In this thesis, an alternative cable lay system was designed that can be used on existing non-cable lay vessels. This design is conceptualized and analyzed for a specific market, following the steps in the engineering design process. The technical and economic analysis investigates the feasibility and competitiveness of the new cable lay system. Nowadays, cables are installed over a chute at the aft of a vessel, which makes the operation very sensitive to motions, inducing high cable loads that result in limited workability. Also, load carrying capacity is limited for existing cable lay vessels within the Boskalis fleet, making the application of joints a necessity in offshore export power cable installation. The design focuses on the international offshore export power cable installation market (both AC and DC), including interconnectors, from shore to substation or shore to shore. This market was selected because of the limited load-carrying capacity of current cable lay vessels, the internationally growing offshore wind market and the relatively low market share of Boskalis in this segment. New concepts were generated and evaluated using multi-criteria analysis, scoring them on technical and economic criteria determined for the selected market. The concept that has been selected for further development focuses on export power cable laying through a moonpool. A Dockwise semi-submersible heavy transport vessel is targeted for this design because it is being converted into a fall pipe vessel already. This conversion includes the installation of a moonpool, opening up the possibility to make the selected vessel a multi-purpose ship. Making use of static stability software, the maximum load-carrying capacity of this vessel has been determined. From this analysis, it can be concluded that the maximum cable load that can be carried by the selected vessel is 9.000 tonnes of cable equivalent, which is approximately 110 km of currently installed export cable length. Conventional cable lay vessels from Boskalis have load carrying capacity up to 5.000 tonnes. Additionally, a deck-layout with the crucial parts of the cable lay system is designed, taking into account all necessary alterations regarding the conversion of the vessel. The main technical challenge for the newly designed system is the second end cable pull-in. With limited space in the moonpool and vertical laying of the cable, the conventional pull-in method cannot be used here. Three solutions have been developed, based on a deployment quadrant or bight lay down. Two of these are already proven in the field, making the concept technically feasible. A model has been made to evaluate both conventional cable lay, and cable lay through a moonpool. With the use of dynamic time-domain analysis, the operational limits have been determined for both methods, keeping the catenary shape of the cable constant. This analysis concludes that cable laying through a moonpool indeed increases the workability for the selected vessel. No significant increase can be seen for moonpool cable laying with a conventional cable lay vessel. This is due to its sensitivity to roll motions, for which the distance towards the center of gravity is equal in both concepts. For the selected vessel, cable laying over a chute is not possible for the chosen catenary shape and sea states because the maximum allowable curvature is exceeded. This is due to the large arm for pitch motions, from the center of gravity to the chute at the aft of the vessel. Cable laying through a moonpool, however, is possible up to 2 meters significant wave height. The limiting factor for all simulations that were done is the maximum allowable curvature. To investigate the competitiveness of the newly designed cable lay concept, an economic analysis is done. Several recently acquired project cases are introduced to compare project costs for both concepts. This analysis concludes that the newly designed cable lay concept is in average conditions 21 to 40% more expensive for all cases than the conventional way of cable laying. For cable installation during wintertime, the newly designed cable lay system is only 6.5 to 27% more expensive than the conventional cable lay system. This illustrates that the newly designed cable lay system is technically feasible but not competitive with conventional methods at the moment according to this analysis. However, the costs and installation time of joints with the conventional methods are not taken into account here due to limited available data. Therefore, the new cable lay concept still has the potential to be competitive for the selected market. Further research must be done to substantiate this.

Context: The shift to green energy offers a need for offshore wind farms. The transition from a relatively cheap energy production from carbohydrates, to a young offshore wind energy industry needs innovations to lower the cost. Increasing the efficiency of wind turbines and lowering the installation cost of a wind farm, will eventually make wind energy cheaper then energy produced by coal. A part of the installation process is the burial of the submerged power cables. The fragile power cables have to be buried a couple meters below the seabed, to be protected from anchors, fallen objects, and fishing activity. Problem definition: To submerge the power cable into the seabed, a trench have to be made. The soil of the seabed can consist of sand, silt, clay, rock or any combination of them. This research focuses on the trenching though clay. Clay has a very low permeability compared to sand, and clay has cohesive strength. For trenching in clay, a narrow plough is mostly used. These ploughs have a small frontal area and are build to cope with the high ploughing forces. A prediction of ploughing forces and velocities are made by models in preparation of cable burial projects. With accurate predictions, the best equipment can be chosen and a planning of the trenching operation can be made. The prediction models take into account the geometry of the plough and the soil characteristics. A reduction in pulling force results in an increase in ploughing velocity and therefore lowering the time and cost of cable installation. Approach: A large part of the pulling force in clay is caused by the adhesive force on the sides of the plough sliding through the clay. The adhesive force is therefore the main focus of this research. There are two main goals: the first goal is to investigate the adhesion force in relation to the strength of the clay; the second goal is to investigate ways to reduce the force caused by the adhesion. The adhesion factor is a parameter that is included in the prediction models. In literature of construction and agricultural engineering, predictions of adhesion factors and ways to reduce the adhesive force can be found. However, the circumstances during subsea ploughing are vastly different then in the other fields of engineering. Therefore, the approach of this study is to use small scale ploughing experiments with different strengths of clay to investigate the two goals under subsea ploughing conditions. For the experiments, a test setup is used. In this setup, a block of clay of a meter long can be hold into place in a water tank and be submerged. On top of the water tank, an electric motor can pull a cart over rails. A small scale plough can be bolted underneath the cart. During an experiment, the plough is pulled through the clay and the velocity and pulling forces are measured with sensors. Results and conclusions: The adhesion factors of three types of clay are found. The softest clay with a undrained shear strength of 25 kPa has a adhesion factor of 0.43. Literature shows that the adhesion force is about 1.0 at 25 kPa. This low adhesion factor could be the result of the frontal cutting that disrupt the clay. The residual shear strength is lower then the undisturbed clay. For the medium (80 kPa) and hard (131 kPa) clay the adhesion factor is respectively 0.68 and 0.53. These values are on the higher side of what literature reports. This could have to do with the relative high velocity during ploughing. The downward trend of adhesion factor with increasing clay strength does correspond to literature. To research the possibility of reducing the adhesion force, three small scale ploughs with modified adhesion surfaces are tested. The Alpha plough, which uses vertical gaps, reduced the adhesion force by 52 percent in both soft and hard clay. The Bravo plough, which uses convex shapes, reduced the adhesion force 39, 72 and 54 percent in respectively soft, medium and hard clay. The Charlie plough, which uses water nozzles at the adhesion surface, reduced the adhesion force by 70 percent in soft clay and 63 percent in hard clay. This experimental study obtained valuable knowledge of the adhesion factor and possibilities to reduce the adhesion force.