Force Variations on Heave Compensating System for Ultra-Deepwater Drilling Risers

This paper discusses modeling aspects related to dynamic analysis of deep water drilling risers. These risers must have a heave compensator that maintains a near constant tension in the riser independent on platform motions. Traditional riser analysis will apply constant top tension or a simple parametric model that may give approximate tension variation. The present paper describes an alternative analysis procedure that consists of the following step: • Global riser analysis including calculation of dynamic stroke of the heave compensator from platform motions and riser dynamics. A “pipe-in-pipe” approach is used to represent the hydraulic cylinders. • Calculation of dynamic tension variation from an analysis of the hydraulic tensioner system. The dynamic stroke found from the first analysis is applied as known piston motions in this analysis. • Identification of parameters in a simple model for dynamic tension variation from the results from the second analysis. • Use of the simple model in a second global riser analysis. The difference between the two riser analyses can hence be found, which represents the error one must expect from a traditional riser analysis with constant riser tension. A case study with realistic data is reported. The conclusion is that the constant tension model is valid for small heave motions only, while the parametric tensioner model can give almost correct results for tension variation. However, the parametric model must be tuned for each case. Hence, an integrated model that accounts for riser dynamics and pressure variation in the tensioner system should preferably be developed.