A New Repair And Strengthening Technique For Steel Offshore Structures
Authors: Williams J.R. , Ma S.Y. 
Published: October 2008

This paper presents a new low cost repair and strengthening method for steel offshore structures which offers potential savings in three areas: weight, installation and inspection requirements. It outlines the evolution and development of the new patented technique (the â??grouted stud/strap connection? from two existing and proven technologies, namely grouted pi/e-sleeve connections and stud friction welding.

A testing programme covering static strength and fatigue behaviour of the connection is described which, on satisfactory completion, led to Certifying Authority approval of the new repair/strengthening method and the associated design procedure.

The above mentioned benefits of the stud/strap technique are quantified in the paper by comparison with currently available grouted repair methods.

INTRODUCTION

Over the last twenty years, a number of alternative techniques have been used (or considered) for the repair and strengthening of underwater members on offshore structures, All techniques, however, have certain limitations in terms of applicability. This may be due to the time/complexity of the subsea operations which are involved during the installation of the repair or to uncertainty in the structural integrity of the repair once it is in-place. â??Repair' in this context is defined as the rectification of damage in such a way as to reinstate the structure to its original form and condition. â??Strengthening' is taken to imply that, in addition to reinstatement, the structure is enhanced to provide extra load capacity.

One option which has become well established in recent years for the repair/strengthening of tubular members is the grouted clamp technique. It is now widely accepted and a track record of successful installations has been established, Development effort has now turned to improving and optimizing the grouted repair method in order to simplify future installation. The â??grouted stud/strap connection', described in this paper, is such a development.

CURRENT APPLICATION OF GROUTED CLAMPS

The origin of grouted clamps lies in a combination of mechanical clamping and pile/sleeve connections. The mechanical clamping technique uses a cylindrical clamp to surround the existing member or joint and relies on the friction capacity of the interface for transfer of load away from the area requiring repair, The friction capacity is created by the application of an external force perpendicular to the friction face. Clamps are installed in two or more sections and bolts are used to join the sections and create the radial force. It is now normal practice to use long (i.e. greater than 600mm) bolts since these have demonstrably better performance when the repair is subject to fatigue loading.

Grouted clamps take a similar form to mechanical clamps except that an annular space is created between the clamp ody and the original member. This annulus is subsequently filled with cement grout, The main advantage of this technique is the virtual elimination of any problems with tolerances or lack-of-fit, since these are accommodated within the annulus dimension.

There are a variety of grouted clamp types and the selection is made on the basis of load carrying requirements, All types rely on the friction/bond capacity of the steel-grout interfaces