Direct Determination of Sling Tensions in Heavy Rigging

ABSTRACT Industry standards governing the design of heavy offshore lifts are limited in scope with regard to analytical techniques, particularly when sling length variation is considered. This paper presents approximate and exact methods for determining sling tensions, regardless of lift configuration. A parametric study examining critical features in lift geometry is presented. INTRODUCTION Throughout its history the offshore industry has moved to heavier and heavier offshore lifts in order to obtain the economies provided by minimizing offshore hookup time. At present there is at least one heavy lift vessel capable of lifting 14,000 metric tons operating. The economic risks associated with these heavier lifts make it imperative that engineers understand the implications of their lift analyses. Current industry design standards, such as those of API and DnV, are adequate for situations which they cover. For situations where sling length variations exceed standard tolerances, which is common, or unusual sling configurations are used, guidelines provided put the burden on the engineer for analyzing and defining acceptance criteria for rigging. To analyze this type of problem, this paper presents both approximate and "exact" techniques for analyzing sling configurations, either regular or unusual, which include slings with length variations. While the techniques presented are applicable to any size lift, the author originally developed them for lifts of 1,000 tons or more. Exact analyses of rigging considering length variations in slings are non-linear. The approximate solution presented herein can be incorporated in an electronic spreadsheet for rapid solution of such problems. It provides results adequate for preliminary design and, under many circumstances, final design. A technique is presented whereby the engineer can make an exact analysis of such non-linear systems with linear finite element analysis programs. Both solution methods were developed for single hook lifts. The exact solution principles can be extended to multi-hook lifts. To quantify some of the variables in a lift analysis, a parametric study was performed. Results of this study can be used by engineers as an aid in recognizing which factors are more important in a lift analysis and which are less important. Help in determining when an approximate analysis is appropriate and when an exact analysis is justified is also provided by the parametric study. Brief comments on load factors for sling design and on in-plane deck bracing are included. The primary aim of the techniques presented is to provide analytical methods for determining sling tensions, not stresses in the lifted structure. Once accurate sling tensions are available, analysis of the lifted structure is straightforward.