Design of Leg Mating Unit for Float-Over Installation of Decks

Abstract

The Leg Mating Unit (LMU) is a critical component in ensuring safe method of installing topsides of offshore oil and gas platforms by the float-over method. Traditionally, topsides are lifted onto the substructure (e.g. jacket) using heavy lift crane vessels. However, the ‘lift’ method of installation is constrained by the availability of a limited number of heavy lift vessels in the region, with high day rates. As an alternative to modular installation with light crane vessels, float-over installation enables installation of a single pre-commissioned integrated deck, minimizing offshore hook-up time and cost. Further, float-over method is particularly suited to shallow water depth locations, remote locations (with no access to crane vessels). In a float-over installation, the deck is transported on a cargo barge to the pre-installed substructure location. The barge is guided into the jacket slot and positioned so that the stabbing cone on each leg is aligned with the corresponding jacket leg. The barge is then ballasted down (aided by the falling tide) so that the topside load is transferred from the barge to the jacket. Once the load is transferred and sufficient clearance is achieved between the deck structure and barge support structure, the barge is withdrawn from the slot. The transfer of load is the crucial step of a float-over installation and should occur in a controlled manner under the dynamic influence of environmental forces. This smooth load transfer is achieved using LMU’s. LMU’s are customized leg and deck mating units, used to dampen the impact loads generated during the mating process. They consist of steel structures with elastomer elements and are designed to perform three primary functions, absorb shocks, limit hammering onto the structures and provide defined stiffness between deck and sub-structure. The objective of this paper is to outline the design philosophy of a LMU and address the behavior of the LMU under the combination of vertical and horizontal loads during the mating process. The paper also recommends guidelines on the selection of elastomer stiffness based on load-displacement relationship. The LMU is analysed in ABAQUS, a commercially available finite element (FE) analysis package considering a non-linear time-domain analysis. The results from the FE analysis are compared with the qualification tests for the elastomer and LMU assembly performed on-site to establish correlation.