4. Discharges to sea
Discharges to sea consist primarily of discharges from well drilling and produced water. Produced water is water that accompanies the oil from the reservoirs. Drilling discharges are mainly comprised of rock particles from the borehole and drilling fluid. Discharges are only permitted from wells drilled using water-based drilling fluid, as well as oil-based drilling fluid with permission from the Norwegian Environment Agency, if contamination from oil-based fluids is less than ten grams of base oil per kilogram of cuttings.
Figure 3 shows that drilling activity in 2023 increased slightly from 2022 to 2023, with a total of 206 wells drilled, of which 35 were started exploration wells. Discharges of produced water reached a peak in 2007 at just over 160 million Sm³. In 2023, total discharges amounted to 113 million Sm³.
4.1 Discharges from drilling
Drilling activity increased slightly from 2022 to 2023. The number of new production wells drilled in 2023 was 171. The number of exploration wells was 35, roughly the same level as the previous year (34).
The drilling fluid used in well drilling serves many functions. It transports cuttings to the platform while lubricating and cooling the drill bit. At the same time, the drilling fluid prevents the borehole from collapsing. Last but not least, it keeps the well pressure stable and prevents uncontrolled oil and gas blowouts.
The industry primarily uses two types of drilling fluids: oil-based and water-based.
The discharge of oil-based or synthetic drilling fluids, or cuttings contaminated with these, is prohibited if the oil concentration exceeds one percent by weight. One percent by weight equates to ten grams of oil per kilogram of cuttings. Drill cuttings discharge contaminated with oil-based or synthetic drilling fluids containing less than one percent by weight of oil is only permitted if authorized by the NEA. Used oil-based drilling fluids and contaminated cuttings are either transported onshore as hazardous waste for carefully controlled treatment or injected into specially designated wells beneath the seabed.
Figure 3: Number of wells drilled on the NCS (Source: Norwegian Petroleum Directorate).
Figure 4: Disposal of oil-based drilling fluid
Field operators use water-based drilling fluids as far as possible to reduce the quantities of waste requiring treatment. Oil-based drilling fluids are more effective from a technical drilling perspective than water-based drilling fluids, and more complex wells will have a greater need for the use of oil-based drilling fluids.
The use of oil-based drilling fluid decreased by over 3 percent from 2023 compared to 2022, as shown in Figure 4. The quantity of oil-based cuttings contaminated by drilling fluids and injected sub-surface has shown a downward trend in recent years but increased slightly from 2022 to 2023.
The quantities of drill cuttings presented in Figure 5 are based on calculations of the rock which has been drilled out. The quantities of drill cuttings registered as hazardous waste transported onshore (see Chapter 8) are, however, significantly larger. This is because cuttings from many fields are slurrified by adding water to make them easier to handle from the platform to the vessel and then to shore. This nonconformity is therefore largely due to water being added to the cuttings before they are received onshore.
The quantity of oil-contaminated drill cuttings transported onshore as waste increased from 2022 to 2023. In 2022, the quantity was just under 70,000 tonnes, and in 2023, it was 88,000 tonnes. The water and drill cuttings are separated onshore. Whilst the water is treated and discharged to the sea, the cuttings undergo further treatment in accordance with current regulations.
Figure 5: Disposal of drill cuttings contaminated with oil-based drilling fluids
Figure 6: Disposal of drill cuttings from wells drilled with water-based drilling fluids
In 2023, discharges from drill cuttings drilled with water-based drilling fluid were approximately 74,000 tonnes, a decrease of 12 percent from the previous year, as shown in Figure 6. Water-based drilling fluids mainly contain natural components such as clay or salts. These substances are classified as green in the NEA’s classification system. According to OSPAR, they pose little or no risk to the marine environment when discharged.
The potential impact of these discharges on the environment is monitored through extensive environmental monitoring (see Chapter 5.3).
Discharges from oily water
Oily water discharges from petroleum operations on the NCS derive from three main sources, with produced water accounting for the largest contribution.
- Produced water
This is water that accompanies oil and gas from the reservoir. Produced water is complex and can contain several thousand different individual components. Routine analyses of the water are therefore conducted. When produced water is injected to enhance production, it will mix with formation water. The produced water will also contain various chemical additives, for example, to inhibit bacterial growth, corrosion, and emulsion. On the offshore installations, the water is treated using various treatment technologies before being discharged to the sea. Different treatment technologies help to keep the oil content as low as possible. The regulatory threshold for oil concentration in produced water discharged to the sea is 30 mg/l. - Displacement water
Seawater is used as ballast in storage cells on some platforms. When oil is to be stored in the storage cells, the water must be treated prior to discharge. The seawater has a limited contact area with the crude, so the quantity of dispersed oil is usually low. The discharged volume depends on the oil production. - Drainage water
Rainwater and water washed off the decks may contain chemical residues and oil. Drain water discharges make up only a small proportion of the total volume of water discharged.
"Other oily water" is also reported. For example, particles and oil-contaminated sand that are collected in separators must be flushed out from time to time, known as jetting. Some oil adheres to the particles after the water has been treated according to the requirements, but the volume of oily water discharged to sea is marginal. Oily water can also occur from hosing down processing equipment, in connection with incidents, or from the oil droplets forming when burning oil during well testing and well maintenance work.
Discharges of produced water
Figure 7 shows the historical development of produced water volumes discharged to sea and reinjected into the bedrock. Projections for discharges of produced water from the NCS pointed upward for many years and were expected to exceed 200 million Sm³ from 2012 to 2014. However, discharges peaked at 160 million Sm³ in 2007 and declined significantly in the following years. From 2012 to 2015, discharges increased to nearly 150 million Sm³. However, after 2015, they were reduced once again, and in 2023, they amounted to 113 million Sm³, a decrease from 116 million Sm³ in 2022. The quantity of dispersed oil to sea was 1,309 tonnes, distributed across all types of discharges. The largest discharges occur on mature fields with large volumes of produced water and produced water accounts for 95 percent of the oil discharged to sea.
On certain fields, where conditions allow, all or parts of the produced water is reinjected into the bedrock. Since 2002, injection has increased significantly and has been around 20 percent in recent years. In 2023, nearly 30 percent of the produced water was injected (46 million Sm³), a slight increase from the previous year (43 million Sm³).
On new fields, produced water consists solely of water already present in the reservoirs. However, the injection of water leads to an increase in produced water volumes as the field matures. Water is injected to maintain reservoir pressure and increase the oil recovery rate from the reservoir. This is primarily treated seawater. The oil recovery rate from fields on the NCS is generally significantly higher than the recovery rate worldwide. Despite this, discharges from the NCS are comparable to international figures.
Figure 7: Produced water volumes discharged to sea and reinjected into the bedrock
Figure 8: Concentration of oil in the discharge of produced water to the sea
Figure 9: Ratio between produced water and oil
Before oily water is discharged to the sea, it is treated. Different technologies are used on different fields. The average oil content in produced water for the entire shelf in 2023 was 11.5 mg/l, while the regulatory threshold is 30 mg/l. The concentration of dispersed oil decreased from 11.8 mg/l in 2022, as shown in Figure 8.
The ratio between the volumes of produced water and produced oil on the NCS, shown in Figure 9, has slightly declined since 2019. This is likely due to the startup of production on several new fields. The startup of the Johan Sverdrup field, for example, contributed to higher oil production while water production did not increase correspondingly.
Both risk-based modelling and environmental monitoring studies have so far not indicated any significant environmental impact caused by discharges of produced water (see Chapter 5.3). An article by Beyer et al. (2020) indicates mild acute environmental impact associated with produced water in the water column, limited to the vicinity of the discharge.
Discharges of other types of water
Figure 10 shows that discharges of other types of water are dominated by displacement water. Discharge volumes decreased steadily until around 2010. Since 2011, discharge volumes have hovered at around 30 million Sm³. In 2023, displacement water discharges totalled approximately 32 million Sm³.
Figure 10: Discharge volumes to the sea of other types of oily water
Discharges of oil with water
The amount of oil discharged to the sea with produced water decreased from 1,370 tonnes in 2022 to 1,305 tonnes in 2023, as shown in Figure 11. A total of 1,377 tonnes of oil were discharged with water from drainage, displacement, produced water, and jetting. In 2022, this figure was 1,451 tonnes.
Figure 11: Oil discharges accompanying water discharges from the NCS
Discharges of other substances accompanying produced water
Produced water has been in contact with the bedrock over a long period of time and therefore contains a number of naturally occurring substances. In addition to oil, a typical composition includes mono- and polycyclic aromatic hydrocarbons (PAHs), alkylphenols, heavy metals, naturally occurring radioactive materials, organic matter, organic acids, inorganic salts, mineral particles, sulphur, and sulphides. The composition will vary between fields depending on the properties of the bedrock.
4.2 Chemical discharges
Chemicals are assessed according to their environmental properties, including their persistence, potential for bioaccumulation, and toxicity (PBT). The Norwegian government has also specified criteria in the Activities regulations and guidelines for reporting from offshore petroleum activities.
Chemical additives that are subject to discharge permit requirements are divided into four categories according to the classification in the Activities regulations:
| Green | Zero or minimal environmental impact. Discharges allowed without special conditions. |
|---|---|
| Yellow | Normally acceptable environmental impact. A discharge permit is required but generally approved. |
| Red | Must be prioritized for substitution with chemicals in the green or yellow category. |
| Black | Discharge is not permitted. Exceptions may be made in special cases, for example, if it is crucial for safety reasons. |
A more detailed description of the classification is provided in the NEA's guideline M-107, Guidelines for reporting from offshore petroleum activities.
Discharges of chemical additives from Norwegian petroleum operations in 2023 totalled approximately 162,000 tonnes. This is a decrease of about 6 percent from 2022. Ninety percent of the discharges were green chemicals. Red and black chemicals together accounted for about 0.3 percent of the discharges, with the distribution shown in Figure 12.
Replacing chemicals with less environmentally harmful alternatives, known as the substitution obligation, is an important part of the environmental initiative to reduce potentially harmful effects from offshore discharges. Operators regularly assess the chemicals used to determine if they can be substituted. The substitution of chemicals has been extensive and has reduced the discharges of the most environmentally harmful chemicals to a fraction of what they were just ten years ago.
Figure 12: Distribution of discharges of chemical additives from the NCS by the NEA’s colour categories
From 2011 to 2014, however, there was a substantial increase in reported discharges of black chemicals. This is primarily because discharges of fire-fighting foam were previously not reported as it was defined as a contingency chemical. There are now alternatives with less environmentally harmful properties, and fire-fighting foam has therefore been included in the substitution requirement. These new alternatives have now been phased in across all fields on the NCS.
The increase observed in 2020 is partly because lubricants leaking from submerged seawater pumps became reportable as black category discharges. Discharges from black category substances are expected to further decrease in the coming years as part of the ongoing substitution efforts. There are now alternatives to the lubricants used in submerged seawater pumps. For some older pumps, the substitution work has stalled due to pump failures, and more knowledge about the reasons for the failures is necessary before the substitution efforts can continue.
Black chemical discharges totalled 4.1 tonnes in 2023, a slight increase from 3.7 tonnes in 2022, as shown in Figure 13. This increase is partly due to boric acid and several borates used in corrosion inhibitors becoming reportable as black category, after receiving a new classification as harmful to health from the European Chemicals Agency. However, boron occurs naturally as an inorganic salt in seawater, and the environmental risk in the marine environment is assessed as low. Several of the chemicals used in freshwater production offshore lack Harmonized Chemical Notification Format (HOCNF) and are therefore classified as black.
Figure 13 shows that chemicals in the red category had a steady increase in reported discharges since 2013, when they were around 8 tonnes. In 2023, 410 tonnes of red chemicals were discharged, a decrease from 419 tonnes in 2022.
The apparent increase in recent years is due to changes in reporting requirements. For instance, the antifouling agent sodium hypochlorite, which is also used in drinking water treatment and indoor swimming pools, was reclassified from yellow to red.
In 2020, a new reporting requirement was also introduced for chemicals used in the production of freshwater. Here, several fields use self-produced hypochlorite, which must now be reported and classified as red.
Furthermore, Figure 13 shows a reduction of 11,000 tonnes in the discharge of green chemicals from 2022 to 2023, while discharges of yellow chemicals remained relatively stable.
Figure 13: Discharges of chemical additives from the NCS categorized by the NEA.
4.3 Unintentional spills
Unintentional spills are defined as unplanned emissions/discharges that occur suddenly and without a permit. The potential environmental impact of such releases will depend on the properties of the substance spilled, the volume, and the time/location of the spill.
Unintentional spills are classified according to three main categories:
- Oil: diesel, fuel oil, crude oil, waste oil, and other oils
- Chemicals and drilling fluids
- Gas emissions to sea and air
The oil and gas industry prioritizes preventive measures. These are measures (barriers) that prevent unwanted incidents from occurring, thereby reducing the number of unintentional spills. All unintentional spills are reported to the NEA in the annual emission/discharge reports.
Unintentional oil spills
The total number of unintentional spills of all types of oil has generally decreased over the past 20 years. The marked decrease in the number of spills from 2013 to 2014 is due to a clarification of regulations, resulting in fewer oil spills of less than 50 litres, while the number of unintentional chemical releases in the same volume category increased correspondingly.
Figure 14: Number of unintentional oil spills to sea on the NCS
Figure 15: Number of unintentional crude oil spills to sea on the NCS.
In 2023, there were 41 incidents involving oil spills compared to 42 in 2022, as shown in Figure 14. There have been around 10 to 15 incidents per year in recent years involving spills of over 50 litres. In 2023, there were a total of 12 spills exceeding 50 litres, of which 3 were greater than 1 m³. The largest isolated spill in 2023 was 64 m³, an incident at Alvheim in November 2023.
Looking only at crude oil spills in Figure 15, there is also a clear downward trend over the past 10–15 years. In 2023, there were 18 such spills.
The total oil spill volume from unintentional oil spills varies significantly from year to year, as shown in Figure 16. The statistics are influenced by large single incidents. In 2007, the second-largest oil spill on the NCS occurred, amounting to over 4,000 m³, while total spills since then have ranged between 10 and 200 m³. In 2023, the total volume was 70 m³, dominated by a single spill.
Figure 16: Spill volume from unintentional oil spills on the NCS.
Figure 17: Total unintentional chemical spills on the NCS distributed across three spill sizes.
Unintentional chemical spills.
The number of unintentional chemical spills does not show the same downward trend as for unintentional oil spills. The marked increase in 2014 to 237 spills was attributed to a clarification of regulations, which led to fewer oil spills and more chemical spills. In 2023, the number of spills was 199, of which 42 were larger than 1 m³. Their distribution is shown in Figure 17.
The total volume of unintentional chemical spills in 2023 was 349 m³, a decrease from 2022 when the spills totalled 398 m³. The unintentional spills were distributed as follows: 89 percent green chemicals, 10.3 percent yellow, 0.8 percent red, and 0.1 percent black.
In the period from 2007 to 2010, the spill volumes were dominated by individual years where leaks from injection wells were detected. These wells are now permanently plugged.
Figure 18: Total volume of unintentional chemical spills.
Unintentional discharge of gas to sea.
From the reporting year 2023, unintentional discharges of gas to the sea are also reported in Footprint. The number of unintentional discharges was 23, with a total volume of 32,907 Sm³.
Unintentional gas emissions to air.
Unintentional gas emissions are mainly small leaks of hydrocarbon gases and refrigerants from process equipment. In 2023, the number of emissions was 155, up from 132 in 2022. However, the total mass was reduced from 24,016 kg in 2022 to 7,206 kg in 2023.
Figure 19: Unintentional gas emissions to air.
Leak detection.
A detection system is an important barrier for identifying leaks and other unintentional spills as quickly as possible. The system is designed to provide the necessary data so that relevant actions can be initiated as promptly as possible, also ensuring that spills are notified, reported, and documented in accordance with regulations. All facilities on the NCS currently have one or several leak detection technologies installed.
The number of incidents involving unintentional spills from subsea installations is low and is highlighted each year in the Norwegian Petroleum Safety Authority's report RNNP Trends in Risk Level.
Large spills can be detected immediately through process monitoring, and daily satellite monitoring and radar monitoring of the sea surface. However, it can be more challenging to detect small spills from subsea installations. Modern subsea installations are equipped with local leak detection systems, but this is not always the case for older installations built before such technology was available.
The NEA and the Norwegian Ocean Industry Authority (formerly the Petroleum Safety Authority) conducted a joint audit initiative in 2020/early 2021 to inspect the operators' routines and equipment for detecting leaks of oil, gas, and chemicals from subsea installations on the NCS. Some common deviations were identified among all operators, particularly related to the detection of minor leaks and performance requirement procedures. A work group was therefore established under the directive of Offshore Norge to investigate this jointly.
The findings from the audits are largely due to insufficient risk assessments of potential spills from individual installations, as well as insufficient documentation and holistic evaluations of the systems’ capabilities/performance. The industry is currently working to address these issues, and assessments are ongoing to identify gaps and how to close them. Consideration must also be given to the limitations of technology; most leak detection systems can only cover a limited area and cannot always detect small spills from a distance. For the very smallest spills, inspections may therefore be the only option for detection. Inspections are carried out on all fields at regular intervals.
There are also several subsea installations on the NCS that have low or negative pressure relative to the surrounding water mass. These fields are more likely to experience water leaks into the system than oil and gas discharges to sea.