Operability of a floating LNG terminal

Abstract

This document focuses on the weather downtime in the offloading cycle of a generic LNG Floating Storage and Regasification Unit (FSRU). Results from these weather downtime assessments are essential for site concept selection and design of FSRU's, and are also used as input for logistics simulating the total gas supply chain. The FSRU is an unproven concept as none yet exists and so far Shell has not designed one in detail. Therefore, the analytical knowledge of and operational insight in operating limits with respect to the FSRU side-by-side offloading cycle is not available. This leads to difficulties in determining downtime and its most sensitive factors. Objective The objective of this project is therefore twofold: • To determine the most sensitive operating limit that causes weather downtime • To create a tool that can assess weather downtime of an FSRU The study focused on a near shore import terminal that is weathervaning around a turret and is supplied by LNG Carriers (LNGC's) with a capacity of approximately 135,000 m3 and offloading the LNG in a side-by-side mooring configuration. The duration of this offloading cycle is 25 hours. These are general characteristics and figures practical in today's industry. Methodology In order to achieve this goal, three building blocks were designed and implemented: • Operating limits of the typical operations in an offloading cycle of the FSRU • A downtime calculation tool • A model that calculates the heading direction of the FSRU As an FSRU has never been built before, root criteria and operating limits could not be derived from historical data, and thus had to be derived from other (concept) projects, literature, guidelines and operational experience on comparable operations like lightering and conventional LNG operations. The downtime model designed for this study has been based on the Downtime Assessment Tool of DMC: this tool calculates downtime based upon the availability of suitable weather windows to perform an offloading cycle. Fleet logistics or other causes of downtime are not included in the model. The model considers a continuous time trace of hindcast environmental data. An essential component of the downtime tool is the rotation model, which has been designed to calculate the heading direction of the FSRU, resulting in the relative angles of incidence of the environmental forces. This model takes into account the transverse forces and yaw moments based on empirical approximation equations of forces on vessels and LNG Carriers, taken from the guidelines of the Oil Companies International Marine Forum (OCIMF). To find the most sensitive operating limit, each operating limit was varied by ±20% for three locations during downtime assessments performed with the FSRU downtime tool. These locations are: • Gulf of Mexico (US) • Hazira (India) • Long Island Sound (US) These locations have also been compared to each other. Comparisons and conclusions with respect to the most sensitive operating limit have been based on two characteristics of operability that emerged from the tool: • Theoretical average uptime, which is the number of calculated potentially available offloading slots divided by the maximum number of theoretically offloading slots (350 sots for a 25h offloading period). This gives an estimate of the theoretical annual throughput. • Waiting time distribution, which can be used to present an indication of the required storage capacity by using the waiting time that has a probability of exceedence of 1%.