Garden Banks 388 Deepwater Production Riser Structural And Environmental Monitoring System
Authors: Thrall D. E. , Pokladnik R.L. 
Published: October 2008

A deepwater production rise to be installed in 1995 in Garden Banks 388 in the Gulf of Mexico, has been instrumented with bonded resistance strain gages so that the remaining operational fatigue life can be quantified. The riser supports flowlines from subsea oil and gas wells to a floating combination drilling/production vessel, and thus is fixed at its lower end and fme to move at its upper terminus. The distribution of bending stresses throughout the riser, which is attached to a template at 2096 foot depth and extends to within 150 feet of the surface, reaches a maximum at the bottom.

The riser is instrumented with strain gages, inclinometers, and accelerometers at five locations along its length so that tension, bending, orientation, and motion of the riser can be monitored at these locations, including the bottom joint. Correlation of the riser response data with the excitation or environmental data, including wave motion, current velocities, wind velocities, and vessel mooning tensions and positions is enhanced by acquiring and archiving all data on a single common system having multiple redundant elements for reliability.

This paper describes the production riser structural and environmental monitoring system used on the Garden Banks 388 project.

INTRODUCTION

As the use of permanent floating production facilities moves further offshore into deeper water, the connection between the template wellheads and product pipelines on the seafloor and the production equipment on the surface becomes more challenging. The Garden Banks 388 project employs the longest freestanding production riser, at nearly 1900 feet, to date. This major system component comprises 37 steel joints, most 50 feet long, and 1 titanium joint at the bottom where riser bending is accommodated. The steel joints are nearly 7 feet in diameter, with the outer 3 feet being syntactic foam to achieve near-neutral buoyancy. Integral air chambers in each joint, together with large air tanks at the upper end of the riser, are used to help control riser tension. A tensioning system is also deployed between the 4-pontoon semi submersible drilling/production facility and the upper end of the riser oth to provide additional stability and to maintain coincidence of the two elements for the flowline catenaries.

Continued successful and reliable operation of the production riser requires information on the riser response to the environmental loads. The primary data requirement is for a cumulative fatigue damage model for the titanium stress joint, which is designed specifically to accommodate riser deviation from the vertical. A secondary but still important use of the data is to provide real time information on the riser during the first few years of operation. This information can only be provided by a structural monitoring system installed on the riser, Since the riser is intended tobe in use for 20 years, the reliability of this monitoring system was one of the primary goals in its design.

RISER FATIGUE MONITORING APPROACH

Since the primary contribution to riser fatigue is bending stress, and since riser bending is primarily concentrated in the lowermost joint, connected to the template on the seafloor.