The Effect of Wave Energy Spreading on the Mooring and Riser System Response of a Spread-Moored FPSO

Abstract

Floating unit, mooring lines and risers comprise an integrated dynamic system that respond to environmental loading due to wind, waves and currents in a complex way. Correct consideration of wave energy spreading may be extremely significant for the floating unit response and the mooring and riser system integrity assessment as the directional spectrum induce vessel coupled responses. The riser and system response of a spread-moored Floating, Production, Storage and Offloading unit (FPSO) is assessed employing a cos2s function for the wind-generated wave spreading acting together with a unidirectional swell system. A coupled dynamic analysis is performed considering simultaneously the dynamics of the vessel and of the slender structural system. A case study is presented and the results assessed considering typical offshore Brazilian conditions.

Flexible Riser Bending Hysteresis Influence on Bend Stiffener Response

“…The case study is carried out considering the same floating unit and mooring-riser system adopted by Caire and Schiller [2]. It consists of a spread-moored FPSO anchored with 20 mooring lines and 15 risers connected to port side, leaving starboard side free.…”

“…For the multidirectional wave system, a SW direction is selected to represent swell and a SE direction is selected for wind waves. More details can be found in Caire and Schiller [2] Case study description and results…”

“…The current industry practice is a two-step global/local approach involving global nonlinear analysis models, which may include bending hysteresis effects, followed by local analysis of a detailed model segment for lifetime assessment. An overview of this methodology can be found in Sousa et al [1] As pointed out by Caire and Schiller [2], the modeling simplifications and assumptions adopted in the global dynamic analysis usually increase the analysis conservatism. They have investigated the effect of wind-wave energy spreading acting on a FPSO-riser-mooring system using a fully coupled dynamic analysis approach.…”