The Northeast Gateway Deepwater Port, Excelerate Energy’s second buoy-based offshore receiving facility, officially was placed in service in May 2008. It lies in Massachusetts Bay 13 miles (21 km) offshore Boston, Mass., and provides access for LNG carriers to Northeastern US markets.

anchor chain

With a gypsy winch installed above the hold and a chain chute to guide the chain over the vessel’s side, the 26,247 ft (8,000 m) of 134 mm anchor chain (total weight of 3,000 metric tons) was loaded onto one section of the tank top and the “‘tween deck.” (Images courtesy of Jumbo Offshore)

The infrastructure of Northeast Gateway consists of a dual submerged turret loading (STL) buoy system and a 16-mile (24-km) pipeline connecting into the existing HubLine pipeline operated by Algonquin Gas Transmission. Specially designed LNG carriers dock on one of two permanently anchored STL buoys that connect via flexible risers to a subsea pipeline system. Each buoy is held in place by eight mooring lines consisting of a chain segment and a 558-ft (170-m) length of spiral strand wire (SSW) attached to the buoy.

The STL system is a crucial technology component in the system. When no vessel is attached, the buoys float 98 ft (30 m) below the surface. When an LNG carrier arrives, it pulls one of the buoys into a conical shaped opening in the vessel’s keel. The re-gasification process begins, and LNG (returned to its gaseous state) is fed through a secure connection into the local gas distribution grid.

Jumbo Offshore (Dutch) and Norway’s APL installed the two STL buoys. In less than 40 days, the Jumbo Javelin installed 16 suction anchors, including the world’s largest, laid 4.9 miles (8 km) of anchor chain, and prepared two 150-ton STL buoys.

Cooperation
In January 2007, Jumbo Offshore signed an agreement with APL to jointly execute the installation of the two APL STL buoys and associated moorings using the DP2 heavy lift vessel, Jumbo Javelin.

The agreement included transporting most of the project materials from Europe to North America, installing the 16 mooring legs, and preparing and overboarding the two APL STL buoys for subsequent connection to the mooring system by an anchor handling tug (AHT).

With the exception of the suction anchors, which were shipped directly from China to the local staging post in Saint John, New Brunswick, all project materials were readied by APL in Rotterdam for transport to site by the Jumbo Javelin.

In June, the vessel arrived in Rotterdam for general project preparation and loading. With a gypsy winch (a chain wheel) installed above the hold and a chain chute to guide the chain over the vessel’s side, the 26,247 ft (8,000 m) of 134 mm anchor chain (total weight of 3,000 metric tons) was loaded onto one section of the tank top and the “‘tween deck.” The vessel’s two 900-ton cranes loaded the eight reels of SSW and the two STL buoys into the Jumbo Javelin’s hold.

On July 6, the Jumbo Javelin arrived in Saint John, New Brunswick, where the six largest suction anchors were loaded onto the main deck and sea-fastened vertically for transportation to the installation site.

Innovative installation
Once the pre-installation survey had been completed on site, the first anchor was prepared for overboarding. Following

crew, lift rigging

The crew attached the lift rigging to the pad eyes located on top of each suction anchor, up to 66 ft (20 m) above deck.

strict safety precautions, the crew attached the lift rigging to the pad eyes located on top of each suction anchor, up to 66 ft (20 m) above deck. Next, the suction anchor was moved to the overboarding zone. The anchor chain was routed out of the hold, up and over the gypsy winch, over the side of the vessel and then back onto deck.

Connection of the chain to the suction anchor was carried out using a platform hung off from strong points located on the side of the anchor. Through this procedure, the 551-lb (250-kg) pin could be safely handled and inserted to secure the connection between the anchor chain and the pad eye on the anchor.

The anchor was lifted and overboarded in a single lift operation. The speed of the heavy lift mast cranes made the overboarding of even the largest anchors a quick and efficient operation.

A passive heave compensation system was used to compensate for excessive dynamic loads and motions while lowering the anchor through the splash zone.

Upon touchdown, the weight of the suction anchor allowed it to penetrate the seabed approximately 3 ft (1 m). Again, the Cranemaster was used to compensate for the vessel motion relative to the stationary anchor. Final penetration was achieved by means of a remotely operated vehicle (ROV) equipped with a suction pump.

Once the anchor was landed on the seabed, the ROV docked the pump suction head onto the suction flange. With the suction pump activated, pressure inside the pile drops, and the external hydrostatic pressure forces the suction anchor further into the seabed.

For this installation, suction anchor embedding took between six and 20 hours, depending on anchor size and local bottom soil conditions. Once the design penetration had been achieved, the anchor chain was laid in the relevant lay corridor. This operation was repeated for all 16 anchors. The size and number of the anchors meant that two trips were required.

Before a buoy could be overboarded, one end of each of the eight spiral strand wires had to be attached to the turret underneath the buoy, and the other end hung off from a spreader beam.

By means of a powered reel drive, each SSW was reeled off via a chute overboard and hung off underneath an outrigger structure to the starboard side at one end. The other end of each SSW was connected to one of two spreader frames, one of which was hung off at the aft end and one at the fore end of the vessel.

When all eight SSWs had been suspended under the outriggers, one of the vessel’s 900-ton cranes was used to move the 156-ton STL buoy from its transit location onto the outrigger structure. Tugger winches pulled each socket upward and into a position where the spelter socket connection with the turret could be made up. When the suction piles were installed and the anchor chain (7,900 m/3,000 metric tons) was laid, the SSWs were unreeled and connected to each STL buoy and subsequently handed over to the AHTs for connection to the pre-laid mooring chains. This complex project was successfully completed in 30 offshore days.

This new installation method allowed the crew to avoid damage to the sensitive coating of the SSWs, as all of them were hung off vertically underneath the buoy making the connection to the turret relatively easy.

With all eight sockets connected, the buoys were handed over to waiting tugs for connection to the previously installed anchor chains/suction anchors.

In employing this installation method for STL buoys for the first time, Jumbo demonstrated that a complex mooring system can efficiently and safely be transported and installed by a DP2 class heavy lift vessel.