Guidelines on Coupled Analyses of Deepwater Floating Systems
Authors: Hansen Vigleik L. , Wang Lihua , Sodahl Nils , Ward E.G. 
Published: October 2008

Within Deepstar (DS), a Guideline (ref. /1/) representing an industry consensus on deepwater design analysis methods and model test verification has been prepared. As the offshore industry move into deeper water, the capacity to analyze and model test deepwater floating systems are challenged. The designers need efficient tools and procedures on how to determine dimensioning responses. For some of the floating systems installed/planned, coupled analyses is considered a must, both with respect to safety and cost. The paper focuses on how to perform coupled analyses efficiently for deepwater floater and riser/mooring systems.

1. Introduction

During the last years, the offshore industry has gained increased experience and knowledge with respect to how integrated, coupled analyses should be performed on floater types like FPSO, TLP, Spar, Semi, with associated riser and mooring systems. The introduction of new floater solutions/combinations, new materials and the fact that the riser/mooring system introduce more non-linearities in deeper water emphasise the need of a unified approach for coupled analyses. Model testing alone can not be regarded as solely suited for final verification/validation of complex floating systems in deep water when e.g. truncations have to be included in the tests. Some years ago, DS invited the offshore industry to perform analyses on some selected floating systems, with FPSO, TLP and Spar as the 3 'Theme Structures'. The relative large scatter in the calculated results led to the execution of model testing these 3 floating systems (ref. /2/). Following these model tests, a smaller group of vendors (3 basins and 4 designers) with DNV and OTRC as the coordinators were established. New analyses were conducted with the purpose to explain and reduce the scatter in results (refs. /3/, /4/ & /5/). This exercise typically involved 'modeling the model' used in the model tests and necessary calibrations (model test results vs simulations). Further details on these calibrations and correlations are given in ref. /6/. In parallel with these analyses and calibrations, a Guideline on how to perform coupled analyses of deepwater floating systems was prepared. The main objectives of the Guideline (ref. /1/) are to provide guidance on:when it is deemed necessary to perform coupled analyses.how to efficiently perform coupled analyses.riser and mooring line analyses.relevant load models for floater and slender structures.important coupling effects from slender structures to the floater.the interface between model tests and coupled analyses.which input parameters are of key importance in simulations.obtaining confidence in critical design and validation cycles. The 10 sections of the Guideline deal with the following issues, respectively:Presents objectives, scope and background.Gives an outline description of different floating concepts, riser and mooring systems and an evaluation of coupling effects.Outlines a typical design process for deepwater developments.Provides guidance on how to identify design conditions.Provides guidance on how to perform de-coupled analysis, covering load models, static analysis, frequency domain and time domain analyses.Global slender structure analysis with focus on slender structure load models and FE analyses.Provides guidance on the methodology and procedures of coupled analysis.Model testing and which effects and parameters are of importance.