The feasibility of offshore LNG terminals exposed to the ocean environment is being studied in various projects. Both fixed, gravity based terminals and floating structures have been proposed for offloading LNG from large carriers, and further pipeline transport to shore.

Model tests in the Ocean Laboratory.

Several technical matters are considered in the design of a terminal, such as:

Anchoring of ships waiting for access to the terminal
Number of assisting tugs and power requirement for berthing
Required mooring system (hawsers, fenders)
Procedures for berthing and departure
Availability of the terminal as limited by environmental conditions for the different stages (arrival/berthing, standby alongside berth, offloading, departure)

To determine the loads in mooring lines and fender systems presents several challenges:

Wave and wind conditions, including effects of sloping seabed, diffraction and reflection from beach and structures.
Interaction between ship and berth structure
Nonlinear characteristics of fender and hawser system

A combination of numerical analyses and model tests is required to obtain a realistic ship response and confident estimates of the design forces in hawsers and fenders. Model testing of gravity-base LNG terminals have been carried out in the Ocean Laboratory at MARINTEK. The tests have given important understanding of the physics involved and the importance of modelling of the actual draft, wave and wind conditions.

Through subsequent numerical analysis the dynamic conditions can be studied further, in several steps:

Hydro- and aerodynamic analysis for estimating the forces on the ship.
Static analysis to establish mean loads, stiffness, etc.
Refraction analysis to establish wave conditions outside the berth.
Calibration of the numerical model, using model test results.
Dynamic response analysis, time domain or frequency domain, to find motions and forces

Simulated berthing assistance.

The numerical analysis based on a calibrated model is a valuable supplement to the model tests, in which a wide range of environmental conditions (e.g. variation of sea state and wind and wave directions) can be studied. In general the non-linear elastic properties of selected fibre ropes and documented properties (compression-force characteristics and internal damping) of alternative types of fenders can be modelled more accurately in a numerical analysis than in model tests. Series of analyses are therefore also well suited for various parameter studies, such as:

Variation of hawser configuration and pre-tension
Variation of fender characteristics, including internal damping and friction
Systematically introduced failure modes (such as line breakage)

Typical calculation results have been:

Extreme tension in all the hawsers, varying with weather direction
Extreme compressive force in all fenders
Ship motion at the offloading point
Recommended number and location of mooring lines and fenders, including safety margin for line breakage
Environmental limits for offloading

In a real-time simulator effects of human actions can be studied, such as hawser connection at arrival or quick release in an abandonment situation.

Contacts at MARINTEK: Peter Chr. Sandvik / Svein Karlsen

(Article in MARINTEK Review No 1-2004)

Published January 26, 2005