Impact Of Coupled Analysis On Global Performance Of Deep Water Tlp'S
Authors: Sircar Sid , Kleinhans J.W. , Prasad Jitendra 
Published: October 2008

ABSTRACT
This paper presents the results and conclusions of a study whose objective was to determine whether a "linear spring" analysis approach can adequately predict the global performance behavior of a relatively large TLP in 3,000 ft of water for a typical Gulf of Mexico (GOM) site. "Coupled" and "linear spring" analyses of the TLP were performed using random time simulation techniques in extreme, reducectextreme and normal environments, with the TLP in the intact, one-compartment damaged and one-tendon removed conditions.
The results presented in this paper show that "coupled" analysis does not have significant impact on the prediction of the total design responses of the TLP. It however significantly impacts the prediction of the dynamic part of design responses. k is further demonstrated that for such TLPs, high-frequency resonant response of the tendons could significantly impact the strength and fatigue design of these tendons.

INTRODUCTION
Conventional approach to global analysis of Tension Leg Platforms (TLP) includes modeling the tendons as simplified "linear springs", i.e., as straight rods having only axial stiffness. It is also assumed that these tendons do not attract any hydrodynamic loads and have no inertia. This approach to TLP design is based on the offshore industry's past experience with design, operation and performance of TLPs in water depths less than 2,000 ft (see reference 1). More complex analytical studies and model test experience on these past projects have confirmed the validity of using the "linear spring" approach. One such complex analytical technique is where the global analysis is performed using a "coupled analysis" technique in which each tendon is modeled as a series of beam elements having axial and bending stiffness and which are exposed to hydrodynamic loads and also have inertia.
In water depths 3,000 ft and more, limited experience exists in analyzing and model testing TLPs, Hence, it is not certain if the simplified "linear spring" approach would produce acceptable global analysis results of such a TLP. API RP 2T (see reference 2) and other codes discuss this issue but do not provide any clear guidance as to when "coupled" analysis is more applicable. Past publications on this topic (e.g. see Reference 3) have compared analytical methods which were different in other ways besides the tendon mathematical model, This has resulted in not knowing with certainty whether the differences between the responses predicted by these two methods are significant enough to impact the design of one or more components of a TLP.
A study was initiated to determine whether global analysis results of a 3,000 ft water depth TLP, using a "coupled analysis" technique would confirm "linear spring" analysis' results. The focus of this study was to investigate the differences in global analysis results solely because of the differences between the two types of tendon mathematical models, as described above. To meet this objective, all other differences, such as time/frequency-domain, wave theory, hydrodynamic theory, etc, were kept the same for both analytical techniques. The results of this study are presented in this paper.