Evaluation Of Marine Pipeline On-Bottom Stability.
The seabed is the pipeline's resting place, literally speaking. The pipeline can be laying on the surface of the seabed or buried in the seabed, or it may be lying on the seabed surface covered with non-local material (rocks, grout bags, etc.). But the seabed is not a hard surface so the pipeline will not necessarily stay where it is initially positioned even when there is no hydrodynamic forcing present. The pipeline may sink into the soft if the downward force exceeds the beating capacity of the bed and may move laterally by sliding if the pipe is installed on a non-horizontal seabed. The movement of the pipeline by seabed failure (slips and slides) is not considered here.
Another hazard for pipelines placed in trenches and covered with backfill material is that they may "float" up through the backfill. The processes responsible for flotation of pipelines have been studied for many years but recently new advances are being made in understanding some of the mechanisms leading to flotation, namely static liquefaction and upheaval buckling. Both these processes are potentially more important now that the pipes are becoming lighter and the hotter products can lead to greater thermal gradients and mechanical stresses.
Subsea developments require the installation of (flexible) pipelines to connect the wellhead to the distribution system and this installation is undertaken by surface vessels ,with diver support or by diverless ROV (Remote Operated Vehicle) technology. In these situations, pipelines are often pulled along the seabed or dragged sideways. Ensuring reliable connection operations requires an adequate knowledge of the seabed soils and configuration of the system being installed. This will help provide accurate predictions of the pulling forces required. Recent advances in the scaling of laboratory scale experiments within the framework of soil mechanics has produced a better understanding of the processes acting and generated better predictions of the loads required to move pipelines over the seabed.
This improved understanding can also be brought to bear on the potential for pipe displacement due to the impact of trawl gear or anchors.
Pipeline Stability Under Hydrodynamic Loading
In this article we will state only one equation--the equation describing the criteria for on-bottom stability of a pipeline (with forces expressed as Newton per meter length of pipeline).
F [is less than or equal to] [Mu] ([W.sub.S]-[F.sub.L]) + [F.sub.p]
where F is the sum of the (horizontal) drag and inertia force on the pipe, [Mu] is the (non-dimensional) friction coefficient, [W.sub.S] is the submerged weight of the pipeline, [F.sub.L] is the lift force (hence the term in brackets is the dynamic buoyancy of the pipeline) and Fp is the passive soil friction (due to pipe embedment or trenching). The force balance is illustrated in Figure 1. So, the equation states that the pipe will be stable if the horizontal hydrodynamic force does not exceed the horizontal component of the soil resistance. If Fp is set equal to zero in the stability equation we get the classical expression (Coulomb) for friction induced by an object over a hard surface. Note that for design purposes, the driving force F is usually multiplied by a safety factor, typically of the order 1.1. In the following we will discuss the soil-related terms on the right hand side of the above equation.
The friction coefficient m is a material constant. Typical values of m for North Sea applications are given in BS 8010: Part 3 (1993). In contrast, the parameter Fp is difficult to determine a priority. The force due to passive soil resistance, Fp emerges due to the fact that the seabed is not a hard surface. The pipeline will usually be slightly embedded in the seabed, even if it has not been trenched, and additional lateral resistance is provided, because in order to shift the pipeline sideways, an amount of soil needs to be moved as well.
The Norwegian Pipestab project and recent physical modeling experiments at HR Wallingford in connection with the Atlantic Frontier Project (Damgaard et al, 1999) revealed some interesting results for passive resistance on sandy (non-cohesive) as well as clayey (cohesive) seabeds. For a cohesive bed an increase in the soil shear strength produces a decrease in the lateral soil friction. The reason for this is that the initial penetration of the pipe into the seabed is important. On hard clay the solid friction analogy is appropriate, an object sliding over a hard surface, and the Fp term vanishes. On less dense clay and mud the loads required to move pipelines will depend on the length of time that the pipeline has spent resting on the seabed. The rate of increase of resistance is important both during subsea installation, where minimum pipe resistance is desirable, and during operation, when maximum resistance may be preferred. A point of practical importance is that the HR Wallingford experiments indicate that physical modeling, i.e. tests performed at model scale and results converted to prototype scale, is a viable option for determining the total soil resistance exerted on a pipeline configuration.
Also the lateral soil restraint for the trenched case has been analyzed at HR Wallingford (Wilkinson et al, 1988). In this approach the influence of the trench is included by modifying the friction coefficient and setting Fp = 0. For trench side slopes at the angle of repose of sandy sediment, for instance 30 degrees, this results in an effective friction coefficient more than twice as large as the non-trenched case.
Behavior Of Sediment Near The Pipe
The presence of waves and currents does not only affect the pipeline structure directly; it also has a profound influence on the seabed sediments near the pipeline.
If the bed shear stress is large enough to mobilize the sediments in the area of seabed around the pipeline, then the presence of the pipe will cause an enhancement of the sediment mobility. Even if the stress is not sufficiently large to mobilize the bed, the presence of the pipe can result in the flow being enhanced locally and the sediment becoming mobile. There are no hard and fast rules about scour around pipelines but the recent advances have been summarized by Whitehouse (1998). The extent to which it happens depends on the magnitude of the wave-current forcing as well as the pipeline and the sediment characteristics. It is important to realize that flow speed-up and enhanced turbulence levels are not only caused by the pipe itself, they can also result from the local flow enhancement caused by rock dump material, and it is therefore necessary to proceed with care whenever introducing protective structures near the pipeline.
Given the design conditions for the bed shear stress and the bottom sediment conditions, the likelihood of sediment movement can be determined. If the sediment is mobile there is the risk of scouting of sediment from around and underneath the pipeline leading to free-spanning with the increased exposure to wave-current forces that this produces. Free-spanning can also occur due to the migration of sandy bedforms, for example sand waves, across the seabed leading to a periodic coverage of the pipeline. A free-span is unlikely to cause immediate rupture of the pipeline unless the pipeline is damaged by an anchor or dropped object. During the three decades of operation of the North Sea fields many free-spans have been observed but to our knowledge none of them have resulted in pipe failure. However, when the natural frequency corresponding to the free-spanning pipe is close to the vortex shedding frequency, the pipe will start to vibrate, thus shortening the fatigue life of the pipe. The amount of shortening depends on the frequency and the amplitude of the vortex-induced vibrations and the sequencing of events.
Another soil process, which is of importance to subsea pipeline engineering, is dynamic liquefaction caused either due to the passing of individual (steep) waves or due to a gradual build-up of pore pressure in the soil due to repeated cyclical forcing, such as wave motion. As a consequence of liquefaction the soil loses its bearing capacity. Hence, a negatively buoyant pipe will sink into the seabed, but what is usually worse is that a buoyant pipe can work its way out of the seabed with potentially hazardous consequences. Although dynamic liquefaction has been studied for more than 20 years, we are still some ways from formulating clear design rules, partially because it is still not dear how important this effect is in the field; the evidence is contradictory. However, via a combination of experience and research done within this topic, it is possible to make a sensible assessment of the liquefaction risks and it is the aim of present and planned research programs to obtain clear engineering guidelines (BGS/ICE Ground Board meeting, 1998).
A number of issues regarding the on-bottom stability of pipelines have been discussed. The research, which has been carried out in the past decade, has reached various states of acceptance and implementation in the pipeline industry. With time it is expected that the recent and relevant research will be included in the industry standards and that the new challenges will generate further requirements for research which in turn will be of benefit to the industry in the future, in terms of more cost-effective design.
BS8010: Part 3. 1993. Code of Practice for Pipelines. Part 3.
Pipelines subsea: design, construction and installation. British Standards Institution. London.
Damgaard. J S, White, J, Worsley, M. 1999. Physical modeling of pull-in loads for subsea jumper installation. Paper accepted for 18th International Conference on Offshore Mechanics and Arctic Engineering--OMAE99, July 11-16, 1999, St John's, Newfoundland, Canada.
Whitehouse, R J S. 1998. Scour at Marine Structures. Thomas Telford, 218pp.
Wilkinson, R H, Palmer, A C, Ells, J W, Seymour, E, and Sanderson, N. 1988.
Stability of pipelines in trenches. Proceedings of the Offshore Oil and Gas Pipeline Technology Seminar, Stavanger.
Jesper S. Damgaard (MSc Eng.) is a Senior Engineer at HR Wallingford responsible for the area of subsea pipeline engineering.
Richard J. S. Whitehouse (BSc, Ph.D.) is a Principal Scientist at HR Wallingford.