Oil's well that ends well: Recovering value, managing costs and reducing emissions during decommissioning

RORY MACKENZIE, OSSO. 

Oil and gas decommissioning is rapidly expanding across the UK, as North Sea assets mature and the energy transition accelerates. Offshore Energies UK forecasts that about £20 billion will be spent on decommissioning North Sea oil and gas installations over the next decade, with the share of expenditure in the oil and gas sector expected to grow from 10% in 2021 to 19% by 2031. 

This brings with it many challenges. Cost is one of the major impediments to successful decommissioning and, in today’s carbon-conscious landscape, minimizing carbon intensity is also a vital concern. Of course, decommissioning efforts are not confined to offshore oil and gas but also extend to a vast network of onshore facilities and equipment, where the same imperative exists to find new cost and carbon efficiencies. 

The industry has made some progress in driving down the projected costs of this work. After steadily revising its decommissioning cost estimates downwards over recent years, the UK’s North Sea Transition Authority established a target to reduce the total projected cost of UK decommissioning 10% further by the end of 2028. However, after a challenging and unpredictable year, the estimated cost has started to creep back up. So where can essential efficiencies be found to get back on track? 

FROM TRASH TO TREASURE 

Significant waste streams are typically produced during cleaning and hydrocarbon removal and encompass a variety of materials from oil sludge to water contaminated with hydrocarbons, and naturally occurring radioactive materials (NORMs). These must be disposed of in accordance with strict regulatory requirements.  

The standard approach to dealing with this contaminated fluid waste is “skipping and shipping,” where waste is shipped or tankered away and sent for further treatment, or even straight to incineration or landfill. However, this is costly, carbon-intensive, and risks operators writing off significant values of hydrocarbons where the correct application of technology could salvage them. Given these challenges, the inefficient and wasteful process represents a significant obstacle in reducing decommissioning costs. 

This is why the effective deployment of separation technology can be game-changing. Technologies, such as disc stack and decanter centrifuges (Fig. 1), can efficiently separate waste streams into their constituent parts by deploying high-speed rotational forces to distinguish between substances of different densities. This means valuable components can be separated from waste streams, and reused or sold, while the total volume of remaining materials for disposal can be reduced by enormous quantities.   

Fig. 1. - A decanter centrifuge used to separate hydrocarbons, solids and water during decommissioning.

SEPARATE TO SUCCEED 

So, what might that look like in practice? It is important to begin by laying the correct foundations. This involves stringent risk assessments, including a Control of Substances Hazardous to Health (COSHH), to ensure safe operations. A collaborative and consultative approach is key to understanding the specific needs of the operator. Then, the correct separation technology, such as disc stack seperators (Fig. 2) or decanter centrifuges, can be identified, tweaked and tailored to meet the needs of the project and deliver operational gains. 

Fig. 2. A disc stack separator used to efficiently recover hydrocarbons from waste streams.

 Effectively cleaning infrastructure to remove all hydrocarbons often requires large volumes of water; for example, to enable the pumping of oil sludge from storage tanks. The alternative to water is a long, laborious manual cleaning process that could greatly increase project timelines, costs, and risk to personnel.  

For example, we recently worked on a project at an oil terminal in Scotland, aimed at fully decommissioning an oil tank. Following a stringent pre-project process, we leveraged our decanter centrifuge and disc stack centrifuge technologies, to treat and separate over 4,540 m³ of waste sludge that had been pumped from the tank, including NORMs, water, solids, and recoverable oil.  

This enabled the on-site disposal of over 3,060 m³ of water through an effluent treatment plant, saving a seven-figure sum in disposal costs and minimising carbon emissions from waste transportation. Crucially, it enabled the recovery of approximately 1,280 m³ of oil, valued at around £450,000.  

Consequently, the total savings and value recovered from the project are estimated to be in the millions, with only 200 tonnes of waste ultimately requiring off-site disposal at an estimated cost of £120,000. This project offers a microcosm of the global economic and environmental benefits if this was approach was replicated industry wide. Yet with a significant proportion of work also taking place offshore, how can these principles be deployed in offshore environments? 

OFFSHORE OPTIMIZATIONS 

Offshore projects still adhere to the broad principles of minimizing cost and emissions, while ensuring that strict timelines are met. However, limited deck space, restricted personnel numbers, and the inherent nature of working in remote and hazardous environments present some different challenges. 

The oil-to-water ratio (Fig. 3) of wastewater discharged into the sea cannot exceed the regulatory limit of 30 parts per million (ppm). In many cases, the onboard technology can’t separate enough oil from the water to meet this standard, meaning that contaminated water must be transported back to shore for further processing or disposal. This adds additional costs and carbon emissions to the decommissioning process.  

Fig. 3. Operators must meet stringent regulations to remove contaminants like oil from water when working offshore.

 While this is currently standard practice, the same separation technologies and principles can be applied to reduce cost and carbon emissions. With offshore decommissioning facing high volumes of wastewater and severe constraints on deck space and shipping capacity, this solution could also offer several benefits over skipping and shipping. For example, in a recent offshore project, we were able to successfully treat approximately 8,000 m³ of waste fluids and solids, around 79% of it below the 30-ppm limit, so that it could be safely discharged at source. This significantly reduced skip and ship requirements, resulting in notable cost and emissions savings for the operator. 

Reducing offsite disposal also significantly simplifies required planning and enhances health and safety by reducing the need for lifting and handling of hazardous waste. Crucially, treating waste at source and allowing the flushing process to run continuously and concurrently also significantly accelerates the decommissioning process. 

At an earlier stage of the process, managing brine and other by-products produced during offshore well plugging and abandonment (P&A) operations presents similar opportunities. The high salt content of brine can corrode equipment, and when mixed with other substances, carries notable safety risks. Typically, these fluids are transported through pipelines to onshore facilities rather than treated offshore. Yet this merely defers the need for treatment to a later stage, while adding risks during transportation, and potentially slowing down the decommissioning process.  

Deploying separation technology directly on rigs can again effectively address this challenge by enabling on-site disposal. OEUK estimates that 50% more wells required decommissioning last year, compared with 2022, and anticipates this figure to continue climbing throughout the decade, so these challenges will only grow in prominence. We’ve already seen the industry show growing interest in this as a solution to P&A byproduct treatment.  

TOWARDS A CLEANER, MORE COST-EFFECTIVE PHASE-OUT FOR OIL AND GAS

Fig. 4. An offshore platform in the process of decommissioning.

 Of the 280 oil and gas fields in the North Sea, 180 will have ceased production by 2030 and will require decommissioning, Fig. 4. Ultimately, embracing separation technology could reduce the costs, risks and environmental impact of decommissioning across onshore and offshore environments. Treating waste at source would enable the industry to minimize the need for costly and carbon-intensive offsite transportation and disposal while also accelerating decommissioning and recovering valuable products for sale or re-use.

This could create a smart decommissioning process, not only aimed at reducing the economic and environmental costs of disposal, but recovering and recirculating value into the global economy. 

 RORY MACKENZIE is the Business Development lead for OSSO's Production & Decommissioning division. With over a decade of specialized experience in waste management and industrial cleaning, Mr. MacKenzie leverages his technical proficiency and strategic insight to deliver innovative, value-driven solutions for the decommissioning industry.