Internal riser inspections on FPSO storage & production facilities by Jurgen F P Elbertse A Hak Industrial Services bv, Rhenen, Netherlands Contents of this Paper: Introduction Preparations Testing Mobilization Riser preparations Inspection Inspection results Consequences Conclusion Acknowledgements Appendix 1: General description of the ultrasonic device Appendix 2: General description of the Pipecat crawler Appendix 3: Example of inspection results Appendix 4: Overview of the riser configuration Copyright 2001 Scientific Surveys Ltd. All rights reserved. 1
Pipeline Pigging and Integrity Monitoring Conference: Amsterdam, 1997 Introduction Since the end of the production lifetime of some of the major offshore oilfields in the North Sea is coming closer, the exploration of what is called `marginal oilfields' is again being taken into consideration. As already captured in the name, these `marginal' oilfields are not big enough to allow the construction and installation of major offshore platforms and pipelines. An economical alternative was found in the installation of FPSO facilities. In principal this is a large oil tanker moored via a buoy on top of, or close by, the production well. The crude oil flows through a network of flexible hoses to the mooring buoy and directly into the tanker. A second tanker will moor alongside and load this oil. The production will last for an average period of 5-10 years after which the FPSO can be transferred to the next location. During the last couple of years a number of these facilities have been installed on the North Sea, and a number of them are still to come. In the Timor Sea, north of Australia, BHP Petroleum has been operating these FPSOs for a number of years. These facilities had been designed (and constructed) for a limited lifetime only; however, although this calculated lifetime was approaching, the oilfields still produce sufficiently to maintain economic production for an additional number of years. In order to gain permission from the various authorities, it was requested that the most critical sections of the complete facilities were inspected and tested rigorously. This involved the tanker itself (this was done during an extensive docking in which we obviously had no part), and the riser systems in the floating buoy. This paper describes the various aspects of the riser inspections on board the FPSO Jabiru Venture, operated by BHP Petroleum in the Timor Sea. Preparations Due to the complexity of the project the preparations took a considerable amount of the total project time. The first decision to be taken was which type of tool had to be used. An important aspect which influenced this was the actual riser configuration, which was as follows: flexible hoses were running from the subsea wellhead to the floating buoy. arriving at the buoy, these flexible hoses are connected with a special coupling to the rigid steel pipework running into the buoy. in the buoy itself, which is inaccessible, various bends and an expansion-loop were configured. on the top of the buoy, the rigid steel pipework was connected to a swivel, from which it ran into the tanker. The only access point to the riser was on top of the floating buoy near the swivel, where a spoolpiece could be removed. Furthermore it was not possible to create a flow in the direction of the wellhead. It was possible, though, to create flow from the wellhead to the top of the buoy with the crude, but it will be obvious that this was only a contingency. Based on this riser configuration the use of regular intelligent pigs was not possible (apart from the fact that all bends were 90 bends with a radius of 1.5D). Therefore the use of a crawler was required. This crawler had to be able to negotiate the 90 bends with a radius of 1.5D, move backwards and forwards, negotiate vertical pipework both upwards and downwards, work in a complete water-filled, pressurized, environment, and carry a certain amount of inspection sensors. Last but not least the redundancy and the ability to retrieve it quickly in case of emergency were also important selection criteria. 2 Copyright 2001 Scientific Surveys Ltd. All rights reserved.
Internal riser inspections on FPSO storage & production facilities After careful evaluation it was decided that the Pipecat crawler in theory was the most suitable crawler. However the theory had to be tested before it could actually be deployed on the FPSO. A second, not unimportant, aspect of selection of a tool was the inspection capability of the tool to be used. In principle two types of inspections had to be carried out. The first was a complete visual inspection of the internal wall of the riser, identifying internal corrosion (pitting) and identifying the internal condition of the various welds. If technically and operationally possible, an attempt would have to be made to reach the connection between the rigid steel pipe and the flexible hose in order to examine its condition. After completion of the visual inspection, the riser had to be completely inspected using a non-destructive method. Due to the fact that small details were required as well as high resolution and high accuracy, it was decided to use the ultrasonic method. Also the ultrasonic measurements had to be available in C-scan format online during the inspection. Both the visual inspection using a video camera as well as the ultrasonic inspection were available, and could be used on the Pipecat crawler. Testing During the preparation phase the Pipecat crawler was selected as the tool to execute the inspections. As no real experience was available in inspecting these difficult riser configurations, it was decided that a full qualification programme had to be set up and executed. This testing programme consisted of the following main items: proof that the Pipecat crawler was capable of negotiating this particular riser configuration. proof that the camera was able to detect the specified corrosion problems and allow a proper identification of the same. proof that the ultrasonic device was able to detect the specified corrosion and allow proper analysis. test the contingency procedures in case of emergency or failure of the crawler. In order to test this as close as possible to the actual circumstances, a mock-up was built with the same diameter and with the same type and number of bends as in the riser system itself. In total the height of this mock-up was over 7m (21ft). The only differences were that the longest vertical section of over 50m (150ft) was reduced to 1m, and that the wall thickness of the mock-up was only 6mm, opposed to the 18.3mm of the riser itself. The results of these tests proved the following: the Pipecat was able to crawl through the complete mock-up without real problems various times, both starting at the bottom of the riser (which was not the intended start point) as well as starting at the top of the riser (which was the intended start point of the inspection). during these tests the standard Pipecat camera was attached to the crawler (which had the ability to tilt and rotate at certain angles). This showed that this camera was not suitable for carrying out these inspections. First of all, the illumination was minimal which would make an inspection in a water-filled riser difficult. Furthermore the tilting of the camera did not allow to examine the welds in great detail. Last but not least, it proved to be very difficult to interpret the video images properly due to the fact that the position of the camera lamps was behind the camera lens. In general one could not distinguish between pitting and `bulbs'. Copyright 2001 Scientific Surveys Ltd. All rights reserved. 3
Pipeline Pigging and Integrity Monitoring Conference: Amsterdam, 1997 testing the ultrasonic device was undertaken at the same time as the commissioning of this device. Although it had been designed, and a prototype built and tested, this was not the same as having a fullytested and proven pipeline inspection system. Therefore, together with the manufacturer (TNO - the Institute of Applied Physics in Delft, Netherlands) and BHP Petroleum, a tight testing and commission programme was set up. For this programme BHP Petroleum supplied an original piece of the riser pipe material and testing man power support. A little ahead of schedule this ultrasonic device was completed, tested, and commissioned, and provided an accuracy and resolution well beyond any available ultrasonic system available at that time. A full description of this system is given in the appendices. the final tests were the contingency tests. For this purpose the Pipecat crawler was lowered from the top into the mock-up and crawled all the way to the lowest end of the riser. Then a power failure was simulated and an attempt was made to retrieve the Pipecat crawler by pulling it out of the riser via the umbilical. This proved not to be possible. A very simple solution was found to overcome this problem. A steel wire was attached to the rear of the crawler, adjacent to the umbilical connection. By pulling on this steel wire instead of the umbilical, the Pipecat crawler was retrieved from the mock-up. An additional advantage was the fact that the tests were carried in a dry environment. The inspections would be carried out in a totally-filled riser which reduces the weight of the tool and the friction of the umbilical. The only problem to be resolved prior to mobilization would be camera images. Specially for this purpose, it was decided to design a new camera which had the ability to tilt 90, rotate 360 and, most importantly, on which the lamps were placed in front of the lens instead of behind. After completion of this camera, a final test run was made which proved that this camera could supply all the details, including weld examination, as required. Mobilization Before mobilization, a complete project plan was written together with BHP Petroleum in which every aspect of the project was described. This included the lists of all parts to be shipped, the calibration procedures on-site, the actual inspection, the safety aspects and all contingency procedures, and the responsibilities of the personnel involved. Riser preparations In order to be able to execute a proper visual and ultrasonic inspection it is evident that the pipeline has to be cleaned. Under normal circumstances a (series of) pig(s) can be sent through, or a thorough flushing with various chemicals could be executed. Due to the fact that this line was not piggable, this posed to be a problem. In close consultation with ourselves, BHP Petroleum, and the cleaning company, it was decided to use chemical cleaning. The main reason for selecting chemical cleaning was the assurance of the cleaning company to be able to provide an absolute clean pipe wall. Furthermore, its experience in cleaning pipelines prior to ultrasonic inspections was considered to be of major importance. Also, for this aspect of the project, a fully-detailed project plan was written. Inspection The inspections on the FPSO went according plan and provided all the information as desired. The only goal not achieved was inspecting the connection between the rigid steel pipe and the flexible hose. These was not possible due to the high friction of the umbilical and as a consequence the reduction of the forward travel speed of the Pipecat crawler. Continuation would have been an additional safety risk which was considered to be unacceptable at that time. 4 Copyright 2001 Scientific Surveys Ltd. All rights reserved.
Internal riser inspections on FPSO storage & production facilities Inspection results The inspection provided a lot of information on both the internal and the external condition of the riser. First off all the visual inspections showed in some areas of the riser a number of areas with minor pitting. Already during the visual inspection the type and average depth of this pitting could be determined. Also near the welds, areas could be detected with minor wash-out. Non of these areas was considered to be of major concern. The ultrasonic results confirmed the visual inspection results. Minor pitting was detected but none of the pitting exceeded a depth of 3mm. In order to be sure (as this was the first inspection with this tool) a detailed analysis was carried out back at our premises together with the Institute of Applied Physics and with the Technical Welding Institute from the UK. The latter organization also made the MAOP calculations based on both the ultrasonic results and the visual inspection. Consequences The inspection results were such that BHP Petroleum was allowed to continue producing oil without even having to drop their pressures. Although the calculated lifetime of the facility has been reached, the Jabiru Venture is still in production. Since the start of this programme, additional inspections on other riser systems at the same facilities on behalf of BHP Petroleum have been executed. Furthermore, based on the high resolution of the inspections, consideration is being given to running the same programme again in order to monitor the corrosion. Conclusion Despite the fact that the FPSO facilities may have a more complex pipeline configuration from an inspection point of view, the inspection method described has proven to be accurate and, very important, quick and cost effective. Also, for less complicated riser configurations, this method has proven to be an accepted and effective method of inspection. Over the last period a wide range of North Sea operators have used this method for inspecting their riser systems. Sometimes even by simply lowering the tool into the riser without even deploying the Pipecat crawler at all. Acknowledgements The author wishes to thank BHP Petroleum in Darwin for releasing drawings and other relevant technical information, as well as for the pictures of the Jabiru Venture on which the inspections took place. Special thanks goes to David Werner of BHP Petroleum who, at the time of the inspections, was project leader and asset co-ordinator of the Jabiru Venture, for reviewing this paper and adding the necessary details. Copyright 2001 Scientific Surveys Ltd. All rights reserved. 5
Pipeline Pigging and Integrity Monitoring Conference: Amsterdam, 1997 Appendix 1: General description of the ultrasonic device The ultrasonic device is jointly developed with TNO (Institute of Applied Physics) in Delft, The Netherlands. Its design is based on an already existing bore hole scanner which makes use of a rotating mirror in front of a transducer. A champagne cork shaped unit was designed in which in the nose a single transducer is positioned. This transducer transmits its signals in axial direction. In front of this transducer, a special parabolical shaped mirror is placed which rotates very quickly. Via this mirror the signals are sent to the pipewall and via the same mirror the reflections are captured as well. Depending on the pipe diameter, the wall thickness and the level of pollution, the type of mirror and the transducer frequency are selected. Transducers are standard available in the range of 2.25 Mhz to 5 Mhz. The presentation of the measurements is done via C-scan images and is shown on-line. Not only the wall thickness is provided, also inner radius, amplitude strength of the first (inner wall) reflection and the amplitude strength of the second (back wall) reflection. Appendix 2: General description of the Pipecat crawler Pipecat The Pipecat is a crawler type inspection tool which is specifically designed to inspect pipelines which contain bends with a bend radius of 1.5 D. In its current design it is furthermore capable of climbing into vertical pipelines and work in waterdepths down to 100 meters. Waterdepths exceeding the 100 meter can be overcome as well, however this requires more preparation time. The crawler's principle of movement is based on the principle of movement of a caterpillar. This means that one section is clamped to the pipesurface while another section is pushed forward. These moving parts are all electrical/pneumatic driven. As the crawler is only considered to be the "carrier tool", it can be equipped with a high resolution colour ccd camera, which has the ability to rotate 360 degrees, tilt 90 degrees, and of which the focus can be remotely adjusted. Due to these options, detailed images can be made of the pipe surface, showing the smallest pitting and weld portrusions in detail. Obviously this camera can work under water. The second inspection sensor, which can be added to the crawler, is a full 100% coverage ultrasonic wall thickness measurement device. With this device, it is possible to measure the remaining wall thickness, internal corrosion, external corrosion, pitting, etc. All these measurements are presented on-line by means of colour diagrams, of which an example is added to this document. The Pipecat crawler is a so called cable operated tool. This means that all data collected from both the camera and the ultrasonic device is transmitted via this cable to a set of operator control units. The operator is thus able to operate the camera, the crawler and the ultrasonic device and always knows exactly what the status of the pipeline is. He is even able to tell the pipeline owner during the inspection what the average condition of his pipeline is. 6 Copyright 2001 Scientific Surveys Ltd. All rights reserved.
Internal riser inspections on FPSO storage & production facilities The maximum cable lengths available for the crawlers depends on the diameter of the pipeline. That overview is given below. Pipecat 4" - 6" Umbilical Length 500 meters Pipecat 6" - 10" Umbilical Length 500 meters Pipecat 10" - 18" Umbilical Length 1.000 meters Pipecat 18" - > Umbilical Length 1.000 meters The maximum number of 90 degree bends which has been passed with one of the crawlers, so far, has been nine on a row. This is also considered to be its maximum.. Applications The Pipecat crawlers are mainly used for inspections of river crossings, road crossings, underground pipe work on chemical and petro chemical industries, offshore riser inspections, on-board piping systems and all other non piggable pipelines. Copyright 2001 Scientific Surveys Ltd. All rights reserved. 7
Pipeline Pigging and Integrity Monitoring Conference: Amsterdam, 1997 Appendix 3: Example of inspection results Fig. 1p: Weld at 74.6 mtr. Min. WT = 21.2 mm Fig. 1q: General wall loss, Min. WT = 20.5 mm Fig. 1r: Weld at 66.0 mtr. One area with Min. WT = 21.1 mm 8 Copyright 2001 Scientific Surveys Ltd. All rights reserved.
Internal riser inspections on FPSO storage & production facilities Fig. 1s: No special features.min. WT = 22.2 mm Fig. 1t: Weld at 56.9 mtr. One spot 21.2 mm. File change. Fig. 1u: Wrong automatic weld detection. Min. WT=22.2 mm. Copyright 2001 Scientific Surveys Ltd. All rights reserved. 9
Pipeline Pigging and Integrity Monitoring Conference: Amsterdam, 1997 Fig. 1v: Weld at 48.1 mtr. General wall loss Min. WT=21.2 mm. Fig. 1w: Side connection at 42.3 mtr. Weld (bend) at 41.3 mtr. Appendix 4: Overview of the riser configuration 10 Copyright 2001 Scientific Surveys Ltd. All rights reserved.