INTRODUCTION

Recent research into the behaviour of Duplex steel under cathodic protection has revealed a possibility of crack initiation and propagation from a point of high tensile strain exposed to the environment that could lead to catastrophic failure in pipeline and related components. This finding has a significant effect on many existing and future subsea installations where Duplex steel is used for critical parts.

THE STUDY

ConocoPhillips’ new Ekofisk development in 2006 included an 18” NB 64.9Bar subsea line made from Duplex steel. This line, and all connectors within, had to be designed considering the findings of the new research work in order to satisfy the requirements of DNV HISC II project evaluations and obtain a license to operate.

Taper Lok® connectors were chosen by ConocoPhillips for use on this line due to their usual superior strengths and accommodating new design requirements for maximum strain concentrations was a new concept that required investigation. In order to investigate the chosen Taper Lok® connector design for areas of possible excessive strain we used 2D axis-symmetric finite element analysis (FEA). The design analysis was split into two parts:

a) consideration of linear material properties to find any area that could exceed 80% yield.

b) for areas that did exceed 80% yield, investigate maximum strain level in the same area considering non-linear material properties.

The ultimate acceptance criteria for maximum principal strain was 0.4% over total section thickness but a maximum peak of 0.6% in an area less than 5% section thickness.

The direction of the strain was also a consideration because an area under dominant compression was not likely to rupture and allow crack propagation and so could be treated differently to an area of high tension.

The initial analysis of the Taper Lok® connector showed areas where we expected to see potential high stress / strain concentrations, i.e. sharp corners, gasket, weld bevel area, but none of these highlighted regions were unable to be reduced by some simple minor geometry change or inclusion of larger radius.

The FEA allowed simple changes to the geometry to be made and re-analysed very quickly so we soon had a compliant design ready for manufacture. The analysis was made very accurate by dividing the flanges into zones and creating a very high mesh density in those zones of interest whilst the other unaffected areas of the flange where relatively coarse.

The Taper Lok® connector turned out to be ideal for the application and new design requirements due to the relatively low bolt pre-tension not creating very high concentrated loads, large area gasket evenly spreading the contact force and no intended flange rotation/deformation during bolt tensioning. The long sweeping hub neck of the Taper Lok® connector allowed stresses to flow evenly from the pipe into the flange body rather than hitting a sharp corner at the flange face and so intensifying the stresses.

In addition to the FEA work performed to ensure all potential high stress/strain
concentrations had been removed, the following disciplines were implemented:

• selection of suitable steelmakers and forgemasters able to meet the stringent microstructure (ferrite content and grain refinement) requirements for forging supply in the selected grade – ASTM A182 F60.
• maintenance of a 'sterile' environment and equipment to ensure that the material remained uncontaminated at all times during the in-house manufacturing and handling processes (including shipment of each individual part for in new wooden cases for pickling and passivating).
• coating of all connectors with a thermally sprayed aluminium (TSA) coating to prevent exposure of the Duplex steel surface to the environment during service.

All work performed by Welding Units was reviewed and accepted by ConocoPhillips and DNV - the flanges subsequently being given license to operate.

The outcome of this work has provided valuable input to the recently published DNV Technical report No. 200503237 – Design Guideline for Duplex Stainless Steel used for subsea equipment exposed to cathodic protection.

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