Mooring

Case Study 1 - Perdido
 

Deepwater Mooring just got deeper

 

Subsea mooring connectors are going deeper. First Subsea's Ballgrab mooring connectors have recently been used to moor Shell's Perdido Development direct vertical access spar in around 8,000 ft (2,438m) of ultra deepwater in Alaminos Canyon in the Gulf of Mexico, setting a new record for the connector's deepest mooring.

 

"A particular challenge Shell faced at Perdido was delivery and installation of the spar and 'storm safe' mooring system prior to the peak of hurricane season in mid-August 2008, and the use of subsea mooring connectors helped to quickly and safely connect each mooring line to the associated anchor pile," said Curtis Lohr, Shell's project manager for the spar and mooring system.

 

Designed specifically for deepwater projects, the Ballgrab subsea mooring connector (SMC) comprises a male connector and female connector. It works on the simple principle of a ball engaged in a taper. The male connector is inserted within the female connector. As the balls roll up the male connector's tapers, the tightness of the grip to the female connector increases in direct proportion to the load applied. In the field, the tool is aligned into position and inserted; once engaged it cannot be released until the load is removed.

 

Nine Leg Polyester Rope Mooring System

 

The Perdido Regional Host truss Spar has a 9-leg taut chain-polyester rope-chain mooring system averaging 13,833 ft (3km) in length with suction anchors. The nine mooring lines are configured in 3 x 3 pattern with polyester rope main sections and chains at each end of the mooring lines. Specifically the off-vessel mooring equipment in each leg included the following: Suction anchor, Pile shackle, Pile chain, D-shackle, Ballgrab subsea mooring connector, D-Shackle, Ground chain, Rope shackle, Polyester rope segments with H-link connecting segments, Polyester insert, H-link, Adapter, D-shackle, Platform chain, Pear link and Tail chain system.

 

Each of the nine male Ballgrab connectors, 2.8m in length and with 11 rows of balls, was designed with a MBL 17,259kN, the maximum load they would ever experience. In typical use the Perdido spar connectors would expect to see 30 % of this load.

 

Heerema Marine Contractors was responsible for installing the Perdido mooring system using its deepwater crane vessel, Balder. "We've had a lot of experience with Ballgrab connectors in deepwater and find them straightforward to install," said Yigal Oosterink, project engineer, Heerema Marine Contractors. "On the Perdido project, this experience allowed us to concentrate on other, more challenging, areas of the mooring system such as handling the long lengths of polyester rope."

 

Ballgrab female connector pre-installed in ple docking porch

 
Suction Pile SMC Docking Porch

 

Each Perdido suction pile was pre-fitted with a female Ballgrab connector, held in a docking porch at the top of the pile, and connected to 22.8 m of 5.28 inch (13.4 cm) ground chain. Once the pile was in position, a ROV was used to fit a receptacle cap, containing anti-corrosion chemicals, to the top of the female connector.

 

To provide a secure fit within the docking porch, each female Ballgrab features a trunnion – cylindrical projections welded to each side of the connector. The trunnion is designed, and FEA analysed, to take the weight of the ground chain, in this case 11.2 tonne (tested to 40 tonne), without the trunnion bending.

 

Ballgrab male mooring connector with mooring chain attached

 

SMC Installation

The male Ballgrab connector is prepared by testing the ball locking mechanism and adding corrosion inhibitors within the body of the connector. This means that the male connector can be connected to the female connector anytime within the next 24 hours.

 

The male connector was attached to 1,170ft (357m) of 5.28 inch (13.4 cm) of chain, and lengths of polyester mooring rope, as the connector was lowered, using the Balder's purpose-built, Mooring Line Deployment Winch, to within 50 meters above the female connector. A ROV removed the female receptacle plug and the male Ballgrab was guided into position directly above the female connector, using the Balder's dynamic positioning, and stabbed into the female connector.

 

Once the male Ballgrab connector is fully inserted into the female connector, the female connector is lifted out of the docking porch. The ROV is used to remove the male connector's T-bar to engage a secondary locking mechanism and so complete the mooring connection. "One of the key benefits for HMC of using the Ballgrab is that you need to insert the male connector vertically to get a connection and without a lot of ROV intervention," observed Yigal Oosterink.

 

Perdido's mooring connections were made sequentially working anti-clockwise per cluster with a male Ballgrab connector attached to each of the three clusters. With the first three mooring lines connected to each of the clusters the spar was 'storm-safe'. The mooring connector installation sequence was then repeated for the remaining connectors. All the Ballgrabs SMCs were installed in 18 days.

 

 

 

Case Study 2 - Thunder Hawk Mooring

Polyester Rope Mooring for Thunder Hawk DeepDraft Semi

 

 

The latest deepwater platform to come on stream in the Gulf of Mexico is the Thunder Hawk DeepDraft Semi in the Thunder Hawk field located 240 kilometres south-east of New Orleans. Designed and constructed by SBM Atlantia, the Thunder Hawk FPU is using a polyester rope from Lankhorst Offshore Ropes Division (formerly Quintas & Quintas Offshore) to give the platform a 'softer' deepwater mooring.

 

Thunder Hawk has a process capacity of 45,000 bpd of oil and 70mmcf/d of gas. Murphy Exploration & Production Company is the field operator and operates the Thunder Hawk Floating Production Unit (FPU).

The Thunder Hawk semi-submersible FPU is moored in Mississippi Canyon Block 736, in a water depth of 6,060ft (1,847m). It is anchored with a polyester rope-chain spread mooring system connected to twelve driven piles by First Subsea, Ballgrab ball and taper, subsea mooring connectors.

 

Thunder Hawk's mooring lines are arranged in four clusters, one for each corner of the FPU, of three lines each. "We chose polyester rope mooring because of the water depth and the type of platform," says Mark Slider, manager, Mooring Installation, Engineering Group, SBM Atlantia. "The FPU motions will be better for the risers with polyester rope than for steel wire rope. Polyester gives us a 'softer' mooring system than steel wire rope and consequently, the FPU motions are more compliant and riser friendly."

 

Thunder Hawk Ballgrab male connector with mooring line chain
 

Lankhorst Ropes and mooring specialist Offspring International, their worldwide sales agent, supplied the polyester rope and project managed its deployment. GAMA 98® polyester rope was selected to meet the SBM requirement for a rope with a minimum breaking load (MBL) of 4,254 kips (1,930 tonnes). GAMA 98® is made up of 12 sub ropes configured with three in the centre and nine arranged around the outside. It is a proven deepwater mooring rope construction which, in addition to Chevron's Tahiti Spar in GoM, has been successfully used on a number of deepwater long-term and MODU projects.

 

Each of Thunder Hawk's 12 mooring lines comprise two 4,230ft (1,289m) sections of GAMA 98® rope connected by a H-link, and with a 52ft (15.8m) rope test insert 800 feet beneath the FPU. Lines 4, 5 and 6 each have two test pieces giving a total of 15 rope insert test pieces. The number of test pieces is determined by the design life of the mooring system, in this case 20 years, and the number of major storms it is anticipated the FPU will be exposed to over this period.

 

Prior to the latest Notice to Lessees and Operators (NTL) on synthetic mooring, the US Department of the Interior, Minerals Management Services (MMS), Gulf of Mexico OCS Region, required a polyester rope insert be tested routinely every five years, and after major storms. The latest NTL allows new facilities that are equipped to record environmental data and mooring tension data during major storms and hurricanes to only remove a test insert if the recorded tension in any line exceeds 70% of MBL.

 

Dave Rowley, director, Offspring International, sees relaxation in MMS requirements as part of a wider acceptance of polyester rope as a preferred deepwater tethering material beyond 4,000 ft (1,219m). "We conduct our own rigorous rope tests for every project. The MMS requirements are an important, and independent, source of on-going rope testing that further strengthens the case for polyester deepwater mooring tethers," he said.

 

After a significant event, the operator has to demonstrate that the mooring ropes are still fit for purpose and one method is to test a rope insert. These tests subject a selection of sub ropes extracted from the insert to break load testing and fatigue analysis. Microscopic analysis is also used to check for yarn-on-yarn abrasion.

 

The mooring lines were installed by first pre-installing the Ballgrab female receptacle, connected to the anchor chain, in a docking porch on each of the piles. Once the Thunder Hawk FPU was in position, the male Ballgrabs, attached to the end of each of the mooring lines, were lowered into the female receptacles.

 

Thunder Hawk Ballgrab make connector with mooring line chain

 

The mooring connections were made sequentially working counter-clockwise per cluster with a male Ballgrab connector attached to each of the four clusters. The connection was completed within a few minutes of the male connector reaching the female connector. With the first four mooring lines connected to each of the clusters, the installation sequence was then repeated for the remaining connectors.

 

The accuracy of the polyester rope lengths was critical to the smooth installation of the mooring lines, noted Mark Slider. "Until the advent of the new rope length measurement system developed by Lankhorst, obtaining polyester lengths with a high level of accuracy has been extremely challenging, if not impossible. However we knew from Technip's experience on Tahiti that Lankhorst was able to make long rope lengths estimated to be accurate to within a metre. Our experience on Thunder Hawk was no different. The rope segments were the correct length which made completing and tensioning the lines straightforward," he said. All of the Thunder Hawk mooring lines can be adjusted to take into account any polyester bedding-in that might occur during a major storm or to adjust mooring line tensions and platform location after a riser installation.

 

Production began from the Thunder Hawk Field on 8 July 2009.