Advancements in Deepwater Drilling Riser Modelling

Experience with modern ultra-deepwater capable drilling vessels and their associated marine riser tensioner systems has led to increased concerns over tensioner load variations in extreme environments. Modern drilling riser tensioners are complex hydro-pneumatic systems whereby the tension applied at the slip-ring can vary significantly with tensioner stroke in response to vessel heave. The aforementioned tensioner load variations that occur with modern tensioner systems can have a significant effect on the loads transferred to the wellhead and conductor/casing. This can lead to fatigue concerns at critical locations. The connectors along the conductor and surface casing can be highly susceptible to fatigue if they are located in regions of high bending loads below the mudline. This paper will give a detailed overview of recent technology advancements that have been incorporated into the latest version of an industry-standard tool for global analysis of drilling risers [1] and that allow these concerns to be addressed. It will focus on two main areas where significant enhancements have been made in tensioner and wellhead & casing modelling. (Note that the version of the software incorporating these capabilities will be made commercially available during 2014.) The advanced tensioner modelling capability consists of a detailed tensioner model that includes individual hydraulic and pneumatic components of the tensioner system that are fully integrated with a non-linear 3D structural finite element model. This tensioner model is capable of fully capturing all transient behaviour and load variations of real world tensioner systems. This paper will describe in detail this unique modelling capability and its application, including riser recoil analysis. Additionally, accurate modelling of the wellhead, conductor/casing and the surrounding soil structure is crucial in order to accurately predict bending loads experienced in this region. This includes modelling individual wellhead sections (high pressure housing, low pressure housing and tapered sections) and multi-pipe structures for conductor/casing sections (including cement layers). The advanced soil modelling includes the capability to specify different soil types at different depths. This paper will describe in detail these advanced modelling capabilities. The detailed tensioner modelling capability represents a highly advanced technical and innovative development and is fundamental to providing a realistic recoil analysis capability. The advanced wellhead and casing modelling allows for accurate prediction of the stresses experienced in this critical region and accurate determination of the fatigue lives of these components. This paper will demonstrate how all of these advanced modelling capabilities can be used to accurately model deepwater drilling risers and provide increased confidence in conducting drilling operations in the harshest of environments.