OTC 24497

Evaluation of Technologies for Inspection of Riser Splash Zone Rafael Wagner F. Santos, Petrobras, Mario P. Filho, Petrobras

Copyright 2013, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference Brasil held in Rio de Janeiro, Brazil, 29–31 October 2013. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract Corrosion at splash zones of risers and spool pieces of subsea pipelines, can be severe reading corrosion rates up to 1mm/year, due to the lack of effectiveness of cathodic protection and coating damages caused either by disbondment or object impact. So, it´s highly recommended that this zone have a specific inspection plan combined with a special inspection program for preventing the occurrence of failure, as well as following up the evolution of any failure mechanism which eventually might be present. Traditionally inspection techniques applied at splash zone are: general visual inspection and local thickness measurement. However these techniques are not practical as they require marine growth and even coating removal. After an event caused by corrosion at splash zone, a literature review was carried out to produce the state of the art of non intrusive technological solutions for inspecting the corroded areas of splash zones of risers and spools. This paper describes the results of laboratory tests carried out with some of the identified technologies on full scale riser samples. Test results indicate that technologies such as guided waves and Saturated Low Frequency Eddy Current (SLOFEC) can be complementary alternatives to the splash zone traditional inspection techniques, although the coating type and wall thickness have significant influence on these technologies´ sensitivity and probability of detection. These technologies have the capability of inspecting/monitoring large areas of splash zones of risers and spools without marine growth and coating removal. This is an advantage when compared to the traditional inspection techniques. 1. INTRODUCTION

Statistical data collected in the last 30 years indicate corrosion as the main failure cause in risers [1]. At splash zone on risers and spool pieces, where cathodic protection is not effective, the higher probability of coating damage due to object impact or coating disbondment turns this zone highly susceptible to corrosion [1,2]. Studies published by U.S. Department of the Interior, Minerals Management Service (MMS) indicate that 92% of corrosion failures occurred in risers in the Gulf of México was caused by external corrosion [1]. In 2000, Chevron communicated a high pressure riser failure in one of their platforms in West Coast of Africa due to severe corrosion at splash zone caused by coating damage. Further studies on all 1027 production risers in that region revealed serious problems in the Integrity Management Program of risers practiced at that time [2]. At Petrobras, a working group was established after events that led to repair/replacement of spools pieces damaged by corrosion at splash zone. This group aimed at searching risers and spools pieces inspection techniques available in the industry, which would be adequate for application in the field. It was identified some technologies, that should be technically evaluated finding their limitations and actual capabilities for further application at Petrobras. This paper describes the laboratory tests results obtained by applying some of these identified technologies on full scale pipe samples.

2 OTC 24497

2. LITERATURE REVIEW

2.1. Inspection Plan for splash zone on risers and spools pieces Risers and spools pieces should be included in a long term Inspection Program. This philosophy must be implemented since design and construction phases in order to assure their inspectionability along their full operational life [2]. According to DNV OS F101 [3] “inspection/monitoring frequency shall be established in such a manner that pipeline may not be subject to failure caused by failure mechanisms that might occur in the interval of 2 programmed inspections in sequence”. API RP 1111 [4] recommends that a visual inspection shall be carried out annually to detect eventual physical damages or corrosion on risers and spools´ splash zones. Total developed a general specification for riser inspection that establishes the inspection frequencies indicated on Table 1 [2]. Petrobras practice establishes that riser and spool pieces at splash zone must be visually inspected and have their thickness measured in a 3 year basis. Risers with splash zones protected with Monel alloy sheet should be visual inspected every 5 years. Exceptions may be applied according to Regulations.

Table 1 - Riser and spool inspection frequency specified by Total. Inspection Technique Frequency (inspection at every x years)

General Visual Inspection 2 Inspection by ROV 2

Thickness Measurement 2 Intelligent Pig 5

2.2. Inspection Technologies

Risers and spool pieces inspection at splash zone is very difficult to be executed due to access restrictions. It represents a gap between the topsides and subsea inspection, when this last one is carried out by ROV. Usually the inspection is carried out by alpinists, inspectors on scaffoldings or by divers, but its progress depends heavily on environmental conditions [2]. Traditionally, riser and spool piece inspection plan includes general visual inspection and UT thickness measurement. In the last 10 years a growing number of new technologies have been utilized for this purpose [1]. DNV F-206 [5] summarizes the methods that can be applied on risers and spool pieces for detecting corrosion in a table. Table 2 is a customized reproduction of the said DNV table.

Table 2 – Methods for corrosion detection on risers and spools pieces. Method Advantages Disadvantages

General Visual Inspection (GVI) Fast and low cost Limited to external damages; subjective

and low accuracy

Local Thickness Measurement by UT A-scan

Good accuracy; Good sensitivity Liquid coupling, clean and smooth surface needed; Limited to coating

thickness up to 6mm

UT C scan Fast, good resolution and sensitivity Liquid coupling, clean and smooth surface needed; Coating removal

Guided wave Fast and Global scanning Does not deliver remnant wall

thickness; Does not discriminate internal and external defects

Magnetic Flux Leakage (MFL) Coatings up to 3 mm do not need to be

removed Limitations on heavy wall riser/spool

and on thick coatings

Pulsed Eddy Current (PEC) Does not need coating removal Low resolution and accuracy on thick

coatings Field Signature Method (FSM) Local Monitoring of Corrosion Very small monitored area, high cost

Digital Radiography Good resolution HSE issues Acoustic Emission Global Monitoring False calls susceptible

Following are presented the testing procedures and the results of the lab tests carried out with Guided Waves, SLOFEC and PEC – Pulsed Eddy Current technologies on full scale riser samples.

OTC 24497 3

3. TESTING PROCEDURES In order to evaluate other technologies than the traditional ones, knowing their limitations and actual feasibility of being applied on the splash zone of risers and spools pieces, tests were carried out onto 3 full scale riser samples. Following are presented the procedures utilized to sample fabrication and to define the probability of detection of defects of the selected technologies: Guided Waves, SLOFEC and PEC. The full scale riser samples were tested at CTDUT facilities in Rio de Janeiro.

3.1. Samples fabrication

3.1.1. Dimension and coatings Three 12 m long riser pipe samples were fabricated and identified by numerals from 1 to 3. Their basic configuration is shown in Figure 1. All pipe samples have 2 m long epoxy coated extremities. This coating was applied accordingly to Petrobras standards. The intermediate body of riser samples was coated with a specific coating which represents systems utilized in the field. Coating schemes and sample dimensions are summarized in Table 3.

Figure 1 – Basic configuration of riser samples. Dimensions indicated in meters.

3.1.2. Defects

In order to evaluate the probability of detection of each tested technology, artificial round defects were inserted in the pipe samples´ walls. These defects have their diameters varying from 5 mm to 80 mm and depths from 10 % up to 80% of wall thickness. These defects have different diameter/metal loss ratios to represent actual corrosion anomalies such as pitting corrosion (ratio up to 10) and general corrosion (ratio above 10) [6].

Table 3 - Identification and characteristics of riser samples manufactured for the study.

(*) Note: Stopaq coating was selected due to its characteristics that are similar to the coaltar/enamel coating, very popular for subsea applications in the past and due to its easy application to the substrate. Figure 2 shows some defects inserted in riser pipe samples which were positioned horizontally on benches for easy handling and application of inspection tools.

4 OTC 24497

Figure 2 – Some of defects inserted in pipe samples wall and on right pipe samples positioning.

3.2. Testing

3.2.1. Guided Waves

Riser pipe samples were tested with guided waves tools supplied by the three largest vendors. Figure 3 depicts the guided waves testing. Testing of guided waves tools proceeded as follows: Day 1 – Data collection after positioning sensor collar at pipe samples 0 – 2 m extremity. Draft report was issued for each pipe sample; Day 2 – Data collection after positioning sensor collar at pipe samples 10 – 12 m extremity. Draft report was issued for each pipe sample; Day 3 – Delivery of consolidated final report considering inspections carried out from both pipe samples extremities. This procedure was adopted because presently riser and spool pieces inspection at splash zone should be carried out attaching guided waves sensor collar only at upper topsides. Subsea/ marinized sensor collar handled by divers were not utilized at this testing. So, with this procedure it was evaluated the influence of distance between the sensor collar and defects on the performance of guided waves tool.

Figure 3 – Testing pipe samples with guided waves tools.

3.2.2. SLOFEC (“Saturated Low Frequency Eddy Current”)

For testing of SLOFEC technology, an 8 sensors marinized tool was selected. Some tool adjustments to fit pipe samples diameters were required to carry out the inspection. Prior to the inspection each tool was calibrated on calibration pieces with similar diameter to the pipe sample to be inspected next. This is shown on Figure 4. Calibration pieces consisted of a half pipe with flat bottom holes of 5mm, 10mm e 20mm diameter. Hole depths ranged from 20%, 40%, 60%, 80% and 100% of pipe nominal wall thickness. Tool sensibility was adjusted in such a way that for the 20 mm diameter hole with 80% metal loss the signal amplitude would be 80% of the screen height.

Figure 4- SLOFEC tool adjusted for: 6” pipe sample (left); 12” pipe sample (center). On the right SLOFEC tool on 6” calibration piece.

OTC 24497 5

Along pipe samples testing, tool was positioned in such way that the sensors were located at edge of the intermediate part of pipe sample in order to be manualy moved over the specific coating. As tool could not cover full pipe sample circumference, for samples 2 and 3 it was performed 4 longitudinal scannings (each at every 90° or 3 o´clock sectors). For sample 1, due to its large diameter, it was necessary 6 scannings (each at every 60° or 2 o´clock sectors).

3.2.3. Pulsed Eddy Current (PEC) Testing of PEC technology was carried out in 2 steps. First, the intermediate section of pipe samples was fully inspected manually. A grid with a mesh to fit the PEC probe was applied on the pipe samples surface. Centerline of PEC probe was positioned coinciding with grid cell area center. On pipe samples 2 and 3, PEC probe was positioned at every 60° (or 2 o´clock), while on pipe sample 1 probe was positioned at every 30° (or 1o´clock). Probe selected by vendor for the first step was AP-50, which is more adequate for coatings with thickness ranging from 40mm to 200mm. Figure 5 depicts probe AP-50 and data collection on the grid cells of pipe sample 1.

Figure 7 – on left, grid for manual inspection with PEC probe, on center probe positioning and on right probe AP-50.

As a second step, another smaller PEC probe (model CP-30) was tested. This probe is more adequate for applications on coatings with thickness up to 20mm. Additional scannings with both probes AP-50 and CP-30 were applied to pipe samples 1 and 3, but only at specific regions selected by Petrobras (close to defects). 4. RESULTS AND DISCUSSION Three pipe samples were tested with guided waves, SLOFEC and PEC. On the following sections, only considerations about the tests will be presented due to paper presentation rules. However, the full tests results are available on reference [7].

4.1. Pipe sample 1 Guided waves reports indicated that a complete inspection of the neoprene coated area was done and it was not observed a high attenuation of guided waves due to the coating. Inspection with the marinized SLOFEC tool was very fast. Average time for each longitudinal scanning (a total of 6) was only 23 seconds. PEC testing took longer than other tested technologies. The grid utilized on the pipe samples for the largest probe (AP-50) had 1068 points. Data collecting at each point took approximately 5 seconds. The report issued by vendor after tests first step included a pipe sample wall thickness mapping in color scale and the results were not considered satisfactory. Inspection testing results demonstrated that the guided waves and SLOFEC technologies had the best performance, as both of them were able to detect almost the same defects (table 4). It must be clarified that the pipe sample 1 characteristics are favourable to guided waves technology, due to its coating, as it is regular and homogenuous, mainly because it was applied at mill and offers less attenuation. At the same time its coating is unfavourable to SLOFEC as both pipe sample wall and coating thicknesses are in the upper limit of this technology application.

Table 4 – Summary of detected defects by GW and SLOFEC on sample 1. Defect diameter (mm) # of defects # of defects detected by GW # of defects detected by SLOFEC

5 2 0 0 10 3 0 0 20 4 2 1 40 6 5 6 60 3 3 3 80 2 2 2

6 OTC 24497

In the second step of the PEC testing, other inspections were carried out on pipe sample 1 but only on areas with defects detected by the other 2 tecnologies. Figure 8 shows graphics with the number of detected defects by probe CP-30 as function of defect diameter and metal loss. None of these defects were detected with the larger probe (AP-50). Based on that, it is evident that PEC technology has less detection capability in comparison to guided waves and SLOFEC, considering the characteristics of pipe sample 1.

Figure 8 – Number of detected defects by PEC tool (probe CP-30) as function of defect diameter and % Metal Loss.

4.2. Pipe Sample 2

Results have shown that for Stopaq coated pipe samples it is fundamental to apply the guided waves sensor collar to the closest of its extremities. This means that in the field sensor collars should be applied the closest as possible of splash zone. Stopaq coating causes a great attenuation of guided waves propagation, limiting its maximum range mainly when testing is carried out only from one side. Besides that, larger defects will not be detected when located further than 6 m from sensor collar. Considering only the points with valid data, i e, for guided waves only the region in the range informed by operators, detection capability of guided waves was similar to the delivered by SLOFEC, as each technology detected 9 out 10 defects in the total, observed minor differences on small defects. Results of PEC technology confirmed that the chosen methodology for the first step of tests was not adequate, as no defects were detected, even the largest ones.

4.3. Pipe Sample 3 Similarly to what was observed on pipe sample 2, the results of testing have shown a strong attenuation of guided waves propagation caused by epoxy coating. For this reason even though large defects may not be detected, if sensor collar is positioned further than 6m from these defects. In the field sensor collar should be positioned the nearest of splash zone. Taking into account only valid data, i e, readings taken in areas within the range defined by operators, the best result in terms of detectability for the technology of guided waves was similar to that obtained from SLOFEC, as both technologies could detect a total of 9 defects, with minor differences at the smallest defects. However, due to the characteristics of the epoxy coating, as rough surface caused by manual application and more viscosity, when compared to neoprene, for instance, vendors reported many false calls. In the field this would cause high costs for unnecessary interventions. PEC testing results have shown again that the methodology selected for the first step was not adequate and that probe selection has strong influence on PEC detection capability. All defects chosen for the second step tests on pipe sample 3 were detected by guided waves and SLOFEC, and it was observed that PEC has a lower detection capability than the one presented by these other technologies, for the characteristics of pipe sample 3. However, comparing PEC testing results on pipe sample 1 against pipe sample 3, these last ones were the best as it was possible to detect defects with 40mm diameter.. The main cause to this better performance can be attributed to the lift-off reduction from 15mm in pipe sample 1 to 4-5mm in pipe sample 3. 5. CONCLUSIONS There are available in the market new inspection technologies like guided waves, SLOFEC and PEC, which can mitigate the risks of failure of risers and spool pieces in the splash zones, delivering reasonable cost/benefit.

5.1. Guided Waves Guided waves technology is attractive for inspecting risers and spool pieces coated with low viscosity coatings like paint, TSA, neoprene and poliethylene. Tests results on pipe sample 1, neoprene coated, have shown the technology can be applied to a range of up to 8 m, from the end of the targeted area.

OTC 24497 7

Higher viscous coatings, for example, epoxy layer, coaltar enamel and Stopaq, mainly with rough finishing on the surface, can lead to reduce guided waves POD, the range and increase false calls. So, if the guided waves inspection plan of splash zone does not consider the deployment of sensor collar by diver or ROV, probably some areas will be not correctly inspected and there will be a risk of large defects not being detected.

5.2. SLOFEC Testings with SLOFEC techonology with marinized sensors have shown that tool can deliver very fast scannings of the areas underneath splash zones coating, observing that some riser or spool pieces accessories like supports or anodes could impose some limitations on the reach of inspection. SLOFEC detection capability was not affected in areas with 4 to 15 mm thick coatings and all defects with 40 mm or bigger diameter were detected, regardless of their respective % Metal Loss (in the range from 10% to 80% of wall thickness). It was also observed a trend to detect 100% of defects of 20 mm diameter with, at least, 80% Metal Loss.

5.3. PEC Taking into account the characteristics of the pipe samples, PEC delivered the worst results when compared to guided waves and SLOFEC, requiring further studies. Key impacting factors on POD are defect diameter and coating thickness. On areas with 15mm thick coatings only 60 mm and larger diameter defects were detected. On areas with 4 to 5 mm thick coatings only 40 mm and larger defects were detected. The main interest on PEC, provided there will be defects in its range of detection, is the possibility to read the remnant thickness underneath the probe. However, analysis of results on remnant thickness readings has shown the read values did not match with the existing condition on pipe samples. PEC Vendor indicated that may have occurred an error in the methodology used to convert signals into wall thickness. They would check with the PEC equipment manufacturer about this, but until the edition of this study no answer was received. 6. RECOMMENDATIONS According to the testing results, an Integrity Management best practice for splash zone of risers and spool pieces is the combination of periodic inspection with marinized SLOFEC sensor and monitoring with guided waves sensor collar. Monitoring performance will be improved if carried out from both ends of splash zones, that is, from the above emerse area and from below immersed with diver or ROV assistance. It is recommended that new risers and spool pieces should be designed with space allowance in the splash zone, for the application of guided waves sensor collar, SLOFEC sensor and PEC probes and assuring minor interference on their inspection and monitoring performances. Risers and spool pieces Integrity Management strategy defined in the design phase of subsea pipelines should consider the type and finishing of coating associated to the inspection and monitoring technologies to be utilized in the field. 7. ACKNOWLEDGEMENTS Authors acknowledge collaboration with technical information utilized in this work of our colleagues at Petrobras, Carla Marinho, Francisco Marques and Sergio Damasceno. 8. REFERENCES [1] Lozev M. G., Smith R. W., Grimmett B. B., “Evaluation of methods for detecting and monitoring of corrosion damage in risers”, Transactions of the ASME, vol. 127, p. 244-254, AUG 2005. [2] Technical Report nº 202034-PES-ZVM-RP-02, p 1-53, DEC 2010. [3] DNV OS F101, “Submarine Pipeline Systems”, OCT 2010. [4] API RP 1111, “Recommended Practice for the Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design)”, 4ª edition, DEC 2009. [5] RP DNV F-206, “Riser Integrity Management”, APR 2008. [6] Galbraith J. M., Williamson G. C., “Practical Considerations for Users of Guided Wave Ultrasonic Testing”, NACE 2007, 2007. [7] Santos R. W. F., Pezzi Filho, M., “Tecnologias para inspeção de zona de variação de maré de riser/spool rígido”, Coteq 13, 2013.