The Reappraisal of Steel Jacket Structures Allowing for the Composite Action of Grouted Piles
Authors: Tebbett Ian E. 
Published: October 2008

Increasingly offshore operators of steel jacket structures are requiring reappraisal of existing installations. This may be in the light of revised design recommendations based on a better knowledge of structural performance, revised environmental loading conditions, revised operations leading to an increase in topside weight, or to assist in the specification of maintenance and inspection works. This paper discusses an approach which can be taken to demonstrate enhanced structural performance due to the presence of grouted piles.

In jacket structures where piles pass through the main legs and are welded to the jacket at deck level the vertical loading from the deck is shared between the piles and the jacket structure. In a global structural analysis, piles within the leg may be modeled as members parallel to the leg and with equivalent support stiff nesses at or some distance below the mud line. Tubular joint forces along the leg are then checked against conventional design formulae capacities allowing for local can thickness.

Ultimate load tests have indicated that where piles are grouted there can be considerable enhancements in joint strength and fatigue performance due to composite action between the leg, the grout and the pile. Further enhancement can also be demonstrated by completely filling the centre of the pile.

In addition to describing recent research on grouted tubular joints, this paper sets out an appropriate approach to the reassessment of existing grouted structures and the assessment of the scope for improvement of ungrouped structures. Static strength, stress concentration factors and fatigue performance are considered and the trends in experimental results are identified.

The influence of local joint stiff nesses on member loads is also considered and the need to consider the effect of the increased stiffness due to grouting is identified.

It is concluded that where existing structures are ungrouped or incompletely grouted, grouting can be used as a relatively inexpensive method of improving strength and structural performance.

Introduction

Recent experience has drawn attention to the requirement for reappraisal of existing jacket structures for many reasons including:–increased topside dead or operating loads–additional environmental loading due to attachments not accounted for in design–recognition of more severe environmental conditions than considered in design–better knowledge of the performance of structural components–the need to specify priorities for inspection and maintenance programmers–introduction of certification in areas not previously covered

Conventional space frame analysis may highlight areas of a structure which are apparently under strength or of inadequate fatigue performance. If the structure has been in-place for some years without observed deterioration and if the problem is not due to proposed increased loading or structural modifications, the probability of damage in the future may be low and strengthening unnecessary. Further, if the analysis predicts damage for the existing structure and this is shown by inspection not to have occurred, some reserve in strength has not been accounted for. The two most likely reasons for this reserve are the influence of local joint flexibility and joint details on member forces and the presence of grouted piles within leg members.