Prevention of pipeline floatation during dredge-based backfilling

Prevention of pipeline floatation during dredge-based backfilling

Biemans, S.

Van Rhee, C. (mentor)
Van Paassen, L.A. (mentor)
Hendrickx, P.H.A. (mentor)
De Jager, R.R. (mentor)
Jacobs, W. (mentor)

Civil Engineering and Geosciences

Geoscience & Engineering

Geo-engineering

2012-08-21

Underwater pipelines have become a vital part of modern civilisation. Transport by pipeline is relatively inflexible compared to other means of transport, however it consumes less energy. Most offshore pipelines carry oil or gas, but they can also transport water or other fluids. More and more oil and gas is produced from offshore fields. The product has to be carried onshore and that is usually done by pipeline. Pipelines can be laid uncovered on the seabed, or can be embedded in the seafloor when stabilisation or protection is required. Pipelines are primarily covered when they are located in shallow waters. The material used to refill an excavated trench is called backfill. The typical material used as artificial backfill is coarse granular soil (rock, gravel, or coarse sand). When there is no suitable backfill material available close to the pipeline route, then material has to be transported from another location. A subsea pipeline should remain stable during its entire lifetime. Normal engineering practice focuses on the operational phase, while the installation phase is almost neglected. However, this study focuses on the installation phase and particularly on dredge-based backfilling. Dredge-based backfilling means using the suction pipe of a Trailing Suction Hopper Dredger (TSHD) as discharge pipe. It is not always clear if a particular soil can be used as backfill material. In the past this lack of knowledge resulted a few times in pipeline floatation during the installation phase. This lack of knowledge can also result in the disposal of perfectly suitable backfill material. A pipeline can be lifted up from the bottom of the trench if the weight of the pipeline is lower than the weight of the liquefied backfill. Pipeline floatation occurs if a soil-water mixture remains liquefied over a too large distance. In general, coarse granular soils remain liquefied for only a short period of time. If the backfill is composed of fine material, it will take a relatively long period of time before the soil settles and develops structural density. G.C. Sills (1998) defines structural density as the density which marks the change from a fluid-supported suspension to a soil in which effective stresses exist. The subject of this study is prevention of pipeline floatation during dredge-based backfilling. It is therefore necessary to understand which characteristics cause pipeline floatation. Due to economic reasons, pipelines have become lighter than a typical soil-water mixture. This is a potential risk if the sedimentation time of a soil-water mixture is relatively long. Sedimentation time is the time required for soil particles to settle out of suspension. The sedimentation time and the backfilling process are analysed using the two dimensional (horizontal and vertical) sedimentation model (2DV model) developed by Van Rhee. The 2DV model is designed to simulate the sedimentation process inside a TSHD. However, in this thesis the model is used to simulate the backfilling of an offshore trench. To simulate this process correctly, it is necessary to implement a moving discharge point in the model. Loads due to waves and currents are not considered, since pipelines are usually installed during relatively calm weather. These considerations make it possible to use the 2DV model. The backfilling process is studied in three steps. First, the process is simulated as a one-dimensional sedimentation test. Secondly, the backfilling of a trench using a stationary TSHD is evaluated. Finally, the backfilling of a trench using a moving TSHD is simulated. The moving TSHD tests produce a distribution of the soil-water mixture density. This distribution is converted to a grid which is used as input for the beam-model (Matlab). The beam-model simulates pipeline displacement at a certain moment during the backfilling operation. The beam-model is based on the theory of the beam on elastic foundation. In this model only static loads are considered; the own weight of the pipeline and the buoyant force caused by the displaced fluid. The beam-model combined with the 2DV model provides an approach to model pipeline displacement during dredge-based backfilling. The 2DV sedimentation model is tested to its limits in this thesis. The simulation of the backfilling operation of a moving hopper requires a model area with a great length and a relatively small height. This results in a model composed of stretched grid cells (in longitudinal direction). The 2DV model is at his numerical boundaries due to these stretched grid cells. The results of the ‘moving hopper’ simulations are therefore questionable. The results of the static simulations can be considered as reliable, since in these simulations the 2DV model remains well within the boundaries.

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Student theses

master thesis

(c) 2012 Biemans, S.