Subsea pipeline are extensively used for the transport of hydrocarbons from offshore wells, to platforms, pump stations and to onshore facilities. Because the installation of pipelines is time consuming it is responsible for a significant amount of the total costs of a project. Thus the workability of the installation is of great importance.When installing a subsea pipeline one always begins with a start-up structure, a FLET (flowline end termination) or PLET (pipeline end termination). The start-up structure is lowered through the moonpool via the pipelay tower until it reaches the seafloor. When it’s close to the sea floor the start-up rigging is coupled to the start-up pile with the use of a remote operated vehicle (ROV). Often the moment the start-up structure transitions from a vertical to a horizontal position with respect to the sea floor the loads on the stem pipe become critical with regards to the structural integrity of the pipe. And as such dictates the workability limits of the start-up structure installation. Pipe integrity is maintained via the use of a unity check equation which is described by the design standard DNVGL-ST-F101 issued by Det Norske Veritas Germanischer Loyd (DNVGL). In this equation, the combined loading criterion, the combination of the effective axial tension, the bending moment load and the water depth is evaluated for the structural integrity of the pipe string. The purpose of this thesis is to decrease the conservatism of the equation by probabilistic modelling of the resistance parameters – yield strength, ultimate tensile stress, wall thickness, outer diameter & ovality – instead of using deterministic nominal values and in the end allowing for higher sea states to operate in which in most situations increases the workability. For start-up structure installations DNVGL aims for a target probability of failure of 10-3¬ ¬.To achieve this first a well-documented load case was found in the Ichthys project. In HMC’s pipeline database the 18” Ichthys pipeline project offered 1106 geometrical and material strength pipe line data points. This data set was filtered analysed and used two describe the (bivariate) probability distributions of the resistance parameters. Analysing the data set it was found that the wall thickness and outer diameter and the yield strength and ultimate tensile strength showed a significance correlation. Dependence models have been defined by the use of copula’s. A performed sensitivity analysis showed that in the shallow water case, which the Ichthys project is, modelling the ovality as a stochastic variable has no significance influence on the outcome of the unity check. To assess the benefits of probabilistic modelling of the resistance parameters in a more general sense the base case Ichthys situation is altered to four different load scenario’s. Two shallow water cases and two deep water cases. For shallow and deep water, one case with the original sea state, in which the unity check is below 1. And one case in which the significant wave height is increased to push the unity check value to its limit of 1. After the input, the (bi-variate) probability distributions, and the test cases were defined the sample size for the Monte Carlo was determined to be 3*106 samples to guarantee the accuracy similar to what is used in current installation analyses. Performing the Monte Carlo simulations the results showed the expected conservatism in the current method. Where DNVGL aims for a probability of failure of 10-3¬, the probability of failure in the base case was calculated to be 10-5. Which allowed for finetuning and decreasing the safety class resistance factor used in the equation by 3% in the shallow water case and 4% in the deep water case. Which makes it possible to operate in heavier sea states and thus increases the workability of a start-up structure installation in certain situations.

The main goal of this project is to determine the optimal location for an OTEC installation with a minimum lifespan of 30 years off the coast of Barranquilla and to make an anchor mooring design for the floater on which this installation is located. Bluerise has identified an area near the coast of Barranquilla for which OTEC can be applied. This area is situated within Colombia’s territorial waters (within 12 nautical miles, or 22.2 kilometres), where two locations have been identified by Bluerise: Location 1: 11.2028 latitude, -75.0003 longitude, Location 2: 11.2772 latitude, -74.9208 longitude.

Environmental conditions -
The daily wind direction is NE-ENE. There is no clear extreme wind direction. The daily waves have a dominant Northeast direction while the extreme waves have a dominant Northern direction. The extreme waves are generated far north of Barranquilla by very high wind speeds which explains the relatively high extreme significant wave heights in the area and the relatively low extreme wind speeds. The yearly average (nautical) surface current direction at the two possible floater locations is predominantly south or southeast. The top 20 strongest current speeds in the past few decades have come from the west or southwest however and therefore these are the normative current directions. The environmental conditions are equal for both possible floater locations. A temperature difference of 20_C is reached at warm water intake and cold
water intake depths of 30 and 763 meters, respectively. The depths at which a temperature
difference of 22_C is reached are 36 and 1023 meters (with temperatures of 27 and 5 degrees, respectively). The influence of the Magdalena river and upwelling is concluded to be negligible.

Marine traffic - The two locations with safety zones are located in a traffic-dense area. The area is getting more traffic intense in the upcoming years. However, it will not pose an immediate threat to the operation. As location 2 has slightly less traffic, it would be preferable from a safety point of view.

Seawater intake- and return pipes - Assuming a cold seawater intake temperature of 5C and a warm seawater intake temperature of 27C, the intake pipe lengths become 1023 and 36 meters, respectively. Based on the equation
of state, the mixed water return flow pipe length becomes 130 m. At this depth, the effects of a difference in density between the surrounding seawater and the mixed returned water are minimized. Also, the depth is outside of the euphotic zone which minimizes algae growth. If the intake water is higher than 27 degrees, the discharge temperature will have a higher temperature. Calculations with the Equation of State reveal that the warmer the discharge temperature,
the lower the density of the discharge water is. Whenever the discharge temperature is higher than the output temperatures, less depth is needed in order for the discharge water to be naturally buoyant. As the intake temperature fluctuates throughout the year, it is therefore advised to design the length of the discharge pipe at 120 meters.

Anchor mooring design - The proposed anchor mooring design consists of a spread-moored 4x3 taut mooring system. The lines are composed of three parts: a 50 meter chain connected to the ship, a 1290 meter fibre line part and another 150 meter chain at the end that is connected to the anchor. The floater is positioned in a 58,05 angle with respect to the north in a northeast direction. This ensures comfortable operation during daily conditions and will reduce fatigue build up. The hurricane conditions were found to be governing. The design complies with the basis of design stated in section 5.3 and with the DNV-OS-E301 code and the API Recommended Practice 2SK.