Drilling, production spar dictates difference in mooring setup

Oryx's Production Superintendent for the Gulf of Mexico stands adjacent to one of the Bardex hydraulic linear chain jack stations on the Neptune Spar. [62,621 bytes]

Simplicity versus multi-functionality

Tom Miller, P.E.
Bardex Corporation
For over 30 years, the deep draft, caisson-type, floating structure known as the spar has been used by the offshore industry as a research vessel, communication relay station, and storage and offloading buoy. Last year with the successful deployment and start of operation of the Neptune Spar in the Gulf of Mexico, Oryx Energy proved that spar technology could be effectively employed in the form of a production vessel for deepwater oil and gas development.

In 1998 Chevron is scheduled to complete what amounts to the next phase in spar development by installing the Genesis Spar, also in the US Gulf deepwater, as a drilling and production spar.

The two structures employ taut catenary mooring lines and hydraulic linear chain jack mooring equipment developed by Bardex Corporation. The configuration of the mooring lines on each consists of short lengths of studless chain at the hull and seafloor ends and steel wire rope in between. Anchoring at the seafloor utilizes high uplift capacity anchor piles.

Chevron's specifications for the Genesis Spar dictated that certain changes and improvements be made to satisfy the mooring requirements of its unique drilling and production configured spar. This article examines the key differences in the main jack equipment for the two and how the design challenges were met.

The noted increase in size and weight of the Genesis Spar requires higher mooring tensions for basic stationkeeping and to accommodate the drilling operations. While the speed rating of each linear chain jack assembly on the two spars is 4.0 fpm (ft/minute) average, the size of chain each assembly will handle on Genesis increases rather significantly - to 5-1/4-in. (from 4-3/4-in. on Neptune) -in order to accommodate the higher chain tensions.

Note that high performance Grade 4 studless chain, which weighs 10% less than Grade 4 stud chain and is equivalent in break strength, is used for the mooring lines of both Spars. With few exceptions, this represents a departure of the US offshore industry's tradition of specifying stud link mooring chain. Studless chain was principally selected to avoid the potential fatigue problems that can be encountered when Grade 4 stud chain is used for high line tensions and long term applications such as Spar mooring.

Since the high strength steel in Grade 4 stud chain is susceptible to hydrogen embrittlement and associated cracking when welded, the stud is not welded to the link. The stud is put into position at the center of the link while the latter is still hot and is pressed into the link. Neurals protruding from each end of the stud help secure the chain in place. During proof loading of the chain, the pulling action effectively "sets" the neurals into the chain bar stock. When the chain is installed and is subjected to repeated use, stress fatigue points can occur in the chain bar stock in and around the neurals. Since the stud ends are not sealed, corrosion can accelerate the degradation of the stud and link causing premature chain failure.

The complete chain jack equipment on the Neptune Spar consists of six 600-kip lift capacity hydraulic linear chain jack assemblies and six combination (chain and wire rope) sheaves, three portable local control panel assemblies, a dual 200-hp hydraulic power unit assembly, a central monitoring panel assembly, and a programmable logic computer (PLC) enclosure.

Mooring system configuration on the Genesis Spar contains over twice as many components: 14 1,050-kip lift capacity chain jacks each with combination sheaves, two 3-by-350-hp electrohydraulic power units, four local control panel assemblies for automatic operation and monitoring, and two local PLC enclosures which interface with the mooring control and advisory system located in the Spar's central control room.

Electrical and hydraulic systems used to control the chain jacks on each spar are fully integrated to provide the necessary safety, operational interlocks and redundancy.

Each mooring system is designed to be an active system - capable of moving its spar about its central position over the seafloor at any time by hauling in or paying out the mooring chains and then holding links during idle periods of operation.

A major difference in the two spars' chain jacks is the degree of automation that is made available for operating the jacks. While Oryx had interest in keeping everything on the Neptune Spar as simple as possible, Chevron is favoring a more sophisticated design for its spar mooring control system. For example, in the normal operating mode of Neptune's mooring system an operator walks to an individual chain jack station and plugs in one of the portable local control panel assemblies in preparation for operating that particular chain jack in either the pull-in or payout mode.

Chevron has specified that the Genesis mooring system be designed to be controlled by a computerized mooring advisory and control system. In this method of operation, the mooring advisory system's computer calculates how much chain needs to be pulled in or paid out at each station based on the difference between its present and new locations.

It was important to increase the total number of chain jacks that could be operated simultaneously from two on Neptune to seven on Genesis to achieve a satisfactory, relatively rapid response to the computer-determined movement of the spar.

Bardex reduced the flow requirements needed to support operation of eight large chain jacks by using a regenerative circuit for the hydraulic actuation of the chain jack cylinders. In a regenerative circuit, the extend speed is increased by taking the fluid exiting the rod end and porting it directly into the back end. This rod flow is added to the pump outlet flow. In a normal, non-regenerative circuit, the hydraulic fluid exiting the rod end flows to the hydraulic power unit reservoir.

This serves to speed up significantly the flow rate of the hydraulic fluid, which in turn allows significantly faster cycling of the pull-in or payout operations with smaller primary pump flow capacity and electric motor horsepower.

Immersion of the mooring system components in a saltwater environment for such a long period of time has presented Bardex with some unique challenges. Protecting against moisture/seawater ingression in the electrical circuitry as well as the hydraulic lines of the linear chain jack stations was required.

In preparation for the transport of the Genesis hull sections, the junction boxes will be taken off the chain jack stations and the ends of the cables will be sealed. Since the Genesis mooring system will use flex I/O technology, only two incoming network cables (about eight wires) will be involved (versus the multitude of cables at each station on Neptune). This will greatly simplify the overall wire sealing task and streamline the re-hookup of the wiring at the deployment site.

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