Increased measures to reduce volatile organic compounds (VOC) emissions during the loading operations for crude oil, both offshore and onshore, are being required by the Norwegian Pollution Control Authority (SFT) and international environmental agreements.
The Norwegian oil and gas industry accounts for approximately 39 per cent of national VOC emissions, of which approximately 90 per cent are believed to be attributable to offshore crude oil loading operations.
The current SFT emission requirements for crude oil loading operations require a minimum recovery efficiency of not less than 78 per cent of non-methane VOC (NMVOC). This particular recovery efficiency requirement is applicable to shuttle tankers operating in the Norwegian sector of the North Sea.
The laws and regulations for onshore sites require that facilities apply for independent concessions under which to operate.
Recovery efficiencies, rather than mass based emission rates, are commonly used for defining crude oil emission requirements because of the very wide range and complexities of crude oil compositions. As the reader may know, crude oils from different areas in the world and indeed within the various sectors of oil producing areas can vary significantly, some high in light VOC compounds others high in heavy compounds and so on.
Because of the wide range of compositions, it has now become an accepted practice to design crude oil Vapour Recovery Units (VRU’s) on a recovery efficiency basis.
StatoilHydro – Mongstad
One case, by way of example, is the recently commissioned crude oil vapour recovery system at StatoilHydro’s Mongstad facility. The VRU is required to recover VOC from all the different crude oil types, with wide ranging compositions and vapour pressures.
The vapour recovery system has been designed to recover vapours resulting from each of the crude oils loaded at the terminal. However, for each of the crude oils loaded, a separate recovery efficiency applies, reflecting the varying composition. In June 2006, Aker Solutions signed a contract for the design and delivery of a VRU to the Mongstad terminal; the contract was completed in June 2008. The contract party for the VRU in Mongstad is Aker Cool Sorption A/S in Denmark, a fully owned
subsidiary of Aker Solutions.
The main benefits of using these systems are to reduce the discharge of environmentally hazardous substances and minimise what would be a significant loss of a potentially valuable energy resource represented by the evaporation of the oil. In addition, significant safety and operator health risks in the distribution and handling of oil vapour are gained.
The Mongstad terminal, located near Bergen in Western Norway, is jointly owned by StatoilHydro and Petoro, the state company responsible for commercial aspects of the government’s involvement in petroleum activities on the Norwegian continental shelf. Crude oil is delivered to the terminal by shuttle tanker and pipeline. Mongstad provides an intermediate storage facility for more than one third of the crude oil produced by StatoilHydro.
At any one time the terminal has a capacity to store 9.4mbbls (1.5million-m3) of crude oil in up to six rock caverns. Exports to customers in North America, Asia and Europe are loaded from one of two jetties, each capable of accommodating tankers up to 380,000dwt. In an average year the terminal loads up to 450 tankers.
The Mongstad VRU’s peak capacity is a vapour rate of 36,000 m3/hr (vapour rate) emissions that were previously released into the air, reflecting positively on the environment.