Restriction Diagrams: How to Work with DP Rigs in Close Proximity
Authors: Juiniti Ricardo , Pallaoro Afonso A. , Ohara Shiniti 
Published: October 2008

Dynamically Positioned (DP) rigs started out in Campos Basin with an exploratory drilling program in waters around 900 meters deep. After this, these vessels were used in shallow water workover and completion jobs, where conventionally moored rigs could not work due to the cluttered sea floor. Despite their success, DP rigs do present failures such as black-outs, drift-offs, and drive-offs. Any such failure can end up in a collision with a surface or subsea obstacles, of catastrophic implications. A mathematical method was established based on probabilistic statistical analysis of vessel characteristics, black-out elapsed times, and time between total power failures to determine the minimum distance that a DP rig can work from an obstacle. This method led to the so called restriction diagram. Safety alternatives were also pursued, in partnership with the contractors, for the cases when it is necessary to operate inside the restriction diagram, such as safety anchors and safety boats. Real scale experiments were performed to test the feasibility of safety anchors for a DP semi submersible working in a close proximity to an obstacle. Furthermore, restriction diagrams, DP vessel characteristics, and safety devices have helped to choose from the available rig fleet the best unit to perform certain tasks such as well completions in a cluttered area. As a result, restriction diagrams have grown to be powerful tools to use in deep water fields development using subsea flowlines.

Introduction

Petrobras started its deep water drilling program using DP rigs in 1984, when the wildcat well 1 RJS 2 19 A was drilled in Campos Basin in 840 meters of water depth. 011 discoveries in waters as deep as 900 meters pushed the geological studies to deep and ultra deep waters and more DP rigs were necessary to develop a fast growing and promising exploratory drilling program. The success of DP rigs also pushed its use to shallow water workover jobs, where conventionally moored rigs could not work due to the cluttered sea-floor. Increasingly deeper water subsea completions naturally followed the discoveries. Between 1984 and December 1992, almost 8,000 days of DP operations were logged in drilling, workover, and completions operations. Moreover, for the incoming years a greater number of DP rigs would be necessary to meet the exploratory and development programs almost entirely held in Campos Basin (Figure 1). This increasing activity of DP rigs, led Petrobras to create in 1992 the Dynamic Positioning Safety Program(1) (DPPS) which is a partnership program with the DP rigs contractors to minimize the risk of station keeping failure to avoid its catastrophic consequences. Among the initial DPPS projects, there was one entitled "Risk Equivalence" that aimed to answer questions like: how safe is a DP rig when working in close proximity to another DP rig, a moored rig, or production unit? Which is the most adequate DP rig to perform certain tasks? What minimal distance a DP unit should maintain from surrounding obstacles to avoid collisions? These questions came from the the initial DP experience in Campos Basin, where 53 emergency disconnections were observed from 1984 to 1992.