Structural Design of Tether Components for the Snorre TLP
Authors: Hannus H.G , Haug S. , Aunemo H. 
Published: October 2008

A structural design procedure of tether components is described and its application shown for characteristic components of the Anchor Latch Assembly and the Top Tie-Off assembly. In the finite element analysis it is shown that prediction of local stress peaks requires an extremely dense local mesh and that threaded connection stress concentrations are largely influenced by machining tolerances. The analysis is verified by good agreement with strain gauge results from full scale structural testing. Finally, the material defect sensitivity is accessed through fracture mechanics analysis, giving consideration to the local stress state.

INTRODUCTION

Offshore developments are moving into water depths beyond conventional platform concepts, and at present the most favoured new concept for larger fields appears to be the Tension Leg Platform (TLP). A TLP will be installed in 1992 on the Snorre field in the Norwegian sector of the North Sea, with Saga Petroleum a.s as operator. For a general description of the platform, reference is made to /1/.

The most unique feature about a TLP is the tether (or tendon) system, which constrains the platform vertical motions while compliant under loading in the horizontal plane. The main components of the Snorre TLP tether system are illustrated in Figure 1. starting from the seabed, the connection to the foundation template is through the Anchor Latch Assembly (ALA), which includes a flexelement to allow for angular deflection. The tether string continues in tether elements, connected by taper threaded couplings. At the bottom of the platform the cross load bearing transfers through a horizontal load component into the platform and absorbs the bending through a flexelement. The tether string continues up to the Top Tie-off Assembly (TTA), which ties off the tether tension and incorporates a tension adjustment facility.

This paper describes the structural design of components of the tether system, the primary design tool being linear and nonlinear finite element analysis. The description is limited to the ALA, which was used for verification of the analysis procedure through full scale structural testing, and the TTA, which through its threaded section sets extreme requirements to analysis and fabrication. Cross sections of these components, identifying the major parts are shown in Figures 2 and 3.

The governing design ultimate loading corresponding to a 100 year storm condition, is a tension load of 35.5 MN for the TTA and a tension load of 34.3 MN with a corresponding flexelement angle of 8.4 degrees for the ALA (these loads include load factors as discussed in next chapter).

In the North Sea environment also the longterm load distribution is of great significance. The tether components are of high strength material to achieve low weight and high resistance against extreme loads. Offshore codes do in general not give credit for increased fatigue strength with increased material strength of welded components. Particular effort is therefore required in detail design and defect sensitivity assessment.