Analysis, Prediction, and Repair of Vertical Wave Fatigue Damage of Conductor Guide Bracing

Abstract

ABSTRACT Fatigue damage has been discovered in the conductor guide bracing of many of Mobil's Gulf of Mexico offshore platforms in water depths ranging from 62 to 432 feet. The damage varied from magnetic particle indications to total failure of the guides. Although the damage was found primarily in the uppermost underwater guides, cracks have been discovered in elevations as low as the -225 foot level. Inspection and analysis revealed the cause to be fatigue due to vertical wave loading. Although generally ignored in the past, vertical wave loading is now recognized as a major factor in conductor guide placement and design. This paper will discuss a simplified fatigue analysis method and various repair methodologies including weld profiling, internal grouting of members, removal of guides, bracing of guides, and the installation of stress grouted clamps. Twenty-eight multiwell drilling / production platforms were analyzed. They were all modern designs installed from 1972-1989 and ranging in water depth from 62 to 760 feet. Potential fatigue life problems were discovered in 23 of the 28 platforms analyzed, indicating that a problem exists in the current method of designing and placing conductor guides. An analysis method is presented that can predict potential fatigue damage before it occurs and without costly inspection methods. Economical repair methods are proposed that can be implemented before fatigue cracking occurs, resulting in significant savings in maintenance costs. This paper also includes guidelines for designing and placing conductor guides on new platforms. BACKGROUND The literature cites problems with fatigue cracking in the first level of submerged conductor guides, primarily in the North Sea as early as 1979. Table One contains a summary of these problems and solutions. The industry began to recognize the problem of fatigue due to vertical wave loading in conductor guides and began to recommend placement of the first submerged guide as low as possible.14 The first problem with conductor guides in Mobil's Gulf of Mexico platforms occurred in 1975 when a -20 foot elevation guide fell out of the East Cameron 286-A platform after a hurricane. The platform was only three years old. Although not noted at the time, it was most certainly accelerated by fatigue due to vertical wave forces. Vertical particle motions had been accounted for in the static design, but their magnitude was negligible. In 1981 cracking was discovered at the -28 foot elevation conductor guides on High Island A 573-A, B and A 572-C platforms. Repairs, consisting of tieing the guides to he conductors were completed, however the guides failed again and were removed. The cracks were surface cracks at the six and twelve o'clock positions, indicating that they were caused by fatigue due to vertical wave forces. Further inspections revealed problems at many of the uppermost underwater guides, some with as little as two years of service. The -27 foot elevation conductor guide at West Cameron 617-A platform completely failed and dropped out of the platform in 1985 after nine years of service.