The methods adopted to separate the mixture of components delivered by the riser are broadly…
The fluid components (oilwaterand gas) need to be isolatedefficiently and reliablyand the solids (usually sand) must be removed to minimise blockage and erosion. These operations are typically performed at elevated pressures and temperatures. Once separatedthe products are removed from the pressure vessel and conveyed to a tanker or shore facility.
There is a varied list of devices to considerincluding flow diverters and distribution devicesliquid coalescing and component separation systems.
For conveniencethe efficiency of all types of
(two-three-or four-phase) separator can be defined in the same waynoting that an ideal separator would remove 100percent of the componentirrespective of the operating conditions. Unfortunatelythis is never achieved in practiceand the efficiency generally worsens with increasing flow rate orin a gas production operationas the liquid fraction rises or a higher concentration of smaller droplets occurs.
As a general ruleeach separation process will be improved if the fluid remains in the pressure vessel for a longer (residence) time. Since this conflicts with the need for minimum vessel size and maximum flow ratean understanding of the physical basis for design and some appreciation of the influence of the operating parameters on performance are required. Failure to satisfy the operator’s specification could lead to severe performance penalties.
Once the size of vessel is specifiedthe designer must meet the process requirementstogether with the usual mechanical engineering and commercial constraintswithin the given dimensions. Consequentlyoverall performancefabricationand costmust all be considered. Given that all components must be inserted through the (typically) twenty-four inch manwayfinal assembly within the vessel is another vital consideration.
Recentlyattention has been focused on the relationship between stresssheet metal thicknessdurabilityand the weight of components. In making the inevitable compromisesthe dimensions of each device are balanced against their weightstructural integrity and predicted performance. The need to allow for erosion and corrosion must also be considered.
Since designs must be rigorouscomputational modelling using finite element analysis (FEA) and computational fluid dynamics (CFD) is usefulallowing examination of those regions where the stress levels and fluid loading might be most demanding. These methodsapplied with due cautionalso provide a possible route towards optimisation and refinement.
The process and mechanical design procedures rely on the application of physical laws and empirical datatogether with a good deal of operational experience. In truthit is almost impossible to measure the performance of the internals directly – but the overall effect of these can be judged by the operators. Because of the close confines in which the internals must operateinteractions between the different stages can be expectedso the designer needs to exercise some judgement about the influence of one component on another. The overall performance of the internals will be largely determined by these interactionswith the potential to cause poor flow distribution or droplet break-up.
In productionthe slowest separation rates will often be associated with extraction of the two liquid phases (water and oil)governed mainly by the rate at which oil droplets rise in water. Thusformation of the floating oil layer determines the allowable inlet flow rate and the dimensions of the liquid/liquid separator. Furthermorethe complex flow conditions within the vessel imply that the internals should be considered in sequencefrom the flow diverters and distribution devices at inlet to the liquid/liquid separation and gas/liquid cleaning systems downstream. Noting this complexityit is not surprising to discover that computational simulation has assumed greater importance in recent years. Howeverthese methods require cautious application and solutions must be validated against real data to have any value. This is not easily accomplished.
Clearlythe client and the supplier have different commercial viewpoints but both are intent on achieving reliability and performance. Herethe development of strong working relationships is often as important as the technical issues.
The reduction in available reservescoupled with reducing output levels from established centres of production and sharply increasing demand from developing nationshas increased the value of petrochemical products dramatically. Consequentlythere is emphasis on locating new reserves and
re-assessing fields that were once considered to be marginal. It is predictedthereforethat a much greater level of sophistication will be required in designing the process internals over the next few years.
The need to discover and exploit new resources has forced operations into even deeper waters and plans are reported of attempts to exploit the polar-regions. Surelyhoweverone of the most exciting challenges facing the industry must be the move to
sub-sea operations. Hereall the processing will be carried out on the sea beddispensing with the need for a platform to provide support and access to the
well-head. Efficiencyreliability and durabilityand the ability to assemble all the different componentseg pumpsvalvesand separation equipmenton the sea bedwill be critical.
Awareness of the manner in which the market is developing has led Zeta-pdm Ltd to forge strategic partnerships with other companies with the aim of developing separation technology for environmental protection systems and to address emerging opportunities for sub-sea operations. Needless to saythis technology poses many new and exciting challenges.o
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Dr John T Turner is Technical Director of Zeta-pdm LtdNewportIsle of WightUK. www.zeta-pdm.com
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