Abstract

A computational method is described for calculating the detailed dynamics of fixed offshore platform cranes based on finite element techniques. The method can be used to calculate stresses in individual areas of the crane during the lift; the calculations are valid for any typical offshore fixed platform boom crane. Transient amplification factors are quoted. The dynamical behaviour of the crane is described including the effect of damping introduced into the boom hoist system and discussed with regard to the operation of crane safety systems. The main conclusion is that crane dynamic motion can delay recognition of an emergency situation and that the introduction of damping can considerably reduce the dynamics related problems and reduce the derating of the crane.

Introduction

The use of cranes offshore, including lifting from moving supply boats, has meant that dynamic effects have become a major problem involving greatly increased stresses on the crane as compared with normal land use.

There are always transient increases in stresses above the static equilibrium values during any crane lift, but they can normally be kept manageably controlled by hauling the load in sufficiently slowly. Lifting offshore from a supply boat requires often that a lift must be made as fast as possible, to avoid danger to personnel and load on the boat deck and such snatched lifts involve the maximum line speed. Additionally, if a lift is misjudged the load may have considerable initial velocity downwards due to deck velocity and this again increases the transient stress amplification. Further, the crane may need to be used with its boom fully extended which increases the moment arm on the crane support system. Because transient amplification may be considered as increasing the apparent load seen by the crane the simplest approach to remove the problem has been to derate the crane, i.e., to reduce the maximum permissible load by a deration factor depending on the configuration of the crane and the sea-state in which the lifts are to be carried out. General formula have been quoted for deration factors, but they have typically been based on a single spring mass model of a crane.

It is important to have more exact methods for calculating the detailed motions and stresses in different parts of a crane during a lift. Firstly, the simple models do not give correct deration factors; for example these vary for different parts of the crane. Additionally, there exist various crane safety monitoring systems which measure a variety of parameters on the crane during lifting such as lift-line tension and boom hoist tension or which monitor the crane hydraulics. To evaluate or intelligently design such systems it is necessary to have mathematical models which can predict the values of these parameters and their time dependent behaviour.

Consequently, we have developed a computer simulation method based initially on a full and detailed finite element respresentation of a crane and its supporting system. From this, as a basis and for economy of computer time, an adequate but simpler mass and stiffness matrix model may be derived, and after calclation of time history response, these can be re-expanded to give detailed stress histories at any required location.

The time calculation is carried out in two parts, before and after lift-off, so that any given prescribed deck motion can be dealt with, and the winding in of cables is fully represented by a set of equivalent forces. This method of calculation allows various features and systems additional to a normal crane configuration to be modeled, for example, sudden changes in stiffness of the hoist cable and damping etc. This facility has been used to discuss crane design.

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