Coupled Dynamic Analysis of Platforms, Risers, and Moorings
Authors: Chaudhury Gautam , Ho Cheng-Yo 
Published: October 2008

Motion analysis of platforms operating in deep waters is quite a challenge. In order to obtain a realistic motion, the analysis must be performed as an integrated system of risers, mooring lines, and the platform. A fully coupled analysis of such a system is very time consuming and modeling simplifications, both structural and hydrodynamic, are often applied thereby introducing uncertainty in the accuracy of the results. For accurate vessel motions such fully coupled analysis may not really be warranted. It is rather beneficial to use a simplified procedure to include the effects of the risers and mooring lines and, if possible, be more accurate in the hydrodynamic modeling.

A simple procedure has been developed in the form of a computer program, NICDAF, (Non-linear Integrated Coupled Dynamic Analysis of Floaters). The coupling effects of the risers and mooring lines are included based on their quasistatic configurations at each time step instead of their actual dynamic equilibrium configurations. The case study results show that full coupling of dynamic equilibrium is not important for accurate platform motion, but it is important for the riser or mooring line motions. Details of the development of the procedure and the case study analysis results from ABAQUS and NICDAF are presented.

INTRODUCTION

Recovery and production of oil from fields in deep and ultradeep waters require the services of compliant floating platforms. Design of the mooring system and risers, connected to floating platforms, are dominated by the motions of the platforms. An over prediction of motion would require costly risers and moorings, whilst an under prediction can lead to inadequate designs of possible catastrophic consequence. Accurate prediction of motions of a floater is very important for the integrity and associated costs of the risers, and moorings.

For platforms in deeper waters, risers and mooring lines generally contribute significant inertia and damping due to their longer lengths, larger sizes, and heavier weights. Accurate motion analysis of platforms in deep waters requires that a fully integrated risers, moorings, and platform dynamic model be used. It is believed that a fully coupled analysis must be performed. This has resulted in recent efforts of the industry focused on the fully coupled dynamic motion analysis of floating platforms (1–4) to be used in deep waters.

The accuracy of predicted motions also depends on accurate computation of wave forces (both wave frequency and low frequency), other hydrodynamic loads, and how the frequency dependent parameters are incorporated in the analysis. For example, wave forces on a large floating body are computed up to the mean water level and at a constant draft. Actual water surface elevation and the instantaneous position of the floater cannot be included in the present wave force calculation. Computation of hydrodynamic loads in a situation where natural periods of the system are changing all the time is complex. Change in natural periods occurs due to changes in mooring line tensions, which generate the system stiffness.