Reliability of TLP Tethers Under Maximum and Minimum Lifetime Loads

Abstract

ABSTRACT This paper describes reliability analyses which were performed to determine the adequacy of TLP tether maximum and minimum tension design criteria. Reliability analyses for other potential tether failure modes such as fatigue were not performed. The calculated reliabilities were compared to typical calculated reliability levels of steel piled jackets. The effect on tether loading of vertical wave impact on the underside of a TLP deck was also considered for the maximum tension case. Tether load analyses were performed using frequency domain analytical tools and reliability was calculated using the advanced, first order, second-moment analysis technique. A probabilistic methodology for combining time-variant and time-invariant tether load components was developed. The results of the study are limited by assumptions discussed in the paper. INTRODUCTION The present economic climate in the oil and gas industry encourages the search for low cost deepwater production systems. One of the challenges in the evaluation and development of such systems is establishing criteria which lead to safe, but not overly conservative designs. Since little or no actual service experience exists for new concepts, a probabilistic reliability analysis is a means of calibrating the design criteria. This is accomplished by comparing the calculated reliability of the new structure with that of conventional offshore structures which have demonstrated their safety over time. The subject of this study is the Tension Leg Platform (TLP). The TLP, Figure 1, is a deepwater production system that has a semisubmersib1e type hull and integral deck permanently moored to the ocean floor by tethers which are kept in tension due to excess buoyancy of the hull. The tethers are the most novel and also the most critical feature of the structure. There are several potential modes of tether failure. The two studied here are maximum tension in which the tensile capacity of the tethers is exceeded, and minimum tension, in which loss of tension leads to unacceptable stresses caused by retensioning or excessive tether bending. While the TLP can still function properly with a tether damaged or removed, it is not known how much damage a ruptured tether falling to the ocean floor could cause to risers, adjacent tethers or foundation and well templates. In this paper, failure of the TLP system is defined as failure of one tether. Reliability analyses for other failure modes such as fatigue, which was not critical for the Gulf of Mexico application being studied, were not performed. TLP TETHER LOADS AND DESIGN CRITERIA TLP Description The TLP analyzed in this study was designed for 762-meters (2500-feet) of water in the Gulf of Mexico. It has a square, four-column, symmetric conf1guration with circular columns and square pontoons. The pontoons have rounded edges. The displacement is 52200 mt (57540 st) and the platform weight is 34330 mt (37840 st). The column center-to-center span is 62.8 meters (206 feet),