluid flow at the nodes.

Along the way from the well to the surface production facility, flow assurance measurements provide accurate flow parameters including flow rate and flow regime, phase fraction, temperature, and pressure. The data provide early warning of deposits of asphaltenes, wax, and hydrates or the formation of scale on the interior surface of tubulars. Most flow assurance systems also use the data to meter the injection of inhibiting chemicals into the flowstream or to control flowline heating elements to forestall flow assurance incidents.

Developing seamless technology
Along with measuring and controlling flow integrity, measurements must be able to determine the integrity of the overall production system.

Optical fiber technology has played a role in enabling system integrity surveillance. Fiber-optics solutions include discrete measurements of pressure, axial strain, curvature, and temperature, as well as distributed temperature sensing and distributed vibration sensing. The latest fiber-optics tool is distributed temperature and strain sensing. Not only can these sensors detect and quantify internal conditions of the flow system, they also can sense external effects such as seabed movements or third-party interference, such as a fishing boat dragging its anchor across the flowline.

To address the challenges specifically related to the flexible pipe domain, Technip and Schlumberger have initiated a collaboration to develop and provide subsea surveillance systems. They recently developed a new series of flexible pipes incorporating an optical fiber conduit within its tensile armor layers. This allows fiber-optic sensors to be deployed after the flexible pipe is commissioned offshore. There are a number of applications for the distributed measurements enabled by the flexible pipe design. For example, distributed vibration can be used to detect tensile armor wire breakages, and distributed temperature measurement can be used to detect annulus flooding.

The new flexible pipe design enables operators to move from relying on periodic visual inspection and testing to real-time monitoring of critical integrity parameters. The operator is then able to rationalize the maintenance requirements for the complete asset. This is condition-based maintenance, where maintenance is driven by actual equipment condition rather than the expiration of a specified time period. Maintenance is performed only if needed and can be planned according to the operator’s time schedule.

Used by the airline industry, NASA, and the military, CBM is based on the idea that calendar- or time-based maintenance either is too late to avoid failures or wastes money fixing things that do not need to be fixed.

The ability to collect continuous data to provide a complete in-service operational history is an important element of the integrated surveillance solution architecture.

The systems currently are being submitted to full qualification programs to demonstrate efficiency and reliability. Tests also are being carried out in Brazil for armor wire rupture detection.

New monitoring approach
Total and Schlumberger have collaborated in the development of a new monitoring approach for flexible pipe integrity called subC-racs. Flexible pipes that flex with variable subsea currents and wave action are used in all offshore fields to transport high-pressure, high-temperature fluids. An issue for flexible pipe integrity is the presence of water in the annulus, either due to condensation or damage to the outer sheath. Seawater, or condensation water in the presence of diffused CO2 or hydrogen sulfide, can lead to corrosion of the armor layers and can impact the service life of the line. It also is possible for oxygen and moisture from the air to enter the annulus if vent ports are not correctly connected.

The subC-racs system relies on reservoir engineering principles using pressure transient analysis, material balance, and sampling, combined with Technip (Flexi France) expertise on flexible pipe annulus, to detect and quantify gas egress or water ingress in the subsea flexible riser annulus. The system is able to measure parameters and access conditions that could cause degradation of the flexible riser life expectancy. Software analysis is able to define accurately the annulus condition. This allows the operator to ask for expert assessment and plan maintenance in advance, thus potentially helping to prevent a failure while minimizing downtime.

During onshore tests, the riser annulus was injected with different volumes of nitrogen and CO2. The tests validated the subC-racs system's ability to determine annular volume within 2% precision.

Ongoing field tests on production and gas export lines demonstrate the system's continual ability to determine the real-time status of the outer sheath, annular volume, and difusion rates.

A technician tests an optical fiber temperature and strain sensor on a test jig (orange).

A floating installation in West Africa was comissioned in December 2009. SubC-racs data used alongside the gas sampling analysis enable continuous assessment of armor layer fatigue life and improved scheduling for life extension.

The subC-racs technology also has been adopted by Technip, which has incorporated it into its manufacturing plants, allowing flexible manufacturing and installation to have a “birth certificate” using the subC-racs. Any variation on the annulus volume and composition together with gas migration rates can be detected from the manufactured and commissioned state early on. This enables mitigation measures to be taken by the operator if necessary.

Complete, proactive package evolves
An integrated surveillance solution is an important decision-making tool for operators. System integrity can be monitored so several problems potentially affecting well or reservoir productivity can be detected, analyzed, and resolved with minimal cost and downtime.

Decision work flows are augmented by simulation software that allows the operator to perform hypothetical scenarios. They are enhanced by the availability of reliable, accurate measurements from the integrated monitoring system to eliminate guesswork and compare alternate approaches to determine the best solution for a situation including financial implications.

Planning integrity monitoring at an early stage is an important aspect of field development. Investing in integrity management can pay for itself by eliminating unnecessary and costly interventions, prolonging production uptime, and eliminating unplanned retrofit monitoring at a later stage.