Hurricane-proof production structure designed for deepwater
Undocked production tanker ship sails away
For deepwater areas prone to hurricanes, LMG Marin of Norway has developed what it claims is a more cost-effective floating production vessel concept than conventional deigns. The vessel is known as the Sea Dock.
Sea Dock provides ease of mooring and disconnection, and simplifies the conversion of a tanker into a production ship.
Sea Dock is a U-shaped steel structure for mooring a production ship with process and storage capabilities. Risers are themselves tied into the Sea Dock, through which the production flows are transferred to the ship. When a hurricane warning is received, the ship is undocked and sails for a safe area. Sea Dock itself is able to ride out the hurricane.
Another benefit of this solution is the simplification of the production ship, which can be converted from a second-hand tanker with the addition of processing plant and possible upgrading of the living quarters.
LMG Marin, a consulting engineer and naval architect firm, based in Bergen, has developed two versions of the facility to date:
- Sea Dock 42, which is 42 meters wide and designed for accommodating vessels of Panamax size
- Sea Dock 71, which is 71 meters wide and intended for ships of VLCC size.
The relatively simple steel construction for the system can be built at a conventional shipyard, according to Kjersem. Its main equipment is a ballasting system consisting of a power pack and pumps.
Sea Dock offers substantially reduced construction costs. The larger version is estimated to cost $8-10 million. The smaller system is $5-6 million. Mechanical outfitting comes on top of this, but would not be expected to cost more than $1-2 million. Meanwhile, the cost of an internal turret for 20 risers, is estimated by Kjersem at $80-120 million.
The main benefit of a turret is to give the production ship a full weathervaning capability. Sea Dock is able to weathervane through +/- 90°, so that when accommodating a converted tanker, it can weathervane through 180°. If required to weathervane through 360°, it could be used in combination with LMG Marin's double-bowed Sea Twin ship design.
In some areas in which a weathervaning capability is not required, production ships can operate without turrets. In such a case, the benefits of Sea Dock are the ease of disconnection of the ship from the dock and of re-connection once the hurricane has passed.
The facility, which is suitable for use in water depths of 60-3,000 meters, can accommodate a large number of risers, depending on the size. In the case of 6-in risers, the number can include up to 22 for Sea Dock 42 and 40 for Sea Dock 71. They can be tied back to both decks of the Sea Dock.
The ship is held within the Sea Dock cradle by an internal locking system such as a pin-and-hole type. Fenders between the dock wall and ship side ensure very soft force transfer between the two. Yokohama fenders or other existing types may be used, according to Kjersem.
Flexible risers are hooked up and linked to the ship's production facilities at main deck level, giving direct access to the connection points in a dry environment. Short jumper hoses and cables in the hookup area may be either manually or automatically operated. Routine disconnection of 30 risers will normally take 35-40 minutes, while an emergency disconnect can be carried out in a matter of minutes. Warning of imminent hurricanes is usually given several hours and maybe a day in advance.
The ship may safely leave the dock facility in a significant wave height of Hs = 7 m, equivalent to a maximum wave of about 13 meters, without the assistance of a tug. However, entering the Sea Dock is more critical and may not take place in sea states exceeding Hs = 2.5m (maximum wave of about 4.5 meters). A single-screw ship would require tug assistance to enter the facility.
Sea Dock itself is held in position by mooring lines. These may be conventional chain/wire lines, or taut-leg moorings. As with semisubmersible structures, it has been designed with a reduced water plane area, and is capable of withstanding partial flooding as a hurricane passes over.
Other benefits stem from the Sea Dock concept. It may be used to accommodate a drillship with test production equipment. In this way, early cash flow can be achieved, while reservoir information gained from the initial wells can be used for optimizing the location of subsequent wells.
The larger version is able to accommodate well intervention equipment of either a permanent or modular type, though the use of such equipment presupposes that the wells are located beneath the floating facilities.
Sea Dock, which has a 20-25 year design life, can be moved from field to field. It can be owned by a contractor or by an oil company, and can be used in combination with a ship belonging either to the same owner or a different one, depending on circumstances. It could also be developed for application in harsher environments such as North-West Europe, where it could offer good economy with in excess of 98% operability, Kjersem says.