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Inspection of offshore flexible riser with electromagnetic and radiographic techniquesE. VEITH
C. BUCHERIE (Bureau Véritas)
J.L. LECHIEN (CEA/CEREM Saclay)
J.L. JARROUSSE (Institut Français du Pétrole)
Bernard RATTONI (CEA/DAMRI Saclay)
Bureau Veritas, CEA and IFP have studied the feasibility of on-site inspection of flexible pipes used by the offshore petroleum industry. The part of these flexible pipes most subject to fatigue is the pressure layer of the dynamic risers which vertically link the offshore platform and the wellhead on the seabed. Operational strain may cause cracking and breaking of these layers. No device is currently available for the inspection of the integrity of the various metal structures of the risers.
We have developed an electromagnetic and a radiographic method for the inspection of the risers.
Risers are the physical link between the seabed and the topside of offshore installations, for production, gas lift or water injection purpose. They can be either rigid or flexible, but overall, they are critical components of this type of installations.
As the offshore exploration and production were developing deeper fields, flexible risers became the key to "floating" offshore facilities, like Floating Production, Storage and Offloading installations (FPSO), as they can comply with movements of these types of surface support without undergoing excessive stress. Consequently, the number of flexible risers in operation has tremendously increased in the recent years.
Compared to rigid, flexible risers are of a new generation. Calculating their fatigue life is very complex, especially when considering the possible interactions of additional degradation mechanisms like erosion and corrosion. These difficulties will increase with depth. To illustrate this, the specification API 17J [ref.1], which came in force on March 1st, 1997 requires a safety factor of 10 versus the service life (maximum calculated Miner ratio of 0.1 where usual components that can be expected only need to satisfy a ratio of 0.3).
Flexible risers also have that particularity to be difficult to inspect and also repair in-service, due to their complex structure and the lack of adapted technique.
hese considerations have driven Operators to be extra-cautions in order to reduce any possible down time linked to a failure of such a component. All cumulated, these aspects have a considerable cost impact.
The objective of this paper is to present the methodology adopted by Bureau Veritas (BV) and the Institut Français du Petrole (IFP) to develop a reliable equipment dedicated to In-Service Inspection (ISI) of flexible risers, serving the need of Operators for :
Such an effective and reliable ISI tool would hopefully drive to a consensus on the reduction of the safety factor in fatigue engineering to a more "standard" value.
Offshore operations including ISI, are never considered as minor. Importance of the results, associated costs and stringent limitations linked to safety of personnel are as many factors that often turn offshore ISI into delicate missions. Going to deeper fields will worsen failure consequences, thus increasing the criticality of these operations. Considering what is at stake here justifies the objective of providing a mean of performing, in-service, a non-destructive examination of offshore flexible rises.
In order to highlight the challenge such a development represents, the possible geometrical configurations and the general architecture of this type of structure are recalled below, together with the possible degradation mechanisms that may occur.
1.1.1 General architecture
A flexible pipe is "An assembly of a pipe body and end fittings. The pipe body comprises a composite of layered materials that form a pressure containing conduit. The pipe structure allows large deflections without a significant increase in bending stresses. Normally, the pipe body is built up as a composite structure comprising metallic and polymer layers." [ref.1]. Each layer has a specific role, preventing either collapse, burst, or leak while allowing some degree of freedom between one another. Metallic layers are made of interlocked (except the outer tensile armour) carbon or stainless steel wires, of various thickness and spiral wounded on different angles depending on the use of the pipe. A schematic view is given in figure 1.
|Fig 1: Schematic view of flexible pipe structure||Fig 2: Areas of concern for ISI on flexible risers|
1.1.2 On-site configurations
The most common configurations found offshore are :
They are identified on figure 2.
1.1.3 Identified failure mechanisms
These mechanisms have been described in detail in [ref.3]. Only considering the metallic components of the risers, limits the amount of degradation types. They all depend on the layer specificity :
In their final stage, each individual damage may result in a total failure of the riser, through leak or burst. However, the time needed to reach this final stage can extend over a relatively long period of time. It has been observed in some cases that even with severed reinforcement wires, flexible pipes could hold the test pressure, and then pass the hydrotest. Operators could take advantage of this period of time between the defect detection allowed by ISI and the complete failure, for planning any remedial action to their best convenience, rather then acting in an emergency. This period of time could even be optimised through a defect monitoring procedure.
1.2 NDE technical concerns
Typically, the following difficulties for inspecting such structures are encountered, which ever technique is chosen :
1.3 Operational constraints
Flexible risers, depending on the structure they are suspended to, can have their upper connection located either in air or in water. The tool will then need to be operated in both environments.
The design of offshore installations rarely accounts for accessibility to various items that may need to be inspected, it is expected that access to areas of concern (top part of the riser) will not be straightforward in a first instance. This limitation will influence not only the physical access to the site of inspection, but also the progress of the inspection because of the close vicinity of fixed elements.
Other stringent limitations are linked to the emission of radiation in air. In some areas, these limitations are such that radiography is no longer used, unless there is no other alternative. This partially explains the preferential development of electromagnetic techniques in the recent years.
As a direct consequence of these :
The ideal equipment used offshore need to be intrinsically safe, limited in size, and use a rapid non-intrusive technique that allows a quick and reliable diagnostic on the integrity of the inspected structure, both above and underwater.
1.4 Present state of the art
API17 J states that "The requirements for the manufacturer to design and implement flexible pipe inspection, monitoring and condition assessment systems and procedures should be specified." By and large, HSE guidelines [ref.3] describe a methodology based on risk assessment for the monitoring of flexible pipes. From each of these, a large freedom is left to the Operator for what concerns the inspection techniques and frequencies.
If we exclude usual tests like hydrostatic tests (rather destructive and not always effective), and the inspection carried out during fabrication, the main inspection which is presently carried out in service on this type of structure is visual examination on the outer sheet and on attachments. It allows to check for possible large damage to the riser, and cannot be used for assessing the integrity of the inner parts. This type of inspection is carried out on the full visible length, by Remotely Operated Vehicle (ROV) or by divers on the underwater parts. As for any visual technique, it requires experienced operator (diver) to faithfully report the status of the riser.
The only NDE technique which has been tentatively applied with an objective of ISI is based on Eddy current. It allows checking the integrity of the outer armour wire, but cannot penetrate deeper into the structure. It is applicable mainly in the above water section, and is somewhat time consuming. When applied from inside, only the first magnetic layer can be inspected, but can we still call this "in-service" inspection?
Radiography remains the ultimate mean in case of big worries. Using this technique is not possible on a regular basis, due to the required exposure time for traditional film radiography, and the safety requirements linked to the use of a radioactive source. Nevertheless, it is the only fully external method that works at the moment.
To the author's knowledge, pigging flexible risers for anything else than geometric verification has never been performed up to now.
In 1998, BV and IFP decided to start the development of a Non Destructive Examination equipment to allow ISI of Flexible Risers. Considering the previously exposed aspects, it was decided to focus on the top part, from the top stiffener down to the first 40 metres.
This decision was made due to the importance of this area for the overall integrity of the riser, but also because it has the following advantages :
2.2 Feasibility study
An initial review of the state of the art of ISI applied to flexible risers proved that a feasibility study was necessary to identify the best possible NDE technique(s) and its(their) limitations to be integrated in a dedicated system.
This feasibility was conducted in close co-operation with french specialists of NDE research at the Commissariat à l'Energie Atomique (CEA - Nuclear Energy Commission) and at BV NDE subsidiary CEP Industrie.
It covered the most likely applicable techniques for such an application i.e.: radiography and electromagnetic techniques. The preliminary results and conclusions are exposed in chapter 4.
2.3 Prototype development
The normal process in the development of such a system is to select one technique and to design a prototype which allows to carry out ISI offshore.
The identified steps to be implemented are :
3.1. Test procedure
In order to best evaluate the capabilities of each envisaged techniques, representative samples of flexible were inspected. A progressive approach was chosen, using flat samples, and starting from scanning a single layer of each type of metallic component, then on coupled layers, and finally on the full assembly. This approach aimed at gaining of better knowledge of the response of each layer, and also helped the construction of an image database in view of future image and data processing through the best adapted methodology, which is not defined at the time of writing.
This procedure was followed both for radiographic and electromagnetic techniques and continued with cylindrical samples, in order to understand the requirements of a real set-up.
To define the limitations of the techniques, it is necessary to have some defects in the examined sample. Therefore, some defects were "inserted" in the sample, which were fully characterised. It was rapidly demonstrated that macro defects are of primary interest in the frame of this study. These defects are of three types :
These are the defects which announce a pipe failure in the near future, and then are critical enough to require remedial action to be carried out within a relatively short period of time, still giving enough notice to allow the Operator to repair before a failure occurs.
Although the influence of the radiographic source is known as an important factor, most of the work has been carried out with X-rays rather than gamma. This a-priori choice is justified by the fact that, from a performance aspect, X-rays are probably inferior, but easier to set-up and to use on-site, and that gamma rays could be tested afterwards, should X-rays be insufficient.
Efforts were focused on detection techniques, which can mark great differences between ISI systems.
184.108.40.206. Standard film radiography
An initial library of films was built-up, using standard method. The capabilities of standard radiographic technique was already known through experience of IFP, BV CEP (see also [ref.2]), and this was chosen as a baseline reference for evaluation of other techniques. All available samples have been X-rayed this way.
220.127.116.11. Imaging plates
Imaging plates (IP) were tested. They showed a very significant time reduction with respect to the standard film method. Detection capabilities are slightly inferior to the films, but acceptable for defects considered within the study. Another advantage of the technique is that the plates are re-useable after storage of the image on a hard disk under numeric format. From an operational point of view, this option could prove interesting as the exposure times were found to be reduced by a factor of 25:1 as a minimum, however the inspection still remains a stepwise process. Drawbacks are that it remains a step by step inspection, and requires change of plates between two shots.
An example of obtained result is shown of figure 3.
|Fig 3: Example of results obtained with Image Plates||Fig 4: Example of results obtained with Linear Detectors||Fig 5: Example of results obtained with Image Intensifiers|
18.104.22.168. Linear detector
Linear detectors have been tested and proved to detect large defects, as for IP. This method is a real time inspection method, the display is obtained on a monitor, and the inspector can proceed to a first diagnosis on-line. In addition, the image is also stored under numerical format on a hard disk, and can be processed on-line or at a later stage, through an image processing software. However, high energies are required to undertake double wall single image inspection. This may prove detrimental when studying the overall in-situ applicability.
An example of result is shown on figure 4.
22.214.171.124. Image intensifier
Image intensifier also showed good performance, giving the best accuracy in the results. Problems were however encountered due to remaining magnetic field in the sample which distorted the image.
This technique has the same main advantages as linear detectors, but suffers from the size of the detector which is more bulky and fragile.
An example of obtained result is shown on figure 5.
126.96.36.199. Additional tests
Additional tests are running on another type of application, but results are not known at the time of writing.
Eddy current performances to detect defects in metallic structures used in the offshore industry are well known to be adapted to the characterisation of the only layer which is closest to the sensors. According to the previous feasibility study, an assessment of the ultimate performance of this technique has been undertaken on flat samples, starting from one to several layers representative of the riser geometry. Three different and optimised eddy current (EC) probes have been operated for this purpose :
EC1 Probe : double functions axi-symetrical probe giving an absolute reading, with a ferrite core and two opposite coils (one for reference and one for measurement).
EC2 Probe : low frequency circular probe comprising one emission coil and a second coil for reception.
EC3 Probe : differential oval probe with separated functions.
The results obtained on flat samples showed that the inspection of the first metallic layer is always possible, but the signal is drastically decreasing in quality when inspecting the second layer. This is due to the sensitivity of the probes to lift-off, and to the shielding effect of the first layer when trying to reach the internal wires. EC3 Probe gives better results than the two others for which no significant signal has been obtained for the second layer. However, the signal to noise ratio obtained on wire breaking type defects remains too small for this technique to be selected as a candidate able to fulfil the requirements of the offshore ISI equipment. Figure 6 illustrates the obtained signal to noise ratio for the three EC probes on a flat mock up with a wire breaking.
|Fig 6: Signal to noise ratios obtained with different techniques on wire breaking type defects in presence (dark) or not (light) of an inner carcass.|
188.8.131.52. Magnetic flux leakage
An alternative method to the eddy current technique has been evaluated in the frame of the feasibility study. A special application of magnetic flux leakage using alternating current (AMFL) and a special built probe, has given rather encouraging results. This technique makes it possible to inspect the pressure layer (figure 1) when operated from the inside of the pipe without any significant influence of the inner carcass. The obtained signal to noise ratio is illustrated and compared with the optimised EC probes on figure 6. Figure 7 represents a C-SCAN view of the AMFL signal obtained on a mock up composed of a 10 mm thick inner carcass, 10 mm thick polymer and the pressure layer in which representative defects (wire breaking and wall thickness reduction) have been introduced.
|Fig 7: Example of results obtained with the AMFL Special Probe. Defects (wire breaking and wall thickness) are represented by the full and dashed line respectively.|
3.3. Conclusion of feasibility
The main conclusion of this study is that it is feasible to inspect a flexible riser in-service using one or another technique as described above. Most of them do detect unlocking, wire break and large cracks to a certain extent. However, operational procedure used in laboratory for each one prove that not all of them will be applicable in a safe and economical way offshore.
A very large amount of work remains, which will validate the choice of technique and design a suitable probe carrier for on-site deployment.
The coming work programme aims at the development of a prototype to be tested on real installations. Prior to starting this development, there will be a choice of technique(s) made on the basis of :
The prototype development will be carried by specialised companies, in NDT, robotics and control command, signal processing and offshore inspection.
The work will be funded through a Group Sponsored Project, calling for Manufacturers and Operator to join and participate to the progress financially and technically.
Specification of the system will define in detail the performance objectives of the prototype. An ideal requirement (but hard to achieve) is that the prototype needs to provide an indication of the integrity of the metallic components of the pipe without interrupting production or operations on the installation. That means a completely external system that can diagnose down to the innermost metallic layer of the pipe with a maximum reliability. Some of the minimum requirements are tentatively identified below :
This step is the critical one in a prototype development. Only Companies with extended know-how of this type of activity will be contracted.
When the prototype is built, first runs will be conducted in laboratory to ensure the proper functioning of the assembly. Only after this crucial stage will the system be tested offshore.
We have presented BV/IFP approach to achieve a technological break-through in offshore In-Service Inspection. This break-through can be considered under a purely non-destructive aspect, with progress in assessment of integrity of multi-layered structures of various physical, mechanical and electro-magnetic properties. Equally important is the possibility given to Operators to improve their Asset Management by supporting the development of and using innovative means of inspection.
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