Upheaval Buckling: A Practical Solution Using Hot Water Flushing Technique

Abstract

ABSTRACT In the development of the Glamis Field, a sub sea satellite to Balmoral, measures were required to avoid the expected problem of upheaval buckling of the long hot insulated flow lines. Following review of various solutions, Sun opted for the novel technique of boosting the temperature of the Balmora 1 produced water and pumping it through the Glamis flow lines as a means of inducing a tensile pre stress. The technique proved successful and could be considered to have a wider application in other satellite developments where similar water supplies are readily available. INTRODUCTION Sun are operators of the Glamis Field in UK North Sea Block 16/21a which has been developed as a Sub sea step out 8km South West of the Balmoral Field (Ref 1). Glamis comprises three satellite wells - 2 production and 1 water injection - tied back to the Balmoral template by individual flow lines. The two production flow lines are insulated to maintain separator inlet temperatures above 25°C. Preliminary analysis indicated that upheaval buckling could occur. Several methods of preventing or stabilizing buckling movements were considered before a decision was made to use the hot water flushing technique which is the subject of this paper. The sequence of events from the evaluation of theupheaval buckling movements, through selection of the hot water flushing technique to the offshore operation are described in detail. The effectiveness of the method is assessed from the results of the post hot water flushing, post trenching and post first oil pipe movement surveys. BACKGROUND Flow lines buckle when the net compressive load in the pipe exceeds the bending stiffness and the frictional restraining force. Vertical movements occur if the pipe's preferential lateral mode of movement is restrained and the compressive load is able to overcome the weight of the pipe and cover weight/resistance. The compressive load is a combination of the internal pressure effects and the temperature induced expansion forces. Experience with an existing Balmoral (l6/21b-12) flow line revealed upheaval buckling movements. The limited extent of the observed movements was at variance with theoretical analysis which indicated that the buckling should be much more widespread on this and other flow lines where no buckling has been observed. The conclusion was that first order analytical techniques were too conservative. The Balmoral experience did however make it clear that some means of preventing or mitigating the effects of upheaval buckling was necessary for Glamis to avoid exposing the flow lines to the risk of trawl gear impacts, over stressing and, in the extreme, pipe crimping.