Effects of topography on process plant

For gas/condensate systems the point of slug formation usually occurs further down the pipeline, depending on the liquid condensation rate and topography. Slugs are not usually formed at the start since the pipeline conditions can be above the dew point. For typical off-shore gas condensate pipelines terminating on land, the greatest uphill gradients can be at the beach, making this a possible location for slug formation. This phenomena was observed in dynamic simulations of the Bruce/Frigg pipeline system where liquid drop-out occurred when Bruce was shutdown. This resulted in a pressure decrease along the pipeline which gave rise to an increase in the liquid drop-out rate due to retrograde condensation. Predictions using

showed that following a 12 hour shutdown it required around 48 hours to re-pack the pipeline (Figure 7.23). During the shutdown liquid drop-out occurred in the pipeline, which was partly swept out when Bruce gas was re-introduced into the system. shows that during the start-up phase some of the liquid evaporates as the pressure increases. That liquid which is swept out arrives as a train of 700 bbl slugs which are easily handled by the process plant (Figure 7.24). Another result from the simulations was the fact that the 12 hour shut-down was not long enough for equilibrium conditions to be obtained, hence the liquid content is less than the equilibrium value. This is contrary to the results obtained using steady state analy-sis of the holdup change for the cases with and without Bruce gas which indicated that around 4000 bbls of liquid could be removed from the pipeline.

The user should take care with this type of analysis to make sure that the rate of condensation and evaporation is realistic, as it is possible that the simple nature of the phase change model in may give rise to large errors. For example, the use of a single composition for the fluid may mean that the condensate is re-evaporated at a much lower pressure than would be expected in practice because the condensed liquid would have a heavier composition than the feed stream.

Cell position = 0 Pipe component 12 hour shut down

Total vapour

Time (Seconds)

Figure 7.23 Pipeline vapour content

Section 7. Transient Flow ^Design

Figure 7.24 Total liquid content showing slugging during re-start

It is seen that there can be numerous transient effects that are caused by the topography.

These can be difficult to predict using steady state analysis techniques. The lack of an inclina-tion term in the present slug sizing methods and the inability to allow for hysteresis effects are both examples and goes part way to addressing these problems. Transient simulators can in theory account for both phenomena. However, in slug flow improved models are required to track the slugs. The effects of slugs discharging should be well suited to transient analysis provided that slug generating mechanisms can be clearly defined. However, if a sys-tem has numerous slug initiation sites, and is susceptible to small perturbations, then one may expect that the long term accuracy of the simulations may be suspect.

Codes such as PLAC and have produced good results with pipe emptying problems and slug generation with topographical changes. However the slugs produced by PLAC are not real slugs. can generate slugs produced by gravity effects but is limited in the way that the slug density and velocity are modelled, hence more work is required.

Dynamic flows produced by topographical changes are important to the design of process plants, as slug characteristics are changed while negotiating the platform riser and

upstream of the receiving vessel. If the slug is long, the additional hydrostatic head requires an increase in the pipeline pressure to force the slug up the riser. As the slug is produced into the separator it may accelerate due to the reducing gravitational resistance and the reduced fric-tional length (Figure 7.25). This can cause large velocity increases which can impact on the process plant control, and can also give rise to large loads on the vessel internals. This is dis-cussed more fully in Section 10 of the Multiphase Design Manual.

Section 7. Transient

= constant

Psep

Slug slows down as line packs to generate hydrostatic head

P+P head

Psep

Excess pressure and reducing frictional lengths lead to slug acceleration

Slug velocity

Time

Figure 7.25 Velocity increase during slug reception

Process plant dynamics can be simulated by a number of codes. BPX presently uses the code which has also been interfaced with a simple slug hydraulic model to enable the simulation of slug effects on process plant to be investigated. The slug size is an input to the simulation. However, it is possible to see the effect of the slug catcher pressure on the passage of the slug. Classical control schemes generally reduce the gas compressor speed if the separator gas flow is reduced, hence when the slug is produced the compressor is decelerating. A better solution can be to increase the compressor speed when the gas flow drops off, hence reducing the separator pressure and sucking the slug up the riser. This has the effect of reducing the gas starvation period and also means that the compressor is acceler-ating when the gas surge occurs.

A recent study using to investigate control strategies to limit the impact of slug dynamics on the process plant has shown that gas outlet flow control can reduce the peak in the gas surge. As the slug is received the gas reduces, by opening the gas outlet con-trol valve the pressure in the slug catcher is reduced, hence sucking the slug in and allowing some scope for increase in the slugcatcher pressure to absorb some of the gas surge.

PLAC can also model a simple separator with controls, including valves. However, it is not capable of simulating the compressor in detail. In the future is is hoped that the transient pipeline and process dynamic simulators will be coupled to facilitate integrated system analysis.

A simple interface already exists between OLGA and D-SPICE.

Section 7. Transient Flow

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Section 7. Transient Flow