Field Measurement of Platform Behavior, Comparison With Design, and Long-Term Monitoring of Responses Stability After Three Years of Service
Authors: Ansquer P.F. , Antalovsky S.A. , Hjelde Erik F. , Agnello Giuseppe F. 
Published: October 2008

Field data from structural, geotechnical and wave measurements made on the concrete gravity platform TCP2 of the Norwegian FRIGG field are disclosed in this paper. Data recorded during three years are interpreted and compared to design values. The stability of invariant parameters has been followed and demonstrated in view of monitoring the long term behaviour stability of the platform.

The measured behaviour differs from the design behaviour and exhibits that design assumptions were conservative. Particularly during this three-year period the platform experienced very severe storms with waves up to 25.7 m, a value close to the maximum design wave (100 year wave: 29 m). It is shown from field data how a bridge connection to a neighbouring platform may influence the actual behaviour compared to the design one and must be taken into account to understand the measured behaviour.

It is finally shown that supposed invariant parameters like natural frequencies and others are stable on a long term basis, and how these invariants are intended to be used for the long term stability monitoring.

INTRODUCTION

Amongst all the efforts deployed on the Frigg Field in trying to understand and monitor the behaviour of platforms, the case of TCP2 is an example of the results which can be obtained after several years of operation.

At the origin of the project, the objectives of the instrumentation were settled as follows :

Short term objectives:to measure real dynamic behaviour and compare it with the behaviour defined by the computation modelto obtain a clear description of the behaviour in order to improve and readjust the computation methods, particularly for the foundation.

Long term objectives:verification of fatiguere-estimation of the extreme wave and determination of corresponding extreme platform responses.

The above objectives met the Norwegian Petroleum Directorate regulations on E (Environmental) and P (Platform) data measurements.

Finally, during the course of the project a third sort of objective was introduced. It appeared of interest to use the results together with those already obtained from regular inspections with conventional non-destructive methods. The main aim was to obtain, from instrumentation, the possibility of verifying the absence of any major defects in the structure or its foundation.

The instrumentation of TCP2 can be broken down into three categories (Fig. 1) :Measurement of the environmental excitation : wavesMeasurement of the structural responses : shaft deflection, accelerations, strainsMeasurement of the foundation responses : shallow and deep pore pressures.

Several other instruments complete the instrumentation of TCP2 (Ref. 1).

SOME FIGURES ON THE PROJECT

TCP2 is a 3-shaft concrete platform designed by CONDEEP. Its apparent weight is 165000 metric tons, and it is installed in 103 metres of water.

A total of 56 channels of Environmental and Platform data are linked to a digital computer acquisition and recording system. The operation is around the clock, with routine recordings of 20 minutes every third hour and, according to environmental conditions or platform responses, this rate is increased.