Subsea oil and gas developments in the Grand Banks region, offshore Eastern Canada, require mitigation techniques to protect against iceberg keel interactions. For example, untrenched infield flowlines incorporate weak link systems designed to fail in the event of flowline snag to protect upstream and downstram assets. Even with these systems, the assumption that any iceberg contact equates to flowline failure means that flowline lengths in excess of approximately 10 km require trenching to meet safety target levels. Furthermore, all subsea wells to date have been installed in excavated drill centers to avoid contact with gouging icebergs. Based on current design practices, these mitigation measures are cost prohibitive and limit the potential for the development of marginal fields. This paper addresses conventional practice to protect against iceberg interaction and proposes alternative solutions that maintain safety, while reducing costs significantly.
Boroojerdi, Marjan Taghi (Memorial University) | Taylor, Rocky (Memorial University) | Mohammadafzali, Soroosh (Memorial University) | Bailey-Dudley, Eleanor (C-CORE) | Turnbull, Ian (C-CORE) | Hossain, Ridwan (Memorial University)
Sea ice rubble/ridge strength and interaction mechanics are highly important in the design of structures and subsea infrastructure for ice prone offshore environments. To better characterize sea ice conditions in northern Newfoundland, a series of field tests were conducted on landfast ice in Pistolet Bay, NL in February 2018. This paper presents a summary of recent shear strength tests on solid and freeze-bonded ice specimens, to help improve understanding of ice rubble properties and behaviour under field conditions. Both horizontal and vertical sea ice samples were tested under dry and submerged conditions, as well as freeze-bonded ice samples under submerged conditions. Sea ice samples were sheared using the Asymmetric Four Point Bending (AFPB) method, which has been shown to produce a near pure shear region at the center of the specimen. For the dry tests, cores were sheared directly after collection so as to test them in conditions as close to in-situ as possible. For submerged tests, cores were submerged for a specific period of time before shearing. Freeze-bonded samples were prepared using a confinement frame which applied a pressure of 25 kPa to the specimens during submergence. These data for AFPB field tests are an important consideration in modelling the strength of ice rubble/ridges and are the first of their kind. From this work it may be concluded that the AFPB method is a promising approach for studying shear strength of both solid and freeze-bonded specimens in the field and additional testing is recommended. New field testing approaches, such as the one presented here, will help improve understanding of in-situ sea ice properties and behavior, which ultimately supports the development of new ice-structure interaction models, which directly benefits oil and gas, shipping, renewable energy, and public works projects in ice prone Arctic and Sub-Arctic regions.
For oil and gas projects offshore Newfoundland, Canada, subsea structures are generally placed in excavated drill centres which lower the equipment below the natural mudline, protecting the equipment from damage due to iceberg impact. This paper introduces a concept of protecting this equipment by utilizing a concrete structure affixed to the seabed using hammer driven piles.
Iceberg loads have been assessed utilizing a Monte Carlo iceberg contact model and a modified version of the Iceberg Load Software (ILS) developed for regions offshore eastern Canada. The Subsea Iceberg Protection Structure (SIPS) was designed using post-tensioned concrete construction. Preliminary concrete design in addition to pile capacity design is performed utilizing FE analysis. Using a hammer driven piled system, the maximum lateral resistance capacity can be determined in addition to the maximum impact energy absorption.
The internal Subsea Production System (SPS) system has been designed to specifically fit inside the SIPS while maintaining full ROV access for operation, maintenance and future well intervention.
The SIPS was designed as an L1 structure in accordance with ISO 19906. This includes impact from free floating and gouging icebergs. The design load for this impact event was calculated based on energy absorbed through ice crushing. The deformation and global movement of the SIPS was not considered as part of the energy absorption mechanism. The maximum ice crushing design load on the SIPS was determined for four locations on the Grand Banks offshore Eastern Canada.
In addition to the structural design of the SIPS, the piling system was analysed to determine the maximum capacity. The total lateral resistance was determined using a combination of a continuum model and a structural beam model (P-y method). The global movement was less than the maximum allowable deformation of the structure. The structure is therefore considered fit for purpose.
The projected construction and installation cost of this structure shows the potential for reduced costs compared to an excavated drill centre, thereby increasing the feasibility of potential tie-backs.
Using updated knowledge regarding iceberg size and geometry, areal density and ice strength, the analysis and design presented in this paper suggests that it may be more economical to install protection structures rather than dredge excavated drill centres, for marginal fields. In addition, the advancement of the internal SPS system is such that the equipment footprint is compact, requiring limited space within the SIPS.
This paper provides the necessary information to show that installing a structure to protect subsea equipment is technically achievable.
Due to their potential instabilities, deploying personnel onto icebergs to make direct in-situ measurement is hazardous. The preliminary results from an investigation into the usage of Unmanned Aerial Vehicles (UAV) for surveying and monitoring icebergs are presented. The project had four objectives: (i) acquisition of imagery for the generation of iceberg topside reconstructions using photogrammetry; (ii) development of a GPS tracking device and a deployment mechanism to place it onto an iceberg; (iii) development of a motion sensor to record the motion of an iceberg and a deployment mechanism to deliver it onto an iceberg; and (iv) iceberg draft measurements from a UAV-mounted ice penetrating radar.
The project has used both commercially available and custom-built UAVs. The sensor packages (cameras, tracking devices, accelerometers and ground penetrating radar) were commercial products that have been modified for this study and, when required, mountings and delivery mechanisms have been designed and manufactured to integrate the system together.
Fieldwork was performed during the 2017 iceberg season in a near-shore environment (Bonavista, Newfoundland and Labrador, Canada) aboard a survey vessel and, in 2018, from an operational supply vessel offshore Newfoundland and Labrador. The field campaigns were conducted in parallel with an iceberg profiling system that uses an integrated multibeam sonar and LiDAR system to generate composite (topside and subsurface) iceberg reconstructions. These reconstructions can be compared with the results obtained from the photogrammetry and the radar survey.
During the 2017 program, iceberg imagery for photogrammetry was acquired and GPS tracking devices were deployed onto icebergs and sea-ice. The longest iceberg track obtained was 21 days. For the 2018 campaign, further photogrammetric data was collected and ground penetrating radar surveys of icebergs were performed. The photogrammetry topside reconstructions and the draft estimates from the ground penetrating radar produced results comparable to measurements from the iceberg profiling system.
This project has explored the capability of UAVs to deliver sensor packages onto icebergs, and to take aerial measurements over and around them. They are an emerging technology that, although challenging to work with in the harsh North Atlantic environment, have proved useful.
During July 2018, an expedition was carried out offshore northern Newfoundland to central Labrador to profile, track, and forecast the drift of icebergs. One of the central goals of the drift modelling work was to test potential improvements in iceberg drift forecast accuracy up to 24 hours when measured iceberg profiles are used as opposed to estimated iceberg draft and mass. During the expedition, 14 icebergs were profiled using a rapid iceberg profiling system which uses a multibeam for the underwater portion of the iceberg and a LiDAR for the freeboard. The 14 icebergs were tracked on the vessel marine radar, and their drift was forecast using a physical model which time integrates the momentum balance of the forces acting on the iceberg. The iceberg profiles were three-dimensional point clouds which provided a highly accurate representation of the iceberg dimensions and shape, and from which a volume and mass could be readily calculated. The point cloud was projected into a two-dimensional plane from 16 perspective angles and averaged into a single projection of iceberg keel and freeboard against which the currents and winds were forced in the drift model, respectively. Average results for the forecast iceberg position versus observed at 24 hours show approximately a nearly 3 km or 18% improvement when iceberg profiles are incorporated into the drift model as opposed to using estimated iceberg draft, shape, and mass. The drift model will become part of an integrated ice profiling, forecasting, and management system for oil and gas exploration and drilling operations on the Grand Banks offshore Newfoundland.
Drifting icebergs can threaten navigation and marine operations and are prevalent in a number of regions that have active oil and gas exploration and development. Satellite synthetic aperture radar (SAR) is naturally applicable to map and monitor icebergs and sea ice due its ability to capture images day or night, as well as through cloud, fog and various wind conditions. There are several notable examples of its use to support operations, including Grand Banks, Barents Sea, offshore Greenland and Kara Sea.
New constellations of satellites and the increasing volume of satellite data becoming available present a new paradigm for ice surveillance, in terms of persistence, reliability and cost. To fully extract the value of the data from these constellations, automation and cloud-based processing must be implemented. This will allow more timely and efficient processing, lowering monitoring costs by at least an order of magnitude. The increase in data persistence and processing capability allows large regions to be monitored daily for ice incursions, thus increasing safety and efficiency during offshore operations in those regions.
The process of automating SAR-based iceberg surveillance involves creating a process flow that is robust and requires limited human intervention. The process flow involves land-masking, target detection, target discrimination and product dissemination. Land masking involves the removal of high-clutter land from the imagery to eliminate false detection from these locations. Target detection usually involves an adaptive threshold to separate true targets from the background ocean clutter. A constant false alarm rate (CFAR) is a standard technique used in radar image processing for this purpose. Target discrimination involves an examination of the distinct features of a target to determine if they match the features of icebergs, vessels or other ‘false alarms’ (e.g., marine wildlife, clutter). The final stage is the production of an output surveillance product, which can be a standard iceberg chart (e.g., MANICE) or something that can be ingested into a GIS system (e.g., ESRI shapefile, Google KML).
The target discrimination phase is one of the most important phases because it provides feedback to operations about the presence of targets of interest (icebergs and vessels). The authors have used computer vision techniques successfully to train target classifiers. Standard techniques usually result in classifier accuracies of between 85%-95%, depending on the resolution of the SAR (higher resolutions produce more accurate results) and the availability of multiple polarizations. To see if new machine learning techniques could be applied to increase classifier accuracy, a dataset of 5000 ship and iceberg targets were extracted from Sentinel-1 multi-channel data (HH,HV). The images were collected in several regions (Greenland, Grand Banks, and Strait of Gibraltar). Validation either came by way of supporting information from the offshore operations, or was inferred by location. An online machine learning competition was hosted by Kaggle, a company that conducts online competitions on behalf of their clients. The detection data were made available by Kaggle to the broad internet community. Kaggle has a loyal following of data scientists who regularly participate in Kaggle competitions. The competition was hosted over a three-month period; over 3300 teams participated in the competition. The competition produced an improved classifier over standard computer vision techniques; the top three competitors had 4-5 stage classifiers that increased classification accuracy by approximately 5%.
Up to present, the annual iceberg contact frequency for short subsea flowline systems designed for offshore Newfoundland and Labrador has been less than the target reliability level. For longer flowlines, iceberg contact rates will be higher and the consequence of such contacts must be considered. It is possible, for example, that the pipe gets pushed into the seabed with acceptable damage to the pipe and/or localized ice failure takes place. If it can be demonstrated that a pipe could survive some impacts, it might be possible to avoid costly protection strategies such as trenching or rock berms. This paper describes physical tests conducted as part of a preliminary investigation to assess the consequence of a free-floating iceberg interacting with a flowline placed on the seafloor. Two scenarios were considered in this testing program. The first focused on understanding the local iceberg failure processes and the second evaluated the transverse flowline motion when a free-floating keel snags a flexible pipe laid on the seabed.
Approaches are presented in this paper for estimating the global mooring loads and response of a semisubmersible drilling rig, as a result of pack ice loading. The focus is on loading events from pack ice conditions relevant to the Grand Banks, where the pack ice typically consists of small floes and limited concentrations. The current practice for semi-submersible drilling operations on the Grand Banks is to avoid contact with pack ice by disconnecting and moving off-station in the event of an ice incursion. From a global loads perspective this may be unnecessary, given that the typical pack ice is of low severity and mooring loads may well be within acceptable limits. To be able to operate in pack ice while moored, operators need to demonstrate that the moored semisubmersible will have sufficient structural and mooring capacity to withstand the ice loads. Some existing semi-submersible hulls have ice strengthening in place as specified by a classification society, with associated allowable operating criteria in terms of ice conditions. These operating criteria are to ensure sufficient structural capacity given the ice conditions. No standardized approaches are currently available to quantify global pack ice loads and associated offsets for moored semi-submersibles, which are needed to assess the required mooring capacity. The objective of this paper is to address this gap and present approaches that can assist in specifying allowable operating criteria for station-keeping in pack ice.
Design of offshore structures for arctic and subarctic regions requires consideration of wave, wind and ice actions. If individual actions are not mutually exclusive, then combined actions also need consideration. ISO 19906 recommends that, when possible, extreme level combined actions should be determined based on the joint probability distribution of the actions. As an alternative, ISO 19906 provides a framework where a user can determine principal and companion extreme actions independently, and sum these with calibrated combination factors applied. While the combination factors in ISO 19906 were calibrated over a range of conditions and platforms, site-specific information is not taken into account when applying the method. In this paper, a procedure is presented for determining extreme level combined actions for sea ice and waves based on site-specific sea ice and wave information, accounting for the joint probability distribution of the actions. The procedure is demonstrated for an example fixed structure on the Grand Banks off Canada's east coast. The results are compared with extreme actions determined using the ISO 19906 combination factors.
The current practice for protecting wellheads and associated subsea facilities from icebergs on the Grand Banks is an Excavated Drill Centre (EDC), which is simply an excavation in the seabed in which wellheads and associated facilities are placed. Free-floating icebergs simply drift over an EDC, with the exception of those that roll as they pass over an EDC and increase draft sufficiently to enter. The risk from gouging icebergs entering an EDC is a function of the clearance between the surrounding undisturbed seabed and the top of the facilities in the EDC, and the distribution of gouging iceberg keel penetration depths. A field program conducted in Bonavista Bay in 2015 was used to estimate iceberg rolling rates, and an analysis of high resolution iceberg profile data collected in 2012 was used to determine the associated distribution of iceberg draft changes that occur due to rolling, and thus the rate at which iceberg keels penetrate an EDC due to rolling events. Modeled iceberg grounding rates and iceberg scour data from the Jeanne d'Arc were used to estimate the rate at which gouging icebergs enter EDCs. Iceberg gouge data from the Jeanne d'Arc and a dynamic time-step iceberg simulation using the 2012 iceberg profile data were used to determine the impact rate for facilities in the EDC as a function of the distance between the midline and the top of the facilities (clearance). The analysis addresses some of the conservatisms in the current approach, allowing for reduced EDC excavation depths.