The fl ight of the condor
A Condor computer cluster is increasingly frequently being used to help analyse downtime events in the LNG sector. Report explains this new method.
Jaap de Wilde
j.dewilde@
marin.nl
FSRUs (Floating Storage and Regasification Units) are a new promising concept for off- shore LNG importing, providing an alterna- tive solution to near-shore jetties. FSRUs receive regular LNG shipments, transfer it to onboard storage tanks and send it to shore in its gaseous phase through a subsea pipeline. The FSRU is permanently moored on station, free to weathervane about a single point mooring, facilitating continuous operation throughout the year and survival during extreme storm conditions.
Floating LNG production units are being developed to produce and process gas in remote offshore locations. This system converts the problem of associated gas dis- posal into a money-making opportunity or an economical means to develop remote offshore gas fields. A large, barge-type floater is positioned over the reservoir and replaces the offshore platform, pipeline to shore, onshore LNG plant and the jetty. When sufficient LNG is accumulated, it is offloaded to an LNG carrier and shipped.
Condor computer cluster method Side-by-side mooring is considered the preferred offloading option in areas with relatively calm seas because it allows conventional LNG loading arms to be used. The availability of floating LNG terminals needs typically to be at least 98%. Model tests and time domain simulation shows that offloading is possible for significant wave heights above 3 m. Thresholds for long period swells (Tp > 14 s) are usually lower and thresholds for waves under an angle are usually smaller than for head waves.
The plot below shows an example of the calculated probability of downtime persistency for berthing and offloading. Three and six hour delays occur with a frequency of 13.7 and 8.5 events respectively per year. These result from high wind speeds prohibiting safe approach and berthing. Twenty-four hour delays occur 10.3 times per year on average. One or more samples showed that the mooring loads or relative motions, are exceeded during the side-by-side offloading operation. Very long downtime events are rare but still more than two events per year on average can be expected with downtime longer than 80 hours. Since the buffer capacity of most facilities is about 80 hours, this can have major consequences for the economics and reliability of the facility.
80 hours storage capacity
A new method for direct calculation of the downtime events in the time domain for N-years of environmental data has been developed. The calculation of tens of thousands of runs has been made possible using 200 fast PCs in a Condor computer cluster. The PC performs the calculations when idle. This method provides much better insight into downtime events, includ- ing occurrences of crossed conditions, beam swells and unstable headings. The complete operation, including approach, berthing, tug operations, side-by-side offloading and departure, can be assessed with properly calibrated numerical models. Persistency and seasonality can be evaluated in a direct and consistent manner.
18 report
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