Risk Based 'Optimum' Inspection for FPSO Hulls
Authors: Xu T. , Bai Yong , Wang Mark , Bea R.G. 
Published: October 2008

ABSTRACT
The increase in deepwater exploration activity has generated increased use of the Floating Production/Storage Offloading Systems (FPSO's). Converting existing tankers is, in many cases, more economically feasible and faster than building new FPSO's for the same purpose. For conversion of tankers to FPSO's, inspection and subsequent fitness for purpose assessment are crucially important.
The objective of this paper is to present the principles and strategies of in-service inspection programs for FPSO's. The paper summarizes the technical basis for three levels of inspection strategies:probability-based inspection method,risk-based inspection method, and'optimum' inspection method.

INTRODUCTION
FPSO's have many attractive features including relative low cost, large working area, large water surface area, and good stability and floatability.
FPSO's in many cases, are converted from existing tankers. It is therefore important to have a rational and reliable inspection method to provide information and knowledge concerning the proposed, present, and future integrity of FPSO's.
FPSO inspections should be focused on:determination of condition of structural elements and structural system,disclosure of defects (design, construction, operation, and maintenance),assurance of conformance with plans, specifications, guidelines, rules, and quality requirements,disclosure of damage, anddevelopment of information to improve design, construction, operation, and maintenance procedures.
FPSO inspections have several levels of intensity:general (global conditions),specific (basic aspects of defects and damage), anddetailed (precise descriptions of flaws and other items of operation and maintenance concern).
FPSO inspections should be life-cycle oriented and include quality assurance and control measures in:design,construction,operation,maintenance, andaccidents / casualties.
FPSO inspections should be full-scope and include quality assurance and control measures of the structure, equipment, facilities, and personnel.
Research in inspection has been conducted in regulatory organizations, universities, and other leading organizations. [1, 2, 3, 4]. Some important guidelines have been developed for offshore inspection practices.
The objective of this paper is to present the principles of development of in-service inspection strategies for FPSO's.

PROBABILITY BASED INSPECTION
A number of limitations of in-service inspections have been identified [3, 4, 5], especially the significant uncertainties in design, fabrication and damage detection, as well as the adequacy of examining only a limited amount of the structural elements. The usefulness of probabilistic models to deal with uncertainties, as well as Bayesian models has been recognized. The probabilitybased inspection method is thus developed to include Bayesian analysis, and probability-based inspection planning.

Bayesian Analysis
The bulk of research on Bayesian analysis in engineering application was first conducted in early 1970's. This approach, defined as the probability of detection (POD) updating method, was widely applied in aeronautical engineering systems, such as airframes, gas turbine engines [6]. In the late 1980s, an alternative Bayesian approach was developed as event updating using First Order Reliability Method (FOSM) in the offshore industry[7]