Molded products protect risers, cut VIV strake volumes

Molded products protect risers, cut VIV strake volumes Lankhorst/Mouldings has been active in the marine industry for nearly two centuries, developing and manufacturing rope and mooring products.

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Lankhorst/Mouldings has been active in the marine industry for nearly two centuries, developing and manufacturing rope and mooring products. Following its merger with the Portuguese Grupo Euronete in 1998, the combined company claims to be Europe's largest producer of rope, yarns, and netting.

Since 1977, Lankhorst/Mouldings has been recycling polypropylene and polyethylene waste – from production fibers and yarns – into molded products using a patented low-pressure extrusion process. Most of these products are designed for heavy-duty applications, requiring a high level of safety and reliability.

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Lankhorst's new piggyback block.
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Over the last two years, the company has addressed the needs of the offshore industry, developing a series of products for risers and flowlines. The riser fin protection system is designed to protect the auxiliary lines on the drilling riser from impact when running or retrieving through the rotary table or moonpool, or during handling operations from the riser storage bay. The fins also help extend the weather window for riser handling operations, and decrease riser running and retrieval time, according to Business Development Manager Martin Esselbrugge.

"The riser fins have an outside diameter (OD) equal to the OD of the joints that have buoyancy elements attached. This means that when a bare riser joint with riser fins passes through the rotary table, it is centralized, and the following joint (also with buoyancy elements) is already centralized to the same position. The fins also remove problematic points where the riser gets stuck, or keeps hanging, such as the metal clamps that hold the auxiliary line in place," Esselbrugge says.

The fins, which can be up to 3 m long and with a weight of 50-70 kg, are manufactured from a slightly buoyant plastic (s.g. of 0.85 g/cu m), which means no weight is added to the riser strings once they are submerged. Attachment to the drilling riser is similar to the method employed for buoyancy modules with either polyester or Kevlar banding. A hybrid is under development involving fins with buoyancy elements in between. Drilling contractors using the riser fin system include Pride International and GlobalSantaFe, while Stena Drilling, Diamond Offshore, Transocean, and Mærsk are all evaluating the system.

Over the past few years, joint industry projects analyzing vortex-induced vibration (VIV) in risers have been trying to assess the optimum strake geometry, in terms of pitch and vane height. Lankhorst/Mouldings has developed its own configuration of the 3-Shell VIV Strakes, a VIV suppression system.

According to Esselbrugge, "assembly is optimized in our design through the production of identical components with a low weight of 7.5-15 kg. These are easier for the operators to lift and handle."

Safety is also improved through having the metal banding enter a port in the vane instead of an open sleeve. Should a seal fail during tensioning with pneumatic tools, Esselbrugge says, the metal band will not fly off into the operatords face.

"Furthermore, we have achieved a big reduction in the volume of VIV strakes to be transported, as our tri-shells with hollow vane can be nested. Previous designs were either a hinged, cylindrical shape – bigger even than the riser – or half shells that could not be nested. If you had to install a length of 3-4,000 ft of cylindrical VIV strakes, with a total diameter 1.5 times the riser's (outside diameter) OD, the containers would occupy all available critical deck-space."

So far, the company has been awarded three contracts for this technology. The first was for the Williams Devils Tower spar project in the Gulf of Mexico, where 14-in. VIV strakes were stipulated for the two steel catenary risers (SCRs), with a total length of 3,150 ft. Technip-Coflexip's CSO Deep Blue vessel performed the installation.

The second contract involves supply of 20-in. VIV strakes to SaiBos CML for the oil offloading line on ExxonMobil's Kizomba A project offshore Angola. Here, the first batch of strakes are due to be delivered in December.

The third project covers delivery of 2,000 ft of 16-in. VIV strakes for SCRs on the Phoenix project from GulfTerra Field Services. Installation will be performed by Allseas vessel Solitaire.

Lankhorst's third offshore development is a piggyback block that provides either free movement or axial restriction to piggyback pipelines. Due to the large amounts of blocks involved in these projects (typically 5,000-15,000 pieces), the emphasis was on achieving a simple design with a low weight while maintaining strength and rigidity.

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Pre-molded anode taper for bracelet anodes.
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"The piggyback block has an open top side with enough clearance for the piggy line to enter the block," Esselbrugge explains. "The closure is a piece of 1-in., plastic sheet that is intended to protect the piggyback line from touching the metal banding around the block."

The piggy line is not clamped and can therefore expand freely. By placing a different "piston-like" closure piece in the same block, the piggy line is actually clamped when the metal banding is tensioned.

The prototype design was tested in close cooperation with Allseas, including lateral pull tests. The feedback was used to further improve the design, and the resultant block is currently being deployed on Burlington's Rivers project in the East Irish Sea, where a 4-in. MEG line is being installed as a piggyback on a 24-in. concrete coated pipe.

Other products soon to be introduced into the market include:

  • Pre-molded anode tapers for bracelet anodes, said to be easy for barge personnel to install, and eliminating the need for onboard storage of reactive polyurethane chemicals that carry a risk in terms of health, safety, and the environment
  • Pre-molded field joint infills, as have been developed for the Repsol-YPF Poseidon project, that can be installed easily as an alternative to hot mastic or low density PUF, which require time-consuming curing
  • Plastic intermediate clamps for drilling risers that have a low dry weight, and that do not add weight to the riser when submerged, unlike metal clamps
  • Pipe-in-pipe centralizers.

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