In Europe, many offshore projects are planned for deeper waters further from the shore, which requires sailing speeds of up to 13 knots and jacking capabilities for water depths of up to 60 m. Most of the specialised vessels now in operation are classed by GL.
GL has accumulated an unrivaled pool of knowledge as well as invaluable practical experience that enables the company to address the complexities posed by these vessels to provide site-specific solutions based on cutting-edge technologies. GL Group services for wind turbine installation ships (WTIS) include:
-combined assurance, consulting, engineering and execution support
-marine warranty services and dynamic positioning assurance
The importance of pre-design expertise
At the pre-design stage, important parameters can be verified by feasibility studies and classification pre-checks, both carried out by GL. As WTIS involve high levels of complexity, early investigations are an effective tool for saving time and money at later stages. Relevant issues include: should the ship be self-propelled, with jacking capability; what is the desired sailing speed, size of crane and lifting appliances, number of compartments and dynamic positioning?
For vessels with dynamic positioning it is particularly important to define the redundancy concept at the earliest possible stage. Flaws discovered at this stage result in no more than extra paperwork, whereas flaws discovered at later stages, may lead to equipment rework,delays and commercial losses.
FMEA & the redundancy concept
The FMEA is a critical safety element for DP2 and DP3. Cost efficiency can be optimised at this stage if the redundancy concept and the worst-case failure design take into account operating modes (e.g. the work to be carried out, such as pipe laying, crane work, supply, transit etc.) and the operating conditions(e.g. environmental conditions for work, lifting etc.).
This allows for appropriate design modifications before they become costly and is provided for in the GL Rules "Guidelines for Vessels with Dynamic Positioning Systems" by early approval of the redundancy concept.
Design and construction of custom-built WTIS
At the design stage it is important to determine the most appropriate design to meet specific requirements. Considerations include: expected turbine size, water depth, distance from shore and the time window for installation.
GL has been involved with three design generations of wind turbine installation ships:
Generation I: These are combined crane and working barges with high lifting capacity, high deck load capacity and a large working deck. They are used quite often in areas close to the shore, or if suitable generation II and III vessels are not available on the market.
Generation II: These are jack-up barges without propulsion. They have high lifting capacity, high deck load capacity, large working deck capacity, a large accommodation area, helicopter deck, and they are self-elevating. The jack-up barge THOR for Hochtief with a 70 x 40 m working deck and a crane capacity of 500 t is an example of this vessel type.
Generation III: These have all the features of generation II and also propulsion with DP2 / DP3 capability. This generation is mainly used in areas further from shore. It significantly reduces the time needed for installation, as the time for transferring foundations, towers, turbines and blades from transportation barge to installation barge can be saved. This also makes operation much safer. HOCHTIEF’s Innovation or the Pacific Orca belong to this category.
With a total length of 161 metres, a breadth of 49 metres and a depth of 10.4 metres, the Pacific Orca is the largest WTIS ever built.
Several business segments of the GL Group had been involved in this project: GL Noble Denton undertook engineering and design of legs, spud-cans and jacking system, including FMEA, as well as integration of these components into the ship’s hull. They also provided the shipyard consultancy and site-specific assessment services, carried out global strength and fatigue analyses for the ship afloat, and were responsible for owner consultancy.
GL as classification society conducted the in-place analysis in jacked-up mode, as well as providing the classification services.
Offshore WIND staff, September 4, 2012; Image: Swire Blue Ocean A/S