Assessments Of High-Consequence Platforms: Issues And Applications
Authors: Craig M.J.K. , Digre K.A. 
Published: October 2008

An API task group has developed a process for the assessment of existing platforms to determine their fitness for purpose. This has been released as a draft supplement to API RP 2A-WSD, 20th edition1. Details and the background of this work are described in a companion paper2.

The assessment of a platform's fitness for purpose involves firstly a definition of the platform's exposure; and secondly, an evaluation of the platform's predicted performance relative to the assessment criteria associated with that exposure.

This paper deals with platforms in the high exposure category. That is, platforms whose potential failure consequences, in terms of potential life loss and environmental damage, are significant.

The criteria for placement of a platform in a high exposure category are explained, as are the performance criteria demanded of these high exposure platforms. In the companion paper, the metocean assessment process and associated API-developed acceptance criteria are highlighted. This paper addresses primarily ice and seismic loading assessments and associated API-developed criteria, which are based on over thirty years of successful offshore operation and field experience, as well as extrapolation of land-based performance criteria. Three West Coast, USA production platforms are used for illustration.

EXPOSURE CATEGORY

‘Exposure’ comprises threats to life safety and to the environment. The platform assessment criteria discussed in this paper deal with that small subset of the industry's domestic production platforms whose loss of serviceability would have both severe life safety and pollution consequences.

A manned, non-evacuated platform category has been defined as a condition in which a platform is actually and continuously occupied by persons accommodated and living thereon, and it is not intended that they be evacuated during an environmental design event1? Assessment of a manned platform in seismic prone regions is suggested to be triggered when on average five or more persons are present on the platform at any one time3. This implies that some of the smaller production platforms offshore California, which are lightly manned or have no sleeping accommodations onboard, and which also have little potential for pollution, do not fall into the high consequence exposure category.

To categorize a platforms pollutive potential as having an unacceptable impact on the environment requires the evaluation of a number of factors. Will the producing wells flow naturally in the unlikely event of the failure of one or more of their subsurface safety valves? Is there normally a large amount of oil stored on the platform? Is the local marine environment particularly sensitive? Is local cleanup equipment likely to be unavailable or ineffective? If the answer to most of these questions is yes, then the platform may be considered one with the potential for "Significant Environmental Impact"4. Note, some production platforms offshore California should not be classified as having ‘significant environmental impact’ potential, due first to the inability of the producing wells to flow freely without the assistance of downhole pumps, second to low onboard oil inventories, and third to the small heavy oil leakage potential from the export pipeline in the event the pipeline or riser is severed.