Model Tests Of An Active Pneumatic Bow Hawser Load Compensator
Authors: Smitt Leif W. , Naess Tor 
Published: December 2007

Abstract
This paper presents the principles of the control strategy and arrangement of a load compensator for reduction of hawser loads in mooring connections. The simulation of the compensator in model scale is described, and results from model tests with the compensator applied to an offshore loading system are discussed.

Background
For mooring systems intended for operation in open sea, it is of vital importance to control dynamic hawser loads exerted by the environment. Especially in exposed areas it may be difficult to absorb the substantial loads which are imposed by wind, wave and current action. To avoid significant operational restrictions with regard to weather conditions or frequent hawser break-down, improvement of existing systems should be considered.
With the intention of increasing the regularity of offshore loading systems, Aker Engineering A/S introduced in 1975 a load compensation concept for a tanker especially adapted for offshore loading. Preliminary model tests were carried out to evaluate the conceptual idea. The tests gave promising results and it was decided to proceed with development of a more refined controller strategy able to cope with various weather conditions. This development was carried out in close cooperation between Aker Engineering A/S and the Danish Ship Research Laboratory, who were commissioned to perform simulation studies and model tests.

Basic Considerations
The basic principle of the compensation unit is to introduce a spring in the mooring system able to accept first order wave effects (wave orbital motions) without introducing large hawser loads. In order to cope with the rapid load fluctuations under various conditions, the spring system is equipped with a controlled pretension to its centre position when subjected to relatively slow fluctuations of load caused by second order wave effects and variations in wind and current forces. In this way the compensator will be able to accept the maximum of rapid excursions symmetrically around a mid position. This is in practice accomplished by a pressure control system for adjustment of pretension according to a frequency dependant gain function. In order to develop a powerful controller it was important to closely examine resonance effects in the total mass-spring system and include a damping strategy at resonance frequencies to avoid self-excitation.
The hardware part of the compensation system can in principle be described as a pneumatic cylinder in which a piston is situated. The hawser is attached to the piston rod and the cylinder is connected to a pressure vessel (accumulator) by means of controlled valves (fig. 1). The spring effect is obtained by compression of gas in the cylinder and pressure vessel.
A compensator with no control of pretension is in the following referred to as a passive compensator, whereas the concept with controlled pretension is described as an active compensator. The model tests presented in this paper were carried out with a 100,000 dwt tanker moored to a floating offshore storage and loading terminal. The compensator unit was situated on the tanker forecastle deck during the tests. As an alternative to this location the unit can be mounted on the offshore loading terminal. Even if the actual tests were carried out for an offshore loading system, the strategy and the principles developed should be applicable to a variety of other concepts where objects are moored in open sea.