Canadians evaluate technologies to manage offshore drilling cuttings

This article presents several treatment and disposal technologies for SBF and proposes the most suitable option for offshore operations. The deterministic, multicriteria decision-making model was described in an earlier publication, while this article focuses on the uncertainty and sensitivity analyses used in the method and their effects on the ranking of the technologies.1

Methodology

The first step of any evaluation is to identify the most important criteria from environmental, cost, and technical standpoints. The criteria, which are the controlling factors under which the options will be scored in the evaluation, were chosen to reflect the issues with respect to drilling cuttings management technologies.

In this study, researchers divided criteria into two major types: threshold criteria and decision-making criteria. Conformity with discharge regulations was the threshold criterion to screen out inappropriate options. The options that were unable to meet the regulatory discharge standard of 6.9% retention of base fluid based on the US Environmental Protection Agency’s (EPA’s) cuttings discharge standards and the Offshore Waste Treatment Guidelines were rejected and not considered further.2 3

For drilling cuttings management, researchers used four important factors in applying a technology offshore as decision-making criteria. These factors were four primary criteria categories: technical feasibility, rig compatibility, environmental and safety, and costs. The unnumbered figure at the top of this page shows the criteria hierarchy.

We then assigned the decision-making criteria weights according to their importance using a direct weighting method. Using this method, we distributed and assigned a total weight of 100 to the criteria.4 The assigned weights appear as the numbers in brackets in the criteria hierarchy illustration.

Technical, environmental, and cost aspects received about equal importance. Therefore, as shown in the box, we assigned weights of 20, 16, 32, and 32 to the technical feasibility, rig compatibility, environmental and safety, and costs, respectively. The sum of the weights of technical feasibility and rig compatibility is 36, which is about equal to the weights assigned to the other two categories of the criteria.

We subdivided the four major categories of criteria into subcriteria and then grouped these subcriteria for simplicity in assigning weights. Each group received appropriate weights according to its relative importance and the subcriteria in the same group received equal weights.

The weights of the subcriteria in each group add up to the weight of the upper-level criteria and the weights of the criteria in the same level add up to 100. For example, as shown in the criteria hierarchy, three subcriteria were grouped into the higher level criterion, ease of operation. We assigned the ease of operation criterion the weight of six and each of the three criteria under the ease of operation criterion received an equal weight of two.

For a technology to be selected as an option in the evaluation, it must meet the threshold criterion. Therefore, the selected options include eight technologies which do not involve ocean discharge or those which are able to reduce the amount of retained base fluids on cuttings to lower than 6.9%: the vertical centrifuge, horizontal centrifuge, thermal desorption, incineration, grinding, stabilization, biological treatment in a bioreactor, and reinjection. Descriptions of the technologies can be found in many publications.5 6

These technologies involve use of various types of treatment or disposal mechanisms including mechanical treatment, thermal treatment, chemical treatment, biological treatment, and reinjection. Options that are currently in use offshore include vertical centrifuge, horizontal centrifuge, and reinjection. The other options are current land-based technologies but have potential for offshore applications.

We then rated and scored the options under each corresponding criterion according to information obtained from various sources, such as journal papers and the industry. We distributed questionnaires to companies that supply drilling cuttings treatment and disposal systems and to their industry users.

In order to score the options, we divided the scoring schemes used in the evaluation into two types according to whether the data were qualitative or quantitative. Quantitative data, such as weight and capacity of management systems, were normalized so that the values for different criteria conformed to an equivalent scale. The normalization employed linear value functions as shown in Fig. 1.

The quantitative data may have values that are either increasing (a higher value indicates a higher ranking) or decreasing (a lower value indicates a higher ranking). Therefore, we assigned a negative sign to the scores with decreasing values so that the least preferred properties were assigned the lowest score of negative ten. This results in two different ranges of normalized scores, 0 to 10 for increasing values and -10 to 0 for decreasing values.

Equation 1 is the general equation used to normalize quantitative values.

On the other hand, where quantitative data were not available, we used subjective rankings to measure or evaluate the option and converted them into numbers within the range of 0 to 10 using conversion charts shown in Fig. 2. These charts were divided and marked with the numbers from 0 to 10 where the higher value represents a preferred characteristic of a technology. As seen in Fig. 2, for the proven technology criterion, because the technology that has been previously used offshore is preferred, we assigned it a score of nine. Accordingly, less preferred characteristics correspond to lower scores in the conversion chart.

Analysis

In this study, we selected the widely used Additive Value Model as the overall value model because of its simplicity and robustness.7 Equation 2 (modified from Parnell) can calculate the overall value for each cuttings management option.4

With the weights assigned and the ranges of the scores as mentioned in the preceding section, the possible range of overall values is -560 to 440. The calculated overall values are relative numbers used to compare and rank the evaluated options, and there is no significance of a number within the range of the overall values.

According to the calculated overall values, the best three options were the vertical centrifuge, horizontal centrifuge and reinjection, with overall values of 291, 285, and 182, respectively. Therefore, based on the overall values alone, we considered these three options the most suitable options for offshore drilling cuttings management based on the established set of criteria.

Currently, these selected three options are the only technologies used offshore. The bioreactor, which ranks the fourth in this evaluation, obtained a slightly lower overall value than reinjection. This means that even though this option has never been used offshore, it is one of the most promising options for potential future offshore applications. Fig. 3 shows a comparison of the overall values of the eight options.

Fig. 4 compares total scores of each major category of the criteria. Centrifuges (vertical and horizontal) scored the best under technical feasibility. This is partly due to their high capacity to process the waste and their ease of operation and maintenance. In addition, the fact that these technologies are well known and have been previously used offshore also ensures their technical feasibility.

Under rig compatibility, centrifuges attained the highest scores, mainly because of their compact size and weight. Previous offshore application also enhanced their high rig compatibility.

Reinjection attained the highest score under the environmental and safety category because of the reduction of toxic substances and future liability. Under the cost criteria, centrifuges scored the highest (least negative).

We compared cost-effectiveness of the options based on the relationship between the options’ offshore applicability and their costs. The offshore applicability, in this case, was defined as the applicability of a treatment option to be used offshore with regard to offshore technical feasibility, rig compatibility, and environmental and safety. Therefore, the sum of the three category scores under technical feasibility, rig compatibility, and environmental impacts and safety represented the offshore applicability value.

Fig. 5 shows the offshore applicability values of each evaluated option plotted against cost category scores to outline the performance of each management option compared with their costs. From the figure, the two types of centrifuges (vertical and horizontal) provided high values of offshore applicability with the lowest costs (least negative and nearly zero scores) and are the most cost-effective.

Reinjection, despite its slightly higher costs than the bioreactor, showed significantly better offshore applicability. Therefore, reinjection was the third most cost-effective option in this case.

In order to identify the reliability of the final results, we also conducted uncertainty analysis.

In the case that data for an option were not available, we assigned a midrange value to the option (a value of 5 for a positive score range and -5 for a negative score range). In addition, we assigned an associated uncertainty of ±5 to the option to reflect the reliability of the final results. The uncertainty of ±5 indicates that each of the assumed midrange values could actually vary from 0 to 10.

The general equation for uncertainty analysis is Equation 3.8 In this case, r is the overall value model as shown in Equation 2. Thus, we substitute and arrive at Equation 4.

Therefore, Equation 5 calculates the uncertainty values of the final overall values.

Table 1 lists the calculated uncertainty values associated with each option. These values are relative uncertainties. The value of zero does not mean that the data for the option has no uncertainty associated with it, but rather that all the data for that option were available and no figures were assumed.

We added the uncertainty value (±U) of each management option to and subtracted it from the option’s overall value, resulting in ranges of overall values (Fig. 6). According to the figure, the centrifuges (vertical and horizontal) provide the highest overall values with the lowest uncertainty.

Reinjection and the bioreactor are both the third best choice. We chose reinjection as the third best option, however, because the bioreactor has a much higher associated uncertainty.

Therefore, with consideration of uncertainty associated with the overall values, the best three options are the vertical centrifuge, horizontal centrifuge, and reinjection. The result is similar to that when overall values alone were used to compare the options.

To determine the sensitivity of the evaluation results to the distributions of the weights among the criteria, we varied the criteria weights to observe new overall values and alternative ranks. We conducted seven sets of analyses (Table 2) by specifying a weight for one criterion or weights for a group of criteria and adjusting the weights of the other criteria proportionally. In all cases, the total weight remained constant. Therefore, once a weight increased, the others decreased, resulting in different tradeoffs among the criteria in each set of the analyses.

As shown in the table, we assigned a criterion or a group of criteria weight(s) that were decreased by up to 75% (0.25 x original weight) and-or increased by up to 150% (2.5 x original weights) in order to determine changes in alternative ranks.

The seven cases of sensitivity analyses showed that the results of the evaluation were not significantly sensitive to the changes in the criteria weights. The ranks of the options, especially those of the best three technologies, were unchanged in most cases of criteria weight distributions. Changes in the ranks of the best three options only occurred when unrealistic criteria weights were assigned.

For example, as shown in Fig. 7, reinjection ranked first when we increased the weights of the environmental and safety criteria by 150%, which resulted in as high as 80% of the total criteria weight being assigned to environmental and safety aspects. Also, in the case that the weights of the environmental and safety criteria were decreased by 75%, the bioreactor became the third ranked option. However, this case also involves impractical criteria weight distribution as only 8% of the total criteria weight was given to the environmental and safety criteria.

Discussion

Based on evaluation of drilling cuttings management options with a weighted deterministic multicriteria decision making, the optimum onsite management technologies for drilling cuttings included the vertical centrifuge, horizontal centrifuge, and reinjection. Offshore feasibility and low cost were the dominant factors in these options emerging as the most favorable. These technologies are also the only technologies widely used in current offshore operations. The bioreactor, which is in fourth place, is the most promising technology for possible future offshore applications.

Many parameters influence the validity of the evaluation results, including the availability of data and the distribution of weights. Therefore, we conducted uncertainty and sensitivity analyses to validate the results of the evaluation. The options’ uncertainty reflects the reliability of the options’ overall scores due to the limited availability of data. The centrifuges and reinjection scored the best when uncertainty values were considered along with the overall scores, as the uncertainties associated with these options were relatively low.

In addition to the best three options, the bioreactor ranked fourth, with larger associated uncertainties due to lack of information on size, weight, energy consumption, and its feasibility for offshore use.

We used a sensitivity analysis to see the effects of changes to the criteria weights on the ranks of the options. Results of the evaluation were not significantly sensitive to changes in criteria weight distribution. This is largely due to the fact that the top three options are markedly superior in important criteria, which results in those options scoring higher regardless of the weight alterations.

Another possible reason is that the weight that was distributed throughout a number of criteria resulted in rankings were less sensitive to changes in any one criterion. Therefore, criteria should be selected so that detailed reassessment using only significant criteria in comparing the options can be conducted.

Finally, this evaluation was designed to provide a simple but comprehensive methodology to initially assess offshore drilling cuttings management technologies. In a real situation, selecting a technology to deal with contaminated drilling cuttings depends on many site-specific issues. Therefore, we recommend a case-by-case analysis involving some modifications to this approach.

Acknowledgments

The authors thank the Natural Sciences and Engineering Research Council, Canada, for financial support.

References

1. Thanyamanta, W., Hawboldt, K., Husain, T., Bose, N., and Veitch, B., “Evaluation of Offshore Drilling Cuttings Management Technologies using Multicriteria Decision Making,” in S.L. Armsworthy, P.J. Cranford, and L. Kenneth (eds.) Offshore Oil and Gas Environmental Effects Monitoring. Approaches and Technologies, pp. 167-179. Columbus, Ohio: Battelle Press, 2005.
2. US Environmental Protection Agency. Development Document for Final Effluent Limitations Guidelines and Standards for Synthetic-Based Drilling Fluids and other Non-Aqueous Drilling Fluids in the Oil and Gas Extraction Point Source Category. EPA-821-B-00-013, 2000.
3. NEB, CNOPB, and CNSOPB (National Energy Board, Canada-Newfoundland Offshore Petroleum Board, and Canada-Nova Scotia Offshore Petroleum Board). Offshore Waste Treatment Guidelines, August 2002.
4. Parnell, G.S., Jackson, J.A., Kloeber, J.M., Jr., and Deckro, R.F., “Improving DOE Environmental Management: Using CERCLA-Based Decision Analysis for Remedial Alternative Evaluation in the RI/FS Process,” VCU-MAS-99-1, 1999.
5. Canadian Association of Petroleum Producers (CAPP), Offshore Drilling Waste Management Review. Technical Report 2001-0007.
6. Cripps, S.J., Picken, G., Aabel, J.P., Andersen, O.K., Heyworth, C., Jakobsen, M., Kristiansen, R., Marken, C., Paulsen, J.E., Shaw, D., Annand, A., Jacobsen, T.G., and Henriksen, I.B., Disposal of Oil-based Cuttings. Prepared for the Norwegian Oil Industry Association (OLF) by Rogaland Research (RF). RF-98/097, Vers. 3, 1998.
7. Hobbs, B.F., and Meier, P., International Series in Operations Research & Management Science. Energy Decisions and the Environment. A Guide to the Use of Multicriteria Methods. Norwell, Mass.: Kluwer Academic Publishers, 2000.
8. Coleman, Hugh W., and Steele, W. Glenn Jr., Experimentation and Uncertainty Analysis for Engineers. New York: John Wiley & Sons Inc., 1989.

The authors

Worakanok Thanyamanta ([email protected]) is a PhD candidate in mechanical (offshore oil and gas) engineering at Memorial University of Newfoundland. She previously worked for Chula Unisearch, Chulalongkorn University, Bangkok, as a research assistant in Bangkok Metropolitan Administration’s Bangkok air quality management project funded by the World Bank. She holds a BEng in environmental engineering from Chulalongkorn University, Thailand, and an MEng in civil engineering from Memorial University.

Kelly Hawboldt (hawboldt @engr.mun.ca) is an assistant professor of environmental engineering at Memorial University of Newfoundland. After her undergraduate degree, Hawboldt worked at Norcen Energy Resources as a process engineer at a sour-gas plant. She also worked at Bel-MK Engineering as an environmental engineer prior to and during graduate school. Hawboldt holds a BSc in chemical engineering from the University of Saskatchewan, an MSc and a PhD in chemical and petroleum engineering from the University of Calgary and is a member of SPE and CSChE.

Tahir Husain ([email protected]) is a professor of environmental engineering at Memorial University of Newfoundland. He also currently holds the position of director of continuing engineering education. Husain worked 16 years for the Research Institute of King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia. Before joining Memorial University, he was a visiting scientist at the Harvard School of Public Health, Cambridge. Husain holds a BEng from Aligarh Muslim University, Aligarh, Uttar Pradesh, India, an MEng from the Asian Institute of Technology, Bangkok, and a PhD from the University of British Columbia.

Neil Bose ([email protected]) is a professor of ocean and naval architectural engineering at Memorial University of Newfoundland and holds a Tier 1 Canada Research Chair in offshore and underwater vehicles design. He taught at Glasgow University 1983-87 and joined Memorial University in 1987. Bose was also director of the Ocean Engineering Research Centre from 1994-2000 and chair of ocean and naval architectural engineering at Memorial University from 1998-2003. He holds a BSc in naval architecture and ocean engineering and a PhD in hydrofoil design, both from Glasgow University.

Brian Veitch ([email protected]) is an associate professor of ocean and naval architectural engineering at the Memorial University of Newfoundland. He currently holds a position of PetroCanada/Terra Nova project junior research chair in ocean environmental risk engineering. Before joining Memorial University, he worked at NRC’s Institute for Marine Dynamics. Veitch holds a BEng and an MEng from Memorial University. He also earned a licentiate of science in technology and a doctor of science in technology from Helsinki University of Technology.