An offshore well structure is shown in Figure 3-92
1. Marine riser
This is a type of structure that is different from those used on land, and it can isolate the drilling (drill string) columns or oil tube from strong corrosive seawater and can also be used to guide the drilling tool, production string, and other tools, and separate drilling fluid with seawater when circulating mud. They are generally made of 762 mm (30 in) diameter casing; the riser is at a depth of a 70∼100 m mud line at sea generally.