OPINION: Tow-to-port - the main (realistic) option for major component repairs in Floating Offshore Wind
By LUKAS CARBOL - Claims Executive
Major component replacement, repairs or exchange (MCR) comprise scenarios when offshore floating wind turbines require repairs or maintenance that are unable to be performed by technicians dispatched to the turbine.
Replacement of a generator, or a gearbox are, therefore, apt examples of such works. There are several main alternatives for conducting an MCR.
Floating to floating
The majority of MCRs in bottom-fixed offshore wind are performed by heavy lift jack-up vessels. However, the numbers of available jack-up vessels have proven to be insufficient when compared to the demand of the segment, due to the growth within offshore floating wind in recent years. Usually, they are employed under long-term T&I contracts for installing new capacities. Sourcing a jack-up vessel with a suitable crane for a single replacement or repair is, therefore, fairly challenging at present. Floating offshore wind introduces an additional challenge related to its location. Since the floating wind is installed further offshore, a possibility to utilize a jack-up heavy lift vessel is very limited. This practically leaves us with semisubmersible crane vessels which, however, needs to be able to operate in harsh sea conditions. As such, this limits this choice to only a handful of capable vessels with extremely high day-rates. A floating-to-floating alternative is possible but remains a rather impractical option.
On-site
Another method, much discussed recently, will be either floater, tower or turbine mounted cranes. Once fully developed tested and implemented, these technologies will probably be the most efficient alternative for MCRs. The technology is still under development or in a testing phase but, to our knowledge, not in the stage of being fully industrialized.
Tow-to-port
The main, and possibly only realistic, option for MCRs within offshore wind is therefore a method termed Tow-to-port. This consists of disconnecting the floater from all moorings and cables, towing it to a nearby port where the necessary repairs are conducted, towing it back and reconnecting it. Being the main alternative, the industry should be aware of the challenges this method entails in order to engage in a dialogue to address them.
When introduced, one of the main benefits of the Tow-to-port method was the possibility to utilize relatively inexpensive and available vessels – anchor handling tugs (AHTS). However, AHTS are mainly utilized in the Oil & Gas industry, where rates and utilization are currently at their peak. In addition, the pool of AHTS is relatively old, with approximately half of them reaching the age of 15-20 years in 2026 and 20+ years in 2030. Signals from shipping brokers do not appear to indicate any significant orders of new tonnage to be currently in place. While this remains hypothetical, this potential bottleneck should be addressed by this sector to avoid the same issues as we have seen in bottom-fixed wind with capable jack-ups.
The major challenge of this method, however, rests in its complexity. Tow-to-port requires extensive planning and cooperation between numerous stakeholders. For a long period of time, this has been perceived by the industry as a benign operation conducted numerous times in other segments. Unfortunately, the latest experience proves this assumption to be rather incorrect.
While the towage is relatively straight forward, the number of interfaces between various parties involved during dis-/reconnection, and weather sensitivity thereof, introduced unexpected risks and challenges.
For dis-/reconnection, you will always need to account for at least three mooring lines and an inter-array cable. The generally recognized operational limit for these operations is approx. 1.5m wave height. In addition, necessary technical personal need to be dispatched onboard the floater, which is also restricted by the same 1.5m limit. All these operations in locations with sea conditions vastly differ than in bottom-fixed wind, basically restricting the possibility of conducting these operations to only summer seasons.
To illustrate this in an example, let’s consider two scenarios: one incident resulting in a need to conduct a MCR occurs in the beginning of December, plus a second in June. Tendering and engineering - basically planning - is estimated to take approx. four months, whilst the disconnection, towage back and forth including repairs and reconnection, approx. two months.
In the first scenario, occurring in December, the entire planning phase will be ongoing during winter months, followed by disconnection early-summer, tow to port and back and reconnection: all in about six months from the time of when the need arose. Six months is already a significant downtime in the context of potential loss of revenue but, at least, it reflects the planned estimates.
In the second scenario, if the event occurs in June, there is a realistic risk that the project won’t be able to start disconnecting the unit before the winter season arrives. As touched upon previously, it is objectively unrealistic to conduct such operations during the winter season due to short weather windows, operational limits and number of activities, for which such limits apply. The second example could therefore lead to completion of the tow-to-port operation in the end of May the following year, directly leading to a downtime of 12 months vs estimated 6 only dependent on when the incident occurred.
See the graphic below illustrating the discussed examples. At this point, it should be also noted that both the estimates and examples were discussed and confirmed by one of the leading marine warranty surveyors as realistic.
There is a certain paradox: if the event occurs during the summer months, there is a significant risk that the completion will be postponed to the next year.
Effectivization of the equipment, technical solutions etc. will indeed contribute to expediting the process but the main downtime driver is predominantly the planning phase. It is a question whether this has been overlooked or –extensive downtime.
Importance of Planning
The discussed scenarios illustrated that any possible reduction of downtime is directly linked to and affected by the project’s preparedness for MCR, and plans and procedures related thereto. It is almost inevitable that every floating wind farm will be forced to tow their units to port, and the following points should therefore be pre-planned, or ideally, pre-contracted.
In simple terms, there should be a drawer with a procedure called MCR, with Tow-to-port being the main alternative, and subsections of all necessary activities including detailed weather and downtime analysis to avoid unnecessary standby time.
Potential contractors should be considered well before the farm enters operation. Ideally, they should be approached with the specifics of the project allowing them to engineer the operation in advance. The real-life experience showed that there was a significant difference between the contractors utilised during installation, who had some knowledge of the project, and those approached post the incident.
Another important subsection of the procedure should be port assessment as not all ports can accommodate these massive constructions. In the UK for instance, there are no ports capable to accommodate any turbine of the currently operating floating wind farms at this point. The assessment should be based on the project specifics within the acceptable range.
Cranes are equally important as ports. After construction is completed, the crane utilized during assembly (fitting) of the turbines is usually demobilised. Assessment of alternatives for crane suppliers and their lead time is therefore another critical point to address.
The entire plan will be reviewed and the process overseen by the Marine Warranty Surveyor (MWS). Involving them and providing them with the necessary specification of the project as early as possible, could lead to further reduction of the downtime. On top of that, MWS often acts as a bridging party, and their early involvement might reduce Interface risks. This appears to be rather high, due to involvement of numerous stakeholders during the entire Tow-To-Port operation. The sooner in the process they are involved, the better they can perform their role and the better the bridging function is maintained.
Despite being the main MCR alternative, Tow-To-Port operation does not fall under the scope of the most utilized insurance conditions. Therefore, the project will most probably need to obtain and tender a specific stand-alone insurance for such operation. Addressing this in advance might again contribute to downtime reduction.
Summary
The past decade has proven that technological development within offshore wind advances at an incredible pace. Hopefully, this will continue to be true and the new methods for MCRs, such as turbine, tower or floater-mounted cranes will be available in the near future. The reality today is, however, that the Tow-to-port method is the main alternative for all MCRs. Despite its benefits, there are some challenges related to it, out of which extensive downtime is probably the most significant one. Effectivization of Tow-to-port by pre-planning, and ideally pre-contracting of the main elements, could therefore be one of the ‘low-hanging fruits’ in how to contribute to better floating offshore wind viability.
