Dynamic simulation of deep water drilling risers with heave compensating system

Abstract

This thesis discusses modelling aspects related to dynamic analysis of deep water drilling risers. These risers must have a heave compensator that maintains a near constant tension independent of platform motions. Traditional riser analysis will apply constant top tension or a simple spring-damper model that may give approximate tension variation. The present thesis describes an alternative improved analysis procedure that consists of the following steps: Global riser analysis including the calculation of the 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. An integrated model of the hydraulic system and marine riser could preferably have been developed. This type of model can be based on the two computer programs applied in this study, but a real time communication between the programs must be established. This approach has not been attempted in the present study.

A case study with data from a real situation is reported. The data shows that the constant tension model is only valid for small heave motions, while a spring-damper tensioner model can give almost correct results for tension variation. However, a parametric model must be tuned for each case.

All data used in this study have been taken from the platform Aker Spitsbergen. The heave compensator on the platform was instrumented for recording of response data. It is primarily the acceleration measurement system on the Direct Acting Tensioner (DAT) that has been of interest. The main conclusion based on these measurements is that the accelerations are higher than 1.5 ms-2 The design criteria for the cylinders in the heave compensating system have been that they should be able to withstand a lateral acceleration of 1.5 ms-2 evenly distributed along the cylinder. The measurements confirm that there is a significant response component that is very likely to be caused by slamming loads. 

As a result of inadequate design criteria all cylinders in corresponding applications are modified and replaced with cylinders that are designed to withstand higher accelerations.

Tension variations from the DAT system are also higher than previously assumed. These tension variations should be taken into account in riser design analyses for existing and future platforms.