Crane Load Moment System For Offshore Crane Operations
Authors: Schaner David S. 
Published: December 2007

History has shown that dependency upon the crane operator to monitor loads and boom angle or load radius do not allow the margin necessary to perform the required crane functions with speed, accuracy and safety. Voluntarily and by legislation it has become necessary to develop automatic methods for monitoring, warning and limiting crane operation during critical lifts. A review was made of all regulatory requirements, desired operator aids, crane load ratings, crane operating configurations, load and boom angle sensing methods, display methods, memory methods and integration methods. From this review and subsequent design effort the Crane Load Moment system described in this paper was developed.

Introduction

For many years it had been the accepted practice to monitor and display crane hook load and boom angle or load radius with analog devices, either mechanical, hydraulic or electronic. The operator would utilize these displays in conjunction with the crane load rating chart and self-determine if he was operating the crane within the established limits of the crane and its operating configuration. This practice was applied to mobile cranes onshore and fixed base cranes offshore. In time, the next generation of devices began to be developed for mobile cranes. The load and boom angle or load radius were monitored and these signals where integrated to provide load moment or percent of rated load comparison to the load rating of each machine. Alarms were provided to warn the operator without his having to watch the load indicator and angle/radius indicator and reference the load rating chart while performing the other required functions of crane operation.

Governments began to recognize the need to provide the offshore crane operator with these devices which would reduce the demands on his attention and provide him with alarms which warned him and determined when restrictions or limits had been reached. These governments, thru their regulatory agencies, have adopted requirements for crane load warning and limiting devices. Though there is not complete uniformity in the values designated for warning and limit signals, there is general uninformity in the functions to be monitored and limits applied.

Regulatory Requirements

The various regulations may be summed up in the following statements:An automatic safe load indicator, or load moment indicator shall be used to give the crane operator a visual and audible 'warning signal' when the load being carried reaches and exceeds the warning value at any operating radius. This warning value could be 85%, 90% or 97.5% of the rated working load depending on the particular regulatory requirement.The automatic safe load indicator or load moment indicator shall give the crane operator and any other person in the immediate vicinity of the crane a continuous visual and audible 'limit signal' when the load reaches and exceeds the limit value at any operating radius. This limit value could be 100% or 110% of the rated working load depending on the particular regulatory requirement.The load being carried must be displayed on an indicator clearly visible to the crane operator at all times.