Instrumentation for Monitoring Behavior of Lena Guyed Tower
Authors: Lamb W.C. , Hibbard H.C. , James A.L. , Koerner W.A. , Rothberg R.H. 
Published: October 2008

The Lena Guyed tower project offered the opportunity for the acquisition of performance data which would be of benefit to the development of technology for more challenging applications in deeper water and harsher environments. The authors describe the instrumentation system built for the Lena guyed tower which provided valuable data used to assist in the tower installation, and which is currently providing performance data for operational monitoring and research objectives.

INTRODUCTION

Planning for a major instrumentation system for the Lena guyed tower began early in the Project. The goals identified for this system were:to aid in the safe, efficient installation of the tower and its guylines,to monitor the structural integrity of certain critica1 components during platform operation andto provide data in support of environmental and structural research activities.

The early decision to instrument the guyed tower made it possible to consider the installation, operational, and research goals together and to integrate the required equipment into the tower structure during fabrication. By treating the instrumentation system as an integral part of the guyed tower, the required sensors and electronic systems could be designed for multiple uses, minimizing duplication of equipment for the three sets of goals.

Instrumentation Goals

Installation - During installation, components of this system were used to verify ballasting conditions, to measure guyline tensions and tower attitude, and to observe the electrochemical polarization of the tower structure by the sacrificial anodes.

Operational - Throughout the operational life of the Lena guyed tower, the same or similar system components will be used to monitor the structure. Two critical structural components have been selected for monitoring the guylines and major buoyancy chambers. Although the guyed tower is designed to be safe even if several neighboring guylines are missing, it is desirable that loss of pretension due to accidental damage or other reasons be detected immediately so that remedial action can be initiated. Similarly, the twelve buoyancy tanks and eight buoyant legs, which together support about 75% of the deck payload, are instrumented to detect water intrusion. Even though the foundation has sufficient reserve capacity to safely carry the entire deck load in moderate seas, operating personnel should be made aware of any flooding that occurs, so that corrective action can be planned to maintain the desired reserve of foundation capacity.

In addition, the history of extreme displacements of the helical flexible wellhead flowlines is monitored to determine if deflections beyond the design conditions have occurred and replacement is necessary. Finally, electrochemical monitoring will continue, to provide early warning of any problems with the corrosion protection system.

Research - The primary research goal is to confirm and refine the analytical tools and methods used to predict the behavior of guyed towers subjected to wind, wave, and current loads.