Journal of Petroleum Technology August 2012 : Page 28

TECHNOLOGY UPDATE Advanced Desander Removes Solids Without Deferring Well Production Colin Sherwood, Stork Technical Services As oil and gas fields mature, production of sand and other reservoir solids can become an increasing and sometimes major impediment to hydrocarbon pro-duction and facility operations. These sol-ids can move through the wellbore into process facilities, causing flow assurance problems that affect system integrity, export line transportation, cleaning, and disposal. As operators seek to optimize production from mature assets, solids management is likely to take on additional importance. Sand and solids production can also be a problem on some new wells. To address these issues, Stork Tech-nical Services developed an advanced online desander (AOD) for managing sep-arator solids, providing thermographic determination of sand accumulation, and enabling clean sand discharge to the sea, with no deferred production (Fig. 1). The system has been used successfully in eight deployments on the Hummingbird float-ing production, storage, and offloading system (FPSO), which supports Centrica’s Chestnut field, operated by Wood Group, in the United Kingdom North Sea. Risks of Sand in Production Vessels The presence of sand in production ves-sels reduces capacity and affects separa-tion performance, which leads to dete-riorating produced water quality and the carry-over of water in the oil phase. If not controlled, the sand can cause a sepa-rator to be taken offline and manually cleaned. This exposes facility crew mem-bers to process hazards and may damage downstream equipment. Conventional sand management techniques require a shutdown of 10 to 14 days for manual sand removal, result-Fig. 1—The advanced online desanding system. 28 JPT • AUGUST 2012

Technology Update • Advanced Desander Removes Solids Without Deferring Well Production

Colin Sherwood, Stork Technical Services

As oil and gas fields mature, production of sand and other reservoir solids can become an increasing and sometimes major impediment to hydrocarbon production and facility operations. These solids can move through the wellbore into process facilities, causing flow assurance problems that affect system integrity, export line transportation, cleaning, and disposal. As operators seek to optimize production from mature assets, solids management is likely to take on additional importance. Sand and solids production can also be a problem on some new wells. To address these issues, Stork Technical Services developed an advanced online desander (AOD) for managing separator solids, providing thermographic determination of sand accumulation, and enabling clean sand discharge to the sea, with no deferred production (Fig. 1). The system has been used successfully in eight deployments on the Hummingbird floating production, storage, and offloading system (FPSO), which supports Centrica’s Chestnut field, operated by Wood Group, in the United Kingdom North Sea. Risks of Sand in Production Vessels The presence of sand in production vessels reduces capacity and affects separation performance, which leads to deteriorating produced water quality and the carry-over of water in the oil phase. If not controlled, the sand can cause a separator to be taken offline and manually cleaned. This exposes facility crew members to process hazards and may damage downstream equipment. Conventional sand management techniques require a shutdown of 10 to 14 days for manual sand removal, resultIng in a significant delay and additional expense. Following this, oil-on-sand discharge rules may limit disposal options and compel storage of some or all of the sand. The Online Desanding Technology The desander is designed to allow more operator uptime and to reduce costs, compared with traditional vessel entry cleanout methods. With the new technology, there is no need to shut down production, and the elimination of manual cleaning improves safety. In addition, the technology enables operators to change their approach to planned shutdown periods. Because the system allows a high degree of cleaning to take place before shutdown, the team of technical experts needed for a shutdown can be smaller and planning for it simpler. The system comprises a number of purpose-designed, integrated components including solids extraction, solids separation, and solids conditioning packages. A high-flow jetting unit with a recovered-liquid package recycles the separator water. The system complies with European Union hazardous area safety requirements and can be used in Zone 2 hazardous offshore environments. The technology deploys jetting heads into vessels accurately and safely, while collecting the recovered material into process facilities in an internationally standardized container that includes a wash tank, a high-flow centrifugal pump, and an inbuilt access. All of the components operate from a single 63A electrical socket. The produced solids are washed, checked, and quantified before overboard discharge. With the assistance of the Macaulay Institute in Aberdeen, a system of field analysis kits was developed to verify overboard discharges for contamination levels in real time. With the design’s modular concept, the existing wash tanks could be used with a separation package equipped to precondition the slurry stream. A coarse “spin filter” on the skid inlet handles sand bursts from the point of startup, significantly reducing the risk of blockage. The sand jetting nozzle is deployed by means of a solids extraction ram with rotational drive to facilitate sand recovery from the production separators. A system of controls and process interlocks ensures that equipment operation poses minimal risk to workers and the installation. Verification studies, including a full hazardous operations analysis, were conducted in the development stage, with the support of independent engineering authorities and client process engineers. The system’s mobility enables sand production problems to be handled cost effectively because there is no need to design or make specialist hardware or shut down the process for installation. The package is connected through the separator drain valves, which has minimal effect on the platform infrastructure. Thermographic imaging is used to monitor sand levels before, during, and after desanding. Removal of the accumulated solids restores performance efficiency to the separator and allows increased throughput, while the extracted oil is returned to the process through the platform’s hazardous drainage system. The recovered material is subsequently removed and treated in situ with no need for onshore transportation or storage reinjection. The portable analysis kit enables onboard, real-time testing of treated sand, followed by overboard disposal where permitted. Operators are only able to discard untreated material overboard if the level of contamination is within limits of best environmental practice. The system guarantees that discharge is washed to below 1% oil on sand, well within the UK Department of Energy and Climate Change (DECC) and United States Environmental Protection Agency guidelines. Meeting this level is vital when applying for discharge licenses. Figs. 2a and 2b show sand samples before and after cleaning. The Technology in Operation The system was used in eight deployments on the Hummingbird FPSO. A well was producing sand to the FPSO’s first-stage separator, posing a risk to production rates if it could not be managed. The operator considered three options: corrective well intervention measures, reduced oil production to restrict sand intake, or online desanding at regular intervals. The third method was chosen. Over the eight deployments, beginning in 2009, 58 tonnes of sand were removed from the separator while it remained online. There was no disruption to production or ongoing operations. Each deployment was completed without harm to people or the environment. All sand was washed to below 1% oil on sand, consistent with DECC discharge requirements. Recent discharges to the sea have averaged 0.07% (700 ppm) oil on sand, which is below the 1% target figure. Removal of the sand also allowed some separator instrumentation problems to be resolved. Further deployments are planned this year as production optimization takes place. Conclusion The AOD provides a cost-effective, environmentally compliant means of removing sand from production streams in small field operations. While many separator vessels have inbuilt desanding systems, they are prone to blocking, expensive to retrofit, and require the vessel to be taken offline in the event of failure. The AOD can be easily deployed by connecting to existing drains, removing accumulated material, extracting oil from the sand, and allowing the recovered oil to be returned to the process through the platform’s hazardous drainage system. Most significantly, the technology reduces the crew size for shutdown periods, thus simplifying the planning process.

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