Optimization Of Drilling Riser Operability Envelopes For Harsh Environments
Authors: Williams Dara 
Published: July 2010

As exploration activity in deepwater and harsh environment regions, and associated vessel costs, increase so too does pressure on drilling contractors and operators to accurately assess the limitations of vessels and drilling equipment. As a result there is a requirement for more refined methodologies and finite element models to verify the operability of drilling riser systems in these environments.

The efficiency of a drilling riser system for connected operations will generally be most influenced by the current regime onsite as this will influence the vessel offsets and nominal flex joint angles. However, as vessel offset from the nominal position increases, the vessel dynamics become increasingly important in the determination of operability limits. Likewise for the storm hang-off, deployment (including both conductor and casing deployment) and retrieval scenarios the accurate assessment of system response is critical to identifying operability windows.

For harsh environment operations the operability and efficiency of the system is a function of both riser component limitations and vessel response characteristics. As a result detailed analysis considering both system response screening (involving estimation of downtime due to extreme weather events) and irregular sea analysis of riser response is required. In addition, an iterative design approach is required in order to balance the competing design drivers associated with connected operations, storm hang-off, riser recoil, deployment and vessel drift-off and resultant weak point response.

This operability assessment requires a detailed model of the riser system to accurately predict the response. This model needs to account for the nonlinear response of riser tensioners, influence of wellhead, conductor and casing, nonlinear soil interaction, detailed storm hang-off arrangements and associated clashing and interference issues. In addition to refined FE models, a detailed knowledge of the metocean data for the region of interest is required. This metocean data includes detailed seastate scatter diagrams and current profiles accounting for seasonal directional variation.

This paper outlines the key issues associated with the modeling and analysis techniques for drilling riser systems intended for deepwater and harsh environment locations such as Atlantic Margin, offshore Norway and Offshore Canada. In addition a number of conservatisms in current practice are identified and optimizations are outlined.

Introduction

Operators are now drilling in up to and beyond water depths of 10,000ft, the deepest ever attempted in the offshore industry. In addition to this recent years have seen an increase in exploration activity in regions of harsh environment such as the Atlantic Margin, offshore Canada and in the Southern Atlantic. Ultra-deep water and harsh environmental conditions place higher demands on both the vessel and the drilling equipment. As a result drilling riser systems are being operated at close to their component limitations. It is therefore of critical importance that an accurate assessment of riser system loads and vessel response be made.