A New And Cost-Beneficial Approach To Tlp Tethering

ABSTRACT A new concept of the tendon system for next generation Tension Leg Platforms, has been developed. The concept highlights an approach in solving primary problems related to TLP tethering in which simplicity is a main issue that provides the basis for a safe and cost effective tendon concept. Main features of the system are:–Prefabrication of complete tendons–Tendon transport by submarin towage in groups–Preinstallation of tendons in groups–Hull to tendon mating Emphasis, has been put into working out competitive tendon to hull interface, making the concept suitable to most designs of TLP's, steel as well as concrete. Additionally the installation method is relatively insensitive to water depth, making the system especially suitable for deep water application. Bouyant thin wall tubes are used as tendons. These are proposed prefabricated to obtain the best fabrication conditions. The tendons are transported in groups by well established submarin technique. At the field the preinstallation to foundation takes place groupwise in just four identical operations. This method offers a number of advantages as e.g. quick installation subdivided in few well separared steps. The installation is relatively weather insensitive and there is minimum risk of handling impact. The mating and tensioning are undertaken in a short period and are judged reliable. The tendons are terminated at the base of the columns and load transfer for each tendon is in a single point. There is provided for easy access for inspection and maintenace of tendon structure, support, load measuring system etc from dry compartments at the bottom of the columns. Results from analyses and model tests of tow and installation show that the proposed method cause low stress levels in the tendons during these operations. By introducing a number of new approaches in TLP tethering and using only common relatively low cost marin vessels as well as omitting temporary mooring of the TLP and other vessels the cost for the system is extremely competitive. INTRODUCTION The cost for the tendon system of a TLP is typically judged to be about 20 percent of the total project cost. Essential to future TLP projects is thus development of a cost effective tendon system. An attractive system is based on first of all reliable and easy installation, each step of which should make short weather window requirements and "waiting for weather" should not cause significant cost increase. A tendon system shall be safe in operation with low risk of failures, simple and reliable inspection systems and effective procedures for tendon change out. The presented system is developed considering these objectives. The system offers a number of advantages compared to other systems. The tendons can be prefabricated to low cost by application of common fabrication praxis. Use is made of the prefabricated tendons suitability of cost effective submarin towage and prefabrication gives project schedule and cost advantages.