Pipeline Free Span Mitigation

During pipeline routing evaluation, consideration has to be given to the shortest pipeline length, environment conservation, and smooth sea bottom to avoid excessive free spanning of the pipeline. If the free span cannot be avoided due to rough sea bottom topography, the free span length must be corrected. Free spanning causes problems in both static and dynamic aspects. If the free span length is too long, the pipe will be over-stressed by the weight of the pipe plus its contents. The drag force due to near-bottom current also contributes to the static load.

To mitigate the staticspan problem, mid-span supports, such as mechanical legs or sand-cement bags/mattresses, can be used. Free spans are also subject to dynamic motions  induced by current, which is referred to as a vortex induced vibration (VIV). The vibration starts when the vortex shedding frequency is close to the natural frequency of the pipe span. As the pipe natural frequency is increased, by reducing the span length, the VIV will be diminished and eliminated. Adding VIV suppression devices, such as strakes or hydrofoils, can also prevent the pipe from vibrating under certain conditions. The VIV is an issue even in the deepwater field since there exists severe near-bottom loop currents. To prevent static and dynamic spanning problems, a number of offshore pipeline spanning mitigation methods in Table 3 have been identified.

Based on soil conditions, water depth, and span height from the seabed, the appropriate method should be selected. If the span off-bottom height is relatively low, say less than 1 m (3 ft), sand-cement bags or mattresses are recommended. If the span off-bottom height is greater than 1 m (3 ft), clamp-on supports with telescoping legs or auger screw legs are more practical.

Scour Protection

Underwater pipeline protection is of major concern for the Oil and Gas operating companies. They carry high pressure crude oil, gas, and products and hence their health is of major concern. Given the amount of threat present in the underwater pipeline, the following protection :

Rock and Gravel Dumping

Rock and gravel dumping provides a protective layer of rip- rap around the pipeline. Various installation techniques have been employed from the surface vessels, namely:

From a side- dumping barge or vessel with individual stones falling to the sea bed, From a split- hopper barge as one big mass, From a barge through a pipe to reduce the fall velocity of the rock and improve placement accuracy.

The type of material req uired and the amount of material required is determined by the site location and hence the method of deployment depends on it. The material used to form the protective layer must offer sufficient resistance to withstand the flow induced forces (enhanced sh ear stress, vortex action). The stability of the local sea bed material can be calculated based upon knowledge of the local flow field around the pipeline.

Mattresses

Prefabricated mattresses have been used in bed protection or preparation schemes. These

mattresses can be installed in a controlled manner as compared to the rock dumping method.

Mattresses are often used to provide the much needed protection to pipelines but it can also be adopted for other sea bed structures. One of the major advantages of using mattresses is that they are flexible and can be laid to suite the local bed contours.

The various types of protective mattress used for pipeline protection are:

• Fascine mattress: it synthetic filter fabric strengthened with synthetic or natural fascines, usually overlain by rock dump material
• Block mattress – it continuous array of concrete blocks held together by cables and laid on the sea bed or individual blocks held in it pattern on the sea bed by synthetic nails
• Cell mattress — mesh baskets filled with sand or gravel, large rocks in large wire mesh also called gabion baskets
• Concrete mattress – the mesh baskets of the cell mattress are filled with underwater concrete instead of ballast
• Stone asphalt mattress – a synthetic filter fabric ballasted with it stone and asphalt mixture
• Ballast mattress — a heavy synthetic fibres woven mattress is double folded at both sides and tilled with sand or gravel.

Whilst the rock rip- rap is held in place due to its own weight and resistance between the rock and the underlying layers. The mattresses are often held in place by the use of soil pins or anc hors.

The resistance to pull out presented by the soil fixings, or the tensile strength of the material joining the mattress to the anchor, is designed to resist the uplift and drag due to hydrodynamic forces. Poorly designed fixings have historically been the most common cause of failure of these types of protection devices. Steps are often taken to fill any unevenness in the bed beneath the structure and this provides additional protection

from scour.

Trenching (Pipelines) Or Increasing Structure

Embedment Trenching of pipeline or increasing the structure embedment in the sea bed provides the much needed sheltering from wave and current forces which cause scour. This increases the pipeline stability and also the margin of safety against the undermining by scour as a result of greater soil-structure interaction.The trenching of pipeline into the sea bed not only provides protection from the scour activity but it also reduces the hydrodynamic load on the pipeline thereby adding stability to it. The increased embedment of the pipeline results in reduced flow fields around it when compared to pipeline resting on initial bed level. When trenching or increased embedment of pipelines are not feasible in harsh environmental conditions it may be necessary to stabilize them with anchors.

In a region of active sediment movement or sand waves, sea bed needs to be ploughed flat prior to the installation. This causes the sand waves to reform and migrate over the pipeline resulting in changes to the pipeline cover over time, thereby providing cheap and effective way of protecting and stabilizing a pipeline. The embedment process can be enhanced with the use of a spoiler placed along the top of the pipe.

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