The Challenges of Pipeline Burial

Damian R Morrow and Peter D Larkin Acergy Ltd

Aberdeen, UK

ABSTRACT Burial of sub sea pipelines provides a number of advantages and as part of the installation process all or part of a sub sea pipeline system may be buried, either with or without provision of backfill. While much of the sub sea environment is largely flat significant local slopes of natural or man made origin can occur in areas where pipeline burial is required, posing a number of challenges. Having reviewed the advantages of pipeline burial and the origin and environment of seabed slopes the pipeline burial assets available to the sub sea installation contractor are considered. The importance of an appropriate bathymetric survey and the role of route optimisation is emphasised. After discussing some consequences that can result from sloping seabeds these consequences are elaborated on and given context by discussing examples from pipeline burial projects. KEY WORDS: pipeline; burial; slopes; sedimentary features; pockmarks plough; trencher; route optimisation. INTRODUCTION As part of the installation process a sub-sea pipeline may be buried over some or all of its length. Post lay burial is particularly prevalent within the North Sea as well as in other parts of the world where it is required to address specific pipeline design issues. When pipeline burial is undertaken trench backfill may also undertaken providing a covering of soil over the pipeline. Although much of the subsea environment can be considered to be flat or shallow gradient, there are however exceptions. When these slopes occur they can have a significant effect on sub-sea construction activities. This paper discusses the challenges posed to pipeline burial in areas with sloping seabed. To give context to these discussions the causes of seabed slopes will be presented along with the range of pipeline burial techniques available to the sub sea installation contractor. ADVANTAGES OF PIPELINE BURIAL

A discussion of the challenges associated with pipeline burial needs to be considered within the context of the advantages pipeline burial provides. The principle advantages of burial to a pipeline system are:

• Protection • Stability Under Thermal Or Pressure Driven Expansion • Hydrodynamic Stability • Thermal Insulation • Freespan Mitigation

Pipeline Protection The protection of pipelines is perhaps the most common driver for pipeline burial. In Northern Europe intensive fishing activity and the large amount of closely spaced infrastructure associated with mature oil and gas developments results in the need to provide protection from trawl gear for small diameter lines. Large diameter trunk lines are typically considered to have sufficient mass and strength remain on the seabed during the course of their design life. In addition to protection from fishing activities, burial can also protect a pipeline from dropped objects and vessel anchors. Protection can be provided by methods other then burial, such as covering the pipeline with flexible concrete mattressing or rock dump berms. However, generally these are only economic solutions for short sections of pipeline. The specification for a pipeline burial solution is typically expressed in terms of depth of lowing (DoL) relative to mean sea bed level (MSBL), depth of cover (DoC) of the soil backfill may also be specified. The design of the burial specification may need to consider other drivers for burial, or the requirement to provide sufficient download to mitigate upheaval buckling (UHB). When considering the issue of protection alone it is appropriate to take a risk based approach, such as that outlined in DNV RP F107 (2002). The nature and frequency of the risk factor is considered (e.g. fishing or dropped object), along with the amount of protection given by the seabed soils the trench is formed in and any trench backfill. Trench geometry can also be a factor. As an example; 0.6 m DoL in a very soft clay seabed provides significantly less protection then 0.6 m DoL in very dense sand. A further extreme example would be a pipeline trenched into a rocky seabed, in this case

Proceedings of the Sixteenth (2007) International Offshore and Polar Engineering ConferenceLisbon, Portugal, July 1-6, 2007Copyright © 2007 by The International Society of Offshore and Polar Engineers(ISOPE)ISBN 978-1-880653-68-5; ISBN 1-880653-68-0(Set); ISSN 1098-6189 (Set)

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even small depth of lowering can provide a significant amount of protection. A more detailed review of this approach to burial protection for the specific hazard of ice scour is give by Palmer (1997). Another example is the analogous situation of burial protection for submarine telecommunication cables discussed in Allan & Comrie (2001). Stability Under Thermal Or Pressure Driven Expansion Many pipelines associated with oil and gas infrastructure are used to transport warm fluids under pressure. This requirement on pipeline systems should also be set against an increasing trend to high pressure high temperature (HPHT) lines. This trend to HPHT is occurring in association with advances in material and welding technology, making possible development projects previously regarded as unfeasible (Collberg and Levold 2005, Spinazze et al 2005). Pipelines are lain on the seabed at ambient temperature. During and following pipeline start up hot fluids travel through the bore of the pipeline, heating the line to temperatures significantly above ambient. This heating leads to thermal expansion of the steel materials of the pipeline. At a small scale this expansion may appear to be insignificant, however, in an axial direction this expansion or the forces associated with the potential to expand are to some extent cumulative. Thermal expansions lead to a complex process of pipe soil interaction that can result in large forces and/or displacements within a given section of pipeline. Axial forces and displacements are also an important consideration for the design of inline structure or the spools that connect pipelines to structures. Depending on the relationship between axial and lateral pipe soil interaction axial expansion may be restrained. This can result in the expansion forces propagating lateral buckling of a pipeline. Buckling of a pipeline is generally an undesirable event inducing stress in the line and potentially plastic deformation and/or fatigue associated with a series of buckling cycles. In extreme cases lateral buckling can over stress a pipeline resulting in loss of integrity. In the case study discussed by Oliveira et al (2005) a pipeline on soft clay seabed buckled laterally 4.1m over a length of 44m resulting in loss of pipeline integrity and a large release of oil into the environment. It has been noted that buckling is generally undesirable. However, in recent deepwater developments buckling has been judged to be unavoidable. In this case buckling is propagated at specific locations using various buckle inducement measures e.g. sleepers, buoyancy (Bruton et al 2005). Burial of a pipeline significantly alters the pipe soil interaction providing stability against lateral buckling and significantly reducing axial expansion. However, it should be noted while pipeline burial can solve lateral stability issues there is the potential to introduction the equally challenging issues of upheaval buckling (UHB). In UHB the axial expansion forces over come the downloaded provided by trench backfill resulting in upwards buckling (Schaminee et al 1990, Bolton and Barefoot 1997). This has the potential to over stress the pipe and/or present a section of pipeline that is susceptible to snag loading by fishing equipment. This discussion on pipeline expansion has been largely limited to rigid steel pipeline and has therefore focused on thermal expansion. Broadly analogous processes can occur with flexible pipelines. Although with flexible lines pressure is more of an influence then temperature effects (Kodaissi 1995). The construction techniques utilised to produce the carcass of a flexible pipeline is also a important consideration.

Hydrodynamic Stability Hydrodyamic stability is generally an issue confined to shallow water depths, although the presence of strong currents over a range of water depth should not be discounted without due consideration. Dependent on the pipe soil interaction with the seabed (friction, adhesion and passive earth pressure due to embedment) the pipeline has a certain resistance to lateral movement. If a pipelines lateral resistance to movement is exceeded by the hydrodynamic loading associated with a given metocean design case e.g. a 25 or 50 year storm, the stability may need to be increased. This could be achieved by increasing pipeline weight either by adding concrete weight-coating or by increasing wall thickness of the steel component of the pipeline. Rock dump berms or concrete matressing could also be used. In place of, or in addition to remedial measures an allowable displacement philosophy may be adopted dependent on pipeline location, one such approach is adopted in DNV-RP-E305 (1988). Horizontal directional drilling (HDD) or cofferdams may also provide a method for shore approaches. However, as with pipeline protection these methods can quickly become uneconomic and alternatives to surface lain lines may need to be considered for longer sections of pipeline. Even trenching a pipeline to half a diameter or placing the pipeline in a open trench can significantly increase its resistance to lateral movement. Lowering a pipeline produces increased lateral resistance either due to passive earth pressure on the embedded section or from the slope of the trench it is placed in. In association with this effect the hydrodynamic loading on a pipeline is also reduced as it is lowered into the seabed. Full burial and backfill effectively removes hydrodynamic forces from a pipeline, ensuring stability. There is however the need for caution. When considering pipeline burial for hydrodynamic stability the seabed by implication is an unstable and dynamic environment. This needs to be fully accounted for in the design process including depth of burial. For example sediment scour and deposition may need to be considered, wave induced liquefaction may also be a factor in pipeline stability as discussed in Sumer et al (1999) and Teh et al (2006). Thermal Insulation As previously discussed many pipelines associated with the offshore oil and gas industry have to transport fluids at high temperature. In addition there is also the requirement to maintain the temperature of this fluid within certain tolerances to prevent the deposition of precipitates (e.g. wax's) within the system. To maintain temperature the pipeline may incorporate a layer of insulation coating within their construction. As the insulation requirements of a pipeline system increases there is an associated increase in cost as well as potential manufacturing and installation issues. In this scenario there can be an advantage in utilising the insulation properties of trench backfill to reduce the thickness of pipeline insulation coating. Finch et al (2000) and Young et al (2001) discuss some of the issues relating to thermal insulation and pipeline burial. Freespan Mitigation On a uneven seabed, the geometry of the seabed, pipe soil interaction and the residual axial tension from the lay process can result in the pipeline being suspended above the seabed i.e. the pipeline is in freespan. This can be undesirable for a number of reasons; a free-spanning pipeline is subject to increased hydrodynamic loading potentially contributing to instability as well as inducing stress in the line. In addition the suspended self weight of the pipeline also applies loads on the line in the vicinity of the touch down points.

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In areas where freespans are likely to occur analysis may be undertaken to assess the allowable freespan length and assess the likelihood of this length being exceeded for a given pipeline route. Holdon et al (2005) describe this process for routing of the Ormen Lange Pipelines in the challenging seabed terrain associated with the Storegga slide in the Norwegian sector of the North Sea. In conjunction with route optimisation, rock dumping, dredging and trenching provide methods of mitigating freespans. ORIGIN OF SEABED SLOPES The sub sea environment can be classified into the following three broad zones: • Continental Shelf • Continental Slope • Deep Sea Environment The continental shelf can be considered to be a shallow water setting including near shore and shore approaches. However, although shallow, continental shelves can extend hundreds of kilometres from the shore in some areas. In addition the definition of shallow water can vary widely; in this case it encompasses water depths of several hundred metres. On a regional scale this zone is extremely flat with a mean slope of < 1°. Transition to continental slope typically occurs at water depth between 130m to >350m with the variation relating to the geographical and geological setting (Friedman et al 1992). It is this zone where most pipeline burial is undertaken. The continental slope represents the transition from the relatively shallow water depths to the deep sea environment. At a regional level continental slopes typically have a slope angle of 3° to 6°, although there is significant variation as well as this being a zone where extreme localised slopes are common. There is also significant variation in the range of water depth this zone extends over (Friedman et al 1992). In recent years sub sea infrastructure associated with the oil and gas developments extends into or crosses this zone, although pipeline burial in this zone is rare. The deep sea environment encompasses a range of settings characterised by reduction in slope angles from the continental slope zone. Slope angles may be a few degrees for features associated with the edge of the continental slope with a trend towards negligible slopes. Traditionally this zone and even the continental slope were of little interest for those considering sub sea infrastructure. However, as the practical depth for oil and gas developments increases the trend towards deeper water has focused attention on these areas in recent years. It can be seen from the discussion above that with the exception of the continental slope much of the sub sea environment tends towards being flat when considered on a regional scale. However, at a local scale there are significant slopes with a variety of origins. Primarily it is these localised slopes that effect pipeline burial the most, partly due to the steep gradients and the change from a relatively flat seabed to a slope over a short distance. It is convenient to discuss the development of localised seabed slopes in terms of their origin. These in turn can be subdivided into natural phenomena and those resulting from the human activities. Natural Slopes

All localised natural slopes result from the removal or deposition of seabed sediments by natural processes. Common causes of natural slopes include:

• Sedimentary Features • Ice Scour • Pockmarks • Slope Processes

Sedimentary Features For the purposes of this paper sedimentary features (bedforms) are defined as seabed features formed by the erosion and deposition of sediment due to hydrodynamic processes. Classification of these bedforms is conveniently approached with a hierarchical system based on height (amplitude) and wavelength, however, there is no universally accepted classification system or agreement on nomenclature. For the purposes of this paper the system used by Passchier & Klienhans (2005) will be utilised. Ripples are not discussed by Passchier & Klienhans (2005) and are not particularly relevant to the subject of this paper. However, for completeness ripples will be considered to have a height of less then 0.3 m and wavelengths of less the 1 m. This classification is summarised below in Table 1.

Name Height Wavelength Sandwaves 1.5 m to 17 m or larger 40 m – 800 m or larger

Megaripples 0.3 m to 1.5 m 1 m to 40 m

Ripples Less them 0.3 m Less then 1 m

Table 1. Bedform Classification A detailed discussion of bedform morphology is outside the scope of this paper, however issues such as bedform asymmetry should be given consideration. When hydrodynamic forces have a prevailing direction bedforms trend towards a morphology consisting of a relatively shallow rising slope (stoss slope) and a steeper down slope (lee slope) (Soulsy 1997). In plan the morphology of bedforms can also be relevant when considering pipeline routing. In addition to active features a site may also contain palimpsest features formed under different hydrodynamic regimes. Example of these are the gravel features found in some areas of the Southern North Sea as discussed in Cameron et al (1992). Ice Scour Ice scour features form when an ice sheet or more commonly an ice burg enters shallower water and its base or keel is grounded. When this happens the wind, waves or currents may continue to move the ice, which in turn scars or ploughs the seabed. This can have a dramatic effect on seabed morphology. In addition to changes in seabed profile the action of ice can also effect the geotechnical properties of seabed soils, effecting soils below the base and adjacent to the walls of a scar, further discussion is given in Palmer (1997). The presence of ice scour is dependent on location, climate and water depths. For example routing a pipeline into deeper water can be an effective protection measure. However, when considering the effect of ice scour features on pipeline burial the presence of relict features formed under differing climatic and sea level conditions may be of influence. An example of active and relict ice burg scour in close association is given Lewis and Woodsworth Lynas (1990). Lewis and

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Woodsworth Lynas (1990) also noted differing ice scour morphology dependent on seabed soils. Stiffer clays exhibit deep (up to 6m) scars with steep sides up to 60°, where as sandy soils exhibited shallower scar (<1m) with low angle slopes of less the 6°. In the northern North Sea relict ice scour features have also been observed, these are typically 2 m deep and 20 m across with some post formation infill with sandy sediments (Johnson et al 1993). As with bedforms the morphology of ice scour features in plan can be important for pipeline routing. Although in heavily scarred areas with variation in scar orientation routing options can be limited. Pockmarks Pockmarks are fluid escape structures that are particularly prominent when gas or water escapes through soft cohesive sediments. Their occurrence is common in soft clay sites around the world where these sediments occur in association with thermogenic and biogenic methane. Pockmarks can also be ground water driven, although these are thought to be less common (Judd 2001). Typical examples of pockmarks found in the North Sea have a diameter of 50m to 100m and are 2 - 3m deep, although larger pockmarks up a diameter of 200m and 6m deep can also occur. Pockmark density greater than 30 per square kilometre are not uncommon (Long 1986). Pockmark morphology typically consists of a steep slope adjacent to the edge which can exceed 10°. The base of the pockmark is typically a flatter area, especially if pockmark formation has been inhibited by firmer soils at depth. In plan, pockmarks can exhibit an asymmetric morphology, which is common in North Sea pockmarks (Judd 2001). In addition to their occurrence in the North Sea, pock marks can also be found in other areas of the world, for example; Gay et al (2006) discuss pockmarks in West Africa and Cifi et al (2002) notes pockmark activity in the Black Sea. Variation in geotechnical properties in association with pockmarks and the presence of cemented material can also influence pipeline burial in pockmarked areas. If the trend towards deeper water continues then in addition to encountering pockmarks deep water geohazards such as mud volcanoes may also be features to consider. Slope Processes Sediment transport associated with these slopes can result in features which have a significant influence. An extreme example would be slope failures and the resulting scaring in the failure zone and the deposition of slide blocks in down slope areas. An example of pipeline routing in this sort of terrain is discussed by Holden et al (2005). Manmade Slopes In addition to slopes resulting from natural processes pipeline burial can also effected by slopes of manmade origin.

• Spudcan Depressions • Anchor Scars • Trawling Scars

Manmade debris has been excluded from this list. The issues surrounding debris are primarily related to the nature of the debris rather then any associated slopes. Spudcan Depressions Jack up rigs are typically used in water depths of less then 100 m although some recently constructed rigs have extended this limit by a few tens of metres. Jack ups act as mobile offshore drilling units (MODU’s) or in a support role to other platforms e.g. flotels. Spud cans

form the foundation interface for the jack up legs. Dimensions and geometry varies, however, circular spud cans with a diameter of 10m to 20m are perhaps most common. Penetration of spud cans during preloading and operation varies with soil conditions, shallow penetration of a few metres is typical for sandy conditions, whilst 10m to 20m is not uncommon for soft clay conditions penetrations of up to 40m recorded in some locations (Clarom 1993). At larger penetrations backflow of soil is likely, however there is significant disturbance of the seabed during penetration and extraction. In areas where there has been jack up activity spud can depressions can form steep localised slopes, generally in groups of three in a triangular geometry. Anchor Scars An alternative to jack up rigs are floating mobile drilling units such as semi-submersible rigs. In shallow water these rigs typically maintain station using a spread of drag embedment anchors. Drag embedment anchors are also utilised by some pipe lay vessels and in more permanent applications for floating production units (FPU). In soft clay conditions penetrations of 10m to 20m are not uncommon for larger anchors loaded close to their ultimate holding capacity (API 1997, Vyhof 2000). Drag anchor installation and recovery can result in scars and depression that have the potential to effect pipeline burial activities. Anchor chains and lines can also result in seabed scaring, however this is generally not significant enough to effect pipeline burial operations. Trawling Scars The use of trawling equipment in an area results in scaring of the seabed. The typical geometry of trawl scars results in their prominence on side scan sonar records. They tend to be less prominent on bathymetric surveys, although this is dependent on the resolution of the survey and the processing techniques utilised. As with many of the other man made features the geometry of trawl scars is highly dependent on the properties of the seabed sediments. Generally speaking trawl scar do not pose a significant hazard to pipeline installation. METHODS OF PIPELINE BURIAL It is convenient to consider pipeline burial assets in two broad categories: • Ploughs • Trenchers Ploughs Ploughs, not surprisingly, bear some similarity to agricultural implements of the same name. A share cuts a V shaped trench with side slopes in the region of 35°, two spoil mounds of excavated material are deposited to the side of the trench. During the ploughing process the pipeline is held in roller cradles within the body of the plough. This protects the pipeline, picking it up from the seabed ahead of the excavation and deposited it back into the trench behind the plough. A pipeline plough is a passive tool albeit with a range of control mechanisms and surveillance equipment. A tow wire from a plough support vessel (PSV) provides the force required to excavate the trench and ensure forward progress of the plough. The PSV also provides all the equipment and personnel for launch, recovery, operation and maintenance of the plough.

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Figure 1: An animated view of the aft of a pipeline plough A high bollard pull vessel is required as a PSV as steady tensions of 200 - 250 Te are not uncommon to ensure adequate progress in sandy soils. Further information on performance predictions of ploughs is provided in Finch et al 2000 and Cathie 2001. When backfill is required over a ploughed pipeline this can be achieved using a backfill plough. This burial asset typically rides in the ploughed trench with skids located either side of the pipeline. Large backfill blades extend in a V from the body of the plough, encompassing the spoil mounds previously deposited when the trench was formed by the pipeline plough. Backfill is swept into the trench as the backfill plough follows the route of the pipeline. As with a pipeline plough a backfill plough is a passive tool towed by a PSV, although the bollard pull requirement is lower.

Figure 2: A tracked jetting trencher being launched. The inset image shows the jetting swords for this trencher Trenchers

The Trencher category describes a wide range pipeline burial equipment. They have been separated from ploughs as they are active assets which do not require towing by a surface vessel. Trenchers can be sub divided into two further categories; jetting trenchers and cutting trenchers. Trenchers operate as a remotely operated vehicle controlled by a umbilical connected to a trenching support vessel (TSV). Jet trenchers achieve pipeline burial by fluidising the soils around and below the pipeline with medium pressure high volume water jets. These water jets originate from a series of nozzles mounted in “jet swords” which are pushed down either side of the pipe. Backfill is achieved over the pipeline when the fluidised/suspended sediment is re-deposited around and above the pipeline. Jet trenchers come in a range of types, sizes and power output and can take the form of tracked trenchers, free flying vehicles or pipe riders. Jet sleds would also loosely fall into this category. .is significantly affected and no progress is likely in higher shear strength soils. The second category of trenchers, cutting trenchers are suitable for higher shear strength clays, some are also suitable for cemented soils and rock. These trenchers mechanically cut a slot into the seabed using a toothed chain, belt or wheel, some trenchers also use jetting in combination to remove cutting waste and provide backfill. ROUTE OPTIMISATION When seabed slopes pose difficulties to pipeline burial there is often scope to mitigate their effect by careful route planning, reducing their influence. This usually, although not always, results in an increase in pipeline route length. Route optimisation balances the cost of increased length against the effect a slope has on pipeline installation and burial. Experience indicates that moderate rerouting around individual features results in a surprising small increase in route length, so the benefits should always be considered. The potential of route optimisation has increased with the advent of advanced software generically referred to as geographical information systems (GIS). The use of GIS in the offshore oils and gas industry has initially been slow with limited uptake, but has gathered pace over recent years. Simple route optimisation can be undertaken by using software to generate themed maps. For example; slopes within a certain range can be highlighted in prominent colours. Routes can then be adjusted to avoid features, slope statistics and route lengths can be produced to quickly compare route options. This approach is generally adequate for short simple routes between two fixed points such as a short tie back between a well and platform. As route length increases, complicated seabed conditions are encountered and variables such as start and end point are introduced optimisation quickly becomes complicated and difficult to interpret by eye. In this scenario least cost algorithms can be utilised. A cost surface is compiled for the seabed, this surface assigns a cost to crossing a given terrain with the algorithm considering a total route cost based on length and terrain. The cost surface and the cost algorithm typically require some "tuning" to account for the characteristics of the pipeline burial asset and project priorities. It is relatively simple to incorporate other elements into this analysis, for example geohazards. SURVEY TECHNIQUES Having discussed some of the causes of seabed slopes and the use of route optimisation the importance of an accurate and appropriate bathymetric survey should be emphasised. A survey that adequately characterises the seabed morphology is an important aspect of pre trenching engineering as well as being relevant to other subsea construction activities. The absence of such a survey can result in

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significant problems, typically occurring when a "off the shelf" or "one survey fits all" approach is adopted. Perhaps the best approach to a bathymetric survey is a phased approach including consultation with all end users or the involvement of a suitably experienced consultant. The initial phase could consist of a reconnaissance survey covering a wide route corridor. A desk study may be utilised for planning of this initial survey with the corridor width dictated by the degree of re-routing that is expected. From this initial survey a course route can be defined. This route then becomes the focus for a subsequent route survey producing high resolution data. Resolution and processing techniques should focus on the nature of the hazards and the proposed burial tools. In some projects there may be an argument for high resolution bathymetric surveys from a remotely operately vehicle (ROV) or autonomous underwater vehicle (AUV). PIPELINE BURIAL ON SLOPING SEABEDS When pipeline burial is undertaken in areas with seabed slopes strategies such as route optimisation may be undertaken as discussed in the previous section. However, at some point the pipeline may need to be buried in an area characterised by a series of localised slopes originating from one or more of the mechanisms discussed in this paper. All pipeline burial assets interact with the seabed to some extent. Typically burial assets are ground bearing to some extent, even so called free flying trencher often operate with skids or tracks and the vehicle ballasted to have a slight negative buoyancy. Even if the a burial asset is able to reduce its interaction with the seabed when making forward progress (free flying) the pipeline still rests on the seabed. The excavation tool (e.g. jet leg, plough share, chain cutter) will, by definition, always interact with the seabed. It is therefore reasonable to consider that all currently available burial assets are affected by seabed slopes. There is a relatively complex interaction between slope geometry and a specific asset. Typical negative consequences are listed in bullet point below. • Out of straightness (OOS) • Loss of DoC and/or DoL • Slopes outside a burial assets capability • Damage to the product and or burial asset Out of straightness (OOS) is a pipeline design term that refers to the profile of the pipe in the vertical plane. OOS is particularly important when considering upheaval buckling (UHB), as OOS incident can result in a concentration of driving forces resulting in a preferential buckling location. An OOS incident can form due to various reasons such as transitions in soil conditions, buried boulders, or trench wall slumping prior to pipeline touchdown, however seabed profile is often considered to be a common cause, especially for smaller OOS incidents. An OOS incident can also be associated with loss of cover. An example of a typical OOS incident would be a pipeline lain over a mega rippled seabed. A burial asset may be able to correct this to some extent, however pitch and roll of the asset will significantly effect this ability. Pitch, and to a lesser extent roll, can also result in a non-optimal orientation of the excavation tool. This is particularly the case for jetting swords where pitch will alter nozzle depth and orientation. Multiple passes may be required to reduce OOS, or alternatively spot rock dumping may be required. As with OOS loss of cover is closely associated with UHB, and as with OOS the pitch and roll of the trenching asset can result in reduced cover height. If a plough is used then spoil mound stability and the

pitch of the backfill plough can also be an issue. Of particular relevance to areas with bed forms is the datum used to define DoL and DoC. This is typically based on mean seabed level (MSBL), however in areas with mega ripples it is common to define (MSBL) as the trough between mega ripples. This makes the DoL and DoC requirement within these areas particularly onerous. Slopes can be outside the capability of an asset resulting in an inability to bury a section of pipeline. An example of this would be a freespan that is elevated over the seabed and out with the travel of a ploughs roller box's or the ability to adjust the height of the front skids. Large free spans can also "ground" trenchers that are operating in a ground bearing mode as the pipeline height exceeds the clearance from the trencher chassis. There are some links with the final point as areas that are out with a trenchers capability may include areas where the risk of asset or pipeline damage is considered to be high. Failure to bury a section of line is particular serious when burial is relied upon to provide insulation. Any remedial measures will need to provide insulation to avoid a "cold spot". This cannot realistically be achieved by using rock dump and a combination of sand and rock dumping may be required. In soft clay terrains the contrast in insulation properties of soft clay and sand may result in the need to provide significantly more DoC then was required for the soft clay backfill. Careful planning and pre-trenching engineering has an important role. It is clearly better to identify areas of concern prior to a trenching campaign. Transitions from trench depth to seabed can be pre-planned and remediation can be given due consideration at an early stage. It is perhaps clear why asset and/or product damage is the most serious consequence that can result from a sloping seabed. In common with most catastrophic or accidental occurrences it likely that other contributing factors play a role, for example poor quality survey data or poor route planning. Damage to a pipeline can occur if a burial asset slides sideways down a slope apply loads to the line. Loads can also be applied at freespans if they are out with an assets capability. Pipeline ploughs pose greater risks of pipeline damage compared to a small trenchers, a pipeline plough can weigh in the region of 150 Te (submerged) and tow forces in region 200 to 250 Te are typical in sandy soils. These large forces can create significant damage in the event of an incident associated with steep seabed slopes. Trenchers are lighter with a low driving force in place of high tow forces. Although clearly trenchers with cutting tools can still pose a significant risk if the cutting tool can contact the pipeline in a slope related incident. Damage can comprise a wide spectrum from minor coating damage to pipeline deformation. Any serious pipeline damage will result in large costs involved with mobilising a repair spread. In addition to direct costs there is also likely to be effects on the schedule of a sub sea development resulting in further significant costs. Damage to a burial asset is also likely to be expensive and time consuming. As with pipeline damage schedule effects can result in significant secondary costs, possibly for numerous other pipeline burial projects scheduled for the same year. EXAMPLES AND CASE STUDIES The previous section provided an introduction to a number of the issues and challenges surrounded pipeline burial. In order to give context and to elaborate on some of these points this section presents examples from pipeline burial projects. As noted, sedimentary features are a common challenge to pipeline burial. The authors have experienced these features on shallow water projects in many areas of the world. Within Europe, sedimentary features are particularly prevalent in the shallow water of the southern

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North Sea both in the UK and Dutch Sector. Three project undertaken in recent years illustrate some important aspects of pipeline burial in this type of terrain. An additional example of a project in a pockmarked terrain is also included. Project A was characterised by sand waves with a wavelength of between 100 m and 500 m and up to 8m high. Project B was characterised by mega ripples with a wavelength of 10m - 15m and heights up to 1m. The importance of the morphology of the seabed rather than the absolute angle or height of features is illustrated by these two projects. Project A with its gradual transitions between slopes was the less challenging project, freespans were rare and the pipeline plough used on this project was able to accommodate the gradual changes in angle between stoss and lee slope. Project B was considerably more challenging, free spans were present at most features and the sharp changes of angles resulted in jet leg rotation for the tracked trencher that undertook burial on this project. Multiple passes were required on project B. Following these multiple passes the required DoL and DoC were successful achieved, in addition there was some evidence of reduction in OOS from the initial as laid profile of the pipeline. Projects A and B also illustrate the issue of survey data quality. On project A the larger features where adequately characterised by a vessel mounted multibeam echo sounder (MBE) survey. In contrast to project B the only information was a survey which it subsequently became clear had failed to adequately characterise the smaller bed forms prevalent on this site, underestimating their size, frequency and the slopes associated with them. An accurate characterisation of these mega ripples was achieved using a narrow corridor high resolution MBE survey undertaken by ROV. The importance of high quality survey data was also illustrated by Site C. For Site C a survey was undertaken some time before offshore operations as part of the project tendering process. During offshore operations it became evident that the initial survey had only characterised the larger feature. Issues surrounding data resolution and data processing had resulted in a failure to characterise smaller bed forms which where associated with the larger bed forms present at this site. Site D was located in a very soft clay area characterised by a pockmarked seabed terrain. Initially a straight line route was adopted for this project. However, the benefits of route optimisation were quickly demonstrated, reducing the angles that the trenching asset had to accommodate with only a moderate increase in route length. The importance of a wide route corridor was highlighted when re-routing went very close to the edge of the survey corridor. A pipeline plough was used at site D to trench a pipeline which was subsequently backfilled with a backfill plough. In addition to the re-routing some benefits in reduced OOS were obtained by making adjustments to the plough when traversing pockmarks. Speeds where reduced to give plough operators time to operate the skid and roller box heights in response to pitch and roll sensors. Skid adjustment resulted in reduced pitch of the asset and changed the trench profile to reduce OOS in these areas albeit with slightly reduce cover height. CONCLUSIONS This paper has discussed the advantages pipeline burial provides and the drivers that can lead to the decision to bury all or part of a pipeline system. The origin of natural and man made slopes have also been presented along with the range of pipeline burial assets that are available to the sub sea installation contractor should burial be undertaken.

If seabed slopes are encountered during a pipeline burial project the problems that can occur range from relatively minor problems that are simple to address with appropriate engineering to catastrophic problems resulting in significant costs and delays. Throughout there has been an emphasis on early front end engineering to address these challenges before they are encountered in the field. As the raw material that feeds into this engineering a route survey that adequately characterises the seabed morphology is extremely important. Time and effort focussed towards this aim is generally well spent. Perhaps the most promising engineering solution discussed in this paper is route optimisation, which in combination with high quality survey data and up front engineering provides a powerful combination to meet the challenges posed by pipeline burial on sloping seabeds. ACKNOWLEDGEMENTS The authors would like to acknowledge the support and encouragement provided by Acergy during the course of writing this paper. In particular we would like to thank the individuals involved with Acergy library services for their assistance in obtaining some references referred to in this paper. REFERENCES Allan, PG, and Comrie, RJ, (2001). “The selection of appropriate burial tools and burial depth,” Suboptic 2001, Kyoto, Japan, May 2001. API (2005). “API Recommended Practice 2SK – Design and Analysis of Stationkeeping Systems for Floating Structures,” American Petroleum Institute. Bolton, MD, and Barefoot, AJ, (1997). “The variation of critical pipeline trench backfill properties” IBC Conference on Risk Based and Limit State Design and Operation of Pipelines, Aberdeen, May 1997. Bruton, D, Carr, M, Crawford, M, and Poiate, E, (2005). “The Safe Design of Hot On-Bottom Pipelines with Lateral Buckling using the Design Guideline Developed by SAFEBUCK Joint Industry Project.” Deep Offshore Technology 2005, Vitoria, Brazil, November 2005. Cameron, TDJ, Crosby, A, Balson, PS, Jeffery, DH, Lott, GK, Bulat, J, and Harrison, (1992). “British Geological Survey – United Kingdom Offshore Regional Report -The geology of the southern North Sea,” HMSO, London. Cifi, G, Dondurur, D and Ergun, M, (2002). “Sonar and High Resolution Seismic Studies in the Eastern Black Sea,” Turkisk J. Earth Sci., Vol. 11, pp. 61 -81. Clarom (1993). “Design Guides For Offshore Structures – Stability and operation of jackups,”. Le Tirant, P and Perol C (eds). Edtions Technip, Paris. Collberg, L, and Levold, E, (2005). “Structural design of HPHT pipelines - new guideline,” Rio Pipeline Conference and Exposition 2005, Rio, 17-19 October 2005. DNV (1988) “Onbottom Stability Design of Submarine Pipelines” Det Norske Veritas. DNV (2002). “RP-F107 Risk Assessment of Pipeline Protection,” Det Norske Veritas.

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