Bird lovers may be horrified, but the use of canaries to save miners from asphyxiation was an effective practice. The sacrificial canaries offered certain benefits. They were far more sensitive to toxic gases than humans, so they provided early warning. They were continuously “on duty,” so they provided full-time monitoring services. And they were extremely economical.

The problems of casinghead gas or gas in the casing/tubing annulus are well known, and producers have been devising ways to prevent its accumulation, or at least to safely bleed it off. Caused by slowly leaking tubulars or packer elements, the leaks themselves are usually so small and slow that they cannot be detected by normal techniques such as temperature logging. Bubble by bubble, the gas slowly makes its way upward until it accumulates just beneath the wellhead. There it builds pressure until it becomes a safety hazard.

In Canada, the problem of casinghead gas accumulation actually spurred legislation that requires producers to systematically check for its presence and take immediate remediation steps.

The challenge offshore

A similar challenge exists with flexible marine risers. Left unmitigated, gas accumulation in the flexible pipe annulus leads to corrosion and reduced fatigue life of the tensile armor layers. This is far more challenging than normal casinghead gas in conventional casing/tubing completions because the corrosion or armor weakening can take place anywhere — usually at the weakest place in the riser. Corrosion is accelerated by mechanical stress or fatigue from flexure, so damage is focused on the point in the riser where it is least wanted.

The flexible riser is the result of a marriage of engineering ingenuity, manufacturing skill, and materials science. Only a few companies have the ability to construct reliably flexible pipe in the diameters and lengths required that can withstand the harsh environment of offshore production operations. Flexible risers must perform flawlessly from near freezing, high-pressure conditions at the seabed to ambient temperatures and pressures at the surface.

In general, flexible pipe consists of a central stainless steel or metal-alloy carcass covered by an outer sheath that provides hydraulic integrity. Numerous layers of flexible armor surround the sheath, or pressure vault, to provide tensile- and hoop-stress strength. The armor layers are usually separated by cushioning layers of composite or thermoplastic material to prevent them from rubbing against one another. The number of armor layers is a function of the pressure and tensile strength specifications imposed by the particular application for which the riser is designed. A final thermoplastic outer sheath provides protection from external sources such as seawater ingress, attack from marine growth, or abrasion.

Micro-leaks and time create hazard areas

The pressure vault is made as impermeable as possible, but high-pressure gases can seep slowly through the material and accumulate in the annulus between the inner and outer sheaths. Also, as the thermoplastic material ages, it can lose some of its flexibility and develop permeable micro-cracks. The gases contain molecular quantities of water that can condense as pressure and temperature drop. These coalesce into measurable amounts of water that form corrosion cells to attack the armor. Finally, the outer sheath can become damaged from a combination of aging and fatigue or from being struck by another object.

Currently, the presence of annular gas is checked by performing periodic vacuum or pressure tests of the flexible riser’s annulus integrity. The objective of the tests is to measure the annulus’ gas-filled volume to detect accumulations of condensation or seawater ingress. The tests are sometimes augmented by continuous measurement of gas that escapes from the riser’s vent ports. Unfortunately, even if the tests are administered faithfully, they are very expensive and can provide inadequate results. Gaps in the data include information regarding gas diffusion rates or water vapor emissions. Also, there is no alarm system to warn of problems in time to perform preventive maintenance.

A new canary is hatched

Engineers at Total and Schlumberger have developed a fully automated “canary” that continually monitors and assesses flexible riser pipe annulus condition and eliminates the need for costly vacuum tests. The new system is called the subC-racs.

In a fundamental sense, the design had to address four key issues to be effective:

• It had to allow continuous monitoring because operating conditions vary almost continuously;

• It had to enable inference of the condition of the internal thermoplastic sheath by accurate diffusion rate measurements;

• It had to allow continuous monitoring of outer sheath integrity; and

• It had to measure the annular environment, including the presence of diffused gas and water (from both condensation and ingress water).

Meeting requirements

The operator requirements were demanding. Whatever the final solution, it had to be external and capable of being installed on any riser equipped with vent ports. It had to incorporate continuous monitoring, have a robust alarm system, and avoid the requirement for vacuum testing. The system had to be capable of measuring annular volume as well as water fill-up rates and detect outer sheath failure in real time. Finally, it had to be capable of measuring the gas diffusion rate and flow rate at the vent ports and verify the proper operation of the ports.

In the end, a small but effective measurement unit was designed to fit on the deck of the production facility. Each unit samples a single riser annulus at the vent ports and exhausts into the flare line. All data are collected, managed, and analyzed, and the system is controlled by a standard PC computer. A remote junction box rated for Ex Zone 1 enables the system to monitor up to 16 risers.

The innovation comes from the operating principle. Standard reservoir engineering methodology is used that is familiar to all petroleum engineers — pressure transient analysis, material balance, and sampling — making the potential users immediately comfortable with the solution.

The way it works is that the migrating gas slowly pressures up the riser’s annulus by about 0.5 bar. Then, a solenoid valve opens, dumping the annulus gas through a highly accurate gas flow meter into the flare line. The annular free volume is derived from the vented gas mass and analysis of the pressure variations.

Electrical analogy validates system

Using an electrical analogy where pressure buildup was simulated by building up an electrical charge on a capacitor through a series of resistors that simulated the riser annulus, the system was modeled and its outputs and system behavior predicted. Once the simulation worked and passed qualification tests in the Schlumberger flow laboratory, the actual equipment was tested on a 6-in. diameter, 558-ft (170-m) long riser at Total’s yard in Pointe Noire, Congo.

The test was simulated electrically; then the actual riser sample was tested at different temperatures and pressures with nitrogen and with carbon dioxide. The subC-racs system calculated volumes compared with an acceptable degree of repeatability to the annular volume obtained from the riser manufacturer. The pressure variations matched the simulation, which allowed evaluation of vent port performance.

The next step was to start an extended field test, which is currently being conducted by Total Congo at one of its offshore production facilities. Two risers are monitored, with annulus volume and gas migration rate accurately measured every few days.

It is expected that the deployment of the subC-racs system will improve safety and reduce maintenance costs for offshore production systems using flexible riser pipe. The early warnings provided by the system will allow time for replacement risers to be mobilized and switched out with minimal production interruption.

Over time, detailed analysis of riser histories using the system’s continuous data is expected to shed light on failure mode analyses, potentially enabling designs to be modified to provide greater longevity and eliminate catastrophic failures.