Krska Chronicles Part 3: Balance the Mass Balance

Late design change is always problematic, so test them thoroughly. Poor modelling can obscure the consequences. Read the bold highlights if you are not technical.

Unfortunately, a lot of our revenue comes from investigating and fixing poor designs. In this case the impact of a late design change was masked by the manner in which the process simulations were constructed.  The process was in fact inoperable with no clean solution.

Like any offshore platform the aim was to produce a stabilised oil.  Disposal of the condensate was initially recognised as a potential problem.  It was part of the original design to re-inject the condensate into the reservoir. In order to cut costs this idea was revisited during the infamous ‘value engineering’ phase and removed.

On start-up the gas compression KO drums flooded due to the accumulation of condensate recycling around the compressors.  John was asked to review the design and suggest possible ways forward.

Sample analysis confirmed the well stream compositions corresponded to those used in the design.  However, the steady state process simulation model provided was very slow and difficult to converge. This is generally a warning sign.

The model convergence appeared to be due to excessive use of ‘recycle convergence blocks’.  Each recycle was independently configured a dedicated convergence block.

Checking the overall component mass balance also showed the cumulative effect of each recycle blocks convergence criteria produced small but significant discrepancies in the mass balance of some key components.  Rationalising the convergence blocks significantly reduced their numbers and scope for errors.

The revised model was still difficult to converge, requiring manual intervention, it did however now show a substantial increase in the condensate recycles sufficient to overwhelm the compressor KO drum condensate level control valves and drain lines.  After start up it wasn’t practical to increase the valve and drain line sizes to the required values.

Reconfiguring the scrubber drains and purging some light condensate to flare, whilst relaxing the crude TVP and export gas specifications provided a less than ideal solution. In other words an offshore process had been built that simply didn’t work.

The problem arose due to not recognising a potential issue when a model is difficult to converge.  Each use of a convergence block introduces convergence criteria that either compounds the error in mass balance or stops the model from converging.  In this case, the errors were sufficient to mask the problem of removing the condensate injection.

Models that don’t converge easily can usually be put down to one or more of four causes:

  • Convergence blocks are incorrectly or inefficiently placed.
  • The model contains ‘Adjusts’ that iterate some operating conditions to provide a solution to some desired criteria. ‘Adjusts’ are unreliable, especially when nested.
  • The process has significant issues that must be resolved. These may be manifest by slow convergence and relative large changes in the flow rate of some streams when relatively small changes to temperature and pressure specifications are made.
  • The simulation package contains an error.

We expect the process simulator to provide a perfect mass balance (Input = Output), they don’t. There are usually some small insignificant imbalances.  In models that have several recycle convergence blocks and/or high recycle flow rates the imbalances may become significant.

If a steady state model is indicating convergence problems, investigate the cause.


Krska’s Chronicles Part 2: Model it All

This is a story about completeness. As the old adage says ‘Short cuts can lead to long delays’.

In this story, mistakes were made in the initial design that created safety issues requiring investigation.  As all too often happens the investigation unearthed still more concerns. The effect of production turndown was found to produce awkward ‘unexpected’ consequences.

The offshore platform was required to produce a stabilised oil which was stored in a gravity-based structure prior to tanker loading.  The original design therefore routed the compression knock out (KO) drum drains in a manner that wouldn’t elevate the stabilised oil TVP (True Vapour Pressure). The design simulations had indicated that the liquid quantity to be discharged from the HP compressor KO drum would be negligible.  The drain was therefore routed to the closed drains under on/off level control.  In practice the drain valve was found to open more regularly than expected.  The temperature drop across the drain valve caused drains to block with ice and produce external ice.  I was then asked to investigate the issue and determined a safe route for this drain that would still provide a stabilised oil.

The company’s initial view was that the well fluids were different from the original design.  Fresh well fluid sampling and analysis determined that this wasn’t the case.

A review of the original design’s heat and mass balances and simulations highlighted a key discrepancy.  The fuel gas take off wasn’t considered in the simulations.  Installing the fuel gas take off in the simulation produced a significant amount of condensate in the HP KO drum, although the amount of gas feeding the KO drum had reduced by the fuel gas take off.

The extra condensate in the HP KO drum was due to the LP KO drum drains recycling to a point downstream of the fuel gas take-off.  The fuel gas was therefore leaner than the gas feeding the compressor.  As the produced gas/fuel gas ratio decreased the compressor gas also became richer as the low MolWt components were preferentially routed to fuel gas.

The current problem of the HP compressor drain icing up was relatively simple to fix, requiring a simple reroute of the HP KO drum drain.

The problem of the fuel gas take-off producing a richer compressor gas was more problematic.  Further simulations of turndown showed that the amount of condensate recycling through compressor and drains would significantly increase as there were insufficient low MolWt components to carry the condensate through to gas export.  This created the awkward situation that the compressors would need to be larger to move less gas.  The condensate drain lines would also need to be a lot bigger to cope with the recycling condensate.

Left as it was, the KO drums would have flooded, and it would have been necessary to flare a significant portion of the gas to purge condensate from the system. The problem was resolved by rerouting some other condensate returns.  This produced a richer fuel gas which required some reconfiguration of the gas turbine controllers.

Missing out the fuel gas take off from the design simulations was a significant design error, especially as effort had clearly been given to routing the condensate to maintain the oil TVP.

The moral? This moral is twofold. We mustn’t assume that lower production rates produce lower internal flow rates.  It is almost inevitable recycles will increase and recycles produce unintended consequences. Is it worth trying to save a few coins by neglecting to model part of the process?