The Glamis Field in UK North Sea Block 16/21a is operated by Sun Oil Britain Ltd as a subsea step out from the Balmoral Field, 8km to the North East.
The relationship between Balmoral and Glamis is shown in Figure 1. Dedicated single flowlines tie the two production wells and the one water injection well back to the Balmoral template. For process reasons, the production flowlines are heavily insulated to maintain fluid arrival temperatures at the separator above 25 °C (Reference 1)
(Fig. 1 is available in full paper)
Pipeline surveys had revealed that an existing Balmoral insulated flowline had been subject to upheaval buckling movements Similar but more widespread movements were expected on the Glamis lines unless steps were taken to prevent oralleviate the conditions causing the movements. Theoretical predictions based on published analytical techniques confirmed these expectations.
Flowlines 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.
There are several methods for preventing or stabilising buckling movements Only a limited number were relevant to Glamis and of those only the most appropriate are described in this paper
The preferred solution was the hot water flushing technique which was developed specifically for this project. The essence of the method is artificially raising the flowline effective installation temperature so that the differential with the operating temperature is reduced to a value which will not cause compressive loads high enough to result in buckling
Theoretical aspects of the hot water flushing procedure are described in conjunction with details of the offshore operation. The effectiveness of the technique is discussed in terms of the observed pipe movements recorded during the post lay, post hot water flush and post first oil surveys.
Containment or alleviation of upheaval buckling movements can be achieved using either of the two general concepts of cure or prevention By definition, those techniques based on preventioa reduce the compressive loads to below the buckling threshold whereas the cure approach contains the pipe movements when the loads occur
The important difference between the two approaches is that a well designed prevention method should resolve the issue for the full design life However a cure method may require regular survey and refurbishment work Before discussing the alternative methods considered for Glamis the general conditions under which upheaval buckling can occur are described briefly.