Failure Paths Identification and Damage Tolerance Definition for Marine Platforms in the Bay of Campeche
Authors: De Leon David , Florentino Saul , De Martinez Gilberto 
Published: October 2008

Abstract
Offshore Marine Platforms have been built in the Bay of Campeche since the 60's. Managers of those facilities have shown a strong interest on keeping the production activities at some of the structures that are close to its lifetime. As a consequence, there is now an important concern about the effect of aging on the structural integrity of these platforms as exposed to environmental loading. A probabilistic approach was developed in order to help to identify the sequence of nodes and level of damage that may be tolerated by the platform before its overall safety is compromised. The referred procedure allows the safety assessment against fatigue and involves the combination of Structural Reliability and Fracture Mechanics concepts. The formulation is applied to the platforms AKAL-N and NOHOCH-B, both located in the Bay of Campeche. Because of the large volume of oil and gas production at risk, a careful and detailed evaluation of these platforms is required. The results show that the structural redundancy degree plays an important role to define the damage level that the platform may tolerate before reaching the minimum limit of the reliability index recommended for its category.

Introduction
Current regulations recognize the fatigue mode as an important failure mode providing design specifications for marine platform (1). The effect of local damage on the global resistance of offshore platforms has been previously addressed (11) and its impact on the platform's lateral capacity has been studied (2). A damage tolerance criterion for welded joints was proposed by using markovian models (6).
Also, reliability tools that probed their effectiveness in aeronautical and structural engineering have been used to develop fatigue design criteria (7,9,14, 15,16).
A technique to identify failure paths was suggested on the basis of the Wirsching's model for nodal fatigue reliability (5). Inspection strategies are suggested on the basis of estimating the fatigue reliability as the platform experiences cyclic loading (3,8,10,12). As a result of the project RISC (Reliability based Inspection Scheduling), coordinated by the UCL (University College London) and where 5 european companies and the IMP took a part, a system to estimate inspection periods based on joint reliability calculations was proposed (13). This system was used to get the nodal reliabilities and then, throughout the estimation of the unimodal bounds for the global failure probability, the global reliabilities were obtained. As the collapse of a marine platform, due to fatigue, is viewed as the progressive failure of a set of joints, a process of assumed incremental damage is used to simulate the structure deterioration. Consequently, the benefitial effect of redundancy on the global safety may be measured. Also, some patterns on the spreading paths of damage to the rest of the structure may be identified. This simulation process provides the reference framework to relate the local damage level to the global failure probability. The reliability indices obtained are relative quantities to represent the criticality degree of individual nodes.