For offshore rig and vessel operators, time spent out of service is money lost. To alleviate some of the pocketbook pain, Ensco found a way to streamline the mandatory hull inspection process, trimming it from weeks of downtime spent in dry dock to very little downtime and no dry dock.
Hull inspections are critical to ensuring a vessel’s safe and continued operation. All classed vessels are required to have their hull inspected a minimum of two times every five years, with no more than 36 months between the inspections. This means that roughly every two to three years the vessel needs to go into dry dock.
An alternative is to request permission from the society the vessel is classed with to perform an underwater inspection in lieu of dry docking (UWILD). UWILDs are performed by divers while at sea, eliminating the need for dry docking while limiting the amount of time a vessel spends out of service.
Earlier this year, Ensco performed its first UWILD using not a diver but a small submersible ROV video camera called the VideoRay Pro 4.
The VideoRay Pro 4 system was deployed for an external hull inspection of ENSCO DS-1 drillship offshore Angola, according to Sachin Mehra, vice president of asset management for Ensco.
“We first learned of this technology about two years ago. We carried out a series of tests on some of our rigs with the VideoRay Company and with class societies, participated in a joint industry project and performed a technology trial to make sure that it would be suitable and accepted by the class societies,” he said.
“It took us almost a year to validate the technology and then communicate to the fleet that this technology was available. The rigs started purchasing the equipment, and in May we had our first successful inspection. It was more than a year and half from the time we conceived the idea that we achieved success.”
While this inspection was just for the external hull, Mehra is certain the system has more to offer.
“We believe that we have great potential to replicate our success using the ROV to inspect inside the hull because of the large size of the drillships’ ballast tanks, but we need to work with class societies to make sure that the tank inspections are credited to us when we perform the inspection with the mini-ROV,” Mehra said.
For Ensco, the benefit of using the system is two-fold: simplified deployment without a diver means safer inspections and a reduced need for the rig to log out-of-service periods. Deploying the VideoRay is as easy as lowering the lightweight (about 14-lb) submersible system into the water and minding the umbilical while a trained operator runs the system.
Safety precautions for the mini-ROV include shutting down the thruster closest to where the ROV is to prevent it from being sucked into the thruster. The other is keeping an eye on the ROV’s umbilical during operation to ensure that it is not damaged or destroyed.
“When we perform UWILDs the conventional way, we use divers. There is an inherent risk with divers because you need to mobilize a large spread of people and diving equipment, which is a challenge for remote locations,” he said. “Then there are safety concerns. For example, divers cannot be in the water while thrusters are running, which means some of the thrusters must be shut down, thus increasing the risk to station-keeping.
“Depending on the day rate of the rig, this could become quite costly. By using the mini-ROV, the logistics is simplified because it is a very small piece of equipment, and there is no need for large expenses to mobilize it to the rig. It is simpler to get one person to the rig to operate the ROV than it is to mobilize an entire diving spread to the rig,” he added.
“Secondly, we have been able to show that we can do this inspection without taking any out-of-service period, which is a huge benefit to Ensco because the rig keeps on working. We are able to provide undisrupted service to our clients while performing this inspection, which is probably the biggest gain.”
The mini-ROV provides the opportunity for surveyors to perform general and close visual inspections. It also is equipped with a thickness measurement probe that can perform thickness measurements of the hull plating. For internal tank inspections, the ROV again demonstrates the safety and cost savings benefits.
“For internal tank inspections, we avoid having to completely de-gas the tank, and no one has to work in a confined space,” he said. “Typically, when we do a tank inspection with a man entry, it takes us about one and a half days. With this ROV, you can do the inspection quite easily by simply filling up the tank and dropping the ROV inside.”
Ensco is in discussion with several companies to develop the technology necessary to inspect the welds.
“We still have some work to do on the technology for weld inspection,” he said. “The ROV technology does not have that capability at this particular moment, so if we continue doing the UWILDS with the current mini-ROV technology, we may have to perform limited diver’s work.
“As part of the inspection, we have to do nondestructive testing of the welds, and the current technology cannot do that. While we’ve had great success with the inspection of ENSCO DS-1, we know the limitations of this technology and realize that to replicate that success going forward, we need to develop new inspection technologies.”
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