Riser Response Based Optimal Positioning of Deep-Water Vessels

The present paper discusses the mathematical modeling of risers and riser-like structures applied in a positioning context for deep-water floating vessels. The main purpose of the paper is to show that an estimate for the optimal vessel position, sufficient for most practical applications, is obtained from measurements of the riser inclinations or related parameters at lower end, and optionally upper end, through a solution based on the variably tensioned beam differential equation. Due to the ease of implementation this solution is well suited for direct application in on-line riser monitoring systems. The method is an attractive alternative to on-line FE-analyses, application of pre-computed regression curves based on idealized loading or black-box neural networks, which has been proposed by others to be applied as basis for interpretation of the measured riser responses. The basic idea behind the method is based on the observation that the riser inclinations or stress-joint moments at upper and lower end have mainly two causes. Firstly an effect caused by the position of the riser top end relative to the wellhead due to permanent vessel offset and slow drift vessel motions, and secondly the effects of transverse current down the riser. The general theory behind the method will be outlined. It will then be shown how the method adapts to drilling-risers with flex-joints, risers with stress-joints and also to the special case of well intervention with coiled tubing in open sea without applying a work-over or marine riser. The performance of the method is illustrated using simulated vessel and riser dynamic response data. The simulations are performed for selected vessel types both for deep-water and shallower waters applying state-of-the-art software for simulation of the riser and vessel dynamic response in random sea states.