There are three issues here.
First, when you have a clause that describes some one or something in a sentence, it may or may not be set off with commas. That is -- and let me simplify the sentence to make my point -- you can say, "The pirates who attacked the tanker had help", or you could say, "The pirates, who attacked the tanker, had help." The difference is that with commas it is called a "non-restrictive clause", meaning that it tells us something about the noun being modified, but does not narrow its meaning. Without commas, it is a "restrictive clause", meaning that these words are necessary to understand who or what is being referred to. In this example, if we say, "The pirates who attacked the tanker ...", "who attacked the tanker" tells us which pirates we mean. If we said, "The pirates, who attacked the tanker, ..." then we would have to have already established in context what pirates we are talking about, and now we are just telling you something additional about them.
For example, consider: "A group of pirates set out in a boat while onlookers watched from the beach. Then men who attacked the tanker wore red shirts." In this case we need to say "who attacked the tanker" to distinguish the pirates from the onlookers. Otherwise it would not be clear whether it was the pirates or the onlookers who wore red shirts. On the other hand: "The pirates boarded a boat and set out to sea. They were dangerous killers who had committed many acts of violence. The men, who attacked a tanker last month, ..." Now we are only talking about one group of men, so we don't need to identify them. saying they attacked a tanker is just supplying additional information.
Second, in very long sentences like this writers sometimes add a comma more to give the reader a chance to catch his breath than for any truly grammatical function. I think that's the case in your example. I'd say that's wrong as the comma does not serve a defined purpose and introduces an essentially random break into the sentence. Others would disagree.
Third, using a semi-colon here would be flatly wrong. A semi-colon can be used in two ways:
(a) To combine two independent clauses. That is, to basically cram two sentences together without using a conjunction. Example: "I ran to the door. Mary was there." Two sentences. "I ran to the door and Mary was there." Two sentences combined into one with a conjunction. "I ran to the door; Mary was there." Two sentences combined into one with a semi-colon.
(b) When giving a list where elements within the list have sub-elements, especially if those sub-elements are separated by commas. Say you were listing the colors of flags you saw. One flag is red, white, and blue. Another is solid green. A third is black, white, and read. If you wrote, "The flags were red, white, and blue, green and black, white and red", a reader could easily be confused where one set of colors begins and another ends. Was there a flag that was green and black and another that was white and red? Or maybe one was blue, green, and black. But if you write, "The flags were red, white, and blue; green; and black, white, and red," then the meaning should be clear.
