assessing hydrocarbon presence in the waters of …

102 The Journal of Ocean Technology, Vol. 15, No. 3, 2020 Copyright Journal of Ocean Technology 2020

ABSTRACT

The waters adjacent to the Port au Port Peninsula, in Port au Port Bay, Newfoundland and Labrador, are known to be subject to release of hydrocarbons from natural oil seeps and old abandoned oil wells. An investigation was done to determine whether there were sufficient oil compounds present for planned autonomous underwater vehicle (AUV) test missions to develop adaptive sampling algorithms to delineate oil spills. Fluorometers were used in-situ to measure oil concentrations. Oil-and-water samples were taken at selected waypoints for chemical analysis in the laboratory to validate the sensor measurements and to provide a ground truth. Only one of the fluorometers was found to have a minimum detection level that was capable of sensing the hydrocarbons in the water column. The water sample results indicated hydrocarbon levels up to almost 30 ppm in the east side of the bay, just to the west of Shoal Point, but no detectable levels on the west side of the bay. It was concluded that it would be possible to operate an AUV on a planned fixed mission with a pre-programmed search path and record the levels of signal detected from fluorometers or other sensors. However, it would be difficult to implement an adaptive mission in this case because of the low levels of sensor signals resulting from the low concentrations of hydrocarbon present.

KEYWORDS

Oil spill investigation; Fluorometers; Hydrocarbon; Gas chromatography – flame ionization detector; Gas chromatography – mass spectrometry; Total petroleum hydrocarbon

ASSESSING HYDROCARBON PRESENCE IN THE WATERS OF PORT AU PORT BAY, NEWFOUNDLAND AND LABRADOR, FOR AUV OIL SPILL DELINEATION TESTS

Jimin Hwang1, Neil Bose2, Brian Robinson3, and Hung Nguyen1

1Australian Maritime College, University of Tasmania, Launceston, Australia2Memorial University of Newfoundland, St. John’s N.L., Canada3Fisheries and Oceans Canada, Dartmouth, N.S., Canada

The Journal of Ocean Technology, Vol. 15, No. 3, 2020 103Copyright Journal of Ocean Technology 2020

INTRODUCTION

This paper reports the results of an experiment conducted in the ocean adjacent to the Port au Port Peninsula, or Payun Aqq Payunji’j – its Mi'kmaw place name and the traditional territory of the Benoit First Nation. This region has been known as a place where oil deposits are prevalent and drilling for oil has occurred since the 1870s [Azmy et al., 2008]. Oil seepage is common in the region of the test site and drilling investigations have been undertaken for more than a century on the west side of Shoal Point just to the east of the test area [Fowler et al., 1995]. According to an investigation report submitted to the Department of Municipal Affairs and Environment, Government of Newfoundland and Labrador, oil sheening and odour was observed at the beach in 2015 in the vicinity of the abandoned wells. Though a temporary repair was done, the Canadian Coast Guard received a report advising that oil seepage had resumed after sea-ice damaged the capped oil and gas well castings early in 2016. Re-capping was done in 2018, which may have resulted in less oil release at the time our test was done.

Our team planned this experiment to evaluate and quantify the concentration of oil in the ocean, in the bay adjacent to the old oil wells and natural seeps. The primary objective of the experiment is to investigate whether the chosen site adjacent to the Port au Port Peninsula has sufficient oil compounds for our planned AUV missions. In addition, the outcomes of the mission are used to evaluate the performance of submersible oil sensors that can potentially be mounted on an Explorer AUV. We also present quantified hydrocarbon

concentration from the collected oil-and-water samples through chemical analysis to support the sensor measurements.

METHOD

This section describes the mission procedure, in-situ sensors, and water sampling equipment, respectively.

Mission DescriptionThe mission comprised of activities over three days including July 11, 12, and 13, 2019. Visual investigation was done to observe if there was any oil slick along the west side of the Shoal point. The background acidity level, about 8 pH, was measured from a water sample from the test site at this location prior to the experiment.

The mission was conducted by following pre-determined waypoints in the bay where oil seeps have been known to exist as shown in Figure 1. Eleven waypoints were selected, and a sensor suite was lowered from a survey boat down to a 5 m maximum water depth at 1 m intervals. At each depth and at each waypoint, we recorded data for one minute to acquire stable oil concentration readings. In total, 180 minutes were recorded for in-situ sensor measurements, collecting water samples, and travelling time between the waypoints. The total distance travelled was about 23 km. The collected water samples were preserved in bottles that contained a small amount of acid. The samples were kept at less than 4° Celsius. Chemical analysis was done by the Centre for Chemical Analysis, Research and Training, the chemistry lab in the St. John’s main campus of Memorial University.


In-situ Oil Sensors and Sampling EquipmentFour types of sensors were selected including a UV AquaTracka, SeaOWL, and two Cyclops-7, one with a refined optic sensor and one with a crude optic sensor. They were the fluorometers that have been the most commonly used in delineating an oil plume using an AUV in the field [Jakuba et al., 2010; Zhang et al., 2011; White et al., 2016; and Johengen et al., 2012]. The oil sensors and their specifications are given in Table 1.

A sampling unit was configured with the fluorometers integrated inside a cage as shown in Figure 2. The unit accommodated the water sampler which consisted of a 10 m sampling tube connected to a drill driven motor. Sample

water was pumped from the depth of 5 m and collected in the amber glass sampling bottles. In order to minimize degradation by marine organisms, the pH level was lowered to approximately 2. To achieve this pH level, 0.1%, or 1 mL, of 37% concentrated Hydrochloric acid was placed in the bottles prior to the samples being collected. In total, 11 water samples were taken, one at each waypoint where fluorometer data was collected.

RESULTS

The measurements from the in-situ fluorometer sensors are given in this section together with the results from the laboratory-based chemical analysis of the water samples collected during the mission.

Figure 1: A map of Port au Port Peninsula Bay, Newfoundland and Labrador, showing locations of documented wells and seeps. The map was adapted and recreated from Wheeler [2015A; 2015B]. The trajectory with waypoints is marked in red.

Table 1: The manufacturer’s specifications of the four selected in-situ fluorometers used in the experiment.


In-situ Sensor ResultsTo obtain the water accommodated fraction (WAF) of the target oil, it is necessary to have a sufficient amount of the target oil to correctly convert the sensor readings to calibrated oil concentration [Saco-Álvarez et al., 2008]. However, when the oil characteristics are unknown, and it is

Figure 2: Sampling equipment: (a) Sensor measurement and sampling configuration; (b) Schematic of the sampling unit.

a

bnot possible to obtain an adequate sample as in this case, alternative measures have to be made. We used an empirical result from previous tests with the same instruments where measurements had been made of hydrocarbons in crude oil for calibration coefficients as given below [Conmy et al., 2014].

(1)

(2)

(3)


The output from the SeaOWL and the Cyclops-7 (both crude and refined optic) fluorometers did not show any meaningful signals to enable us to discriminate between the waypoints with and without hydrocarbons present. The outputs from these sensors displayed little variation over depth at all waypoints where measurements were taken. Therefore, we concluded that the overall hydrocarbon level in the region was out of the detection range (below the detection limits) of these sensors and this was corroborated by the levels of readings from the UV AquaTracka sensor.

The Chelsea UV AquaTracka showed varied readings at waypoints #2-#5 and #7-#11, showing a variation over depth at waypoints #3 and #11 as shown in Figure 3. At those locations, the measurements received higher levels of returns at depths up to 3 m, which indicates a higher density of fluorescent substances in the range of 360 nm-440 nm wavelength values (the sensitivity range of the instrument) at these depths. The signals dropped back to the background level in deeper water between 4 m and 5 m. These positive returns near the surface imply a higher possibility of the presence of an oil plume at shallower depths.

Figure 4 compares the measurements at waypoints #2 and #10 that represent the locations where hydrocarbons were found

and not found, respectively, from the subsequent chemical analysis results. The first plot at waypoint #2 shows a number of consecutive peaks at 1-2 m depth that are six times higher than the background level. The highest measurements in the second plot at waypoint #10 remain close to the average level, yet with a slightly higher variation near the surface. In summary, a distinction could be made in fluorometer measurements over depth between the waypoints where our chemical analysis showed there to be hydrocarbons present or not. The chemical laboratory tests with our water samples were needed to confirm this.

Chemical Analysis ResultsTwo chemical analysis methods were used toquantify the hydrocarbon concentration in the collected samples: Gas Chromatography –Flame Ionization Detector (GC-FID) and Gas Chromatography – Mass Spectrometry (GC-MS).

Figure 3: The UV AquaTracka fluorometer results for which a relatively higher vertical variance was observed near the surface compared with deeper water: (a) At waypoint #3; (b) At waypoint #11.

Figure 4: Comparison of the UV AquaTracka fluorometer results: (a) At waypoint #2 at which hydrocarbon was found from our chemical analysis results; (b) At waypoint #10 at which hydrocarbon was not found. The dotted line corresponds to the averaged background level.

A B

A B


GC-FID is conventionally used to obtain the total petroleum hydrocarbon (TPH) values from an oil-and-water mixture, while GC-MS is used to obtain the hydrocarbon spectrum which gives more detailed information about the hydrocarbons present.

Total Petroleum Hydrocarbon (TPH) ResultsThe certified TPH standard mixture, 2000μg/mL in methylene chloride (C8-C40), was used for calibration as a reference hydrocarbon mix. The amount of TPH in most of the samples we obtained was below the minimum detection limit of the GC-FID instrument except for waypoints #1, #2, and #5. The TPH values for these waypoints were 20.3 ppm, 27.9 ppm, and 4.3 ppm, respectively. Significant levels of hydrocarbon were found only between the range of C24-C30, which are the straight-chain alkanes. The TPH results detailing each component and their amounts are listed in Table 2. The TPH results are plotted along the trajectory as shown in Figure 5.

The total petroleum hydrocarbon concentrations found at waypoints #1, #2, and #5 from the TPH analysis were much higher than measured from the in-situ sensor results. Fluorometers detect the hydrocarbon compounds with a benzene ring structure; whereas the hydrocarbons detected by GC-FID were mostly straight-chain alkanes that would not be detected by the fluorometers. The TPH results suggest that the in-situ sensors detected

some parts of molecules of hydrocarbon compounds. In summary, the GC-FID results matched the sensor measurements in terms of the locations of the presence of oil.

Gas Chromatography – Mass Spectrometry ResultThe Gas Chromatography – Mass Spectrometry results from our water samples revealed a

Table 2: Total petroleum hydrocarbon (TPH) results.

Figure 5: (a) The survey trajectory plotted using Google Maps; (b) Total petroleum hydrocarbon (TPH) results.

A

B


clear positive hydrocarbon presence at waypoints #1, #2, and #3. Relatively lower yet a noticeable concentration of hydrocarbons was detected at waypoints #5 and #6 as well. The water samples from waypoints #7 and #11 showed no sign of hydrocarbon presence. The chromatogram plots at waypoints #2 and #11 are compared in Figure 6. The x-axis represents analysis time, while the y-axis represents the corresponding abundance for each hydrocarbon molecule. Due to limitations

with the analytical procedure that was used, it was not feasible as a part of this study to convert the abundance into the concentration values in ppb (or ppm) units. However, the peaks characterize the type of hydrocarbon and the total hydrocarbon concentration for each sampled region by a comparison with the chromatogram of the certified TPH reference. In summary, a hydrocarbon plume appeared to exist near the old abandoned wells (Shoal Point #1 and #2) and any possible nearby

Figure 6: Overlay of chromatograms of samples at waypoint #2 (black) and #11 (blue).

Figure 7: Chromatogram of sample at waypoint #2. Alkanes ranging from C22 to C32 were identified.


seeps, which dispersed towards the east, but did not quite reach the west side of the Port au Port Bay and the Long Point peninsula.

The mass spectrometer results revealed that petroleum hydrocarbons in the range C22-C32 existed in the collected water sample at waypoint #2 as shown in Figure 7. These are simple straight chain aliphatic hydrocarbons; they do not have a ring structure that is found in polycyclic aromatic hydrocarbons (PAHs). Aliphatic hydrocarbons are readily biodegradable by marine micro-organisms and relatively less toxic compared with PAHs [Hodson, 2017].

DISCUSSION

As the primary objective of the project was to delineate oil spills, we sought a location where there was a much higher concentration of hydrocarbons in the water. Ambiguity in fluorometer sensor outputs tells us that high resolution of in-situ oil sensors is essential for successful AUV adaptive missions. Despite the low concentration levels of the in-situ fluorometer measurements, the chemical analysis results revealed that the collected water samples contained much higher concentrations of oil compounds. Therefore, there is evidence of persistent oil seepages in the east side of the bay resulting from abandoned wells or natural seeps. The bay would be valuable as a test ground for adaptive missions with an autonomous underwater vehicle equipped with in-situ sensors capable of recording low oil concentration levels and for work focused on identifying natural oil seeps in the environment where hydrocarbon concentration

levels in the water column remain low.Fluorometers have commonly been a choice of in-situ sensors for AUV missions to detect hydrocarbons in the water column [Baszanowska and Otremba, 2017; 2019; Wen et al., 2018]. Yet the results from the four fluorometers used in this experiment indicated that there would be challenges if we were to use these sensors for autonomous interpretation. This is because of the low concentration of hydrocarbon in the water which led to low signal to noise ratios. However, fluorometers can be useful to indicate the presence of oil in water provided that there is a sufficient concentration of oil above the minimum detection limit of the fluorometers in water [Kukulya et al., 2016]. If the fluorometers are used as the primary sensor to rely on to autonomously aid the navigation of an underwater vehicle in the detection of an oil plume, it would be beneficial to also precede the tests with a WAF test to accurately track the actual subject oil prior to a mission.

There were no visually obvious oil particles in the water. An in-situ particle size analyser (such as a Laser In-Situ Scattering and Transmissometry) can measure micron sized oil particles, but there would be difficulties in distinguishing oil droplets from other particulate organic matter in the water column.

To successfully delineate an oil plume, an AUV should be outfitted with a range of oil sensors; for example, an underwater mass spectrometer (UMS), which is capable of providing more information on hydrocarbon compounds without the need to collect water samples, might be used when concentrations are low such as with this


plume. The UMS can decrease ambiguity as it can distinguish hydrocarbons from other micro-size fluorescent and/or non-fluorescent organic compounds. The current generation of UMS only measures compounds up to 200 atomic mass units and so cannot provide a full picture of the hydrocarbon content in the seawater in an in-situ test. Therefore, it is important to also collect water samples to obtain a ground truth and it is advantageous to use an adaptive sampling technique to take samples only once oil is detected by other sensors [Hwang et al., 2019]. An adaptive sampling algorithm has been developed for a forthcoming AUV mission in the ocean using the knowledge acquired from this sensor test. Example adaptive mission designs are illustrated in Figure 8. In this figure, the dashed line shows the AUV trajectory which starts at the left-hand side in a survey mode: different forms of survey mode are depicted in the top and bottom plots. In the top plot, the AUV is programmed to transect the oil plume and identify its extent, returning to the plume after each transect and marking the centre point of each transect. In the bottom plot, the AUV is programmed to identify the boundary of the oil plume by traversing in and out of the plume edge.

CONCLUSION

A field experiment testing submersible oil sensors was done to assess levels of hydrocarbon present from natural oil seeps and abandoned drilling wells in the Port au Port Bay area of Newfoundland and Labrador. The work was done to assess the site as a potential field location to test the performance of adaptive algorithms to control an AUV to delineate an oil spill.

Four types of in-situ oil sensors were tested. The UV AquaTracka, the most sensitive fluorometer among those tested, indicated that the water may have contained fluorescent hydrocarbons such as PAHs which are typically of major concern due to their toxicity when an oil spill accident takes place.

A chemical analysis was done to obtain more information from the collected water samples than it was possible to assess from the in-situ fluorometer readings. Two types of analyses were undertaken: a TPH analysis for the total hydrocarbon concentration and mass spectrometry for the wider hydrocarbon spectrum. GC-FID analysis showed a

Figure 8: AUV adaptive sampling mission designs where the vehicle transects a simulated oil plume (top) and follows the plume boundary (bottom).


petroleum hydrocarbon concentration of 20.3 ppm and 27.9 ppm at waypoints #1 and #2, respectively. The mass spectrometer results identified detected petroleum hydrocarbons in the range of C22-C32. Both results confirmed that there was a distinct hydrocarbon plume at the waypoints at #1-#5 adjacent to the eastern shore of Shoal Point, and no sign of the plume was observed at waypoints #6-#11 on the western side of Port au Port Bay.

ACKNOWLEDGMENTS

This research was supported by the Australian Research Council’s Special Research Initiative under the Antarctic Gateway Partnership (Project ID SR140300001) and by Fisheries and Oceans Canada through the Multi-partner Oil Spill Research Initiative (MPRI) 1.03: Oil Spill Reconnaissance and Delineation through Robotic Autonomous Underwater Vehicle Technology in Open and Iced Waters. The first author acknowledges the Australian Government Research Training Program Scholarship in support of this higher degree research.

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