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.

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In this paper CFD results are presented for the thruster-hull interaction effects for a drillship with 6 azimuthing thrusters. The results using different approaches to model or simulate the propeller are compared and their advantages and disadvantages are discussed. The approaches investigated are the so-called Frozen Rotor approach, where the propeller rotation is modeled, the Actuator Disk approach with prescribed body forces and the unsteady Sliding Interface approach where the motion of the propeller is simulated in time. First, open-water calculations for a tilted thruster are carried out using the Frozen Rotor approach. The open-water calculations are repeated using the Actuator Disk prescribing the propeller thrust and torque distribution obtained from the Frozen Rotor calculations. The results with Actuator Disk are very similar for the unit thrust and nozzle thrust compared to the results using the Frozen Rotor approach. Furthermore, the results using the Frozen Rotor or the Actuator Disk are very close to the experimental results for the nozzle thrust. The thruster-hull interaction of one active thruster under the drillship is investigated using the Actuator Disk approach, the Frozen Rotor Approach and the Sliding Interface approach. A comparison to experimental results is presented for the thruster-hull interaction coefficients. Using the Actuator Disk a good agreement with the experiments is obtained. The results using the Actuator Disk and Sliding Interface are very similar to each other, but the computational costs for the Sliding Interface method are at least a factor 20 higher. The results using the Frozen Rotor deviate due to an unphysical wake behind the thruster. Based on the results presented in this paper we conclude that, using the steady-state approach with the Actuator Disk, CFD can be a cost-efficient and accurate method to determine the thruster-hull interaction effects at bollard pull conditions for a typical offshore vessel.

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