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Annulus Testing for Condition Assessment and Monitoring of Flexible Pipes
Abstract
The Norwegian operator Norsk Hydro has more than 80 flexible dynamic risers and service lines in operation at different platforms. Riser integrity monitoring programs have been established for the flexible risers in order to ensure safe and reliable operation. SeaFlex has performed annulus testing on a large number of these risers as a part of the programs. The free annulus volume of a flexible pipe is defined as the volume between the extruded internal pressure barrier layer and the extruded external sheath subtracted the volume occupied by pressure- and tension armor, tape and eventual other layers. Two methods are presently used by the industry for annulus free volume testing of flexible pipes, namely nitrogen pressure testing and vacuum testing. Both methods identify trends of volume reduction with time and to detect annulus flooding. Annulus testing has proven to be an efficient and reliable tool for detecting annulus flooding, blocked vent ports and outer sheet damages. This paper address the challenges related to annulus testing of flexible pipes, advantages, experiences and how such tests and the results are used for condition assessment and monitoring of the risers.
... Reports of methods for annular flooding detection include pressure measurements and vacuum testing in the near end-fitting region and general visual inspection for outer sheath rupture detection [9][10][11]. However, these either do 1 3 60 Page 2 of 10 not provide coverage of the entire length of the structure or have limited reliability and sensitivity to small punctures or holes in the sheath. ...
In recent years a number of high-profile cases have shown that flexible pipes are particularly susceptible to the aggressive environments occasionally found in deep-water oil and gas exploration. Ruptures in the outer polymeric layers of the flexible pipes are relatively common in these structures; this often leads to seawater ingress and, under specific conditions involving pressure and CO2, stress–corrosion cracking of inner metallic layers may occur. In this scenario, a non-destructive technique that is capable of detecting flooding of the annulus region of flexible pipes becomes desirable. Radio frequency identification (RFID) is a well-established technology that allows wireless reading of dedicated low-profile tags. Furthermore, these tags, when designed appropriately, can also detect changes in the surrounding environment, and could in principle be used to detect, for example, seawater ingress in the annular space of flexible pipes. In this paper, a preliminary version of a RFID sensor was designed in order to test this concept through the use of finite-element modelling, and a prototype was used for validation and testing. Results show a marked change in frequency–response between dry and wet conditions. The sensor also allowed differentiation between water or oil-flooding of the surroundings, and also showed to be capable of quantifying proportions in water–ethylene glycol and water–CO2 mixtures if adequately calibrated.
Corrosive environments are responsible for the highest degree of degradation and failure in marine structures. The presence of sea water in marine structures such as flexible pipes can cause a significant reduction in their operational life, especially when associated with permeated gases, which could lead to corrosion related failure mechanisms such as corrosion-fatigue and hydrogen cracking. The ingress of sea water into flexible pipes can occur either due to ruptures in their external polymeric sheath or to permeation of condensed water from the pipe bore. This event since flooding of the so-called annular space of flexible pipes is the trigger for all knows corrosion assisted failure modes, it is clear that a system that is able to reliably detect the presence of water in the structure is highly desirable. This work will describe a radio frequency identification (RFID) system designed for this purpose; it relies on the measurement of shifts in the resonance frequency of specially-designed tags which would be inserted within the layers of the flexible pipe during manufacturing. This paper shows the design and validation process of these tags and also of a reader which is meant to be scanned along the outside surface of the pipe by a remotely-operated vehicle (ROV). The study was performed through a finite element analysis and a test in which the tags were inserted within a full-scale mock-up of a flexible riser, which was then flooded with synthetic seawater. Results show that the shift in response due to sea water is clearly identifiable and distinguishable from other effects.
One of the main concerns regarding flexible pipe integrity is its annulus condition, as a flooded annulus can lead to excessive corrosion and reduce fatigue life of the armor layers. The current approach to address this is to periodically perform a vacuum or pressure test to check the annulus integrity and to measure its gas-filled volume, in order to detect an accumulation of condensation water, or the ingress of sea water (Bondevik, 2004). These measurements are sometimes complemented by a continuous measurement of the flow rate of gas escaping the flexible riser's vent ports (MCS International, October 2002). The vacuum or pressure test is a costly operation, performed intermittently, while the conventional vent-gas monitoring does not provide reliable information on gas diffusion rates or water vapor emissions.
To address these issues, TOTAL and Schlumberger have developed the subC-racs* riser annulus condition surveillance system for continuous monitoring of flexible riser integrity, which eliminates the need for vacuum tests.
The gas that permeates the riser pressure sheath is depressurized while measuring its pressure, temperature, and flow rate. As in a well production test, the pressure drawdown and buildup curves are analyzed to give detailed information about fluid content and connectivity. The instrument's resolution and accuracy allow frequent calculations of gas diffusion rate and of the volume of liquid that may have entered the annulus, weekly, daily, or more frequently, depending on gas diffusion rate and riser parameters.
In this paper we describe the measurement principle and hardware, modeling of the gas diffusion in the annulus compared with experimental results, and field test results on various risers in a West Africa field. Emphasis is placed on the measurement results, but the implementation in hardware and real-time software for alarms and remote monitoring is also shown.
Introduction
Monitoring of flexible pipe integrity is a main concern for all offshore fields. It has become more significant as the number of flexible risers increases, and as they age.
The main issues for flexible risers are the status of outer sheath and the presence of water in the annulus due either to condensation or by damage to the outer sheath (Figure 1).
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