When exploratory wells find commercially viable natural gas or petroleum deposits, it is economical to build a permanent platform from which well completion, extraction, and production can occur. These large, permanent platforms are extremely expensive, however, and generally require large expected hydrocarbon deposits to be economical to construct. Some of the largest offshore platforms are located in the North Sea, where because of almost constant inclement weather, structures able to withstand high winds and large waves are necessary. A typical permanent platform in the North Sea must be able to withstand wind speeds of over 90 knots, and waves over 60 feet high. Correspondingly, these platforms are among the largest structures built by man. There are a number of different types of permanent offshore platforms, each useful for a particular depth range. Because of their size, most permanent offshore rigs are constructed near land, in pieces. As the components of the rig are completed, they are taken out to the drilling location. Sometimes construction or assembly can even take place as the rig is being transported to its intended destination.