Publications

Abadi Floating LNG (FLNG) plant will be constructed in the Masela Block, Arafura Sea, Indonesia. Five (5) wells via three (3) subsea flow-lines will supply the natural gas to the FLNG plant to produce 2.5 MTPA of LNG. Unlike feed gas pipeline for onshore LNG plant, these subsea flow-lines are relatively short, consequently it will have relatively small gas inventory in the flow-line. This adds some operational challenges related to production control since it is known that LNG production will fluctuate as the ambient air temperature changes. An increase in LNG production will demands more feed gas. In conventional onshore LNG plant, to fulfill such demand, it is not necessary to open the wellhead choke due to large inventory in the feed gas pipeline. Abadi FLNG will apply the same strategy, i.e. no interference to the subsea well head choke valve as it will be an operational burden and quick response cannot be guaranteed. The challenge is that only small inventory is available within the subsea flow-line. This creates new challenges to production control strategy. Dynamic simulations were conducted using UniSim Design® R400 software to investigate the effect of gas inventory in the flow-line on the FLNG production system. It is anticipated that LNG feed gas demand fluctuation can be accommodated by adjusting the topside inlet control valve. A sudden 10% increase of natural gas demand in the FLNG has been simulated to represent an increase LNG production rate while pressure at the inlet facility is kept constant. The quantity of mass inventory in the flow-line is expected to be sufficient to accommodate the increase in LNG production demand without requiring any interference to the subsea wellhead choke valve. The study confirms that at a certain opening of sub-sea choke valve, the gas flow increases along with the opening of topside choke valve up to a point where the pressure between downstream and sub-sea choke reaches the ratio approximately 0.55. When downstream pressure of subsea choke reaches the critical pressure, gas flow rate will not increase any longer. Since Abadi wellhead pressure is high compared to topside process pressure, critical flow may occur. Therefore, it is better to operate the flow-line within the pressure ratio range of 1 to 0.55 to avoid critical flow.