It was proposed to apply a modified external corrosion direct assessment process to a small station. This paper describes the technical approach which was developed in order to overcome the expected challenges. 

Shale oil companies are currently building state-of-the-art gathering infrastructure to transport oil and natural gas from the wells through an oil gathering system to a central oil stabilization facility to eliminate truck traffic and condensate tank emissions. A portion of fracturing fluids and sand that returns to the surface (slickwater and proppant backflow) may accumulate in this new-built pipeline infrastructure and cause severe pitting corrosion. Also, paraffin and wax deposits may cause operational problems in various parts of the oil gathering system. In offshore production systems, production streams frequently contain organic/inorganic solids and water causing corrosion and wax buildup in subsea pipelines, which are unpiggable in many cases.
We propose a novel system for removing solids, wax, sludge, and other unwanted fluids causing corrosion and/or flow assurance issues in oil and gas gathering systems, including manifolds and unpiggable process lines. The system comprises one component that generate fluid batches at the inlet of the flowlines and a model-based controller component that determines the launch time of fluid batches such that the batches generated in the flowlines are merged into one batch in the gathering pipeline or production manifold. The controller simulates the fluid batches moving along the flowlines using a real-time simulation model and provides automatic control of the systems generating the fluid batches. The dose of corrosion or wax inhibitors is automatically adjusted to maintain a predetermined concentration of the chemical in response to a variation of the rate of water production in the well.
Scale-model test results and real-time simulations of system operation are presented. The stationary bed of solids formed in the production manifold and process lines is converted into a series of solids dunes that slowly move toward the separator. This effect dramatically reduces the likelihood of internal corrosion. Also, the risk of wax deposition reduces because the internal surface of the pipe is continuously flushed by hot water batches. This technology is applicable for existing and new-build pipeline infrastructure and virtually does not have limitations regarding the design of the oil or gas gathering system, operating pressure and temperature. As a result, the production manifold itself and unpiggable process lines are efficiently flushed with produced water.

According to a survey of corrosion releases in hazardous liquids pipelines, of the 52 internal corrosion releases reported in 2009-2012, 31 occurred in intermittently operated lines. Pigging operations cannot be performed in many of these pipelines for a number of reasons, such as limitations imposed by pipeline design features, pigging cost, risk of the pig getting stuck in solids or sludge accumulated in front of it. For unpiggable  pipelines, direct assessment using the liquid petroleum internal corrosion direct assessment (LP-ICDA) method is a widespread industry practice that helps operators detect pipeline sections damaged by internal corrosion.  The models and correlations referenced in the standard are for steady-state, oil-water flow or liquid-solids flow, however the flow of transported fluids is transient (unsteady) in intermittently-operated, crude-oil pipelines. Furthermore,  the critical inclination angle defined in the standard is applicable to a pipeline having a piecewise elevation profile. Nevertheless, the local slope of the pipeline changes continuously along its entire length because the local slope of an elevation profile of the landscape or seabed changes continuously in most cases. In this paper, a pipeline diagnostic survey using transient, ultra-high definition simulations of three-phase oil-water-solids flow is performed to identify ICDA regions and develop solutions to extend the useful life of a 48-in. diameter pipeline transporting crude oil from an onshore tank farm to a Single Point Mooring/Pipeline End Manifold (SPM/PLEM) system. Transient free water and solids holdup profiles along the pipeline during loading operations and shutdown periods were predicted based on historical cargo data (including detailed loading plans and loading rates), basic sediments and water (BS&W) data, the rate of conversion of emulsified water into free water caused by the residual concentration of demulsifier in crude oil, solids properties, and tanker loading schedules. It was found that significant accumulations of free water and solids occurred only in a few sections of the offshore portion of the pipeline. This was attributed to the fact that in each loading operation the cargo officer requests the terminal to reduce the loading rate and adjust it until the final cargo transfer quantity is reached. As a result, during this time the pipeline is operated at a low flow condition at which free water ceases to enter PLEM, while it is still displaced from the onshore portion into the offshore portion of the pipeline. Two solutions were proposed to prevent microbiologically influenced corrosion (MIC) and extend the useful life of the pipeline. The first solution is based on the optimization of the loading plan to minimize the volume of free water accumulated in the offshore portion on completion of cargo transfer. In the second solution, a system generating batches of drained water taken from the storage tanks is connected to the pipeline inlet. A biocide is injected into the water batches. The concentration of the biocide and speed to kill are selected based on the water batch residence time determined using the transient flow simulations. The number of water batches to be launched depends on the volume and properties of solids that can enter the pipeline.   

Upstream oil and gas companies operate oil gathering systems comprising a flowline network and process facilities that transport the flow of produced fluids from the wells to a main processing plant. The frequency of corrosion related leaks has increased recently despite a corrosion inhibitor is injected at the wellhead into all flowlines. A root-cause analysis conducted by several companies revealed that severe internal corrosion was caused by a low fluid flow velocity an increasing water cut and the presence of sulfate-reducing bacteria (SRB) in the production streams. Nevertheless it was not clear why some of the flowlines may leak while others do not leak despite the composition of produced fluids principal design parameters (diameter and length) dosage of corrosion inhibitor and environmental conditions of the flowlines are similar. A diagnostic analysis of different oil flowlines of was carried out to gain an understanding of why a first group of oil flowlines is developing leaks and why a second group of flowlines has not experienced leaks. The methodology used for the diagnostic analysis comprises 1) Ultra-High Definition simulation of 3-phase or 4-phase flow of gas oil water and solids; 2) 3D imaging of phase distributions inside critical sections of the oil flowlines as per NACE ICDA; 3) mapping adverse operational conditions; and 4) the determination of probability of failure in the critical sections based on criteria depending on the severity of operating conditions inside and outside the flowlines. It was found that multiple sections were exposed to stagnant water and/or had a fraction of internal surface area covered by a stationary bed of solids (formation solids produced from the well). The identified causes of potential leaks comprise the following failure mechanisms: a) metal loss caused by colonies of SRB b) composed load acting on the pipe wall and c) cyclic" thermal expansion/contraction of the flowlines due to seasonal ambient temperature variations. One of the surprising findings of this study was that a shorter flowline with a lower water cut may have multiple leaks while a longer flowline with a higher water may not leak at all approximately for the same period after commissioning. This result was explained with help of maps of adverse operational conditions constructed for the two groups of flowlines. Immediate corrective mitigation actions and preventive actions were implemented to reduce leak frequency including the installation of a novel automatic flushing system.