Pressure Grouting: An Improved Method of Offshore Structure Grouting

Abstract

ABSTRACT A new method of grouting the legs of offshore oil platforms has been evaluated with respect to resultant grout quality, economics and reliability. The experience gained from more than 40 actual grouting operations in the Gulf of Mexico was considered. A scale model of a typical jacket-piling assembly was constructed. The model was constructed from clear plastic material that would allow visualization of the process. A series of tests was conducted by displacing water in the annulus with compressed air and introducing a Class A cement slurry into the annular space. These tests were observed and recorded on color movie film. The results from the scale model tests and from field operations show that a high quality grout column can be obtained by the new method and that the process is feasible from an economic and operational standpoint. INTRODUCTION The methods of grouting the legs of offshore oil structures that are commonly used today inject the grouting material into the bottom section of the legs near the ocean floor. This procedure requires that grouting lines extend from the surface to the bottom of each leg. The bottom of the leg must also be sealed. Generally, inflatable subsurface packers or grout seals are used. As the grouting material is pumped into the bottom of the leg, the water inside the leg is displaced through a surface vent. The injection is continued until grouting material of the desired specification is noted at the surface vent. In some situations, divers are used to remove the grout lines and packer inflation lines. Satisfactory results are not always achieved with these conventional methods because:The sealing system could sustain damage during the installation of the platform that would not allow it to support the full column of grout. This would result in incomplete filling of the leg with grout.The injected grouting mixture could overrun static water in the leg and leave water pockets or water channels. This overrun water would signify cantle reduce the strength of the grout in those local areas.The platform jacket might have sustained unknown damage that would allow the grouting material to leak out after placement. A new method of grouting has been developed that will overcome these potential problems, that will offer substantial economic savings in most cases and that promises improved reliability. This paper discusses the results of a scale model study of this method, describes the field experience gained to date and briefly describes the operational procedure. This method is called the "pressure method". The details of the method have been previously disclosed in U. S. Patent No. 3,601,999. The authors believe this to be the first published report concerning the laboratory and field performance of the method. PURPOSE" OF MODEL Since the inside of a platform leg could not be observed during the grouting operation, it was necessary to build a transparent scale model to demonstrate the viability of the pressure grouting method.