PRACtically Speaking
The Newsletter of Petroleum Research Atlantic Canada - October 2006Research Profile
Stability and Drift of Icebergs Under Tow
Freeman Ralph, C-CORE
Icebergs pose a unique risk to petroleum production platforms in the waters of Atlantic Canada. Current iceberg management tactics rely heavily on iceberg towing using offshore supply vessels to alleviate the risk of disconnecting a facility from its mooring. This risk is among the most important considerations for facility selection for oil and gas field developments.
Knowledge of an iceberg’s stability characteristics is very useful when attempting to perform towing operations. This information helps the towing vessel’s master decide the appropriate towline configuration to use and the maximum amount of bollard pull that can be applied without rolling the iceberg.
Figure One. Performing iceberg towing operations. Note tow cable in centre of iceberg.
Project Organization
Freeman Ralph is Director, Ice Engineering at C-CORE in St. John’s, Newfoundland Labrador. Together, with partners at Petro-Canada, Memorial University, Ocean Inc. and Provincial Aerospace Limited (PAL) Environmental Services, backed with two-year funding from PRAC, he conducted a study designed to close the gap in knowledge associated with iceberg shape, motion and resultant stability while under tow.
A lost iceberg tow, whether through instability or tow rope slippage, results in lost time and increased risk. Increasing the understanding of the state of stability of an iceberg, guidance on optimized tow configurations and the limiting tow force that can be applied will result in tremendous benefit to ice management operations.
Work Scope
The key focus of the project was to analyze the relationship between iceberg stability and towing parameters such as point of application, tow force, acceleration and tow speed. In addition, the project focused on whether tow strategy can be optimized using a model that integrates the forces on the iceberg through the water column to predict drift based on forecasted conditions.
A numerical model has been developed to estimate the stability of an iceberg under tow as well as the subsequent tow direction and speed based on iceberg and environmental particulars, tow force and direction. Practical guidelines for improving the effectiveness of iceberg towing operations were also developed.
Iceberg Stability Under Tow
In practice, ice management is a time critical operation. Completing a tow and having the vessel available for another tow is important. The maximum power available to tow an iceberg is often applied without giving consideration to the size of the iceberg. This means a small iceberg relative to the vessel powering ability will likely roll.
The model used for simulating iceberg stability under tow is based on the work of Liang (2001). The model considers iceberg stability and deterioration and includes the effect of towing forces on the iceberg. Since selecting a particular iceberg orientation for a tow (relative to the desired tow heading) may not always be practical, critical forces for profiled icebergs would be computed as the minimum force for all directions around the body in the horizontal plane. Iceberg profile data were used for verifying the results including Dynamics of Iceberg Grounding and Scouring (DIGS) iceberg profiles (Hodgson et al, 1988) and Oceans profiles (Oceans, 2002, 2003).
Iceberg Tow Direction
An iceberg tow trajectory model has been developed based on work by Dunderdale (2000) to estimate the influence of the magnitude and direction of the tow force on the towed iceberg speed and course made good (CMG). The purpose of the model is to provide a tool to aid in the development of a towing strategy for improved management of threatening icebergs. The model is based on maintaining a balance of the drift forces acting on an iceberg.
Data were obtained from the PERD (Program of Energy Research and Development) Comprehensive Iceberg Management Database (PERD, 2004) to evaluate the performance of the model. Single line tows, defined as successful in the database, with all the required model inputs defined and resulting CMG and tow velocity C V recorded were selected for comparison to the model predictions.
Based on the results of the regression analysis, the model slightly underestimated the deflection angle on average (92 per cent of observed). On average, modeled speed made good was 74 per cent of the observed value.
The tow model was used to evaluate the predicted track deflection based on applied tow force and direction for icebergs ranging in size from 20 m to 250 m and freely drifting at speeds of 0.1 m/s to 0.7 m/s. Tow forces considered ranged from 40 tonnes to 120 tonnes applied at relative tow angles from 10 degrees to 180 degrees.
The predicted change in trajectory (?CMG) and time required to achieve it were plotted against tow force and tow angle. An example has been provided in Figure Two for an iceberg with a waterline length of 140 m traveling at a speed of 0.5 m/s. It shows that a tow force of 80 tonnes applied at an angle 50° offset from the iceberg’s drift heading will result in a 13° ?CMG. The time required to attain this new heading is 2.1 hours.
Figure Two. Variations in CMG and time to CMG for a 140m iceberg drifting at 0.5m/s based on tow angle and force.
Towing Catenary and Iceberg Orientation
In the past, a number of iceberg towing strategies have been used with varying degrees of success. The influence of the towing catenary was considered for improving towing effectiveness. The influence of increased angle of tow force application was assessed to illustrate the improvement in iceberg stability. The results concluded that the tow force that can be applied without rolling can be increased to in excess of 100 per cent with an increase in the tow angle relative to the surface of 20 degrees.
The assessment of towing catenary considers the length of the tow rope (or steel towing hawser), the depth of the monkeys face plate relative to the seafloor. Key factors that influence the catenary include the specific weight tow hawser and monkeys face, applied bollard pull and tow speed.
One limitation with the work is the ability to accurately calibrate models with full scale data. It is imperative that any field work, whether it be for specific research interests or strict operations, should make efforts to collect quality data.
A model has been developed to estimate the catenary of a single line towed cable system.
A captain could estimate and graphically display the geometry of the towed cable system allowing him to optimize the towing parameters to maximize the angle of the towline relative to the horizontal without the cables making contact with the sea floor.
Calibration of the stability model is limited due to the lack of high resolution iceberg profiles. It is also uncertain the state of iceberg stability once a tow is hooked up. Current data suggests that no correlation exists, however, rationalization of the problem indicates that this is incorrect. In lieu of limited data, an alternative mechanism to calibrate and verify improved deflection algorithm would be beneficial.
Future efforts will focus on integrating the stability software with iceberg profiling software, as well as integrating the tow deflection and catenary algorithms into Grand Banks ice management service providers (PAL Environmental Services) iceberg tracking and management software. Where appropriate, software algorithms will be made available to vessel captains. The final product will allow offshore ice observers and vessel captains to strategically plan tows that consider towing stability (including catenary arrangement), and tow deflection.
References
For complete details, please contact (902) 494-2762.
PRAC News
PRAC Welcomes New Member
PRAC would like to welcome Environment Canada as its newest member. Mr. Jim Abraham (Director General Atlantic) will be Environment Canada’s representative. PRAC looks forward to working with Mr. Abraham and the positive contributions he and his organization will bring to PRAC’s research and development (R&D) efforts.
Board of Directors' News
There have been some recent changes to PRAC’s Board of Directors. Former Chairman of the Board, Liam Mallon (President, ExxonMobil Canada), has moved on to a new posting in Malaysia. PRAC would like to thank Mr. Mallon for his contribution as Board Chair and wishes him well in his new position. Replacing Mr. Mallon as Acting Chair is David Collyer (Shell Canada) who will remain in the position until PRAC’s Annual General Meeting (AGM) in the spring of 2007. Glenn Scott of ExxonMobil Canada has replaced Mr. Mallon on the Board. PRAC wishes to thank David Collyer and Glenn Scott for their support.
Staff Changes at PRAC
Anjlee Bhatt-Standley (Manager, Records and Information Systems) left PRAC in August to pursue her education through the MBA program at Saint Mary’s University. PRAC thanks her for her two-years of service with the organization and wish her well in her studies.
PRAC would like to welcome its newest staff member, Jennifer Barnable (Manager, Communications) to the St. John’s office. Jennifer brings an impressive variety of work experience in professional writing, communications and special event planning. She holds an Advanced Diploma in Public Relations and has worked for the Historic Sites Association and Parks Canada Agency over the past four years. Jennifer says, “I’m looking forward to learning more about the petroleum industry, the role PRAC plays and how to build upon our communication strategies.” You can reach Jennifer by phone at (709) 726-5640 or by e-mail at Jennifer.barnable@pr-ac.ca.
Nadine MacDonald has been hired as Administrative Assistant in the Halifax office where she will be responsible for a wide range of administrative duties. Nadine comes to PRAC with 11 years experience in assisting various executives with daily business. Her background includes the computer services sector, the wholesale business related to interior design as well as property management. She can be reached by phone at (902) 494-2960 or by e-mail at Nadine.macdonald@pr-ac.ca.
2006 Energy R&D Forum
On behalf of the Nova Scotia Department of Energy and Atlantic Canada Opportunities Agency, PRAC has prepared and released a synthesis of presentations and evaluation report for the 2006 Energy R&D Forum. For more information visit www.energyresearch.ca and click on the “Energy Research and Development Forum 2006” banner.
Roadmap Unveils Vision for Unconventional Gas
Petroleum Technology Alliance Canada (PTAC), along with project partners and sponsors, is pleased to announce the completion of Filling the Gap: Unconventional Gas Technology Roadmap.
This Technology Roadmap identifies challenges and opportunities for future development of unconventional gas in Canada, and the technologies needed to maximize that development. The ambitious vision proposed focuses on accelerated, environmentally-responsible development of Canadian unconventional gas resources, predicting that to meet rising domestic and North American demand, natural gas production will need to rise to 7.5 Tcf/year by 2025. To achieve this vision it is estimated that by 2025, 40 per cent of Canadian domestic natural gas production will have to come from unconventional sources.
The roadmap is not in itself a detailed technology plan but an analysis of the role unconventional sources can play in the future energy requirements of Canada. Subsequent to the release of the roadmap, attention will need to be devoted to further long-term research and development planning. The roadmap is an important, necessary first step towards identifying future opportunities and development needs that industry and governments can incorporate into their planning. It provides lead time for the efficient and effective development of the resource.
PTAC is a not-for-profit organization with a mission to facilitate innovation, technology transfer and collaborative research, development, demonstration and deployment of technology for the responsible development of Western Canada’s upstream hydrocarbon energy industry. PTAC and PRAC work closely in the interest of increasing collaborative R&D and Technology Transfer throughout both Western and Atlantic Canada.
The Unconventional Gas Technology Roadmap is available for purchase through the PTAC Knowledge Centre. Within a few weeks time, an electronic version of the roadmap will be available for download from the PTAC website (http://www.ptac.org) free of charge. You can request your copy by contacting the PTAC Knowledge Centre at (403) 218-7712.
Comments and Questions Are Welcome!
Send an e-mail to communications@pr-ac.ca with any questions or comments about this newsletter. Visit www.pr-ac.ca for more information about PRAC.
