Analysis of Brief Tension Loss in TLP Tethers

Abstract

The avoidance of “slack” tethers is one of the factors which may establish the required tether pretension in a tension leg platform (TLP) design. Selection of an appropriate safety factor on loss of tension depends on how severe the consequences may be. It is sometimes argued that if tethers go slack, the result may be excessive platform pitch or roll motions, tether buckling, or “snap” or “snatch” loading of the tether. The results reported here show that a four-legged TLP would not be susceptible to larger angular motions until two adjacent legs lose tension simultaneously. Even then, this analysis shows that a brief period of tether tension loss (during the passage of a large wave trough) does not lead to excessive platform motion. Similarly, momentary tension loss does not cause large bending stress in the tether or significant tension amplification as the tether undergoes retensioning. This paper presents TLP platform and tether response analysis results for a representative deepwater Gulf of Mexico TLP with large-diameter, self-buoyant tethers. The time-domain, dynamic computer analysis included nonlinear effects and platform/tether coupling.