SCALE ASSESSMENT PIGGING OF THE TOTAL DUNBAR 16 MULTI-PHASE PIPELINE

Transcription

1 OPT Conference & Exhibition March 1 & 2, 2005 Amsterdam SCALE ASSESSMENT PIGGING OF THE TOTAL DUNBAR 16 MULTI-PHASE PIPELINE Gordon Allan Pipeline Inspection Manager Weatherford Pipeline & Specialty Services Unit 3 Newhailes Industrial Estate Musselburgh EH21 6SY UK T: F: gordon.allan@eu.weatherford.com Jon Hawes Lead Project Engineer TOTAL E&P UK plc Crawpeel Road, Altens Industrial Estate Aberdeen AB12 3FG UK T: F: jon.hawes@total.com 1. ABSTRACT The 16 Dunbar Pipeline, operated by TOTAL E&P UK PLC, transports raw multi-phase fluids 22km from the Dunbar platform to the North Alwyn Bravo platform in the North Sea. The pipeline was commissioned in 1994 and a metal loss inspection with an intelligent pig was successfully completed in Video inspections of the pipeline conducted in recent years indicated a significant deposition of scale in the topside and riser pipework at the Dunbar platform. Initial attempts to assess the extent of the scale build-up in the risers and pipeline were only partially successful, and by the summer of 2003 the deposition was estimated to have restricted the remaining internal diameter at the top of the riser to less than 10. By late 2003 TOTAL required that a further metal loss inspection be conducted in To enable an intelligent pig inspection of the pipeline, it was necessary that it be cleaned of scale. To enable the formulation of a cleaning methodology that minimised the risk of blocking the pipeline during cleaning, it was critical that further details be established as to the quantity and distribution of the scale within the pipeline. Following a review of available inspection techniques, TOTAL contracted Weatherford s Pipeline & Specialty Services (P&SS) to develop a multi-diameter, multi-channel caliper pig for the Dunbar pipeline. Weatherford P&SS provide pipeline inspection services to the international oil and gas industry with their SAAM smart utility pig and range of caliper tools. This paper, jointly presented by Weatherford and TOTAL, describes the development work undertaken, as well as summarising the results of the inspection runs carried out with both SAAM and the caliper pig during 2004.

2 2. INTRODUCTION & BACKGROUND The Alwyn and Dunbar Fields are located to the east of the Shetland Islands, and are operated by TOTAL E&P UK PLC. The Dunbar export pipeline runs from the Dunbar Platform to the North Alwyn Bravo Platform, a distance of some 22km in metres of water (Figure 1). Figure 1 Dunbar to Alwyn Export Pipeline Installed in 1994, the pipeline was the first Pipe-in-Pipe (PiP) commissioned to a design by ITP. Prefabricated in 24m-long double joints, the Dunbar PiP system consists of an inner 16 pipe (406.4mm by 17.5mm), an insulating layer of fibreglass and argon, and a 20 outer pipe (508mm by 11.91mm). At the end of each prefabricated joint, the outer and inner pipes are connected by a tulip-type sealing arrangement, which is joined by the interpipe weld. During installation, (from a conventional pipelay vessel), each field joint is insulated using a prefabricated screwed sleeve arrangement. This gives an overall U value of 1.22 W/m2K. The Dunbar Platform does not possess any processing facilities, and therefore exports raw unprocessed fluids, primarily gas with condensate and water. Velocities of the fluids vary from 1m/s in the liquid phase to >10m/s for the gas phase. This makes for a flow regime that is difficult to predict. In 1997 the pipeline was intelligently pigged, using an Ultrasonic Inspection Pig. After three years operation, the Ellon and Grant Fields were brought on stream and routed via the Dunbar platform through the export pipeline to Alwyn. Within a very short time, scale deposition became evident in the topsides pipework. Subsequent investigation revealed that this was caused by the combination of the produced water from one of the Ellon wells and the produced water from some of the Dunbar wells. The scaling worsened over the following years despite the use of a program of scale inhibition and the separation and re-injection of Ellon produced water on the Dunbar topsides. In 2002, with concern growing over the inability to verify the integrity of the pipeline, TOTAL initiated an investigation to try and determine the extent of the scaling in the pipeline. A program was developed to run successive foam pigs of varying size and density, to then run a gauge plate and finally a caliper pig. After the gauge plate became detached from its pig, a retrieval operation determined that it had lodged in a mass of scale located at the top bend of the riser. After retrieval of the gauge plate, a video survey was carried out on the riser to approximately 70 metres down the riser. This showed heavy scale deposition covering 100% of the riser inner surface to this depth (Figure 2). In 2003, a study was undertaken to determine the best methods for removing the scale to facilitate an intelligent pig inspection. During the study, further inspection of the riser was carried out. This took the form of the following three activities: During the dewatering of the pipeline, a radioactive isotope was injected at Dunbar and time of flight to Alwyn was measured. With the known flowrates, an estimate of the remaining pipeline volume was obtained. When compared to theoretical volume, an estimate of scale volume could be determined. A repeat video survey was carried out, this time to the bottom riser bend (Figure 3). This showed that the scale was continuous to the bottom bend, but that the most severe area of deposition was at the top bend. A sonar head was deployed to measure the minimum bore of the riser. This gave a worst case measurement of approximately 10, backed up by the physical size of the camera assembly, which was 9 across.

3 Figure 2 Scale at Riser Top (2003) Figure 3 Scale near bottom of Riser (2003) The conclusion of the study was that the preferred scale removal method for the pipeline was hydro-mechanical cleaning. However, in order for this method to work effectively, a full three-dimensional map of the scale was required. Existing conventional single channel caliper pigs would only give the minimum diameter at any given point along the pipeline, and would not be able to pass through the restricted area at the top of the riser. Therefore, the decision was taken to develop a customised caliper pig with dual diameter and multi-channel functions and capable of running in a multi-phase production flow regime with minimum disruption to production. Based on the information gained from the riser and pipeline surveys, sufficient parameters were available to allow TOTAL to go the marketplace for the development of such a pig. Cognisant of the fact that a solid bodied pig had not transited the pipeline since the intelligent pig run in 1997, TOTAL issued a tender for the design, construction, testing and operation of a caliper pig with dual diameter and multi-channel capabilities. Two options were short listed, with a mechanical pig (Weatherford) being favoured over an ultrasonic pig because of the operational difficulties presented by the requirement for a liquid contact medium. Weatherford Pipeline & Specialty Services provide pipeline inspection services to the oil and gas industry using their patented SAAM and caliper inspection pigs. The SAAM technology provides the operator with a low cost, low risk means of inspecting their pipeline and comprises a package of electronics and instrumentation that can be retrofitted to a traditional cleaning or utility pig, rather than being a special standalone inspection pig. The SAAM tool (Figure 4) measures and records the behaviour of its carrier pig as it travels along the pipeline. The results are then analysed, providing information on the following: Pipeline out of straightness [1] Wax/debris deposition Internal corrosion [2] Mechanical damage or other bore restriction [3] Process parameters The Weatherford caliper tool (Figure 5) is designed to directly measure the internal diameter of a pipeline using an array of spring loaded mechanical arms. The minimum diameter is then measured and recorded by the on-board electronics package. The caliper tool is used in both newly constructed and operational pipelines to determine the presence, size and location of any dents, ovalities or other restrictions. Weatherford s experience with the design and operation of these inspection tools, along with an extensive track record in pig design was crucial to meeting the challenges of the Dunbar scale assessment project.

4 Figure 4 A Typical SAAM Pig Figure 5 The Weatherford Caliper Pig 3. DESIGN REQUIREMENTS FOR THE CALIPER PIG The key challenges identified by the project team comprised the following: Running a solid bodied pig in the pipeline during production with the least practicable reduction in pipeline flowrate. As well as the lost revenue associated with reduced production rates, TOTAL were keen to avoid the problems associated with the start-up of wells following shut-in. Previous pigging of the Dunbar pipeline had been carried out during shut-downs as multi-phase pipelines such as Dunbar are notoriously difficult to pig due to problems with slug flow and pig speed excursions. Designing an inspection pig which could successfully negotiate the severe restriction due to the scale build. This effectively required a multi-diameter pig capable of operation in a diameter range from 10 to 16. The fact that the most severe restriction was believed to be in a bend increased the risk of the pig sticking or stalling. A related challenge was the expected high pig speed through the restrictions in the pipe. The high degree of accuracy required for any scale measurement. An error of 1mm in scale thickness measurement over the length of the pipeline could equate to 26m 3 or 100 tonnes of scale, a substantial amount of material to remove from a pipeline 1. The need for high accuracy would typically require the carrier pig to be held precisely in place in the pipeline, a requirement at odds with the need for a multi-diameter capability. The short lead-time. The project work kicked off in late December 2003, TOTAL required the results early in the second quarter 2004 in order that the scale removal and subsequent pipeline inspection programme could proceed in the summer of that year. The relatively high operating temperature of 81 C was at the upper limit of readily available electronic components as well as pig elastomer parts. In addition the abrasive nature of the scale build-up required the use of highly abrasion resistant elastomer parts. 4. DUMMY CALIPER & SAAM PIGGING Due to the severe consequences of getting the caliper pig stuck in the Dunbar pipeline, and relatively high risk of it occurring in comparison to other more routine pigging operations, the following decisions were taken: To run the caliper pig in a pigging test facility with simulated scale build-up based on the worst case expected in the Dunbar pipeline. To build a dummy caliper pig designed to pass a greater bore restriction than the caliper pig. To run this dummy in the test facility as soon as possible to enable any lessons learnt to be incorporated in the design of the caliper pig. To run this dummy caliper in the Dunbar pipeline fitted with gauge plates 2. This section describes the test facility and experiences with the dummy tool both in the test facility and in the Dunbar pipeline. The Pigging Test Facility As well as enabling the proving of the bore passing capability of the pigs, the test facility (Figure 6) was designed to enable the proving of the caliper tool accuracy. The following features were included: 1 With scale removal by pigging the preferred method for the Dunbar pipeline, one of the key risks is the pipeline becoming blocked by a build-up of the removed scale. 2 Gauge plates are used to witness the minimum bore a pig has passed without a significant risk of the gauging pig becoming stuck.

5 A 16 5D bend internally lined with concrete to a diameter of 10 as shown under fabrication in Figure 7. Sections of pipe with internal diameters of 12, 14 and 16. Several sections of simulated scale build-up of between 5mm and 30mm fabricated from both steel and concrete. Pig launching and receiving facilities, water storage tanks and pumping facility. Figure 6 Layout of Pigging Test Facility Figure 7 Fabrication Of 16 Bend with Simulated Scale Build-Up Dummy Caliper Pig Testing The testing of the dummy caliper pig (Figure 8) took place in early February. The initial runs were unsuccessful, resulting in damaged gauge plates. These plates, however, had been sized as large as possible to allow the designers of the caliper pig more scope to maximise the size of the tool. Further runs with smaller gauge plates were successful, enabling limits to be imposed on the physical size of the caliper pig. Figure 8 Dummy Caliper Pig Figure 9 SAAM Results From Dummy Pig Run in Dunbar Dummy Caliper Pigging In Dunbar After testing, the pig was mobilised offshore where it was successfully run in the Dunbar pipeline in late February There was no damage to the gauge plates although the pig did lose several petals from the rear disc pack, which was believed to have happened in the pig receiver so was not considered to be a significant problem. During both the testing and the run in Dunbar, the pig was fitted with a SAAM inspection tool. Subsequent analysis of the SAAM data from the pigging run in Dunbar gave the following results: Based on the behaviour of the pig, the most severe scale build-up was in the bend at the top of the riser, with the build-up tailing off over the first few kilometres. Specifically, there appeared to be no other severe restrictions elsewhere in the line, something that had been of significant concern to the cleaning contractor. Figure 9 shows the

6 SAAM data traces for the first 2km of the pipeline, the higher differential pressure and pig vibration indicating the scale build-up. The speed of the pig was controlled reasonably well for the majority of the line with few significant speed excursions that would likely cause problems with the accuracy of the caliper pig. 5. DUNBAR CALIPER PIG DESIGN The final design of the caliper pig (Figure 10) is presented in this section with descriptions of the major parts or sub-systems. Caliper Arms PU Discs Electronics Pig Body Odometer Wheel Figure 10 The Dunbar Caliper Pig Caliper Arm Assembly This sub-system comprises 8 independent arms designed to be held in place against the internal surface of the pipe by the spring system visible at the very rear of the pig. Each arm assembly includes its own linear position sensor for the measurement of the arm position. The springs and sensors are located centrally and to the rear to enable the arms to fold back fully when the pig is negotiating restrictions in the pipe. Electronics Package This includes the battery power source, the processor and power supply electronics, flash memory data storage as well as interfaces for connection to the caliper arms, the odometers and for USB communication with the host computer. The package also included additional instrumentation comprising: Gyroscopes for measurement of the radius and angle of in-line bends; this information being required for the cleaning and inspection contractors. Accelerometers, which in conjunction with the gyroscopes were designed to record pipeline out-of-straightness. All the components were housed within a stainless steel pressure vessel which also formed part of the structure of the pig body. Odometer Wheels The odometer provides a regular pulse as it rotates. This is used to trigger the recording of the caliper arm measurements and to measure the distance travelled by the tool. The tool included a spare odometer to provide backup in the event of damage to

7 the other, something that was considered a possibility in this particular project due to the likely speed of the pig when it first encountered the severe build up of scale. Polyurethane Disc Packs Each of the identical front and rear disc packs comprise a slotted support cup, a spacer and twin slotted sealing discs. All parts were manufactured in-house by Weatherford from high grade polyurethane (PU). The function of the support discs is to hold the pig centrally in the pipe, therefore they are manufactured from a relatively hard and stiff PU compound. The sealing discs are slightly over-sized relative to the internal diameter of the pipeline, thereby providing a seal and causing the pig to be driven by the line fluid. The slots in both types of discs allow them to fold back when the pig encounters reduced diameters. The slots in the seals are offset to ensure an effective seal is maintained at all times. Pig Body The pig body, comprising the electronics package and a spacer piece, required to be narrow in diameter in order for the pig to successfully negotiate the severe scale build-up in the bend. This is shown in Figure 11, which also shows the cut-outs in the pig body that enable the odometer arms to retract fully. Figure 11 Dunbar Caliper Tool in 16 5D Bend with 3 thick Scale Build-up Figure 12 Example Screenshot of the Caliper Software Bumper Nose and Isotope Housing The PU bumper nose included a housing for a radioactive pig tracking system. Immediately prior to launching the pig, a radioactive source was fitted enabling the pig to be tracked whilst in the pipeline. This method of tracking was chosen in preference to alternatives as it was considered inherently more reliable than other methods, did not rely on limited life battery power and was proven to give reliable detection in subsea pipe-in-pipe designs such as Dunbar. Given the higher risk of a stuck pig, in comparison to other more routine pigging situations, the additional cost and safety issues were considered to be justified. Host PC Software The software included functionality to enable communication with the new tool, visualisation of the raw data and analysis of the caliper to data to calculate scale volumes, an example screen shot of the software is shown in Figure TESTING OF THE DUNBAR CALIPER PIG The design and manufacture of the new caliper pig was completed using the size limits determined by the experiences with the dummy pig. Prior to the pigging trials, a series of acceptance and qualification test were carried out, including: Basic operation and calibration of all instruments. Measurement repeatability. This proved the accuracy of the measurement system in a static situation without the variables arising during pigging. Pressure, temperature and vibration testing to prove ability of the tool to withstand the environment expected in the pipeline.

8 The pig was then run in the pigging test facility. The first test of the new pig resulted in it becoming stuck in the reduced bore bend. The reason for this was quickly identified as being due to the slots on the sealing discs being too small such that they jammed together when fully folded back as shown in Figure 13. Once this was rectified, the testing progressed satisfactorily with a total of 6 runs being completed. The resultant data was then analysed and a comparison made between the caliper pig results and the true shape of the defects in the test loop. The outcome can be summarised as follows: The caliper pig was well within the measurement accuracy requirement in most cases for concentric reductions in pipe ID. For non-concentric simulated defects fabricated by welding a curved metal plate to the inside of the pipe, the measurement accuracy generally met requirements. For larger defects fabricated from concrete as shown in Figure 14, and including the restriction in the bend, the accuracy was generally out-with the required tolerance. Closer examination of the concrete defects showed the defects sizes did in fact vary and that the errors were not as great as initially thought. On this basis and with the caliper pig having successfully negotiated the loop without damage, it was deemed to have passed the qualification testing and the project proceeded to the next stage - caliper pigging in the Dunbar pipeline. Figure 13 The Dunbar Caliper Tool During Testing Figure 14 Simulated Scale Build-Up in Test Spool 7. CALIPER PIGGING & SCALE ASSESSMENT The caliper pig was run in the Dunbar pipeline in mid-april Initial examination of the data showed that the pig had logged data for the duration of the run. However, several of the caliper arms gave erroneous readings at certain times during the run. Examination of the pig speed showed an average of approximately 3m/s but with momentary speeds far in excess of this. The erroneous readings were attributed to these speed fluctuations, and it was decided to conduct a re-run at a slower speed. Some modifications were also made to the caliper pig to improve its ability to accommodate the higher speeds experienced. The re-run was completed at a lower average speed of just under 2m/s with a far better performance from the caliper pig. As was the case with the dummy pig and the first caliper pig run, several petals from the rear disc pack were damaged, and in addition, one of the odometer wheels had lost its tyre. Figure 15 shows a photo of the caliper pig after the re-run. The pig still experienced significant speed variations as shown in Figure 16, and examination of the data showed that it had actually stopped at the top of the riser when it encountered the scale build before being forced through the obstruction as the differential pressure increased.

9 Figure 15 Caliper Pig following the Re-run Figure 16 Caliper Pig Speed during the Re-run The caliper pig data was then analysed to provide an assessment of the quantity of scale in the pipeline. This is presented as a graph against distance as shown in Figure 17. This clearly shows the severe scale build-up present at the start of the pipeline, gradually reducing over the first 6 km. The slight increase in quantity of scale shown from 6km to the end was considered not to represent any actual scale, but to be a consequence of either inaccuracies of measurement or variations in the actual inside diameter of the pipeline. Figure 18 shows the theoretical effects of any error in the measurement of the scale, illustrating the importance of the accuracy of all measurements of this type. The final calculated scale volume of 43.5m 3 equates to a weight of 170 tonnes of scale. Figure 17 Scale Assessment Results Figure 18 Scale Assessment Sensitivity to Measurement Error 8. SUBSEQUENT CLEANING & INSPECTION OF THE PIPELINE Following the work detailed in this paper, the pipeline was successfully cleaned using specialist pigging equipment. It was not possible to measure the amount of scale removed, so no accurate feedback has been available on the accuracy of the scale assessment. Subsequently the pipeline has been inspected using a UT intelligent pig. The results indicated that approximately 97% of the scale had been successfully removed. The only traces of scale remaining showed that the scale extended at least 9km into the pipeline, but was most likely less than 1mm thick, below the threshold of detection of the caliper pig. 9. CONCLUSIONS The successful completion of the scale assessment pigging project enabled the line to be cleaned and subsequently inspected. During the project a number of significant successes were achieved: A number of solid bodied pigs were run in the pipeline without the need for a shutdown, and with minimal disruption to production activities. The caliper and SAAM tools successfully recorded data under extremely severe operating conditions.

10 The scale assessment was carried out within a very short period of time enabling the scale assessment, cleaning and intelligent pig inspection to proceed in the one season. The use of the SAAM tool on board the dummy provided early information as to the extent of the scale, in particular that the build up in the riser was in all likelihood the most severe. There were many lessons learnt with relevance to future specialist pigging or pipeline inspection projects, in particular: The approach of using a dummy tool was critical to the success of the project. Without this, the likelihood of the worst case scenario, namely getting a pig stuck, would have been much higher. The use of onshore full scale pigging trials of both the dummy and actual caliper pig was similarly crucial to meeting the project objectives. The post dummy pig analysis of the SAAM data enabled TOTAL to improve operational parameters for each subsequent run of the caliper pig, particularly to minimise slugging induced by the caliper pig while in the pipeline. 10. REFERENCES [1] Moore, David; Snodgrass, Bob; Nicholson, Barry. Smart Utility Pigs Used To Determine And Monitor Pipeline Out-Of- Straightness, With Specific Reference To Inspection Of BP Alaska s 10 Northstar Crude Oil Pipeline, International Pipeline Conference, Calgary, Alberta, Canada [2] - Short, Gordon and Flett, Dave. New Approach To Pipeline Condition Monitoring Of The Beatrice 16'' Oil Export Line. 4 th International Pipeline Conference, Calgary, Alberta, Canada [3] - Russell, David and Short, Gordon. Pipeline Mechanical Damage Detection, Assessment and Monitoring Using the SAAM Pipeline Inspection Tool. Brazilian Petroleum & Gas Institute - IBP 3rd Seminar on Pipelines, Rio de Janeiro, Brazil, 2001.