The current expansion of the container terminal market is leading to an unprecedented quantity of new cranes and a strong trend towards refurbishment. This affects not only the big players but also smaller terminals which often are government owned or owned by a group of local investors.
This class of terminal operator is operating under tight budget with the pressure to keep profits at a maximum and thus faces the challenge of increasing its productivity while reducing capital costs.
A viable option to increase productivity without purchasing new container handling equipment is to extend the life of existing STS cranes.
Refurbishment normally involves both the mechanical and the electrical system. An optimal solution is to balance technical aspects, comply with all local and international norms and regulations as well as to keep maintenance costs to a minimum while remaining cost competitive.
It is common practice to do a structural test of the crane before deciding to refurbish the electrical installation. Once the mechanical structure is found worthwhile for refurbishment, decisions need to be taken as to the refurbishment scope of the electrical system.
The focus of this article is to outline considerations for the design of a safe electrical system for the refurbishment of STS container cranes. The starting point of the refurbishment activity is an updated risk assessment as outlined in ISO12100 ‘Safety of machinery:
Basic concepts, general principles for design’ and ISO14121 ‘Safety of machinery: Risk assessment.’ The design of the control system is to comply with IEC60204-32 ‘Safety of machines: Requirement for hoisting machines’ (issued in 1998). Safety first: Evaluation and assessment of risks
Any crane refurbishment, with respect to changes in the drive technology, e.g. analog to digital or increase in rating and speeds, constitutes a major change in the system configuration. As such, the risk assessment of the original machine must be reviewed in the context of new risks introduced by the modification and with
consideration of the current local and international standards.
The risk assessment in combination with requirements formulated in IEC60204-32 allows the detailed design of safety circuits. IEC60204-32 in section 5 provides a concrete requirement for the power supply configuration with switching and disconnecting devices for the motion drive system and special circuits (lighting, socket outlets, crane elevator, maintenance hoist). It also regulates handling of the system response in case of activation of emergency stop pushbuttons (see 5.3.7 ‘Crane switch’).
Section 9.4 ‘control functions in the event of failure’ provides guidelines to circuit design with regard to usage of proven circuit techniques, redundancy and diversity.
Section 9.5 requires one to determine stop categories for safety-related control circuits which detect excessive travel, speeds and load based on the risk assessment.