FLNG (Floating Liquefied Natural Gas) as a means of monetizing offshore gas is now a reality with Technip being involved in the first two major projects. Both these projects incorporate side-by-side offloading of LNG from the FLNG to the LNGC (LNG Carrier) using loading arm technology. As FLNGs move into more severe environments, the side-by-side operation is no longer applicable and a solution providing greater environmental thresholds is required. Several tandem LNG offloading systems are proposed by the industry. They provide great safety separation distance between the vessels and high environmental thresholds, but they require the use of a dedicated LNGC fleet with Bow Loading Systems or the deployment of at least three floating LNG flexible lines of sufficient length, more than 300 meters, to reach the midship manifold of conventional LNGCs. Since early 2015, Technip has worked with HiLoad LNG AS to develop a LNG loading system that combines the advantages of tandem stern-to-bow and side-by-side offloading solutions. The system is based on already proven or qualified technologies, allows a vessel separation greater than 100 m, high environmental thresholds and minimum length of LNG transfer lines without any modification to the LNGC. The system uses the HiLoad LNG DP (dynamically positioned) vessel for the position keeping of the LNGC parallel to the FLNG and Technip's cryogenic flexible pipes for the transfer of LNG and vapor. The main focus areas in the studies have been the risk assessment of the parallel loading configuration and the assessment of the DP station keeping capabilities within predefined operating sectors, with the system being exposed to both steady and dynamic/changing environmental conditions. Black-out scenarios have also been investigated as part of the simulation work. The HiLoad DP system was able to keep the LNGC inside a tight operating ‘green' envelope during all investigated challenging metocean conditions. The HiLoad LNG Parallel Loading System has been concluded as a feasible, well suited solution, enabling a safe and robust LNG loading method with use of conventional LNGCs.
Coral South Floating Liquefied Natural Gas (FLNG) unit is designed to offload its product to LNG Carriers (LNGC) moored in a Side-by-Side (SBS) configuration, using Marine Loading Arms (MLA) technology. With such a method, multiple design aspects require accurate hydrodynamic simulations at an early stage of engineering phase for a large number of environmental conditions, including wind sea and swell: sizing of FLNG mooring outfitting, design of the MLAs for the candidate LNGC fleet and availability of LNG offloading. Complex phenomena like multi-body hydrodynamic coupling and LNG sloshing in partially filled tanks must be accounted for, and non-linear berthing characteristics require time-domain simulations. However the computing time for all the environmental conditions described in the available 23-year hindcast databases combined with all the possibilities of FLNG heading obtained with thruster assistance, for multiple LNGC and loading conditions, is not compatible with the project engineering phase timeframe, so the simulations must be first performed on a suitable-size sample of environmental conditions, creating a database which can then be used for predicting the data related to any environment. The selected sample of environmental conditions must meet the constraints of computing time while keeping a sufficient resolution to be reliable. In other projects, the time-domain quantities subject to operability criteria, derived from this limited number of simulations, were used to predict the behavior for any unknown environment by interpolation. This approach presented some limitations like the overfitting of maxima dependent on the wave realization (seed), which is seen as a noise, and was not best suited for generalizing the results to non-simulated environments out of the sample. In this paper, the improved methodology used for the Coral South FLNG project is presented. A Radial Basis Function (RBF) Artificial Neural Network (ANN) is used to model the variables impacting the SBS offloading operability. The ANN learns from the results of simulations performed on a sample defined with a K-means clustering algorithm. The RBF is modified to be adapted to the specifics of the driving parameters, of which some are periodic (wave direction) and the rest non-periodic. A proper smoothing of seed-dependent maxima and accurate estimations for unknown environments (generalizations) are achieved. The learning process does not require significant computing time and fewer preliminary time-domain simulations are needed. This design methodology represents a significant improvement for the calculations performed during the project's engineering phase, but it may also be applied later once offshore, to assist in decision making relative to the weekly forecasted SBS LNG offloading operations. When Coral South FLNG operates, the learning database may be completed with on-site measurements to further improve its accuracy. The principle may also be extended to other offshore operations constrained by environmental conditions.
In order to properly design the mooring and offloading equipment, and assess the availability of Floating Liquefied Natural Gas (FLNG) units during the engineering phase, TechnipFMC has paid a particular attention to the operability prediction of LNG offloading operations. For the FLNG projects currently under construction, the chosen offloading solution is the Marine Loading Arms (MLA) technology with the LNG carrier (LNGC) moored in side-by-side (SBS) configuration. In this paper, an improved methodology using Monte Carlo simulations is proposed to create a representative sample of a typical hindcast for North West Australia, assuming the case of a 147,000 m3 LNGC moored side-by-side to a TechnipFMC in-house design FLNG. It is shown that the random sampling is very well suited for large number of dimensions implied by the driving parameters (e.g. significant height, peak period, relative heading) of multiple wave partitions. In a second step, different multidimensional scattered data interpolation techniques are evaluated in order to further improve the accuracy. Since this would not be practical in time-domain and because the objective for this study is to compare the sampling and interpolation techniques to a benchmark on the whole hindcast, the vessels’ motions and SBS offloading operability are calculated in frequency-domain. The conclusions could also be applied to time-domain simulations. The paper also presents the sensitivity studies to the different interpolation methods and their key parameters and discusses the suitability of the proposed solutions.
A numerical solution is proposed for the design analysis of the mooring system of an FSRU in shallow water. Previously. such analysis relied on second-order diffraction theory with viscous damping empirically calibrated from physical model tests. However, both experimental and theoretical methods had to introduce uncertainties in the predicted mooring load because of their physical and theoretical limitations. A complicated procedure had to be introduced to derive design loads considering the uncertainties and limitations. The proposed numerical solutions are developed to minimize those uncertainties by introducing the state-of-the-art numerical tools to accurately model the flow field near the FSRU and the surrounding wave field. A CFD-based numerical wave basin, MrNWB, and a potential-based higher-order Boussinesq wave model, HAWASSI, are coupled together to simulate the near- and outer-field free-surface flows around the FSRU hull. This paper describes the framework of the proposed numerical method, followed by preliminary verifications of the accuracy and effectiveness of the proposed solution. A benchmark model test of an FSRU moored in a shallow sloping beach is used to validate the generation of the low-frequency wave and the slow-drift motion of FSRU from CFD simulation. The numerical results show significant improvement in the low-frequency FSRU responses compared to the conventional theoretical methods.
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