The paper gives a brief history of pipeline burial in the North Sea to establish the parameters for deburial and indicates why this activity is increasing. A review of past and present deburial techniques and equipment is given together with their most Important features. The paper concludes with a look at future developments.
Since the first North Sea gas export pipeline was laid some 21 years ago, oil and gas companies have been obliged to bury their pipelines for reasons of stability in strong currents, protection from impact damage, thermal insulation and to meet governmental requirements.
The earliest pipeline burials were carried out in the mobile sand and shell soils of the shallow Southern North Sea. Any visitor to the Southern Sector In the late sixties and early seventies will recall the familiar pipelay barge followed several miles later by the unmistakable profile of a jet barge with its'pipe burying "claw".
These units, when towed along the pipe, blasted huge volumes of high pressure water below and to the sides of the pipe and disgorged fluidised sand through large airlifts either side of the pipe. The effect was to half trench, half fluidise the ground and allow the pipeline to settle 1 to 2 metres below the natural seabed. The weight of the "claw" often assisted by pressing the pipe into the fluidised soil. This work was normally only undertaken on unpressurised pipelines.
Later more sophisticated low pressure soil fluidisation machines were developed, like that of the Netherlands Offshore Company that fluidised large areas of seabed and allowed the pipeline to sink under its own weight. Some pipelines were thus burled even more deeply, up to 3 metres on occasions.
Subsequent movement of large sand waves across these areas resulted in pipelines being covered by a total of 5 metres or more of soil. At other times subsea conditions reversed this trend and left pipelines exposed on the surface and the battle was on to backfill scours, support freespans and rebury exposed sections.
At a time when the objectives were always to lay, bury and test pipelines to bring fields on stream as fast as possible and to keep pipelines burled, there was little incentive to develop methods of reversing the process or to dig up live pipelines.
When oil and gas production moved northwards into the much deeper Northern North Sea different seabed materials were encountered, silts, silty clays and boulder clays.
These soils with shear strengths often between 100 and 200 KPA (eg Fulmar 130–160 KPA) but sometimes exceeding 400 KPA and resembling tough, hard plasticine, could not be trenched with the traditional let barge and demanded a different approach to burial.
A new generation of remotely controlled caterpillar tracked trenching machines and towed ploughs evolved which employed mechanical digging devices and furrow forming blades much more suited to these cohesive soils.