Energy Implications and Environmental Analysis of Oil Rigs Decommissioning Options Using LCA Methodology

Abstract

The decommissioning of offshore oil rigs presents complex environmental challenges and opportunities, particularly in the context of energy transition goals and marine ecosystem protection. This study applies a Life Cycle Assessment (LCA) approach to evaluate the energy and environmental impacts associated with two different decommissioning approaches: full removal and partial removal. The analysis considers greenhouse gas emissions, energy consumption, material recovery, and long-term waste management. The study demonstrates important energy savings through the recovery and recycling of steel, which offsets energy-intensive operations such as cutting and marine transport. In addition, the analysis underscores the potential of integrating decommissioned infrastructure into offshore renewable energy systems, highlighting synergies with circular economy principles and the decarbonization of offshore operations. The findings highlight the importance of site-specific assessments and integrated policy frameworks to guide environmentally sound decommissioning decisions in offshore energy infrastructure. The analysis shows that full removal results in 14,300 kg CO₂ eq emissions during cutting and transport, compared to 3090 kg CO₂ eq for partial removal. Meanwhile, steel recycling generates environmental benefits of −3.80 × 10⁶ kg CO₂ eq for full removal and −1.17 × 10⁶ kg CO₂ eq for partial removal.

1. Introduction

In recent years, the global oil and gas industry has faced increasing pressure to responsibly manage the end-of-life phase of offshore infrastructures, as a growing number of platforms approach decommissioning due to resource depletion, aging, or regulatory mandates [1]. Currently, over 7500 offshore oil and gas installations exist worldwide, many of which are in mature basins such as the North Sea, Gulf of Mexico, and Southeast Asia [2]. In the North Sea alone, more than 1200 structures are expected to be decommissioned by 2040, representing a significant technical, economic, and environmental challenge [3]. The process is not only cost-intensive, estimated to range from EUR 30 to EUR 40 billion only for Europe, but also full of uncertainties related to the assessment of environmental consequences and regulatory evolution. It is important to outline that decommissioning activities request the use of enormous amounts of energy for dismantling, cutting, and transport but, at the same time, allow for energy savings through material recovery, such as steel recycling, avoiding primary production emissions [4,5]. This study introduces a novel contribution by presenting the first site-specific LCA of offshore decommissioning in the Mediterranean Sea using primary industrial data from ENI on the Clara platform [6]. Furthermore, it adopts the EF 3.0 method for impact assessment and conducts sensitivity analyses on key parameters such as steel mass and recycling efficiency.

Offshore oil rigs consist of many heavy and complex components, including topsides, jackets, and subsea pipelines, made primarily of steel and alloys and requiring high amounts of energy for their management, either during their operational life, for removal, or for reuse as, for example, renewable energy infrastructures, such as wind farms or hydrogen production units [7,8,9]. Any decommissioning choice also needs to consider its energy-related consequences.

The topside is the above-water structure that houses oil and gas processing equipment and supports operational infrastructure such as drilling rigs, helidecks, and cranes. Supporting the topside is the jacket, a lattice framework fixed to the seabed, often anchored via massive footings, which include pile clusters for stabilization and a drilling template through which wells are accessed. Surrounding the base, a pile of drill cuttings composed of rock fragments and drilling fluids accumulates as a byproduct of extraction activities. In addition, pipelines are installed for the transportation of extracted oil and gas to onshore facilities. The scale of a platform has a very wide range of variation: from small platforms weighting around 200 tons to larger topsides that can exceed 50,000 tons, with gravity-based structures that can reach several hundred thousand tons, depending on environmental conditions and operational requirements.

Decommissioning strategies vary significantly depending on regional policies, technical constraints, and environmental sensitivities. The most common approaches include (i) total removal, which involves dismantling and transporting the entire structure to shore; (ii) partial removal, where parts of the structure—typically the topside and upper jacket—are removed, leaving the lower section in place; and (iii) toppling, in which the structure is rotated and laid on the seabed to serve as an artificial reef habitat [10,11]. Each of these options carries different trade-offs in terms of energy use, emissions, marine habitat alteration, and long-term liability.

To support evidence-based decision-making and repurposing scenarios, Life Cycle Assessment (LCA) is increasingly being applied to evaluate the energy implications and environmental impacts of decommissioning options from a cradle-to-grave perspective [7,12,13]. LCA allows for the quantification and comparison of impacts such as greenhouse gas emissions, energy demand, material recovery, and effects on biodiversity, offering a more comprehensive framework than traditional cost–benefit analyses [14,15]. By integrating environmental criteria into the evaluation process, LCA provides a critical tool for optimizing decommissioning strategies in line with the principles of sustainable development, energy transition goals, and circular economy.

The data presented in this study were derived from documentation provided by ENI, related to the decommissioning activities of the Clara platform, an offshore oil rig located in the Mediterranean Sea. The materials contained within the platform and therefore recoverable for waste treatment processes were estimated based on two different decommissioning alternatives. The recovery process for these materials, including structural components and processing equipment, is critical in evaluating the overall environmental and economic impacts of decommissioning operations.

The Clara NW platform is located at geographic coordinates 14°01′23.862″ E, 43°48′07.723″ N, positioned in a water depth of 75.9 m. The estimated removal weight of the integrated deck and wellhead module is approximately 956 tons. The jacket substructure has an estimated total removal weight of 1580 tons and consists of four vertical legs and four foundation sleeves anchored to the seabed. The platform uses dry-tree drilling technology and is equipped with essential processing units, utility systems, and accommodation modules. Natural gas is extracted from four wells and processed through individual gas–water separators—one for each production string. After separation, a continuous glycol injection system is applied downstream of the separators to prevent hydrate formation before the gas is transferred to the Calipso platform.

The platform employed dry-tree drilling technology and includes processing units, utilities, and accommodation modules. The Clara NW platform is supported by a four-legged steel lattice substructure (jacket) that houses conductors, casings, and risers and four foundation sleeves for seabed anchoring. The integrated deck includes the essential processing units required for gas extraction, water separation, and transfer operations. Natural gas is extracted from four wells, processed through dedicated gas–water separators (one per production string), and then treated with a continuous glycol injection system downstream of the separators to inhibit hydrate formation.

This paper explores recent advancements in decommissioning strategies that consider both the energy and the environmental implications with a particular focus on the role of LCA in guiding sustainable decision-making.

2. Literature Review

The decommissioning of offshore oil and gas platforms has become a pressing issue worldwide due to aging infrastructure and growing environmental concerns. Traditionally, three main decommissioning options are considered: total removal, partial removal, and toppling. Each of these alternatives presents unique challenges in terms of environmental impact, energy consumption, safety, and cost-effectiveness. While much of the existing literature pertains to the North Sea and the Gulf of Mexico, this study incorporates Mediterranean-specific parameters such as shorter transport distances, milder sea conditions, and Italian regulatory standards. These aspects are embedded through foreground data from ENI and national guidelines.

Recent studies have shown that total removal, although often the default regulatory requirement, can have high environmental and economic costs due to the intensive use of vessels and heavy-lift equipment, the high amount of energy necessary, and the processing of waste materials [7,16]. Partial decommissioning and toppling, which leave parts of the structure in place, have gained attention for their potential benefits in terms of cost savings and the preservation of artificial reef habitats, but they raise concerns about long-term environmental risks and regulatory compliance [17,18,19].

To quantify and compare the environmental impacts of these options, Life Cycle Assessment (LCA) has emerged as a robust and widely accepted methodology. LCA provides a framework to evaluate direct and indirect environmental effects throughout the life cycle of decommissioning—from material extraction and transport to cutting, lifting, waste treatment, recycling, and overall energy used [20]. Studies in the Gulf of Mexico, the North Sea, and Southeast Asia have demonstrated that using LCA can guide better decision-making and policy development [8,21].

In recent years, LCA methodology has been applied also to address decommissioning options in the Adriatic Sea, where Italy has a dense network of offshore gas platforms. These structures have been built starting around 1960 and many of them are reaching their end of life. The Italian Ministerial Decree of 15 February 2019 introduced comprehensive national guidelines for the decommissioning of offshore platforms and associated infrastructure. This decree ensures the quality of environmental impact assessments and requires annual reporting on assets authorized for decommissioning. It promotes alignment with environmental sustainability, circular economy principles, and the reuse of materials and components where feasible [22].

The northern Adriatic Sea represents a particularly sensitive area due to shallow waters, extensive marine biodiversity, and its proximity to densely populated coastal zones. The study by Colaleo et al. [23] applied LCA for the total removal of the “Viviana 1” platform and found that most environmental impacts were associated with vessel operations and fuel production that dominated the energy inputs. However, the recycling of steel and other recoverable materials provided significant offsets in terms of avoided emissions and energy use.

Other researchers have proposed frameworks for evaluating reuse and repurposing options as viable alternatives to full decommissioning. Zanuttigh et al. [24] developed a multi-criteria approach that integrates LCA with economic, social, and regulatory considerations for the sustainable reuse of platforms in the Adriatic Sea, including conversion into sites for aquaculture, offshore renewable energy production, and eco-tourism. The PLaCE project (Platform Lifecycle and Circular Economy) explores similar themes, promoting integrated and sustainable reuse strategies for offshore assets based on circular economy principles and energy production [25].

These initiatives, alongside evolving regulatory frameworks and improved LCA methodologies, demonstrate that decommissioning is not solely a technical issue but a multidimensional issue that involves environment, energy, economic, and social factors. The incorporation of LCA into national decommissioning policies and project evaluations represents a key step toward minimizing environmental impacts and identifying more sustainable offshore platform end-of-life pathways.

3. Materials and Methods

Life Cycle Assessment methodology is applied to evaluate the environmental effects of two different decommissioning strategies. The attributional approach is used and the reference normative (ISO 14040-44:2021 [26]) followed.

3.1. Goal and Scope

The objective of the analysis is to evaluate, under the environmental perspective, two decommissioning strategies, partial and total removal, respectively, identified in the following sections as Alternative A and Alternative B. The analysis refers to the Clara platform, an offshore oil rig located in the Mediterranean Sea. These two strategies realize the same objective of performing an oil ring decommissioning activity, compliant with regulations and normatives.

The reference flows include all the processes and activities related to the decommissioning of the two strategies in terms of resources used, energy needs, transport emission and fuel consumption, waste produced, and potential material recovery.

3.2. System Boundaries

The system boundaries included in the study encompass cutting activities, the transportation of removed materials, and consequent waste treatment (End of Life, EoL). Different scenarios were considered for materials. Background processes were used to model EoL processes, while foreground processes were used to describe and model cutting activities and transportation. Foreground data were related to the energy consumption of cutting machines and the emission factor and fuel consumption of vessels used to transport decommissioned materials.

3.3. Environmental Impact Categories, Models and Indicators

The selection of impact categories must be comprehensive to cover all relevant environmental issues related to the analyzed system. The Environmental Footprint 3.0 Method (EF 3.0) was used to calculate environmental impact (

Table 1. Impact category selected from the Environmental Footprint 3.0 Method.

Impact Category	Unit
Acidification	mol H+ eq
Climate change	kg CO2 eq
Ecotoxicity, freshwater	CTUe
Particulate matter	disease inc.
Eutrophication, marine	kg N eq
Eutrophication, freshwater	kg P eq
Eutrophication, terrestrial	mol N eq
Human toxicity, cancer	CTUh
Human toxicity, non-cancer	CTUh
Land use	Pt
Ozone depletion	kg CFC11 eq
Photochemical ozone formation	kg NMVOC eq
Resource use, fossils	MJ
Resource use, minerals and metals	kg Sb eq

). It includes the normalization and weighting factors published in November 2019. Concerning the allocation procedure of the EcoInvent v.9.10 datasets, the following model is suggested: ‘allocation, cut-off by classification’.

3.4. Life Cycle Inventory Analysis

Data were derived from ENI documentation relating to the decommissioning activities of the Clara platform [22]. Materials contained in the platform and consequently sent to waste treatment processes refer to Alternative A and B.

Table 2. Material quantity and cutting resources for Alternative A and B (decks, equipment, and structures).

Alternative A				Alternative B
Origin	Quantity	Unit	Material Type	Origin	Quantity	Unit	Material Type
Deck	342	ton		Deck	342	ton
	300.96 (88%)	ton	Steel		300.96 (88%)	ton	Steel
	34.2 (10%)	ton	Stainless steel		34.2 (10%)	ton	Stainless steel
	6.84 (2%)	ton	Cement		6.84 (2%)	ton	Cement
Equipment	276.5	ton		Equipment	276.5	ton
	207.37 (75%)	ton	Steel		207.37 (75%)	ton	Steel
	41.47 (15%)	ton	Stainless steel		41.47 (15%)	ton	Stainless steel
	27.65 (10%)	ton	Cast iron		27.65 (10%)	ton	Cast iron
Piping	196.2	ton		Piping	196.2	ton
	137.34 (70%)	ton	Steel		137.34 (70%)	ton	Steel
	19.62 (10%)	ton	Stainless steel		19.62 (10%)	ton	Stainless steel
	19.62 (10%)	ton	Nickel alloys		19.62 (10%)	ton	Nickel alloys
	19.62 (10%)	ton	Plastic		19.62 (10%)	ton	Plastic
Structure	133.5	ton		Structure	133.5	ton
	106.8 (80%)	ton	Steel		106.8 (80%)	ton	Steel
	20.025 (15%)	ton	Stainless steel		20.02 (15%)	ton	Stainless steel
	6.675 (5%)	ton	Aluminum		6.675 (5%)	ton	Aluminum
				Jacket	1411.6	ton
					1333.5 (94%)	ton	Steel
					78.1 (6%)	ton	Cement
Cutting phase				Cutting phase
Deck	24	h		Deck	24	h
Jacket	102	h		Jacket	516	h
Energy consumption	6930	kWh		Energy consumption	29,700	kWh

presents materials recovered from the decks, equipment, and structures and the related cutting times and energy consumption;

Table 3. Material quantity for pipeline for Alternative A and B.

Alternative A			Alternative B
Material Type	Quantity	Unit	Material Type	Quantity	Unit
Steel + anod	9.7	ton	Steel	1493	ton
Cement	12.5	ton	Cement	2022	ton
Polyethylene coating 159	0.3	ton	Polyethylene coating	44.25	ton
			Anod	16.35	ton

contains the materials recovered from the pipeline. Data related to the transport phase are presented in

Table 4. Transport data for Alternative A and B.

Alternative A			Alternative B
ton·km	1012.5	ton·km	ton·km	160,902	ton·km

.

The data relating to the disposal phase are shown below (

Table 5. EoL treatments for recovered materials.

Material	Treatment	Efficiency
Steel	Recycle	88.1%
Steel	Not Hazardous Landfill	11.9%
Aluminum	Landfill	100%
Plastic	Landfill	100%
Cement	Landfill	100%

); for each material category, one EoL treatment was defined (recycling or landfill) according to average data provided by a company. Uncertainty is related to the real conditions of components, both corrosion and contamination with oil, and especially contamination due to marine fauna and flora, like mussels, algae, etc. The total efficiency of the treatment and separation phase for steel is assumed to be equal to 99.8%, assuming 1 kWh of electricity consumption for each ton of material treated [27]; datasets to model steel recycling were derived from ref. [28] and already used in a similar assessment [23], where a recyclability efficiency equal to 88.1% was assumed for the steel contained in oil rigs.

Landfill disposal is assumed for aluminum, plastic, and cement; these materials present high recyclability rates for industrial waste. A company provided the real waste flow related to materials that belong to a very specific and unusual category.

3.5. Results

Environmental burdens were calculated using the software SimaPro 9.6, applying the EF 3.0 Method.

The impacts contained in

Table 6. Cutting and transport impact for Alternative A and B.

Impact Category	Unit	Cutting and Transport A	Cutting and Transport B	EoL Treatment A	EoL Treatment B
Acidification	mol H+ eq	3.80 × 101	1.90 × 102	−4.35 × 103	−1.75 × 104
Climate change	kg CO2 eq	3.09 × 103	1.43 × 104	−1.17 × 106	−3.82 × 106
Ecotoxicity, freshwater	CTUe	4.16 × 103	1.95 × 104	−8.68 × 107	−3.47 × 108
Particulate matter	disease inc.	6.57 × 10−4	2.85 × 10−3	−1.06 × 10−1	−4.33 × 10−1
Eutrophication, marine	kg N eq	1.40 × 101	6.61 × 101	−8.38 × 102	−7.30 × 102
Eutrophication, freshwater	kg P eq	9.10 × 10−2	4.27 × 10−1	−4.81 × 102	−1.87 × 103
Eutrophication, terrestrial	mol N eq	1.54 × 102	7.28 × 102	−1.06 × 104	−4.18 × 104
Human toxicity, cancer	CTUh	1.15 × 10−5	5.37 × 10−5	−3.37 × 10−1	−1.42 × 100
Human toxicity, non-cancer	CTUh	5.96 × 10−6	2.88 × 10−5	−4.49 × 10−3	−9.30 × 10−3
Land use	Pt	2.89 × 103	1.34 × 104	−1.43 × 106	−3.86 × 106
Ozone depletion	kg CFC11 eq	4.67 × 10−5	2.15 × 10−4	−1.66 × 10−3	−4.35 × 10−3
Photochemical ozone formation	kg NMVOC eq	4.51 × 101	2.12 × 102	−3.53 × 103	−1.36 × 104
Resource use, fossils	MJ	3.96 × 104	1.82 × 105	−9.16 × 106	−3.65 × 107
Resource use, minerals and metals	kg Sb eq	1.63 × 10−3	8.15 × 10−3	−7.92 × 100	−3.30 × 101

are related to the transport and cutting activities performed in each alternative. Environmental impacts or benefits are characterized by a uniform distribution for all the impact categories included in the study. This means that the following proposes a global discussion that applies to all categories of impact. Due to a higher number of ship trips and cutting activities, Alternative B indicates a higher impact in all the impact categories considered. In fact, Alternative B indicates the recovery of an additional 1411.6 t of material from the jacket and about 3500 t from the pipeline. This indicates an increase in cutting activities (from 102 h with Alternative A to 516 h with Alternative B) and transport (from 1012.5 t*km with Alternative A to 160,902 t*km with Alternative B).

Between cutting activities and transport, the first have the highest impact, and the diesel burned in the engine to power cutting machines represents the main contributor for all the impact categories considered in the study.

EoL impact presents negative values for both the alternatives, i.e., benefits for the environment. The benefits related to the recycling of steel, i.e., the avoided impact obtained by the production of secondary raw materials, determine an overall benefit on the environment for both of the alternatives (A and B).

This result is mainly guided by two factors: the quantity of steel contained in the platform (average value of 80%) and the high recycling efficiency assumed for steel recovery, which is equal to 88.1% [23,27]. This means that for each kg of steel retrieved from the decommitted platform, 0.881kg of secondary raw material is produced. The impact, in fact, has a negative value, e.g., it represents a benefit for the environment. This is valid for all the impact categories included in the study. All the impact categories included in the analysis present the same environmental profile, in which the production of secondary raw materials, assumed as avoided products, is the main benefit contributor.

Table 7. Cutting, transport, and EoL treatment impact for Alternative A and B.

Impact Category	Unit	Total A	Total B
Acidification	mol H+ eq	−4.31 × 103	−1.73 × 104
Climate change	kg CO2 eq	−1.17 × 106	−3.80 × 106
Ecotoxicity, freshwater	CTUe	−8.68 × 107	−3.47 × 108
Particulate matter	disease inc.	−1.05 × 10−1	−4.30 × 10−1
Eutrophication, marine	kg N eq	−8.24 × 102	−6.64 × 102
Eutrophication, freshwater	kg P eq	−4.81 × 102	−1.87 × 103
Eutrophication, terrestrial	mol N eq	−1.04 × 104	−4.11 × 104
Human toxicity, cancer	CTUh	−3.37 × 10−1	−1.42 × 100
Human toxicity, non-cancer	CTUh	−4.49 × 10−3	−9.27 × 10−3
Land use	Pt	−1.43 × 106	−3.84 × 106
Ozone depletion	kg CFC11 eq	−1.61 × 10−3	−4.13 × 10−3
Photochemical ozone formation	kg NMVOC eq	−3.48 × 103	−1.34 × 104
Resource use, fossils	MJ	−9.12 × 106	−3.63 × 107
Resource use, minerals and metals	kg Sb eq	−7.92 × 100	−3.30 × 101

presents the total impact of the decommissioning activities for Alternative A and B. Total values are obtained by adding the impacts of the cutting and transport phase with the impacts relating to the disposal and recovery of waste from the oil rig.

It is possible to observe the environmental benefits related to the production of secondary steel from wastes, represented by numbers one or two orders of magnitude higher than impacts related to cutting and transport activities. As a consequence, the total impact results in environmental benefits (negative value).

3.6. Sensitivity Analysis

The sensitivity check has the purpose of assessing the reliability of the final results. In this case, it is applied to the mass of steel contained in the platform and the efficiency of the steel recycling process, which are strongly influenced by the condition of recovered parts (e.g., corrosion, mussel growth, contamination from marine ecosystems, etc.). Reductions in steel mass of 10% and 15% were analyzed, respectively, illustrated by S1 and S2, to which a reduction in recycling efficiency from 88.1% to 70% was added (S3).

The results show an increase in impact in all the scenarios analyzed, as demonstrated in

Table 8. Sensitivity analysis results for Alternative A.

Impact Category	Unit	S1-A	S2-A	S1+S3-A	S2+S3-A
Acidification	mol H+ eq	24%	32%	50%	56%
Climate change	kg CO2 eq	32%	44%	57%	66%
Ecotoxicity, freshwater	CTUe	24%	33%	41%	48%
Particulate matter	disease inc.	23%	31%	48%	54%
Eutrophication, marine	kg N eq	60%	81%	90%	109%
Eutrophication, freshwater	kg P eq	25%	34%	51%	58%
Eutrophication, terrestrial	mol N eq	25%	34%	51%	57%
Human toxicity, cancer	CTUh	22%	30%	38%	44%
Human toxicity, non-cancer	CTUh	46%	63%	107%	117%
Land use	Pt	39%	53%	104%	112%
Ozone depletion	kg CFC11 eq	40%	54%	131%	136%
Photochemical ozone formation	kg NMVOC eq	26%	35%	52%	58%
Resource use, fossils	MJ	24%	33%	54%	60%
Resource use, minerals and metals	kg Sb eq	22%	30%	52%	57%

for Alternative A and in

Table 9. Sensitivity analysis results for Alternative B.

Impact Category	Unit	S1-B	S2-B	S1+S3-B	S2+S3-B
Acidification	mol H+ eq	15%	21%	45%	50%
Climate change	kg CO2 eq	24%	33%	59%	66%
Ecotoxicity, freshwater	CTUe	15%	21%	35%	40%
Particulate matter	disease inc.	14%	20%	42%	46%
Eutrophication, marine	kg N eq	155%	205%	318%	358%
Eutrophication, freshwater	kg P eq	16%	23%	47%	52%
Eutrophication, terrestrial	mol N eq	15%	22%	46%	51%
Human toxicity, cancer	CTUh	13%	19%	31%	36%
Human toxicity, non-cancer	CTUh	51%	69%	188%	197%
Land use	Pt	34%	46%	147%	152%
Ozone depletion	kg CFC11 eq	35%	48%	198%	200%
Photochemical ozone formation	kg NMVOC eq	16%	23%	48%	53%
Resource use, fossils	MJ	15%	21%	50%	54%
Resource use, minerals and metals	kg Sb eq	13%	19%	47%	51%

for Alternative B. Percentage refers to the difference between basic scenarios (Alternative A and B). The impacts are almost equally sensitive to both the reduction in steel mass and the reduction in the efficiency of recycling processes. When they are combined toghether, the variation becomes significant. The analysis identified these parameters as particularly critical, also due to the uncertainty related to their values.

The analysis did not consider the impact of weather, sea state, or technical failures that could increase the duration of operations and related emissions. These aspects present a very low level of predictability; therefore, they were not included in the sensitivity analysis.

4. Discussion

The implementation of LCA methodology and tools for the analysis of two different decommissioning alternatives of oil platforms has allowed us to quantify the implications associated with these activities in terms of environmental burden and energy efficiency. Decommissioning operations involve the recovery of materials from the platform and their successive dismantling operations. The materials involved are steel (about 80%), aluminum, alloys, cement, and plastics. Based on data provided by a company, steel is recycled, while the other materials are sent to landfills.

The inclusion of benefits from recycling activities that, in environmental terms, correspond to avoiding the production of primary raw materials means that decommissioning operations determine the overall environmental benefit for all the impact categories included and considered in the present paper.

The benefits are registered even if the cutting and transport operations necessary for dismantling of the platforms are included.

The results are strongly influenced by the high percentages of steel contained in the platform and, consequently, by the large amount of secondary raw materials produced. This result, although different from a similar case study already addressed in the literature, is in line with other findings. In the study of Colaleo et al. [23], the percentage of steel is about 21%, and therefore, its recovery, to which environmental benefits are connected, fails to compensate for the environmental damage caused by cutting and transport activities.

The results were also analyzed in terms of sensitivity, which shows significant variability in relation to the variation in the mass of steel and to the efficiency of the recycling process assumed. The recycling process could in fact be influenced by the conditions of the materials themselves, contaminated by exposure below sea level for a prolonged period. It is worth outlining that recycling efficiency has a significant impact on the overall energy benefits. The real condition of components and materials, e.g., the level of contamination, represents an important parameter to measure and then include in future works; its influence on the results, included in the sensitivity analysis as a general cause of recyclability reductions, is significant. Direct measures of the contamination level could represent an element for refining the analysis.

In addition to the quantifiable environmental impacts assessed through Life Cycle Assessment, it is fundamental to also consider several non-quantifiable or context-specific effects that may have significant ecological consequences. Among these, noise and vibrations produced by the engine units powering the generator sets of drilling rigs represent a major source of underwater acoustic pollution, with potential impacts on a wide range of marine organisms. Marine mammals rely heavily on sound for navigation, communication, and foraging and are highly sensitive to anthropogenic noise. Prolonged or intense exposure to low-frequency vibrations and continuous engine noise can lead to altered behavioral patterns, stress responses, auditory masking, and even physical harm [28,29]. Fish populations, benthic invertebrates, and zooplankton have also shown evidence of behavioral disruptions, migration delays, and impaired reproduction linked to underwater noise exposure. While regulations increasingly demand Environmental Impact Assessments (EIAs) to account for noise during offshore operations, the transient yet intense nature of sound during decommissioning—especially during cutting, lifting, and material processing phases—necessitates more detailed modeling and real-time monitoring to mitigate acoustic disturbance.

Moreover, the discharge of polluting gases from internal combustion engines remains a concern for localized air quality and greenhouse gas emissions, particularly in low-dispersion marine zones. The management of onboard liquid waste—such as glycol, lubricating oil, and platform drainage water—also poses risks of marine contamination if containment and treatment systems fail. Similarly, greywater from purification plants and solid waste akin to urban refuse must be managed to avoid discharge into the marine environment. Waste streams originating from drilling activities, including contaminated muds and cuttings, may contain heavy metals and hydrocarbons with ecotoxic potential. These factors, although often outside the direct scope of LCA models, underscore the necessity of integrating qualitative and precautionary measures into environmental management frameworks during decommissioning. Proactive noise mitigation strategies—such as using quieter equipment, acoustic shielding, or operational timing restrictions during sensitive biological periods—can significantly reduce the risk to marine fauna and should be embedded in best practice protocols [9,30].

5. Conclusions

The present paper quantifies the environmental impact of two different alternatives for the decommissioning of offshore oil rigs. The LCA approach was applied to evaluate the environmental consequences of full removal and partial removal. The analysis considers emissions, energy use, material recovery, and long-term waste management.

The results show that both strategies offer net environmental benefits due to high steel recycling rates. However, trade-offs emerge depending on the definition of strategic priorities. If the main goal is a reduction in emissions, partial removal (Alternative A) is preferable due to lower energy use and a cutting time of 3090 kg CO₂ eq versus 14,300 kg CO₂ eq for full removal; if, instead, the maximization of metal recovery is prioritized, full removal (Alternative B) may be more suitable due to greater avoided impacts from secondary steel production, quantified as −3.80 × 10⁶ kg CO₂ eq versus −1.17 × 10⁶ kg CO₂ eq.

The sensitivity analysis demonstrated that the quantity of steel recovered and recycling efficiency are both critical parameters. A decrease in recycling efficiency from 88.1% to 60% led to an increase in impact of up to 66% in the climate change category, showing the importance of ensuring high-quality material recovery. In scenarios where recycling efficiency drops below 60%, full removal still offers environmental benefits, but with substantially reduced margins.

Another aspect that emerged from the study is the importance of considering dismantling delays, particularly those driven by weather, sea state, or equipment failure. While such delays were not quantified in this LCA, they may result in extended vessel operation times and increased fuel consumption, ultimately raising total emissions. A sensitivity scenario including 20–30% longer cutting and transport durations is recommended for future analyses to refine emissions forecasts.

Moreover, non-LCA side effects such as underwater noise, localized air pollution, potential leaks, and waste management inconsistencies (e.g., glycol, lubricants, or contaminated cuttings) can significantly affect marine ecosystems and require integration into environmental impact frameworks.

In summary, the complexity of the operations highlights the need to include aspects normally outside the boundaries of LCA. This would allow us a more complete view of environmental impacts, providing more robust support in decommissioning decisions for offshore energy infrastructure. Total removal could in fact cause significant alterations in the ecosystem formed around the platform over time, which currently cannot be accounted for with the traditional LCA method.