Gene Kliewer, Technology Editor
Flexible riser design is an engineering process based on complex assumptions about the product, environment, and the specific application. To compound this challenge, flexible pipe is a complex product with unique design and construction issues, particularly as water depths increase. Even with industry experience, failures do occur, and the attendant cost of such failures and the problems they induce, are enormous.
Flexible riser integrity management systems are available to reduce the problem. These systems can include hardware monitoring, data acquisition, and data interpretation to reduce the probability and severity of failures. It also can lead to more accurately tailored maintenance and intervention plans.
Failure issues
Once flexible pipe risers are installed, a number of problems can affect their performance and field life. External damage can arise from pipe contact with platform braces, bend stiffener contact with the I-tube, localized compression in the bellmouth, and sharp corners inside the bellmouth helmet. Accelerated armor wire failure can result from fatigue at the I-tube and as a result of corrosion after external sheath damage and sour annulus environment.
Schematic illustrating the collection and transmission of riser monitoring data.
Over the past decade, a significant number of flexible risers are believed to have failed structurally during operations. In most of these cases, the damage was located in the top section of the riser near the bend stiffener. This included damage to the external sheath, corrosion and/or fatigue induced damage to the tensile armor, and torsional instability. Contact between the riser and platform as well as riser hitting riser are common in semisubmersible applications because of the amount of drift incurred. At the I-tubes, damage commonly is found in the form of external sheath abrasion caused by interference with the bend stiffener internal insert. As large diameter flexible risers approach today’s construction technological limits, the armor wires become more sensitive to fatigue in high-stress areas such as interiors of end fittings.
Drawing on its riser design and construction history, 2H Offshore has developed a system to proactively monitor flexible riser performance. With this system, an operator can optimize inspection and maintenance planning and detect progressive failures before complete compromise to improve safety and prevent environmental impact. The process also can provide early detection of unexpected motion-induced damages and can verify the design suitability for a particular application.
Whether for FPSO, spar, fixed platform, etc. the 2H Offshore approach to riser integrity management starts by collecting design, fabrication, and installation details. That information is used to establish work parameters for the riser system. Then, those details are incorporated into a set of key performance indicators. The indicators are used to set trigger points for inspections and interventions. In order to set these triggers to action, the defined performance indicators are compared to data sets on a risk matrix incorporating the chances of a given event occurring and the degree of criticality to the entire system resulting from that event.
This presumptive data is compared to the actual field returns, and that is used to update the working parameters and defined performance indicators over the life of the riser. This cycle of feedback is continually updated to optimize the installation’s operating plan, inspection routines, and intervention prediction and scheduling.
Observations and data routinely collected as part of the integrity management program come from several sources and can be stored in several places. Riser monitoring hardware, ROV inspection video, and metocean and production history data all can be collected, saved, and analyzed, with the data points being collected into a database for future reference. The data can go into an intelligent management system onboard the offshore production facility where it is processed and produces alarms at predefined threshold points. The data also can be sent anywhere via satellite or Internet for storage, additional monitoring and processing, management reports, and determination of long-term trends.
The 2H monitoring system integrates 2H INTEGRI products using sensors to monitor motions, angle fluctuations, tension variations, armor wire failure, hydrocarbon leakage, and annulus pressure. Location of the various sensors is critical to collecting useful data.
Armor wire failure sensing is new and provides direct indication of pipe degradation and loss of integrity. A single wire failure is not so critical, but can lead to a knock-on effect where load redistribution causes subsequent wire failure. Detecting this is an important step in flexible pipe monitoring. This capability has been developed by 2H and qualified by Petrobras in recent full-scale tests in Rio de Janeiro, Brazil.
Petrobras employs flexible risers in most of its offshore and deepwater operations. There are some 1,200 flexible risers in use in Brazil to transport about 80% of all oil and gas. Outside of Brazil, there were more than 1,000 flexible risers at use in the UK section of the North Sea in 2001.
Damage history
When failures have occurred, research indicates that most of the damage has been found in the topmost 30.5 m (100 ft) of water. Of those, 66% of the failures that occurred during installation were due to contact with the platform or another riser. Damage in the tensile armoring came from corrosion and fatigue.
Wire rupture test monitoring results.
An armor wire failure test was conducted at the Federal University at Rio de Janeiro where a 4.3-m-long (14-ft) unbonded flexible pipe section was tensioned to 8-100 Te, and 30 wires were mechanically notched to simulate failure. The monitoring system sensors detected wire failures for all 30 breeches. The system also detected pipe rotation in both clockwise and counter clockwise directions. Detectable peaks in the logged signal were recorded when the wires broke. The breaks were identified even above the noise levels recorded during hammer tests used to determine changes in the riser acoustic properties.
The 2H monitoring system tracks as many aspects of riser response as necessary for confidence. The data goes into an intelligent personal computer algorithm that “watches” for evidence of riser structure deterioration, excessive tension, leaks, and vortex-induced fatigue.
The computer alerts vessel personnel to areas of concern and also can send the data to 2H for analysis and comparison with historic data resulting in reports issued periodically to the operator regarding riser integrity at a point in time.
The system can:
- Detect armor wire failure
- Monitor riser top section motion
- Monitor VIV and other riser vibration
- Monitor annulus pressure
- Detect damage to the outer sheath leading to flooding of the annulus
- Provide early warning of metallic layer deterioration due to sour gas diffusion
- Detect gas leakage into the pull tube
- Monitor top tension
- Monitor top angle
- Monitor differential angle across bend stiffener
- Monitor hangoff motion of the riser top section
- Monitor vessel and riser hangoff position.
The programmed collection of this data coupled with evaluation of the monitored data, allows for the integrity of the flexible riser to be determined with confidence.
System components
The system has four parts. Monitors are mounted in low-profile housings onto the riser below the bellmouth. They record riser motion, inclination, and armor wire failure. A second monitor is mounted on a riser end termination that is rigidly connected to the vessel. This monitor records the movement and armor wire detection at the end termination to allow vessel motion to be accurately accounted for at the termination point. Gas detectors remain online to detect hydrocarbons in the I-tube annulus, which provides another early warning of pipe failure either of the outer carcass or of a progressive rupture. Pressure sensors are fitted to the annulus vent line for constant monitoring of the over-pressure buildup that occurs in the annulus if gas permeates the pressure barrier. The pH and composition of the gas sampled at the vent valve is monitored to detect potential corrosion of metallic layers in the presence of H2S, CO2, or a combination.
The monitors are hardwired to a personal computer containing global positioning and satellite communication systems. Software combines signals from the sensors and processes them to detect anomalies. If variances are detected, the computer also notifies both offshore and onshore personnel in a predefined response. The data collected also is stored for each riser, which allows review and reporting, identification of key events, and long-term trend analysis.
