Operational feedback shows that SOV crews choose to operate the vessels in astern mode up to 75% of the time at offshore wind farms. This number is very likely to increase further on the ‘Windea La Cour’ and the ‘Windea Leibniz’ vessels due to their X-STERNs.
A vessel's ability to back is not usually tested at sea trials. However, the crew on current operating Service Operation Vessels (SOV) report that they prefer to manoeuver stern first rather than bow first during their stay at the offshore wind farms.
The SOV owners give various reasons for astern operations:
- It leads to a better overview of the vessel’s positioning, as the gangway, the turbine and the ship’s side can all be observed from the bridge
- Most power is installed in the aft part of the ship, enabling it to keep positioned more easily
- The vessel doesn’t have to turn around before leaving for the next turbine, saving manoeuvering time between turbines
First X-STERN® vessel
With the new, patented, X-STERN, firstly implemented on the Windea La Cour, the preference numbers for operating astern will rise.This 88m-long SOV is an SX175 design from ULSTEIN. The X-STERN aft design has been developed for the SOVs to work close to the offshore wind farm structures, disregarding the directions of wind or waves, or where it is headed next. The X-STERN° is a follow-up of the X-BOW® fore ship design, both originating from Ulstein’s long experience in designing for the offshore industry.
The X-STERN was designed for increased operability and station-keeping whilst in dynamic positioning mode. The vertical transom plate, a very common aft shape, will create slamming when going backwards or when being positioned with the stern towards waves, wind and current. The X-STERN, however, is a pointed aft. The sloping and higher stern, with its sharp stern shape, improves offshore wind farm service and maintenance. This has an importance when it comes to the flexibility and efficiency of the vessel. The X-STERN improves the crew and passenger comfort and increases the operability through positive effects on station keeping, wave response and safety in harsh conditions. It operates with reduced power and fuel consumption while staying positioned.
Keeping up backward speed without reducing comfort
A vessel with the X-STERN can keep almost the same speed in backward mode as in forward mode without the occurrence of slamming or vibrations. ‘Windea La Cour’ has demonstrated the ability to sail astern at a speed of 12.1 knots, just 1.8 knots shy of her forward speed, and she is easily manoeuvred both ways. The crew management has noted the vessel’s smooth motions and the very low levels of noise and vibrations, all these being important factors in a vessel in which one of the main tasks is to transport and accommodate service technicians.
The sister vessel to the ‘Windea La Cour’, the ‘Windea Leibniz’, was delivered from Ulstein Verft in February 2017.
Three deliveries of SOV designs
Ulstein has developed several SOVs, of which three have been delivered, one to Siem Offshore Constructors and two to BS Offshore. A fourth offshore wind vessel to be built for Acta Marine, is also to be equipped with the X-STERN.