Non-Linear Response Analysis Of Anchorage Systems For Compliant Deep Water Platforms
Authors: Wilhelmy Viktor , Fjeld Svein , Schneider Stefan 
Published: October 2008

With the need of production platforms for ever increasing water depths, the compliant type concepts become more relevant regarding feasibility and cost - effectiveness. The mooring systems of compliant type platforms play a key role in the platforms safety and operational characteristics. Therefore, a better understanding of the mooring systems behaviour becomes necessary.

A complete dynamic analysis of two different mooring systems is presented, where all nonlinearities are considered, including variable boundary conditions introduced by up and down motions of clump weights.

The effect of inertia and drag forces due to the motion of components through water are considered in addition to static loads and dynamic excitation. As results, clump weight and line motion, line forces at upper end plus anchor forces are obtained as a function of time and plotted vs. excitation. Also, line force distributions for specific instants of time are available.

The analyses presented were performed on two actual concepts of platforms being studied for production in 350 m (1150 ft) water depth, one a floating concrete structure and the other, a guyed tower. Therefore, the usefulness of the method presented is demonstrated on realistic design situations rather than on examples of academic nature.

Results obtained differ from those of quasistatic or other approximate analyses. Important tension amplifications but also zero line forces can be obtained. Clump weights exhibit larger motions and also phase lag effects with respect to excitation. Their touch down can be a violent impact on the seafloor instead of a gradual settling, while lift-off causes additional amplifications in line forces.

These results indicate that a complete nonlinear dynamic analysis of the mooring systems as presented is important for development and reliability of future concepts. Quasistatic methods can lead to serious under-design.

Further analyses including such effect as ground suction and friction and also global system analyses with several mooring lines at a time can be performed with the tools presented.

Introduction

The analyses presented are based on two radically different deep water production platform concepts whose mooring systems are, however, quite similar in principle.

For the floating concrete platform Conprod the function of the mooring system is to keep the platform in place and to limit motions, while for the Guyed Tower it is also to keep the structure upright.

These operational and structural requirements lead to composite designs with catenaries, clump weights and trailing lines of dimensions never used before. These parts interact with each other, the structure and the surrounding water, which together with the nonlinear behaviour make the systems difficult to analyze.

For both concepts, overall platform responses to environmental conditions were obtained in previous analyses by simulating the effect of mooring lines with nonlinear massless springs having the static line characteristics known for each system (Fig. 5). The line forces were subsequently obtained by quasi-static considerations which greatly underestimate motions and forces.