Texts: QUESTION 1
A three-phase separator on a new offshore oil and gas installation with sour fluids is to be constructed from an ASTM 304 18Cr-05Ni stainless steel. The material has a higher proof stress and ultimate tensile strength than carbon steel, as well as a much higher fracture toughness. A thinner wall thickness can then be used for the same pressure service, which has a number of benefits for the design of the installation. Complete the following:
a) The separator vessel will be horizontally oriented with a 1.8 m external diameter and a tan-to-tan length of 2.5 m. The range of normal operating pressures for the vessel is likely to be 10 – 12 barg. The PSV set-point is 18 barg, and the hydraulic pressure test will be completed at 1.5 times the design pressure. The material’s elastic-plastic behavior is described by the 0.2% proof stress of 250 MPa and ultimate tensile strength of 570 MPa. Use reasonable assumptions to confirm the vessel wall thickness that keeps the maximum stress in a safe range.
b) The project manager has asked the design team to confirm through calculations whether the vessel could be made with the same dimensions but using cheaper carbon steel rather than super-duplex, and also to confirm the likely difference in weight. Use the density of carbon steel as 7950 kg.m-3 and the stainless-steel density as 7850 kg.m-3, together with a yield stress of carbon steel of 200 MPa and ultimate tensile strength of 450 MPa. Set out your assumptions clearly and give your recommendation for material selection if the cost ratio for the carbon steel to stainless steel is 1:5.