Subsea process design guideline for reliability
Authors: Sunde Leif Tinnes 
Published: October 2008

This paper presents a new design guideline, based on a computerized tool for assessment of the reliability and cost of subsea process systems. The guideline and the tool have been developed as a joint industry project. The guideline is based on three predefined generic subsea processing station (SPS) base cases. For each base case, a number of realistic technical and operational alternatives to increase the SPS reliability and availability are studied. The computerized optimization tool has been developed based on an Excel model. The tool has been used to estimate the reliability parameters and the cost of each base case together with all the realistic alternatives (mitigating actions). A ranking list of the various options is provided.

Introduction

Background.

Optimal technical safety and reliability must be designed into new products, and integrated into the design process through all design phases. This is particularly true for subsea systems that are not accessible for normal maintenance and repair, other than through high cost vessel operations. In traditional design processes, safety and reliability aspects are generally not considered until some verification of equipment or components is required. This is usually too late in the system design process to obtain an optimal design. Hence, there is a need for a subsea process design guideline to support the early project development.

During the last years, approaches like "concurrent engineering" and "design for safety and reliability" have been used in some industries to integrate safety and reliability aspects in the design process - from the outset of the concept phase. According to ref. /1/, .."early identification and prediction of regularity (productionavailability) enables the decision-maker to balance regularity aspects against LCC (Life Cycle Cost) and other relevant aspects. Decisions made early in project development have a much greater influence on regularity as well as LCC than those made later. Early activities should therefore focus on optimisation of overall configuration while attention in detail will increase in later phases...."

However, the objectives of the new approaches like "concurrent engineering" and "design for safety and reliability" are not always reached. Ref. /2/ supports this, and states that: "Current RM&S [reliability, maintainability and safety] analysis techniques are too limited and fragmented seen in this perspective, since the findings resulting from the current analysis techniques cannot easily be related to given properties, and the output from the different analysis techniques are strongly diversified."

The reason for this is that the current analysis of production availability and LCC are often too cumbersome and time-consuming to support "design for safety and reliability". Hence, there is a need for a new and more effective design guideline and computerized tools to support the design process.

Objective.

The objective of the JIP has therefore been to develop a guideline that can be used to integrate reliability considerations into the conceptual design process of subsea process systems, and thus representing a pro-active approach. It has also been an objective to develop a userfriendly computerized tool that may be used to assess the reliability and production availability versus cost of the suggested SPS bases cases and alternatives to these.