Storage tank capacity of Liquefied Natural Gas (LNG) is one of the key designaspects for a Floating LNG production unit (FLNG). Because of the limitedfootprint of an offshore floating facility, a FLNG cannot have the same amountof redundancy in tank capacity as an on-shore LNG plant. In addition, theside-by-side LNG offloading operation in open sea is more onerous thanconventional offloading methods. Therefore the risk of the filling levelreaching the top of the tanks is higher for a FLNG than for an on-shore LNGplant, resulting in higher risk of shut down of the LNG production andassociated economical loss.
To overcome the aforementioned problems, this paper proposes a new methodologyto determine the design LNG storage capacity for side-by-side offloading. Itconsists of the following four steps:
- –
Two (2) body motion analysis: for ten (10) year time series environmentaldata to determine motions based on model test;
- –
Offloading criteria analysis: incorporating offloading operational scheduleand berthing criteria determined by navigation simulation;
- –
Tank inventory calculation: incorporating production rate, LNG carrier size, FLNG capacity, LNG carrier arrival schedule, LNG offloading rate and initialFLNG storage inventory; and
- –
Cost benefit analysis to determine the design FLNG storage size
Since FLNG is new for the industry, there is no generally establishedmethodology for analyzing the (weather) related downtime and the associatedrisk of tank top. This paper will exemplify the analyzing methodology for theAbadi FLNG project currently planned by INPEX Masela Ltd. in the Indonesiansea.
Generally, the storage tank capacity of a FPSO is sized based on productionrun-down rate, offtake frequency and other design parameters such as offtaketanker size, number of tank compartment and so on. For a FLNG, the samestrategy basically can be applied. However, some FLNG specific issues need tobe considered.
Due to the cryogenic nature of LNG, the LNG transfer with floating hose cannotyet be carried out. Therefore, cryogenic LNG loading arms will be used for LNGoffloading between a FLNG and a LNG carrier in a side-by-side configuration(side-by-side offloading) which is a similar configuration for LNG offloadingbetween LNG carriers and a jetty. The availability of side-by-side offloadingin offshore is affected more by metocean conditions than in nearshore orharbor. Generally metocean conditions in offshore are more severe than fornearshore or for a harbor, causing larger motions and interactions between theFLNG and the LNG carrier. The large motions and relative motions may exceed theworkable limits for side-by-side offloading systems such as loading arms, fenders and mooring lines. The factors mentioned above shall be taken intoconsideration for the evaluation of side-by-side offloading availability.
