TLP Global Motion Performance Analysis Procedures
Authors: Natvig B.J. , Vogel H.S. , Johnsen Øyvind 
Published: October 2008

The paper gives an overview of the analysis procedures involved in analyzing the global motion performance of tension leg platforms such as the Snorre and Heidrun platforms.

The Aker Engineering TLP analysis system involves a suite of specially adapted purchased computer programs as well as in house developed programs. These programs are assembled into procedures for efficient and highly accurate analysis of all motion dependent parameters important for TLP design such as:Offset, set-down, air-gap etc. due to wave, wave drift and wind gustRiser and Tether extreme loads and fatigue lifeTemporary phases such as towing, station keeping and tether installationStructural analysis

The paper addresses the hybrid panel/space frame model where all radiation and diffraction effects are accounted for and superimposed on stochastically linearized viscous drag which is particularly important to achieve correct damping values for the important slowly varying effects of wave drift and wind gust.

Sum-frequency excitations and its effect on tether fatigue life and extreme forces are also discussed.

The implementation of coupled TLP/tether dynamic analysis is outlined.

INTRODUCTION

To the hydrodynamicist, the award of the Snorre TLP Main Engineering Contract represented one of the most challenging involvements that one could hope for in a life time. The compliancy of such a structure, the considerable dynamic amplification of all 6 degrees of freedom resonances as well as its critical dependency on the tethers meant that state-of-the-art analysis techniques had to be stretched to the limit. For some aspects of the global motion performance, techniques only available in research institutions had to be transformed into practical design analysis tools for the first time.

Presently, the Aker Engineering hydrodynamics are engaged by Norwegian Contractors, which is another Aker Company, to assist in all major analysis wrt. the Heidrun Tension Leg Platform. Due to the deep draft of the Heidrun TLP. which will be the first concrete floating production platform ever to be constructed, the demand for even further refinements of the hydrodynamics analysis procedures has been pressing.

This paper reviews some of the more important adaptations made to the procedures and it outlines how the suite of tightly connected computer programs performs all hydrodynamic and global motion performance analysis tasks required for the TLP hull, tethers and risers.

TLP ANALYSIS TOOLS

The following is a summary of the major components of the TLP motion performance analysis tools:(available in full paper)

GEOMETRY MODELLING

All radiation and diffraction data is derived from a wetted surface discretization of the TLP hull, whilst the viscous drag is represented by projected areas in combination with drag coefficients according to the well known Morison equation. This means that the TLP hull has to be modelled by both a panel and a space frame model. All global motion analysis will, however, be performed on a space frame representation of the TLP hull. To achieve a transformation of panel based radiation and diffraction pressures to a space frame Morison type coefficient representation a table of panel numbers versus space frame members must be established.