Feasibility Stage Assessment of Side-By-Side LNG Offloading Operation

A key driver in assessing the economic viability of floating LNG terminals is the marine offloading operations uptime. Marine offloading operations uptime is the percentage of time on site for which weather conditions are such as to permit offloading operations to be undertaken. Physical model testing or time domain numerical simulation techniques can model these marine offloading operations to a very high level of fidelity. However it is not practical for reasons of time and cost to apply such high fidelity modeling to the long duration data sets necessary to make reliable uptime estimates. Simpler solution methods, which can be used to carry out rapid what if studies as well as provide uptime assessment based on very long data records are therefore required. This paper illustrates that a reliable and fast numerical approach based on frequency domain analysis can be developed and used as a pre-screening tool to identify key marine operations uptime drivers. In this method the process of determining the marine offloading operations uptime involves the following steps: 1. Collect and collate site-specific environmental data. The typical starting point for an uptime analysis will be 5 to 10 years of hindcast environmental data, consisting of records of the average wind, wave and current amplitude over successive 3-hour sea states. 2. Evaluate the expected vessel heading in each successive 3-hour sea state throughout the hindcast record. 3. For each 3-hour sea state, estimate the relative motions between the FPSO and LNGC, at the previously determined vessel heading. From the relative motions estimate the envelope of motions of the loading arms and the maximum tensions in the mooring lines between the FPSO and LNGC. 4. For each 3-hour sea state compare the estimated loading arm motion envelopes and maximum mooring line tensions with the maximum acceptable design values to determine whether offloading would be feasible in this 3-hour sea state. 5. Identify times when there are sufficient consecutive 3-hour sea states to allow the offloading operation to be completed (weather window). Determine the percentage uptime from the ratio of the total of these periods to the total environmental data length. A range of sensitivity analysis can also be performed using this methodology, thereby allowing critical cases to be identified for further examination using the high fidelity model testing or time domain numerical simulation programs.