The performance of pipeline ploughs traversing seabed slopes
Introduction
Offshore pipelines are often buried within the seabed in order to protect them from fishing activity, boat anchors, environmental loading and to mitigate upheaval buckling (UHB). One method of post-lay burial is to create a trench into which the pipeline is lowered by pulling a large “V” shaped plough through the seabed (Palmer et al., 1979) using a trenching support vessel. Burial of the pipeline can be achieved by running a backfill plough over the pipeline route and pushing the spoil heaps, generated at the edge of the trench, back into the trench (Cathie et al., 1998). The method has the advantage of being able to bury a plough in a wide range of soil conditions, in a continuous process with little mechanical intervention. Typical ploughing rates may be in the range of 150–1000 m/h, but will be slowed if more difficult soil conditions are encountered such as dense, silty sand (Cathie and Wintgens, 2001) where mobilisation of the maximum support vessel tow force (bollard pull) will limit the rate of progress. To avoid slow progress (or reduce tow force) it is normal in these soil conditions to limit the ploughed depth to 1.5 m below seabed. Where deeper ploughing is required a multi-pass approach is often used to avoid these issues (Machin, 1995) or the use of new generation high bollard pull vessels.
Following installation, the pipeline is likely to be used to transport high-temperature hydrocarbons. This causes a temperature change which may lead to upheaval if insufficient vertical soil restraint is supplied by the soil cover (Morrow and Larkin, 2007). Upheaval buckling is exacerbated with initial imperfections of the pipeline due to ‘out-of-straightness (OOS)’ (i.e. a lack of level trench leading to reduced soil cover) of the trench base (Cathie et al., 2005). Consequently, the required depth of cover is dependent on the properties of the soil backfill and the out-of-straightness achieved during trenching. The out-of-straightness may be significantly influenced by the plough encountering various seabed geohazards such as sand waves or sloping seabeds for instance.
The current understanding of offshore pipeline plough behaviour has been gathered from small-scale beach tests (Grinsted, 1985, Reece and Grinsted, 1986), back-analysis of ploughing projects (Cathie and Wintgens, 2001) or by laboratory testing at various scales (Bransby et al., 2005, Brown et al., 2006, Lauder et al., 2013). Initial attempts have been made to investigate behaviour using numerical simulations (Peng and Bransby, 2010) but these efforts are limited by the large strain nature of the problem (Cortis et al., 2017) or have focused on structural performance of the plough (Wang et al., 2015). Beach tests and back analysis of trenching projects have the uncertainty of unknown soil properties (e.g. typical geotechnical site investigation may only involve one cone penetration test (CPT) per kilometre for a pipeline route) and the difficulty of deconvoluting the elements of plough behaviour. Alternatively, 1 g scaled laboratory simulations and testing has the advantage of allowing careful control of both soil conditions (e.g. soil types, relative density, surface profile) and plough settings (e.g. plough depth and velocity) to allow parametric study, albeit introducing the need to consider scaling issues (Lauder and Brown, 2014). By combining these and other findings there is relatively good understanding of the force-trench depth-velocity relationships in uniform, level seabed conditions (Palmer, 1999, Cathie and Wintgens, 2001, Lauder et al., 2012, Lauder et al., 2013).
To date though very little work has been carried out examining the effect of geohazards for example either non-uniform seabeds (i.e. layered soils) (Bransby et al., 2005) or non-level seabed surfaces on pipeline plough performance. The study of non-level or inclined seabeds has previously focused on the investigation of the performance of ploughs in sand wave fields (Allan, 2000, Chen et al., 2001, Bransby et al., 2010) but has not considered seabed slopes. There is little current guidance in the public domain on the offshore industry's approach to ploughing on slopes (down or across) or how decisions are made on the viability of ploughing on slopes. Anecdotal evidence suggests that normally cross slope ploughing on slopes up to 5° does not pose any significant risk to operations but that at slopes steeper than this specific assessment of viability is required. With this apparent lack of guidance to allow industry to make informed decision making on cross slope ploughing (which might be encountered on continental slopes or in smaller geohazards, e.g. pock marks, iceberg scour etc.). The response of ploughs traversing slopes has been investigated by performing a series of 1/50th-scale laboratory tests in which the slope angle was varied from 0 to 30°. This paper reports the experimental methods used, the results obtained and discussion of implications for ploughing practice.
Section snippets
Results: flat bed case
The result from the shallow depth testing for the flat seabed case (β = 0°) are presented initially to show typical plough behaviour and allow later comparison with the sloping seabed results (Fig. 4, Fig. 5). Fig. 4a shows the plough depth and tow force variation as the plough is towed through the soil. Plough depth is defined as the vertical distance between the deepest part of the plough and the initial soil surface level. The plough starts with a small plough depth (Dplough ≈ 3 mm) before
Tow forces
Fig. 6 shows the measured relationship between tow force and plough movement for the tests with different slope angles ploughing at medium depth. The general response of each test shows an increasing tow force before steady-state towing conditions are reached. However, there seems to be a reduction in average tow force for the larger slope angles and this behaviour is compared in Fig. 7a and b with both trench depth and plough depth. The average tow force was determined by averaging the tow
Discussion and implications for practice
Despite the very high slope angles approaching the soil's angle of friction, the plough remained stable. The plough roll was found to be approximately equal to the slope angle (roll, α ≈ slope angle, β). This implies that the skids remain flat on the inclined soil surface and that the ploughing operation itself is possible. Unfortunately, although the plough is stable the plough runs parallel to the towing point but offset down the slope so that there is an out of plane towing angle (η) which
Conclusions
A series of laboratory model tests were conducted to investigate the performance of pipeline ploughs on sand slopes. For the conditions tested, the tests revealed that although the model plough could operate up to slope inclinations of 30° the depth of the final trench formed was severely reduced due to trench collapse at slopes inclined at 10° or steeper, whereas plough tow forces and depth were relatively unaffected. Where trench collapse occurs prior to pipeline touch down this would result
List of symbols
- Dtrench
- Trench depth
- Dplough
- Plough depth
- F
- Tow force
- L
- Tow wire length
- Vsup
- Spoil heap volume up-slope
- Vsdown
- Spoil heave volume down-slope
- Vt
- Trench volume
- Atc
- Volume of collapsed soil into trench
- x
- Horizontal position along trench line
- y
- Horizontal position perpendicular to trench axis
- Δy
- Down-slope plough or trench movement
- z
- Elevation compared to the box base
- α
- Plough Roll
- β
- Slope angle
- δ
- Plough yaw
- η
- Out of plane tow angle
- θ
- Plough pitch
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- 1
Present address: Fugro, Level 15 Alluvion Building, 58 Mounts Bay Road, Perth WA 6000, Australia. [email protected]
- 2
Present address: NYC Constructors Inc., 110 East 42nd Street, Suite 1502, New York, NY10017, USA. [email protected]
- 3
Present address: Cathie Associates Limited, Collingwood Buildings, 38 Collingwood St, Newcastle upon Tyne, NE1 1JF, UK. [email protected]