Design Aspects of SPM LNG Terminals in Shallow Water
Authors: Naciri Mamoun , Poldervaart Leen 
Published: October 2008

With the LNG market booming, the need for a reliable and safe means of transferring LNG from a producing floating facility to an LNG carrier and from this carrier to a near-shore terminal is becoming acute. In this context, the SYMO® (Soft Yoke Mooring and Offloading) system has been developed and recently tested in MARIN's offshore basin.

Important lessons have been learnt from these model tests and from the calibration of numerical tools performed thereafter. One of the main issues is the inherently weakly damped nature of a moored LNG carrier. Consequences in terms of system design, mooring analysis methodology and model test program will be discussed. Another issue specific to nearshore import terminals is the shallow water effects on the wave drift forces. These effects will be discussed and guidance regarding their importance will be provided.

The paper will also address the evolution of the concept based on the model test findings. In particular, the thorough design work performed with a view to obtaining an Approval in Principle (AiP) from one of the leading classification societies will be discussed with emphasis on the HAZID conclusions.

Introduction

Prompted by a strong demand in energy from both developed and developing countries, by tougher environmental constraints related to flaring as well as by safety issues (NIMBY), the fast growing LNG market has stimulated the industry into finding a number of innovative ways for LNG transfer. The aim is to transfer the LNG between near-shore or offshore fixed or floating structures and LNG carriers. Central to all these transfer systems is the standard 135,000m3 capacity LNG carrier with its distinctive hull shape designed to achieve cruising speed up to 20 knots and its almost constant draft whether laden or empty.

Firstly a brief description of an innovative solution for LNG transfer will be given before covering in some detail the model tests performed in 2003. These tests were conducted to assess the feasibility of the concept from three viewpoints: connection, offloading and disconnection. Next will follow a detailed discussion of the calibration of numerical tools against model tests emphasizing the issue of damping, the shallow water effects on wave drift forces and the ability of numerical tools to capture the physics and therefore match the test results. Lastly will be briefly discussed the Approval in Principle sought from a leading Classification Society and its recommendations, especially those related to shallow water. Conclusions will be drawn as to where more efforts should be directed to in the near future.

SYMO Concept Description

In general terms, the SYMO system has been designed to provide a disconnectable mooring of an LNG carrier to a fixed or floating structure in a harsh offshore environment and to ensure a safe and reliable offloading of LNG at industry standard transfer rates with maintainable service and minimum downtime.

The SYMO concept consists of the following main components (see Figure 1):A: JacketB: Revolving craneC: Mooring legsD: Yoke A-frameE: Yoke noseF: Upper connectorG: Lower connectorH: Fluid transfer piping