Motions and Mooring Loads of an LNG-Carrier Moored at a Jetty in a Complex Bathymetry

Abstract

The Yemen LNG Company Ltd. is working on the design and construction of an LNG plant in the Republic of Yemen. The LNG plant, located at Balhaf on the Gulf of Aden, includes a jetty approximately 750 m from the shore to allow loading of LNG carriers. The bathymetry around the jetty is very complex and includes a large variation in water depth along the berth. Furthermore a cape near the jetty affects the incoming wave conditions. Deltares (formerly WL | Delft Hydraulics), together with MARIN, carried out a study of combined hydrodynamic scale model tests and computer simulations. The aim of the project was to determine the limiting environmental conditions for safe mooring of the LNG carriers at the jetty. The hydrodynamic scale model tests at Deltares focused on an accurate modeling of the wave conditions at the jetty and the motion response of the moored ship. To achieve this, the bathymetry around the jetty was modeled in detail, including the cape partially shielding the jetty from incoming waves from the open sea. Wind was applied to the moored ship as constant forces. The time-domain computer simulations were carried out by MARIN, using their TERMSIM simulation model. After calibration against scale model test results, the numerical model was used to quantify the effect of gusting wind for all environmental conditions and all ships as tested in the basin. The results of the scale model tests, corrected for the effect of gusting wind, indicated that the vessel can stay safely moored at the jetty in quite severe conditions. This leads to a high jetty availability, which is a favorable outcome of the project. After describing, in a general way, the methodology and results of the project, this paper focuses on the comparison of the results of the model tests with those of the computer simulations. This comparison showed that the low frequency effects, both excitation and response, in the complex bathymetry that was considered here, are very complex and beyond the present numerical modeling capabilities. Therefore, in the short term, physical model testing will remain necessary for an accurate prediction of the moored ship’s response in such situations. For the longer term the development of additional analysis and simulation methods is required.

Experimental and numerical study of the effect of variable bathymetry on the slow-drift wave response of floating bodies

“…The standard deviation of its measured slow-drift sway motion is compared with state-of-the-art calculations. It is found, alike in [2], that calculations overpredict measurements at the two shallower waterdepths, while a fair agreement is observed at the two deeper ones.…”

“…with K = Ω 2 /(α 2 g), J 0 and Y 0 the Bessel functions of order zero, and the constants γ 1 and γ 2 determined in such a way that η (2) matches (in value and x-derivative) the upstream flat bottom solution η (2) L in x = 0. This expression is also valid for a negative slope.…”

“…In such cases, when K y > K 00 , the subharmonic free wave is only propagating in the direction parallel to the depth contours and evanescent in the normal direction (so-called edge wave). The solution is finally obtained by matching the total secondorder potentials ϕ (2) Pj +ϕ (2) Fj and their x-derivatives at the successive boundaries, and stating that there are only outgoing propagative and evanescent modes at the upwave and downwave boundaries of the variable depth zone, superimposed with the locked potentials given by Eq. (4).…”

“…This result can easily be extended to an irregular bottom by idealizing it as a series of upward or downward rectilinear parts, with slopes α j . The coefficients γ 1j and γ 2j are determined so that both the second-order elevation η (2) j and its slope ∂η (2) j /∂x is continuous at the slope discontinuities. The solution can then be propagated from the upwave region all the way to the end of the domain.…”

“…When its flat bottom expression is introduced in the calculations, it has often been observed (e.g. see [2]) that the slow-drift response is overestimated. Part of the discrepancies can be imputed to directional effects (e.g.…”