This paper presents a methodology in order to perform a drift off calculation for drillships according to given parameters such as: environmental conditions and water depth. Drift off occurs when there is insufficient thruster force so that the vessel is drifted away from the target position by the environmental forces. For a safe operational drillship it is expected that the drifting off will be resumed in due time when blackout recovery system starts running and, therefore, enough thrust takes place. Water depth plays an important role when considering the default maximum release of Lower Flex Joint (LFJ) angle for physically disconnecting, which is 10 degrees in the majority of suppliers. This methodology is intended to be applied to drillship design, by comparing the time to stop drifting and the distance from the reference point after a total blackout occurs. Electrical generators sets installed in drillships are designed to work with extreme environmental conditions. Since there is an excess of installed power for the majority of the operational time, drillships often operate with all high voltage busbars connected to each other improving engine efficiency, decreasing levels of pollution emissions and reducing maintenance. The use of this electrical power configuration is possible because there is no need to turn on all generators at the same time, but only the ones that are needed on that particular moment. However, when a single failure such as a short circuit occurs and the system is not prepared to disable and segregate the failure, all electrical system will crash, causing a total blackout and the drillship will start to drift off. The drifting off time was obtained by numerical simulations conducted by modeling a standard drillship using time domain software. The model took into consideration the vessel hydrodynamics under environmental conditions (wind, current and wave), the drag force in marine drilling riser, and the thrusters in Dynamic Positioning (DP) operation. The simulation is divided into three steps: First, the behavior of the DP system in full operation is simulated until system is stable. After that, all thrusters are turned off to simulate a total blackout. Finally, since the ship will not stop immediately because of its inertia, a time range for the ship’s inertia was also considered and this time is added to the pre-established blackout duration. The conclusion of the study shows how parameters as water depth, environmental conditions, and blackout recovery time affect the necessary time to stop drifting off, so as to foresee that after a total blackout the standard drillship will remain in safe limits.

In drilling vessels, the dynamic positioning (DP) system has a great importance for the operation, since it ensures the station-keeping ability for the drilling operation. However, an emergency situation involves ungoverned drift due to problems associated with the DP system failures, such as thrusters, generators, powerbus, or control system. During this situation, the vessel drift is subjected to the influence of environmental conditions and the drift can lead to collisions with floating obstacles or submerged systems, wellhead emergency disconnection, damage to equipment and potentially causing major environmental disasters. It is then necessary to define a safety region for the drilling ship operation and to determine the limiting operation offset that the drilling vessel can disconnect from the wellhead without damage to any equipment. This offset limit is obtained through a riser analysis and drift-off study, important inputs for the Well Specific Operating Guidelines (WSOG). A validated time-domain simulator is required and able to predict the vessel drift trajectory after the DP failure under several environmental conditions. The aim of this work is to present a large set of model and full-scale drift tests and the validation of a time-domain numerical simulator (Dynasim), based on the main parameters of the drift tests: drift distance, heading variation, and trajectory. The comparisons between the numerical simulation results with full- and model-scale data demonstrated the accuracy of the numerical model, confirming that the simulator is a reliable tool to predict the motion of a drilling vessel after a blackout.

Special station-keeping requirements must be defined for a safe operation of a DP pipe-laying and crane barge. When a pipe is being laid in shallow waters, small displacements of the launching ramp may induce large forces on the pipe or even to deviate it from the defined route. Offshore crane operations are performed in close proximity with other vessel or platform, and large loads are transported in a pendulum configuration. Again, a precise positioning of the barge is required, in order to avoid unsafe relative motions, as well as keep the load being transported on a stable position. Due to these special DP requirements, it has been shown in the present work that a simple static analysis of the DP System is not adequate in this case. Dynamic effects related to wind gusts, slow drift forces, propellers response, DP filtering and time-delay must be considered since the initial stages of DP specification. The common approach of considering an amount of 20% extra power to compensate for dynamic effects may underestimate the necessary power. A fully non-linear dynamic simulator was then used to carry out a complete analysis of the barge. Thrust utilization capability plots were obtained for the DP design environmental condition, considering the occurrence of a single failure. After that, an analysis of the environmental conditions in the Brazilian waters was carried out, and a comprehensive set of more than 1700 conditions were obtained. Dynamic simulations were then used to define the operational window of the barge as well as the estimated downtime for each operation. The barge is also able to operate in a DP-mooring assisted mode. The simulations were used to define under which operational and environmental conditions that such mode must be used.

Currently, most offshore oil exploration operations are performed by Dynamically Positioned (DP) units; in Brazil, the use of these units is traditional in drilling operations (DP drilling rigs). The advent of DP systems, which allow the vessel to maintain a certain position without the need for anchor lines, has brought great flexibility to the oil field; however, usually in drilling areas, there are a large number of operations being performed simultaneously and subject to weather conditions and a failure of the DP system may cause these vessels to drift, which may occasionally result in a collision with other field equipment or vessels, causing material, personal and environmental damage. In this context, it is necessary to analyze what would be the best positioning points for these units according to the configuration of potential obstacles present in the area (such as risers, anchor systems and floating production systems), the characteristics of the DP unit and the expected environmental conditions. It is vital to know the risk of collision associated with the positioning of these units. The risk of collision will depend mainly on the meteo-oceanographic variables of the operating region, the hydrodynamic characteristics of the unit, the DP system reliability and its repair time, and the distribution of obstacles in the area. The objective of the ongoing research is the development of a methodology to define the risk associated with the positioning of the DP units, through a statistical method and a validated drift mathematical model under the influence of environmental agents. The proposed methodology allows us to demonstrate compliance with a widely accepted RAC (Risk Acceptance Criterion). The developed methodology proposes the use of two instruments: Location Iso-Probability Maps (MIL) and Operational Iso-Risk Maps (MIRO), to synthesize the information to the decision making about the operation of the DP units at a specific location, considering the overall collision risk (at MIRO) and the probability of the rig being at a specific location (at MIL).

This paper and the companion paper (Rateiro et al., 2011) present an illustrative case of the joint application of experimental tests and numerical simulations for the proper analysis of a complex offshore operation (launching of a sub-sea equipment using one or two vessels). The main idea of the whole study is to compare two methodologies and operational procedures for the installation of the equipment in the seabed, using either one vessel (conventional operation) or two vessels in a synchronized operation in a Y-configuration. The experiment was conducted under a simplified configuration, and uses ODF (one degree of freedom) servo-actuator to emulate the vessels induced motion. The hydrodynamic properties of the equipment was then calculated, and some preliminary conclusions about system dynamics could also be drawn. After that, numerical simulations were conducted, considering the coupled dynamics of the vessels, cables and equipments under irregular sea state. Those simulations were used for determining the limiting environmental condition for a safe operation, and are described in the companion paper. This paper describes the reduced scale experimental setup used for evaluating the hydrodynamic properties of the equipment during a subsea installation under waves excitation. The reduced scale model of the equipment was attached to one or two servo-actuator, that emulate the wave-induced motion. The tests were conducted at the physical wave basin of Numerical Offshore Tank (Tanque de Provas Nume´rico – TPN). The experiments enabled the preliminary evaluation of the dynamic behavior of the equipment when submerged by one or two launching cables. In the later case (two launching cables), several tests considering phase shifts between the servo-actuator have been conducted. The reduction in the dynamic amplification of cable traction could also be experimentally verified.