Snorre TLP Tethers and Foundation

ABSTRACT The mooring system for the Snorre TLP, including four concrete foundation templates and tether components for the 16 tether strings is described. The design requirements are listed, and the significant development and testing work to document function, strength and reliability of the various components are also presented with emphasis on key results and comparison with theoretical predictions. Manufacturing and installation aspects of foundations and tether components are also presented, highlighting the very strict requirements to manufacturing and installation tolerances in order to meet the design requirements. INTRODUCTION One of the most challenging aspects of a TLP project is the development of the mooring system, which is of vital importance for the safety and structural reliability of the entire TLP. It is, therefore, of importance that all aspects related to loading, structural behaviour and resistance of the mooring system are considered in detail at an early stage of the development, and that sufficient overcapacity and redundancy is built in to allow some margins for unforeseen changes in later project phases. This paper describes the work related to development of the mooring system for the Snorre TLP, i.e. the foundations and the tethers. Most of the paper is concentrated on the work to develop and document a structurally sound and safe mooring system, while aspects related to fabrication and installation are more briefly covered. MOORING SYSTEM DESCRIPTION The Snorre TLP mooring system is based on a through column installation method with mooring flat tie-off above water level (Fig. 1). It consists of a total of 16 tether strings, 4 in each corner of the TLP. Each group of 4 tethers is anchored to one concrete foundation template (CFT) on the seabed. Each CFT has a multicelled geometry with skirts penetrating 12 meters into the seabed (Fig. 2). In the upper part, the skirts are closed by a dome structure, and in the centre part above the domes, a prestressed solid concrete section provides the support for the tether anchoring components (abutment rings), which are cast and reinforced into the concrete structure for transfer of tether loads. The outer skirt walls are extended above the domes forming retaining walls for the solid ballast which is placed in compartments on top of the domes. The CFTs will be installed with the tether anchoring supports at the same elevation, and the inclined seabed is compensated for by preadjusting the height of the center solid part of the different CFTs according to the actual seabed topography. The anchor latch assembly (Fig. 3) provides the means of securing the tether to the CFT. The assembly consists of the anchor latch and the template insert which is the receptacle in the foundation. The loads are transferred from the anchor latch through the template insert to the abutment ring. An important part of the anchor latch is the flexelement (Fig. 5) which accommodates the angular displacement of the tether at the seafloor and, therefore, prevents excessive bending stresses in the tether string itself.