As the universe approaches the point of “ peak ” oil, offshore oil and natural gas are still really of import as a beginning of energy for the universe ( ref**** for peak oil ) . Safe and dependable transit of this dwindling supply of oil and gas through offshore grapevines has hence become of greater significance to the care of supply of these of import resources in the seas and ocean. Grapevines are used for several intents in offshore hydrocarbon resources development including: export grapevines to reassign oil and gas, grapevine packages, flow-lines to reassign merchandise from the point of recovery to export lines, H2O injection or chemical injection flow-lines, flow-lines to reassign merchandise between platform, subsea manifolds and orbiter Wellss ( Bai, 2003 ) . Offshore boring is expected to cover more than one tierce of planetary growing in oil and gas boring, doing the offshore grapevines development an highly of import subject in the energy industry ( Guo, 2005 ) . The constituents of a typical mature field are shown in Figure 1.

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

Background

As oil Fieldss mature and more hydrocarbons come from fringy and “ stripper ” good beginnings, smaller “ in-field ” flow-lines tend to be used alternatively of big diameter bole pipe lines. These little diameter grapevines are normally installed with reel-lay techniques. With this technique, the grapevine to be laid is manufactured in a uninterrupted length on board of the pipe-laying vas and so spooled onto a big reel. During the pipe-laying procedure the grapevine is normally straightened and passed over an inclined incline. Tensioners and/or clinchs are used for keeping the antecedently launched grapevine ( Rodenburg et al. , 2008 ) .

This method normally uses little diameter pipes, but requires thicker walled pipe to avoid local buckling during the bending and unbending procedure. Offshore grapevines are normally buried beneath the ocean floor for safety, operational and environmental concerns e.g. protection against, hydrodynamic forces, fishing activity, icebergs, scouring and to supply on bottom stableness and bettering thermic insularity of the grapevine system ( Bransby et al. , 2001 ) .

Since grapevines are laid in remote and potentially hostile environments ( frequently at great H2O deepness ) the cost of puting and keeping the grapevine can be highly high, in footings of the existent work required, equipment mobilisation times and costs, and reduced end product. Therefore, offshore inhumed grapevines must be constructed as rapidly and expeditiously as possible, whilst keeping the highest degree of certainty against failure for the continuance of their usage.

To accomplish high flow rates in grapevines, the gas or oil must be kept at high temperature and force per unit area. Normally, these grapevines are laid with close zero axial tonss, at the ambient temperature. On warming, the grapevine will see important axial strain, which is resisted by seabed clash so that compressive forces addition in the pipe. These compressive forces are on occasion big plenty to bring on perpendicular upheaval ( upheaval buckling ) of trenched lines, with the pipe emerging from the dirt or going significantly distorted, so that its ability to defy farther burden is compromised. Upheaval clasping may go on on start-up or as a progressive turbulence clasping during operation. These phenomena are due to cyclic conditions brought approximately by chilling and heating due to line breaks, which bit by bit ‘ratchet ‘ the pipe upwards, or from initial ballad imperfectness ( or a combination of the two ) . The dirt above the grapevine and the floaty weight provide opposition to this uplift force and the embedment deepness must be sufficient to forestall the perpendicular pipe motion from happening.

Although there have been legion reported instances of upheaval clasping happening within the industry, it is intelligibly rare that any of these are reported in the proficient literature. One of the few illustrations of a good documented instance of UHB is the 17 kilometers long Rolf “ A ” to Gorm “ E ” grapevine in the North Sea ( Nielsen et al. , 1990 ) .

Impinging and burial is typically achieved by specialized H2O jetting, plowing and cutting equipment. Knowledge of the in situ mechanical belongingss ( before and following impinging operation ) of these dirts is highly of import for the design of inhumed grapevine systems ; burial techniques can bring forth considerable perturbation to the construction of seabed deposits, taking to alterations in their behavior. Perturbation of the ocean floor in the locality of the trench depends on the dirt type and province, and the manner of operation of the trencher.

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

Ploughed soft and stiff clay backfill can be “ chunky ” in nature with big pieces of integral clay, making the heterogenous construction supplying a macro construction. Stiff clay is believed to hydraulicly fracture and really soft or silty clay can liquefy. The exact behaviour between these two extremes is non clear yet. Homogeneity of the subsequent backfill will besides be a map of clip to commissioning of the grapevine ( Cathie et al. , 2005 ) . The surfaces of the clay balls will be remoulded and soften due to exposure to free H2O during plowing. The nothingnesss between the balls will be filled with H2O, slurry and sand fractions if present. This double porousness stuff will consolidate much faster than a homogenous stuff consisting of purely integral stuff and a suited theoretical account for carry oning analysis of the consolidation procedure is that proposed by Yang and Tan ( 2005 ) .

Of peculiar concern to industry are trenches that have been H2O jetted ( see Fig 2 ) in soft powdered silt and clay dirts, due to the potency for important alterations in construction and the associated uncertainness of the trench backfill belongingss around the grapevine. A remotely operated tracked ‘trencher ‘ is driven over the ocean floor. The trencher has a series of noses mounted in frontward confronting jet-legs, which penetrate the ocean floor below. Water is pumped out of these jets at high force per unit area to destruct the construction of the clay, so the grapevine will drop into it. During jetting, the construction of the seabed dirt is likely to be broken down and may liquefy wholly. It is besides possible that some integral balls of clay could stay ( although these may be capable to some remoulding ) and these can increase the strength of the ensuing backfill.

Determining the grade of liquefaction or hydraulic break and the conditions under which these phenomena occur is an country of ongoing research. In peculiar, the province of the backfill and strength addition will lend well as to whether drained or undrained conditions occur during upheaval clasping events due to the different drainage features of slurried and ‘lumpy ‘ backfill ( Cathie et al. , 2005 ) . Likewise, the resulting clip dependant backfill behaviour following jetting will be different ; both soil provinces will consolidate and derive strength bit by bit, but this will happen much faster in the ‘lumpy ‘ backfill ( Cathie et al. , 2005 ) . This is peculiarly important in dirts with a high per centum of clay where the consolidation procedure can take many months, particularly after full liquefaction.

Due to recent involvement in the country of upheaval buckling, a figure of analytical and numerical theoretical accounts have been developed to foretell the perpendicular opposition to shriek motion provided by the dirt and grapevine system ( Ref *** ) . These theoretical accounts incorporate assorted false failure mechanisms for the behavior of the soil-pipeline system during upwards gesture through the trench backfill. The theoretical accounts are preponderantly flat strain ( 2D ) representations that assume dirt distortion and failure surfaces that either extend to the seabed surface ( shallow ) or are to the full contained within the backfill stuff ( deep ) . The uplift capacity of the soil-pipeline system will depend on the geometry of this deforming system, the mobilised shear strengths and organic structure weights, the comparative rate of burden and the potency for withdrawal of the dirt to happen behind the pipe during upheaval.

Recently a recommended pattern papers of the enfranchisement administration Det Norske Veritas [ DNV-RP-F110, 2007 ] has been published for the anticipation of the upheaval buckling opposition of seaward grapevines in offshore dirts. This papers has extended the province of the art and made recommendation for UHB analysis for sand, crushed rock, clay and superimposed stuffs. However, there seems to be some contention with the industry as to the most appropriate manner to find some of the parametric quantities in this design usher and so whether the calibration/validation of these design methods ( non certainly, ca n’t read it ) are sufficient.

Aim of thesis

Therefore despite the aforesaid organic structure of research bing in the literature, much confusion still exists as to the appropriate design parametric quantities and failure mechanisms involved for different instances. Existing design attacks assume that deep failure does non happen for the trench deepnesss and grapevine geometries that are found in the field, nevertheless jumping malleability solutions based on the upheaval of strip ground tackles suggest that this may non needfully be the instance ( Merifield et al, 2001 ) .The most appropriate attack for UHB design in superimposed stuffs are besides ill-defined. This research presents both numerical finite component and experimental survey that examines the opposition of slurried homogenous and superimposed clayey dirts against upheaval buckling of inhumed grapevines. The specific aims of the thesis are:

To carry on a literature reappraisal on the upheaval buckling of inhumed grapevines in cohesive stuffs, with the position of placing its importance in the design procedure and failure of grapevines ;

To carry on a plan of scaly physical theoretical account trials look intoing the uplift behavior of buried offshore grapevines through individual and superimposed stuffs ;

To carry on numerical finite component analyses to analyze the opposition of undrained cohesive dirts against upheaval buckling of inhumed grapevines to supply and help reading of the research lab disengagement trial and behavior farther parametric survey ;

To measure the current state-of-the-art in UHB grapevine design, to supply counsel for the design of inhumed grapevines for soft and layered backfill dirts and to clear up some of the facets of uncertainness in this subject.

Organization of thesis

This thesis is subdivided into six chapters. Chapter 1 introduces the country of offshore grapevines, puting techniques, impinging and describe job associated with upheaval buckling. Chapter 2 is dedicated to a reappraisal of analytical, experimental and numerical surveies in the literature on the uplift opposition of inhumed objects, failure mechanisms for shoal and deep embedments, overburden effects, fluctuations with raggedness and fluctuations with suction/adhesion. Chapter 3 covers the methodological analysis of scaly physical theoretical account trials conducted by the writer, including a description of basic stuff trials, readying methods, pullout trial setup and pullout trial plan. Soil distortion measurings utilizing Particle Image Velocimetry ( PIV ) is described and application of this technique in failure mechanism survey and the resulting supplanting Fieldss are illustrated. Besides a description of numerical methodological analysis is presented in this subdivision. Chapter 4 addresses the quantification of the uplift behavior of buried offshore grapevines with scaly physical theoretical account trials. Model trials were conducted utilizing man-made clay ( Glyben ) , sand and crushed rock, both in individual homogeneous stuffs and in superimposed stuffs. The opposition forces for perpendicular pipe disengagement and the mobilisation distance for peak opposition were investigated for changing theoretical account geometries and dirt belongingss. The consequence of overburden was investigated and soil distortion observed with Particle Image Velocimetry ( PIV ) for assorted embedments and overburdens. Load-displacement consequences were compared with the bing numerical and analytical surveies. Chapter 5 is devoted to a numerical finite component survey that examines the opposition of undrained cohesive stuffs to upheaval buckling of buried offshore grapevines. The normalized load-deflection behavior for a scope of embedments, raggedness and breaking away are presented. Fictile parts and speed Fieldss at prostration and the effects of overburden force per unit area are besides demonstrated. Chapter 6 nowadayss a sum-up of the consequences and decisions of the survey, and besides identifies countries for future research work.

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