Installation Of Grouted Pile-Sleeve Connections:A State-Of-The-Art Review

Abstract

ABSTRACT This paper identifies the important stages involved in the offshore pile/sleeve grouting process and reviews the problems experienced. Technical advances and improvements over the years have, in certain instances, produced new problems; these are highlighted and discussed. Common problem areas are investigated and the remedial measures used to overcome these are discussed, together with their effect on the final connection integrity. The paper also considers the implications of current design trends towards shorter grouted lengths. For some time now approaches have been made by industry to reassess the relatively large safety factors employed in pile/sleeve grouting design. However, any reassessment and reduction in safety factors should take into account the practical aspects of installing the grouted connections. INTRODUCTION As a Certifying Authority, Lloyd's Register has been involved with several hundred offshore structures, the majority being jacket structures with grouted pile connections. In their capacity as surveyors to Lloyd's Register the authors have, between them, been involved with the on site installation of over 150 of these structures worldwide and have drawn on their experience, and that of their colleagues, to produce the paper. The practices outlined in this paper are based predominantly on North Sea procedures. MIXING AND PUMPING GROUTS To ensure an adequate grouted connection, the annulus between pile and sleeve should be full of good quality cement grout. With the cements and mixing plant commonly used today, the production and pumping of high strength grouts is rarely a problem. Gulf of Mexico practice (1) tends to use grouts with a specific gravity (SG) of about 1.88 (15.7 pounds/U.S. gallon) yielding a characteristic cube strength of 35 to 40 MPa (5000 to 5800 p.s.i.). Typical North Sea specifications utilise grouts with an S.G. of 1.98 to 2.04 (16.5 to 17.0 pounds/U.S. gallon) depending on cement type, which yield strengths in excess of 50 MPa, with 60 to 70 MPa regularly achieved. Mechanical breakdowns during pumping do occur, but with most installation specifications calling for duplication of all major plant, these rarely create delays. Grout Mix Quality Control The quality control of mixed grout is now relatively well maintained, due primarily to the improvements in monitoring equipment fitted to the mixing plant. Most mixers have accurate radioactive density gauges installed, that once adjusted and calibrated by comparison with a pre-calibrated pressurised density balance(2), require little or no further attention. These enable the operator to continuously monitor the mix and, where necessary, make a rapid adjustment to keep within specification. Most offshore grouting operators in the North Sea supply well equipped laboratories with suitably trained personnel capable of making, curing and testing cube (or cylinder) samples in accordance with recognised standards. Once again this part of the overall grouting procedure tends to produce few problems and needs no further discussion