Pipeline Towing – The lengths we go to!

Land & Marine is one of the most experienced contractors in the world in the use of towing methods for pipelines, flowlines and bundles for water, oil, gas, LPG or LNG applications. In this article Tim Ley and David Reynolds explain the opportunities and advantages opening up by new developments for shallow water applications.

What are towed bundles?

Towed bundles have been installed for several decades and bundling is a well known technique for oil and gas developments. The method originated in the North Sea and Gulf of Mexico and has been used offshore West Africa, Australia and South East Asia. Today, shallow water applications are of particular interest. As the requirement for LPG and LNG import and export terminals grows, so does the interest for multiple pipeline towed installations.

The demand for loading and offloading lines for oil, gas, LPG and LNG is increasing as is the requirement for water intake lines and waste water discharge pipes or outfalls. Whether these pipelines are steel or polyethylene, if they can be towed into position there can be great advantages to a project. Fabricating and testing the entire assembly on land before commencing any marine operations can: minimise the time spent working offshore
give the contractor the opportunity to select the best weather window possible.
capitalise on using local skills and resources so that costs and risk exposure can be better controlled.
mean that specialist installation pipe-lay vessels are not required.

What is the best towing method?

All tow methods assume that an onshore site is available with suitable access to launch the pipeline into the sea. The bottom-tow method, commonly used for crossings and landfalls, is the simplest and best known. For stability and to prevent damage to any anti-corrosion layers, a concrete weight coating is often specified. With the off-bottom tow method, the pipeline or bundle is designed to float a short distance above the seabed, thereby keeping the pipe well clear of any abrasive soils, rock or debris. Pull forces can be minimised and concrete coatings eliminated.

For longer distance installations, either the Controlled Depth Tow Method (CDTM) or Land & Marine’s Flow-Lay® method may be more appropriate, especially if there is existing seabed infrastructure to cross. With CDTM, a number of flowlines (and umbilicals) are installed within a steel carrier pipe, and can be up to 7.5km long. Fitted with towhead-manifolds at either end, bundles of this type are designed to be towed below the sea surface to reduce the effects from waves. They are ‘delivered’ and installed in a short time scale in comparison with conventional methods. The steel carrier pipe mitigates the effects of upheaval buckling by constraining the flowlines. Being relatively large in diameter, the carrier pipe is resistant to damage from ice flows or fishing activities and provides protection for the internal production lines against dragged cables or dropped objects. With the Flow-Lay® method, the pipelines or bundles are designed to be towed on the surface with the technique being particularly suited to shallow water installation.

What recent pipe towing developments have taken place?

In regions such as the north Caspian Sea, towed pipelines or bundles solutions are seen as offering great potential. Land & Marine’s Flow-Lay® method was originally developed for the North Sea, to install flowlines or bundles of flowlines up to 12 km long into 150m water depths. The technique is extremely adaptable and appropriate for very shallow water because:
the steel carrier pipe can be replaced by temporary plastic buoyancy pipe.
a flat-pack cross-section may be a most effective arrangement depending on the nature of the product lines and the duties involved. as a surface towing technique, Flow-Lay® only requires tow and trail tugs.
accurate laying of pipelines is achieved by controlled flooding.

To date the longest pipeline installed by Land & Marine using the Flow-Lay® method, is a 5.5km long 20” offloading pipeline at La Libertad in Ecuador. With 3” of concrete coating and an overall diameter of 26” (660mm), the pipeline was laid from the beach out to a Single Point Mooring buoy along a lay corridor that was curved to avoid rock outcrops. This project proved that the Flow-Lay® technique is ideal for remote locations, where other methods may not be as flexible or available, and where the use of local resources is highly desirable from an economic stand-point. The method also offers the advantage that installation is quick and economical, minimising disruption to others in the field.

Towed pipeline techniques such as Flow-Lay® can be successfully utilised in the Caspian and Black Sea and many other areas around the world, where lay-barges and other specialist vessels may not be readily available. The Flow-Lay® method is suitable for long lengths of multiple pipeline and umbilical installations.

What about applying the new methods for LPG or LNG/offloading systems?

LPG import and export facilities require very high performance insulation systems and these are often specified as pipe-in-pipe, where fabrication is often more suited to onshore construction which allow long lengths to be assembled and tested before being launched. The Camisea Project, Peru, was one of the first contracts to utilise subsea pipelines to export propane, butane, naphtha and diesel. A 3.2km flat-pack bundle comprising 4 pipelines – including twin 20”/24” pipe-in-pipes - and umbilical, was constructed onshore and once assembled, the entire structure weighed over 5,500 tonnes. Land & Marine installed the bundle in a dredged trench in less than 3 days! No trestle or jetty was required and the pipeline was safely buried into the seabed. Today tankers berth at the offshore terminal mooring dolphins which are the only visible sign offshore that the pipeline off-loading facility exists.

The towing method required will depend on the application – all the techniques described above have been successfully executed by Land & Marine. Please visit our web site www.landandmarine.com for further details.