With its new 30m-high by 17m-diameter internal turret and new above-deck structures, the 350m-long, 275,000 dwt Espadarte floating production, storage and offloading (FPSO) vessel recently began operations in 800m water depths in the prolific hydrocarbon-producing Campos Basin off the coast of Brazil.

In redesigning the former crude oil tanker for its current FPSO duty, the original idea was to use one winch, located on the winch deck of the vessel’s upper turret superstructure, for installing the 45 production risers as well as for hauling in and tensioning the vessel’s 10 mooring lines.

Each mooring line has three sections. The bottom length consists of studless 115mm (41/2 inch) chain, which is connected to the line’s anchor. The line’s mid-section is polyester rope. The top section is the studless chain again.

The chains are routed into the turret through curved hawsepipes. These hawsepipes are located around the circumference of the turret, between risers, in two groups of three and one group of four.

A crossarm with integral chain stopper is permanently attached to the top of each hawsepipe. The turret structure includes a horizontal pipe located 5m above each of the chain stoppers. The pipe serves as a turning shoe that allows the chain to pass vertically upward through the hawsepipe and stopper, over the turning pipe and back down into the moonpool inside the turret.

Due to the deep water and high mooring loads that would be encountered in the Campos Basin, it became apparent that the diameter of the winch drum that would be required to accommodate the mooring lines would be too large to realistically fit within the confines of the vessel’s turret. Additional complexities, due to the high loads, included very high wind loads and the need for movable sheaves, which would be required for aligning the winch drum with the load. Given these considerations, it was determined that a simpler more compact solution was needed.

Bardex Corporation, a hydraulic heavy load handling system manufacturer in Goleta, California, USA, was contracted to provide the chain tensioning equipment. The company designed a compact, high capacity hydraulic chain jack, that was small and light enough to be lowered into the moonpool area inside the turret, and slipped between the risers onto each of the 10 chain stoppers (see Figure 2).

Following chain tensioning, the chain jack and its control console would be disconnected, lifted out of the moonpool (since the chain deck would be submerged during long-term mooring periods) and secured out of the way for long-term storage.

As a contingency, chain jacks would remain on board in case re-tensioning was required due to potential stretch in the polyester rope over the long-term mooring periods.

Chain jack design

The compression style chain jack comprises two hydraulic cylinders, a travelling crossarm with integral latches, a cylinder connecting crossbeam with hydraulic manifold, and a separate cart-mounted control console. Travelling latches are opened and closed using an integral hydraulic cylinder. The travelling crossarm is bolted to the top end of each cylinder rod.

The travelling crossarm is slotted like a horseshoe in shape. This allows the entire chain jack to be installed or removed from a chain stopper without disturbing the chain. The chain jack is temporarily attached to the fixed crossarm/stopper using 12 high-strength M30 bolts.

Although one chain jack could be used at any of the 10 chain stoppers, three chain jacks (and three consoles) were provided – one per group of chain stoppers – so that chain haul-in could be performed simultaneously at more than one chain station. To simplify movement of the chain jack, the turret was fitted with a circular monorail and trolley mounted 5t hoist for moving the 4.3t chain jack from one stopper to the next.

The hydraulic manifold mounted on the cylinder crossbeam contains the relief, control and overcentre valves for the unit. Jumper hoses with quick-disconnect (QD) fittings were used to connect the manifold to the turret ring main hydraulic supply and the return permanent piping. Short length jumper hoses and QDs were also used for connecting the portable control console to the hydraulic manifold.

Operator control of the chain jack is via the hydraulic control console, which includes main cylinder lift/lower control, travelling latch open/close, and fixed stopper open/close. A control console pressure gauge provides indication of loads in tonnes and pressure in bar/psi.

The tension in each mooring line is measured by instrumented links; tensions are confirmed by angle measurements.

Chain jack lift capacity was specified to be 325t at a speed of five feet per minute, with a stall capacity of 400t and capacity fully retracted (structural) of 456t. With a 1m stroke, the extended height of the chain jack is 3.8m. Retracted height is 2.5m. The envelope area of the chain jack in the horizontal plane is 1.6m by 810mm.

In addition to the benefit of compactness, the linear design of the chain jack provides a reduction in interlink chain stress and wear, when compared to a chain winch or wildcat. In the linear chain jack, chain links are loaded in tension only, exactly as the chain is tested by the chain fabricator. The straight pull design of the unit thus avoids the added bending stress and interlink wear inherent with chain winches in which individual chain links are hinged under tension.

In operation

The installation of each mooring line began with the Espadarte ballasted up at the bow. This attitude raised the chain deck in the turret about 2m above the water, creating a dry, safe work place. Six messenger chains had been pre-installed. The inboard end was seated in the chain stopper. From the stopper, the chain trailed down through the hawsepipe.

The outboard end was lifted back up to the ship’s foredeck where it was secured. The remaining sections of the mooring line had been deployed earlier and were buoyed-off for retrieval. Each chain jack and control console was lowered from the winch deck to the chain deck (Figure 3) and installed on one of the 10 chain stoppers in each of the three groups.

The first anchor handling vessel (AHV) retrieved the first anchor line. Its related messenger chain was released from the bow of the Espadarte and passed over to a second AHV. The second AHV carried the end of the messenger chain to the first AHV where the messenger chain was passed across. On the first AHV, the messenger and mooring chains were connected and released overboard.

The messenger chain was then hauled in using the chain jack. It took about an hour to haul in the 90m messenger chain. Messenger chains were disconnected and transferred across the moonpool for use with one of the other mooring lines. The time required to shut down a jack, move it, and prepare it for hauling the next chain ranged from one to two hours.

On several occasions it was decided to wait for two symmetrically opposed chains to be prepared. These were then hauled in simultaneously with the advantage of minimising movement of the ship. In all instances, the chain jacks were repositioned and ready to haul chain well before the anchor handling vessels had completed the chain hookup.

Each of the 10 chains was connected and hauled in to pretension level. At this point, the mooring spread (AHVs and positioning tugs) was released. The FPSO was now securely moored. Following the successful pre-tensioning, each chain was further tensioned to the final prescribed tension level. Chain jacks and consoles were then disconnected and lifted from the moonpool and secured out of the way for long-term storage.