The design and performance of offshore gas oil water separation processes
Creators: name: Abia-Biteo Belope, Miguel-Angel
Publisher: name: University of Surrey
Year: 2010

description: Three phase gravity separators, degassers, hydrocyclones and coalescers are commonly used in
description: the oil and gas industry as primary and secondary means of separating well-head fluids, which may
description: appear as a mixture of gas/oil/water. Although the design and principles of operation behind these unit
description: operations are well known; assessing their performance as part of an integrated system to predict oil in
description: water, water in oil, and perhaps liquid in gas flowing out of them is complex due to the variability of
description: the inlet flow pattern, the fluid characteristics entering the production facility, and due to the difficulty
description: in predicting the droplet size distribution entering the system and the droplet size distribution inside the
description: separators. A system model capable of designing and assessing the performance of the production
description: separation system of an offshore platform regardless of the design configuration and arrangement of
description: the production equipment installed has been developed to overcome these complexities; no such
description: system model has been published before.<br>
description: <br>
description: A design model for primary gas/oil/water gravity separators has been built using a
description: comprehensive procedure found in the literature, and the results have been compared to the installed
description: industrial separators in the Alba Field in Equatorial Guinea, property of Marathon Oil Corporation.
description: Results obtained showed that the design procedure in the open literature may be a little bit more
description: conservative than that used for primary separators in production fields, mainly when setting limits for
description: the holdup and surge times in the oil and water compartments of separators. However, it can be
description: concluded that the procedure found in the open literature does correspond to that of primary separators
description: in production fields. Assessment of the design of the Alba Field primary separators showed that the
description: field separators will allow water in the oil stream leaving the separators due to insufficient oil
description: residence time in the gravity separation area, while insufficient surge time in the oil bucket may result
description: in oil spill over the oil bucket to the water side and out of the separator with the water stream.
description: Inadequate holdup times in both the oil bucket and water compartment could also result in gas leaving
description: the separator with both the oil and water streams. Evidence for all these potential problems has been
description: observed in the downstream production facilities.<br>
description: <br>
description: A performance model has also been built and used to assess the performance of the Alba Field
description: separators. The results obtained have been published (Abia-Biteo Belope &amp; Thorpe 2007). Results
description: from the field when cross-plotted against the performance model produced a clear relationship between
description: oil contain in water and water contain in the oil streams leaving production separators. The
description: performance model results also produced a guideline on how the Alba Field separators should be
description: operated depending on the degree of separation required for the downstream processes. It is hoped that
description: the general method of plotting performance versus a dimensionless variable (θ / t) may be widely
description: applicable and may in time yield a universal curve for all separators. Finally, predictions of the model
description: for changes to key separation variables such as flow rate, temperature, pressure and increase in
description: dispersed phase droplet size are found to be consistent with results from the literature.<br>
description: <br>
description: The effectiveness of the dissolved gas flotation technique for the treatment of oily waste water
description: from an oil production water system had been demonstrated in the study by Hanafy &amp; Nabih (2007), in
description: which they showed that high separation efficiencies of about 80% could be obtained for the removal of
description: oil droplets. However, residence times in the order of 20 minutes were required, which are simply too
description: high for offshore production platforms due to limited space. An experimental study was conducted at
description: the University of Surrey, using part of a dissolved gas flotation rig previously used to investigate the
description: flotation of solid particles (Hague 2002) and the determination of the local rate of release of air in the
description: contact and flotation zones of a dissolved air flotation tank (Fontain 2003); to investigate the efficiency
description: of the dissolved gas flotation technique at residence times comparable to those of other gravity
description: separation equipment installed in offshore production platforms (3 – 6 minutes). The study consisted of
description: creating an emulsion of droplets of size less than 100 μm in a continuous water phase using a Sulzer
description: mixer, and then separating the emulsion using gas micro-bubbles of an average diameter of 50 μm with
description: standard deviation of 5.5 μm. Results of the study showed that the principle of dissolved gas flotation
description: could be successfully applied to offshore degassers to separate oil droplets of size less than 100μm,
description: which may have been missed by the primary separators in an offshore separation process, achieving
description: efficiencies as high as 80% at residence times of about 5.5 minutes. The results have also provided, for
description: the first time, plots showing the relationship between droplet size and separation efficiency in a
description: dissolved gas flotation system. Finally, it is hoped that the study conducted could also be used as a
description: platform for further studies that could prove that oil separation efficiencies comparable to those
description: achieved in hydrocyclones (&gt; 90%) can be achieved using the dissolved gas flotation technique, if the
description: right gas bubbles size distribution to attach a known oil droplets size distribution are present in the
description: water stream, and if the required gas saturator pressure and gas saturated water flow rate are set.<br>
description: <br>
description: The offshore separation system model built also incorporate the prediction of the droplet size
description: distribution throughout the production system, from the choke valve to the water stream to be
description: discharged to the sea or to be injected back to the reservoir. Results obtained when applying the
description: maximum stable droplet size distribution model of Van der Zande et al (1999b) to the Alba Field
description: system showed that very small oil and water droplets in the region of 10 μm are present in the
description: production flows, which may not be separated in the primary production separators, as they are much
description: smaller than the critical droplet size often designed for these separators, which is about 100 μm. Some
description: of these droplets may coalesce in the production pipeline between the choke valve and the primary
description: gravity separators. The model by Rozentsvaig &amp; Pergushev (1981) was used to estimate the maximum
description: droplet diameter likely to leave the production pipeline as a result of coalescence due to turbulent
description: stirring during motion through the pipeline. Results obtained when applying the model of Rozentsvaig
description: &amp; Pergushev (1981) to a surface production pipeline between the choke valve and the primary
description: production separator in the Alba Field showed that the small oil and water droplets generated across
description: the production choke valve will coalesce to sizes in the region of 60 μm and 30 μm respectively.
description: However, these droplets will still be smaller than the critical size for separation in the primary
description: production separator. If sufficient residence time is not allowed in the production separator for these
description: droplets to coalesce to a size equal or larger than the critical size, then they are likely to be carried out
description: to the produced water cleaning system where they may be separated, depending on the efficiency of
description: the produced water cleaning system. The current produced water system design in the Alba Field is not
description: efficient for droplets of such size. The introduction of dissolved gas floatation to the existing degassers
description: could provide efficient separation of such droplets.<br>