Time-Domain Analysis of the Relative Motions Between Side-by-Side FLNG and LNGC Under Oblique Waves

Abstract

Strong hydrodynamic interactions during the side-by-side offloading operation between floating liquefied natural gas (FLNG) and liquefied natural gas carrier (LNGC) can induce high risks of collision. The weather vane effect of a single-point mooring system normally results in the satisfactory hydrodynamic performance of the side-by-side configuration in head seas. Nevertheless, the changes in wave directions in real sea conditions can significantly influence the relative motions. This article studies the relative motions of the side-by-side system by using the theoretical analysis method and the numerical calculation method. Based on the three-dimensional potential theory modified by artificial damping-lid method, the frequency-domain hydrodynamic coefficients can be improved to calculate the retardation functions for the multi-body problem. An in-house code is then developed to perform the time-domain simulation of two vessels, through which the relative motions are subsequently obtained. A range of oblique waves are chosen for the extensive calculation of relative motions between the two vessels, which are further analyzed in terms of the phase shift of motion responses induced by specific resonant wave patterns. Investigation results show that wave directions have a significant influence on the relative sway, roll, and yaw motions. Under the circumstance that the absolute phase shift between the roll motions of two vessels approaches 180°, stronger relative motions are induced when LNGC is on the weather side. Moreover, the gap water resonances at high frequencies tend to cause the dangerous opposed oscillation of two vessels in the sway and yaw modes, whereas FLNG reduces the gap water resonances and relative motions when located on the weather side.

Keywords

Floating liquefied natural gas (FLNG) / Multi-body system / Time-domain simulation / Oblique waves / Relative motion / Gap resonance / Shielding effect

References

[1] Buchner B, van Dijk A, de Wilde J (2001) Numerical multiple-body simulations of side-by-side mooring to an FPSO, Proceedings of the 11th International Society of Offshore and Polar Engineers conference, Stavanger, Norway, 343–353

[2] Bureau Veritas (2007) Hydrostar for experts user manual, Research Department of Bureau Veritas

[3] Chen XB (2005) Hydrodynamic analysis for offshore LNG terminals, Proceedings of the 2nd Offshore Hydrodynamics Symposium, Rio de Janeiro, Brazil

[4] Chen XB. Offshore hydrodynamics and applications. The IES Journal Part A: Civil and Structural Eng, 2011, 4(3): 124-142

[5] Chen JP, Zhu DX. Numerical simulations of wave-induced ship motions in regular oblique waves by a time domain panel method. J Hydrodyn, 2010, 22(5): 419-426

[6] Choi YR, Hong SY (2002) An analysis of hydrodynamic interaction of floating multi-body using higher-order boundary element method, Proceedings of the 12th International Society of Offshore and Polar Engineers Conference, Kitakyushu, Japan, 303–308

[7] Cummins WE. The impulse response function and ship motions. Schiffstechnik, 1962, 9: 101-109

[8] Faltinsen OM. Sea loads on ships and offshore structures, 1990, New York: Cambridge University Press

[9] Fang MC, Chen GR. The relative motion and wave elevation between two ships advancing in waves. Int Shipbuild Prog, 2002, 49(3): 177-194

[10] Fournier J R, Naciri M, Chen X B, 2006. Hydrodynamics of two side-by-side vessels experiments and numerical simulations, Proceedings of the 16th International Society of Offshore and Polar Engineers Conference, San Francisco, USA, 158–165

[11] Huijsmans RHM, Pinkster JA, Wilde JJD (2001) Diffraction and radiation of waves around side by side moored vessels, Proceedings of the 11th International Society of Offshore and Polar Engineers conference, Stavanger, Norway, 406–412

[12] HYDROSTAR for Experts (2014) User Manual. BUREAU VERITAS

[13] Kim KH, Kim Y, Kim MS (2008) Time-domain analysis of motion responses of adjacent multiple floating bodies in waves, Proceedings of the 18th International Society of Offshore and Polar Engineers conference, Vancouver, BC, Canada, 301–307

[14] Ogilvie TF (1964) Recent progress toward the understanding and prediction of shipmotions. In: 5th symposium on naval hydrodynamics, Bergen, 3–80 1964

[15] Pauw WH, Huijsmans RHM, Voogt A (2007) Advances in the hydrodynamics of side-by-side moored vessels. Proc. of the 26th Offshore Mechanics and Arctic Engineering Conference (OMAE), San Diego, USA, 597–603. DOI: https://doi.org/10.1115/omae2007-29374

[16] Pérez T, Fossen TI. A derivation of high-frequency asymptotic values of 3D added mass and damping based on properties of the Cummins’ Esquation. J Marit Res, 2008, 5: 65-78

[17] Qi Q (2011) Hydrodynamic numerical calculation and analysis of FPSO-LNG multi-body system. Master thesis, Huazhong University of Science and Technology, Wuhan, 46–47. (in Chinese)

[18] Sclavounos PD, Ma Y (2018) Wave energy conversion using machine learning forecasts and model predictive control. 33th international workshop on water waves and floating bodies, France

[19] Shivaji GT, Sen D. Time domain simulation of side-by-side floating bodies using a 3D numerical wave tank approach. Appl Ocean Res, 2016, 58(2016): 189-217

[20] Soares CG, Taylor RE, Ewans K, . Safe offloading from floating LNG platforms. Appl Ocean Res, 2015, 51: 252-254

[21] Van Oortmerssen G (1979) Hydrodynamic interaction between two structures floating in waves, Proc. 2nd Int. Conf. On behavior of offshore structures (BOSS’79), London, 339–356

[22] Watai RA, Dinoi P, Ruggeri F, Souto-Iglesias A, Simos AN. Rankine time-domain method with application to side-by-side gap flow modeling. Appl Ocean Res, 2015, 50: 69-90

[23] Xu X, Yang JM, Li X, Xu LY. Time-domain simulation for coupled motions of three barges moored side-by-side in floatover operation. China Ocean Eng, 2015, 29: 155-168

[24] Zhao WH, McPhail F. Roll response of an LNG carrier considering the liquid cargo flow. Ocean Eng, 2017, 129: 83-91

[25] Zhao WH, McPhail F, Efthymiou M. Effect of partially filled spherical cargo tanks on the roll response of a barge-like vessel. J Offshore Mech Arct, 2016, 138(3): 031601-0311-9

[26] Zhao WH, Wolgamot H, Taylor PH, Taylor RA, . Gap resonance and higher harmonics driven by focused transient wave groups. J Fluid Mech, 2017, 812: 905-939