Probability-Based Estimate of Cumulative Displacements at Pot Bearings for Floating Platforms

Abstract Pot bearings have been used in offshore platforms to support topsides structures. For guided and mobile pot bearings, low friction Teflon (PTFE) is commonly used between steel plates at the interface to allow the topsides to slide freely to prevent hull pry-squeeze loads from transferring to topsides structures. However, Teflon is relatively soft compared to steel. This softness makes it prone to wear when subjected to high friction, especially when in contact with harder surfaces. Moreover, Teflon can deform under continuous pressure, which can exacerbate wear over time. This deformation makes it more vulnerable to being worn away under frictional forces. Damage to the Teflon in guided or mobile pot bearings can cause them to lock and lose their ability to slide freely. This may result inadditional stressesbeing imposed on the topside structures that may be over and above the original design loads, therefore potentially leading to structural failure. One of the parameters in the evaluation of wear is cumulative displacements Teflon will experience. For the pot bearings infloating platforms, the wave-induced platform motions exert accelerations on the topsides. These accelerations, as a result, will cause the topsides to slide on Teflon surfaces at the guided and mobile pot bearings. This paper will propose a method for estimating the wave induced displacements at the interface based on a wave scatter diagram. The method considers two scenarios: one assuming no friction at the Teflon surface and the other accounting for a small friction coefficient between Teflon and steel plate. The wave scatter diagram, which characterizes the statistical distribution of wave heights and peak periods at a specific location, is used as the basis for assessing the platform's response to varying sea states over time. By performing hydrodynamic analysis for each sea state represented in the wave scatter diagram, the corresponding motion response of the platform is calculated. The resultant displacements at the Teflon interface are then determined and weighted by the probability of occurrence of each wave condition. The cumulative displacement can be calculated by summing these weighted displacements, providing an overall measure of the platform's expected motion-induced deformation over its design life. This approach offers a practical and statistically robust means of evaluating the cumulative displacements at the Teflon surface and is demonstrated with a case study.