Usfos, part of the Sesam suite of programs, has been successfully used by Zentech in their reassessment study of fixed offshore platforms.

Typical plastic utilisation plot of a jacket at collapse load.

Zentech, Inc. is a Houston-based engineering consulting company which has been providing diverse engineering services to the offshore industry for over 20 years. Zentech was awarded a contract by Pemex Exploración y Producción to reassess the service life of fixed offshore platforms.

Pemex owns and operates several fields in the Bay of Campeche, located in the south Gulf of Mexico, for oil and gas production. Many of the fixed offshore platforms used here were built during the 1970s and 1980s and have already exceeded their design service life. In order to meet the growing demand for oil and natural gas, it is necessary to extend the service life of these platforms by at least another 15 to 30 years. To achieve this, thorough and systematic reassessment studies need to be conducted leading to identification of any structural weakness.

Twenty eight platforms were identified for this reassessment study. The platforms cover the categories of Drilling, Production, Gathering and Habitation. Depending on the particular activity, Pemex specifications assign different levels of acceptable reliability and reserve strength ratio. In order to determine this ratio, one must evaluate the ultimate strength of the platform. The ultimate strength is determined using a detailed finite element model of jacket, piles and deck structures and by performing a nonlinear progressive collapse analysis technique, also commonly known as ‘pushover analysis’. This is accomplished by using the state-of-theart computer program Usfos.

In order to capture the ultimate strength behaviour of these jacket structures accurately, the models should include geometric non-linearity and material plasticity and soil-pile interaction. They should also include information on damage and deterioration obtained from inspection reports, such as dents, bents, cracks etc. All these features are available in Usfos and are employed very effectively in this study.

Another important aspect for these types of analyses is the inclusion of joint flexibility and joint capacity in the accurate determination of ultimate strength. The joint flexibility and capacity modelling is performed using the latest available procedures of Usfos which uses non-linear load-deformation curves. There are two joint strength models available in Usfos: the API model or the MSL Model. The API model, developed prior to the MSL model, uses the joint capacities based on the API equations. The MSL model is based on a semi-empirical method developed from a database of tubular joint tests and FE analyses. The MSL P-d curves are based on the mean values of the joint
capacities. Both models use load-deformation (P-d and M-q) curves developed considering the joint geometry and material properties.

Author: Sharma Mukkamala, Zentech