In 1995 a study was undertaken to predict the likely condition of the presently not operated Forties 32′ subsea pipeline. As part of that study a preliminary probabilistic reliability assessment was performed. The study found that there is scope to re-use the pipeline at reduced operating pressures, and the methods and findings of that reliability study are discussed and presented. The vital importance of reliable as built data becomes evident and is still relevant as in 20 years time our successors will be asking similar, questions of pipelines presently being installed.
The Forties 32′ subsea pipeline was one of the earliest northern North Sea pipelines. It provided safe and reliable transport of crude oil from the Forties and other North Sea fields from 1975 until 1992 when it was replaced by the new 36′ pipeline and mothballed with treated seawater, as it remains to this day.
The pipeline was first inspected by intelligence vehicle in the summer of 1988, when severe internal corrosion was observed. This was re-confirmed by subsequent internal inspections carried out in spring 1989 and autumn 1989 when the decision was taken to embark on a campaign of de-rating and to provide a replacement pipeline This paper presents the methods and findings of a detailed but preliminary probabilistic reliability analysis made by the authors to establish the technical feasibility of re-using the 32′ pipeline.
The principal characteristics of the pipeline are presented in Table 1 below.
Table 1 - Description of the Forties 32′ MOL (available in full paper)
The first step in assessing the possibility of re-using the pipeline was the establishment of a team comprising representatives from BP, BP's alliance partners Brown and Root and AMEC, British Gas On-line-inspection centre (OLIC), and Andrew Palmer and Associates.
The structure of the assessment team benefited from the new alliance and incentivised type contracts that BP is developing with its support companies and there was a close and completely open co-operation between each, which allowed the best possible realisation of the objective, to advise as to whether to proceed to a further stage or not.
There are two principal stages to performing a reliability analysis, these are:
Data Acquisition and Analysis
Reliability Modelling and Interpretation
The single most important aspect of any reliability analysis is the quality of the information upon which it is based In this respect, the pipeline industry is fortunate because the high quality requirements and historical legislative requirements that must be satisfied to be able to operate a pipeline result in a substantial body of ‘as built’ data and material certification.
The data acquisition and analysis work concerns primarily three data sets, inspection data, ‘as built’ data and operational records, where the objective is to find physical parameters and processes that will allow the best engineering model to be selected for the subsequent reliability modelling.