Offshore Oil And Natural Gas Commerce Essay

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As the world approaches the point of "peak" oil, offshore oil and natural gas are still very important as a source of energy for the world (ref**** for peak oil). Safe and reliable transportation of this dwindling supply of oil and gas through offshore pipelines has therefore become of greater significance to the maintenance of supply of these important resources in the seas and ocean. Pipelines are used for several purposes in offshore hydrocarbon resources development including: export pipelines to transfer oil and gas, pipeline bundles, flow-lines to transfer product from the point of recovery to export lines, water injection or chemical injection flow-lines, flow-lines to transfer product between platform, subsea manifolds and satellite wells (Bai, 2003). Offshore drilling is expected to cover more than one third of global growth in oil and gas drilling, making the offshore pipelines development an extremely important topic in the energy industry (Guo, 2005). The components of a typical mature field are shown in Figure 1.

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Figure 1- components of a typical mature field (ref***)

Background

As oil fields mature and more hydrocarbons come from marginal and "stripper" well sources, smaller "in-field" flow-lines tend to be used instead of large diameter trunk pipe lines. These small diameter pipelines are usually installed with reel-lay techniques. With this technique, the pipeline to be laid is manufactured in a continuous length on board of the pipe-laying vessel and then spooled onto a large reel. During the pipe-laying process the pipeline is usually straightened and passed over an inclined ramp. Tensioners and/or clamps are used for holding the previously launched pipeline (Rodenburg et al., 2008).

This method commonly uses small diameter pipes, but requires thicker walled pipe to avoid local buckling during the bending and straightening process. Offshore pipelines are commonly buried beneath the seabed for safety, operational and environmental concerns e.g. protection against, hydrodynamic forces, fishing activity, icebergs, scouring and to provide on bottom stability and improving thermal insulation of the pipeline system (Bransby et al., 2001).

Since pipelines are laid in remote and potentially hostile environments (often at great water depth) the cost of laying and maintaining the pipeline can be extremely high, in terms of the actual work required, equipment mobilization times and costs, and reduced output. Therefore, offshore buried pipelines must be constructed as quickly and efficiently as possible, whilst maintaining the highest level of certainty against failure for the duration of their use.

To achieve high flow rates in pipelines, the gas or oil must be kept at high temperature and pressure. Normally, these pipelines are laid with near zero axial loads, at the ambient temperature. On heating, the pipeline will experience significant axial strain, which is resisted by seabed friction so that compressive forces increase in the pipe. These compressive forces are occasionally large enough to induce vertical uplift (upheaval buckling) of trenched lines, with the pipe emerging from the soil or becoming significantly distorted, so that its ability to withstand further loading is compromised. Upheaval buckling may happen on start-up or as a progressive upheaval buckling during operation. These phenomena are due to cyclic conditions brought about by cooling and heating due to line interruptions, which gradually 'ratchet' the pipe upwards, or from initial lay imperfection (or a combination of the two). The soil above the pipeline and the buoyant weight provide resistance to this uplift force and the embedment depth must be sufficient to prevent the vertical pipe movement from occurring.

Although there have been numerous reported cases of upheaval buckling occurring within the industry, it is understandably rare that any of these are reported in the technical literature. One of the few examples of a well documented case of UHB is the 17 km long Rolf "A" to Gorm "E" pipeline in the North Sea (Nielsen et al., 1990).

Trenching and burial is typically achieved by specialised water jetting, ploughing and cutting equipment. Knowledge of the in situ mechanical properties (before and following trenching operation) of these soils is extremely important for the design of buried pipeline systems; burial techniques can produce considerable disturbance to the structure of seabed sediments, leading to changes in their behaviour. Disturbance of the seabed in the vicinity of the trench depends on the soil type and state, and the mode of operation of the trencher.

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Figure 2-Tracked Trenching System T1 (courtesy of Acergy (permission))

Ploughed soft and stiff clay backfill can be "lumpy" in nature with large pieces of intact clay, creating the heterogeneous structure providing a macro structure. Stiff clay is believed to hydraulically fracture and very soft or silty clay can liquefy. The exact behavior between these two extremes is not clear yet. Homogeneity of the subsequent backfill will also be a function of time to commissioning of the pipeline (Cathie et al., 2005). The surfaces of the clay lumps will be remoulded and soften due to exposure to free water during ploughing. The voids between the lumps will be filled with water, slurry and sand fractions if present. This dual porosity material will consolidate much faster than a homogeneous material consisting of purely intact material and a suitable model for conducting analysis of the consolidation process is that proposed by Yang and Tan (2005).

Of particular concern to industry are trenches that have been water jetted (see Fig 2) in soft fine-grained silt and clay soils, due to the potential for significant changes in structure and the associated uncertainty of the trench backfill properties around the pipeline. A remotely operated tracked 'trencher' is driven over the seabed. The trencher has a series of nozzles mounted in forward facing jet-legs, which penetrate the seabed below. Water is pumped out of these jets at high pressure to destroy the structure of the clay, so the pipeline will sink into it. During jetting, the structure of the seabed soil is likely to be broken down and may liquefy completely. It is also possible that some intact lumps of clay could remain (although these may be subject to some remoulding) and these can increase the strength of the resulting backfill.

Ascertaining the degree of liquefaction or hydraulic fracture and the conditions under which these phenomena occur is an area of ongoing research. In particular, the state of the backfill and strength gain will contribute considerably as to whether drained or undrained conditions occur during upheaval buckling events due to the different drainage characteristics of slurried and 'lumpy' backfill (Cathie et al., 2005). Likewise, the resulting time dependent backfill behaviour following jetting will be different; both soil states will consolidate and gain strength gradually, but this will occur much faster in the 'lumpy' backfill (Cathie et al., 2005). This is particularly significant in soils with a high percentage of clay where the consolidation process can take many months, especially after full liquefaction.

Due to recent interest in the area of upheaval buckling, a number of analytical and numerical models have been developed to predict the vertical resistance to pipe movement provided by the soil and pipeline system (Ref ***). These models incorporate various assumed failure mechanisms for the behaviour of the soil-pipeline system during upwards motion through the trench backfill. The models are predominantly plane strain (2D) representations that assume soil deformation and failure surfaces that either extend to the seabed surface (shallow) or are fully contained within the backfill material (deep). The uplift capacity of the soil-pipeline system will depend on the geometry of this deforming system, the mobilised shear strengths and body weights, the relative rate of loading and the potential for detachment of the soil to occur behind the pipe during uplift.

Recently a recommended practice document of the certification organisation Det Norske Veritas [DNV-RP-F110, 2007] has been published for the prediction of the upheaval buckling resistance of offshore pipelines in offshore soils. This document has extended the state of the art and made recommendation for UHB analysis for sand, gravel, clay and layered materials. However, there seems to be some controversy with the industry as to the most appropriate way to determine some of the parameters in this design guide and indeed whether the calibration/validation of these design methods (not sure, can't read it) are sufficient.

Objective of thesis

Therefore despite the aforementioned body of research existing in the literature, much confusion still exists as to the appropriate design parameters and failure mechanisms involved for different cases. Existing design approaches assume that deep failure does not occur for the trench depths and pipeline geometries that are found in the field, however bounding plasticity solutions based on the uplift of strip anchors suggest that this may not necessarily be the case (Merifield et al, 2001).The most appropriate approach for UHB design in layered materials are also unclear. This research presents both numerical finite element and experimental study that examines the resistance of slurried homogeneous and layered clayey soils against upheaval buckling of buried pipelines. The specific objectives of the thesis are:

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To conduct a literature review on the upheaval buckling of buried pipelines in cohesive materials, with the view of identifying its importance in the design process and failure of pipelines;

To conduct a program of scaled physical model tests investigating the uplift behaviour of buried offshore pipelines through single and layered materials;

To conduct numerical finite element analyses to study the resistance of undrained cohesive soils against upheaval buckling of buried pipelines to provide and aid interpretation of the laboratory pullout test and conduct further parametric study;

To assess the current state-of-the-art in UHB pipeline design, to provide guidance for the design of buried pipelines for soft and layered backfill soils and to clarify some of the aspects of uncertainty in this topic.

Organization of thesis

This dissertation is subdivided into six chapters. Chapter 1 introduces the area of offshore pipelines, laying techniques, trenching and describe problem associated with upheaval buckling. Chapter 2 is dedicated to a review of analytical, experimental and numerical studies in the literature on the uplift resistance of buried objects, failure mechanisms for shallow and deep embedments, overburden effects, variations with roughness and variations with suction/adhesion. Chapter 3 covers the methodology of scaled physical model tests conducted by the author, including a description of basic material tests, preparation methods, pullout test apparatus and pullout test program. Soil deformation measurements using Particle Image Velocimetry (PIV) is described and application of this technique in failure mechanism study and the resulting displacement fields are illustrated. Also a description of numerical methodology is presented in this section. Chapter 4 addresses the quantification of the uplift behaviour of buried offshore pipelines with scaled physical model tests. Model tests were conducted using synthetic clay (Glyben), sand and gravel, both in single homogenous materials and in layered materials. The resistance forces for vertical pipe pullout and the mobilization distance for peak resistance were investigated for varying model geometries and soil properties. The effect of overburden was investigated and soil deformation observed with Particle Image Velocimetry (PIV) for various embedments and overburdens. Load-displacement results were compared with the existing numerical and analytical studies. Chapter 5 is devoted to a numerical finite element study that examines the resistance of undrained cohesive materials to upheaval buckling of buried offshore pipelines. The normalized load-deflection behaviour for a range of embedments, roughness and breakaway are presented. Plastic regions and velocity fields at collapse and the effects of overburden pressure are also demonstrated. Chapter 6 presents a summary of the results and conclusions of the study, and also identifies areas for future research work.

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