The Influence of Drilling Rig and Riser System Selection on Wellhead Fatigue Loading

Abstract

With drilling and exploration activity currently high in both deep and shallow water regions rig availability and selection is an issue for operators to consider in order to achieve the desired exploration schedule. At present the industry focus is on the development of 6th generation drilling rigs with the capacity to operate in increasing deep water. However despite the focus on deepwater exploration and the associated demand for deepwater drilling rigs there still exists demand for drilling rigs that can operate in shallow to moderate water depths (100m–500m). In addition, certain field development scenarios may exist where planned water depths for drilling activities vary significantly and therefore a drilling rig and riser system is required that can operate satisfactorily in both shallow and deep water depths. For a given drill site, rig availability or well location, may be such that an operator may have to select a modern deepwater 6th generation rig for shallow water activities where a 3rd generation rig would appear to provide a better solution. Other considerations such as vessel station keeping requirements may lead to selection of a 6th generation rig over a 3rd generation rig, as the former tend to have improved DP thrusters capacity. However it is also important to note that while the 6th generation rigs may have been proven to be robust systems for operation in deep water, the response of a 6th generation drilling system in shallow water depths can be very different to that of an older 3rd generation rig and drilling riser system. Thus careful consideration must be made by the operator when considering the selection of drilling vessels for shallow to moderate water depths. Fatigue life of the wellhead is shown to be affected when one compares the response of the 6th generation and 3rd generation drilling systems in shallow to moderate depths. This also needs to be accounted for when selecting rigs for workover or intervention operations on older infrastructure. This paper presents a discussion on the various parameters such as BOP stack size, riser, flex joint and vessel design that influence the response of the drilling system in shallow to moderate water depths (100m–500m). A number of case studies and parametric studies have been carried out and the results of these are presented in order to compare the wellhead fatigue damage from the older 3rd generation systems with the 6thgeneration systems and also to identify the critical drivers for this fatigue life reduction.