Abstract

A comprehensive set of basin model test results is presented for a realistic severe multi-directional wave climate. The scale 1:50 basin model tests provide insight into the relative motions and the forces in the mooring lines between a LNG carrier moored and a floating (turret-moored) LNG production or regasification barge. Side-by-side mooring arrangements, portside as well as starboard side, have been compared with three different stern-to-bow (tandem) mooring options: single hawser, bridle-type hawser, and soft yoke. The directional wave data measured in the field have been separated out into spectra for wind-sea (short-period waves) and swell (long-period waves) from different directions. The multi-directional wave climate, which has strong directional bias, has been accurately modeled in the model test basin. The segregation of these sea states proves to be of vital importance for accurate analysis of the response of the system and the subsequent selection and design of the preferred mooring option. The roll motions of both the turret-moored floater and the LNG shuttle carrier are mainly determined by the approach angle of the long-period swell. The use of thruster capacity to reduce the swell approach angle clearly shows the reduction in motions and mooring line forces. However, operationally this may prove to be more problematic if thrusters are being relied upon to continuously provide an optimum heading. Operational experience suggests that side-by-side mooring in severe multi-directional wave climate will not result in sufficient uptime. Stern-to-bow mooring with a single hawser resulted in considerable sideways motions of the bow of the LNG carrier with a high potential for unstable 'fish-tailing'. This is unfavourable for operating an LNG transfer system. There were much less sideways motions of the LNG carrier with the bridle hawser, while the LNG carrier was most stable with yoke mooring. However, dynamic behaviour of the yoke itself makes it less attractive for use in a severe environment. In summary, the mooring options considered are not ideal for ship-to-ship transfer of LNG in a severe environment. Following the developments in the oil industry in the North Sea, a potential future solution for mooring an LNG carrier to a weathervaning barge in a severe environment could be utilizing full dynamic positioning (DP), without any physical mooring arrangement between the units.

Introduction

Production, regasification, and transfer of Liquefied Natural Gas (LNG) are increasingly considered for an offshore environment. In the case of an LNG production facility, this may be a Floating Production Storage and Offloading system (LNG FPSO, or FLNG), and in the case of an LNG receiving terminal this may be a Floating Storage and Regasification Unit (FSRU). The Floater considered in the present study is a weathervaning barge with an external turret. For this system to be workable, it must be possible to moor a LNG shuttle Carrier (LNGC) to the production barge and to transfer LNG via a loading arrangement. Two mooring options have been extensively studied: side-by-side (Figure 1) and stern-to-bow (tandem, see Figure 2). This has been done for a severe multi-directional wave climate.

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