To Improve the Mooring of a LNG Carrier to a Jetty Exposed to Swell Waves

Abstract LNG terminals are often built further away (outside) port and residential areas, often in a more remote location without any protection (breakwaters), but close to the shore for connection purposes. This often puts the ship in shallow water over a sloping foreshore. In such location this ship is not only subject to incident sea and swell but also to incident and reflected infragravity waves. The downtime of such a terminal, exposed to swell waves, can become high. The horizontal ship motions are driven by low-frequency waves (also called infragravity waves, 30–300s). This paper shows how the efficiency of a LNG terminal exposed to swell can be increased by improving the mooring. The question is if the mooring can be improved to such an extent that a breakwater is not required? To investigate this, the response of a 174,000 m3 LNG carrier (in loaded condition) has been determined based on numerical modelling. The ship is moored perpendicular to the shore, bow towards sea (without a breakwater). The relative swell wave direction is head on. The incoming bound long waves are directly included in the modelling; an estimate of the reflected infragravity waves has been made and included. The results in this paper show how the (surge) motion response and the mooring line forces can be reduced by adding extra lines or by applying the ShoreTension® mooring system. However, in many situations a breakwater may still be required to guarantee sufficient berth availability throughout the year.