Dynamically Positioned Rig Used for Shallow-Water Drilling Operations

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 231686, “Shallow-Water Drilling Operations Using DP Rig: From Design to Execution in the Pre-Salt Campos Basin,” by Artur Schlinker, SPE, Acácio S. do Amaral, and Magno A. Silva, Petrobras, et al. The paper has not been peer-reviewed.

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This work introduces a novel predictive methodology combining historical metocean data with real-time riser analysis for shallow water dynamically positioned (DP) rig operations, addressing a gap in industry literature. The integrated approach of predictive nonproductive-time (NPT) estimation, real-time monitoring validation, and structured decision protocols for critical scenarios provides a replicable framework for similar challenging operations in shallow-water environments.

Objectives

The focus of this article is on an exploratory location in the pre-salt region of the Campos Basin. The vertical location, in water depths of less than 300 m, revealed the presence of coral in the area, making moored rigs unfeasible. In search of an alternative that would allow the use of DP rigs, studies were conducted to identify viable locations. The alternative locations identified presented offsets exceeding 3,000 m, resulting in trajectories that were deemed practically unworkable. Given this scenario, attention was refocused on potential locations in water depths of less than 450 m.

In this context, the use of DP rigs for drilling the location in water depths of less than 300 m became a potential enabler for this project. However, that goal necessitated in-depth studies regarding operation of a DP rig under these conditions.

First steps, detailed in the complete paper, included a technical feasibility study with multiple collinear environmental conditions, simulating drift-off scenarios following a complete loss of propulsion of a dynamic positioning rig, and a real-time riser analysis founded on three main pillars: validated software, well-established operational procedure, and high-quality input data.

Well Design

The final location for the project in shallow water achieved a balance between environmental constraints and operational requirements. This location enabled a more-conventional well design, featuring a five-section well configuration. Despite the adoption of standard drilling practices and well architecture, significant challenges remained. The distance from the wellhead to the target exceeded 1,000 m, requiring comprehensive studies on drilling parameters and well design. Moreover, the post-salt section was designed with a length of 3,480 m, setting a record for the operator in this diameter. Additionally, a trajectory in S-mode enabled reaching the target with an inclined section of 28°.

The adoption of a subsea wellhead system with high-bending capacity design was a key technical solution to maximize the maximum allowed bending moment at the wellhead. The H-4 connector significantly improves wellhead bending-moment resistance through an optimized load-distribution mechanism. The result is a substantial increase in fatigue life compared with conventional connectors, particularly when paired with the chosen wellhead system.