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?