OCS Study 

•

MMS 2003-015 

Risk Analysis 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

 
Final Technical Summary - Lick 

2
Risk Analysis 

Final Technical Summary 

Final Study Report 

Author 

Wilbert J. Lick 

Principal Investigator 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-30471 

by 

Southern California Educational Initiative 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region March 2003
Final Technical Summary - Lick 

4

Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the author, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/SCEI/ 

Suggested Citation 

The suggested citation for this report is: 

Lick, Wilbert J. Risk Analysis. MMS OCS Study 2003-015. Coastal Research Center, Marine 

Science Institute, University of California, Santa Barbara, California. MMS Cooperative Agreement 

Number 14-35-0001-30471. 18 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY 1 

FINAL STUDY REPORT 3 

Abstract 3 

Introduction 3 

 Transport Equations 6 

 Numerical Calculations 7 

 Discussion of Results 14 

 Conclusion 16 

 Acknowledgements 16 

 References 17 

List of Tables and Figures 

Table 1. Sediment Bed Parameters 10 

Figure 1. Geographical map and computational region for Platform Hidalgo 4 

Figure 2. Geographical map and computational region for Eugene Isle 5 

Figure 3. Drilling mud accumulation for a source at Platform Hidalgo 8 

Figure 4. Bottom shear stress for Hs of 250 cm 8 

Figure 5. Bottom shear stress for Hs of 900 cm 9 

Figure 6. Sediment deposition ten days after subsidence of waves of 900 cm 11 

Figure 7. Drilling mud accumulation for a source at Eugene Isle 12 

Figure 8. Bottom shear stress for Hs of 150 cm 13 

Figure 9. Bottom shear stress for Hs of 850 cm 14 
Final Technical Summary - Lick 

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Risk Analysis 

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FINAL TECHNICAL SUMMARY 

STUDY TITLE: The Transport and Fate of Drilling Muds 

REPORT TITLE: Risk Analysis 

CONTRACT NUMBER: 14-35-0001-30471 

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 89, FY 90 

COMPLETION DATE OF REPORT: December 1991 

COST(S): FY 89: $24,000; FY 90: $17,090 

CUMULATIVE PROJECT COST: $41,090 

PROJECT MANAGER: Wilbert J. Lick 

AFFILIATION: University of California, Santa Barbara 

ADDRESS: Department of Mechanical and Environmental Engineering, University of 

California, Santa Barbara, CA 93106 

KEY WORDS: drilling muds; offshore oil; pollution; transport; fate; deep-water; 

shallow-water. 

BACKGROUND: The disposal of drilling muds from offshore oil platforms is of 

concern because of the potential effects of pollution from these muds. Both drilling muds 

and fine-grained bottom sediments are similar in their general physical and chemical 

characteristics and can be studied by similar methods. 

OBJECTIVES: To determine the transport and fate of drilling muds off the California 

coast. 

DESCRIPTION: In order to investigate the transport and fate of drilling muds, a 

numerical model of this transport and fate has been developed. The model consists of a 

two-dimensional, vertically-integrated, time-dependent hydrodynamic and mass transport 

model coupled with a three-dimensional time-dependent model of the sediment bed and 

its properties. A deep-water environment and a shallow-water environment were studied. 

Calculations show that the non-linear effects of shear stresses on sediment resuspension 

dominate both regimes. Waves generated by large, low-probability storms result in burial 

of drilling muds deposited in deep water. By contrast, sediments deposited in shallow 
Final Technical Summary - Lick 

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water are easily eroded by typical storm waves due to the higher bottom shear stress 

generated in this shallow-water environment. 

STUDY RESULTS: For the deep water site, small storms were found to resuspend 

sediments from areas with minimal drilling mud deposition, and to subsequently deposit 

this material in deeper water where drilling muds are more likely to be found. Typical 

storms in shallow water generated bottom shear stresses large enough to erode significant 

quantities of drilling mud deposited in these environments. During large storms, the 

amount of sediment resuspended increases dramatically. 

It must be noted that the extent of erosion is strongly dependent upon the choice of 

sediment bed properties. Due to a lack of available field data for these study regions, these 

results must be viewed as general descriptions of the transport and ultimate fate of drilling 

muds in marine environments. However, the non-linear relationship between 

resuspension/deposition and bottom shear stress leads to the conclusion that large storms 

account for the most significant quantities of sediment transport in both shallow and deep 

water environments. 

SIGNIFICANT CONCLUSIONS: Determination of the ultimate fate of drilling muds 

discharged from offshore oil platforms requires a proper evaluation of the hydrodynamic 

character of the discharge region as well as an accurate calculation of suspended solids 

transport and sediment resuspension/deposition behavior. A numerical model possessing 

these characteristics has been employed to analyze the fate of drilling muds discharged 

from a platform sited in deep water at Platform Hidalgo on the California coast, and at a 

shallow water location at Eugene Isle in the northern Gulf of Mexico. Calculations for 

both sites reveal that large storms can resuspend drilling muds deposited in both deep and 

shallow water, although the influence of these storms in shallow water is considerably 

more pronounced. 
Risk Analysis 

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FINAL STUDY REPORT 

ABSTRACT

The disposal of drilling muds from offshore oil platforms is of concern because of the potential 

effects of pollution from these muds. A related problem is the concentration of pollutants from 

produced waters and other sources in bottom sediments. Fine-grained sediments (silts and clays) 

are especially important because of their great capacity for absorbing contaminants which are 

then transported with the sediments. Both drilling muds and fine-grained bottom sediments are 

similar in their general physical and chemical characteristics and can be studied by similar 

methods. 

In order to investigate the transport and fate of drilling muds, a numerical model of this transport 

and fate has been developed. The model consists of a two-dimensional, vertically-integrated, 

time-dependent hydrodynamic and mass transport model coupled with a three- dimensional timedependent model of the sediment bed and its properties. The sediment bed is assumed to be 

layered in the vertical direction. Properties of each layer depend on tune after deposition and the 

relative fraction of fine and coarse sediments. The number of layers and their thicknesses can be 

arbitrarily specified at the beginning of each calculation. Sediment settling speeds and 

resuspension parameters needed in the model were determined from laboratory and field tests. 

A deep-water environment, represented by the Hidalgo Platform west of the Santa Barbara 

Channel, and a shallow-water environment, represented by Eugene Island in the Gulf of Mexico, 

was studied. Relevant hydrodynamic parameters modeled include current velocities, tidal 

incursions, and storm-generated wind waves. 

Calculations show that the non-linear effects of shear stresses on sediment resuspension 

dominate both regimes, with a few large storms causing almost all of the resuspension of bottom 

sediments. Waves generated by large, low-probability storms resuspend buried sediment layers 

that remain stationary during ordinary winter storms in the deep-water regime near Hidalgo. By 

contrast, sediments in the Gulf of Mexico are much more easily eroded by typical storm waves 

due to the higher bottom shear stress generated in this shallow-water environment. 

INTRODUCTION 

The study of the transport and ultimate fate of drilling muds discharged from offshore oil 

platforms is of interest primarily due to the possibility of ecosystem sensitivity to drilling mud 

components. Similarly, pollutant concentrations due to the discharge of produced waters must be 

examined in light of their potentially negative impact on the marine environment. In order to 

gain insight into the transport processes of these effluents, it is necessary to accurately model the 

hydrodynamic and particle transport characteristics of the region in which the platform is 

located. Although drilling sites are highly variable in terms of geographical and bathymetric 

parameters, important conclusions can be drawn from the analysis of two extreme types of 

drilling environments. Specifically, the transport of drilling muds from a platform located in 

deep water will be compared to results obtained for a platform in shallow water. 
Final Study Report - Lick 

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The deep water site selected, Platform Hidalgo, is located 10 km southwest of Point Arguello 

along the California Coast (Figure 1). This region has experienced minimal drilling and 

production activity. The platform itself is located in approximately 120 m water, with steep, 

irregular bathymetric contours dominating the topography. Currents along the California coast 

are highly variable in magnitude and direction, are complicated by strong diurnal and semidiurnal tides, and are inherently three-dimensional. However, during major storms and hence 

high turbulence, the water column is well-mixed in the vertical. Because of this, significant 

insights into the transport of drilling muds can be obtained utilizing a two-dimensional, vertically 

integrated numerical model. This model and the approximations integral to the model will be 

discussed below. 

Figure 1. Geographical map and computational region for Platform Hidalgo 

Eugene Isle, an oil platform located approximately 20 km south of Atchafalaya Bay in the 

northern Gulf of Mexico, was selected for the shallow water site (Figure 2). The platform itself 
Risk Analysis 

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has been in production since 1981, with effluents in excess of 65,000 gallons/day (Neff, 1988) 

discharged to the surrounding waters. In many respects, Eugene Isle is typical of the thousands 

of platforms erected in this oil-rich region. Located in only 7 meters of water, depths do not 

exceed 13 m throughout the study region. The water column is well-mixed in terms of 

temperature and salinity, and currents flow westward, averaging 5 to 15 cm/s through most of the 

year (Murray, 1976). Additional hydrodynamic parameters to consider are tidal effects, which 

are weak in this region, and winter storms in which wind-driven waves become important. 

Because the water column is well-mixed, this site is an ideal candidate for the two-dimensional 

numerical model employed for the Platform Hidalgo site. 

Figure 2. Geographical map and computational region for Eugene Isle 

Contaminants in aquatic systems are frequently associated with fine-grained, cohesive sediments 

due to the capacity of these sediments for sorption of contaminants. While the precise nature of 

this association has yet to be adequately described, an accurate description of the transport of 

cohesive sediments has been developed by Gailani et al (1990). Since drilling muds frequently 

consist of bentonite and other fine-grained clay minerals, it may be assumed that the transport of 

these effluents can be adequately described by this model. Recent laboratory experiments 

(Huang, 1992) support this assumption, as the flocculation and resuspension relationships 

utilized in Gailani’s model for fresh-water cohesive sediments are shown to apply to drilling mud 

behaviour as well. 
Final Study Report - Lick 

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The present study utilizes a two-dimensional, vertically integrated, time-dependent 

hydrodynamics model developed by Ziegler et al (1987). This model is coupled with a threedimensional time-dependent model of the sediment bed and its properties. The sediment bed is 

assumed to be layered in the vertical direction. At the onset of a calculation, each layer is 

assigned an initial thickness, age, and critical shear stress. The critical shear stress is defined as 

the value of shear stress at which resuspension will be initiated. Resuspension will be discussed 

further in the next section. In addition, it is possible to specify the fraction of fine-grained 

(cohesive) material and the fraction of coarse-grained (sandy) material present in the sediment 

bed. It should be noted that flows and sediment bed parameters were selected such that a general 

description of the ultimate fate of drilling muds could be derived. Real-time simulations were not 

included in this study. The equations and necessary assumptions for the development of the 

numerical model are described in detail by Gailani (1991) and will not be repeated here. 

However, certain aspects of the model are critical to the understanding of the transport of drilling 

muds from these locations, and these aspects are discussed in the next section. 

TRANSPORT EQUATIONS

Using the appropriate boundary conditions for currents and tidal activity, the model calculates 

current velocities in each region. However, wind wave generation represents a somewhat more 

challenging problem. Marine environments generally experience the effects of wind waves on a 

daily basis. Due to computer time and memory constraints, it is not presently possible to 

continuously model these systems. However, significant insights into the hydrodynamic 

character of these environments can be obtained by comparison of results obtained from a 

selection of wind wave events. For this purpose, values of significant wave heights and periods 

for the regions of interest were obtained from the National Oceanic and Atmospheric 

Administrion (National Climatic Data Center, 1990) sources. Buoys deployed near the drilling 

sites record on a regular basis both the significant wave height and dominant wave period. Data 

obtained for the period from 1979 through 1988 were analyzed to obtain appropriate values for 

these parameters. Linear wave theory was employed to obtain the maximum bottom orbital 

velocity for these wave events. Specifically, 

where uwbm is the maximum bottom orbital speed, k is the wave number, Hs is the significant 

wave height and d is the local water depth. 

Having obtained the relevant fluid velocities, it is now necessary to calculate the appropriate 

bottom shear stress. Christoffersen and Jonsson (1985) derived friction factors for systems in 

which the bottom shear stress is due to the coupled effects of wind waves and current velocities. 

Utilizing a two-layer eddy viscosity model, they found that the total shear stress can be expressed 

as: 

where τbm is the maximum bottom shear stress, fw is the friction factor due to waves, ρ is the 

fluid density, and m is a factor determined from the relative magnitude and direction of the 

current and wave velocities. 
Risk Analysis 

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Having derived the bottom shear stress, appropriate relationships between this stress and 

sediment behaviour must be applied. For these calculations, settling speeds and resuspension 

criterion must be available. Addressing resuspension first, the probability that material deposited 

to the sediment bed will remain in situ depends strongly upon the magnitude of the bottom shear 

stress. Experiments performed by Xu (1991), Tsai et al (1987), and MacIntyre et al (1990) have 

resulted in the following relationship: 

where ε is the maximum amount of sediment entrained per unit surface area (gm/cm2), τ is the 

local shear stress (dynes/cm2

), τcr is the critical shear stress (dynes/cm2) below which no 

resuspension occurs, td is the time since deposition (days), and a0 and n are experimentally 

determined constants. Laboratory studies for fine-grained, cohesive sediments show that a0 and n

are approximately .008 and 3, respectively. The non-linear relationship between shear stress and 

resuspension will prove to be a determining factor for accurate calculation of sediment transport. 

In general, a shear stress of less than 1 dynes/cm2 will produce negligible resuspension. The 

critical shear stress for the fine sands and clay sediments found in both study regions must be 

determined through field experiments. Since this information was not available for this study, it 

was assumed that measurable resuspension only occurs for shear stresses in excess of 1 

dynes/cm2

. The validity of this assumption will be addressed in the discussion section. 

Additional laboratory results are used in the model to obtain settling speeds of sediments. These 

are discussed in detail by Burban et al (1990), Gailani et al (1990) and Xu (1991). In general, 

these studies found that settling speeds of flocculating particles exhibit a non-linear dependence 

upon the suspended solids concentration and the fluid shear stress. Fine-grained cohesive 

sediments generally have settling speeds from 50 to 150 μm/s. 

Having developed equations to accurately model sediment deposition and resuspension, it is now 

possible to perform numerical calculations of the flow field and sediment transport in the regions 

of interest. 

NUMERICAL CALCULATIONS 

Current velocities and directions at Platform Hidalgo are highly variable in the spatial and 

temporal domains (Minerals Management Service, 1990) and are further complicated by tides 

which may exceed 1 m in height. For this study, two general flow regimes were compared. In the 

first, a mean northwesterly flow of 10 cm/s was specified along the boundaries, and the results 

were compared to those for a southeasterly flow of the same magnitude. The maximum bottom 

shear stress for these flows was calculated as less than 1 dynes/cm2 throughout the study region. 

Calculations for drilling mud ejected at a rate of 7.5 kg/s over a 30 day period for either flow 

regime indicate that deposition is heaviest within 4 km of the platform (Figure 3). However, due 

to the water depth, these effluents were dispersed over a spatially wide area with a significant 

percentage convected out of the study region. Results for a mean southward flow of 10 cm/s and 

a 50 cm diurnal tide are shown in Figure 4. The influence of the tide was to transport drilling 

mud toward the near shore environment. In spite of the tidal effect, over 50 percent of the 

sediment deposited within the study region settled in water in excess of l00 m depth, and 80% 

settled in water more than 50 m deep. Calculations for a northward flow produced quantitatively 

similar results. 
Final Study Report - Lick 

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Figure 3. Drilling mud accumulation for a source at Platform Hidalgo. Discharge rate of 7.5 kg/s for 30 

days. Contour interval is 0.0001 g/cm2

. 

Figure 4. Bottom shear stress for Hs of 250 cm. Contour interval is 10 dynes/cm2. 
Risk Analysis 

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A moderate winter storm in the vicinity of Platform Hidalgo generates a significant wave 

height of 250 cm. Due to the water depth at the platform, maximum bottom shear stresses 

for this event do not exceed 1 dynes/cm2, except in the near shore region. As seen in 

Figure 5, shear stresses in excess of 10 dynes/cm2 occur in areas less than 50 m depth. The 

sediment bed for this calculation was specified as containing four layers. Relevant 

parameters for this sediment bed are illustrated in Table 1. Note that all but the bottom 

two layers were initially empty. This permits accurate modeling of sediment dynamics 

during resuspension and deposition (Gailani et al, 1990). In addition to the parameters 

listed in Table 1, it was also assumed that the composition of the sediment varied from 

100% coarse material for shallow water (<l0 m), to 100% fine-grained material in water 

over l00 m in depth. For these conditions, a maximum of 1 gm/cm2 of sediment was 

resuspended in regions between 50 m and l00 m depth, with less than 0.1 gm/cm2 in 

regions greater than 52 m depth. The amount of sediment resuspended in water more than 

l00 m deep was negligible. Significant net erosion (>10 gm/cm2

) was present only very 

near to shore, where depths are less than l0 m and accumulated drilling mud sediment is 

comparatively small. Calculations showed that, after the storm ended, material 

resuspended during the event settled primarily in water over 50 m deep. Deposition at 

Platform Hidalgo was 0.4 gm/cm2 24 hours after waves subsided. Thus drilling muds 

deposited prior to the storm experienced minimal resuspension, and were subsequently 

buried by material transported from shallower areas. Statistically, a storm of this nature 

will occur at least 20 times annually in the Platform Hidalgo region. Note that the units of 

erosion and deposition are in mass per unit area. The actual thickness of these layers is 

dependent upon the water content of the sediment bed. While it is not theoretically 

possible to evaluate this quantity, a rough approximation is that the thickness is twice the 

amount of material eroded/deposited in gm/cm2. 

Figure 5. Bottom shear stress for Hs of 900 cm. Contour interval is 10 dynes/cm2. 
Final Study Report - Lick 

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Table 1. Sediment Bed Parameters 

 Sediment Bed Particles 

Layer Age Thickness Critical Shear Stress 

 (days) (gm/cm2) (dynes/cm2) 

1 0.25 0.0 1.0 

2 0.50 0.0 1.0 

3 1.00 1.0 1.0 

4 7.00 100.0 10.0 

According to NOAA buoy data, a storm exceeding 900 cm significant wave height sweeps 

the Point Arguello region once every ten years. Calculated bottom shear stresses for an 

event of this magnitude are shown in Figure 5. Clearly these shear stresses are sufficient 

to cause erosion of sediments deposited in l00 m of water or less, with a maximum bottom 

shear stress of just less than 1 dynes/cm2 at Platform Hidalgo, and 10 dynes/cm2 at a 

distance of 2 km east of the platform. The maximum value of bottom shear stress for this 

storm was calculated at 184 dynes/cm2 in the near shore region. Assuming the same 

sediment bed as that used for the smaller storm, resuspension of up to 10 gm/cm2

sediment was resuspended in water of 50m depth during the storm event. No sediment 

resuspension took place at Platform Hidalgo, since the shear stress was less than the 

critical value. Calculations show that in excess of 100 gm/cm2 was resuspended in regions 

of less than 50 m depth during this huge storm. Since the upper layer was specified as 

quite thin, most of this represents material resuspended from a layer having a critical shear 

stress of 10 dynes/cm2. Locations in deep water experienced resuspension of up to 2 

gm/cm2

. Note that this is less than the accumulated deposition for several small storms. As 

the storm subsided, large quantities of suspended solids were transported with the 

prevailing currents. Deposition of these solids occurred primarily in deeper water, as 

illustrated in Figure 6. Net deposition at the platform was 10 gm/cm2 24 hours after the 

storm ended. Drilling muds resident at the platform would thus be buried under 

approximately 20 cm of sediment after a huge storm. 
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Figure 6. Sediment deposition ten days after subsidence of waves of 900 cm. Contour interval is 10 

gm/cm2

. 

For the Eugene Isle platform, a mean northwesterly flow of 10 cm/s was specified along 

the boundaries. This yielded current velocities of 8 cm/s in the vicinity of the platform. 

Imposition of a 50 cm daily tide did not produce appreciable change in the velocity profile 

at the platform; tidal influences were most pronounced within the Atchafalaya Bay region. 

Since this is not an area of interest for this particular study, these findings will not be 

addressed in detail. The maximum bottom shear stress resulting from these conditions was 

less than 1 dynes/cm2 at the platform. As mentioned previously, shear stresses less than 

this critical value result in only negligible amounts of resuspension. 

Calculated deposition for drilling mud discharged from the platform at a rate of 7.5 kg/s is 

shown in Figure 7. The shallowness of the water coupled with a low bottom shear stress 

resulted in heavy deposition of these sediments in the immediate vicinity of the platform. 
Final Study Report - Lick 

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Figure 7. Drilling mud accumulation for a source at Eugene Isle. Discharge rate of 7.5 kg/s for 30 days. 

Contour interval is 0.01 gm/cm2

. 

Vigorous storms in the Gulf of Mexico occur most frequently during the winter months. 

Currents during this season flow to the northwest. A storm of moderate intensity will 

result in a significant wave height of 150 cm; statistically, waves of this magnitude will be 

present at least 50 times per year. Calculations for this typical storm generate a bottom 

shear stress of 35 dynes/cm2 at Eugene Isle and up to 83 dynes/cm2 in the near shore 

regions (Figure 8). The sediment bed parameters for the Gulf of Mexico were the same as 

those employed for Platform Hidalgo, except that the bed was assumed to consist of 70% 

coarse-grained material throughout the study region. For these conditions, approximately 

6 gm/cm2 was resuspended at the platform during the storm. As the waves subsided, settling of these suspended solids, as well as those eroded from the near shore regions, took 

place. Twenty-four hours after the storm ended, total deposition at the platform of 8 

gm/cm2 was calculated, resulting in a net deposition of 2 gm/cm2 at Eugene Isle. Areas of 

net erosion occurred only in the shallowest region near shore (>50 gm/cm2). 
Risk Analysis 

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Figure 8. Bottom shear stress for Hs of 150 cm. Contour interval is 10 dynes/cm2

. 

Drilling mud eroded near the platform during the storm was transported with the 

prevailing currents, settling out when the shear stress declined to less than 1 dynes/cm2. 

According to NOAA data, a storm event having a significant wave height of 850 cm will 

occur only once in a ten year period. For this very large storm event, bottom shear stresses 

exceed 150 dynes/cm2 at the platform. The distribution of calculated bottom shear stresses 

for this event is shown in Figure 9. Extensive erosion of bottom sediments occurs during 

an event of this magnitude. Assuming a critical shear stress of 1 dynes/cm2 for all layers, 

over 5 m of sediment will be resuspended at the platform. Since this is not a realistic 

occurrence, a five layer sediment bed was assumed with very high critical shear stresses of 

10 dynes/cm2 and 100 dynes/cm2 and thicknesses of 100 gm/cm2 and 1000 gm/cm2 for the 

fourth and fifth layers of the sediment bed, respectively. All other parameters for the 

sediment bed were unchanged from the previous calculation. During the storm, the 

calculated erosion was 119 gm/cm2 at the platform, decaying to a net erosion of 19 

gm/cm2 twenty-four hours after the storm ended. As for the smaller storm, resuspended 

material was transported by the currents. 
Final Study Report - Lick 

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Figure 9. Bottom shear stress for Hs of 850 cm. Contour interval is 10 dynes/cm2

. 

DISCUSSION OF RESULTS 

Numerical calculations for the transport and fate of drilling muds have been performed for 

offshore oil platforms located in both shallow and deep water. Conditions examined 

included normal current and tidal phenomena, typical winter storms, and severe, lowprobability storms. Results of these calculations coupled with experimental studies show 

that the amount of material resuspended is strongly dependent upon the magnitude of the 

bottom shear stress. To illustrate this point, consider a uniform bed located in the vicinity 

of Eugene Isle having a critical shear stress of 1 dynes/cm2

. A significant wave height of 

150 cm in l0 m of water generates a bottom shear stress of 13 dynes/cm2 and resuspension 

of 1.2 x 10-1 gm/cm2. Increasing Hs by a factor of roughly 5 results in a non-linear 

increase in the bottom shear stress to approximately 200 dynes/cm2 with a concomitant 

increase in the amount of sediment resuspended by a factor of l04. In deep water, the 

erosional influence is less severe due to decreased values of uwbm. However, significant 

quantities of material will be transported from shallow regions, resulting in heavy 

deposition of near-shore sediments during large storms. To illustrate this point, consider 

that increasing Hs by a factor of 3.5 in the Platform Hidalgo region resulted in an increase 

in deposition by a factor of 50. If the sediment bed had been assumed uniform, the amount 

of deposition in deep water would have been significantly greater. Clearly, the 

relationship between sediment deposition and shear stresses is also highly non-linear. 

In real physical systems, sediment beds are not uniform. If field data is not available, the 

properties of these beds must be approximated using a reasonable set of assumptions, and 
Risk Analysis 

15

through numerical experimentation to evaluate the correspondence of calculations to 

probable physical outcomes. The sediment beds assumed for these calculations were 

developed utilizing the following: 1) laboratory experiments for river, lake, and oceanic 

sediments (Xu, 1990; Tsai et al, 1987; MacIntyre et al, 1990), 2) field data for continental 

shelf sediments (Larsen et al, 1980), 3) field data for study regions (Neff, 1988) and 

(MMS, 1990), and 4) numerical experimentation to determine reasonable sediment 

layering. 

There are several factors which must be considered with regard to these assumptions, 

most significant of which is the dearth of laboratory studies for resuspension of buried 

marine sediments subjected to high shear stresses, and for drilling muds in marine 

environments. In particular, the empirical relationship utilized (Eqn 3) was derived for 

sediments under low (<15 dynes/cm2) shear stress conditions. While current studies 

indicate that the general form of the relationship for resuspension versus shear stress is 

valid for a wide range of sediments, the problem of high shear stresses has yet to be 

resolved. 

Yet another concern relates to the actual layering of the sediment bed and the assumed 

composition of this bed. Numerical experimentation is highly subjective, especially in 

studies such as this in which complete field data is unavailable. Values of the critical shear 

stress for the upper layers have been documented for the California continental shelf in the 

literature (Larsen et al, 1980; Drake, 1976; MMS .1990). Some studies have indicated that 

lower threshold values may apply (Miller et al 1977). Thus the assumption for the upper 

layers may be considered within the range of available data for Platform Hidalgo, and may 

even be too conservative. The assumption that the critical shear stress increases with depth 

is certainly reasonable from a physical standpoint, but exact values must be found from 

field studies. Similar studies were not available for the Gulf of Mexico. It was therefore 

necessary to assume that conditions were similar to those found along the California coast. 

Results for this region would be more satisfactory if parameters for Eqn 3 were available 

for sandy material. 

The issues addressed in the preceding discussion must not obscure the fact that this study 

resulted in quantitative evaluation of the transport, deposition, and resuspension of drilling 

muds and marine sediments. While refinements to the process are needed, the nonlinearity of the relationship between resuspension and shear stress represents a 

fundamental parameter in the accurate calculation of sediment transport. Integrating 

laboratory studies, field data, and numerical modeling, it has been found that large storms 

are responsible of disproportionately greater amounts of sediment and pollutant transport 

when compared to smaller storms. 
Final Study Report - Lick 

16

CONCLUSION 

Determination of the ultimate fate of drilling muds discharged from offshore oil platforms 

requires a proper evaluation of the hydrodynamic character of the discharge region as well 

as an accurate calculation of suspended solids transport and sediment 

resuspension/deposition behaviour. A numerical model possessing these characteristics 

has been employed to analyze the fate of drilling muds discharged from a platform sited in 

deep water at Platform Hidalgo on the California coast, and at a shallow water location at 

Eugene Isle in the northern Gulf of Mexico. 

Calculations for both sites reveal that large storms can resuspend drilling muds deposited 

in both deep and shallow water, although the influence of these storms in shallow water is 

considerably more pronounced. For the deep water site, small storms were found to 

resuspend sediments from areas with minimal drilling mud deposition, and to 

subsequently deposit this material in deeper water where drilling muds are more likely to 

be found. Typical storms in shallow water generated bottom shear stresses large enough 

to erode significant quantities of drilling mud deposited in these environments. During 

large storms, the amount of sediment resuspended increases dramatically. 

It must be noted that the extent of erosion is strongly dependent upon the choice of 

sediment bed properties. Due to lack of available field data for these study regions, these 

results must be viewed as general descriptions of the transport and ultimate fate of drilling 

muds in marine environments. However, the non-linear relationship between 

resuspension/deposition and bottom shear stress leads to the conclusion that large storms 

account for the most significant quantities of sediment transport in both shallow and deep 

water environments. 

ACKNOWLEDGEMENTS 

This research was supported by the National Science Foundation and the United States 

Department of the Interior, Minerals Management Service. 
Risk Analysis 

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REFERENCES 

Burban, P.Y., Y. Xu, J. McNeil, and W. Lick, 1990. Settling speeds of flocs in fresh and 

sea waters, J. Geophysical Res.,Vol.95, C10, PP. 18213-18220. 

Christoffersen, J.B. and I.G. Jonsson, 1985. Bed friction and dissipation in a combined 

current and wave motion, Ocean Engng., 12, No. 5, pp. 387-423. 

Drake, D.E., 1976. Suspended Sediment Transport and Mud Deposition on Continental 

Shelves, Marine Sediment Transport and Environmental Management, pp. 127-

158, John Wiley and Sons, New York. 

Gailani, J.Z., C.K. Ziegler and W.J. Lick, 1990. The Transport of Suspended Solids in the 

Fox River, J. Great Lakes Res., 17, in press. 

Gailani, J.Z., 1991. The Transport of Sediments in the Lower Fox River, Ph.D. Thesis, 

University of California, Santa Barbara. 

Grant, W.D. and O.S. Madsen, 1979. Combined Wave and Current Interaction with a 

Rough Bottom. J. Geophysical Res., 84, pp. 1797-1808. 

Huang, H., 1992. Transport Properties of Drilling Muds, Ph.D. Thesis, University of 

California, Santa Barbara. 

Larsen, L.H., R.W. Sternberg, N.C. Shi, M.A.H. Marsden, and L. Thomas, 1980. Field 

Investigations of the Threshold of Grain Motion by Ocean Waves and Currents, 

Sedimentary Dynamics of Continental Shelves, Developments in Sedimentology, 

pp. 105-132. 

Lick, W. and J. Lick, 1988. On the aggregation and disaggregation of fine-grained lake 

sediments, J. Great Lakes Res., 14(4), pp. 514-523. 

MacIntyre, S., W. Lick, and C.H. Tsai, 1990. Variability of entrainment of cohesive 

sediments in freshwater, Biogeochemistry, 9, pp. 187-209. 

Miller, M.C., I.N. McCave, and P.D. Komar, 1977. Threshold of Sediment Motion Under 

Unidirectional Currents. Sedimentology, 24, pp. 507-527. 

Minerals Management Service, 1990. California OCS Phase II Monitoring Program, Year 

Three Annual Report. U.S. Dept. of the Interior, Minerals Management Service, 

Los Angeles, CA, MMS 90-0055. 

Murray, S.P., 1976. Currents and Circulation in the Coastal Waters if Louisiana, Sea 

Grant Publication No. LSU-T-76 -003. 

National Data Climatic Center, 1990. Climatic Summaries for NDBC Buoys and Stations, 

Update 1, National Oceanic and Atmospheric Administration. 
Final Study Report - Lick 

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Neff, J.M., T.C. Sauer, and N. Maciolek, 1988. Fate and Effects of Produced Water 

Discharges in Nearshore Marine Waters, Final Report to American Petroleum 

Institute, Battelle Ocean Sciences, Duxbury MA. 

Tsai, C.H., and W. Lick, 1987. Resuspension of sediments from Long Island Sound, Wat. 

Sci. Tech., Vol. 21, No. 6/7, pp. 155-184. 

Xu, Y., 1991. Transport Properties of Fine-Grained Sediments, Ph.D. Thesis. University 

of California, Santa Barbara. 

Ziegler, C.K., and W. Lick, 1986. A numerical model of the resuspension, deposition, and 

transport of fine-grained sediments in shallow water, UCSB Report ME- 86-3. 
Risk Analysis 

19

The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most 

of our nationally owned public lands and natural resources. This includes fostering sound use of our 

land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; and providing for the 

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.