Method For Rating High Pedestal Cranes For Lifting From Supply Boats In Various Sea States
Authors: Stenhouse George A. , Hilton Robert J. 
Published: December 2007

This paper describes a method of quantifying the impact effects throughout an offshore platform crane structure when lifting from supply boats in various sea states. The method employs Multi-Degree of Freedom dynamic analysis techniques. It is particularly relevant to cranes mounted on high (flexible) pedestals and in addition to providing a method for rating the cranes, it can be used to generate fatigue spectre for the various crane components.

An earlier published method for determining the impact effects is also described. This method ignored the effect of the masses in the crane structure and some comparison of results of the two methods is included.

Introduction

Offshore platform cranes, especially in the North Sea area, have a significant accident and failure record. A number of these failures can be related to the large impact effects to which the cranes are subjected while picking up moving masses during the offloading of cargo from supply boats. There is, therefore, a requirement for an accurate method of quantifying these impact effects in order that they may be taken into account in the design or rating of the cranes. This paper describes such a method and comments on possible inaccuracies in an earlier published method. Some discussion is also included on other applications of the method employed.

Description of the Problem

When a supply boat or other vessel is being offloaded by a platform crane, the boat and the items to be offloaded are not static, but moving vertically with the heave of the vessel caused by the waves.

If the vessel is dropping when the crane hoist line becomes tight, the load being lifted will be snatched off the deck of the vessel and the crane will experience an impact effect due to this snatch loading. As these impact effects could overload the crane and cause a failure, it is desirable that these impact effects can be quantified and the crane rated accordingly.

It is fairly obvious that vertical motion of the vessel is an important factor in determining the impact effect. For ease of computation, and with minimal error, this vertical motion is assumed sinusoidal, and therefore, if we can determine the amplitude and period of the motion, all other characteristics of the motion can be computed.

The motion of the vessel is closely related to wave motions, but there is no fixed relationship between wave height and period. However, certain general relationships do exist between wave height and period for given geographical areas and Line A on Figure (1) shows a suggested relationship between significant wave height and period for the northern North Sea area. This graph was generated by averaging a number of relationships published in certain oceanography books and reports. (Note: The significant wave height is defined as the average of the highest third of the waves. It is also the normal sea state reported by an experienced observer). In order to account for the heave and pitch of the vessel about the water surface, some amplification of the wave motion may be taken into account. An arbitrary 30% increase in the amplitude of the motion has been used in the past to account for this effect.

Having defined the amplitude and period of the motion, we can compute the maximum vertical velocity of the vessel in any sea state and this is shown on Line B on Figure (1). (Note: This relationship is based on the wave height/period relationships shown and the assumptions stated above).