Decommissioning

Decommissioning Solutions for Offshore Structures Address Reliability, Cost

This paper discusses various best practice technical strategies for decommissioning which can be applied in the Southeast Asia region, and throughout the world. The strategies proposed can result in an estimated operator saving of more than 30% over the entire decommissioning execution expenditure. The processes discussed are included in the typical steps of preliminary design, detailed design, and execution.

Offshore oil rig drilling platform
There are thousands of offshore oil and gas structures approaching the end of their operating life globally.
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The author outlines current decommissioning guidelines and typical practices and explores cost-effective ways by which companies can navigate this multifaceted process. The complete paper provides a case study that illuminates an optimal, cost-effective decommissioning methodology for offshore facilities and structures, which is closely aligned with emerging decommissioning guidelines and regulations and with industry best practices.

Introduction

Decommissioning of offshore facilities will become increasingly important in Southeast Asia. Therefore, the promulgation of standard practices jointly developed by regulators and operators are key to the best outcomes for the environment, affected communities, and owner/operator economics.

Three well-known global conventions address broad guidance of decommissioning activities: International Marine Organization Resolution A.672, the United Nations Convention on the Law of the Sea, and the Oslo-Paris Convention. The coastal state having jurisdiction over the installation or structure should ensure that it is removed in whole or in part in conformity with these guidelines and standards once it is no longer serving the primary purpose for which it was originally designed and installed, or serving a subsequent new use. The most widely accepted international regulations allow members great flexibility in establishing decommissioning guidelines and may establish reefing or other disposal options based on economics and local conditions after proper consideration of safety standards and the marine environment.

The Decommissioning Process

In general, decommission phases may be divided broadly into steps, phases, and cycles. For the purposes of this paper, the process can be divided into the following three phases:

  • Phase 1: Data acquisition, project kickoff, planning, and site survey
  • Phase 2: Well plug and abandonment (P&A), pipeline abandonment, platform and substructures removal, and debris removal
  • Phase 3: Project closeout

Each phase can result in overall liability to the concession operator, considering the regulations imposed onto decommissioning requirements for assets. In this paper, the focus is limited to Phase 2, where alternative selections and approaches to asset removal regarding high-impacs activities can reduce expenditure significantly while still meeting regulatory requirements.

The strategy recommended is to engage all stakeholders in order to achieve the optimal result. Multiple work-breakdown structures (WBS) can be identified, while certain components within the WBS framework can be further selected and analyzed.

The methodology is broken down into separate sections pertaining to different stages of the decommissioning effort. The paper discusses the three highest-impact WBS categories: well P&A, pipeline removal, and jacket and topside removal.

Platform Well P&A

The costs of well P&A are usually one of the highest-cost WBSs. Two well-known methods are used when abandoning platform wells: drilling-rig-based, and rigless. The rigless method requires a much smaller spread of equipment and fewer personnel and provides for much faster mobilization time. In addition to streamlined equipment and reduced mobilization time, the rigless operational personnel are also streamlined and consist of a cross-trained and multidisciplinary crew, including a supervisor, pump operator, two riggers, and a wireline/electric line operator.

The planning and actual abandonment process includes data collection, preliminary inspection, selection of abandonment methods, and submittal of an application for regulatory approval. At this stage, the type of wells (deviated, horizontal, or water-injection wells) can be identified and properly planned for execution.

Pipeline Removal

If pipelines are to be removed from the seabed, this can be performed either by a reverse S-lay exercise or cutting and lifting sections of pipe of equal length onto the cargo deck of a support vessel. Both methods require significant preparations and execution efforts, including the mobilization of a derrick pipe-laying barge or suitable diving support vessel (DSV).

A typical requirement is that pipelines must be flushed, cut, and plugged before abandonment. A dynamically positioned DSV with a saturation diving spread is used to perform the work. This allows for the recovery operations to continue with periodic backloads of recovered pipe when space on the working deck of the DSV becomes limited. It is also a common practice in some areas to allow pipeline to be left in place with the condition that they must be buried 3 ft below the seabed. In this scenario, trenching may be required, or it is possible that the pipelines may have been self-buried in the soft mud in which they are located.

It should be noted that the work scope is dependent on country-specific regulations.

Structure Removal

Topside (Single-Lift vs. Multiple Smaller Lifts). A typical activity in the preparation of topside removal is mobilization of cargo barges and transportation to a fabrication yard. Load spreaders and steel pads to support the point loads of the deck are installed. Tugboats are selected based on bollard pull requirements and transportation analysis. More than one cargo barge may be outfitted depending on the size of the deck and jacket.

Selection of the derrick barge with the right lifting capacity plays a crucial role. Typically, the lifting range may fall within a range from 500 to over 2500 MT. The lifting-strength categories represent significant step changes in mobilization costs and day rates for the charter. Typically, the derrick barge shall be sized and selected to be capable of lifting the deck and the remaining equipment in a single lift to place the structure onto a prepared cargo barge. In other instances, to dissection of the topside into separate lifting modules is more cost-effective.

During the initial installation phase, deck components are sometimes installed in a single lift, or are installed in modules after the main deck is installed onto the jacket. Any equipment on the top deck of the platform installed after the topsides installation must be removed separately and placed on a cargo barge. The deck is then removed by cutting the welded connections between the piles and the deck legs with oxygen-acetylene torches or diamond wire cutting.

The preparation work for a modular deconstruction approach is more extensive and includes severing of members and preparation of modules to be removed. This work can limit the critical path duration of the derrick barge on location and saves significant costs. The modular approach requires cargo barges to be prepared for all key lifts. A generic approach for preparing the cargo barges may be used as smaller lifts are positioned onto the barge. For all small lifts, where possible, the auxiliary or whip line can be used for faster removal operations based on each individual lift weight. The cargo barge or barges transporting the deck and equipment then move to the onshore disposal yard, where individual equipment modules are lifted with cranes from the barge to the yard. Finally, the larger deck components are skidded off the barge into the yard.

Jacket Removal. As a general requirement, the jacket is severed from the seabed and removed in a single lift, but first, the piles are severed at five meters below the mudline. As with the topsides, the selection of the most-effective removal strategy, either single-lift or modular, also shall be considered. The piles may be removed from the jacket legs if they are not grouted to reduce the lift weight. After the main piles are severed, the jacket can be lifted, set on the sea floor, rerigged, lifted again, and secured onto the cargo barge. The cargo barge then transports the jacket to an onshore disposal yard. In the yard, the jacket is cut into small pieces and disposed of as scrap, if a full-removal option with onshore disposal is selected.

Case Study

In the case study detailed in the complete paper, the total decommissioning costs for the typical marginal field could have reached $150 million, considering maximum potential expenditure based on piecewise topsides removal, the tow-to-reef job for the jacket, sectioning and recovering the pipeline with a DSV, burial of the drill cuttings, site clearance, and disposal costs of a reefing option.

The most expensive options are based on tow-to-reef options for jackets and recovery of pipelines. This greatly increases the duration of derrick-barge operations for each platform and duration of DSV operations for pipelines.

Removal of the section and the pipeline-recovery option increases operational duration over 730% from the abandon-in-place option of burying each end. The platform options are sensitive to the number of mobilizations. Each mobilization will increase the difference in cost between the different options.

Once the key WBSs are optimized, following the optimal-approach selection, the decommissioning cost of a typical marginal field can see a cost savings of over 30%. It is important strategically to ensure that the decommissioning methods on offshore facilities are planned in line with regulatory requirements while not overspending.


This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 203250, “Cost-Effective Decommissioning Liability Solutions for Offshore Structures,” by Isara Boondao, Mubadala Petroleum, prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference. The paper has not been peer reviewed.


Technical Paper Synopses in this Series

Decommissioning Solutions for Offshore Structures Address Reliability, Cost

Alternative Method of Planning Decommissioning Reduces Costs

Study Determines Environmentally Superior Decommissioning Options

Stakeholder Engagement in the Decommissioning Process