Achieving Subsea Pipeline Burial And Stability With Spoilers
When offshore pipelines were first laid in the North Sea, all pipelines in the Dutch sector had to be buried. The pipelines were buried by jetting or trenching methods. Pipeline burial ensured that damage to trawl gear and pipelines should not occur.
Trenching and jetting to bury pipelines are costly operations and cause turbidity due to the silt contained in the sandy seabed. Silt clouds may travel over great distances by means of thermoclines and currents, enlarging the area affected by trenching or jetting. The seabed environment may suffer from the detrimental effects of silt precipitation.
Problems are also created by buried subsea pipelines becoming exposed, particularly after violent wave action associated with storms. This may cause stability problems or free spanning which are both potential hazards for the pipeline. An unstable pipeline may fail during violent storms and a free spanning pipeline may lead to fatigue damage due to vortex-induced vibration. Additionally, free spanning pipelines may hinder fishery operations.
New Solutions In The North Sea
In recognition of the problems associated with mechanical trenching, methods of self-burial were considered for use in the North Sea. The idea of applying a vertical fin, or spoiler, on top of a submarine pipeline was considered. An extended period of research and development ensued in The Netherlands during the eighties.
For four years, the process involved small-scale laboratory tests and later flume tests with full-size pipeline sections. From the beginning, there was a need to prove both the functional performance and the cost-effectiveness of the spoiler concept.
Seabed sediment characteristics are important in determining the suitability of pipeline spoilers. Successful self-burial of a plain pipeline (without spoiler) requires a sandy seabed with a maximum of 20 percent silt. Obviously the seabed must be erodible in order for self-burial to occur. Seabed soils containing significant amounts of silt, up to 40-50 percent, may still allow self-burial if the pipeline is fitted with a spoiler. Seabeds with substantial clay or gravel contents are not suitable for self-burial.
But what is a spoiler and what does it look like? Spoilers are assembled on top of a pipeline and consist of two components, the template and fin. These pans are manufactured from impact modified UPVC and are first hand-assembled by engaging the base of the fin into slots of the template (See Fig. 1). The spoiler is then fastened to the pipeline by bands. This is a practical and efficient method which normally takes place after the tensioner and just before the pipe leaves the lay barge.
[Figure 1 ILLUSTRATION OMITTED]
How does the spoiler function? It affects the local flow and pressure field around the pipeline. Mounting a spoiler on top of the pipeline causes the hydrodynamic force coefficient for drag to increase, but the lift coefficient to decrease. Flow over the pipeline is hindered by the spoiler causing flow increase and accelerated scouring underneath the pipeline. Waves and current stimulate the initiation and the rate of development of temporary self-burying free spans produced by the scouring. This results in both sagging of the pipeline in the free span shoulders and deflecting the pipeline into the scour hole.
These following hydrodynamic force coefficients are utilized in stability analysis determinations:
[C.sub.D] = Drag Coefficient
[C.sub.L] = Lift Coefficient
[C.sub.I] = Inertia Coefficient
In conventional on-bed plain pipeline stability calculations, the values [C.sub.D] = 0.7, [C.sub.L]=0.9 and [C.sub.I]=3.29 are widely employed. A large
uncertainty, however, is encountered in applying the CD and CL coefficients. Reasons for this are varying ambient water conditions, determining the current velocity and consistency, the Reynolds number effects, fluctuations of the pressure field near the pipeline and variations in the geometric layout.
Test results of the spoilered pipeline led to the conclusion that the CL lift coefficient reduces significantly during the self-burial process, even to negative lift (becoming a downward force).
During the self-burial process, the pipeline is sustained by rapidly changing sand supports in its self-made trench. The reversed hydrodynamic lift force pushes and pulls the pipe downward. This adds to the overall stability of the pipeline.
See Fig. 2 for the initial position of the spoilered pipeline on the seabed. For tests conducted at ambient current velocities of 0.5 m/sec (1 knot) and higher, the pipeline was lowered by 40-80 percent of its diameter in a matter of hours. As the burial continued, the process of under-tunneling slowed down, as the top of the pipeline and the spoiler assembly protruded less and less above the seabed (Fig. 3). When the first tunnel erosion concluded, the pipeline and spoiler had reached a stable position in its trench (Fig. 4).
[Figures 2-4 ILLUSTRATION OMITTED]
As the testing progressed, communications with pipeline owners, fishery representatives, pipeline engineering companies and the Dutch authorities provided valuable feedback and suggestions which were incorporated into the spoiler design.
As a result of all the testing and dialogue with interested persons in the marketplace, design criteria were developed for the spoilers. The spoiler must:
* Maintain its shape and position on top of the pipeline so as to perform its hydrodynamic functions
* Be manufactured in accordance with internationally accepted standards
* Be light and easy to handle
* Assemble easily on the pipeline without delaying the lay large operations
* Require little storage space
* Not cause any damage to fishing gear
* Withstand crossings by fishing gear
* Be environmentally friendly
* Allow future pipeline repairs, inspection and maintenance
The spoiler design (Shown in Fig. 1) meets the established design criteria. The spoiler is strong, yet flexible so that any interaction with fishing gear before complete pipeline burial, will not damage the spoiler. If the spoiler is pushed aside by fishing gear, it then springs back to its upright position. The ends of the spoiler FLUS are designed with 45 degree beveled edges to minimize potential interference with fishing nets.
Positive Results Of Testing
Summarizing the results from laboratory testing and field testing in the North Sea, pipeline spoilers, in many circumstances, proved capable of solving the problems previously described:
The use of pipeline spoilers enables self-burial of pipelines with erodible seabeds with minimal impact on the seabed environment. Negative effects of silt clouds due to mechanical burial methods are eliminated. The spoiler does not damage fishing gear, and is environmentally friendly to the fishing industry.
Spoilered pipelines on erodible seabeds bury rapidly and reach stable, embedded positions. Therefore, the pipeline is more secure against storms, even in cases where a storm develops within a short period after laying a spoilered pipeline. Increasing near-bed current and orbital velocities from storms actually assist in maintaining burial of spoilered pipelines.
3. Cost Savings
Even more important are the cost savings. The use of spoilered pipelines is less expensive than artificial trenching methods. Additional savings come through a possible reduction in the concrete weight coating, which can, under certain conditions, be reduced as much as 50 percent. In addition, this results in easier handling and transport because the pipe is lighter.
Subsequently, a substantial number of subsea pipeline projects in the North Sea, approximately 100 kilometers, have been furnished with pipeline spoilers with good results. The spoiler has consistently produced accelerated tunnel formation and pipeline burial after pipe laying. The adjusted hydrodynamic forces assist the self-burial process and reduce free spanning problems.
Later Spoiler Design Innovations
Soil anchors were developed and successfully applied to flexible pipelines. This feature increased the soil friction after pipeline embedment to reduce the risk of fatal upheaval buckling. The soil anchors appear to have an added .benefit of stimulating the burial process.
Another spoiler development has been the piggyback spoiler (See Fig. 5). This design effectively closes the space between a main pipe and its piggyback line. This spoilered approach provides the same benefits as a standard spoiler and additionally protects the piggyback.
[Figure 5 ILLUSTRATION OMITTED]
New Challenges/New Solutions
Now that the effectiveness of spoilers has been successfully proven and applied to pipelines in the North Sea, the process may be extended to many worldwide pipeline projects.
More often, pipelines and cables will be laid through environmentally sensitive areas which are frequently morphologically dynamic as well. In many cases, dredging is considered but is less attractive due to the negative environmental effects. Regulatory design parameters of pipelines now allow alternative designs, providing the option to avoid dredging if a spoiler is applied.
Cable burial design and installation methods can be optimized and be less expensive if spoilers are applied in combination with advanced monitoring systems.
Although the major purpose of this article is to explain how spoilers effectively achieve subsea pipeline and cable burial and stability, it is evident that spoilers can be utilized in other subsea applications. Such opportunities will emerge in the future.
[1.] Det Norske Veritas (DnV): “Rules for Submarine Pipeline Systems,” 1981.
[2.] OTC 6154 “Effect of Spoilers on Submarine Pipeline Stability,” by C.H. Hulsbergen and R. Bijker, Delft Hydraulics, Offshore Technology Conference, 1989.
[3.] Stimulated Self-Lowering of Submarine Pipelines (based on 10 years experience), by Romke Bijker and Jaap de Vries, OPT, February 1995.
Romke Bijker is General Director of Alkyon Hydraulic Consultancy & Research, Emmeloord, The Netherlands, and an expert on pipeline/seabed interaction. His e-mail address is firstname.lastname@example.org
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