Jonathan Burns, Adam Bishop, Timothy Bradshaw and Phillip Cole
The increased use of dynamic riser systems on offshore installations worldwide brings with it complex integrity issues for the operators. In particular, the multi-layer PVDF type flexible risers are prone to a number of failure mechanisms, namely: (i) pressure sheath pull-out from the end termination; (ii) carcass fatigue; (iii) carcass collapse; (iv) carcass pull-out; and (v) carcass unwinding. The latter, also referred to as carcass tearing or un-spiraling, is thought to be the newest form of failure of this type of flexible pipe.
This paper reports on the work carried out by MISTRAS and an International Oil Company on the development of an online Acoustic Emission continuous monitoring system to monitor for riser carcass pull-out and/or unwinding. The two year research project resulted in the deployment of an intrinsically safe system installed on two North Sea platforms. Results from the research trial and continuous monitoring will be presented and discussed in this paper.
Un-bonded flexible risers have been in operation in the North Sea since the early 1990’s, when production continued into deeper waters where there was a need to have dynamic riser systems to carry the hydrocarbons from the seabed to the floating oil platforms. The un-bonded flexible risers are constructed of multiple layers of helically wound metallic wires and thermoplastic materials. The metallic wires serve two main purposes dependent upon their relative positioning within the risers: (i) the internal carcass provides the riser with a collapse resistant layer; and (ii) the tensile wire layer(s) provide tensile strength to the riser. The thermoplastic material layers provide barriers to fluid ingress/egress depending on their positions as well as anti-wear properties. In the project presented herein, the polymer was a PVDF type. Figure 1 provides a schematic of a typical flexible riser construction.
Over the last decade, failures of flexibles operating in the moderate pressure range of 80-90 bar have been observed due to internal carcass collapse. Various theories for the causes of carcass collapse are proposed in the literature with references to pressure build up in the different layers, rapid depressurization of the riser and more recently, mechanical failure theories relating to pull out from the welded connection due to a potential weak point from excessive grinding at manufacture, and carcass tearing and unravelling from below the weld . The most severe implication of failure of the carcass can be a total platform production shutdown and evacuation due to gas/hydrocarbons leaks.
Figure 1 Typical construction of a flexible riser .
Following a number of observed cases of carcass collapse, an International Oil Company approached MISTRAS with the aim of developing a continuous monitoring system using acoustic emission (AE); the monitoring system was designed to offer continuous monitoring for carcass collapse based on a mechanical failure, e.g. unravelling and pull-out. This paper reports on the work undertaken as part of this joint collaboration project to develop a monitoring system along with examples of data gathered from an operational system following deployment on offshore production risers in the North Sea.
ACOUSTIC EMISSION MONITORING
Acoustic emission is the term used to describe the high frequency acoustic signals that result when materials crack or yield. Cracking or yielding of materials produces transient stress-waves which travel out from the source like an earthquake and can be detected by appropriately placed surface mounted piezoelectric sensors. Cracking in steels produces signals which may be detected over a broad frequency range, however the typical operating frequency range is 60-1000 kHz; the most effective monitoring frequency is dependent upon the process noise spectrum. Given the high operating frequencies involved with AE monitoring, the approach is generally insensitive to low-frequency background noise.
APPLICATION FEASIBILITY TRIALS
Laboratory Test Trials
The first stage of the collaboration project involved feasibility trials to assess the typical signal characteristics of a carcass catastrophically failing at the welded connection. Carcass test samples were tested to failure under continuous loading in a custom test fixture. Loading was controlled using a hydraulic hand-pump configuration; two different loading rigs were used for different carcass diameters. The carcass samples were “bare”, i.e. no additional layers covered the carcass during the test. AE sensors with different frequency peak sensitivities were coupled to the custom end fitting of the carcass sample (see Figure 2). The frequency range of interest was from 20 kHz to 300 kHz. A multi-channel PCI-2 AE system manufactured by Physical Acoustics Corporation was used for the feasibility trials.
Figure 2 Photographs of a typical bare carcass and the large testing rig used to carry out a pull test with sensors mounted on the end fitting.
Failure loads of 6 to 12 tons were recorded for the 5 and 8 inch carcass samples respectively and therefore as a result, the intensity of the AE resulting for the failure was significant. The amplitude level across all frequencies was close to saturation, i.e. >100-120 dB, with very high energy release just prior to failure. A series of high amplitude signals were recorded prior to final failure; these emissions were associated with the unravelling of the tightly wound interlocking connections of the carcass prior to catastrophic failure. The signals associated with final failure had short rise times, typical of emissions associated with this type of dynamic failure mechanism.
Figure 3 Screen shot of AE software showing amplitude readings across different frequency ranges (60 kHz-ch2, 150 kHz-ch3, 300 kHz-ch4) leading up to failure of the carcass and a photograph of an unraveled carcass.
Sea Base Test Trials
A second set of mechanical trials was performed using a loading rig setup similar to the laboratory trials. In the second set however, the loading rig was attached to a riser topside termination, i.e. a connection piece used to secure the dynamic riser in place on the platform. AE sensors were installed on the neck of the connection piece on the opposing flange to the carcass test rig (see Figure 4). The purpose of this set of trials was to evaluate the attenuation effects of AE signals resulting from carcass failure travelling through the riser topside termination.
Figure 4 Photograph showing the carcass test rig attached to the riser topside termination (inverted connection piece) and close up photograph showing location of sensors installed on the neck section.
Figure 5 Screen shot of the AE software showing activity associated with unravelling and final load bearing failure and a photograph of an unravelled/failed carcass.
The carcass samples were loaded to catastrophic failure using a continuous load ramp controlled via a hand pump. Flow inducted noise associated with the pumping action was observed during the test, however the AE intensity level was low compared with activity from the carcass failure.
The AE activity plot presented in Figure 5 confirms that the attenuation of the signal as it travelled through the large flanged connection and into the neck section of the topside termination was minimal; the emission level recorded in these trials was at a similar level to the laboratory tests as shown in Figure 3. The first indication of unravelling occurred around 115 seconds into the test with a sudden high energy release. Additional AE bursts were recorded shortly after the first indication of onset of unravelling before the loading carrying performance of the carcass significantly deteriorated.
Background Noise Site Trials
Following successful characterisation of the signal features associated with riser carcass failure, site trials to assess the operating background noise levels (ultrasonic frequency range) associated with riser operation were performed on a number of North Sea platforms. AE measurements were made on several risers on the neck section of the topside riser termination (see Figure 6). An example of the process noise levels from the offshore survey are presented below for several risers.
Figure 6 Photograph showing example riser topside termination (highlighted region shows area where background noise measurements were performed) and typical AE signal levels from riser operation.
Low background noise levels were reported across all risers that were evaluated. Some variation in noise level from riser to riser was observed, which was attributed to variations in riser operation characteristics, e.g. flow, pressure etc.
RISER MONITORING SYSTEM
General configuration of the AE monitoring system
Following successful completion of the feasibility work in the development of a riser carcass monitoring system, MISTRAS were commissioned to install two acoustic emission monitoring systems (AEMS) to permanently monitor seven production risers across two North Sea platforms. The AEMS’s that were installed on the two platforms were designed using intrinsically safe hardware supplied by Physical Acoustics Corporation. Four AE sensors of varying frequency sensitivity were installed on the neck section of each riser topside termination (see Figure 7). The sensors were permanently installed using a high strength structural adhesive and reinforced with a mechanical strap. The sensors were cabled back to the multi-channel data acquisition system located in a safe control room via in-field signal amplifiers and isolating safety barriers. Installation and commissioning of each riser monitoring setup was completed, on average, within two working shifts.
Figure 7 Drawing showing neck section of riser topside termination with sensor locations marked and a photograph of sensors installed on the riser.
The AEMS is configured with a remote restart capabilities in the event of general computer failures and operates with RAID redundancy to minimize the likelihood of terminal system failure. Data is stored locally on the acquisition system. The configuration of the AEMS allows for secure remote access to enable monitoring personnel to gain access from any location to review the recorded activity and system performance. Autonomous alarming is configured on each AEMS to provide early warning of indications of interest; the alarm characteristics are based on the feasibility work undertaken and reported within this paper.
Live monitoring data
Owing to the refined acquisition setup of the monitoring system, minimal activity is recorded on a daily basis (see Figure 8). Any activity of interest associated with potential carcass failure can therefore be clearly distinguished from general activity; indications of interest can therefore be relayed back to platform personnel with minimal delay.
Figure 8 Screen shots of AE software showing examples of typical data (absolute energy vs. time); note that the activity shown is below the first stage alarm triggers (red line on plot). Fluctuations in ASL on the RHS graphs indicate typical activity associated with well intervention on the platform.
During continuous monitoring the autonomous alarming highlighted an indication of interest. This was then discussed with platform personnel and correlated to riser shutdown-startup process. Further analysis was performed to identify the signature of the alarm activity; this was seen to be a distinct and isolated signature outside of normal operation (see Figure 9) matching the research trial results. This was correlated to pressure and temperature data provided by the platform staff (see Figure 10).
Figure 9 Screen shot of AE software showing indication of interest on platform riser.
Figure 10 Process related activity associated with AE alarm event.
The use of different frequency sensors combined with autonomous alarming and continuous monitoring allows for a system that can quickly and accurately identify indications of interest for potential carcass failure. The results show a clear burst of high absolute energy coupled with high average signal level across the frequency band used in the sensors. These indications are distinct and outside normal operational background signal levels, allowing platform staff to be contacted and alerted to the activity with minimal delay.
- A. Farnes, C. Kristensen, S. Kristoffersen, J. Muren, N Sodahl. “Carcass failures in multilayer PVDF risers. Proceedings of the 32nd International Conference on Ocean, Offshore and Artic Engineering, OMAE 2013, June 9-14 2013, Nantes, France.
- Bahtui, H. Bahai and G. Alfano. “Numerical and Analytical Modeling of Unbonded Flexible Risers”. J. Offshore Mech. Arct. Eng131(2), Mar 26, 2009.