A Review on Topsides Float-Over Configurations (Longitudinal Vs. Transversal) Based on Structural Performance
Authors: Wey Liong Sin , Nee Er Chin , Mohamed Ali Rafee Makbol , Zukni NorBahrain Ahmad 
Published: February 2024

This paper presents a review on longitudinal and transversal float-over configurations for installation of offshore platform topsides on its jacket substructure. Structural assessments of a selected platform topsides with a dry weight of approximately 16000 metric tonnes are performed in a case study to determine the suitable float-over configuration. The selected 42 meters float-over barge which is common for typical offshore operations is considered based on the barge forward/aft loadout mode. Structural analyses of the selected topsides to suit the longitudinal and transversal float-over configurations, are performed for in-place and pre-service conditions. The analyses have adopted preliminary facility layout arrangements for the respective float-over configurations to perform the assessments and for comparison purpose. Besides the topsides weight, structural load path efficiency, fabrication condition, facility layout arrangement, and installation aids requirements are considered in evaluating the structural performance and recommending the more suitable float-over configuration.

From this study, it is found that transversely recessed substructure framing for the topsides that is in transversal float-over configuration is less sensitive to dynamic effects as the column/support legs are effectively braced laterally compared to the longitudinally recessed framing condition which requires high portal frames at the float-over slot. A transversely recessed substructure could lead to a reduction in the number of marine installation aids, i.e., fewer numbers of float-over fendering devices, e.g. leg mating units (LMUs), deck support units (DSUs) and jacket leg fenders. In this configuration, the caisson(s) and riser(s) can be located within the jacket framings thus additional protector(s) are not required. However, this configuration imposes constraints on the layout arrangements and affects the topsides weight due to limited available space in the longitudinal direction.

On the other hand, the topsides structural framing associated with a longitudinally recessed substructure framing is found to be more suitable for heavier topsides with larger equipment footprints. This configuration is more efficient in distributing the topsides weight onto the transportation barge and the quayside during loadout as the associated Load-out Support Frame (LSF) can span at a greater extent longitudinally contributing to shorter port and starboard cantilevered sections of the topsides onboard the transport barge.

The selection of longitudinal or transversal float-over configurations for installation of a topsides on its jacket substructure depends on many factors. This paper discusses the pros and cons of longitudinal and transversal float-over from the aspects of the topsides structural performance and associated structural steel weight. Findings from this case study could be used as guidelines for structural design engineers to assess the important aspects of the design criteria in determining the optimum float-over approach and topsides structural configuration, at the start of a design project.