Measured And Predicted Wave Forces On Offshore Platforms

ABSTRACT Measured wave force data from the Conoco Test Structure in hurricane Carmen have been analyzed on a wave by wave basis to determine the total hydrodynamic forces exerted on the platform. These measurements are compared with calculated total forces. The calculated forces are based on a conventional API wave-current force determination procedure. Observations made on the basis of the Conoco Test Structure data are compared with those from the Exxon Ocean Test Structure. INTRODUCTION During the last 20 years, there have been substantial increases in our understanding of hydrodynamic (wave) forces. These increases in understanding have been accompanied by large increases in wave forces used to design platforms. In the Gulf of Mexico, the first major increment (mid-1960's) was due to increases in design wave heights [1,2]. The second large increment (early 1970's) was due to the addition of storm currents to wave kinematics, and to increases in drag coefficients (to recognize marine fouling effects). The result for a typical 8-1eg platform in 100 feet of water (Figure 1) was to increase the total wave forces by a factor of about 2.8. In the mid-1960's, there were about 700 major drilling and production platforms in the northeast Gulf of Mexico. Since that time, the number has grown to over 3,000. Ten major hurricanes have moved through the northeastern Gulf since 1964. Seventeen platforms have failed: seven in hurricane Betsy (1965) due to wave overloading and one due to a storm-caused collision, three in hurricane Camille (1969) due to storm induced mudslides [3], one in hurricane Carmen (1974), three in Hurricane Allen (1980), and two in hurricane Juan (1985); all due to wave overloading [4]. Of the thirteen major platforms that have failed due to wave overloading, all but one were designed with "25-year" or pre-1964 wave loading criteria [4]. Many of the platforms that were directly in the path of severe hurricanes such as Betsy, Camille, Carmen, Allen, and Juan experienced green water in the decks [1,4,5,6], yet none of them had major failures. The majority of these structures were designed with pre-1970 or "100-year" criteria that did not include hurricane current forces and increased drag coefficients (to recognize marine fouling) (Figure 1). The foregoing experience suggests two questions regarding present levels of design wave forces. The first concerns new platforms: Are the present levels of design wave forces warranted? The second concerns requalifications of existing platforms: Does experience justify present reference level wave forces? In the remaining parts of this paper, we will examine two sets of platform wave force measurements that will help answer these questions. MEASUREMENTS PROGRAMS Since the mid-1970's, the industry has conducted a number of platform environmental and structural monitoring programs. One of the principal objectives of these programs has been to gather data on wind, wave, and current conditions. Another objective has been to gather platform response data, particularly data on local and total wave forces exerted on the structures.