Dynamic interaction of semi-submersible drilling platform modules with each other and with the ship during platform float-over in waves

Abstract

Object and purpose of research. This paper studies semisubmersible drilling platforms assembled at sea as per float-over technique. The purpose of the study is to analytically estimate the forces generated by dynamic interaction of modules with each other and with the ship in the process of platform assembling, as well as to compare these forces with the ones that arise during sea transportation of modules in more intense waves, and also to develop an approximate calculation method for dynamic forces acting on ship deck and substructure pillars of semisubmersible. Materials and methods. The forces caused by dynamic interaction of modules with each other and with the ship are assessed numerically, by means of well-known Anchored Structures software package. Main results. Analysis of numerical calculation data for dynamic interaction of substructure modules with the top-sides of Polyarnaya Zvezda semisubmersible during its assembly in weak waves, wind and current has shown that the forces transferred from the topsides to ship deck and substructure pillars are unsteady and highly-variable randomprocesses observed only at initial and final stages of assembling, and the forces arising here are commensurable with the loads suffered by the topsides during sea transportation in high seas. Analysis of the results is used to suggest an approximate assessment method for maximum dynamic loads on ship deck and substructure pillars during semisubmersible floater assembling at sea. Conclusion. It is shown that semisubmersible floater assembling in weak waves might be accompanied by a strong dynamic interaction of its modules with each other and the ship, which could result in damage or destruction of modules and deck during sea operation, as well as create high residual stresses and strains in these structures worsening their furtheroperability and reliability. All these factors can seriously or even unacceptably deteriorate the quality of assembling. These outcomes can only be prevented through careful planning of sea assembling operation, with calculation of dynamic interaction forces as suggested in this paper. The results of these calculations may serve to justify assignment of weather limitations for semisubmersible floater assembling. Thus, calculation method for dynamic loads suggested in this paper is an important element in in preparing a high-tech marine operation: assembling of oil & gas production rigs.

Keywords

• semisubmersible floaters
• assembling of marine platforms
• dynamics of floating structures