A flow assurance design was recently carried out for the Talisman Energy Norge AS-operated Varg oil field in the Norwegian sector of the North Sea.
Oil production on Varg is accompanied by a lean gas, which has been reinjected into a dedicated reservoir and accumulated over time. A Varg gas export pipeline is being installed to produce this excess gas to the nearby Rev field, located 6 km (4 miles) from Varg and connected to the Armada platform in the UK sector.
The gas will be transported using a 6-in. flexible pipeline to the Rev manifold, where the fluids will be commingled and transported through an existing 12-in. pipeline to Armada.
Kinetic hydrate inhibitors (KHIs) will be introduced into the field for hydrate protection, with gas production from Varg to start within two years of identification of the KHI concept.
KHI selection
In order to meet the arrival pressure conditions at Armada, it is necessary to choke back on the Varg topsides, which leads to significant temperature drop downstream of the choke due to Joule-Thomson cooling. As a result, hydrate formation in the Varg-to-Rev pipeline is likely.
The use of methanol or methanol/glycol is restricted in daily levels to specific operations only, such as restart. Because of this, the use of a KHI was considered as an alternative to prevent hydrates from forming. The Rev fluids are warmer than the Varg fluids, so after the commingling point the mixture warms up and the fluids will be outside the hydrate formation region in the Rev-to-Armada pipeline.
For this development typical hydrate prevention techniques – thermodynamic inhibitors, insulation, and so on – were not possible due to high capex, restricted operability, and regulations imposed at the receiving facilities.
The objective of a KHI is to prevent or delay hydrate formation for a certain amount of time. A KHI is normally selected after a laboratory performance testing program to determine induction times, but this experimental campaign can add up to a year to the schedule. This project was fast-tracked to reduce project execution time and costs. Therefore, laboratory testing of the candidate KHI did not take place until after completion of FEED.
Flow assurance challenge
From a flow assurance perspective, hydrate management was considered the main challenge in this project. Based on new techniques to evaluate KHIs and an understanding of mechanisms of hydrate nucleation/growth, it was concluded that a KHI would perform well in the Varg system, which is rich in propane and butane, even though high subcoolings are predicted. The experimental campaign is designed with the aim of determining the time that the KHI will be protecting the system before hydrates nucleate. Measuring this time is difficult since the crystallization process is inherently stochastic.
Recent research has shown that KHIs are capable of delaying nucleation and also crystal growth. New techniques for evaluating KHIs have been developed based on nontime-dependent inhibition properties, which circumvent the stochasticity of the crystallization process. These new methodologies provide a useful decision-making tool to manage the uncertainty of the KHI performance during the different project phases.
The design was bound by the need for all operating modes to be within the expected constraints of KHI performance. Confirmation of performance took place during a laboratory program through mid-2012. The results confirmed that the Varg design was within the performance limits of the KHI, with startup planned for late 2013.
The design takes advantage of the fact that the “as-built” Rev-to-Armada pipeline has shown that actual insulation performance and hence operating no-touch times are significantly greater than the original design. The intention is to exploit this overdesign with the tie-in of the cooler Varg fluids.
Tie-back selection and insulation
The value of insulating the 6-in. Varg-to-Rev pipeline also was investigated to assess possible improvements in no-touch time, turndown rate, and operational flexibility that can be gained by improving the heat retention in the system. Such improvement could eventually yield a better KHI performance since the required subcooling during normal operation would be reduced.
However, the benchmark analysis showed that the effect of applying relatively heavy foam insulation to the Varg gas export pipeline is minimal under all foreseeable operating conditions. Furthermore, settle-out conditions following shutdown were identified as the critical design point for the KHI performance.
Hydrate management philosophy
The two parts of the Varg/Rev-to-Armada production system are protected against hydrate formation by two different methods. The Varg gas enters the pipeline relatively cold due to Joule-Thomson cooling from the high-pressure FPSO compression train and has a relatively high hydrate dissociation temperature.
The hydrate prevention strategy for this section during normal operation is continuous injection of a KHI to delay crystal nucleation and growth. In the Rev-to-Armada section the commingled fluids comprising cold Varg fluids and warm Rev fluids are kept outside the hydrate formation region due to insulation and burial of the pipeline; there is no need, therefore, for any additional actions to avoid hydrate formation. During normal operation, the total residence time of the fluids in the pipeline will be shorter than the induction time provided by the KHI.
Finally, the expected water content in the pipeline is very low; hence in case of failure of the KHI, the risk of forming a fullbore hydrate plug is unlikely.
In the event of a short-term shutdown, the system will be protected by the KHI (Varg-to-Rev tie-back) and by retention of heat (Rev-to-Armada pipeline). After approximately 36 hours the system would be depressurized to keep the fluids outside the hydrate formation region.
The performance of the KHI will vary depending on the operating conditions and the subsea ambient conditions. However, long induction (hold) times are expected (possibly exceeding the no-touch Rev-to-Armada section) since the KHI will show a high performance in the hydrate metastable region.
For startup and restart, methanol will be used to inhibit hydrate formation. The thermodynamic inhibitor will be continuously dosed for the first few hours of production until normal operation has been reached. At this point methanol dosage will be halted, and the KHI will be continuously injected at the inlet of the flexible pipeline.
Performance testing results
The qualification program determined that the adequate required dosage of a KHI is 3.5% vol/vol. Also, the preselected KHI is compatible with the corrosion inhibitor and offers a good hydrate inhibition for the Varg gas under flowing (mixing), shut-in, and restart conditions. At high subcoolings greater than 15°C (59°F) the performance is more tenuous (with hydrate growth observed at moderate rates).
Tests using a specifically developed shut-in, cool-down, and restart protocol showed that the KHI can prevent hydrate formation during shut-in and subsequent cool-down of up to one week. These tests also showed that it may be possible to restart without problematic hydrate formation as long as the system is warmed to temperatures within the complete inhibition region within a few hours.
Technological milestones
The general methodology described is applicable to any development considering the use of a KHI for hydrate inhibition. The efficacy of a KHI-based solution can be conservatively assessed reliably and rapidly without laboratory testing.
Two technological milestones have played an important role in this flow assurance-driven design. First, the identification of a suitable KHI has been important when developing the design with the adequate level of conservatism. These new KHIs have been formulated to achieve high performance even at high subcoolings.
Second, the advances in the understanding of KHI inhibition have led to the development of more reliable performance evaluation techniques, providing a useful decision-making tool for managing the uncertainty of the performance during project phases.
Material used within this article originates from a paper presented at the 2013 Multiphase Production Technology Conference.
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