Structural Safety Assessment of FLNG Cargo Containment Systems Against Sloshing Loads
Authors: Park Tae Hyun , Kim Jong Min , Moon Joong Soo , Lee Myung Sup , Sim Woo Seung , Oh Min Han 
Published: May 2013

The Hyundai FLNG has been developed in 2011 and designed to be installed at the North-western sea of Australia with turret mooring system. The Cargo Containment Systems (hereinafter, " CCS") of the FLNG is designed to withstand the sloshing impact loads for any LNG partially-filled condition in the site specific environmental condition. The GTT Mark III membrane type is adopted as a LNG CCS. Since the sloshing load is one of the most important design factors for this membrane type CCS, the longitudinally 2-rows containment systems are applied to the Hyundai FLNG.

In this paper, the practical procedure was proposed to assess the structural safety of Hyundai FLNG CCS. Sloshing model tests were conducted to define the characteristics of sloshing impact pressure in forms of peak and rising time. To consider dynamic response of structure such as dynamic amplication factor (DAF), one-way transient FE analysis was performed against sloshing impact pressure. The Utilization Ratio (UR) was proposed as a quantified structural safety index. Each UR of the Hyundai FLNG CCS and existing LNGCs were calculated to compare their safety concerning operation and sailing condition respectively. Conclusively, the structural safety of the FLNG CCS was demonstrated through the comparison of UR between FLNG and existing LNGCs.

Introduction

The FLNG is a floating unit with Natural Gas Liquefaction Plant at topsides. The LNG production and offloading operations in FLNGs will be continued during the design life and it will, therefore, experience partial filling condition. Recently, the capacity of LNG CCS is increasing according to expansion of FLNG market. Depending on the installation site and CCS capacity, the sloshing loads of FLNG CCS might be larger than those of LNGCs in world wide operation. The safety of CCS under sloshing impact load is important in the FLNG design.

Many organizations have proposed the strength assessment procedures under sloshing impact load during past decades[01–08]. A common point of these procedures is that they have proposed the comparative assessment methodology instead of the absolute assessment methodology. It is because the absolute value of sloshing pressure is difficult to measure in real scale and the strength assessment is not easy due to cryogenic temperature and irregularities of pressure occurrence.

Recently, Fluid-Structure Interaction analysis was introduced. However, this method needs the considerable computational resources and time. Therefore, one-way procedure which calculates the sloshing impact loads and structural responses separately was adopted in this research.

The objective of this research is the development of simplified structural strength assessment methodology and the application to the Hyundai FLNG. Sloshing model test data from SNU and MARINTEK were used to extract the sloshing impact pressure data. Utilization ratio (UR) which is the quantified structural safety index is proposed to assess the CCS structural safety between FLNG and LNGCs.