Tensioned buoyant platform tether response to short duration tension loss

Abstract

In conventional design practice, the tethers of tensioned buoyant platforms are operated at a sufficiently high pre-tension so as not to go slack in combinations of extreme environmental conditions—such as a 100 year return period sea state combined with low tide and high platform variable load. This high pre-tension imposes a significant payload and structural weight penalty and is a motivation for investigation of TBP tether performance at low tensions. TPB tethers operated at lower mean tension would be prone to short duration tension losses in extreme design case conditions. This is likely to remain true despite the possibility of lower anchor connectors being able to alleviate this tension loss by dropping down from their restraints. This paper presents the results of an investigation into tether behaviour under transient tension loss by solving the governing equation of lateral tether motions both analytically and numerically. The resultant amplification functions and preferred modes of tether deformation are used for an example tether to obtain criteria for allowable tension loss and duration time to first attainment of maximum stress in the tether material. It is shown that tension loss lasting a few seconds during the passage of an extreme wave can be designed to be acceptable for typical tether structures.