Abstract

Goal of this project is the development and realization of a software and hardware system for the in-situ movement reconstruction of a top-tensioned marine riser, an enabling technology that aims to improve the performance of the dynamic positioning system and of the BOP control system.

During drilling processes a subsea riser is subject to external forcing due to vessel motion, subsea currents and internal forcing given by the mud flow in the riser and the drill head. Excessive riser vibrations may lead to structural damage and dangerous operative conditions. As a result, it is common practice to use a riser monitoring system which can estimate the riser state and communicate it to an on-board control unit to help engineers making the right decisions. Nonetheless, the riser state is currently computed by systems available on the market by using as unique inputs the upper and lower flex joint inclinometers and the telescopic joint stroke: the objective of this study has been to improve the reliability of state of the art systems by adding distributed measuring point on the riser string.

The precise knowledge of the position of each riser joint allows the determination of the vessel location without relying on the currently used satellite or acoustic technologies. Furthermore, the subsea wireless communication integrated in the riser joints can be used as emergency or additional mux cable for the BOP control system to guarantee increased reliability of the BOP operations. The same technology can be used also to perform distributed measures along the riser string, such as the sea current at various water depth.

Introduction

The riser monitoring system proposed in this paper is organized into subsystems which implement the fundamental functions of sensing, raw data processing, communication, riser dynamic data processing and visualization. Fig 2 shows the system layout, whereas Fig 3 illustrates the main functions and correspondent subsystems. The system is characterized by a subsea assembly and a surface assembly. The subsea assembly is a chain of nodes (pods) installed inside the buoyancy modules wrapped around the riser segments. Each segment is equipped with a node close to the top flange and a second node close to the bottom flange. Each node comprises the electrical and optical devices to measure the local lateral linear accelerations and angular velocities of the riser and thermodynamic state variables of the node (namely temperature, pressure and humidity). The chain is designed such that the data collected by the nodes are sent to the surface assembly processing unit, which computes information on riser dynamic motion for the engineers. The data communication from each node up to the control unit is accomplished by transferring the data from the deepest node up to the vessel node by node. As the information moves upwards the data sampled by a node are added to the existing data flow and the overall size of the data package increases.

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