A merchant vessel is a ship that transports cargo and passengers.
Most countries of the world operate fleets of merchant ships. However, due to the high costs of operations, today these fleets are in many cases sailing under the flags of nations that specialize in providing manpower and services at favourable terms. Such flags are known as 'flags of convenience'. Currently, Liberia and Panama are particularly favoured. Ownership of the vessels can be by any country, however.....WikipediA

A nation's shipping fleet (merchant navy, merchant marine, merchant fleet) consists of the ships operated by civilian crews to transport passengers or cargo. Professionals are merchant seaman, merchant sailor, and merchant mariner, or simply seaman, sailor, or mariner. The terms "seaman" or "sailor" may refer to a member of a country's navy.
According to the 2005 CIA World Factbook, the world total number of merchant ships of 1,000 Gross Register Tons or over was 30,936. Statistics for individual countries are available at the List of merchant marine capacity by country......WikipediA

Year 2007, the global offshore petroleum industry marks the 60th anniversary of completion of the first commercial offshore oil well drilled by a 'mobile' rig out of sight of land.
Drilled in October 1947, few at the time acknowledged the unique aspects of the well - drilled in 14 ft of water in the open Gulf of Mexico'S Ship Shoal Area off southeastern Louisiana. Later, however, the occasion was declared an important event in the history of the petroleum industry - one that created a whole new phase of finding and producing crude oil and natural gas.

Working offshore is different from working in an office onshore - The long working day (12 hours) 7 days a week, for 2, 4, 6 weeks at a time and the harsh weather conditions, especially in the North Sea, the remoteness, and the reliance on helicopter travel do not suit everyone. Others find it a challenging but refreshing environment, quite different from the nine-to-five routine and the rush-hour commute.
A lot depends on the installation. Whether it is a drilling rig, a production platform, jack up rig, or an FPSO (Floating Production Storage and Offloading vessel) a Dive Support Vessel, Supply/Standby Vessel, Floating Hotels, Construction Vessel, Tanker & etc.. they all differ. Some are large, some huge, others small. As a result, the facilities they all offer vary.

Offshore oil drilling is an oil extraction technique which allows oil companies to access deposits of oil buried under the ocean floor. Most typically, offshore oil drilling sites are situated over the continental shelf, although advancements in drilling technology have made platforms even further out to sea economically and physically feasible. Many people are opposed to offshore oil drilling, due to concerns about its impact on the environment, and the unaesthetic appearance of oil rigs off the coastline.
Many sections of the Earth's oceans have massive deposits of oil buried deep beneath their surface, and these oil deposits are extremely appealing to many oil companies. The first offshore oil drilling operation was established in 1938 in the Gulf of Mexico, and other producers quickly started to follow suit in other regions of the world. By the 1970s, many communities had enacted specific bans against offshore drilling, and the issue became a bone of contention in some areas.

An offshore platform, often referred to as an oil platform or (somewhat incorrectly) oil rig, is a large structure used to house workers and machinery needed to drill wells in the ocean bed, extract oil and/or natural gas, process the produced fluids, and ship them to shore. Depending on the circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island, or may float.
Most offshore platforms are located on the continental shelf, though with advances in technology and increasing crude oil prices, drilling and production in deeper waters has become both feasible and economically viable. A typical platform may have around thirty wellheads located on the platform and directional drilling allows reservoirs to be accessed at both different depths and at remote positions up to 5 miles (8 kilometers) from the platform.