Abstract

In August 1998, a 10-inch Steel Catenary Riser (SCR) was installed on the Petrobras XVIII semi-submersible production platform, moored in 910 meters water depth in the Marlim field, Campos Basin, offshore Brazil [1]. This was the first SCR ever installed on a floating moored platform.

In order to evaluate and verify the methodologies and to calibrate the numerical models used in the riser, mooring system and platform design, it was necessary to establish a monitoring program for all the relevant parameters (environmental, platform positions and motions, riser loads and stresses at the top and at the touch down point (TDP) and vortex induced vibrations) [2].

The TDP monitoring system was described in Edwards 1999 [3]. In February 2000, the Vortex Induced Vibration Monitoring System (VIVMS) was added for measuring and recording vibrations of the SCR induced by flow and by the motions of the semi-submersible platform P XVIII.

This paper describes the VIVMS, which was developed to measure, at up to four locations along the SCR, accelerations in three orthogonal directions and angular rates in two directions. The system records this information locally in subsea enclosures, transmits it to the surface on command and includes the software to transform the riser fixed accelerations and rates to earth fixed linear and angular velocities and displacements.

Introduction

The measurement of the vibratory response of the SCR without connections to the surface presented interesting challenges among them were:

  • High Precision/Dynamic Range to permit "capture" of the low frequency motions induced in the riser by the slowly varying and wave frequency motions of the P XVIII while not over ranging during VIV events.

  • Ability to retrieve large data files over a wireless link.

  • Ability to attach Vibration and Inclination Subsea Measurement Units (VISMU) to the riser with a Remote Operated Vehicle (ROV).

  • Endurance-maximization of the time between battery changes.

Measurement Objectives and Constraints

The requirements for the SCR VIVMS were prepared by the Petrobras R&D Center and were as follows:

  • Measure the static inclination in the catenary plane with an accuracy of at least 0.05 degrees.

  • Measure the dynamic inclination of the riser in an orthogonal to the catenary plane with an accuracy of 0.05 degrees.

  • Measure the static azimuthal orientation of the VISMU with respect to the earth.

  • Measure the transverse displacements and velocities of the riser in and orthogonal to the catenary plane.

  • Provide data at two (2) samples per second.

  • Provide an ability to locate the VISMUs anywhere along the catenary with an ROV.

  • Provide the orientation of the VISMU on the pipe without external measurements.

  • Provide an ability to access the data recorded by the VISMUs at the surface and to assess the "health" of the units.

  • Remain submerged for at least 30 days acquiring data at three-hour intervals for periods of 5 minutes transmitting full time histories of the vibration data to the surface every 3 hours.

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