Large amounts of oil exists in old shipwrecks worldwide, both as cargo and bunker. This oil will eventually enter the marine environment as the ship hulls deteriorate or as other types of activities affect the wrecks. Oil being a complex mixture of hazardous substances will when released into the marine environment be a source of both lethal and sub-lethal effects to organisms. Costs of an oil spill in the marine environment, including clean-up actions, socioeconomic and environmental costs is often substantial. Sweden has a ten year nationally funded program where oil removal operations on shipwrecks are performed. From a list of 300 potentially polluting shipwrecks, 31 wrecks have initially been selected for oil removal operations. In a first stage extensive gathering of information was performed regarding each wreck, both archive data and in-situ data at the wreck site. Secondly, a risk analysis was carried out. Based on the probability of oil leakage, amount of oil in the wreck and sensitivity of recipients, a prioritization for oil removal operations was made of the 31 wrecks. Based on the prioritization, time of the year and cost of an operation wrecks are finally selected for oil removal operation. So far, since 2017, five operations have been performed.

During 2019 and 2020, two successful oil removal operations were carried out. The ship Lindesnäs wrecked 1957 in a snow storm close to the lighthouse Norra Kränkan on the Swedish east coast with a cargo of kerosene and diesel as bunker fuel. The operation from mobilization to demobilization lasted for 22 days, and 299 m3 of oil and a large ghost net was removed from the wreck. Secondly, Finnbirch, which wrecked in 2006 east of the island of Öland and started to leak oil during the end of 2018, was salvaged in a two-part operation. In 2019, 60 m3 of diesel fuel and lubricant oil were salvaged, during a fourteen-day operation. In 2020, 114 m3 of heavy fuel oil (HFO) was salvaged from the wreck during a fifteen-day operation. The costs per ton of removed oil were far less than cost for oil clean-up operations in Swedish waters. In conclusion, using a risk-based approach for prioritization of potentially polluting shipwrecks and the subsequent proactive removal of oil from shipwrecks is a cost-effective approach to alleviate the problem.

Sources of input of oil into the marine environment are diverse, for example, oil extraction processes, shipping activities, land based sources and shipwrecks (Andersson et al., 2016, Farrington & McDowell, 2004). The total number of shipwrecks in the marine environment worldwide has been estimated to 8600 (>400 GT), where 1600 of these are tankers. In total it is estimated that these wrecks contain somewhere between 2.5 – 20.4 million tons of various oil products (Michel et al., 2005). Sources to shipwrecks are for example accidents, including collisions and groundings, but the major source is ships sunk during wartime, e.g. World War II. Due to deterioration of the ship hull over time, or activities e.g. bottom trawling, construction work or storms or extreme weather the shipwrecks will start to discharge their oil content affecting the marine environment. This process and rate before discharge will depend on factors such as salinity, biological fouling on the hull, construction of the ship, damage during wreckage, sea-floor currents and intensity of the activities mentioned above (Landqvist et al., 2017). Type of discharge can then be chronic discharges, episodic or the one-time release of the total content in the wreck.

Oil can affect the marine environment in different ways, depending on the type of oil, volume spilled, resilience of affected habitats, weather and season at the time of spill and availability of oil degrading microorganisms. Large, one time spills can have acute toxic effects on organisms, hinder oxygen and UV-light transfer into the water column and disable the protection granted by the feathering and fur on such animals against low temperatures and limit their buoyancy in water (Rogowska and Namiesnik, 2010). In contrast, small but continuous oil spills commonly have sub-lethal effects in organisms, caused by the most toxic components in oil, Polycyclic Aromatic Hydrocarbons (PAHs). Consequences in organisms can then be carcinogenic effects, lowered growth and fecundity and lowered taxonomical, genetic and ecological diversity (Lindgren et al., 2017, Rawson et al., 2010). Both types of spills can originate from shipwrecks, through large holes in the hull created by for example a trawl board resulting in a release of the total content of oil or a continuous minor discharge through a small hole due to for example pit corrosion in the hull.

### Risk assessment

As a starting point the 31 wrecks reported in the governmental remit were used, since they were reported as an acute environmental threat (Fig. 1). The wrecks are located at all sides of the coast of Sweden, in water depths between 20- 140 m, salinity ranges from approximately 6- 32 Practical Salinity Units (PSU), and human activities that affect the wrecks differ widely in intensity. In a first stage the wrecks were risk assessed using the tier 1 of VRAKA; estimating the probability of a leakage times the volume of oil in the wreck. That resulted in an initial prioritization list for oil removal operations. In a second stage, corresponding to tier 3 in the VRAKA risk assessment (the tier 2 approach, which combines probability of leakage with amount of oil and distance to the shoreline was not used), oil spill trajectory modelling together with sensitivity of receptors of an oil spill were performed. The Swedish Metrological and Hydrological Institute (SMHI) provides a tool for oil spill trajectory modelling, SeaTrackWeb (Ljungman & Mattsson, 2011). This is used for example by the Swedish Coast Guard when an oil spill is detected and predictions are made for where oil remediation actions are needed. However, these predictions are valid four days in the future and six days back from present time. Therefore, in addition to the usage of SeaTrackWeb the SMHI also performed oil spill trajectory modelling from the wrecks over all seasons. Starting points for the simulated oil leakage was varied from six different years (PADM model data from 2005–2011), with the horizontal resolution of two nautical miles. In total 432 oil spill simulations were performed for each wreck and each simulation lasted for 10 days (Höglund, 2019).

Figure 1.

The 31 shipwrecks that is included in the national program.

Figure 1.

The 31 shipwrecks that is included in the national program.

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Sensitivity of receptors was assessed using the tool Digital Environmental Atlas. In the tool the Swedish coastline is mapped according to physical characteristics. It takes ecological aspects into account, but is mainly based on the difficulty of remediating a specific type of shore. However, areas with high ecological aspects, e.g. Nature 2000 areas gives higher rating in the tool. Altogether, a coastal area can have a rating between 1–18 where high numbers should be prioritized in case of an oil spill accident (Henriksson et al., 2018). In addition, in the tier 3 risk assessment the project used the tool Protected Areas (https://skyddadnatur.naturvardsverket.se/), a GIS based tool hosted by the Swedish EPA that allows a user to easily visualize various forms of protected areas e.g. Nature 20001 and marine national parks, on a map over the area where the shipwreck being risk assessed is located.

Based on the results from the risk assessment and the subsequent prioritization shipwrecks are selected for oil removal operations. Provided that there are no legal barriers that hinders an operation, e.g. liability of an owner to perform the operation and that enough funding is available for the project a call-off from SwAMs framework agreement with contractors is completed.

Due to confidentiality, the prioritization list of the 31 wrecks is not listed. In total five oil removal operations have been carried out since SwAM, in 2016 was given the responsibility for coordinating risk assessment, investigations and recovery operations of oil from shipwrecks in Swedish waters. Two case studies are presented here, of the wrecks Lindesnäs and Finnbirch, providing information regarding the workflow from risk assessment to oil removal operation.

### Lindesnäs

#### Background

The ship Lindesnäs was built in 1949 at Lindholmen shipyard in Gothenburg, Sweden. It was delivered in 1950 to the shipping company Nordstjernan in Stockholm. The vessel weighed 1,265 gross tonnes and was just over 67 metres long. On April 17, 1957, Lindesnäs was sailing between Nynäshamn and Norrköping, on the east coast of Sweden. It was loaded with 1732 cubic metres of kerosene and unknown amounts of diesel for propulsion. There was a snowstorm and the lighthouse Norra Kränkan had been snowed over, with the consequence that the light displayed the wrong colour. Lindesnäs then passed on the wrong side of the lighthouse and ran aground. All 20 crew members was rescued to safety in the lifeboats. Later on the ship drifted off the ground and sank in deeper waters. The wreck has today been lying for 63 years on the sea floor. It is positioned on its port side at a depth of about 70 metres in an area where the sea-floor sediment consists of compact clay. The wreck is located about 10 kilometres off the coast of the town Oxelösund (Fig. 1). During an in-situ investigation in early 2019 using photogrammetry the wreck was shown overall be in good condition and oil was expected to still be present (Fig. 2.).

Figure 2.

In-situ image of the shipwreck Lindesnäs produced using photogrammetry.

Figure 2.

In-situ image of the shipwreck Lindesnäs produced using photogrammetry.

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#### Risk assessment

The wreck was selected for oil removal operation as the risk assessment using VRAKA placed it at second place in the prioritization of the 31 wrecks. This was mainly due to the factors deterioration, military activity in the area and the possible large amounts of oil in the wreck. One wreck has a higher risk level, but it is not possible to perform an oil removal operation today due to lack of funding. Oil spill trajectory modelling using the SeaTrackWeb tool and modelling over the seasons (se methods) showed that an uncontrolled discharge of oil from the wreck will likely end up southwest of the wreck′s position, affecting two Nature 2000 areas – Bråviken and Hävringe-Källskären (Fig. 3). Areas that are listed as prioritized in Digital Environmental Atlas. Bråviken is a marine nature reserve, which includes hundreds of islands and islets. It has a rich plant and animal life, both on land and in water. The area is also known for its outdoor activities and recreational fishing. Hävringe-Källskären is one of the most important areas for grey seals (Halichoerus grypus) in the Baltic Sea. It is also an important area for the Caspian tern (Hydroprogne caspia), as large numbers breed on the islands.

Figure 3.

a. Oil spill trajectory modelling of a spill from the wreck Lindesnäs using SeaTrackWeb and b. over the four seasons. Top left is summer, top right autumn, lower left winter and lower right spring. The scale represent amount of oil after 10 days simulation per km2.

Figure 3.

a. Oil spill trajectory modelling of a spill from the wreck Lindesnäs using SeaTrackWeb and b. over the four seasons. Top left is summer, top right autumn, lower left winter and lower right spring. The scale represent amount of oil after 10 days simulation per km2.

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#### Cost Comparison -- Overall

Oil spill clean-up at sea and on the coastline after a spill is very costly, even without including cost of loss of tourism and aquaculture or damage to the environment. Equipment, vessels and personnel costs are examples of direct clean-up costs (Andersson et al., 2016). An oil spill in areas with high environmental values or major tourism activity would highly increase the costs of the spill. An example of costs of an oil spill affecting the Swedish coastline, is the so-called Tjörn spill in 2011. In September 2011 a collision in Skagerrak between the cargo ship Golden trader (192 × 32 × 15.7 m) and a fishing vessel led to an oil spill of bunker fuel. The oil was transported by currents to west coast of Sweden and affected the archipelago of the municipality of Tjörn. An intense clean-up operation, both on sea and shoreline, over several months recovered some 500 tons of oil at costs of $16.25 million (MSIR, 2012). An international example is the Prestige oil spill in 2002 outside the north-western coast of Spain were 60 000 tons of crude oil were spilled and the clean-up costs amounted to$560 million (Loureiro, 2006). Of course clean-up costs depends on several factors, such as type of oil, characteristics of the spill location, weather and sea conditions, amount spilled, time of the year and effectiveness of clean-up (Etkin, 1999, 2000, White & Molloy, 2003). Hence, costs will differ between spills and in general clean-up at sea is less costly than clean-up at the shore.

In addition to direct clean-up costs there are socioeconomic and environmental costs. Aquaculture, fisheries and tourism are the main industries connected to socioeconomic costs. Harvested fish and other seafood might become inedible or not allowed to sale due to restrictions. Oiled beaches results in economic losses for stakeholders in the tourism sector, as the number of tourist's decreases. In the case for the Prestige accident socioeconomic costs was estimated to $135 million (Loureiro, 2006). Finally, environmental costs of an oil spill results in temporal degradation of natural resources and services. Assessment of costs in this area is mainly focused on beaches, birds and seals. Not taking into account other habitats or other effects in animals besides lethal consequences, e.g. productivity, fecundity or decreased ecosystem services (Andersson et al., 2016, Lindgren et al., 2012, Liu et al., 2006). Altogether, costs of oil spills, including all types of costs, are very high. Comparing costs for oil removal from the wrecks Lindesnäs and Finnbirch,$7476/ton and $17542/ton (weigh of marine diesel 1150 litres/ton, HFO 1008 litres/ton) respectively, to only the direct clean-up costs for the oil spill at Tjörn and the Prestige,$47000/ton and $14900/ton, in 2020 monetary terms you realize that there is most probably a cost benefit in applying oil removal operations from shipwrecks proactively. Furthermore, Etkin (1999) compared oil spill clean-up costs at different continents. Using these numbers, including inflation, in 2020 monetary terms results in clean-up costs in United States at$38142/ton (excluding Exxon Valdez), Canada $99958/ton, Europe$13924/ton and Asia $24310/ton. However, the costs for clean-up have increased at a higher rate compared to inflation. Firstly, there is now a greater expectation from public and government authorities on the thoroughness of the clean-up. The clean-up operations have then become more thorough and complex, and therefore costly. Secondly, there is now a larger dataset of clean-up operations to base the estimations on, compared with 1999. Clean-up costs for a spill in Europe, offshore, of light oil (as in the case study shipwrecks Lindesnäs and Finnbirch part 1) 2020 ranges between$495000 – 49500/ton and for heavy oil (as in the case study shipwreck Finnbirch part 2) it 2020 ranges between $1050000 – 105000/ton, depending of the volume of the spill (pers. comm. D. Etkin, 2019). Hence, clean-up costs of oil is in most cases larger comparing to proactively removing oil from a shipwreck. Of course, costs also depends of the state's ability or willingness to perform clean-up operations. The costs are then instead displaced to socioeconomically and environmental costs. This is based on the two case studies presented here. However, taking into account other successful oil removal operations where external contractors performed the work, the costs is comparable, average cost$26598/ton (st. dev. ±$21297, updated to 2020 monetary terms), to the operations presented here (NOAA. 2013). One could argue that there is an uncertainty in the complete removal of all oil from the shipwreck, and the input of oil into the marine environment cannot completely be avoided. Or that the shipwreck has, on the contrary to the information at hand, no or nearly no oil on board when an oil removal operation is carried out. With the consequence of very high cost per removed ton of oil, e.g. Solar I and Palo Alto with costs of$1737000/ton and $1370000/ton (updated to 2020 monetary terms) of oil respectively (NOAA. 2013). However, in oil spill clean-up operations, the total amount of the oil spill is rarely recovered, with large volumes of oil affecting the environment. For example from the Exxon Valdez spill only 14% of the oil was recovered (McNutt, 2014, Wolf et al., 1994). If also socioeconomic and environmental costs is included in the cost-benefit estimate, it will strongly indicate that acting proactively instead of reactively to oil spills is beneficial. Tourism is many times a multimillion industry in coastal and near-coastal areas. The industry is then negatively affected by an oil spill as tourists reject contaminated areas. This is also accurate for Sweden and the Swedish coastline, which is one of the longest in Europe. The value of tourism in Sweden 2018 was$33 billion (SAERG, 2018).

Besides economic costs, there is further benefits of a proactive approach, in removing the threat of oil entering the marine ecosystem. The environmental stress on the ecosystem is avoided, as the subsequent effects on marine biota. Marine ecosystems is today often exposed to large number of various anthropogenic stressors e.g. eutrophication, hazardous substances, climate change, that affects marine organisms in additive or even synergistic manner. This is especially true for the sensitive and heavily stressed Baltic Sea (HELCOM. 2018), in which most of the Swedish oil containing shipwrecks is located. By removing oil from shipwrecks, one potential anthropogenic stressor is avoided.

Here we present the Swedish national program addressing the problem with polluting shipwrecks; the inventory, risk assessment and case studies. This risk-based approach and prioritization of oil removal operations is applied to minimize the risk of oil entering the marine environment. During 2019 and 2020, two operations were performed, with the result of 473 m3 (2975 barrels) of oil were prevented from eventually entering the marine environment. By proactively removing oil from shipwrecks, the anthropogenic stress caused by the oil spill is avoided. The cost of oil removal operations are substantial, however, oil spill clean-up operations, socioeconomic and environmental cost are, when summed, higher. Moreover, you could argue that there is a moral obligation of ameliorate or remove threats to the marine environment caused by man.

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