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Scale buildup due to water production can choke oil production and require repetitive scale treatments across entire fields. In subsea wells, the common solution employs a deepwater rig to conduct either workover operations or large-volume scale inhibitor squeezes. Less frequently, coiled tubing (CT) is used from a moonpool vessel. However, current oil prices required a custom solution for subsea well treatments that was more cost effective than either a rig or a moonpool vessel. Similar previous operations successfully used 1 ¾-in. and 2-in. (44.4 mm. and 50 mm.) CT at the same time from a moonpool vessel. A remotely operated vehicle (ROV) in the open water connected the CT to the subsea safety module (SSM) through a dynamic conduit and connected the SSM to the wellhead. An engineered solution to change to 2 7/8-in. CT and use high-rate stimulation pumps was planned to deliver subsea treatments at up to 15 bbl/min. The equipment layout was designed for a multipurpose supply vessel with chemical storage tanks; to increase the available selection of vessels, the CT was designed to run overboard rather than through a moonpool. This project was initiated after accelerated scale buildup occurred because of a pressure decrease close to the bubble point, which happened when the drawdown was increased for aggressive production targets. To effectively inhibit scale in this environment, treatments required thousands of barrels of inhibitor. For wells with more-severe scale conditions, acid treatments were planned. These treatments were delivered with one complete CT package, stimulation pumping fleet, and subsea equipment, which were all installed on the spare deck space of the available vessel. A custom overboard CT deployment tower was designed. The new tower improved the dynamic bend stiffener (DBS) placement, which allowed the clump weights to be deployed with the bottomhole assembly (BHA) and simplified the rig-up. The chosen vessel worked well for the operation; however, the equipment layout and the local weather conditions combined with the response amplitude operator (RAO) of the vessel shortened the projected fatigue life of the CT. CT integrity monitoring with magnetic flux leakage (MFL) measurement was introduced here, and the vessel’s motion reference unit (MRU) provided an input to a fatigue calculator, based on the global riser analysis (GRA). The measurements and the analysis were utilized successfully to prevent CT pipe failures in the open water and deliver the required well treatments. To allow further improvements in deepwater operations, the new engineering work-flow was carefully documented.
Scale buildup due to water production can choke oil production and require repetitive scale treatments across entire fields. In subsea wells, the common solution employs a deepwater rig to conduct either workover operations or large-volume scale inhibitor squeezes. Less frequently, coiled tubing (CT) is used from a moonpool vessel. However, current oil prices required a custom solution for subsea well treatments that was more cost effective than either a rig or a moonpool vessel. Similar previous operations successfully used 1 ¾-in. and 2-in. (44.4 mm. and 50 mm.) CT at the same time from a moonpool vessel. A remotely operated vehicle (ROV) in the open water connected the CT to the subsea safety module (SSM) through a dynamic conduit and connected the SSM to the wellhead. An engineered solution to change to 2 7/8-in. CT and use high-rate stimulation pumps was planned to deliver subsea treatments at up to 15 bbl/min. The equipment layout was designed for a multipurpose supply vessel with chemical storage tanks; to increase the available selection of vessels, the CT was designed to run overboard rather than through a moonpool. This project was initiated after accelerated scale buildup occurred because of a pressure decrease close to the bubble point, which happened when the drawdown was increased for aggressive production targets. To effectively inhibit scale in this environment, treatments required thousands of barrels of inhibitor. For wells with more-severe scale conditions, acid treatments were planned. These treatments were delivered with one complete CT package, stimulation pumping fleet, and subsea equipment, which were all installed on the spare deck space of the available vessel. A custom overboard CT deployment tower was designed. The new tower improved the dynamic bend stiffener (DBS) placement, which allowed the clump weights to be deployed with the bottomhole assembly (BHA) and simplified the rig-up. The chosen vessel worked well for the operation; however, the equipment layout and the local weather conditions combined with the response amplitude operator (RAO) of the vessel shortened the projected fatigue life of the CT. CT integrity monitoring with magnetic flux leakage (MFL) measurement was introduced here, and the vessel’s motion reference unit (MRU) provided an input to a fatigue calculator, based on the global riser analysis (GRA). The measurements and the analysis were utilized successfully to prevent CT pipe failures in the open water and deliver the required well treatments. To allow further improvements in deepwater operations, the new engineering work-flow was carefully documented.
Early attempts to monitor coiled tubing (CT) fatigue in open-water interventions relied on basic statistical methods with a constant damage rate and broad generalizations of sea conditions. The new CT fatigue management system (FMS) comprises hardware and software that improve fatigue tracking. It tracks high-cycle fatigue (HCF) and low-cycle fatigue (LCF) simultaneously and in real time, elevating the accuracy, immediacy, and reliability of CT fatigue monitoring in open-water interventions. The hardware monitors vessel pitch and roll in real time and relays the data to an acquisition system where they are combined with CT depth and circulating pressure. The new acquisition software consumes these data as well as a CT string fatigue life database and geometry and positions of the CT string, work drum, and payout sheave. LCF and HCF are accurately modeled, and the string fatigue life is updated in real time. That information allows the operation to strategically cycle the CT pipe to manage fatigue more efficiently and extend the time before a trim is necessary. The traditional statistical method recorded a cumulative CT fatigue damage of 16% over a 14-hour job. In that same timeframe, the CT FMS measured a cumulative fatigue of 3.8%. Since the CT FMS updates damage rate continuously in response to vessel dynamics and uses a nonlinear methodology to combine HCF and LCF, it reduces inaccuracy and eliminates unnecessary safety factors associated with the statistical method. By improving precision and reliability, the real-time CT FMS safely extended the useful life of CT pipes by 75%. The CT FMS also improves operational integrity by introducing alarm threshold settings and custom material parameters. Alarms are raised reflecting fatigue life growth, tubing payout, and changes to sea conditions. This provides the operator with real-time, actionable data to ensure a more even distribution of the fatigue along the CT. Besides extending useful life, this reduces the risk of CT failure, which can lead to costly downtime, reputational damage, and production losses. The string's operating limits are more accurately monitored, which extends the capacity of the CT into harsher environments, longer operations, and riserless interventions of wider workscope. The CT FMS automates workflows that previously required a dedicated technician, reducing crew size on board. The introduction of CT in open-water interventions brought with it the risk of CT failure, which is inherent to any operation that handles CT pipe. This industry first, real-time CT FMS combines HCF and LCF monitoring and is a step change from the traditional methods to address these CT-related risks. Furthermore, the CT FMS elevates overall operational integrity, increases operational efficiency, and extends the operational envelope and application of CT in open-water interventions.
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