Estimation of extreme response for drilling risers

Abstract

The marine riser is an important and indispensable part of the offshore oil industry as they are used to convey fluids from the sea bed to a platform or floater. Drilling risers are used during drilling of new wells, while production risers are employed for fields of production. The different concepts of riser configurations all possess different fields of application and characteristics. Steel pipes are the most common structure of riser systems, but also flexible and titanium pipes are utilized. Failure may have severe consequences and it is of crucial importance that a riser system is reliable and properly sized. Hence, estimation of extreme dynamic response of risers is necessary. The purpose of this study was to investigate how these loads can be estimated. The normal way to estimate extreme dynamic response is to carry out a full long term response analysis of the riser configuration. If the response of the structure is of a nonlinear nature the problem will be rather complex and time consuming. In this thesis the environmental contour line method is applied to a riser configuration. It is a simplified method used to estimate long term extremes response. The great advantage of this method is that the environmental loading and the response of the structure is decoupled. Thus the response analysis is only required for a set of design sea states. Determination of the short term extreme response distribution of the problem can be used to estimate the long term distribution of extreme response. This thesis comprises descriptions of riser configurations. The mathematical theory used for modeling the wave environment. The statistical methods utilized for long and short term analysis is presented. A thorough description of the environmental contour line approach is also presented.An analysis of a drilling riser configuration has been carried out for the semisubmersible drill vessel Scarabeo 8. The analysis is based on the procedure for the environmental contour line method. The riser configuration was modeled in SIMA, based on FE-program RIFLEX. Several nonlinear response analyses were carried out to identify the most critical sea state. The most critical case is analyzed through 24 analyses of 3 hour duration. The time series for the critical area of the riser with relevant response quantities where stored for post processing. Gumbel distribution where used in evaluation of the results. The bending moment and the axial force were estimated in the wave zone area of the drilling riser. Proper estimates of the response for the short term variability were accounted for by choosing a higher percentile value for the extreme value distribution. In this thesis the percentile values used were 0.85, 0.90 and 0.95. Normally 0.90 is considered a reasonable choice for most structural problems. To assess reasonable percentile values a full long term analysis should be performed. Although there are uncertainties, the contour line method is an approximate method. The numbers of observations, the choice of distribution, the parameter estimators, all illustrates this. There is a growing interest in development of reliable response calculations and this thesis might be viewed as a small contribution to that field.