The Perdido spar is the world’s deepest direct vertical access spar and is operated by Shell on behalf of its partners BP and Chevron. The platform is a massive structure, weighing 55,000 tonnes, nearly as tall as the Eiffel Tower, and moored in 2,450 m (8,000 ft) of water.

It acts as a production hub for three fields – Great White, Tobago, and Silvertip in the Walker Ridge area of the Gulf of Mexico – and has been producing since March 2010. The spar receives, processes, and exports oil and gas from reservoirs located within a 48-km (30-mile) radius. The Tobago field, in 2,925 m (9,596 ft) of water, is the world’s deepest subsea completion.

With 22 direct vertical access wells connected to the spar and an additional 13 tiebacks from subsea completions, this means the equipment must be changed out and serviced at regular intervals.

Rather than deploying offshore service vessels to the remote development, located approximately 322 km (200 miles) off the coast of Texas, the Perdido spar’s design includes a winch system that allows the platform to service the subsea equipment itself.

Payload challenge

Handling payloads up to 43 metric tons in 2,925-m water depth can be a challenge. A traditional steel wire rope to reach this length would weigh almost 40 metric tons. This would double the necessary lifting capacity of the winch, resulting in an increase in its required size and therefore causing significant structural modifications to the spar – something that Shell did not want as it focused throughout the Perdido project on lightening the topsides weight as much as possible.

To solve this problem, it was decided to move ahead with a traction winch using fiber rope.

The winch was designed from concept all the way to installation by Logan Industries, based in Hempstead, Texas. Logan is part of Doedijns Group International (DGI) of the Netherlands. The traction winch that was designed for the Perdido spar is comprised of the following components:

  • Storage reel with 3,048 m (10,000 ft) of synthetic rope;
  • Traction winch drum with an 80-metric-ton line pull capacity, a line speed of 15.24 m/minute (50 ft/minute) at full load, and special design capstan and idler sheaves to accommodate elongation changes in the rope;
  • Hydraulic power unit (HPU);
  • Turndown sheave with load indicator; and
  • Controls.

Storage reel

The storage reel is driven by a hydraulic motor and planetary gear reducer in conjunction with a bullgear. Outer diameter (OD) is based on a minimum D/d ratio of 20:1.

Samson provided 2.4-in.-diameter Quantum 12-fiber rope weighing 85% less than wire rope of a similar size and length. This greatly reduces the deck weight as the rope is neutrally buoyant and adds no weight to the payload. This allows the winch to work at full capacity regardless of depth.

It also has a break strength of 240 metric tons and 12-strand construction and uses DSM Dyneema SK-75 fiber as the main strength member.

Traction winch drum

The drum has rope parallel grooving with the drums offset for proper reeving and spooling. The drum OD is designed to consider the dynamic minimum-bend radius. The idler drum has a through-shaft and is mounted with spherical roller bearings on each end.

The driven drum consists of independent sheaves for the rope to lie in. The number of grooves is based on the tractive effort required for a single drum drive with the other traction roll sheaves acting as an idler.

The idler roll has independent groove sections like sheaves to allow the rope to pre-stretch and float as much as possible with minimal slippage.

Turndown sheave

The rope is routed over the turndown sheave to the main powered traction roll on the traction winch portion. On Shell’s Perdido spar the turndown sheave is placed next to the traction winch, and the rope path will take it directly down to the water so that items can be picked up from a boat and deployed directly subsea. The turndown sheave has a load pin that measures loads being picked up or lowered, line count, speed, and direction.

A dedicated HPU was provided for all system controls and functions. The HPU contains a suitably sized reservoir, heat exchanger, various hydraulic components, and plumbing.

This unit is equipped with dedicated motor starters and circuit breakers enclosed in an explosion-proof enclosure. The unit is tested with the winch and storage reel for overall system performance. The entire system, including HPU and controls, is rated for Class 1/Division 2.

Controls

All functions are controlled by a programmable logic controller (PLC) and are shown on the operator screen. They also can be recorded or tied to a remote readout.

The PLC also monitors the low-side tension and the high-side tension to keep the take-up reel tension up to the proper pull rate to prevent slippage. The overboarding sheave is instrumented to measure load tension and line speed as well as line direction. This system is supplied with weights for the rope free end to keep the line tight when retrieving the rope under light or no load since the normal operation will be mostly paying out with a load, then coming up unloaded. This tractive drive is controlled to operate only when retrieving the rope and is open in the payout mode.

The controls are set up for manual operation of the take-up reel and the traction system independently. When shifted to traction system mode they are synchronized with the load sense of the reel and the PLC, but the unit is still controlled with a single joystick for payout and retrieve.

Performance so far

To date the winch on the Perdido spar has made more than 250 runs down to the seabed with a 100% satisfactory level. The winch also is still using the original rope.

A recent “pull to failure” test showed the rope is still at its original breaking strength.

DGI/Logan is now looking to develop winches with higher capacities. Nowadays more and more operators look for winches with a higher capacity as recent installations clearly show the many advantages they offer for the efficient (and affordable) operation of offshore vessels and platforms.