Goliath crane inspection
14 March 2013
It is a myth that steel structures of Goliath gantry (and other) cranes require only a minimum amount of inspection and that steel structures of such cranes do not, in general, suffer from fatigue. IC presents an abstract of a full paper titled Goliath Gantry Cranes: Their Steel Structure – a Neglected Element
Goliath gantry cranes have become more common in the shipbuilding industry over the last 50 years. Their size and lifting capacity have risen as the size of ships and their building blocks has increased. Existing designs of these very large metallic structures can have a lifting capacity of 1,500 tonnes or more, lifting height between 50 and 115 metres and a rail span of up to 210 m.
Many of these cranes were built in the 1960s and 1970s and are approaching the end of their nominal design life.
The aim of this article is to demonstrate that while steel structures of Goliath cranes may over their lifetime carry a somewhat divergent kind of risks, these risks, by their nature, may be significant with potential consequences far larger than those applicable to other cranes.
The authors, experienced in the investigation, repair, refurbishment and development of Goliath cranes, are aware that this general topic applies to all cranes. To illustrate the problem, however, this type of crane was chosen because the ratio of the structural component to other components of the crane is the highest in these cranes and the general opinion of the corresponding industry rates the steel structure of these cranes as least susceptible to potential problems.
Three types of service are usually applied on a Goliath crane structure during its lifetime. This is in periods superimposed on each other, for example, maintenance, reconditioning or refurbishment and enhancement of technical and operational life. The first type of service, which lasts over the entire life of the structure, is not a subject of this paper. The second, commencing at about 20 to 25 years of age is applicable for approximately 15 years, superimposed on the first. Structural refurbishment is usually being performed in conjunction with renewal of electrical and mechanical equipment and mechanisms. The third service type during the lifetime of a crane involves interventions that, while in progress over several years, are still in an exploration and experience-collecting stage. To the best of the authors’ knowledge, in this category of cranes only one project of this kind has been attempted and, while in its final stages, it is still in progress.
Before dealing with the inspection procedure, requirements to carry it out should first be examined. The most important part is the inspector. He or she must have engineering qualifications, experience, physical fitness, tolerance of dirt, bruises, inclement weather and, above all, easy adaptation to work at height and in confined space. Good and safe access is another primary requirement.
Regarding the inspection and its objectives, what an inspector should primarily look for are the consequences of material defects, design errors or omissions, fabrication defects, erection defects, corrosion, operational damage, unauthorised interventions and, for so-called “ghost” items that could come up.
So, how to localise these or, in broader terms, which are of interest through the crane structure? A good starting point is a review of structural drawings and calculations. Although of limited help, such a review is always useful even if two drawings alone cannot reveal the complete picture of potential problem areas. In addition, availability of “as-built” drawings is unusual.
A skilled engineer can normally anticipate potential problem areas but, after that, it is experience that takes over. A protective coating provides the best “cover-up” and defects are not always evident. Generally speaking, a proper visual inspection should always, right from the start, include a sound and well planned approach. This includes a good eye, attention to detail and intuition. In addition, understanding and correct interpretation of some “signs” of damage, in conjunction with an inductive reasoning regarding the way the structure is operating, can lead to localisation of further areas of interest.
In summary, inspection is the most important element in condition monitoring, refurbishment and life-enhancement service periods, providing an information basis for any one of these. Its quality is paramount for the success of these services.
Based on the results of an in-depth inspection, refurbishment represents the first stage in reconditioning a structure during its lifetime. It is to improve the overall condition of the structure and bring it as close as possible to its original status. Refurbishment can be and often is combined with other major intervention. These include: modification of the crane, for example, geometry, capacity, performance; modernisation, including electrical equipment; and transfer of the crane to other location.
Based on the results of inspection, priorities of work are established and, unless immediate interventions are required, the schedule of works is fixed according to crane availability. The second and equally crucial parameter is the weather allowing satisfactory and safe execution of works. In addition, two major parameters to consider are conditions of access and the cost. Taking into account all the above, refurbishment works can be split into three categories.
Corrosion is a frequently appearing issue and its origins are not only in poor design. Negligence in conservation is mostly the cause. Last, but not least, the issue of damage. Cranes are likely to suffer damage from operations. It may have serious consequences so it is imperative for it to be reported.
Staff that directly witnesses such accidental damage is, most often, unqualified to assess its importance. In addition, human nature tends to hush up such incidents and leave the situation as it is. It is, therefore, recommended to encourage staff to report all accidental damage and leave the decision on its importance to those with relevant qualifications. Finally, equal emphasis should be given to the issue of damage caused by unauthorised and/or inappropriate intervention. Welding, drilling, oxy-cutting, etc without previous proper consideration and supervision can result in considerable damage, which is often difficult or impossible to repair.
Regarding defects in the steel structure, the most common categorisation is linking the defects to their origin. Thus, defects can be subdivided into several major categories, for example, material defects, i.e. defects having their origin in production of steel and of steel products; design errors or omissions having their origin either in oversight during design or in lack of sophisticated design tools, like modern finite element software codes; fabrication and erection defects introduced during these two stages of construction, rooted in poor workmanship or insufficient supervision; corrosion problems caused by poor design or improper maintenance; operational damage, most often caused by collisions with surrounding structures or equipment; unauthorised interventions in the structure by inexperienced personnel; and, finally, “ghost’ items – a general category, including hard to imagine cases, usually as a result of a combination of all the above.
Figure 1: Remnants of gussets or brackets. Three parameters and beyond.
Figure 2: Poor quality of edge cutting.
Figure 3: Extensive corrosion at a friction-grip bolted joint.
At all times the structure must remain under surveillance, especially after refurbishment. This is true, not only to safeguard safety and integrity and verify correctness and adequacy of the measures taken but, equally, as a wise cost-cutting measure. Current codes are relatively vague on the subject of intervals between these inspections. Many factors influence a decision like this, including the initial condition of the crane on delivery, quality and frequency of maintenance, the rate of exploitation, climatic conditions (corrosion rate), damage and, inevitably, the cost factor.
In this context, a question may be raised whether it is better to have these inspections carried out by different inspectors, or whether it is advantageous to keep them in the same hands. While the first approach may bring the benefit of difference in viewpoints and experiences, the latter option is favoured and recommended. Knowledge of case histories of individual defects by a single inspector is of considerable advantage. A long-term “relationship” with a structure can develop and result in a “feel” for it.
Aging is a natural process, at some stage bringing life to an end, for all things. Within this, however, it is necessary to distinguish between “life” and “useful life”, the end of which may come much earlier. Aside from technical aspects, the extent of useful or operational life depends on the adequate compatibility with operational requirements at a given time, and on the cost of operations. If the first can be anticipated and satisfied over an extended period of time, then it is of interest to examine the option of enhancement of operational life of a given crane structure. The first condition to be satisfied is the economy of such action, further condition being reasonable (restricted) maintenance cost during the time so gained.
The prevailing view of the industry is that the currently operating generation of Goliath gantry cranes has a very long life. This is essentially correct, although it is still not exactly known how long is “long”. Software is available for residual life calculation that can be used in estimating the life of the structure. Whatever may be established on the basis of a theoretical calculation, it will only be an upper boundary of the residual life of the crane.
We arrive, therefore, at the fundamental difference between refurbishment and enhancement of operational life, be it in the technical measures to implement or in their costing. In refurbishment, after a thorough inspection, the inspector knows virtually all and it only remains to add it up. In the second case the issue is practically the same but this is only the starting point. A major part of “enhancement of operational life” works consists, firstly, of correctly anticipating one or more sequences of local deterioration that could potentially lead to local failures and, thereafter, of developing and implementing measures to counter them. All in all, “enhancement of operational life” is a task of constantly keeping ahead of problems before they develop or, at least, before they become unmanageable and this at a cost that can be justified. No doubt a daunting task but it can be done. Additional experience from current and future projects shall refine the tasks, improve their technical and financial success and add to their attractiveness to the industry.
Dispelling a myth
The article presents fundamental steps in taking care of a steel structure of a Goliath gantry crane, including expected problems and principles to deal with them. The aim was to dispel widely established myths still pervading the industry, namely that steel structures of Goliath gantry cranes require only a minimum amount of inspection because they are largely “inert” to risks, and that steel structures of such cranes do not, in general, suffer from fatigue.
Regarding the first point, it has been demonstrated that the situation is different and that structural components of these cranes need at least as much attention as those of any other crane. Here we are not talking only about magnitude and value of such cranes, hence of potential cost of a major technical problem. Far more important appear specific consequences of such problem for operations.
Contrary to other crane equipment, many of these cranes operate over a dock as a single unit offering little or no possibility of rapid substitution by another crane. In case of their immobilisation, lost production not only means disruption and delays but, all too often, financial losses.
As far as the second item is concerned, it should be clearly stated that this point of view is as much in error as the first one. The answer to the question, “In what part of the crane, if any, can fatigue damage evolve?” the response is, “In those areas where the number of cycles required for fatigue to develop can be collected.”
Footnote: This article is an abstract of the full paper available here
Vladimir Nevsimal-Weidenhoffer, Intercrane Pte. Ltd, Singapore.
Nicholas Tsouvalis, associate professor, and Vassilios Papazoglou, professor, National Technical University of Athens, School of Naval Architecture and Marine Engineering, Athens, Greece.