Industry News - Offshore Engineer Reports - Sea-sick separation systemsSea-sick separation systems
from: Offshore Engineer
by: Matt Straw
Monday, April 07, 2008
Calculating the effects of separator sloshing on floating production systems can be a tricky business. OE invited Matt Straw, director of engineering partner Prospect, to discuss the state of the art.
Floating production raises many engineering challenges, not least in the area of separation of produced fluids. Multiphase fluids and sand subjected to the motion of a floating installation raise additional challenges beyond those on fixed installations in the design and operation of separation systems. Advanced computer-aided engineering (CAE) tools can be put to use as design aids and as important means of diagnosing and remedying poor performance in existing equipment. This article describes how one such CAE tool, computational fluid dynamics (CFD), can provide a powerful means of understanding the complex motioninduced flows, induced structural loads and separation mechanisms of separators on floating installations.
Added complexity
Providing effective separation of produced water and recovered hydrocarbon gas and liquid on the host facility is a necessity. Floating production systems have the added complexity, over fixed installations, that they must operate effectively under normal offshore conditions and must also be designed to withstand the significant loads induced during extreme storm conditions. Flow augmenting structures in separators (used to aid separation) therefore have an additional function; to help dampen sloshing that occurs due to the installation motion and must be designed to withstand the loads induced by the sloshing fluids for significant periods of operation.
Prospect began its decade long floating production separation analysis in 1998 when approached by a client whose first stage, three-phase (oil, water and gas) separator, installed on a North Sea FPSO, was performing unsatisfactorily. During production it had been noted that pieces of the separator’s coalescing medium were identifiable in the water outlet, and upon shut-down widespread damage to the vessel’s internal configuration was noted. It was extrapolated that this damage had been caused by liquid sloshing forces generated under the waveinduced motion of the host FPSO.
A separator vessel’s motion depends both on installation motion (the response amplitude operators) and the distance of the separator from the installation’s centre of motion. This distance can significantly exacerbate separator motion to create extreme displacements under severe sea-states. This motion can result in highly unsteady sloshing of liquids in the separator which in turn can impart significant loads on the vessel and any internal separation equipment.
With its background in engineering design and analysis of process and separation systems using computer-aided engineering tools, Prospect recognised that the application of computational fluid dynamics (CFD) could be extended, from analysis of separators on fixed and onshore installations, to simulate the effect of FPSO motion. The CFD technique developed was capable of producing unparalleled insight into motion-induced fluid responses within a separator vessel and provided a means of direct quantification of liquid-induced forces on the vessel and its internals. The predicted loads on the vessel saddles and internals could subsequently be used in the vessel’s mechanical design. In addition, by coupling the calculated liquid-induced loading with a finite element analysis Prospect developed a comprehensive method for both the liquid behaviour and structural design and response.
 Any method aiming to reproduce the response of a separator’s fluid inventory must allow both for the motion of the installation and for the position of the separator. For example, heave at the shipfs bow is amplified proportionally to the shipfs pitch and the distance from the shipfs centre of motion to the bow. While CFD tools provide a means of analysing any fluid flow and thermal related phenomena (including solids transport), by solving the governing equations of fluid flow, modelling externally-induced motion of a whole system has not always been possible. A novel method of representing the effect of a moving separator vessel was therefore developed. The method coupled the floating installation responses to a given sea-state and the subsequent motion of the separator under consideration. The technique provided a means of calculating the additional momentum imparted on the contained fluids induced by the separator motion . it essentially fooled the separator fluids into thinking the vessel was moving.
Any CFD analysis begins with the construction of a geometrically faithful model of the separator, in the same way that a 3D CAD model would be created. This information can be directly imported from CAD information so that the same data is used for the mechanical design drawings and separator analysis. In the analysis of the liquid sloshing within a separator, the longest duration activity has historically be the simulation time required. An analysis must be carried out in a timedependent manner over a period of a number of wave cycles in order to reach a repeating sloshing motion ie until any starting transients have died out. This can typically take between three and five wave cycles with typical wave periods of 10 to 15 seconds. The computational expense comes from the fact that the transient and multiphase CFD analysis must track the fluid motion over very short time-scales (of the order of 0.01 seconds) to track the highly transient flows reliably.
Significant advances
Ten years ago, sloshing analyses of this type may have taken up to three weeks for a single separator under a single seastate. The significant advances in computational power and parallel computing have reduced this type of analysis manifold making this type of analysis suitable within operational project time-scales that have in the past been too tight for many engineering analysis tools. Using project-proven modelling methods, Prospect is now able to use this technique to optimise a system by performing analyses in a matter of hours giving project teams a tool that can really impact on system operation as well as design.
 As with any analysis method, validation is critical. A series of sloshing tests were undertaken by Heriot-Watt University and the tests were recreated using Prospectfs CFD analysis technique. The test rig used a cuboid container and simulated surge and roll for pitch about the tank centre i.e. a seesaw motion with displacement of 300mm and pitch angle of ±4°. The water depth was recorded for the test tank and the CFD analysis during the sloshing process. An image of the sloshing during one CFD analysis is shown inFigure 1 along with the water depths measured during the CFD analysis and experiments in Figure 2. Agreement between the test results and CFD analysis were excellent, with the water depth variation with time corresponding very closely between the two. The findings of the validation added to Prospectfs (and their clientsf) confidence in the technique.
 The analysis of a separator designed for a floating production installation can provide immense benefits if undertaken early in the design process and can also be highly valuable in understanding unexpected performance of existing vessels.Without this form of analysis process engineers must rely on empirical and simplistic approaches in vessel design that are not able to reproduce the complex interactions that take place between different fluid phases driven by the installation motion. The image in Figure 3 illustrates the behaviour of the liquid interface in a separator at a snapshot in time during sloshing conditions. As well as highly visual understanding of the fluid mechanics it is possible to use this type of analysis to:
- optimise the internal baffles to minimise the sloshing process '
- quantify the separation performance under sloshing conditions;
- understand reasons for unexpected separation performance;
- calculate the required weir height and profile;
- predict the behaviour of level monitoring equipment; and
- quantify the forces induced on the baffles, vessel and vessel saddles for mechanical design (Figure 4 shows the fluctuating forces on an internal baffle of a separator in the North Sea under 100 year storm conditions).
Since Prospect's first study of separator sloshing, in excess of 20 studies have been performed for separators located (or to be located) on floating production installations across most oil producing regions and Prospectfs clients have included operators, major engineering contractors and separation equipment vendors. These studies have helped solve performance problems and optimise new-build designs in some of the worldfs most challenging offshore environments. OE
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