RAM analysis of dynamic positioning system: An approach taking into account uncertainties and criticality equipment ratings

Currently, a Dynamic Position (DP) System is commonly used for offshore operations. However, DP failures may generate environmental and economic losses; thus, this paper presents the Reliability, Availability and Maintainability (RAM) analysis for two different generations of DP system (DP2 and DP3) used in drilling operations. In addition to the RAM analysis, the approach proposed herein considers the uncertainties present in the equipment failure data and provides more information about criticality equipment ratings and probability density functions (pdf) of the repair times. The reliability analysis shows that, for 3 months of operation, the total failure probability of the DP2 system is 1.52% whereas this probability for the DP3 system is only 0.16%. The results reveal that the bus-bar is the most critical equipment of the DP2 system, whereas the wind sensor represents the priority equipment in the DP3 system. Using 90% confidence level, each DP configuration was evaluated for a 1-year operation, finding a reliability mean equal to 70.39% and 86.77% for the DP2 system and the DP3 system, respectively. The DP2 system asymptotic availability tends to present a constant value of 99.98% whereas for the DP3 system, it tends to be 99.99%. Finally, the maintainability analysis allows concluding that the mean time for system repair is expected to be 3.6 h. This paper presents a logical pathway for analysts, operators, and reliability engineers of the oil and gas industry.

Control of Dynamic Positioning System with Disturbance Observer for Autonomous Marine Surface Vessels

The main goal of the research is to design an efficient controller for a dynamic positioning system for autonomous surface ships using the backstepping technique for the case of full-state feedback in the presence of unknown external disturbances. The obtained control commands are distributed to each actuator of the overactuated vessel via unconstrained control allocation. The numerical hydrodynamic model of CyberShip I and the model of environmental disturbances are applied to simulate the operation of the ship control system using the time domain analysis. Simulation studies are presented to illustrate the effectiveness of the proposed controller and its robustness to external disturbances.

Dynamic Positioning of An Oceanographic Research Vessel Using Fuzzy Logic Controller in Different Sea States

A dynamic positioning system is computer controlled system which maintains the positioning and heading of ship by means of active thrust. A DP system consist of sensors, observer, controller and thrust allocation algorithm. The purpose of this paper is to investigate the performance of proportional derivative type fuzzy controller with Mamdani interface scheme for dynamic positioning of an oceanographic research vessel (ORV) by numerical simulation. Nonlinear passive observer is used to filter the noise from the position and orientation. A nonlinear mathematical model of the ORV is subjected to the wave disturbance ranging from calm to phenomenal sea. Robustness and efficiency of the fuzzy logic controller is analysed in comparison with the multivariable proportional integral derivative (PID) and the linear quadratic regulator (LQR) controller. A simplified constrained linear quadratic algorithm is used for thrust allocation. The frequency response of the closed loop system with different controllers is analysed using the bode plot. The stability of controller is established using the Lyapunov criteria.

Initial Design and Analysis of a Compressed Natural Gas Transport System

Offloading hoses/flexible pipes are used to transfer compressed natural gas (CNG) from an intermediate floating storage and offloading unit (FSO) to CNG vessel (GASVESSEL). The floating hoses are subjected to environmental loads that are mainly waves, current, and vessel motions from both the FSO and the CNG vessel. Preliminary design of loading/unloading system and dynamic positioning system has been performed. Dynamic analysis of the loading/offloading hose and positioning analysis of the FSO and CNG vessel have been carried out numerically in this study. It is verified that the designed loading/unloading system and positioning system of the two vessels (FSO and CNG vessel) are able to operate safely under the sea-state Hs = 6 m within the defined ESD1 zone.

Dynamic positioning analysis and comfort assessment for the early design stage of large yachts

In some specific environmentally protected areas, conventional mooring systems cannot be used by large yachts for stationing at anchor and therefore, the adoption of a dynamic positioning system is required. It becomes then necessary to evaluate the station keeping capabilities of a yacht since the early-design stage. Adopting a quasi-static approach, it is possible to perform a standard capability analysis, as commonly done for the offshore industry, obtaining a capability plot as output. However, capability plots are referring to specific wind-wave correlation that are not covering all the possible wave combinations present in a sea area. Here, it is proposed to use a scatter diagram approach for the dynamic positioning analysis of a large yacht, considering the specific sea areas where the yacht shall operate, in order to figure out the downtime period of the DP system per each sea area. The proposed method can be coupled with traditional ship motions analysis, leading to a combination between comfort assessment and DP predictions. In the present work, use has been made of a traditional displacement yacht 72 m long, comparing five different DP system configurations and evaluating an enhanced comfort ranking combining ISO AWI-22834 guidelines for large yachts with ISO AWI-22822 DP analysis.