Assessment of Platform Cognac Using Instrumentation Data
Authors: Gur Turel , Choi Jae Woong , Soyoz Serdar , Abadie Randall J. , Barrios Andre C. 
Published: July 2009

Hurricanes in recent years produced waves significantly larger than those considered in the design of offshore oil and gas facilities in the Central Region of the Gulf of Mexico (GOM). As a result, the Minerals Management Service (MMS) issued a Notice to Lessees (NTL No. 2007-G27) [1] for the assessment of the essential facilities in the central GOM and the adjacent transition regions. Platform Cognac, which is located in the central region, is subject to the requirements of the NTL. The code-specified metocean criteria for the region were updated by API in Bulletin 2INT-MET [2]. The MMS required assessments to utilize either the updated metocean criteria provided in Bulletin 2INT-MET, or updated site-specific metocean data.

An assessment of Platform MC194A (Cognac) was performed in accordance with the procedure outlined in API 2INTEX [3] and API RP2A [4]. The platform has experienced 10 feet of seafloor subsidence. The combined effects of larger wave height and seafloor subsidence result in 7.7 feet of deck wave inundation at the lower deck, generating significant wave impact force.

Per API 2INT-EX, the acceptance criteria is based on the reserve strength ratio (RSR), which is the ratio of the lateral load capacity to the demand of the environmental loads.

The analyses performed on Platform MC194A (Cognac) were based on the best available information describing the current condition of the platform. The initial dynamic analyses of the platform indicated that the wave loads were amplified by 24% resulting in an RSR estimate of 1.03, which is below the minimum acceptable RSR of 1.2 specified by API. The platform was instrumented to verify the modal periods and the dynamic amplifications in the analyses. The measurements showed that the actual period of vibration of the structure is much shorter than the initial estimates.

The dynamic base shear demand on the platform was re-calculated based on the measured vibration frequencies. The resulting base shear demand was significantly reduced, thus increasing the RSR.

Introduction

NTL 2007-G27 by the MMS requires that all operators with high consequence platforms (exposure categories L-1 or A-1) located in the Central Region of the Gulf of Mexico assess their platforms with respect to the requirements of API Bulletin 2INT-EX and API RP2A. On behalf of Shell Exploration and Production Company, MMI Engineering performed an assessment of Platform MC 194A (Cognac) to assess its conformance to these requirements. This work was completed in two phases. In Phase 1, an analysis was performed of the structure based on its current condition using available inspection data. This analysis indicated that the structure did not meet the minimum acceptable requirements. In Phase 2, additional analyses were performed to improve the definition of total load to achieve conformance with the requirements, or to define mitigation measures that would be required to achieve conformance. The Phase 2 results show that the structure does conform to the requirements of API Bulletin 2INT-EX and API RP2A.