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"Pencil Sharp" solution for subsea installation

The drive for innovation and efficiency in offshore technology is taking Aker Marine Contractors to new frontiers. Having established a firm track record for tow and installation of offshore fixed and floating platforms, the company has extended its expertise into devising a leading-edge technique for the installation of subsea structures. Known as the Pencil Buoy Method, the unique and patented system brings significant benefits and cost savings to clients.

Conventional offshore installation methods involve the use of heavy offshore lift vessels (that come at a premium). Already well known for its expertise in marine operations, Aker Marine Contractors has developed a cost-effective and safe technique, designed specifically for the installation of subsea structures, known
as the Pencil Buoy Method. According to Jan Gramnas, Aker Marine Contractors’ VP Engineering who has been closely involved in the project from the outset, the idea came about as a result of several factors.

“There is always a need to minimize time, and therefore costs, offshore. With the Pencil Buoy Method, we are able to lift the structure through the splash zone in calmer inshore waters and perform safe and controlled overboarding. This not only minimizes the risk but also saves on costs by not having to use the significantly more expensive offshore heavy-lift vessels. These vessels are in short supply and may not be available in the right location at the right time. Advanced anchor-handling vessels (AHVs) are normally more accessible in the market due to the number of such vessels.”

Contact


Communications Manager
Products & Technologies
Kristin Wallem Timenes
T: +47 38 05 73 07 M: +47 950 42 117
 

References
Read more about Aker Marine Contractors  here
Read more about Aker Kvaerner's marine operation services here
 
"Pencil Sharp" solution for subsea installation
"Pencil Sharp" solution for subsea installation
"Pencil Sharp" solution for subsea installation

Designing for Innovation
The principal difference between using the Pencil Buoy Method versus a conventional semisubmersible offshore crane vessel (SSCV) is the transportation to the offshore location. While an SSCV will carry the structures onboard or on a separate flat-top barge, the Pencil Buoy Method utilizes a sub-surface tow. A dynamically positioned
AHV is used as offshore vessel in combination with specialized equipment.

The method comprises three principal steps. The first of these involves transfer of the
weight of the subsea structure from crane barge to pencil buoy in calm inshore waters, bringing the buoy into a vertical position. The pencil buoy is a cylindrical structure with an outer diameter of nearly 15 ft (4.5 m) and the hull is constructed
from steel plate with internal stiffeners. The pencil buoy is subdivided into watertight compartments to ensure that the buoy meets the requirements to sufficient residual buoyancy in a damaged condition, in accordance with international marine operations standards. The towing operation is then conducted using the pencil buoy to carry the submerged weight of the structure and rigging to the field. The third and final step is the transfer of the load from the pencil buoy to a winch on the vessel for lowering to and installation on the seabed. The length of towline is approximately 1,312 ft (400 m), subject to adjustments according to observations of structure behavior during
tow. Average towing speed is about three knots. As Gramnas comments, “This is a slower transit speed than with a conventional transport operation using an SSCV. However, the subsurface tow is classified as unrestricted and as a result the longer transportation period does not increase the schedule risk. In addition, the
day rate of the tow vessel is lower.”  On arrival at field, the towing wire is hauled
in and the structure weight is transferred from the pencil buoy back to the towing wire. The pencil buoy is then pulled onto the deck of the vessel and disconnected. During transfer of the structure’s weight from pencil buoy to the winch/winch wire, the vessel moves slowly ahead to ensure no contact between buoy and vessel stern until the buoy is fully unloaded and horizontal. When lowering the load towards the seabed, the wire from the anchor-handling winch to the structure is routed through a heave compensator.

Hands-On Experience
The pencil buoy technique has been developed from several other installation techniques over the years. In 1994, four large tether foundations were installed for the Heidrun tension leg platform using a combination of lowering the structures
and fine-tuning their position above seabed using pencil buoys before the touch
down. Similar methodology was used in 1995 to install the 110,000-ton Harding gravitybased structure for BP in the UK North Sea sector. The installation technique for setting large suction anchors for floating production storage and offloading (FPSO) vessels from the deck of an AHV has also given practical experience that was useful for the development.
One of the first projects to be carried out using the subsea wet-tow method was the
recovery and re-installation of the Heidrun E template. “We were requested to remove the manifold and original template from the seabed and transport them to shore. The pencil buoy system was designed and selected for the job. After a new template was made ready at the fabrication yard, it was installed offshore the reverse way – both operations in combination with an anchor-handling vessel with A-frame,” adds Gramnas. The method has also been deployed to transport and install two subsea
protection structures destined for the Hydro Vigdis field as subcontract to Stolt Offshore.
Last year, the Pencil Buoy Method was selected for the Snøhvit and Norne projects for
Statoil for subsea equipment with up to 250 tons weight-in-water. For these projects the new construction vessel BOA DeepC was selected as the offshore vessel rather than an AHV, making a unique combination to install additional equipment using the vessel’s heave compensated crane.

As part of the Snøhvit engineering and planning, model tests were conducted to assess the motion behavior and adjust the analysis tool. Strain gauge measurements of structural details have also been performed to monitor
high-stress details on the buoy. These measurements have been used for further development of the method and procedures. One of the most recent projects to be
awarded to Aker Marine Contractors is the installation contract for the Langeled Subsea Valve Station (SSVS) – a 180-ton weight-in-water structure that will connect the Langeled pipeline to the Sleipner riser jacket on its route to the UK. The unit will be installed with the Pencil Buoy Method in early 2006. For this operation, a Maersk A class AHV will be used as tow and installation vessel.

Focus on Safety
One of the critical factors for success in developing and safely implementing such a novel technique is an in-depth understanding of offshore technology as well as compliance with and dedication to follow the appropriate standards for
health, safety, environment & quality assurance. “Extensive operation experience is needed for this kind of development. It doesn’t just happen overnight or without a genuine knowledge of the elements involved,” says Gramnas.
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