Design and Sea Test of Flexible Riser System
Authors: Umehara M. , Kudo K. , Tsuruoka M. , Yamaguchi T. 
Published: October 2008

With offshore oil development projects advancing into deeper offshore waters, flexible risers suitable for floating oil production platforms are in increasing demand. So, we have developed various pipes and design methods for flexible riser systems. We have recently conducted sea tests in order to confirm the validity of our design method for flexible riser systems and to verify installation technology and measurement technology with ultrasonic device.

1. Introduction

In the present test, We used a flexible riser system of j-inch dynamic pipe. First, we ascertained the movement characteristics, an important design condition, of a floating offshore structure (semisubmersible floater named "POSEIDEN") to which flexible risers are moored. Next, in order to determine a flexible riser configuration suited to the expected movements of the floating offshore structure over a long period, static and dynamic behavior analyses were conducted to establish an appropriate configuration. After design work the flexible riser system was installed to verify the practicality of installation technology, and a one-year sea test was conducted in the Japan Sea known for its severe conditions during the winter season. The usefulness of the design method was verified by comparing the observed and design values. After the sea test, the flexible riser system was recovered and the condition of the riser pipe was investigated. As a result, it was found that the pipe retained satisfactory performance.

2. Flexible Riser System Design

2.1 Design Procedure

The design procedures for a flexible riser system are described below (see Fig. 1).The depth of water, currents, horizontal movement and dynamic motion of the floating offshore structure, and the weight, bending rigidity, etc. of a riser pipe are the main design conditions.A flexible riser system which would meet the design conditions was selected.Design parameters that would provide the desired flexible riser configuration were chosen.Static behavior analysis was performed for the flexible riser system based on the chosen design parameters, to check whether the horizontal movement of the floating offshore structure, could be absorbed.Similarly, dynamic behavior analysis was performed to check whether the dynamic motion of the floating offshore structure could be absorbed.

When the requirements were not met in steps (4) and (5) above, the design parameters were again reviewed.

2.2 Studies of Design Conditions

The design conditions, such as the depth of water and currents in the sea installation zone and the weight and bending rigidity of the riser pipe, were predetermined on the basis of data on the weather and sea conditions and the pipe specifications. The horizontal movement and dynamic motion of the floating offshore structure are very important for design. Therefore, studies were conducted to accurately ascertain these movements.

2.2.1 Horizontal Movement of Floating Offshore Structure

The floating offshore structure was loosely moored by six mooring lines. So, it was moved horizontally by the waves, wind, and currents. In order to determine the behavioral characteristics of the floating offshore structure, investigations were made from land using an electric distancemeter and the odolite.