Safety Consideration of TLP on Earthquake Wave Propagated Through Seawater

Abstract

ABSTRACT Seaquake is caused by propagation of the seismic motion of seabed through seawater. Althou h aseismatic design has been providing ap licab5 e marine structures for earthquake zones: e1f?ects of seaquake maybe one of items to be taken mto further consideration about the safety of TLP (Tension Leg Platform). Hydrodynamic ressure caused by sea uake is analyzed and its ef ?ects to a hull of T1P is investigated. Effects of seaquake de end on the water depth, seismic intensity and prei ominant period and size of seabed in vibration. Seaquake may impose impact load on a hull and is advised to be taken into consideration of the safety of TLP. INTRODUCTION Deep sea is one of new frontiers for oil production, Emerged aa measures to cope with this new frontier, TLP has been performin the same functions of oil rigs effectively as those of conventional jacket-type platforms done for shallow sea. Namely, TLP has been applied ractically to offshore oil reduction at the Hutton Fielc?in North Sea in 1984 anf at the Green Canyon Field in the Gulf of Mexico in 1989, which are followed by their counterparts under construction at the Snorre Field and the Heidrun Field in North Sea. TIJ? is now considered to be a promising structure to conquer the new frontier of deep sea. The Japan Ocean Industries Association (JOIA) has organized a technical committee of TLP of a new concept which can be installed in offshore oil fields of earthquake zones. The committee has worked out a concept of a sde and economical TLP for deep sea of earthquake zones and performed research and development on fundamental techniques sustaining the concept. PROTOTYPE STRUCTURE The outline of the prototype TLP prototype which the JOIA committie has stuchedis shown in Fi .1. A steel hull with a mounted deck is moored with four sets of tendons stretched downward in tension to a template of hybrid structure. The steel hull is composed of a frame deck structure, columns and pontoons. Six columns with circular cross section support a pair of longitudinal and transverse main girders of the deck and four corner columns are anchored with tendons to a template placed on the seabed. The total of 16 tendons made of steel pi es with the diameter of 609.6mm are provided with Four tendons per a corner. The template is a avity-type caisson of steel-concrete hybrid structure %a! ving a square cross section with rounded corners for scour protection. This TLP is applicable for offshore oil field with flat seabed having proper bearing capacity and water depth equal to or more than 500m. Environmentalconditions used for design and construction plan of the TLP areas summarized in Table-1. RESEARCH AND DEVELOPMENT To co e with effects of seismic motion for the seabed toa Tb installed in earthquake zane is taken into consideration for a transmitting path of seismicmotion from seabed, template, tendons to hull, aseismatic design of its structure.