An experimental and time-domain numerical analysis of gap resonance effect of an flng system for side-by-side offloading operation

Abstract

The side-by-side configuration of an FLNG (Floating Liquefied Natural Gas) facility and LNGC (Liquified Natural Gas Carrier) during LNG offloading operation has become of increasing importance in the offshore gas industry. This side-by-side arrangement brings about the formation of a narrow and long gap region between the two vessels. This PhD project aims to propose a method to predict the occurrence of gap resonance for a side-by side offloading configuration and to analyse the dynamic performance of the FLNG-LNGC system. The FLNG-LNGC system includes the two vessels and the connecting system (hawser, fenders, mooring lines and offloading arms) with the investigation conducted for different loading conditions subjected to a variety of environmental conditions. Predictive techniques were proposed for the estimation of nonlinear wave elevations based on the parameters of gap distance, vessel draft, wave direction and wave frequency. The numerical analysis was carried out with the potential flow solver, with HydroD in the frequency domain and SIMA in the time domain. To estimate the gap resonance RAOs numerically, a damping lid was introduced into the potential flow solver, to damp the overestimated wave elevation in the gap. The predicted gap resonance RAOs from the potential flow solver is analysed with the addition of a calibrated damping factors and compared with the model test data and show satisfactory agreement for different gap distances. Besides, a gap resonance experiment was conducted in Hydrodynamics Lab at Newcastle University using a new combination of ship sizes with the large model representing the FLNG and smaller model representing the LNGC. The results from the experiment data were used to determine the specific frequencies where resonance occurs. At this resonance frequencies there exist wave elevations in the gap that are extremely higher than that of the incident waves. It was deduced at the resonance frequencies that wave amplification factors can be obtained for the test cases. The generated data from the model test was further analysed to validate the estimated prediction of gap resonance phenomenon. It was indicated that, for the models test at beam sea and oblique sea in regular waves, there are amplification of the surface elevation in the gap at certain specific frequencies which are called resonance frequencies. These frequencies are related to the scantlings of FLNG and LNGC models. It is possible to estimate the amplitude of gap resonance using the different parameters in the model test analysis. Another set of numerical analysis was conducted in SIMA, to study the hydrodynamic characteristics of the floating FLNG-LNGC system, with the FLNG moored using a turret mooring system whilst connected to the LNGC via hawsers and fenders. The damping in the gap region was calibrated from the model test and tuned to obtain a suitable damping factor. Various test cases consisting of different loading conditions for the two vessels and environmental factors which combined wind, wave and current was used for the analyses and evaluation. The relative motions and force responses of the mooring system and connecting hawsers and fenders are analysed. The effects of loading conditions and environmental factors on the hydrodynamic performance of the vessels and the forced on the connecting system including the offloading arms are also investigated. The main in innovation of the research is the provision of an experimental study that better reflects the industry operation by using a new combination of ship sizes for the model test with numerous test cases, because previous gap resonance experiments used models of identical sizes. Secondly, the lack of efficient numerical methods to predict the gap resonance and to guide the operation hence, a novel Artificial Neural Network ANN approach has been proposed and the data from the validated model test and numerical simulation was used for the prediction, Finally, with the limited knowledge on the effect of responses of the FLNG-LNGC mechanical coupling effect, numerical simulation with different test cases such as gap distance, loading conditions and wave direction have been carried out, because FLNG is a relatively new technology the results add to the body of knowledge for improved safety and industry operation during side-by-side offloading.