Assessment Of Vermilion 46-A Platform
Authors: Petruska D.J. , Berek E.P. , Ingersoll R.W. , Valdivieso J.B. , Day M.D. 
Published: October 2008

The assessment of aging Gulf of Mexico platforms, some of which have reached or exceeded their original design life, is a timely topic. Mobil operates over 250 platforms in the Gulf of Mexico, of which a third are 20 years or older. Many of these platforms still have significant economic value due to improved reservoir engineering techniques and advanced secondary and tertiary oil recovery methods. Therefore, evaluation of these platforms to determine their fitness-for-purpose is necessary. Hurricane Andrew and its effects on the Gulf of Mexico platforms have added further impetus to this activity.

This paper discusses the assessment of a 1950's vintage shallow water platform, Vermilion 46-A. The original design was based on the 25-year return period environmental conditions. To assess the platform capacity, both linear and non-linear ultimate strength analyses were performed. These results were used to determine the platform response under the full range of loading and the probability of failure. The results were used to determine the fitness-for-purpose adequacy of this platform.

PLATFORM DESCRIPTION

Vermilion 46-A is a 40 pile structure installed in 1956 as two separate 20 pile jackets in 32 feet of water (Figure 1). It was originally a tender assisted drilling platform but today functions as a gathering and treating facility. In 1969, a blowout occurred in one of the wells leaving a 60 feet deep scour bowl. Sea shell was used to infill the scoured soil. The original set of structural drawings was available, indicating the structural framing, member sizes and detailing and foundation design.

The Vermilion 46-A platform consists of a single level deck, jacket and pile foundation. The 52 by 90 ft. deck is composed of cap beams across the piles and stringer beams spanning the cap beams. The jacket consists of two 20 leg structures connected by the deck and a spacer frame above the waterline. The jacket legs are ungrouted. The horizontal and diagonal braces range in diameter from 4 to 8 inches. One half inch doubler plate wraps were used in place of joint cans. The original piles are 18 inches in diameter with an average penetration of 130 ft. Six 20 inch diameter piles were added to strengthen the platform after the blowout. The platform also has four 30 inch diameter conductors.

MODELLING ASSUMPTIONS

The three dimensional structural model of the platform is illustrated in Figure 2. The piles were modeled to their full penetration and conductors were modeled to 200 feet of penetration below the mudline. Non-linear beam-column elements were used to model the piles in the ultimate capacity analysis and the soil was modeled with p-y and t-z curves. Soil properties were developed to simulate both the virgin soil and the shell infill material.

The wall thickness of all jacket members below waterline was reduced by 10% to account for degradation from corrosion [1]. Members above the waterline that were observed to have moderate to severe corrosion were modelled as non-structural elements generating wave loads but not contributing to the strength and stiffness of the platform.