Current market trends in the construction of newbuild drilling rigs indicate that the market is driven by demand for ultra-deepwater capacity semi-submersible rigs and drillships. These drilling vessels have capacity to drill in water depths of up to 12,000ft and possibly beyond in the near future. With increase in water depth capacity, more complex and heavier BOP stacks are required. Many modern drilling vessels are now incorporating BOPs with capacities of 20ksi pressure and up to 7 shear/seal rams incorporated. This leads to increased height and weight in the BOP. Whilst newbuild drilling vessels will be required to operate in water depths from 1,500ft to 12,000ft whilst on DP mode, deepwater semi-submersible drilling rigs will also have capability for operation in water depths <1,500ft using conventional mooring. Recent experience with modern deepwater rigs with large BOP stacks in water depth of 1,500ft or less suggests increased risk of fatigue when compared to 3rd generation rigs. If future trends continue with larger BOP stacks being designed then the problem of wellhead fatigue with modern deepwater drilling vessels is likely to become more acute.
As noted in previous studies the water depth at drillsite has a major impact on the level of fatigue accumulated in the wellhead system. The main driver for this has been found to be the height and weight of the BOP. With requirements for newbuild drilling rigs for 12,000ft water depth capacity being the industry norm, and with increased requirements for BOP functionality, the gap between wellhead loading from 3rd generation and 6th generation rigs is widening. Given that many 3rd generation rigs will likely be decommissioned in the coming years then the usage of 6th generation rigs for shallow water operations will only become more commonplace due to rig availability. Thus, unless market conditions dictate the construction of smaller and lighter BOP stacks, the design of shallow water wells will be critical to ensure fatigue loading on the wellhead and conductor is kept to a minimum.
This paper presents a summary of the results of a series of parameter studies carried out to assess a range of options for optimisation of casing and conductor design for 6th generation rigs in shallow water. Various recommendations are made as part of this study as to the addition of supplemental casing and conductor strings of varying sizes and wall thickness to ensure a robust conductor system design for fatigue performance.