The dismantling of offshore structures, called decommissioning, is a complex job in the oil and gas business, driven by strict environmental criteria. As the environments in which decommissioning is necessary become more challenging, freedom presents itself as a remedy to run this work as safely and economically as possible, particularly by accepting Remotely Operated Vehicles (ROVs) and divers from the equation.
We review three paths are researched in service of Autonomous Underwater Vehicles (AUVs) for decommissioning: navigation, attachment, and manipulation. To begin with, the Iterative Closest Point method (ICP) is investigated as a means to correct place ramble of an inertial navigation system, by using a previously obtained coarse map. Using actual sonar information from a current decommissioning site from the Gulf of Mexico, the algorithm can reconcile the internally dead-reckoned pose of the vehicle together with that calculated via ICP, to an accuracy of 7cm from a lO0k point sonar scan. Second, to attach lifting points into subsea crap without bracing onto it, one mechanism was designed to either drill and affix anchors in one penetration, from a car in flight.
A prototype was fabricated and its own functionality confirmed. Third, to encourage strong and stable robotic interactions utilizing an industry standard non-backdrivable manipulator, a control legislation has been developed to possess the vehicle-manipulator system interact with its environment, by mimicking an arrangement of springs, masses, and dashpots. This control law has been examined and analyzed in a simple experiment that attained a 90 percent reduction in settling time.
Background
The Gulf of Mexico has become a hotspot for international production in the oil and gas industry, with roughly 3,500 offshore facilities in the Gulf of Mexico [45], con tributing 17% of the total crude oil production for the United States [22]. Dozens of those offshore structures in the Gulf of Mexico, such as the one shown in Figure 1-1 achieve the ends of their operational lives every year [9], and decommissioning, i.e., dis mantling and eliminating, these structures in increasingly difficult environments has pushed the limits of Remotely Operated Vehicle (ROV) and diver capacities. De commissioning is often required for reasons such as: irreparable harm from natural events, degradation of structural integrity due to rust, or the absence of financial benefit to maintain production.
Natural Events are a significant contributor to a lot of the damage sustained by overseas centers in the Gulf of Mexico. Loop currents and hurricanes, two organic occurrences characteristic of the area, can impede, or even completely halt, an otherwise fully functioning overseas facility in a matter of days [20]. This loop current retreats in latitude in a frequency of each 4-16 months, and will drop a warm-core Eddy generally between 150-450km in diameter, which translates southeast at roughly 20km / day on average [11] across countless offshore facility sites. These eddies are especially warm and bring about some of the funniest currents found in the Gulf of Mexico even at depths of several hundred meters.
Hurricanes Bring some of the same acute subsea conditions to offshore structures as the Loop Current, as shown in Figure 1-3, but also contribute wind conditions so hazardous that centers inside their paths must often be evacuated. A hurricane is a tropical cyclone which has a maximum sustained surface wind of at least 74 mph.
U.S. refining capability; 22,000 of 33,000 miles of pipelines, and 3.050 of 4.000 platforms were exposed to hurrica ne conditions, causing the destruction of 115 platforms, leaving 52 others damaged [31]. In 2008, hurricanes Gustav and Ike resulted in the loss of sufficient platforms to halt 1.05% and 1.3percent of the daily production oil and natural gas in the Gulf of Mexico. The Gulf of Mexico sees around 100 tropical di turbances every calendar year, and on average this results in 10 tropical storms and 5 hurr icanes, bringing sustained winds up to 175 miles 133].
Hurricanes And the Loop Current play a massive part in designing criteria for offshore facilities in the Gulf of Mexico. The strength of those natural events influence mooring design and station-keeping, such as in determining the number and size of lines operations. The Loop Current is a particularly important element in the decision to lower a riser from the platform, because these strong currents induce vortex-induced vibrations/ moves (VIV and VIM). Which lead to fatigue damage in risers· additionally, excessive current load-induced curvature can impair drilling entirely [20].
Such ecological dangers put offshore structures in a higher risk for damages that often require Intervention or perhaps decommissioning. A lot of the choice rests upon riser and structural integrity, because risers play a vital role in the petroleum production process, but are also among the very risky assets associated with the platform. They are subject to fatigue loads with no assurance of redundancy in design. Which means that any failure could be not just devastating, but also extremely expensive concerning surroundings al remediat ion, bodily repair, and downtim e in production [10]. As previously mention ed, risers are often subjected to vortex-induced vibrations because of exte rnal dist urbances brought on by loop curr ents and hurricanes, which may shut down production for th duration of this natural event. Afterwa rd, the riser must be completely scrutinized, and repa ired if needed.
Decommissioning Outline
Decommissioning Isn’t only necessitated by natural events; it’s an Unavoidable end to most structures due to either the finite production timeline of a well, or even a construction not passing certification to create past its planned operational life. A detailed procedure exist s through which decommissioning is executed. In accordance with Business (Chevron EMC), decommissioning includes ten specific phases :
- Planning and Engineering – CAD document and drawing hunts, site visits for photo verification, underwater inspection, pipeline research
- Out of riser and permanent sealing of the wellhead
- Of conductors from the water that were utilized to give subsea power
- Of topsides to be removed from the remaining part of the structure
- Subsea Preparation – Inspection of coats To check for structural integrity to be lifted out of their water, determination of severing points, justification for possible reefing
- – Flushing of pipelines, left in position if cov ered, but must be raised out of water if unburied or uncovered
- Deck Removal – Heavy Lift vessel is utilized to remove deck from the rest of the structure
- Trunnions are connected and used to lift jacket from water
- Disposal – Major pieces are taken to world websites or scrapyards
- Site Clearance: Trolling boats and ROVs are used to
Out of these ten phases Of decommissioning, specialists in Chevron EMC possess explicitly stressed the importance of this very first stage of the decommissioning process engineering and planning – as it Determines the proper personnel, equipment, time, And money required to successfully perform the subsequent steps [13]. Thorough underwater inspection is vital, given that lots of platforms are bought from other companies without technology drawings. Even for structures with adequate instruction, decades of severe environmental conditions could change the subsea landscape in unexpected manners.
Decommissioning today Is both expensive and life threatening. The cost of ROV Missions Can cost on the order of hundreds of thousands of dollars every day, and if this speed is maintained over the course of a typical decommissioning timeframe of several months, the costs become rather exorbitant [13]. In addition, by the essence of the work, there is no guarantee of having an ROY-friendly work surroundings, as unforeseen obstructions could potentially sever the mind. What’s more, near death experiences are not uncommon for commercial divers [19]. Severed oxygen umbilical cords [40] and also a high pressure water jet accidents [41] are just a few several ways a regular business dive can fail. Sometimes, such as arc-oxygen flashlight explosions [2], these accidents can be fatal.
As Decommissioning surroundings Become more severe over time, the industry is beginning to turn more towards autonomy to conduct these projects as Safely and economically As possible, namely by eliminating ROVs and divers In the process. On one Hand, AUVs must keep The robust features Of their ROV counterparts. On the other hand, They Need to also Demonstrate their ability to sense and control Their surroundings at least as well as the divers which are Hoped to be substituted. Chapter 2 discusses a way to correct AUV navigation ramble using real time 3D Sonar scans that achieves an accuracy of 7cm from a l00k-point sonar scan. . Chapter 3 introduces a mechanical Design to attach lifting points into subsea scrap metal in a single penetration, where a prototype was fabricated And its performance verified. Chapter 4 investigates a Passive control strategy for free-floating contact Tasks using a non-backdrivable manipulator which Settling time by 24% and 91%, respectively.
