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Subsurface blowouts: Results from field experiments
SpillScience& TechnologyBulletin,Vol.4, No. 4, pp. 239-256,1997 ~
© 1998Publishedby ElsevierScienceLtd All rights reserved. Printed in Great Britain 1353-2561/98$19.00+0.00
Pergamon PII: S1353-2561(98)00021-8
Subsurface Blowouts: Results from Field Experiments H. RYE, P. J. B R A N D V I K & T. S T R O M
S I N T E F Applied Chemistry, 70 34 Trondheim, Norway
A field experiment was carried out in June 1996 with a subsurface release of air and oil close to the Frigg Field in the North Sea. One of the purposes of this sea trial was to increase the knowledge concerning the behaviour of the oil and gas during a subsurface blowout. This was done by simulating a subsurface blowout at 106 m depth with a realistic gas oil ratio (GOR = 67) and release velocity of the oil/gas. In addition to the main oil release (43 m3 Troll crude), other releases with water and air ( G O R = 7-65) were performed. Important and unique data were collected during these subsurface releases that can be used to improve numerical models for such releases.The main conclusions from these trials are: • The field methodology used to study blowout releases in the field appears to be appropriate. • The surface oil slick formed by subsurface releases is significantly wider and thinner than if it was caused by a surface release. • Only 15-20% of the released oil was detected at the surface. This percentage is expected to depend on factors like depth, GOR, oil type and release velocity. • Some discrepancies between existing models and field results were found. The field data may thus serve as a basis for model improvements. © 1998 Published by Elsevier Science Ltd. All rights reserved
The oil and gas industry is presently moving their activities towards deeper waters. At the same time, the technical development also favours sub-sea solutions rather than installing platforms that penetrate the sea surface. These developments enhance the probability of having a blowout with releases of oil and gas from the sea bottom rather than at the sea surface. As a part of the contingency planning, a realistic description of blowout scenarios is essential, which involves the use of numerical models to describe the behaviour of the subsea plume as well as the dimensions of the resulting slick on the sea surface. Different models for prediction of behaviours of subsea blowouts have been proposed. A review of various model approaches that have been attempted in the past have recently been summarised by Yapa and Zheng (1998). A model for subsea blowout plume behaviour and the resulting surface slick dimensions is operable presently at S I N T E F Applied Chemistry. Although the main features of this model
will shortly be presented in this paper, the main objective is to present and describe field data on subsurface plume behaviour and the resulting slick formation. A comparison between field results and model results may therefore reveal where the largest potential for improvements of existing models is expected. In order to obtain relevant field data, 43 m 3 of stabilised Troll oil was released at 106 m depth over a time span of 40 min (approximately 17 l/s) jointly with air. The release velocity and G O R (gas oil ratio, or, in this particular case, the 'air oil ratio') were chosen to be representative for a potential blowout at the Troll oil and gas field, located outside Bergen on the west coast of Norway.
Objectives The subsurface release was carried out as a part of the N O F O (Norwegian Clean Seas Association, 239
RESEARCH Stavanger, Norway) sea trial at the Frigg field in the North Sea for 1996. The main objective of the experiment was to increase our knowledge concerning the behaviour of the oil and gas during a subsurface blowout by: 1. simulating different subsurface blowout conditions at 106 m depth with a realistic G O R and release velocities. 2. extensive monitoring of the subsurface oil/gas/ water plume and the resulting surface oil slick. 3. forming a basis for improving existing blowout models using the field data collected during these subsurface releases.
Experimental test programme Several types of measurements were carried out, both for collecting background information and for making recordings of the plume and slick properties. Examples of such activities are R O V (Remote Operating Vehicle) monitoring of the subsurface plume with sonar, video of the release at the outlet opening, sampling of oil in the slick, remote sensing of the surface slick, temperature and horizontal current recordings close to the plume as well as horizontal currents, temperature and salinity recordings of the ambient water masses. In addition to the oil and gas release, a number of other subsurface releases were carried out, using water instead of oil. The main activities during the subsurface blowout field trial are illustrated in Fig. 1.
Release arrangement The oil and gas or the water and gas were released from a device made especially for this purpose prior to the sea trial. The outlet arrangement consisted of a
H. RYE et al.
U-shaped release pipe, directing the release vertically upwards. The diameter of the outlet opening was 4'. The oil and gas (air) were pumped down to the release platform through 4' and 6' flexible hoses. Three large capacity air compressors were located on the afterdeck on the ship 'GULLBAS' (see Fig. 1) to deliver sufficient air pressure and capacity. Details concerning the different releases are given in section 'Results and discussion'.
Remote sensing The main role of using aircraft during this sea trial was to obtain remote sensing data of the surface slick. The Norwegian State Pollution Control Authorities (SFT) generally uses a surveillance aircraft, in combination with imagery from the ERS satellite, to detect illegal oil discharges. Remote sensing of oil slicks from the air has proved to be an essential part of any oil spill response or experiments carried out at sea. Aircraft provide an oversight of the entire slick area. This is impossible to achieve from the bridge of a ship. Aerial remote sensing enables the full extent of the slicks to be imaged by various sensors. The SFT aeroplane is equipped with remote sensing systems including SLAR (Side Looking Airborne Radar), which detects the change in sea surface roughness caused by oil being present. SLAR images can be obtained up to a range of around 20 km from the aircraft position and show the entire area covered with oil, without any discrimination of oil film thickness. Low clouds do not affect SLAR, although rain showers may interfere with oil detection. The SFT aeroplane is also equipped with UV/IRLS (Ultra-Violet/ Infra-Red Line Scanners). These are dual channel units that can produce simultaneous images of the sea surface as observed in the UV and IR region of the spectrum. The UV scanner essentially detects all the oil area as sheen. The IR scanner detects thicker oil layers as two distinct regions: 1. Those areas of oil that are cooler than the sea surface, referred to as IR 'black', are generally about 10-100/~m thick. 2. Those areas of oil or w/o emulsion that are warmer than the sea surface, referred to as IR 'white', that are thicker than 100 ~m and generally w/o-emulsion in millimetre-range.
Fig. 1 Illustration of the activities during the subsurface blowout simulations.
240
The use of remote sensing aircraft is the most effective way of measuring the size of the oil slicks, how they drift and spread and the distribution of thick and thin areas of oil within the slick (Lewis et al., 1995). In addition, a German aircraft from the German Marine Pollution Control Unit (57+01) participated Spill Science & Technology Bulletin 4(4)
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SUBSURFACEBLOWOUTS:RESULTSFROMFIELDEXPERIMENTS in the sea trial. This plane is similar to that of the SFT aircraft. The purpose of this air plane was to provide a comprehensive and near-continuous record of the development of the oil slicks. Aircraft 57 + 01 is equipped with a modern and comprehensive remote sensing suite including SLAR and UV/IRLS. In addition, this aircraft has Scanning M W R (MicroWave Radiometry). This is a passive technique that detects changes in the microwave brightness of the sea in the presence of oil slicks in a similar manner to IR. Calculations of microwave brightness of oil on water for a given microwave frequency show alternating maxima and minima as the oil thickness increases. The ROV and video camera recordings The R O V was operated from t h e ship G U L L B A S and was mainly used during the subsurface releases. The R O V equipment and operating personnel were hired from the company A/S Maritex of Bergen, Norway. The R O V was a small lightweight R O V of type Sprint 101. The approximate dimensions were 61 cm (length), 61 cm (width) and 70cm (height). The total weight of the R O V is 68 kg. During the exercise the R O V was equipped with a side scanning 360 ° sonar and a visual low light (SIT) video camera. The signals from the sonar and the visual video camera were transferred to the research vessel through cable and continuously recorded on VHS tape. A self-recording video camera was mounted on the release frame and was used to record the release condition and the exact start/stop of oil, water and gas flows. Other measurements In addition, ambient conditions were recorded (winds at 3 m above sea level, waves, ambient currents at 3, 10 and 60 m depth) as well as regular measurements of the vertical distribution of the temperature and salinity (conductivity) with a CTD
The model that was used for comparison with the field results is described in (Rye, 1996). The model is a combination of two other subsurface release models published in the literature (Koh and Fan, 1970; Fannel~p and Siren, 1980). These two models are complementary in the sense that they both model subsurface releases, but they cover different aspects Spill Science & Technology Bulletin
4(4)
-
x
1
Calculation of subsurface plume An illustration of the subsurface plume is given in Fig. 2 (from Fannel~p and Siren, 1980). Note that the velocity profile is somewhat broader than the buoyancy profile. By introducing the radial gaussian shape profile on the subsurface plume, the velocity, radius and the path of the plume can be determined from the conservation equations on the following form (deduced from Koh and Fan, 1970): Q~= ~(8~zM M~ --
General
•
of the problem. Both models simulate the mixing of a subsurface jet based on the principle of conservation of mass, momentum and buoyancy. The Koh and Fan model includes arbitrary stratification combined with an arbitrary orientation of the outlet opening. The Fannelop and Sjoen model includes the features of an expanding gas present in the plume as well as a description of the resulting surface flow and oil slick thickness generated by the subsurface plume. None of the models include effects from the ambient currents on the subsurface plume (bending plumes), but this approximation may be acceptable when the rise velocity of the subsurface plume is significantly larger than the magnitude of the ambient currents. The effect of the surface currents on the resulting slick on the sea surface is included, however. A general feature of a subsurface blowout is that oil and gas under pressure are released and an intense mixing between the oil, gas and the water mass takes place. Except for the initial jet phase, the gas content in the subsurface plume will dominate the buoyancy. The model assumes a constant G O R (gas oil ratio, Sm3/Sm 3) throughout the water column, although some evaporation of the most volatile components must be expected during the rising of the subsurface plume. In the model, adiabatic expansion of the gaseous components is assumed until the water and gas temperatures are equal. From that point, the gas temperature is assumed equal to water temperature.
sensor.
Description of the SINTEF blowout model
A
I ~,]~~'~ ~ ; ~ , ~ 9
(1 +22)gQF sin r
2paM
(1) (2)
(1 + 22) gQF cos z ~s =
2paM2
~P~ F~= 6z Q sin r where Q is the volume flux of the jet, M momentum of the jet, z is the direction of relative to the horizontal, F is the buoyancy of is the entrainment coefficient, index s is the
(3)
(4) is the the jet the jet, deriva-
241
H. RYE et al.
Surface flow
Interaction zone I
Assuming that the density difference between the ambient and the plume is due to the presence of gas, eqn (5) can be approximated by: F = ApOgas
(6)
where Qgas is the volume flux of gas and Ap is the density difference between gas and sea water. The extra term on the right-hand side of eqn (4) may then be approximated by:
Velocity profile
w(r,z) Plume Buoyancy
Fs=--6s ApQg,s W~Ws
profile, p (r,z)
Sea bed . : . . ~..: :~..',. ,~:_..,.-:--~-~----
Virtual origin
_~
~
= r
An illustration of a subsurface plume as presented by Fannel0p and Sj0en (1980).
Fig. 2
tive with respect to the path s, 2 is the Schmidt number (expressing different widths between the velocity and buoyancy distributions in the plume at the same depths), Pa is the ambient density and g is the acceleration of gravity. The terms on the right-hand side of the equations express the conservation principles (mass, momentum and buoyancy). The equations are formulated in such a way that the changes of these quantities are calculated along the plume path s. The initial conditions of the mass flux, the momentum, the angle with the horizontal plane and the buoyancy (density) have to be specified. The equations are then solved numerically along the path s by means of a numerical integration method (Runge-Kutta of the fourth order). Equations for the co-ordinates of the path s, plume width and velocity, temperature and salinity of the water in the plume are all derived from the equations given above. Details are given in Koh and Fan (1970). For this particular application, the release is directed in the vertical. The angle ~ is then equal to 90°, which causes eqn (3) to be zero. The introduction of gas in the plume influences the buoyancy of the plume. An extra term is therefore added to the right-hand side of eqn (4) in order to account for this. The buoyancy term F is defined as: P
F= t. (pa--p)w dA
(5)
where A is the cross-sectional area across the plume, p is the density of plume and w is the vertical velocity of plume. 242
(7)
where Ws is the velocity of the gas bubbles relative to the plume. The correction term W / ( W + W s ) was pointed out by Fannelop and Sj~en (1980), and is due to the motion of the gas bubbles relative to the plume. The buoyancy factor F is not a conservative quantity, but decreases along the plume path due to the presence of stratification in the water masses, and, at the same time, increases along the plume path due to the expanding gas. Both the density difference between gas and water, as well as the volume flux of. gas are increasing along the vertical. The buoyancy term is in fact doubled in the last 10 m before the surface is reached due to the gas expansion effect (neglecting stratification). The stratification in the water mass tends to suppress the buoyancy, but generally the gas lift dominates the buoyancy. The amount of water that is mixed into the plume is described by an entrainment coefficient (assumed to be proportional to velocity and the contact area between the plume and the surrounding water). A comparison between this type of modelling and the experiences from the 1985 Haltenbanken gas blowout and the 1979 IXTOC I oil and gas blowout was presented in Rye (1994). Other types of modelling approaches have also been attempted. One approach was presented by McDougall (1978). His theory was developed further and also confirmed by laboratory experiments by Asaeda and Imberger (1993). Their approaches are applicable in stratified water mass in particular. They found that the plume showed an undulating behaviour in the vertical direction due to the stratification. The plume showed a tendency to regenerate itself once the stratification tends to prevent further penetration upwards. One of the purposes of the field experiments was therefore to investigate such behaviours of the subsurface plumes. Calculation of the slick contour and slick thickness
For oil and gas plumes, Fannel~p and Sj~en (1980) also calculated the radial outflow of the plume generated at the surface. Based on laboratory experiments, an entrainment coefficient for the radial outflow was Spill Science & Technology Bulletin 4(4)
1:1 ~"J~,~ : ~ ~I
SUBSURFACE BLOWOUTS: RESULTS F R O M FIELD E X P E R I M E N T S
Xo= r cost
(10)
From these equations, the upstream penetration is determined to be:
Xupst ....
-- --
m - 21zu~
vm --
r--
(11) u~
r u
i
I
Fig. 3 The contour of a blowout plume at the sea surface. From Fannelop and Sjcen (1980).
determined. From conservation of mass and momentum for the radial outflow, it is possible to determine the depth and the velocity of the radial outflow as explained in the following. According to Fannelop and Sj0en (1980), the combination of the (undisturbed) ambient currents close to the sea surface and the velocity field at the surface generated by the radial outflow of the blowout plume will form a parabolic shaped contour of the spill as shown in Fig. 3. Fannelcp and Sjoen (1980) formulated the equation of the contour of the surface slick by means of what they defined as a stream function if: m Ip = - -
2n
(8)
z + u~y
where ff is the stream function, m is the source strength (equal t o 27~rVm), r is the distance from origin of the subsurface plume area (see Fig. 3), Vm is the outflow velocity at the sea surface generated by the blowout plume (assuming no ambient currents), z is the angle with the x axis directed along the ambient current direction (denoted 0 in Fig. 3), u~ is the ambient current velocity at the sea surface (denoted u in Fig. 3) and y is the lateral horizontal co-ordinate. Assuming that the source strength (the product of r and Vm) is not varying with the distance, r, the equation for the contour of the oil slick (given by the co-ordinates x0 and Y0) becomes Y0= r s i n z =
--
1--
2u~
Spill Science & Technology Bulletin 4(4)
(9)
where the relation m = 2rtrv m has been used. Equation (11) may be interpreted as the distance upstream where the velocity Vm equals the ambient velocity u~. However, the basis for this equation is that the source strength (2rCrVm) is kept constant for increasing r from the source origin. The oil slick thickness at some distance downstream is calculated from a balance between volume flux of oil in the blowout release and the outward flux of oil on the sea surface within the slick area. The slick thickness may then be approximated by the flux of oil in the release divided by the product of the width of the contour and the ambient surface current velocity. In the present version of the model, the oil slick thickness upstream is assumed to be equal to the slick thickness calculated for x = 0.
Results and discussion This chapter presents and discusses the main findings from the subsurface releases performed. Ambient conditions
Five different release conditions were studied, one with the oil and air ( G O R = 67) and four with water and air (gas water ratio = G W R between 67 and 7.25). All releases were performed slightly east of the Frigg field in the North Sea (close to 60 ° 02' N and 2 ° 30' E). The oil release was initiated at 12 June 1996, close to 1103 local time (0903 GMT). The other subsurface releases (with water and air) were conducted during the period from 11 June, 2032 local time (1832 GMT), to 12 June, 1610 local time (1410 GMT). Some of the experiments were only performed as a check of the experimental set-up. Winds and waves recorded during the time period of the releases (11-12 June 1996) are presented in Fig. 4 and Fig. 5. The maximum wind velocity was 11.3 m/s and occurred at some hours before the release of the oil. The velocity during this release was 9-10 m/s. The wind direction during the release was from W - N N W (260-330 dog). The significant wave height was close to 1.8 m during the oil spill release, which was also close to the maximum wave height recorded (1.9 m). Mean wave period was close to 5 s. 243
•
~
•
" A
WIND
SPEED
at 3 m
12 10
2 0
~.-
jun 10 12:00 jun 11 00:00 jun 11 12:00 jun 1200:00 jun 12 12:00 jun 1300:00 Date and time 1996 (GMT) WIND
DIRECTION
at 3 m
The stratification in the water mass remained relatively stable during the experiment. The profile recorded at 0800 G M T at 12 June 1996 (one hour prior to the oil release) is shown in Fig. 7. The profiles show the presence of a salinity gradient at 2 5 - 5 0 m depth, increasing by about 0.45 ppt over this range. The temperature shows a more gradual variation, with homogeneous water mass in the upper 20 m of the water column. The stratification reflects some gradients due to summer heating and also some weak brackish water influence.
360
• 300 e~ 240 d
Description of the different subsurface releases
180
o
~ 120 60 0 iun 10 12:00 jun 11 00:00 iun 11 12:00 iun 1200:00 iun 12 12:00 jun 13 00:00 Date and time 1996 (GMTI
Fig. 4 Wind velocity and direction at 11 and 12 June 1996. Wind in m/s recorded 3 m above sea level. Recordings performed by Oceanor.
The outlet arrangement consisted of a U-shaped release pipe, directing the release vertically upwards. The outer diameter of the outlet opening was 4' (0.1016 m). The inner diameter was 0.090 m. With a pumping rate of 1 m3/min of oil or water, the release velocities are estimated to be as shown in Table 1.
(a) Figure 6a and b show the current velocity and direction, respectively, for the time period 11-12 June 1996. The recordings show that the currents rotated due to tidal action (and possibly other osciilations with periods close to the tidal period of 12.4 h). At the time of the subsurface oil release, the currents were fairly uni-directional from N (direction towards 163-181 deg) with velocities of the order of 16-18cm/s. Thus, the winds and currents have different directions, with the currents heading towards S during the oil release while the winds are acting on the sea surface generally towards SE. The stratification (the vertical variation of temperature and salinity) in the water column was recorded by a CTD sensor (Conductivity, Temperature and Depth recording unit) from a nearby ship• The stratification wasmeasurement r e c o r d e d w i tperiod h r e g u l11-12 a r t i m June e i n t e1996. rvals during the
Ambient currents
I--3m--10m~6Om[
a5 i
~ 20
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:'
~ 15 "~ 10 d s ..... I ..... I ..........
0
6
0
12
18
I .......... 24
I ..........
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36
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11 -12. June 1996
(b)
Ambient currents
I--3m --10m --60ml
36o. 3270~--15~ i
i~
~ "
'
'ili~i
160 SIGNIFICANT
1.5
e-,
w'~
WAVE
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'"
135 ~-I
~
4 .
1.0
o
i
o.6 o.o
jun 10 12:00 jun 11 00:00 iun 11 12:00 jun 1200:00 jun 12 12:00 jun 1300:00 Date and time 1996 (GMT)
Fig. 5 Significant wave height (H~) recorded at 11-12 June 1996. Wave height in metres. Recordings performed by Oceanor. 244
0
.
6
.
.
.
.
.
'
12
.
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.
.
.
18 24 30 11 - 12. June 1996
.
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42
48
Fig. 6 (a) Current velocity recorded at 3, 10 and 60 m depth at 11-12 June 1996. Time of the oil spill at 12 June 1996, 0903-0946 GMT, corresponding to hour No. 33-34 in the figure. (b) Current direction recorded at 3, 10 and 60 m depth at 11-12 June 1996. Time of the oil spill at 12 June 1996, 0903-0946 GMT, corresponding to hour No. 33-34 in the figure. Recordings performed by Oceanor. Spill Science & Technology Bulletin 4(4)
S U B S U R F A C E BLOWOUTS: RESULTS F R O M F I E L D E X P E R I M E N T S
The release opening was mounted on a frame, which was lowered from the boat ' G U L L B A S ' down on the sea bottom close to the stern of the ship. A photograph of the release of oil, taken from the video camera mounted on the frame, is shown in Fig. 8. An outline of the release arrangement is shown in Fig. 9.
Temperature, =C
7
6
8
9
10
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.E •" 60
~ 60 lOO
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134.4
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a=
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,,~ 80, 11313.
120 -
Density in sigmaT unit
26.4 0.
26.6
26.8
27
27.2
27.4
20, E
c
.¢:: ~k
40, 60"
lOO
12o Fig. 7 Temperature, salinity and corresponding density recorded on the site at 12 June 1996 at 0800 GMT. Temperature in °C. Salinity in ppt. Density in ~rT units ( = density in kg/m 3 minus 1000 kg/m3). Recordings performed by Oceanor.
Table 1 Release fluxes and release velocities for various water and air releases (GWR) used during the experiment
GWR
Flux of water m3/min
Flux of air at 106 m depth, m3/min
Release velocity, m/s
7.25 18.6 47 67
1 1 1 1
0.725 1.86 4.7 6.7
4.5 7.3 14.7 20.2
The oil release is similar to the G W R = 67 case.
Spill Science & Technology Bulletin 4(4)
~l~;t :[ll~
¢I
The list of tests performed is shown in Table 2. Note that the oil release test was performed jointly with the G W R = 67 test (test No. 3 in Table 2). This was done intentionally in order to avoid transient effects when the oil release was initiated. The rising time of the oil was observed by taking the time difference between when the oil first appeared at the mouthpiece and then arriving at the sea surface. The oil was observed at the mouthpiece at 0903 G M T and then at the surface 124 s later, which gives an average rising time of 0.85 m/s. The last oil left the mouthpiece at 0946 GMT. After that, the release continued until about 1000 G M T with water instead of oil. The oil used in the experiment was a Troll crude oil with a density of 893 kg/m 3. Further characterisation of the oil is given in Str0m-Kristiansen et al. (1996) and Str0m-Kristiansen et al. (1995).
Comparison between calculated and measured plume widths
Salinity, ppt
34.3 0'
B :3 ~t
Five different subsurface blowout simulations creating a subsurface plume were detected with the R O V equipment, four with air and water in the release and one with the oil and air. The same procedure was applied each time as follows. The recording camera was placed on the release frame before deployment. The R O V was deployed at 106 m depth before the start of each release. The position was initially selected to be some distance downstream from the outlet opening (see outline of the set-up a r r a n g e m e n t in Fig. 10). When the release was started, the R O V recorded the plume, both with video and with sonar. The R O V ascended vertically, while the plume was scanned with the sonar at 10 m depth intervals. This procedure was repeated at every 10 th meters from the lowest depth (102 m) all the way up to 22 m depth below the sea surface (or as long as it was possible to follow a distinct plume). For each depth, a number of side scans were performed (12-35) in order to assess the statistical variability (standard deviation). The data estimated for every depth were the distance from R O V to plume, plume diameter and R O V heading. The determination of the gas plume diameter with the sonar will in this case be dependent on the sensitivity of the sonar to the bubble density in the water mass. The exact sensitivity was not determined analytically. However, the error introduced by the uncertainty connected with this sensitivity.is assumed to be smaller than the uncertainty in the sonar readings. The typical standard deviation within the 12-35 sonar recordings at each depth was 2 - 5 m. The five blowout simulations that were carried out correspond to the five cases indicated in Table 3. The 245
H. R Y E et al.
Fig. 8
Image taken by the video camera m o u n t e d on the release frame during the release of the oil and air.
time of start and termination of each vertical scan are as follows. The results from the plume diameter measurements for the oil and air release case are shown in
v
Fig. 11 along with the corresponding calculated plume diameter. The measured plume diameters for the GWR cases (GWR = 7.25, 18, 47 and 65) are shown in Figs 12-15. The results show that the actual plume diameter was wider than the calculated diameter. The width of the plumes in Figs 12-15 are calculated in the model to be equal to ,86, where a is the standard deviation of the velocity distribution about the centre of the plume.The entrainment coefficient used in the model calculations is 0.1. The calculated width of the plume indicates that the chosen value of the entrainment coefficient for the plume (0.1) may be too low. An increase of the entrainment coefficient would widen the plume in the calculations, in accordance with observations. It may also be that a larger entrainment coefficient should be applied initially (during the jet phase of the release). Such a procedure has been recommended by Rowe
Table2 Listing of the different test cases run experiments with subsurface releases Experiment No.
Fig. 9 Outline of the release arrangement. Diameter of release opening 90 m m . Distance from the beginning of the oil/air mixing zone and the release opening is 0.8 m. 246
1 Test 1 Test 2 2 3 3 (cont.) 4 5
Date 11 June 11 June 12 June 12 June 12 June
1996 1996 1996 1996 1996
Time (GMT)
18.32-19.30 21.10-21.25 05.55-06.10 07.45-08.35 08.40-10.00 (09.03-09.46) 12 June 1996 11.15-11.53 12 June 1996 12.20-14.10
GOR or G W R 7.25 G W R 65 G W R 65 G W R 18 G W R 67 G W R 67 G O R 65 G W R 46 G W R
during the
Oil or water Air/water Air/water Air/water Air/water Air/water Air/oil Air/water Air/water
Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD
EXPERIMENTS
and Laureshen (1988), who observed similar discrepancies between model and experiment.
I ~ ~t ~
:~1~ ~ I
The relative increase of the horizontal extent of the plume as a function of depth compares reasonably
I Hose
f/OO
I
ROV
Hose
Plume
10
Sonar
ROV 360 ° polarscan Fig. 10
Illustration of the monitoringthe subsurfaceplume with use of sonar.
Table 3
Listing of the different sonar side scans carried out during the subsurface release cases
Test case No.
Date
1 2 3 4 5
11 June 12 June 12 June 12 June 12 June
1996 1996 1996 1996 1996
Time (GMT)
Depth interval
G O R or GWR
18.32-19.27 07.43-08.25 09.09-09.41 11.14-11.44 12.21-12.45
102-42" 102-22 102-22 102-22 102-22
7.25 G W R 18 G W R 67 G O R 65 G W R 46 G W R
aAt 62 m depth, this plume was reported by the R O V observers to drift towards W. At 52 m depth, the plume was reported to be 'considerably diminished'. At 42 m depth, there was no sign of the plume.
Spill Science & Technology Bulletin 4(4)
247
q
?
* Modelled diameters / / • Measured diameters (Sonar)] /
20
I
/ /
•
/
I
•
/'•/ 40
/ /
/
J:
I
//
I
60 /
_
80 /
// Error bars on measured plume diameter are one STDEV on each s d e
/
100
// i
5
0
10
15
20
25
30
35
Plume diameter (m) Fig. 11
Comparison between measured (with error bars) and calculated plume diameter. Oil and air release G O R = 67. Test case No. 3 in
Table 3.
well between measurements and observations, which indicates that the entrainment rate is probably of the right order. However, there is a large 'offset' value of the width of order 5 - 1 0 m close to the bottom. It seems unlikely that the plume will increase to this size just a few metres above the bottom. The reason why the measurements show this large initial width is not properly understood. Inspection of the ROV film pictures taken close to the bottom indicates an increase of the width corresponding to a 20 ° opening
of the cone representing the release close to the bottom. Above 102 m depth, the angle of the cone reduces down to 12-13 °, which corresponds closely with the cone angle indicated by the calculations (about 14.4° for all release cases considered). This increase of the cone angle close to the release site is consistent with the observations reported by Rowe and Laureshen (1988). However, this increase of the cone angle close to the release opening (representing an increase of the
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Spill Science & Technology Bulletin 4(4)
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SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
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entrainment of sea water into the plume as well) is not sufficient to explain all the discrepancies between the model and observations shown in Figs 11-15. Another explanation could be influences from the motion of the R O V during the horizontal scanning of the plume. This will lead to a 'wider' plume in the sonar signal, because the plume will appear to 'move' during the sonar scanning, relative to the ROV. The same 'offset' effect was found in the data from the 1995 N O F O sea trial as well, where the same procedure was applied. However, the offset value was
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Spill Science & Technology Bulletin 4(4)
249
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Figure 16 shows the calculated plume widths for all the G W R cases. The width tends to increase to infinity for the G W R = 7.25 case in the calculations, which indicates that the plume ceases to rise at about 30 m depth. The reason why the plume stops rising to the sea surface in the G W R = 7.25 case is the effect of stratification. The parameter F in Section 3 (see eqns (5)-(7)) turns negative at a certain depth due to the stratification in the water mass, indicating that the
buoyancy force is negative and the plume will tend to sink. Also, for the G W R = 18 case, the calculations indicate some depth interval where the parameter F is weakly negative. The F parameters for the two cases G W R = 7.25 and 18 are shown in Fig. 17. Note also that for the G W R = 18 case, the plume was not observed at the surface either. From the subsurface plume recordings, the plume seems to diminish or be considerably reduced in diameter above 22 m depth.
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Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
This depth is at approximately the top of the dominant stratification in the ambient water mass (see the temperature and salinity profiles in Fig. 7). Therefore, the G W R = 18 appears to be close to the turning point of the release for not reaching the sea surface. There appears to be good correspondence in the location of this turning point (along the G W R scale) between measurements and calculations. On the other hand, the actual depth for maximum
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1
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penetration of the plume (for the G W R = 7.25 and 18 cases) is closer to the sea surface in the calculations, compared with measurements, in particular for the G W R = 7.25 case.
Comparison between calculated and measured vertical
plume velocity In order to determine the vertical velocity of the ascending plume, six current meters were deployed close to the outlet arrangement as'shown in Fig. 18. Two moorings were deployed at positions D1 and D2, close to the stern of GULLBAS. On each mooring, the current meters were mounted at 20, 30 and 40 m depth. The meters sampled the vertical current component integrated over 10 min. The meters were of the Aanderaa type with a Savonius rotor. The data analysis showed two occasions where the current meters recorded vertical velocities caused by the ascent of the subsurface plume. In each case, the duration of the intersection of the current meters and the plume is limited to be of order 30-50 rain. This is partly due to the changing ambient currents, causing the plume to pass through the location of the current recordings.
Underwater release 12. June 1996 situation at 08:10 GMT
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Fig. 18 The location of the moorings for the recording of vertical currents (positions DI and D2) and horizontal currents (positions O1 and 02). Spill Science & Technology Bulletin 4(4)
251
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These two occasions occurred on 12 June at approx. 0600 G M T and 12 June at 1300-1400 GMT. These time intervals correspond to a test case conducted with air and water with at G W R = 65 and to the last release case with a G W R = 46. See Table 2 for an overview of the cases run. The vertical velocities were recorded to be at maximum 5 0 - 60 cm/s at the D1 location ( G W R = 65 case) and 25-30 cm/s at the D2 location ( G W R = 4 6 case). The vertical velocity of the centre of the plume calculated by the numerical model is shown in Fig. 19 for the G W R = 67 and 46 cases. The calculated velocities are larger than those measured. However, the average velocity of the rising plume is expected to be lower than the velocity in the centre of the plume. A factor of 1.4 between the average velocity and the centre velocity of the rising plume is sometimes used. When this reduction is taken into account, the correspondence improves somewhat. It is, however, difficult to state the precise correspondence between the measurements and the calculations, because the instrument may record the currents at some distance from the centre of the plume.
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Table 4
The horizontal motion of the plume on its way through the water column
The horizontal velocity profile was recorded at 3, 10 and 60 m during the field trial. If these recordings are interpolated (between the depth interval 3 and 6 0 m ) and extrapolated outside this interval, an estimate of the horizontal current velocity profile in the vertical can be made. By taking into account the calculated velocity of the vertical motion of the plume, the expected horizontal advection of the plume can be calculated. Reading off representative current velocities at the time where the five different releases were performed (cases 1-5 in Table 3), the actual numbers used are shown in Table 4. This table also shows the results from the expected position of the subsurface plume, when arriving at the sea surface (except for the G W R = 7.25 case). The locations of the plume at the sea surface for the two lowest G W R cases (see Table 4) are not relevant because the plume was never observed at the sea surface. If the plume for the G W R = 18 case had reached the sea surface, it would probably have been observed because the location of the plume should have been far away from the influence of the thrusters on GULLBAS (typical area of influence of order 10 m in all directions). For the G W R = 46 and 65 cases, the plume was observed at the sea surface, but the locations were close to the area of the most intense propeller action. The case with the clearest separation between the propeller action at the stern of the ship and the upstream area generated by the plume is the oil and air release case ( G O R = 67). A photograph of this particular case is shown in Fig. 20, showing the location of the plume at approx. 20 m in the direction SSE relative to the stern of the ship. (Also see the location of the release arrangement, as indicated in Fig. 18). The observed location of the plume is thus in good agreement with the calculated position where the subsurface plume hits the sea surface, as shown in Table 4.
Observations of current velocities and directions and calculated positions of the plumes at the sea surface
Time G M T 3 m vel. 3 m dir. 10 m vel. 10 m dir. 60 m vel. 60 m dir. Pos. plume, distance Pos. plume, direction
G W R = 7.25
G W R = 18
G W R = 46
G W R = 65
G O R = 67
11/6 at 1830 0.176 343 0.153 327 0.047 020 Not at the sea surface Not at the sea surface
12/6 at 0800 0.147 169 0.138 175 0.152 155 28 m 163
12/6 at 1230 0.035 208 0.101 234 0.070 208 10 220 a
12/6 at 1130 0.129 205 0.171 209 0.123 172 15 190
12/6 at 0930 0.170 177 0.160 181 0.182 163 19 170
All positions are indicated relative to the position of the outlet arrangement. Velocities in m/s and directions in 0-360 degrees. Time in GMT. "The direction of the currents was changing from S to N during the recording period, see Fig. 6 for the ambient current recordings. 252
SpillScience& TechnologyBulletin4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
The width of the subsurface plume was calculated by means of the mathematical model, see Fig. 16. The width of the plume at the location where the subsurface plume reaches the sea surface was calculated to be in excess of 20 m diameter. This appears to be of the same magnitude as the observed width, as shown in Fig. 20 (see the whiter shaded area at the parabola centre close to the stern of the ship).
Comparison of calculated and measured surface oil slick dimensions The release was intended to simulate a subsurface blowout event. The equations (8)-(11) predict a surface contour and a thickness of the resulting oil slick at the sea surface. Using a surface current velocity of 0.16 m/s and the results from the subsurface plume calculations at the point where the plume reaches the sea surface, the resulting contour of the slick and the thickness can be calculated. Figure 21
shows the calculated surface contour of the surface slick with the Fannel0p and Sj0en (1980) algorithm, using the data for the 1996 sea trial ( G O R = 67): The observed width of the surface slick 200 m downstream from the front is approximately 300 m (see Figs 20, 22 and 23). The modelled width in Fig. 21 is approximately twice as large, 650 m at the same distance from the front of the surface oil slick. In this case, the calculated slick width is approximately twice as large as the observed width. According to calculations, the thickness of the oil in the slick was expected to be close to 100-160/~m. These calculations assume that all the released oil surfaces and that it is not emulsified. Because the size of the slick was somewhat overestimated by the model, one should expect that the measured thickness of the oil would be somewhat larger than calculated. On the contrary, the thickness was found to be of order 5-20 #m on average (occasionally above 30/~m, see Fig. 24), accounting for only 15-20% of the total
Fig. 20 Photo of the contour of the sea surface oil slick taken during the 1996 oil and air subsurface release. Length of ship is 80 m. The slick is drifting southwards. Wind is from the W N W .
Spill Science & Technology Bulletin 4(4)
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H. RYE et al.
amount of oil released. The rest of the oil did not surface during the experiment time. The thickness of the slick and the amount of oil in the slick were also estimated by image analysis of the IR and UV images taken by the German remote sensing aircraft. The contours (slick areas) of the different parts of the resulting surface slick were determined by image analysis. A film thickness of 25/~m was used as a conservative estimate (see Figs 22 and 23). The reason why the bulk of the oil did not reach the sea surface is not clear. Drop size measurements were not performed during the oil release. One possible explanation is that the release arrangement is creating large release velocities, estimated to be close to 20 m/s. In addition, the air is mixed with the oil by releasing the airflow into the oil stream with a velocity of the order 50m/s within the release arrangement itself. These velocities may be large enough to create strong velocity shears that will cause the oil to disperse into small droplets. When this mixture approaches the sea surface, the smaller oil droplets may then have a corresponding small rise
velocity with a low ability to reach the sea surface. Thus, the oil droplets may be trapped within the subsurface plume instead of rising to the sea surface.
Summary and conclusions Different features of the subsurface model have been compared with results from the field trials on the Frigg field in June 1996. The comparison between the measurements and the calculations shows that the model represents many of the features of the release fairly well. However, some other features of the release were not sufficiently well reproduced. The subsurface plume dimensions seem to be of the same proportions as the computer modelling results. The diameter of the subsurface plume was
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Fig. 21 Calculation of the surface contour of the slick and the expected thickness of the oil in the slick, applying the Fannel~ap and Sj~aen (1980) algorithm to the subsurface oil and gas release experienced during the June 1996 sea trial. The oil slick thickness is calculated on the basis that all oil is contained in the slick, with no evaporation or emulsification. 254
Fig. 22 Remote sensing picture taken from the German aerial surveillance aircraft showing the oil slick during release (30 min after start) indicating a surface slick width of approximately 300 m. Distance between crosses is 200 m. Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
calculated to be smaller than what was measured. The reason for this is not clear, but the motion of the R O V during the sonar scan may have contributed to some of the discrepancies. The vertical velocity of the subsurface plume was calculated to be of the order 1 m/s in the centre of the plume. The measurements indicated somewhat lower velocities. It is difficult to say anything more specific about the correspondence between the measurements and the calculations, because the correspondence will depend on how far away from the centre of the plume the recording instruments have been located. The recorded rising time of the plume is found to be in reasonable agreement with the calculations. The calculated rising times were slightly larger than the observed ones. The simulations indicate a reasonably good correspondence between the observed location
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where the subsurface plume penetrated the sea surface and the calculated location, based on the observed horizontal currents. The results from the present field trial resulted in a slick thickness that was less than predicted by theory. The reason for this is probably that the bulk of oil did not reach the sea surface at all, but was contained within the water plume below the slick. One possible explanation for this is that details in the release arrangement or the size of the release velocity contributed to create a large amount of small droplets with a small ability to rise through the water mass in the plume. The velocities used in the release were of the order of 20-50 m/s, which are within the typical range of blowout velocities anticipated for subsurface releases, in particular for deeper waters. Thus, it is reasonable to assume that similar behaviour of the oil can be expected for real underwater blowouts. There was no definitive evidence of regeneration of plumes, after the plume was trapped in the stratified water layers. Some of the plume widths determined from the sonar signal showed that the plume diameter was reduced at the top of the water column. However, the measurements cannot be said to be conclusive at this point. Additional investigations are therefore recommended. Acknowledgements--This work could not have been carried out without the close co-operation and the financial support from a number of different bodies and organisations. The financial support from NOFO and Norsk Hydro is hereby acknowledged. Assistance from NOFO, Norsk Hydro, Oceanor, Maritex, SFT (aircraft) and the German Marine Pollution Control Unit (aircraft) is also greatly acknowledged.
References
Fig. 23 Remote sensing picture taken from the German aerial surveillance aircraft showing the oil slick 20 min after release was finished. Distance between crosses is 200 m.
Spill Science & TechnologyBulletin 4(4)
Asaeda, T. & Imberger, J. (1993). Structure of bubble plumes in linearly stratified environments. Journal of Fluid Mechanics, 249, 35-57. Fannel~p, T. K. & Sj~aen, K. (1980). Hydrodynamics of subsurface blowouts. Norwegian Maritime Research, 4, 17-33. Koh, R. & Fan, L. (1970). Mathematical models for the prediction of temperature distribution resulting from the discharge of 255
H. RYE et al. heated water into large bodies of water. EPA Report, Water Quality Office, October 1970. Lewis, A., Str0m-Kristiansen, T., Brandvik, P. J., Daling, P. S., Jensen, H. & Durell, G. (1995). Dispersant Trials-NOFO Exercise 6-9 June 1994-Main Report, IKU report no: 22.2050.00/14/95. McDougall, T. J. (1978). Bubble plumes in stratified environments. Journal o f Fluid Mechanics, 85, 655-672. Rowe, R. D. & Laureshen, C. J. (1988). An investigation of bubble groups from an undersea oilwell blowout. Proceedings from the l l t h AMOP seminar, Vancouver, 7-9 June 1988. Rye, H. (1994). Model for calculation of underwater blowout plume. Proceedings, 17th Arctic and Marine Oil Spill Program (AMOP), 8-10 June. Vancouver, Canada.
256
Rye, H. (1996). Subsurface oil release field experiment - - observations and modelling of subsurface plume behaviour. Proceedings, 19th Arctic and Marine Oil Spill Program (AMOP), 12-14 June 1996. Calgary, Canada. Str0m-Kristiansen, T. et al. (1995). Forvitringsegenskaper p~ sj0en for Troll r~olje. En h~ndbok for Norsk Hydro a.s. Written in Norwegian. IKU/SINTEF Report No. 41.5132.00/.01/01/95. 104 pages. Strom-Kristiansen, T. et al. (1997). NOFO 1996 Oil on Water Exercise - - Analysis of Sample Material. SINTEF Report STF66 F97050. SINTEF Applied Chemistry. Yapa, P. D. & Zheng, L. (1998). Modelling oil and gas releases from deep water. A review. Spill Science and Technology, 4(4), 189-198.
Spill Science & Technology Bulletin 4(4)
© 1998Publishedby ElsevierScienceLtd All rights reserved. Printed in Great Britain 1353-2561/98$19.00+0.00
Pergamon PII: S1353-2561(98)00021-8
Subsurface Blowouts: Results from Field Experiments H. RYE, P. J. B R A N D V I K & T. S T R O M
S I N T E F Applied Chemistry, 70 34 Trondheim, Norway
A field experiment was carried out in June 1996 with a subsurface release of air and oil close to the Frigg Field in the North Sea. One of the purposes of this sea trial was to increase the knowledge concerning the behaviour of the oil and gas during a subsurface blowout. This was done by simulating a subsurface blowout at 106 m depth with a realistic gas oil ratio (GOR = 67) and release velocity of the oil/gas. In addition to the main oil release (43 m3 Troll crude), other releases with water and air ( G O R = 7-65) were performed. Important and unique data were collected during these subsurface releases that can be used to improve numerical models for such releases.The main conclusions from these trials are: • The field methodology used to study blowout releases in the field appears to be appropriate. • The surface oil slick formed by subsurface releases is significantly wider and thinner than if it was caused by a surface release. • Only 15-20% of the released oil was detected at the surface. This percentage is expected to depend on factors like depth, GOR, oil type and release velocity. • Some discrepancies between existing models and field results were found. The field data may thus serve as a basis for model improvements. © 1998 Published by Elsevier Science Ltd. All rights reserved
The oil and gas industry is presently moving their activities towards deeper waters. At the same time, the technical development also favours sub-sea solutions rather than installing platforms that penetrate the sea surface. These developments enhance the probability of having a blowout with releases of oil and gas from the sea bottom rather than at the sea surface. As a part of the contingency planning, a realistic description of blowout scenarios is essential, which involves the use of numerical models to describe the behaviour of the subsea plume as well as the dimensions of the resulting slick on the sea surface. Different models for prediction of behaviours of subsea blowouts have been proposed. A review of various model approaches that have been attempted in the past have recently been summarised by Yapa and Zheng (1998). A model for subsea blowout plume behaviour and the resulting surface slick dimensions is operable presently at S I N T E F Applied Chemistry. Although the main features of this model
will shortly be presented in this paper, the main objective is to present and describe field data on subsurface plume behaviour and the resulting slick formation. A comparison between field results and model results may therefore reveal where the largest potential for improvements of existing models is expected. In order to obtain relevant field data, 43 m 3 of stabilised Troll oil was released at 106 m depth over a time span of 40 min (approximately 17 l/s) jointly with air. The release velocity and G O R (gas oil ratio, or, in this particular case, the 'air oil ratio') were chosen to be representative for a potential blowout at the Troll oil and gas field, located outside Bergen on the west coast of Norway.
Objectives The subsurface release was carried out as a part of the N O F O (Norwegian Clean Seas Association, 239
RESEARCH Stavanger, Norway) sea trial at the Frigg field in the North Sea for 1996. The main objective of the experiment was to increase our knowledge concerning the behaviour of the oil and gas during a subsurface blowout by: 1. simulating different subsurface blowout conditions at 106 m depth with a realistic G O R and release velocities. 2. extensive monitoring of the subsurface oil/gas/ water plume and the resulting surface oil slick. 3. forming a basis for improving existing blowout models using the field data collected during these subsurface releases.
Experimental test programme Several types of measurements were carried out, both for collecting background information and for making recordings of the plume and slick properties. Examples of such activities are R O V (Remote Operating Vehicle) monitoring of the subsurface plume with sonar, video of the release at the outlet opening, sampling of oil in the slick, remote sensing of the surface slick, temperature and horizontal current recordings close to the plume as well as horizontal currents, temperature and salinity recordings of the ambient water masses. In addition to the oil and gas release, a number of other subsurface releases were carried out, using water instead of oil. The main activities during the subsurface blowout field trial are illustrated in Fig. 1.
Release arrangement The oil and gas or the water and gas were released from a device made especially for this purpose prior to the sea trial. The outlet arrangement consisted of a
H. RYE et al.
U-shaped release pipe, directing the release vertically upwards. The diameter of the outlet opening was 4'. The oil and gas (air) were pumped down to the release platform through 4' and 6' flexible hoses. Three large capacity air compressors were located on the afterdeck on the ship 'GULLBAS' (see Fig. 1) to deliver sufficient air pressure and capacity. Details concerning the different releases are given in section 'Results and discussion'.
Remote sensing The main role of using aircraft during this sea trial was to obtain remote sensing data of the surface slick. The Norwegian State Pollution Control Authorities (SFT) generally uses a surveillance aircraft, in combination with imagery from the ERS satellite, to detect illegal oil discharges. Remote sensing of oil slicks from the air has proved to be an essential part of any oil spill response or experiments carried out at sea. Aircraft provide an oversight of the entire slick area. This is impossible to achieve from the bridge of a ship. Aerial remote sensing enables the full extent of the slicks to be imaged by various sensors. The SFT aeroplane is equipped with remote sensing systems including SLAR (Side Looking Airborne Radar), which detects the change in sea surface roughness caused by oil being present. SLAR images can be obtained up to a range of around 20 km from the aircraft position and show the entire area covered with oil, without any discrimination of oil film thickness. Low clouds do not affect SLAR, although rain showers may interfere with oil detection. The SFT aeroplane is also equipped with UV/IRLS (Ultra-Violet/ Infra-Red Line Scanners). These are dual channel units that can produce simultaneous images of the sea surface as observed in the UV and IR region of the spectrum. The UV scanner essentially detects all the oil area as sheen. The IR scanner detects thicker oil layers as two distinct regions: 1. Those areas of oil that are cooler than the sea surface, referred to as IR 'black', are generally about 10-100/~m thick. 2. Those areas of oil or w/o emulsion that are warmer than the sea surface, referred to as IR 'white', that are thicker than 100 ~m and generally w/o-emulsion in millimetre-range.
Fig. 1 Illustration of the activities during the subsurface blowout simulations.
240
The use of remote sensing aircraft is the most effective way of measuring the size of the oil slicks, how they drift and spread and the distribution of thick and thin areas of oil within the slick (Lewis et al., 1995). In addition, a German aircraft from the German Marine Pollution Control Unit (57+01) participated Spill Science & Technology Bulletin 4(4)
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SUBSURFACEBLOWOUTS:RESULTSFROMFIELDEXPERIMENTS in the sea trial. This plane is similar to that of the SFT aircraft. The purpose of this air plane was to provide a comprehensive and near-continuous record of the development of the oil slicks. Aircraft 57 + 01 is equipped with a modern and comprehensive remote sensing suite including SLAR and UV/IRLS. In addition, this aircraft has Scanning M W R (MicroWave Radiometry). This is a passive technique that detects changes in the microwave brightness of the sea in the presence of oil slicks in a similar manner to IR. Calculations of microwave brightness of oil on water for a given microwave frequency show alternating maxima and minima as the oil thickness increases. The ROV and video camera recordings The R O V was operated from t h e ship G U L L B A S and was mainly used during the subsurface releases. The R O V equipment and operating personnel were hired from the company A/S Maritex of Bergen, Norway. The R O V was a small lightweight R O V of type Sprint 101. The approximate dimensions were 61 cm (length), 61 cm (width) and 70cm (height). The total weight of the R O V is 68 kg. During the exercise the R O V was equipped with a side scanning 360 ° sonar and a visual low light (SIT) video camera. The signals from the sonar and the visual video camera were transferred to the research vessel through cable and continuously recorded on VHS tape. A self-recording video camera was mounted on the release frame and was used to record the release condition and the exact start/stop of oil, water and gas flows. Other measurements In addition, ambient conditions were recorded (winds at 3 m above sea level, waves, ambient currents at 3, 10 and 60 m depth) as well as regular measurements of the vertical distribution of the temperature and salinity (conductivity) with a CTD
The model that was used for comparison with the field results is described in (Rye, 1996). The model is a combination of two other subsurface release models published in the literature (Koh and Fan, 1970; Fannel~p and Siren, 1980). These two models are complementary in the sense that they both model subsurface releases, but they cover different aspects Spill Science & Technology Bulletin
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x
1
Calculation of subsurface plume An illustration of the subsurface plume is given in Fig. 2 (from Fannel~p and Siren, 1980). Note that the velocity profile is somewhat broader than the buoyancy profile. By introducing the radial gaussian shape profile on the subsurface plume, the velocity, radius and the path of the plume can be determined from the conservation equations on the following form (deduced from Koh and Fan, 1970): Q~= ~(8~zM M~ --
General
•
of the problem. Both models simulate the mixing of a subsurface jet based on the principle of conservation of mass, momentum and buoyancy. The Koh and Fan model includes arbitrary stratification combined with an arbitrary orientation of the outlet opening. The Fannelop and Sjoen model includes the features of an expanding gas present in the plume as well as a description of the resulting surface flow and oil slick thickness generated by the subsurface plume. None of the models include effects from the ambient currents on the subsurface plume (bending plumes), but this approximation may be acceptable when the rise velocity of the subsurface plume is significantly larger than the magnitude of the ambient currents. The effect of the surface currents on the resulting slick on the sea surface is included, however. A general feature of a subsurface blowout is that oil and gas under pressure are released and an intense mixing between the oil, gas and the water mass takes place. Except for the initial jet phase, the gas content in the subsurface plume will dominate the buoyancy. The model assumes a constant G O R (gas oil ratio, Sm3/Sm 3) throughout the water column, although some evaporation of the most volatile components must be expected during the rising of the subsurface plume. In the model, adiabatic expansion of the gaseous components is assumed until the water and gas temperatures are equal. From that point, the gas temperature is assumed equal to water temperature.
sensor.
Description of the SINTEF blowout model
A
I ~,]~~'~ ~ ; ~ , ~ 9
(1 +22)gQF sin r
2paM
(1) (2)
(1 + 22) gQF cos z ~s =
2paM2
~P~ F~= 6z Q sin r where Q is the volume flux of the jet, M momentum of the jet, z is the direction of relative to the horizontal, F is the buoyancy of is the entrainment coefficient, index s is the
(3)
(4) is the the jet the jet, deriva-
241
H. RYE et al.
Surface flow
Interaction zone I
Assuming that the density difference between the ambient and the plume is due to the presence of gas, eqn (5) can be approximated by: F = ApOgas
(6)
where Qgas is the volume flux of gas and Ap is the density difference between gas and sea water. The extra term on the right-hand side of eqn (4) may then be approximated by:
Velocity profile
w(r,z) Plume Buoyancy
Fs=--6s ApQg,s W~Ws
profile, p (r,z)
Sea bed . : . . ~..: :~..',. ,~:_..,.-:--~-~----
Virtual origin
_~
~
= r
An illustration of a subsurface plume as presented by Fannel0p and Sj0en (1980).
Fig. 2
tive with respect to the path s, 2 is the Schmidt number (expressing different widths between the velocity and buoyancy distributions in the plume at the same depths), Pa is the ambient density and g is the acceleration of gravity. The terms on the right-hand side of the equations express the conservation principles (mass, momentum and buoyancy). The equations are formulated in such a way that the changes of these quantities are calculated along the plume path s. The initial conditions of the mass flux, the momentum, the angle with the horizontal plane and the buoyancy (density) have to be specified. The equations are then solved numerically along the path s by means of a numerical integration method (Runge-Kutta of the fourth order). Equations for the co-ordinates of the path s, plume width and velocity, temperature and salinity of the water in the plume are all derived from the equations given above. Details are given in Koh and Fan (1970). For this particular application, the release is directed in the vertical. The angle ~ is then equal to 90°, which causes eqn (3) to be zero. The introduction of gas in the plume influences the buoyancy of the plume. An extra term is therefore added to the right-hand side of eqn (4) in order to account for this. The buoyancy term F is defined as: P
F= t. (pa--p)w dA
(5)
where A is the cross-sectional area across the plume, p is the density of plume and w is the vertical velocity of plume. 242
(7)
where Ws is the velocity of the gas bubbles relative to the plume. The correction term W / ( W + W s ) was pointed out by Fannelop and Sj~en (1980), and is due to the motion of the gas bubbles relative to the plume. The buoyancy factor F is not a conservative quantity, but decreases along the plume path due to the presence of stratification in the water masses, and, at the same time, increases along the plume path due to the expanding gas. Both the density difference between gas and water, as well as the volume flux of. gas are increasing along the vertical. The buoyancy term is in fact doubled in the last 10 m before the surface is reached due to the gas expansion effect (neglecting stratification). The stratification in the water mass tends to suppress the buoyancy, but generally the gas lift dominates the buoyancy. The amount of water that is mixed into the plume is described by an entrainment coefficient (assumed to be proportional to velocity and the contact area between the plume and the surrounding water). A comparison between this type of modelling and the experiences from the 1985 Haltenbanken gas blowout and the 1979 IXTOC I oil and gas blowout was presented in Rye (1994). Other types of modelling approaches have also been attempted. One approach was presented by McDougall (1978). His theory was developed further and also confirmed by laboratory experiments by Asaeda and Imberger (1993). Their approaches are applicable in stratified water mass in particular. They found that the plume showed an undulating behaviour in the vertical direction due to the stratification. The plume showed a tendency to regenerate itself once the stratification tends to prevent further penetration upwards. One of the purposes of the field experiments was therefore to investigate such behaviours of the subsurface plumes. Calculation of the slick contour and slick thickness
For oil and gas plumes, Fannel~p and Sj~en (1980) also calculated the radial outflow of the plume generated at the surface. Based on laboratory experiments, an entrainment coefficient for the radial outflow was Spill Science & Technology Bulletin 4(4)
1:1 ~"J~,~ : ~ ~I
SUBSURFACE BLOWOUTS: RESULTS F R O M FIELD E X P E R I M E N T S
Xo= r cost
(10)
From these equations, the upstream penetration is determined to be:
Xupst ....
-- --
m - 21zu~
vm --
r--
(11) u~
r u
i
I
Fig. 3 The contour of a blowout plume at the sea surface. From Fannelop and Sjcen (1980).
determined. From conservation of mass and momentum for the radial outflow, it is possible to determine the depth and the velocity of the radial outflow as explained in the following. According to Fannelop and Sj0en (1980), the combination of the (undisturbed) ambient currents close to the sea surface and the velocity field at the surface generated by the radial outflow of the blowout plume will form a parabolic shaped contour of the spill as shown in Fig. 3. Fannelcp and Sjoen (1980) formulated the equation of the contour of the surface slick by means of what they defined as a stream function if: m Ip = - -
2n
(8)
z + u~y
where ff is the stream function, m is the source strength (equal t o 27~rVm), r is the distance from origin of the subsurface plume area (see Fig. 3), Vm is the outflow velocity at the sea surface generated by the blowout plume (assuming no ambient currents), z is the angle with the x axis directed along the ambient current direction (denoted 0 in Fig. 3), u~ is the ambient current velocity at the sea surface (denoted u in Fig. 3) and y is the lateral horizontal co-ordinate. Assuming that the source strength (the product of r and Vm) is not varying with the distance, r, the equation for the contour of the oil slick (given by the co-ordinates x0 and Y0) becomes Y0= r s i n z =
--
1--
2u~
Spill Science & Technology Bulletin 4(4)
(9)
where the relation m = 2rtrv m has been used. Equation (11) may be interpreted as the distance upstream where the velocity Vm equals the ambient velocity u~. However, the basis for this equation is that the source strength (2rCrVm) is kept constant for increasing r from the source origin. The oil slick thickness at some distance downstream is calculated from a balance between volume flux of oil in the blowout release and the outward flux of oil on the sea surface within the slick area. The slick thickness may then be approximated by the flux of oil in the release divided by the product of the width of the contour and the ambient surface current velocity. In the present version of the model, the oil slick thickness upstream is assumed to be equal to the slick thickness calculated for x = 0.
Results and discussion This chapter presents and discusses the main findings from the subsurface releases performed. Ambient conditions
Five different release conditions were studied, one with the oil and air ( G O R = 67) and four with water and air (gas water ratio = G W R between 67 and 7.25). All releases were performed slightly east of the Frigg field in the North Sea (close to 60 ° 02' N and 2 ° 30' E). The oil release was initiated at 12 June 1996, close to 1103 local time (0903 GMT). The other subsurface releases (with water and air) were conducted during the period from 11 June, 2032 local time (1832 GMT), to 12 June, 1610 local time (1410 GMT). Some of the experiments were only performed as a check of the experimental set-up. Winds and waves recorded during the time period of the releases (11-12 June 1996) are presented in Fig. 4 and Fig. 5. The maximum wind velocity was 11.3 m/s and occurred at some hours before the release of the oil. The velocity during this release was 9-10 m/s. The wind direction during the release was from W - N N W (260-330 dog). The significant wave height was close to 1.8 m during the oil spill release, which was also close to the maximum wave height recorded (1.9 m). Mean wave period was close to 5 s. 243
•
~
•
" A
WIND
SPEED
at 3 m
12 10
2 0
~.-
jun 10 12:00 jun 11 00:00 jun 11 12:00 jun 1200:00 jun 12 12:00 jun 1300:00 Date and time 1996 (GMT) WIND
DIRECTION
at 3 m
The stratification in the water mass remained relatively stable during the experiment. The profile recorded at 0800 G M T at 12 June 1996 (one hour prior to the oil release) is shown in Fig. 7. The profiles show the presence of a salinity gradient at 2 5 - 5 0 m depth, increasing by about 0.45 ppt over this range. The temperature shows a more gradual variation, with homogeneous water mass in the upper 20 m of the water column. The stratification reflects some gradients due to summer heating and also some weak brackish water influence.
360
• 300 e~ 240 d
Description of the different subsurface releases
180
o
~ 120 60 0 iun 10 12:00 jun 11 00:00 iun 11 12:00 iun 1200:00 iun 12 12:00 jun 13 00:00 Date and time 1996 (GMTI
Fig. 4 Wind velocity and direction at 11 and 12 June 1996. Wind in m/s recorded 3 m above sea level. Recordings performed by Oceanor.
The outlet arrangement consisted of a U-shaped release pipe, directing the release vertically upwards. The outer diameter of the outlet opening was 4' (0.1016 m). The inner diameter was 0.090 m. With a pumping rate of 1 m3/min of oil or water, the release velocities are estimated to be as shown in Table 1.
(a) Figure 6a and b show the current velocity and direction, respectively, for the time period 11-12 June 1996. The recordings show that the currents rotated due to tidal action (and possibly other osciilations with periods close to the tidal period of 12.4 h). At the time of the subsurface oil release, the currents were fairly uni-directional from N (direction towards 163-181 deg) with velocities of the order of 16-18cm/s. Thus, the winds and currents have different directions, with the currents heading towards S during the oil release while the winds are acting on the sea surface generally towards SE. The stratification (the vertical variation of temperature and salinity) in the water column was recorded by a CTD sensor (Conductivity, Temperature and Depth recording unit) from a nearby ship• The stratification wasmeasurement r e c o r d e d w i tperiod h r e g u l11-12 a r t i m June e i n t e1996. rvals during the
Ambient currents
I--3m--10m~6Om[
a5 i
~ 20
"
:'
~ 15 "~ 10 d s ..... I ..... I ..........
0
6
0
12
18
I .......... 24
I ..........
30
36
42
48
11 -12. June 1996
(b)
Ambient currents
I--3m --10m --60ml
36o. 3270~--15~ i
i~
~ "
'
'ili~i
160 SIGNIFICANT
1.5
e-,
w'~
WAVE
..j~.
HEIGHT
'"
135 ~-I
~
4 .
1.0
o
i
o.6 o.o
jun 10 12:00 jun 11 00:00 iun 11 12:00 jun 1200:00 jun 12 12:00 jun 1300:00 Date and time 1996 (GMT)
Fig. 5 Significant wave height (H~) recorded at 11-12 June 1996. Wave height in metres. Recordings performed by Oceanor. 244
0
.
6
.
.
.
.
.
'
12
.
.
.
.
.
18 24 30 11 - 12. June 1996
.
.
.
.
.
.
.
36
.
.
.
.
.
.
42
48
Fig. 6 (a) Current velocity recorded at 3, 10 and 60 m depth at 11-12 June 1996. Time of the oil spill at 12 June 1996, 0903-0946 GMT, corresponding to hour No. 33-34 in the figure. (b) Current direction recorded at 3, 10 and 60 m depth at 11-12 June 1996. Time of the oil spill at 12 June 1996, 0903-0946 GMT, corresponding to hour No. 33-34 in the figure. Recordings performed by Oceanor. Spill Science & Technology Bulletin 4(4)
S U B S U R F A C E BLOWOUTS: RESULTS F R O M F I E L D E X P E R I M E N T S
The release opening was mounted on a frame, which was lowered from the boat ' G U L L B A S ' down on the sea bottom close to the stern of the ship. A photograph of the release of oil, taken from the video camera mounted on the frame, is shown in Fig. 8. An outline of the release arrangement is shown in Fig. 9.
Temperature, =C
7
6
8
9
10
o
20 E
4O
.E •" 60
~ 60 lOO
12o
134.4
34.5
.
.
i
, A
A
34.6
34.7
34.8
34.9
,
,
I
I
35
20E
40.
a=
60-
,,~ 80, 11313.
120 -
Density in sigmaT unit
26.4 0.
26.6
26.8
27
27.2
27.4
20, E
c
.¢:: ~k
40, 60"
lOO
12o Fig. 7 Temperature, salinity and corresponding density recorded on the site at 12 June 1996 at 0800 GMT. Temperature in °C. Salinity in ppt. Density in ~rT units ( = density in kg/m 3 minus 1000 kg/m3). Recordings performed by Oceanor.
Table 1 Release fluxes and release velocities for various water and air releases (GWR) used during the experiment
GWR
Flux of water m3/min
Flux of air at 106 m depth, m3/min
Release velocity, m/s
7.25 18.6 47 67
1 1 1 1
0.725 1.86 4.7 6.7
4.5 7.3 14.7 20.2
The oil release is similar to the G W R = 67 case.
Spill Science & Technology Bulletin 4(4)
~l~;t :[ll~
¢I
The list of tests performed is shown in Table 2. Note that the oil release test was performed jointly with the G W R = 67 test (test No. 3 in Table 2). This was done intentionally in order to avoid transient effects when the oil release was initiated. The rising time of the oil was observed by taking the time difference between when the oil first appeared at the mouthpiece and then arriving at the sea surface. The oil was observed at the mouthpiece at 0903 G M T and then at the surface 124 s later, which gives an average rising time of 0.85 m/s. The last oil left the mouthpiece at 0946 GMT. After that, the release continued until about 1000 G M T with water instead of oil. The oil used in the experiment was a Troll crude oil with a density of 893 kg/m 3. Further characterisation of the oil is given in Str0m-Kristiansen et al. (1996) and Str0m-Kristiansen et al. (1995).
Comparison between calculated and measured plume widths
Salinity, ppt
34.3 0'
B :3 ~t
Five different subsurface blowout simulations creating a subsurface plume were detected with the R O V equipment, four with air and water in the release and one with the oil and air. The same procedure was applied each time as follows. The recording camera was placed on the release frame before deployment. The R O V was deployed at 106 m depth before the start of each release. The position was initially selected to be some distance downstream from the outlet opening (see outline of the set-up a r r a n g e m e n t in Fig. 10). When the release was started, the R O V recorded the plume, both with video and with sonar. The R O V ascended vertically, while the plume was scanned with the sonar at 10 m depth intervals. This procedure was repeated at every 10 th meters from the lowest depth (102 m) all the way up to 22 m depth below the sea surface (or as long as it was possible to follow a distinct plume). For each depth, a number of side scans were performed (12-35) in order to assess the statistical variability (standard deviation). The data estimated for every depth were the distance from R O V to plume, plume diameter and R O V heading. The determination of the gas plume diameter with the sonar will in this case be dependent on the sensitivity of the sonar to the bubble density in the water mass. The exact sensitivity was not determined analytically. However, the error introduced by the uncertainty connected with this sensitivity.is assumed to be smaller than the uncertainty in the sonar readings. The typical standard deviation within the 12-35 sonar recordings at each depth was 2 - 5 m. The five blowout simulations that were carried out correspond to the five cases indicated in Table 3. The 245
H. R Y E et al.
Fig. 8
Image taken by the video camera m o u n t e d on the release frame during the release of the oil and air.
time of start and termination of each vertical scan are as follows. The results from the plume diameter measurements for the oil and air release case are shown in
v
Fig. 11 along with the corresponding calculated plume diameter. The measured plume diameters for the GWR cases (GWR = 7.25, 18, 47 and 65) are shown in Figs 12-15. The results show that the actual plume diameter was wider than the calculated diameter. The width of the plumes in Figs 12-15 are calculated in the model to be equal to ,86, where a is the standard deviation of the velocity distribution about the centre of the plume.The entrainment coefficient used in the model calculations is 0.1. The calculated width of the plume indicates that the chosen value of the entrainment coefficient for the plume (0.1) may be too low. An increase of the entrainment coefficient would widen the plume in the calculations, in accordance with observations. It may also be that a larger entrainment coefficient should be applied initially (during the jet phase of the release). Such a procedure has been recommended by Rowe
Table2 Listing of the different test cases run experiments with subsurface releases Experiment No.
Fig. 9 Outline of the release arrangement. Diameter of release opening 90 m m . Distance from the beginning of the oil/air mixing zone and the release opening is 0.8 m. 246
1 Test 1 Test 2 2 3 3 (cont.) 4 5
Date 11 June 11 June 12 June 12 June 12 June
1996 1996 1996 1996 1996
Time (GMT)
18.32-19.30 21.10-21.25 05.55-06.10 07.45-08.35 08.40-10.00 (09.03-09.46) 12 June 1996 11.15-11.53 12 June 1996 12.20-14.10
GOR or G W R 7.25 G W R 65 G W R 65 G W R 18 G W R 67 G W R 67 G O R 65 G W R 46 G W R
during the
Oil or water Air/water Air/water Air/water Air/water Air/water Air/oil Air/water Air/water
Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD
EXPERIMENTS
and Laureshen (1988), who observed similar discrepancies between model and experiment.
I ~ ~t ~
:~1~ ~ I
The relative increase of the horizontal extent of the plume as a function of depth compares reasonably
I Hose
f/OO
I
ROV
Hose
Plume
10
Sonar
ROV 360 ° polarscan Fig. 10
Illustration of the monitoringthe subsurfaceplume with use of sonar.
Table 3
Listing of the different sonar side scans carried out during the subsurface release cases
Test case No.
Date
1 2 3 4 5
11 June 12 June 12 June 12 June 12 June
1996 1996 1996 1996 1996
Time (GMT)
Depth interval
G O R or GWR
18.32-19.27 07.43-08.25 09.09-09.41 11.14-11.44 12.21-12.45
102-42" 102-22 102-22 102-22 102-22
7.25 G W R 18 G W R 67 G O R 65 G W R 46 G W R
aAt 62 m depth, this plume was reported by the R O V observers to drift towards W. At 52 m depth, the plume was reported to be 'considerably diminished'. At 42 m depth, there was no sign of the plume.
Spill Science & Technology Bulletin 4(4)
247
q
?
* Modelled diameters / / • Measured diameters (Sonar)] /
20
I
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/ /
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/
100
// i
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10
15
20
25
30
35
Plume diameter (m) Fig. 11
Comparison between measured (with error bars) and calculated plume diameter. Oil and air release G O R = 67. Test case No. 3 in
Table 3.
well between measurements and observations, which indicates that the entrainment rate is probably of the right order. However, there is a large 'offset' value of the width of order 5 - 1 0 m close to the bottom. It seems unlikely that the plume will increase to this size just a few metres above the bottom. The reason why the measurements show this large initial width is not properly understood. Inspection of the ROV film pictures taken close to the bottom indicates an increase of the width corresponding to a 20 ° opening
of the cone representing the release close to the bottom. Above 102 m depth, the angle of the cone reduces down to 12-13 °, which corresponds closely with the cone angle indicated by the calculations (about 14.4° for all release cases considered). This increase of the cone angle close to the release site is consistent with the observations reported by Rowe and Laureshen (1988). However, this increase of the cone angle close to the release opening (representing an increase of the
Modelleddiamaters • Measureddameters (Sonar) 20
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Error bars on measured plume diameter are one STDEV on each side
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Plume diameter (m) Fig. 12 248
Measured width of the plume (with error bars) and calculated width of the plume for G W R = 7.25. Test case No. 1 in Table 3.
Spill Science & Technology Bulletin 4(4)
I •,~i ~ I
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
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entrainment of sea water into the plume as well) is not sufficient to explain all the discrepancies between the model and observations shown in Figs 11-15. Another explanation could be influences from the motion of the R O V during the horizontal scanning of the plume. This will lead to a 'wider' plume in the sonar signal, because the plume will appear to 'move' during the sonar scanning, relative to the ROV. The same 'offset' effect was found in the data from the 1995 N O F O sea trial as well, where the same procedure was applied. However, the offset value was
/
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found to be smaller in the 1995 field trial case (Rye, 1996), of order 1-3 m increase in the diameter when compared with the calculated results. It may therefore be that the present 'offset' value in the 1996 field data results from a combination of contributions from these two sources (a larger cone angle close to the outlet opening and effects from the motion of the R O V during the sonar scanning). Another feature of both calculations and observations is that the plume tends to stop rising through the water column for the smallest air release rates.
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100 0
5
10
15
20
25
30
35
Plume diameter (m) Fig. 14
Measured width of the plume (with error bars) and calculated width of the plume for G W R = 46 cases. Test case No. 5 in Table 3.
Spill Science & Technology Bulletin 4(4)
249
B :l ~'~l~/'~1:(lil~'tl •
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20
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Plume diameter (m) Fig. 15 Measured width of the plume (with error bars) and calculated width of the plume for GWR = 65. Test case No. 4 (without oil) in Table 3.
Figure 16 shows the calculated plume widths for all the G W R cases. The width tends to increase to infinity for the G W R = 7.25 case in the calculations, which indicates that the plume ceases to rise at about 30 m depth. The reason why the plume stops rising to the sea surface in the G W R = 7.25 case is the effect of stratification. The parameter F in Section 3 (see eqns (5)-(7)) turns negative at a certain depth due to the stratification in the water mass, indicating that the
buoyancy force is negative and the plume will tend to sink. Also, for the G W R = 18 case, the calculations indicate some depth interval where the parameter F is weakly negative. The F parameters for the two cases G W R = 7.25 and 18 are shown in Fig. 17. Note also that for the G W R = 18 case, the plume was not observed at the surface either. From the subsurface plume recordings, the plume seems to diminish or be considerably reduced in diameter above 22 m depth.
o.°
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Modeli.d dlametem GWR, 46
~
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$
•
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10
15
20
25
30
35
Plume diameter (m) Fig. 16 The width of the plume calculated for the four GWR cases indicated by Table 3. Experimental results from GOR = 67 also shown. Entrainment coefficient in the model is assumed equal to 0.1. 250
Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
This depth is at approximately the top of the dominant stratification in the ambient water mass (see the temperature and salinity profiles in Fig. 7). Therefore, the G W R = 18 appears to be close to the turning point of the release for not reaching the sea surface. There appears to be good correspondence in the location of this turning point (along the G W R scale) between measurements and calculations. On the other hand, the actual depth for maximum
Buoyancy parameter F 0 -
-20
-10
0
1
0
2
0
6
0
-20
E r::
GWR = 7.25
GWR = = 18 18 GWR
E
,,
1L-~
Fig. 17 The buoyancy parameter F as a function of the depth in the GWR = 7.25 and GWR = 18 cases. Results from calculations with the entrainment coefficient =0.1. GWR= 7.25 to the left, GWR = 18 to the right.
1
1
penetration of the plume (for the G W R = 7.25 and 18 cases) is closer to the sea surface in the calculations, compared with measurements, in particular for the G W R = 7.25 case.
Comparison between calculated and measured vertical
plume velocity In order to determine the vertical velocity of the ascending plume, six current meters were deployed close to the outlet arrangement as'shown in Fig. 18. Two moorings were deployed at positions D1 and D2, close to the stern of GULLBAS. On each mooring, the current meters were mounted at 20, 30 and 40 m depth. The meters sampled the vertical current component integrated over 10 min. The meters were of the Aanderaa type with a Savonius rotor. The data analysis showed two occasions where the current meters recorded vertical velocities caused by the ascent of the subsurface plume. In each case, the duration of the intersection of the current meters and the plume is limited to be of order 30-50 rain. This is partly due to the changing ambient currents, causing the plume to pass through the location of the current recordings.
Underwater release 12. June 1996 situation at 08:10 GMT
41516400~gneffrelease~.a16
Fig. 18 The location of the moorings for the recording of vertical currents (positions DI and D2) and horizontal currents (positions O1 and 02). Spill Science & Technology Bulletin 4(4)
251
B ~I $ ~ ~ I : ~ q ~E
H. RYE et al.
These two occasions occurred on 12 June at approx. 0600 G M T and 12 June at 1300-1400 GMT. These time intervals correspond to a test case conducted with air and water with at G W R = 65 and to the last release case with a G W R = 46. See Table 2 for an overview of the cases run. The vertical velocities were recorded to be at maximum 5 0 - 60 cm/s at the D1 location ( G W R = 65 case) and 25-30 cm/s at the D2 location ( G W R = 4 6 case). The vertical velocity of the centre of the plume calculated by the numerical model is shown in Fig. 19 for the G W R = 67 and 46 cases. The calculated velocities are larger than those measured. However, the average velocity of the rising plume is expected to be lower than the velocity in the centre of the plume. A factor of 1.4 between the average velocity and the centre velocity of the rising plume is sometimes used. When this reduction is taken into account, the correspondence improves somewhat. It is, however, difficult to state the precise correspondence between the measurements and the calculations, because the instrument may record the currents at some distance from the centre of the plume.
Cer~m v e i m = ~ d p l m l e fm¢ G W R : (ff and 4 6 c a n s
2
3
4
5
6
7
8
-2O -4O E
!
rt -8o
-100 -120
Fig. 19 The centre velocity of the rising plume calculated by the numerical model. Entrainment coefficient = 0.1. G W R = 67 and 46 are shown. G W R = 46 to the right, G W R = 67 to the left.
Table 4
The horizontal motion of the plume on its way through the water column
The horizontal velocity profile was recorded at 3, 10 and 60 m during the field trial. If these recordings are interpolated (between the depth interval 3 and 6 0 m ) and extrapolated outside this interval, an estimate of the horizontal current velocity profile in the vertical can be made. By taking into account the calculated velocity of the vertical motion of the plume, the expected horizontal advection of the plume can be calculated. Reading off representative current velocities at the time where the five different releases were performed (cases 1-5 in Table 3), the actual numbers used are shown in Table 4. This table also shows the results from the expected position of the subsurface plume, when arriving at the sea surface (except for the G W R = 7.25 case). The locations of the plume at the sea surface for the two lowest G W R cases (see Table 4) are not relevant because the plume was never observed at the sea surface. If the plume for the G W R = 18 case had reached the sea surface, it would probably have been observed because the location of the plume should have been far away from the influence of the thrusters on GULLBAS (typical area of influence of order 10 m in all directions). For the G W R = 46 and 65 cases, the plume was observed at the sea surface, but the locations were close to the area of the most intense propeller action. The case with the clearest separation between the propeller action at the stern of the ship and the upstream area generated by the plume is the oil and air release case ( G O R = 67). A photograph of this particular case is shown in Fig. 20, showing the location of the plume at approx. 20 m in the direction SSE relative to the stern of the ship. (Also see the location of the release arrangement, as indicated in Fig. 18). The observed location of the plume is thus in good agreement with the calculated position where the subsurface plume hits the sea surface, as shown in Table 4.
Observations of current velocities and directions and calculated positions of the plumes at the sea surface
Time G M T 3 m vel. 3 m dir. 10 m vel. 10 m dir. 60 m vel. 60 m dir. Pos. plume, distance Pos. plume, direction
G W R = 7.25
G W R = 18
G W R = 46
G W R = 65
G O R = 67
11/6 at 1830 0.176 343 0.153 327 0.047 020 Not at the sea surface Not at the sea surface
12/6 at 0800 0.147 169 0.138 175 0.152 155 28 m 163
12/6 at 1230 0.035 208 0.101 234 0.070 208 10 220 a
12/6 at 1130 0.129 205 0.171 209 0.123 172 15 190
12/6 at 0930 0.170 177 0.160 181 0.182 163 19 170
All positions are indicated relative to the position of the outlet arrangement. Velocities in m/s and directions in 0-360 degrees. Time in GMT. "The direction of the currents was changing from S to N during the recording period, see Fig. 6 for the ambient current recordings. 252
SpillScience& TechnologyBulletin4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
The width of the subsurface plume was calculated by means of the mathematical model, see Fig. 16. The width of the plume at the location where the subsurface plume reaches the sea surface was calculated to be in excess of 20 m diameter. This appears to be of the same magnitude as the observed width, as shown in Fig. 20 (see the whiter shaded area at the parabola centre close to the stern of the ship).
Comparison of calculated and measured surface oil slick dimensions The release was intended to simulate a subsurface blowout event. The equations (8)-(11) predict a surface contour and a thickness of the resulting oil slick at the sea surface. Using a surface current velocity of 0.16 m/s and the results from the subsurface plume calculations at the point where the plume reaches the sea surface, the resulting contour of the slick and the thickness can be calculated. Figure 21
shows the calculated surface contour of the surface slick with the Fannel0p and Sj0en (1980) algorithm, using the data for the 1996 sea trial ( G O R = 67): The observed width of the surface slick 200 m downstream from the front is approximately 300 m (see Figs 20, 22 and 23). The modelled width in Fig. 21 is approximately twice as large, 650 m at the same distance from the front of the surface oil slick. In this case, the calculated slick width is approximately twice as large as the observed width. According to calculations, the thickness of the oil in the slick was expected to be close to 100-160/~m. These calculations assume that all the released oil surfaces and that it is not emulsified. Because the size of the slick was somewhat overestimated by the model, one should expect that the measured thickness of the oil would be somewhat larger than calculated. On the contrary, the thickness was found to be of order 5-20 #m on average (occasionally above 30/~m, see Fig. 24), accounting for only 15-20% of the total
Fig. 20 Photo of the contour of the sea surface oil slick taken during the 1996 oil and air subsurface release. Length of ship is 80 m. The slick is drifting southwards. Wind is from the W N W .
Spill Science & Technology Bulletin 4(4)
253
H. RYE et al.
amount of oil released. The rest of the oil did not surface during the experiment time. The thickness of the slick and the amount of oil in the slick were also estimated by image analysis of the IR and UV images taken by the German remote sensing aircraft. The contours (slick areas) of the different parts of the resulting surface slick were determined by image analysis. A film thickness of 25/~m was used as a conservative estimate (see Figs 22 and 23). The reason why the bulk of the oil did not reach the sea surface is not clear. Drop size measurements were not performed during the oil release. One possible explanation is that the release arrangement is creating large release velocities, estimated to be close to 20 m/s. In addition, the air is mixed with the oil by releasing the airflow into the oil stream with a velocity of the order 50m/s within the release arrangement itself. These velocities may be large enough to create strong velocity shears that will cause the oil to disperse into small droplets. When this mixture approaches the sea surface, the smaller oil droplets may then have a corresponding small rise
velocity with a low ability to reach the sea surface. Thus, the oil droplets may be trapped within the subsurface plume instead of rising to the sea surface.
Summary and conclusions Different features of the subsurface model have been compared with results from the field trials on the Frigg field in June 1996. The comparison between the measurements and the calculations shows that the model represents many of the features of the release fairly well. However, some other features of the release were not sufficiently well reproduced. The subsurface plume dimensions seem to be of the same proportions as the computer modelling results. The diameter of the subsurface plume was
IB. 2
km
IR UU
Contour plot for slick, theoretical model
E
o
Downstream distance in
m
Oil slick thickness in mm
; '~,:
0.06 t
" ~
0.04 0.02 O r
-500
0
500
1000
1500
2000
Downstream distance in m
Fig. 21 Calculation of the surface contour of the slick and the expected thickness of the oil in the slick, applying the Fannel~ap and Sj~aen (1980) algorithm to the subsurface oil and gas release experienced during the June 1996 sea trial. The oil slick thickness is calculated on the basis that all oil is contained in the slick, with no evaporation or emulsification. 254
Fig. 22 Remote sensing picture taken from the German aerial surveillance aircraft showing the oil slick during release (30 min after start) indicating a surface slick width of approximately 300 m. Distance between crosses is 200 m. Spill Science & Technology Bulletin 4(4)
SUBSURFACE BLOWOUTS: RESULTS FROM FIELD EXPERIMENTS
calculated to be smaller than what was measured. The reason for this is not clear, but the motion of the R O V during the sonar scan may have contributed to some of the discrepancies. The vertical velocity of the subsurface plume was calculated to be of the order 1 m/s in the centre of the plume. The measurements indicated somewhat lower velocities. It is difficult to say anything more specific about the correspondence between the measurements and the calculations, because the correspondence will depend on how far away from the centre of the plume the recording instruments have been located. The recorded rising time of the plume is found to be in reasonable agreement with the calculations. The calculated rising times were slightly larger than the observed ones. The simulations indicate a reasonably good correspondence between the observed location
35
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o 1,....*....i
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20
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180
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. . . .
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Distance acrossslick (m) Fig. 24 Film thickness measured across the oil slick 150-200 m downstream from the front of the slick during the last part of the release, using absorbing pads placed on the sea surface.
where the subsurface plume penetrated the sea surface and the calculated location, based on the observed horizontal currents. The results from the present field trial resulted in a slick thickness that was less than predicted by theory. The reason for this is probably that the bulk of oil did not reach the sea surface at all, but was contained within the water plume below the slick. One possible explanation for this is that details in the release arrangement or the size of the release velocity contributed to create a large amount of small droplets with a small ability to rise through the water mass in the plume. The velocities used in the release were of the order of 20-50 m/s, which are within the typical range of blowout velocities anticipated for subsurface releases, in particular for deeper waters. Thus, it is reasonable to assume that similar behaviour of the oil can be expected for real underwater blowouts. There was no definitive evidence of regeneration of plumes, after the plume was trapped in the stratified water layers. Some of the plume widths determined from the sonar signal showed that the plume diameter was reduced at the top of the water column. However, the measurements cannot be said to be conclusive at this point. Additional investigations are therefore recommended. Acknowledgements--This work could not have been carried out without the close co-operation and the financial support from a number of different bodies and organisations. The financial support from NOFO and Norsk Hydro is hereby acknowledged. Assistance from NOFO, Norsk Hydro, Oceanor, Maritex, SFT (aircraft) and the German Marine Pollution Control Unit (aircraft) is also greatly acknowledged.
References
Fig. 23 Remote sensing picture taken from the German aerial surveillance aircraft showing the oil slick 20 min after release was finished. Distance between crosses is 200 m.
Spill Science & TechnologyBulletin 4(4)
Asaeda, T. & Imberger, J. (1993). Structure of bubble plumes in linearly stratified environments. Journal of Fluid Mechanics, 249, 35-57. Fannel~p, T. K. & Sj~aen, K. (1980). Hydrodynamics of subsurface blowouts. Norwegian Maritime Research, 4, 17-33. Koh, R. & Fan, L. (1970). Mathematical models for the prediction of temperature distribution resulting from the discharge of 255
H. RYE et al. heated water into large bodies of water. EPA Report, Water Quality Office, October 1970. Lewis, A., Str0m-Kristiansen, T., Brandvik, P. J., Daling, P. S., Jensen, H. & Durell, G. (1995). Dispersant Trials-NOFO Exercise 6-9 June 1994-Main Report, IKU report no: 22.2050.00/14/95. McDougall, T. J. (1978). Bubble plumes in stratified environments. Journal o f Fluid Mechanics, 85, 655-672. Rowe, R. D. & Laureshen, C. J. (1988). An investigation of bubble groups from an undersea oilwell blowout. Proceedings from the l l t h AMOP seminar, Vancouver, 7-9 June 1988. Rye, H. (1994). Model for calculation of underwater blowout plume. Proceedings, 17th Arctic and Marine Oil Spill Program (AMOP), 8-10 June. Vancouver, Canada.
256
Rye, H. (1996). Subsurface oil release field experiment - - observations and modelling of subsurface plume behaviour. Proceedings, 19th Arctic and Marine Oil Spill Program (AMOP), 12-14 June 1996. Calgary, Canada. Str0m-Kristiansen, T. et al. (1995). Forvitringsegenskaper p~ sj0en for Troll r~olje. En h~ndbok for Norsk Hydro a.s. Written in Norwegian. IKU/SINTEF Report No. 41.5132.00/.01/01/95. 104 pages. Strom-Kristiansen, T. et al. (1997). NOFO 1996 Oil on Water Exercise - - Analysis of Sample Material. SINTEF Report STF66 F97050. SINTEF Applied Chemistry. Yapa, P. D. & Zheng, L. (1998). Modelling oil and gas releases from deep water. A review. Spill Science and Technology, 4(4), 189-198.
Spill Science & Technology Bulletin 4(4)