Materials Problems in Offshore Platforms

ABSTRACT Tubular joints in Gulf of Mexico fixed platforms must undergo local plastic strains to carry their design loads, placing severe demands upon the material. Failures in ordinary steel, and criteria for joint design and materials selections are discussed relative to static strength, fatigue resistance, notch toughness, weldability, homogeneity, and economy. Current practices and an applicable steel specification are presented. INTRODUCTION Template-type permanent platform, such as the one shown in Figure 1, are the backbone of the offshore oil industry. When installed In open waters of Gulf of Mexico, these platforms are subjected to wide variations in loading conditions which place unique demands on the structure and the materials of which they are built. Loading history typical of selfcontained drilling and production platform is shown in Figure 2.(1) The frequency of occurrence of large lateral loads is somewhat exaggerated in this illustration. A typical design storm is one that is expected to occur on the average of only once per hundred years. About 95 percent of the time the sea conditions are not capable of producing a wave that would cause more than ¼ the design load. During the latter periods, the static vertical load may stress the beams and trusses of the superstructure fairly close to their static allowable; however, the legs and piles of the structure are at a fraction of their design load, and in compression. There is very little stress in the template or jacket-the underwater portion of the structure which ties the piles together and resists lateral forces. Furthermore, the jacket id highly redundant; there is a multiplicity of more or less independent structure systems to carry applied loads and the loss of one or two members will not cause failure of the structure. During hurricanes the loading condition change substantially. It is during such periods that the structure is subject to large lateral loads. See Figure 3. The tubular joints of the jacket are highly stressed and must perform satisfactorily under these severe (but rare) loading conditions. This paper will place particular emphasis on considerations for the selection of structure steel for use in the tubular joints. Johnston's state-of-the-art review(3) shows that six years ago there was already considerable empirical information on the static strength of tubular joints. Based on these tests data and field experience, it was known that tubular joints required some kind of reinforcement. Since 1960, designers have used gusset plates, negative eccentricity, extra thickness in the jacket leg, or some combination of these in new design. As a result of intensive research by the offshore industry, sufficient information as available upon which empirical design procedures can be based to assure adequate static strength. In addition, there is a good basic for defining the required mechanical properties-including notch toughness-of the steel used in construction. A well engineered structure requires that a number of factors be in reasonable balance. These factors, which will be discussed relative to the selection of material, are:Static Strength;Fatigue Resistance;Notch Toughness;Homogeneity;Weldability; andEconomics. While fabrication/inspection is not within the scope of this paper, the importance of this factor should be clearly recognized.