Topics on the design of TLP platforms: a study of two-dimensional geometries of minimum heave excitation and dynamic modelling of tethers under parametric excitation.

Author(s)

Alexandre Nicolaos Simos

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 1997-11-28

Examining board members

Celso Pupo Pesce; Jose Augusto Penteado Aranha; Celso Kazuyuki Morooka

• Advisor: Celso Pupo Pesce

Abstract

The Tension Leg Platform (TLP) represents one of the most promising solutions for the ultra-deep water oil exploitation. Research concerning this concept started in the early seventies but, until now, only a few platforms of this kind have effectively been built. Based on the previous history that distinguishes the development of this kind of system, its now a consensus that the TLP design has a great potential for further improvements. The work here presented deals with two important topics related to present and future TLP designs. The first objective is to propose alternative two-dimensional submerged geometries that minimize the first order inertial forces acting upon them due to wave effects, within a certain range of frequencies. The work is based on the formulation proposed by ARANHA; PINTO (1994) in which a extension to the well-known inertial term in Morisons formula is derived, valid for the whole range of frequencies. Through the representation of high frequency diffraction effects, a direct association between the potential flow field and the vertical inertial force behavior is performed. A methodology for the determination of the searched associated geometries is here proposed, based on analogy between the potential flow field and conservative dynamic systems. Various cross-sectional forms obtained through this process are evaluated and their potential application in actual TLP designs is also discussed. As a second topic, the tethers transversal dynamics under vertical parametric excitation is investigated. As the TLP utilization moves towards deeper waters with a desired reduction of the usually high values of tethers pretension, the effect of the mentioned parametric excitation on the tethers dynamic behavior becomes more significant.Also, under these circunstance, the consideration of sinusoidal modal forms of tethers lateral vibrations does not lead to an accurate dynamic representation. The variation of static tension along the tethers length exerts significant influence on the considered dynamics. Based on this forethought, an analytical model for the representation of tethers modal forms of lateral vibration considering static tension variation along the length is then proposed. Through a Mathieu representation, the problem associated with the dynamic stability is analyzed. The effect of the modern trends in TLPs design on the tethers dynamics is also verified. Finally, by comparison with results obtained under a constant static tension approach, one may attest the relevance of the proposed model concerning the considered dynamic phenomenon and also point out its possibly pertinent application to other related offshore engineering problems. (AU)