This report investigates the potential reduction of steel weight for offshore wind turbine supporting jacket structures, if conventional welded joints are replaced by innovative wrapped FRP joints. This new type of connection is under development by Dr. Marko Pavlovic at the Delft University of Technology, and shows outstanding fatigue performance compared to welded counterparts. As jacket structures suffer highly cyclic load, member thickness of current jackets is governed by the fatigue performance of welds. Due to the superior fatigue performance of wrapped FRP joints, substantial weight benefit is expected to be made. The study examines a jacket supported 5 MW wind turbine located in 50-meter water depth in the North Sea. The structure and model are based on the UpWind project. The model includes soil-structure interaction by non-linear depth-dependent springs along with foundation piles. Fatigue limit state (FLS) and ultimate limit state (ULS) are simulated by respectively five and three scenarios. The scenarios consider different combinations of wind (speed and direction), waves (height, period and direction) and current (speed and direction). Six 10-minute simulations are performed for each scenario with different wind turbulence and wave irregularity seeds. Wind and waves are applied in a single simulation, and normal force N and bending moments Mip and Mop time series are recorded at a selection of elements. The time series are post-processed in a self-written MATLAB procedure. For FLS, detailed fatigue analyses of welded joints are performed by evaluating crown and saddle hotspot stress, according to DNVGL-RP-C203. For every time step, the hot spot stress is calculated by applying geometry and load-dependent stress concentration factors (SCFs). Rainflow counting is applied, and the resulting stress range is projected on the details’ S-N curve to evaluate the damage. Linear Palmgren-Miner is applied to accumulate damage. A similar procedure, including stress concentration at thickness transition, is applied to calculate fatigue of elements. For ULS, welded joints are checked for chord face and punching shear failure. Members are checked for tension yielding, local buckling and global buckling. ULS calculations are performed for all time steps and according to Eurocode manuals. The unity check of both FLS and ULS is calculated for each individual member. Next, the member thickness is manually optimised to obtain the most optimal use of material. This optimisation is performed for three different cases with both mild S355 steel and high strength S690 steel. The welded steel structure, case 1, acts as a reference. The unwelded structure, case 2, is the lightest structure if joint fatigue does govern design. Case 3 gives the wrapped FRP structure and includes fatigue results obtained from small scale lab tests. Additionally, due to limited production length of steel tubular elements, it includes circumferential welds in the legs. The potential jacket weight reduction if wrapped FRP joints are applied is large, and the governing unity check shifts from fatigue to global buckling. For mild steel, the reduction of steel weight is more than 50%. The additional reduction of mass for high strength steel is low and not economical. The eigenfrequency of the wrapped FRP structure is viable, as it is outside operating frequencies. The results for the wrapped FRP structure are based on two major assumptions. Firstly, satisfactory joint performance can be obtained, and secondly, this can be accomplished by increasing wrapping thickness only. These assumptions should be verified by future experiments to support the weight reduction statement. In conclusion, the potential benefit of wrapped FRP joints to offshore wind turbine supporting jacket structures is large, and future experiments will show if, or to what extent, the full potential can be exploited. ... Read more

Tourism is one of the main sources of income of Jamaica. . However, at the moment the beaches are retreating. An example is the erosion of Hellshire Beach, showing a retreat of ten meters in only seven years. To preserve the beaches effectively, a new concept is requested. The main requirements of the system are wave attenuation and the marine life enhancement.

The literature study showed, a variety of coastal management techniques exist. However, none of those solutions are capable of attenuating waves and enhancing marine life in an effective way. Ranking criteria are given and the following concept groups are generated: boulders, gabions, marine blocks, big (open) blocks and 'lego' (interlocking) blocks. The Multi-Criteria Analysis shows that the big (open) blocks are the most viable and two concepts are designed within this concept group: a triangular and a hexagonal block structure.

In the hydrodynamic and wave models (Delft3D), a study is performed to find the relation between breakwater dimensions and wave attenuation. Also, three different conditions are modelled: daily conditions, hurricane conditions, and one-year storm conditions at the Hip-Strip in Montego Bay. Using the results from the hurricane model, the flow- and wave forces are calculated using the Morison equations for lift and drag. Three lay-outs for submerged breakwaters are tested in the model. This led to the final lay-out which is a combination of the three tested breakwaters.

Following the Delft3D models, a structural analysis is done with the flow- and wave forces from the Morison equation. The structural analysis focuses on the sliding and uplift of the submerged breakwater. The hexagonal structure shows a better stability than the triangular blocks in hurricane conditions and therefore chosen as the final concept. A sensitivity analysis is performed with regard to the friction coefficient, the force-time profile and the placement errors. The placement errors turn out to be crucial and a connection between the top block and the base is needed to retain stability.

The final dimensions (l x w x h) of the hexagonal blocks are 3 x 0.75 x 0.93 meter. The blocks can be made from a low strength class concrete and reinforcement is needed to provide strength during lifting. To enhance the marine life enhancement properties, fish condos of 4" and 6" are provided, the surface is made more permeable and the pH of the concrete is altered by curing.

The structure shows great future potential and can be built soon. It is a state of the art structure, the stability is high, it enhances the marine life, the final dimensions will precisely agree with the drawings and there is no need for a nearby quarry. However, to all this benefits, there is also a drawback; the cost. The cost is a multiple of the conventional armour stones. Recommendations are given to bring expenses down. Placement in shallow water is preferred and replacing steel reinforcement by fibre reinforcement is worth investigating. Those recommendations will decrease the cost and will increase the viability of the Honeycomb block concept. ... Read more