Piece small decommissioning of substructures

Piece small decommissioning of substructures: Optimizing the cutting strategy to ensure a cost effective decommissioning solution

Eisses, D.A.

Sliggers, P.G.F. (mentor)
Hoving, J.S. (mentor)
Van Dalen, W.R. (mentor)
Kwa, M.E.H. (mentor)

Mechanical, Maritime and Materials Engineering

Hydraulic Engineering

Offshore and Dredging Engineering

2016-06-28

Fixed steel structures in the North Sea that are approaching end of life are increasing in mass and dimensions, this imposes challenges on the current decommissioning strategies. The main challenges arise during removal of the barge-launched substructures. In contrast to the topsides of these large structures the substructures cannot be easily removed by reversed installation methods. For these large structures, a single lift removal is not possible as heavy lift vessels (HLVs) will be limited in lifting height and/or lift capacity constraints. Therefore, offshore disassembly in multiple sections, also known as piece small decommissioning, is required for large substructures from approximately 4,000 tonnes and up. Over one hundred fixed steel structures are currently located in the North Sea with a substructure mass of over 4,000 tonnes and so far twelve have been removed. According to the OSPAR Convention, decision 98/3, all fixed steel structures in the North Sea region must be decommissioned at the end of their useful lives. Business opportunities for Seaway Heavy Lifting lie in this growing decommissioning market of large structures. The objective of the thesis is set to optimize piece small decommissioning of large substructures such that Seaway Heavy Lifting can offer more competitive removal solutions. Reference projects have been analysed in order to obtain a better understanding of piece small decommissioning. These projects have shown that approximately half of all HLV-time is spent on cutting operations. The day rate of the HLV is one of the main expenses in decommissioning projects; therefore it is preferable to minimize the HLV-time. The thesis goal is set to optimize piece small decommissioning of large fixed steel structures (> 4,000 Mt). The focus is put on optimizing the cutting strategy to ensure a cost effective decommissioning solution. The result is that Seaway Heavy Lifting will be able to provide more competitive removal solutions. The focus of optimizing piece small decommissioning is put on decreasing (HLV) cutting time whilst maintaining structural predictability and operational feasibility. The processes and results of all steps for selecting an optimal cutting strategy are combined to form a decision support tool (DST). This tool consists of constraints, guidelines and a data processing file. A generic overview of the DST is given in figure 1. The tool optimizes the piece small decommissioning process by focussing on the most time efficient solution whilst maintaining structural integrity and operational feasibility by selecting the optimal cutting strategy. This is achieved by integrating the results of SACS structural analysis into the decision tool. Selecting the key factors of decommissioning and decomposing them into manageable steps in the DST has decreased the complexity of piece small decommissioning. The DST gives Seaway Heavy Lifting the opportunity to offer more competitive removal solutions for piece small decommissioning. This vastly improves the opportunity to enter the growing market of piece small decommissioning of large structures. The DST presents a novel optimization method in the form of phased substructure decommissioning and is universally applicable in the piece small decommissioning market.

decommissioning
offshore decommissioning
piece small decommissioning
piece meal decommissioning
offshore cutting
phased decommissioning

http://resolver.tudelft.nl/uuid:bb4f9c2b-02a1-4caa-b30b-3267dafa0fec

2021-06-28

Student theses

master thesis

(c) 2016 Eisses, D.A.