Answer

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).

Requirements for the Jacket Platform

            Nizamani (2015) noted that the jacket platform has to be able to firmly support the platform at the top side (which has the rig and the utilities), support the tension of the riser while still remaining resistant to corrosion as it is directly in contact with the ocean water. Dean, added that the jacket should also be able to withstand metocean effects so as to guarantee sustainability of the operations. He later acknowledged that due to the weight of the platform, the weight due to the jackets steel assembly and forces caused by the metocean effects, the jacket’s load has to be well supported by the soil and therefore the recommended depth for maximum weight sustainability should be 100meters within the ocean floor. According to the Eurocode 7 (2008), the depth is mostly determined by two soil charactersitics depending on its movement when the pile load is exerted (Bond & Haris, 2008). In case an increase in the load causes a downward drag of the pile, the Eurocode 7 (2008) refers to this as compression and recommends positioning the jacket at the same time into the soil at a relatively less depth of about 60meters. However, the same code states that in case increasing load causes an upward displacement, the pile is considered to be under tension and thus the structure has to be positioned deeply a maximum depth of 100meters (Bond & Haris, 2008). To determine soil characteristics, the soil is collected and tested in situ through the geotechnical survey and thus compression and tension characteristics described along with the shear resistance and deflection properties due to axial or lateral loads. To prevent corrosion Dean recommends the use of sacrificial anodes which consist of zinc or aluminum covering the entire jacket so that the total load of the plate accounts to 5% of the total load by the jacket. Nizamani (2014) also cautions that the splash zone has to be heavily plated to a thickness of 12mm in order to avoid wear and tear due to the action of water on the surface.

Requirements for the Pipeline

            The characteristics of the oil transportation pipeline are dependent on the nature of the seabed in terms of surface evenness and relative flatness (Inkpen & Moffett, 2011). Specifically, areas with seabed mobility have to be avoided since the moving marine substance may be deposited on the pipe’s surface reducing its lifespan, areas prone to submarine landslide may also cause pipe bending and thus should be avoided, high currents at shallow tidal seas mostly along straits hinders the pipe from maintaining floor positions leading to frequent displacement that may weaken its structure and lastly, ice gouges can settle on the pipe’s surface leading to deformity when they hit the pipe’s surface (Kenny, 2014). Other human initiated activities include: pipes crossing over each other, ships anchorage on the pipes and fishing nets hooking to pipes. All these factors have to be considered while installing the piping system. Since this system will transport hot oil under pressure from the rig to the holding tanks, the pipes have to be thick at about 72 inches in order to transport high amounts of oil. Due to the relatively high amounts of pressure within the pipe, its thickness throughout its length has to be relatively high at 75mm (Kenny, 2014). Dean recommends the use of high-yield steel due to its ability to withstand high pressure and coated with sacrificial anodes just like the pipes in the jacket assembly in order to prevent corrosion. Fiberglass should be added at the top of the cathodic protection to reduce abrasion effects and concrete applied to reduce buoyancy (Dean; Kenny, 2014). The Eurocode 3 (2007) supports the listed recommendations as an improvement of the 1993 set guidelines (Bernuzzi & Cordova, 2016). Since the Scottish west coast oil deposits are located relatively deep within the seabed at more than 1000meters but less than 2000meters, the S-lay system may be used as it is be able to allow the deep settlement of the pipeline on the seabed but also cause the pipe to rise at the shore leading the oil to the storage tanks.

Requirements for the Holding Tanks

            Since the storage facility will have four holding tanks, each being 10meters diameter and imposing maximum load of 5000N, the earth’s seismic structure should be able to support this load. Eurocode 8 (1998) adds that in order to clearly identify the most recommended standards that would support the tank designs, the soil factor, the design acceleration spectrum and damping factors have to be carefully calculated (Gosh, 2011). Although the location can utilize either the flexible or the rigid walled structures, the rigid walled tanks have a simpler design that can easily be incorporated successfully to contain the oil. The International Conference on Petroleum and Mineral Resources and Khoshnaw (2013) noted that the Eurocode 8 provides for installation instructions for both the rectangular and cylindrical shaped holding tanks but due to the application of the rigid walls in this case, the cylindrical design will be most ideal. Finally, these tanks have to be able to dissipate heat in order to balance the internal pressure and therefore painting the tanks white reduces the heat retention.

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.