By Mark Goalen, Offshore Engineering Director, Houlder Ltd
The offshore wind market is maturing rapidly as the world transitions to cleaner energy. Indeed, the BP Energy Outlook 2019 anticipates significant growth of the sector, suggesting that the percentage of generated wind energy in the renewable market will be more than double by 2040. Meeting this rapid growth in demand presents huge challenges in design, technology and engineering. To make the transition economical, wind farm developers and operators are prioritizing efficiencies in cost and performance across all aspects of the operation.
Sea fastening is the routine practice of fastening cargos to the ship for transport, either to the site of installation or transit from port to port. As component sizes increase, the challenge of transportation becomes more acute. Executed properly, sea fastening enables the safe and efficient transportation of project equipment, minimizing the number of trips required to install the wind farm equipment. This critical stage of the installation process must not be underestimated — not least in terms of the value it has the potential to deliver.
Sea fastening, but not as you know it
Over the last 30 years, the blade diameters of offshore wind turbines have more than quadrupled. Indeed some of the newest blade designs are double the length of a Boeing 747, and it is anticipated that this growth will continue. As experience is gained, and the technology develops, the turbines get larger to generate more power.
As the components increase in size and at a significant rate, the industry is pondering the optimum design for the next generation of installation vessel. In the meantime, however, the existing fleet is being pushed to the limit. Particularly as there is commercial pressure to maximize the amount of equipment on the vessel per trip to help decrease the overall cost of offshore wind farm installation. This significantly increases the project risks associated with transportation and installation.
The importance of sea fastening in this context is often undervalued — this is not as simple as a standard offshore container sitting on a deck well within the allowable variable deck load. For today’s wind farm installations, the components do not only differentiate in size and weight, but also shape. The blades need to be transported in racks, the monopiles in cradles, and the towers on grillages; all of which must be designed to fit. This is sea fastening, but not as we knew it.
Stability and motion analysis informs effective design
When particularly large equipment is secured to a vessel, the loading conditions must be checked to ensure the vessel remains stable and within the operationally compliant restraints of draught and trim.
Additionally, the weight and height of these components adjusts the vessel’s motion characteristics. Therefore, bespoke vessel motion must be derived to determine the forces the equipment will impart into the hull as the vessel rolls and pitches while at sea. The length of the blades and sometimes monopiles means that they can overhang the edges of the hull, meaning additional green water analysis may be required.
The structural analysis, design and engineering work follows confirmation of stability and the determination of vessel motions. It is essential that the structural interface for each piece of equipment is designed to transmit the loads into the vessel structure without overstressing and damaging the hull or the connecting interface.
For jack up vessels, it is also important to check hull strength in the jacked-up condition, and that the forces pushing down on each of the legs does not exceed the allowable seabed limitations. The leg forces can vary significantly when the vessel crane lifts the wind farm components, and so several scenarios must be considered.
Integrated thinking across structural analysis, design and engineering
Experienced, practical analysis is essential to ensure proper securing and sea fastening of high-value cargoes to guarantee a project’s success. Developing an offshore wind farm involves specific and expert engineering, from concept design to installation, into operation and finally decommissioning. Every element is closely interlinked and therefore decisions must not be taken in isolation — the wider picture must always be considered.
This is why analytical capability alone is insufficient; structural analysis is just one piece of the puzzle. There is a seamless chronology between understanding stability and vessel motions before then delivering on design. For example, what are the practicable options when an allowable vessel limit is exceeded or is so-close to exceedance that further calculation is required to prove it is acceptable? Every decision has a knock on effect, which is why — throughout the entire process — considerable experience is needed across each and every element to engineer reliable solutions which facilitate safe, timely, and cost-efficient delivery.
Progress necessitates change in the swiftly advancing offshore wind space, particularly as global societal pressure increases the move to cleaner energy sources, while development costs continue to be driven down. To safeguard investment, protect assets and maximize efficiencies, integrated design and engineering remains critical in navigating the evolving challenges of this swiftly emerging sector.
Houlder has completed multiple fixed bottom offshore wind sea fastening projects for various clients. During the course of these projects it has designed the structural interfaces between all of the primary offshore wind components and the vessel — tower grillages, blade racks, substructures, TP grillages, monopile cradles — as well as various ancillary equipment that goes along with the mobilization. Each project is different and presents its own unique challenges.
When designing the primary support structure for any of the main components, the main interface is usually a straight forward process. The challenge is transferring the loads into the vessel to avoid underdecks strengthening or fatigue issues while working within fixed vessel parameters such as crane reach and height, loading condition ballast limits, and avoiding clashes with areas that require access or walkways for safe operations, etc. Careful consideration to each limitation, and a multidisciplinary team that can work effectively together and in parallel is the key to a successful outcome within the planned timeframe.
There are always additional requirements that present themselves where an efficient engineering team can add real value to the project. Houlder has designed lifting arrangements (including spreader beam) to improve mobilization times of blade rack substructures that were not originally designed to be lifted in one unit. It has also added and repositioned boat landing platforms, and relocated crane boom rests, for example. In addition to back deck equipment, Houlder has relocated life rafts and FRCs to avoid project equipment and is familiar with the requirements for class approval of safety-related equipment.
Houlder has also provided quick responses to queries during mobilization. Wrong bolt grades being delivered, slings that are shorter, and a lower SWL than requested, uneven decks, are all things that have arisen and been resolved quickly without holding up the mobilization.