INTRODUCTION

Part of the contract awarded to Welding Units by Shell for the supply of pipeline flanges to their L09 project included special 16” 2500lb Weld Neck and Swivel Ring flanges. The same flanges had been previously designed and supplied by Welding Units for other Shell projects and on those occasions had used an API type BX gasket and groove due to there being no standard ASME B16.5 arrangement available for that size.

All the previously supplied flanges have operated very successfully but it was suggested by Shell that future supply of similar flanges should incorporate an R type gasket and groove therefore some detailed study into the gasket behaviour was necessary.

THE STUDY

Welding Units developed an initial design using an R67 gasket and groove taken from the 16" 1500lb flange but understood this was preliminary until the gasket and groove could be confirmed suitable to accommodate the extra forces induced by a 2500lb rated flange. We used finite element analysis (FEA) to study the behaviour of the gasket and groove when subjected to the required loads but results gained were difficult to interpret and enable a conclusion to be drawn from because other similar analysis and findings are not well documented. Shell understood this and so to proceed further they agreed to allow us to perform a full scale test of a Weld Neck and Swivel Ring to gain real data that would lead to a conclusion being made on the suitability of the R67 gasket and groove.

The test procedure was to apply bolt pre-tension to the joint in pre-determined stages while being monitored by strain gauges and other measuring equipment. All parts were also accurately measured pre and post test by a co-ordinate measuring machine to record the amount of permanent deformation experienced during load application.

Other sacrificial testing was performed on the flange and gasket material to determine exact stress strain curves for both tensile and compression for use in the FEA. CTOD testing was also performed to provide maximum strain experienced in the test specimen at the point of fracture.

Work is still ongoing but preliminary results of the work are encouraging and the possibility of the R67 gasket / groove being used is still alive. The practical test showed expected material behaviour with the soft iron gasket showing significant deformation due to the high compressive loads but no visual signs of rupture or excessive deformation, while the grooves (plain F65 no clad) showed no significant signs of permanent deformation that would impair future assembly. The FEA work was found to tally with actual results obtained from the test and so it has been used to extend the scenario from bolt up only onto operating / hydrotest conditions.

Final conclusion on suitability depends on interpreting CTOD test results from the gasket material to decide whether maximum strain levels expected within the gasket during its lifetime are sufficiently below the strain in the CTOD test specimen at the point of failure. This will provide a margin of safety and form the final approval.

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