Floating vessels such as floating production platforms have been widely used in oil and gas exploration/production as these mobile platforms may be moved easily from one site to another. There are more than 200 ageing floating platforms operating worldwide. These platforms are commonly deployed at offshore sites off Brazil, West Africa and Australasia. In the Brazil offshore market alone; there are a total of 29 floating platforms that are currently being deployed. Of these, more than 65% of these floating platforms employ turret mooring systems. Turret mooring systems allows a platform to freely weather vane around, adopting the direction of the least resistance against waves, wind and currents. For drilling operations, this is advantageous as the platform is able to automatically adopt a heading that provides the least resistance against the raging elements.
As these platforms age, maintenance and upgrading works need to be carried out on these platforms. Furthermore, platform operators are also constantly trying to extend the sea-life of the deployed platforms. This can be achieved by replacing worn out equipment and machinery with newer models or upgrading existing models thereby extending the lifespan of the platforms. The difficulty faced by the platform operators is that these maintenance and upgrading works must be carried out while the platforms are still out at sea, with minimal disruptions to the operations of these platforms. If the platforms were to be disconnected or taken offline for maintenance works, the loss of earnings cause by the down time of the platforms would be insurmountable.
A solution to the above issues is to use vessels equipped with the necessary machines and tools to carry out the necessary upgrading and maintenance works out at sea. However, most vessels are not able to withstand the rough seas and harsh environmental conditions in which floating platforms operate in. One type of floating vessel that is able to operate under such harsh and unpredictable operating conditions is a floating vessel designed for offshore work such as a semi-submersible.
The present market trend is to build semi-submersibles with displacements between 30,000 Metric Tonnes to 55,000 Metric Tonnes. Semi-submersibles are getting heavier and bigger in size as more semi-submersible operators are inclined to load the semi-submersibles with as many features as possible, such as accommodation units, emergency relief centres, helicopter landing pads, storage areas and such other space consuming activities. As the semi-submersibles become larger and heavier, more power is required to move these semi-submersibles. As such, the size of the power thrusters or the size of the engines increases as well. Due to the imbalance in weight and available power, semi-submersibles that are presently available are not able to react fast enough to the yaw motions of the turret moored platform as the platform weather vanes. This is because presently available semi-submersibles are either too heavy to react to the yaw motions or the thrust power of the semi-submersibles engines are fully utilized for station keeping, that is keeping the semi-submersible stationary in the harshest weather conditions. Therefore, it is a challenge to design a semi-submersible that is able to operate safely alongside a turret moored floating platform.