Since the early 1970s, risk assessments of older Gulf of Mexico platforms have been performed to recommend and implement remedial measures which improve their chances of hurricane survival. This process has become known as platform requalification. Amoco's requalification process consists of two steps: platform screening to identify potential high risk platforms, and risk assessments of these platforms to evaluate remedial measures to reduce the risk. As a result of a Gulf of Mexico platform screening effort, Amoco's most recent assessment work has focused on its South Timbalier 161A platform. The assessment led to implementation of remedial measures in 1991. These remedial measures, aIong with those implemented earlier as a result of previous risk assessments, were tested when the platform received a direct hit from Hurricane Andrew in August of 1992. The successful structural behavior of the platform in this 100 year storm lends credibility to the platform requalfication process.
Hurricane wave height and wave force knowledge was limited when many of the first generation of offshore platforms were designed in the early 1960s. Consequently, the decks of many older platforms are closer to the water than today's designs. Instead of large hurricane waves passing underneath the decks of these platforms, some waves may impact the decks resulting in large overturning loads. This is the main reason why older platforms are at a higher risk than today's designs. Other reasons include the lack of joint cans and the use of K-brace framing, which has less reserve strength than today's more widely used X-brace design1.
Since the early 1970s, Amoco has performed risk assessments of its older platforms. The purpose of these assessments has been to identify and recommend remedial measures to improve an older platform's chance of survival if its relatively low design load is exceeded in a hurricane. The assessments have considered only the structural risk due to environmental overload in a hurricane, and not risks due to boat impact, tire and blast, and human error.
Platform risk assessments consider numerous failure modes including failure of deck legs in bending near the cellar deck, tensile yielding or compressive buckling of brace members, axial pile-soil failure, pile bending failure, and joint failure. The challenge of the early risk assessment work was to predict a complex nonlinear phenomenon with simple linear analysis tools.
The remedial measure which Amoco found to be most economically feasible in those early years was grouting of the jacket leg to pile annulus. This measure was implemented on a number of older Gulf of Mexico platforms. The strength provided by the grout enabled these platforms to survive events such as Hurricane Carmen (1974) with no damage, and contributed to the fact that Amoco has not lost a platform due to a hurricane.
