An operation to free a giant container ship stuck in the Suez Canal is continuing, with warnings it could take days or even weeks. The Ever Given, the length of four football pitches, is lying across the southern end of the canal preventing other ships from getting through one of the world's busiest waterways. Tug boats Up to nine tugs have been deployed to free the ship, according to the company which manages the running of the vessel, Bernhard Schulte Shipmanagement (BSM). The ship is 400 metres long, wedged diagonally across a canal not much more than 200 metres wide. Using cables or placing themselves directly alongside the stricken ship, the tug boats have been attempting to move it off sand banks on both sides of the canal. BSM says an attempt to refloat the ship on Thursday morning failed and that they would try again soon. As it's firmly grounded on both banks, all efforts so far have proved extremely difficult to shift it, says Sal Mercogliano, an expert in maritime history at Campbell University in the US. Dredging There is also an operation underway to dredge the canal around the ship. The Netherlands-based dredging company Boskalis is attempting this, clearing sand and mud from around the hull. The chief executive of Boskalis, Peter Berdowski, says "it's an enormous weight on the sand" that will take a combination of dredging, tugging and removing weight from the ship to free it. Maritime expert Sal Mercogliano says the Suez Canal is continually being dredged anyway to keep it passable. "Large machines stick down into the water and basically pull dirt up from the bottom, which you can then deposit onshore." The channel was expanded in 2015 to allow vessels the size of the Ever Given to pass through. Lightening the load The next stage in efforts to re-float the 200,000 tonne vessel would be to remove fuel and cargo. A ship the size of the Ever Given can carry as many as 20,000 twenty-foot containers. There are, however, dangers associated with doing this, as it could cause damage and even unbalance the ship. It would also be a delicate and time-consuming operation. "You would have to bring large floating cranes - but anything you do right now you would have to calculate the stability of the vessel, you have to determine how it would affect the stability," says Dr Mercogliano. "Worst case scenario is that she breaks in half because of [uneven] weight distributions." Also, trying to remove freight containers using floating cranes could be tricky given how high they are stacked up on the ship. Draining fuel from the ship's tanks would be an easier operation, but may well not reduce the weight sufficiently.