Calculation Of Module Reactions In Accordance With Api Rp2A Clause 2.5.6.
Authors: Bunce James W. , Boswell Laurence F. , Taylor Michael L. 
Published: October 2008

Clause 2.5.6 of API RP2A states, ‘Consideration should be given to the effect of deflections on the distribution of load between the platform and supported rigid equipment packages’.

This important clause has been overlooked in the design of a number of major offshore platforms and as a direct consequence. serious strength and serviceability problems have occurred.

The paper begins by describing a method for the analysis of module to support structure interaction which takes account of the practical difficulties of data communication between the designers of the topsides facilities and the designers of the support structure. The method is based upon a matrix formulation which properly considers the relative stiffness of the modules and the support structure.

The paper continues with a review of results obtained from case studies of the Shell-Esso Brent C and the Petrobras PCE-1 platforms. These case studies underline the practical importance of the interaction phenomena - serious errors in the module reactions can occur if the effects of differential deflections are not properly accounted for. Attention is drawn to the fact that the same kind of problem can occur on both concrete gravity and steel jacket supported platforms.

Finally, some basic design guidelines are suggested so that major problems of module to support structure interaction can be avoided.

1.0 INTRODUCTION

A problem facing designers of the topsides facilities modules and the module support frame (MSF) on an offshore platform is to determine the correct distribution of forces acting between the modules and the MSF and the corresponding deflections. The problems arise because the forces in question are usually statically indeterminate so their magnitudes will depend not only on the module weights but also on a number of other inter-related factors including:the stiffness of the module structures compared with the MSF stiffness;the extent and magnitude of fabrication tolerances and shimming errors;the amplitudes and shape of the MSF deflection profile due to self weight, environmental forces. cellar deck loadings etc.

A structural analysis which ignores these factors may give errors in the forces and deflections for the design of the MSF. Since individual module reactions will usually be of the order of several hundred tones, serious local strength problems can arise if they are calculated incorrectly. Moreover, should substantial deflection of the MSF occur during the passage of wave trains through the structure, fatigue problems can arise for the module reactions will fluctuate.

Although the importance of these phenomena now seems to be recognised by the major oil companies. they are not well documented and present codes of practice provide no specific guidance on how they should be treated. In fact, the API RP2A code is the only one that specifically requires any consideration of these effects.

Clause 2.5.6 of the code reads:

'Consideration should be given to the effect of deflections on the distribution of load between the platform and supported rigid equipment packages'.