ABSTRACT

Designers of offshore structures have recently realized the advantages of cast design instead of welded constructions, but the lack of cast steels with high toughness, high strength and good we1dabi1ity has prevented from cast modules becoming more general. Low profit margins have emphasized the importance of reducing the life cycle costs of drilling platforms, and this has led to a revaluation of the most important constructional details like nodes in the leg constructions. This article introduces three new offshore steels and describes their properties. Their high toughness values are based on high purity which has been made possible by the application of vacuum melting tehnology. A yield strength up to 690 MPa is obtained by solution strengthening. Low carbon content increases plasticity of the lath marten site during welding, and therefore improves we1dabi1ity. The design temperature of the new steels is -40°C, and all the properties are specified up to the wall thickness of 200 mm.

INTRODUCTION

The increasing importance of the maintenance and repair costs of oil drilling platforms against a background of low profit margins, and the necessity for these structures to function reliably, especially in arctic fields, have emphasized the importance of material selection and design. For example, welded design in leg structures utilizing tubes and other standard wrought products has given rise to increasing fatigue problems. Fatigue fracture is the most common type of failure in the welded nodes of leg constructions. Several factors in combination influence the fatigue life of a welded node:

  1. high stress concentrations

  2. residual stresses in the HAZ

  3. decreased properties in the HAZ

  4. small linear defects or cracks in the fusion line

  5. lamellar tearing

These difficulties can be reduced by better design and improved materials and fabrication. However, particularly in arctic environments, the construction must be 100 ~ safe against failure. Reliability and total life cycle costs can be optimized by the use of cast modules which are fabricated and inspected in controllable circumstances at a factory. Underwater inspection and related costs form an essential part of the total yearly production costs and may amount to as much as 20 %of the productional costs. Cast nodes are considered to have such a long fatigue life that there is obviously no need for NDT inspections at regular intervals.

An example of this kind of advanced design is a cast node where the fatigue life is effectively increased by decreasing the local stress concentration factors (SCF). The local SCF at the weld toe can be lowered from the typical value of SCF = 3 to a value of SCF = 1.4 /1/ The reduction in SCF obtained increases the expected fatigue life by a factor of 100 based on the tests carried out by Det norske Veritas 12, 3, 4/. Furthermore, the joint welds are removed out of the stress hot spots to less critical areas of the joint.

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