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Evaluation Of Marine Pipeline On-Bottom Stability.

Many of the world's large established offshore oil and gas fields are being depleted and nearing the end of their productive phase. Therefore the hydrocarbon production is shifting its emphasis to deepwater fields and small marginal fields that were previously financially and technically unviable. This development has led to an increase in pipeline activity to transport the products and world wide offshore pipeline length is expected to exceed 100,000 kilometers by 2005 (Jee, 1998). Coupled with this is the trend for the development of less heavy pipes transporting higher temperature and higher pressures products. These factors pose new technological challenges to subsea pipeline engineering. In addition some of the existing fields are transformed or extended by satellite production, using pipelines to connect subsea completion units, while the abandonment of other fields is undertaken within the statutory framework for long-term stability and integrity of pipelines left on the seafloor.

The financial and technological constraints provide strong incentives to seek improvements in designing, installing, maintaining and abandoning (or removing) pipelines in the most cost-effective and appropriate manner. The technological gains enable us to explore and eventually formulate better design and maintenance principles, resulting in cheaper and better pipelines. Since the early '80s the cost of subsea pipelines has decreased by approximately 8 percent (Taylor, 1997) per year. The benefits of research are to reduce conservatism in design by narrowing the safety factors on existing methods without posing a safety hazard or by developing new approaches. The `limit state design' and the 'strain based design' for determining structural and material properties of pipelines are manifestations of this design approach.

In this article we will focus on one important aspect of subsea pipeline engineering, namely the processes and problems related to on-bottom stability of the pipeline. The purpose of the article is twofold: to provide the reader with an overview of the issues involved in determining on-bottom stability of pipelines, and to highlight recent advances that will be of use to the industry in view of the preceding comments on design improvements.

The article is divided into two parts. Part I deals with the hydrodynamics aspects, that is, the environmental conditions and the hydrodynamic forces. Part II, which will be published in an upcoming issue of P&GJ, is dedicated to soil/sediment issues. A schematic view of the various inter-related processes is given in Figure 1. We have deliberately kept the number of references to a minimum but we will be more than happy to provide more background information, including references, on request.

Definition Of The Environment

The hydrodynamic forces on the pipeline and on the seabed are functions of the wave and current climate. It is of paramount importance to correctly predict the forces imposed on a pipeline since they have a direct bearing on the safety and the economy of the project. Therefore the first step in an assessment of the on-bottom stability is a definition of the environment along the pipeline route. To this end reliable and accurate prediction methods must be applied to avoid overdesign.

The waves can be either locally generated wind waves or swell and the currents can be tidal currents, as e.g. in the southern North Sea, or circulation currents prevalent in some deepwater.

Only rarely are actual measurements of the wave and current-climate available for more than a short period of time compared to the design life. Therefore wave height, period and direction, and water level have to be estimated in some manner. At HR Wallingford this wave hindcasting (Hawkes, 1987) is carried out routinely to determine the wave climate on the basis of local wind-speed, direction and the seabed bathymetry.

Assuming that the tidal flow is the main cause of the current experienced at a particular site, the relevant quantities can be obtained via computational modeling of tidal flow or estimated as a first approximation from tide tables and tidal streams atlases or Admiralty Charts.

Hydrodynamic Forces

The waves and currents exert forces on

[Sigma] The pipeline,

[Sigma] The seabed sediment.

The forces are essentially the same: drag (skin friction and form drag), inertia and lift. However, the force on the sediment is expressed as a bed shear stress, i.e. force per unit surface area of the seabed. The hydrodynamic forces on the pipeline and the bed shear stress are determined in quite different ways. In either case the process is non-linear and it is not a trivial task to calculate the magnitude of these parameters.

The hydrodynamic forces on pipelines are most often calculated using the Morison equation. This approach has now been in use for more than three decades and the feed-back from the industry and the operators indicates that generally the predictions obtained via use of the Morison equation corresponds fairly well with what has been observed. At HR Wallingford the coefficients used for the Morison equation were determined through prototype scale experiments and field measurements (Wilkinson and Palmer, 1988).

One area that has received much attention is the (non-linear) way in which near-bed currents and the wave-induced oscillatory motion interact and the resulting input conditions for calculating the forces. When the bottom orbital velocity is less than the magnitude of the steady current then the flow direction does not reverse and the mean current profile is advected back and forth. However, the largest wave-current interaction is found with relatively large wave motion in the presence of small currents when the flow direction is actually reversed during a part of the wave cycle. The Morison coefficients have been evaluated directly for the combined wave current scenario both for the case where the flow direction does not reverse and where it does reverse (Wilkinson et al, 1988).

Recent research at HR Wallingford has resulted in simple methods of predicting the bottom shear stress under a given set of input conditions for waves and currents (Soulsby, 1997) which has also been implemented in a methodology for determining scour potential (Whitehouse, 1998).

Design of pipelines require information on the maximum level of forcing expected within the design life of the pipeline, say, 100 years. It is quite rare that information on the joint probability of wave heights, periods and current speeds is available from measured records of greater than a few years duration and a degree of extrapolation is required. In this situation design practice (DnV: On-bottom stability design of pipelines, 1988) prescribes the use of the 100 year return condition for wave-induced bottom particle velocity combined with the 10-year return current condition, in the case of wave dominated forces, or vice versa in the case of current dominated forces. Assuming that the waves and currents are statistically independent--and this is indeed a justified assumption in most offshore conditions--this design condition is typically many orders of magnitude rarer than the 100-year condition for the joint probability of waves and currents. The result is overdesign, the degree of which is determined by local conditions.

In order to address this problem, HR Wallingford has recently developed a model that uses a Monte Carlo simulation technique to yield results of the return conditions for the combined wave and current induced forces and stresses. The results so far look promising and could lead to potential improvements in pipeline design. P&GJ

REFERENCES

Det Norske Veritas (DnV). 1988. RP E305: On-bottom stability design of submarine pipelines.

Hawkes, P J. 1987. A wave hindcasting model. Conference on modeling the offshore environment. Society for Underwater Technology, April 1987.

Jee, T. 1997-1998. Subsea Pipeline Engineering. Course notes. Trevor Jee & Associates.

Soulsby, R L. 1997. Dynamics of Marine Sands. Thomas Telford, 250 pp.

Taylor, K. 1997. Developments in the pipeline: an oil company perspective. The Pipeline Industries Guild Prestige Lecture. Pipes & Pipelines International, Vol. 42, No. 6. Scientific Surveys Ltd.

Whitehouse, R J S. 1998. Scour at Marine Structures. Thomas Telford, 218 pp.

Wilkinson, R H and Palmer, A C. 1988. Field measurements of wave forces on pipelines. Proceedings 20th Offshore Technology Conference, Houston, 389-398.

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 is a Senior Engineer at HR Wallingford responsible for the area of Subsea Pipeline Engineering. He has carted out consultancy studies around the world, coving the various aspects of on-bottom stability of pipelines. He is also doing research on hydrodynamics and sediment transport mechanics.

Richard J. S. Whitehouse is a Principal Scientist at HR Wallingford. He works on a wide variety of projects related to marine sediments with particular specialisation on seabed scour around pipelines and other seabed structures. He has recently written the book "Scour at Marine Structures".
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Author:Damgaard, Jesper S.; Whitehouse, Richard J.S.
Publication:Pipeline & Gas Journal
Geographic Code:00WOR
Date:Mar 1, 1999
Words:1448
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