Dynamically positioned offshore drill-ships and semisubmersibles are commonly used for drilling operations in deep water (more than 3000ft water depth). For shallow water depth (500ft (152.4m) to 2000ft (609.6m)), moored semisubmersibles were often used to perform the drilling operations. Because of the shallow water depth, in case of an emergency, dynamically positioned drill-ships and semisubmersibles may have limited watch circles to enable safe disconnection of the riser system from the wellhead.
With superior mobility and loading capacity, dynamically positioned drilling vessels are used more frequently for offshore drilling. In recent years, there are growing interests in using them for shallow water drilling. In this paper, the feasibility and limitation of using dynamically positioned drilling vessels for shallow water operation are studied. A drill ship was selected to perform the riser and drift off analysis. The water depth was varied from 500ft to 2000ft, with mud weight at 8.56ppg and 16ppg, respectively. Typical Gulf of Mexico met ocean conditions were applied to a global riser model using DeepRiser Software. Based on the results of the study, a minimum water depth was found with sufficient watch circles for drilling operation. Below the minimum water depth, the watch circle may be insufficient, or not available.