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Introduction
Fossil fuels, especially natural gas and oil are the most widely used forms of energy across the world. Inkpen and Moffett (2011) note that for over 150 years in which oil and gas have been extracted in different parts of the world, the periods between the year 2000 to this year (2016) has seen some of the highest production levels. The authors further acknowledge that the increasing demand has also compelled oil and gas industries to advance the extraction of this important resource in areas previously considered expensive and difficult: the offshore fields. Randolph and Gourvenec (2011) estimate that there are about 7000 or more offshore platforms operating across the world to date. However, they also admit that the effective establishment of an offshore platform to extract oil at 2000meters below the sea requires effective geotechnical engineering for safety, environmental conservation and efficiency. This study seeks to investigate some of the requirements to be considered for the construction of a pipeline system to transport oil and gas extract to an onshore storage facility with three holding tanks, waiting further processing of the oil. The study focuses on the Scottish west coast which is the fastest growing oilfield due to the newly discovered potential of the location (Oil & Gas in Europe, 2016).
Foundations of Offshore Geotechnical Engineering
Geological studies on the distribution of oilfields note that there are 932 oil and gas fields classified as gigantic due to their naturally high resource potential accumulating to 500million barrels in total (Inkpen & Moffett, 2011). Inasmuch as some of these giant oilfields have begun exhausting their resources, new are being discovered with majority of these fields being located in the offshore areas (Randolph & Gourvenec, 2011). Scotland’s coastline is one of these zones whose newest oilfields are being discovered offshore and thus the need for effective infrastructure to enhance successful extraction in the region. According to Cheon and Gilbert (2014) and Dean, effective construction of oilfield related structures in water basins requires proper geotechnical engineering in order to ensure the structures are able to support operations as required, they are safe to both humans and sea life, they can be sustainable for a long time and can enhance any expansion needs within the offshore oilfields. In this regard, careful analyses have to be done concerning the nature of soil and bedrock, where this soil has to be tested for any carbonates or the bedrock analyzed for strength abilities to confirm whether it can be withstand the weight of the standing structures (Randolph & Gourvenec, 2011). Kenny (2014) adds that some areas are more prone to geoharzards (geographical processes that can impact the earth’s surface) and therefore a study has to be conducted in order to avoid such zone or erect structures that can guarantee effective use of the rig structures even in these zones.
Furthermore, there are numerous regional and international requirements for the construction of offshore and onshore oil and gas extraction infrastructure. The International Regulators’ Forum and the International Society for Soil Mechanics and Geotechnical Engineering are the main regulators at the international level. These regulators not only set the standards through the provision of industrial rules, they also conduct research on behalf of member states towards supporting the states in identifying effective zones and infrastructure for the offshore and infield operations (Dean). In Europe, the Eurocode is one of the most important regulatory codes used in the construction of geotechnical infrastructure (Cobb, 2014). Since Scotland is part of the European Union, it also uses this code and other international regulations in its oil and gas industry and therefore this study will apply the code towards explain some of the important factors to be considered in the construction of the pipeline from the offshore rig to the mainland where the oil will be stored in holding tanks awaiting further processing. Evidently, this section introduces the important factors to be considered in the investigation of the requirements for the construction of the pipeline. The approach will be to compare the company requirements to the industrial standards while also applying reviewing literature on soil structures of Scotland’s west coastline.
Investigation and Design Parameters
The plan in this project is to construct a pipeline system for resource (oil) transportation from the production rig located 15 miles offshore to the four holding tanks onshore. The holding tanks are 10meters in diameter and impose a total weight of 5000KN each. For effective handling of the pipeline, a jacket platform has to be constructed in 20meters of water close to the production rig. In an offshore rig, a jacket is a steel structure with a tubular design used to support the deck assembly and hold it in a fixed position above the water level (Nizamani, 2015). Although offshore platforms can either be fixed vertically on the seabed or allowed to float then supported by tension belts, jacket platforms are normally fixed on piled foundations (Hosseinlou & Mojtahedi, 2016). The average water depth for the jacket platform is 400meters which according to Nizamani (2015) is relatively low compared to other platform designs like the compliant tower that goes to a depth of 600meters. Nizamanai (2015) also provides some of the design characteristics of the jacket platforms and therefore notes that, the steel pipes used in jacket construction have to be thick and have a diameter of 2meter. He adds that the pipes should be positioned at least 100meters below the seabed for better stability of the entire structure and an average height above sea level can be 15meters and over. In this case the preferred height above water is 20meters. The Block 11 in South Pars, Iran has two mixed use jacket platforms and thus a good illustration of what the jacket platform design and functions entail.
The design characteristics as discussed by Nizamani (2015) and Dean illustrate that the construction of jacket platforms requires extensive understanding of the geological factors and technological interactions between the design or materials and the environment within which it will exist. There are three types of surveys that guide the nature of the investigation when exploring the seabed or onshore geographical location for oil and gas infrastructural installation. To begin with, Randolph and Gourvenec (2011) note that the desk study has to be conducted in the initial and combined with other survey techniques as the investigation progresses. As the name suggests, this study involves the survey of available information from diverse reliable sources like academic journals and conference reports. Randolph and Gourvenec (2011) add that the study may last up to 6 months and important concepts studied in the period include: ocean floor depth using the sonar depth sounding (bathymetry), meteorological factors like wind and waves likely to affect the research while at sea and the operations upon initiating the project (metocean study), geological structure and composition of the seabed, obstacles and existing, potential or previously noted geohazards within the targeted region.
The ocean seafloors are occasionally filled with stationary objects and irregular formations that affect the construction of important offshore oil extraction infrastructure. Such objects may include: ship wrecks and ice gouges and therefore in order to determine the availability of such object and review the nature of variability in the location, geophysical surveys are normally conducted (White et al, 2011). Remotely operated vehicles are mostly used in this study but sonar devices can also be used especially for the depth statistics. After a successful geophysical analysis, the actual floor bed contents have to be tested in a laboratory for deeper understanding of the predetermined characteristics (Randolph & Gourvenec, 2011). This stage seems to query why the land formations are as illustrated by the sonar device, it also explores the history of the soil composition and even its strength and later guides future decisions. Based on the various content compositions, this final survey allows the surveyors to understand potential interactions when using different mechanical equipment in the location and thus referred to as the geotechnical survey. Randolph and Gourvenec (2011), note that this survey helps to understand both physical factors of the seabed like the levels of salinity and the mechanical factors like the soil density. In order to successfully conduct this survey, the surveyors use direct field studies. This involves drilling and soil tests in situ (Inkpen & Moffett, 2011).
Conclusion
Offshore extraction of heat is rapidly increasing especially in the Scottish West Coast. This is causing rapid demand for offshore drilling services thus the need to effectively understand the dynamics of establishing an effective infrastructure. Geotechnical engineering services allow oil companies interested in offshore activities to effectively evaluate their target operational areas in order to guarantee efficient production in a safe and clean environment. Consequently, the three main surveys done through the geotechnical research are: desk study which involves the review of documented materials about the location, geophysical surveys that review the geographical features in relations to the ocean floor and the geotechnical survey that focuses on the relationship between the physical and mechanical features expected or noted within a location. Based on these investigations, the jacket platforms have to be placed at about 100meters or less within the seabed depending on the pile’s compressibility and tension. Cathodic plating should also be used to control the metal’s corrosion. Similarly, the pipeline should be able to apply cathodic plating for corrosion but other features like cement coating should be added to increase its weight so as to prevent buoyancy. The pipes are however under several threats like seabed mobility and therefore such areas have to be avoided. The holding tanks are greatly dependent on the seismic structure of the planned construction site and therefore the Eurocode 8 should be applied just like the Eurocodes 7 and 3 were used to guide the installation of the jacket and pipelines.