Concept study of modification alternatives for the High Pressure Cap Running Tool

Abstract

This thesis is a result of a request from Aker Solutions regarding a modification project of a high pressure cap running tool. This tool is used to set and retrieve the high pressure cap from the flow line mandrel when the xmas tree is not installed. Aker Solutions has identified challenges concerning the complexity, costs and time associated with the operation of the tool, and come up with four concepts that may solve these challenges. The task of this thesis is to look into the engineering challenges and the life cycle costs of each of the concepts. The goal is to provide a recommendation for which concept Aker Solutions should pursue through the research question: What is the best-suited modification solution for the Aker Solutions high pressure cap running tool?

The four identified concepts that have been analysed and evaluated, seek to solve complexity, cost and time issues concerning the current method of operation, which utilises an umbilical for hydraulic power supply for the tool. The first is a pre-charged accumulator concept, which uses an internal hydraulic reservoir to supply the tool. Concept number two uses a topside hotline to provide the hydraulics. Concept number three uses a subsea hotline from an ROV. Concept number four uses a hotline from a subsea powerpack.

The different concepts are analysed and evaluated against the current method on nine different scored and weighted criteria, based on 36 interviews. The criteria reflect the identified issues and are as follows: feasibility, physical parameters, maintainability, reliability, complexity in use, economic impact, development cost and time, degree of risk reduction and environmental impact. These unbiased and weighted criteria scores, reflect the impression of how each criterion’s criticality are perceived in Aker Solutions as an organisation. The analysis and evaluation utilises a Pugh matrix for pairwise comparison as a decision-making tool and delivers a best-suited concept suggestion.

Based on this analysis and evaluation the Pugh matrix suggests that the pre-charged accumulator concept is the best alternative, with a significantly better result than the opposed concepts. The Pugh matrix does however not consider which of the “+” rated concepts perform best on each criterion in relation to the other concepts in the matrix. This is addressed in a qualitative evaluation where the pre-charged accumulator concept is compared to the other concepts on each criterion and the nuances are highlighted. It further shows that the pre-charged accumulator concept has a lower life cycle cost that mitigates the slightly lower performance on maintainability and development cost and time. The only innuendo is that it does not reduce risk as much as the hotline subsea concepts. This can however be accounted for with a thorough operational procedure.

The conclusion is that the best-suited modification solution for Aker Solutions Subsea high pressure cap running tool is the pre-charged accumulator concept. This contributes with potential savings for Statoil of 72.7 MNOK, increased rig time efficiency of 57.9%, increased redundancy, reduced operational risk and a more eco-friendly solution.