Environment and Environmental Load of Offshore Oil Engineering
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Transcript of Environment and Environmental Load of Offshore Oil Engineering
The Environment and Environmental Load of Offshore Oil Engineering
Abstract
The objective environment of Offshore Oil Engineering differs significantly. It is influenced not only by the climate (similar to the land), but also the severe environmental conditions of the sea, for instance, waves, currents, sea ice, and typhoon, monsoon, etc. These marine environment and environmental loads may cause enormous damages, and even affect the normal operation of offshore oil engineering and the production in offshore oil and gas fields. Based on this topic of environment and environmental load of offshore oil engineering, Chapter 01 is intended to introduce the combinations of wind and wind load; tide, current and current force; wave and wave force; sea ice and ice load; and earthquake force and the combination of it with other wave forces. And in this chapter, it also discusses the theory and method of calculation of the marine environmental load acting on the offshore oil engineering structures.
Keywords
Environment and environmental load; Current force; Random wave spectrum; Wave theory; Stokes wave theory; Ice load; Ice-induced vibration; Tsunami; Earthquake force
Although collectively referred to as the “sea,” seawater with depth less than ∼ 2000–3000 meters is commonly known as the sea, while that with a depth more than 3000 meters is known as the ocean. Ocean is the core of the sea, and accounts for approximately 89% of the total area of seawater. The total volume of seawater nowadays is about 13.7 × 108 m3, making up around 97% of the total water on the Earth, and the total area of seawater across the world occupies 78.9% of the Earth's surface, the area of which is 5.11 × 108 km2. The ocean is rich in natural resources, among which oil and natural gas are some of the most important strategic resources. The world’s offshore oil production began in the 1940s. In the 1960s, offshore oil production reached 1 million barrels per day. In 2005, it reached 25 million barrels per day (almost 1.25 billion tons per year), which accounts for one-third of world crude oil production. At present, more than 2000 offshore oil and gas fields are found all over the world. According to the statistics by the Cambridge Energy Research Associates, offshore oil production accounted for 33% of global oil production in 2009, and this proportion will rise to 35% in 2020. Offshore gas production accounted for 31% of global gas production in 2009, and this is estimated to rise to 41% in 2020.
China is a large country in terms of both land and sea. China has a coastline of more than 18,000 km, and the island coastline of Hainan, Taiwan, etc. reaches 2,300 km. According to the United Nations Convention on the Law of the Sea, the sea area under China’s jurisdiction is around 488 × 104 km2, and the amount of oil and gas reserves reaches about (40 ∼ 50) ×108 t. The continental shelf is the extended perimeter of each continent. After preliminary investigation, more than 300 sedimentary basins available for exploration (covering around 450 × 104 km2) have been found in the immense area of the continental shelves of the China Bohai Sea, China Yellow China Sea, and East China Sea, of which the marine sedimentary layers occupy an area of about 250 × 104 km2. About 10 large-scale offshore oil and gas basins have been found in China, namely, Bohai Basin, North Yellow Sea
Basin, South Yellow Sea Basin, East China Sea Basin, West Taiwan Basin, Pearl River Mouth Basin of South China Sea, the southeast part of the basin near the Ryukyu Islands, the Beibu Gulf Basin, Yinggehai Basin, and Taiwan Shoal Basin. According to the third petroleum resource assessment results in 2008, China’s offshore oil resources measure 24.6 billion tons, which accounts for 30% of the total oil resources in China. And the offshore natural gas resources are 16 trillion cube meters, which accounts for 30% of the total gas in China. In the above-mentioned offshore oil and gas resources in China, 70% of them are in the deep-sea area.Though only since the late 1950s has China begun shifting oil and gas research and exploration from the land to the sea, the country has seen rapid progress in this process. By the end of 2002, oil and gas investigations in all seas and waters and the evaluation of oil and gas resources in the major basinshave been completed. Now, 235 104 km seismic lines have been acquired (with the three-dimensional lines being 153 104 km long, and the two-dimensional lines 82 104 km); 817 wildcat and appraisal wells have been completed (with 513 wildcat wells and 304 appraisal wells); 207 geological structures containing oil and gas have been discovered; and 52 oil and gas fields have been found. As of the end of 2008, there are more than 80 oil and gas fields in China’s sea area. In 2008, the annual oil production was 189 million tons and the annual nature gas production was 74.49 billion cubic meters. To the end of 2008, cumulative oil production is 5.02 billion tons and cumulative nature gas production is 940 billion cubic meters. It is predicted that by 2020, China's offshore oil production will reach 37 to 41 million tons.
Offshore oil engineering consists of a process that starts with offshore exploration for oil and gas, including exploration, development, and production, and continues through to the transport of the oil and gas to land. The environment of offshore oil engineering can vary greatly. It is influenced not only by the climate (similar to the land), but also the severe environmental conditions of the sea, for instance, waves, currents, sea ice, typhoons, monsoons, etc.Since offshore oil engineering is carried out in the harsh conditions of the marine environment, engineering structures, such as drilling platforms, oil extraction platforms, floating production platforms, production storage and offloading vessels, single point mooring devices, and submarine pipelines and cables, have to withstand the marine environment load coming from the wind, waves, currents, sea ice, and even earthquakes, tsunamis, etc., either in its construction or in use. These marine environmental loads may cause enormous damage, and even affect the normal operation of offshore oil engineering and the production in offshore oil and gas fields. For example, the wind load that hurricanes on the sea exert on offshore platforms can severely affect the security of the platforms. On November 25, 1979, China's “Bohai Sea II” drilling platform was capsized due to the wind load during towage for well relocation. Similarly, on December 25, 1983, the “Java Sea” drillship rented by the U.S. Atlantic Richfield Company was turned over in the South China Sea during its operation because of a typhoon. In the Gulf of Mexico, the hurricane in October 1964 destroyed more than 30 platforms and the hurricane in October 1967 led to many cases of platform collapse. Moreover, wave loads can have a destructive impact on offshore platforms. In August 1980, four drilling platforms in the Mexico Gulf were destroyed by great tidal waves; in November 1989, the American drillship “Seacrest” was also overturned by huge waves. By 1989, statistics showed that more than 50 offshore drilling platforms and ships worldwide have been engulfed by the sea. Based on past
experience, the marine environment conditions of where the platform settles in has an important impact on the fatigue of the structure. For instance, the wind and waves are generally stable in the Gulf of Mexico, but hurricane can occur during a certain part of the season, so the low cycle fatigue should be taken into consideration. And in the North Sea, the marine environment condition is difficult, so the high cycle fatigue should be considered. In 1966, when periodically checking the four-legged fixed platform that settled in the North Sea after 6 years, it was 600 mm lower than the low-tide level waterline, and the pole was entirely out of the chord, which is caused by high-cycle, low-stress fatigue crack in the section observed. At the same time, there was a large amount of corrosion products attached to the pole, which made the out diameter of the pole change from 300 mm to 700 mm (with a wall thickness of 12 mm). Consequently, the resistance force was greatly increased. Also, the other attachments on the platform increased the resistance. In addition, the destructive force of the sea ice load cannot be ignored. The China Bohai Sea is an ice zone, and its ice-frozen period lasts approximately three months in winter. During the wintertime, the minimum temperature is about –22 °C, and the maximum wind speed is about 32 m/s. approximately every 10 years, there will be a normal/severe ice regime in the China Bohai Sea. Its sea ice thickness is normally ∼20–40 cm, and sometimes it can be ∼40–60 cm, and even up to 80 cm.In the China Bohai Sea, there have been profound lessons learned from the ice load acting on China’s offshore oil engineering structures. Take the old “No.2” oil platform as an example. This platform is composed of a life platform, an equipment platform, and a production platform. In 1969, the China Bohai Sea was severely frozen. In February of that year, the life platform was the first to be overturned by the ice, and then the equipment and production platforms had both been turned over by March 8th. In 1974 a beacon tower in the China Bohai Sea was torn down by ice. Also, in 1979, one of the legs of a self-elevating drilling platform was broken by the ice load, resulting in a serious accident. All the above-mentioned incidents illustrate the significant influence of ice load on engineering structures.In conclusion, obviously it is of great necessity and importance to study the environment and the environmental load of offshore oil engineering. Therefore, based on this topic, the first chapter is intended to introduce the combinations of wind and wind load; tide, current, and current force; wave and wave force; sea ice and ice load; and earthquake force and the combination of it with other wave forces.