On the Prediction of Wave Resonance Behavior in a Gap Between Vesseles During Ship-to-Ship Transfer

Abstract

International Energy Agency (IEA) predicts an increase of 62 % over that of 2008 in the demand for natural gas in 2035. For the stable supply of natural gas, construction of the Floating LNG (FLNG) is now being discussed worldwide as one of the promising ways to exploit undeveloped gas fields.

The Ship-to-Ship (StS) transfer using flexible hoses is one of the methods to transfer LNG from FLNG to LNG carrier in the ocean. In 2011, the Society of International Gas Tanker and Terminal Operators (SIGTTO) published an operative guideline for the StS transfer of LNG, but it did not refer to the safety and workability evaluation in the StS transfer.

During the StS transfer operation, water level has been reported to rises to the height equivalent to several times as much as incident wave amplitude in the gap (so called gap resonance) between the side-by-side moored vessels. The flexible hoses are frequently beaten by waves because of the gap resonance during the operation, resulted in an injury to the hoses or in hitting against the ship hull. Thus, the gap resonance causes an important problem because such unfavorable conditions directly affect the safety and workability characteristics during the StS transfer operation.

In this paper, we investigated an evaluation method of the gap resonance during the StS transfer. We estimated the wave resonance behavior in the gap between FLNG and LNG carrier moored by side-by-side configuration using the boundary element method with parameters such as loading conditions, wave period and wave incident angle. In addition, we tested a tendency of the gap resonance using the 1/140 scale models at Ocean Engineering Basin in National Maritime Research Institute (NMRI). We confirmed that the result of estimation based on the boundary element method coincided well with that of the actually measured gap resonance behavior.