Grounding out explained
Within the first six days after Sam Taylor posted this image of the Maersk Connector on his LinkedIn page, the photo had been viewed more than 90,000 times and had received around 1,000 likes. Some people were impressed, others were just plain curious about the story behind the picture. There was even disbelief as one user went so far as to post the comment of ‘Fake picture!’
One thing is for sure, the photo is not fake. Mr Taylor, DeepOcean’s Technical Sales Manager, has not used Photoshop to attract more views to his LinkedIn page. He posted the photo to show the vessel at work on a recent project in the north-west of England.
Mr Taylor knows the Maersk Connector like the back of his hand. In his prior role of DeepOcean Senior Engineer, he was involved from the original design process, during construction at Damen Shipyards Galati in Romania, all the way to final delivery to owners Maersk Supply Service in February 2016. “This vessel has been part of my life for almost four years,” he says. On long-term charter to subsea services provider DeepOcean, the vessel’s first port of call was to install the 7,000-tonne carousel system.
Source: DeepOcean Group YouTube channel
The explanation
“The photo was taken during a shore pull – the connection of the offshore export cable to the land-based cable of the national grid – for an offshore wind farm that is being built off the UK’s west coast. We bring the vessel in at high tide using her thrusters and DP, using tugs to position her 7-point anchor system. The offshore cable comes in and is spliced at a Transition Joint Bay, which is normally around 500 metres to 1 kilometre behind the high water mark on the beach.”
“With this project we used excavators on the beach to bury the first 3 kilometres of cable. And from then out we used our cable plough which lays and buries the cable in one simultaneous operation. The vessel’s anchor system was a big development on this project – providing up to 150 tonnes of bollard pull to pull the plough.”
The matter of tidal range
So, the question remains, why is it necessary to beach the vessel? “It depends on the tidal range. In some parts of the world the tidal range is not large, so we can get in position close to the beach. Then we can float the cable to shore and pull the cable in. It’s all nice and simple.”
“But it’s more challenging where the tidal range is huge – this current project for instance. It means that, with a vessel with the very substantial cable carrying capacity like the Maersk Connector, at low tide, we would be sitting 15 kilometres away from the shore end. Of course, we can’t pull the cable in that far, so we have to get closer.”
One option to get the cable in to shore is to use a barge equipped with additional Dynamic Positioning and accommodation capabilities.
"You can put five or six kilometres of cable on board a barge, take it up on the beach at high tide, ground it, then lay away and join up with the offshore installation vessel in deeper water. This would involve splicing the cable in deeper waters."
Maximising the weather window
In negating the use of a barge, the Maersk Connector (which is based on the Damen Offshore Carrier 8500 vessel design) is bringing some valuable advantages to the table. “The aim of using the Maersk Connector here is not only to simplify installation, but also to avoid additional splices or joints in the cable. These increase the risks of potential weak spots and, therefore, the operational life of the cable.”
Importantly, removing the need for additional splices saves time as well as maintaining cable integrity. “One of the biggest areas of concern when doing an export cable installation is being able to find a big enough gap in the weather,” he adds. “You might need a two-week window and it’s very difficult to get that type of forecast. For example, an offshore splice or joint for this type of cable can take at least six days – so avoiding additional splices saves us time. Therefore, we need a smaller weather window, and we can perform operations in a greater range of time across the year.”
Ground preparation
Because grounding is not an activity that can be taken lightly, the Maersk Connector has a few special features up her sleeve to make sure the whole process goes as smoothly and safely as possible. “First of all, we carefully select and prepare the area where we are going to ground. This involves removing boulders or debris, for example. And, if necessary, we would flatten the seabed with excavators or dredgers.” A similar preparatory way of thinking has been used when considering protection of the hull: “Of course we have to protect the keel during the grounding process, and normal ship’s paint would suffer damage, so it is painted with icebreaker-class paint.”
Ballast control
“And then we have to be aware of vessel requirements while we are grounded. Under normal conditions we have sea chests which are open to the sea to cool the main engines. Of course you can’t do that when you’re out of the water. So we use ballast water to cool the main engines – first assessing what engine loads will be required and also the temperature of the sea water to determine how much ballast water we will need.”
The ballast system is also called into play when the vessel touches down on the seabed. Consisting of four 1,000m³ per hour pumps, DeepOcean calls this its quick ballast system. “When we are in position and the tide is starting to come out, we flood the ballast tanks to increase the draught of the vessel by half a metre in 30 minutes. In this way we land the vessel ourselves instead of going down with the tide and possibly bouncing. The reverse happens when we lift back up; as the tide comes in, we can empty the tanks just as quickly to float off in a controlled manner.”
"We look at it in terms of using an offshore installation vessel with the added capabilities of a shallow water vessel. We have the offshore performance, we keep a huge amount of carrying capacity, and we have proper DP2. But we can do the shallow water work too,"
Mr Taylor concludes.
