Method and apparatus for installing tethers on a tension leg platform

Tension leg platform arrester apparatus for restraining the vertical motion of a tension leg platform during installation and tensioning of mooring tethers between a subsea foundation and the tension leg platform, the platform having a work deck supported on columns above a buoyant hull and a hoist system including support structure and a hoist line connected to a winch, the arrester apparatus comprising;

• a platform arrester housing connected with the hoist support structure to move with the support structure and platform during dynamic vertical motions of the platform,
• slip means movably mounted in said housing through which the hoist line slidably passes,
• actuating means operatively connected with said slip means for selectively moving said slip means between a retracted position allowing the hoist line to slide therethrough and a biased locking position engaged with the hoist line to allow only downward movement of said platform arrester relative to the hoist line, such that
• when the tether is secured at its lower end in the foundation and connected at its upper end to the hoist line and constant tension is applied thereto through the hoist line, said platform arrester moves with the platform during dynamic vertical motions of the platform, and
• when said slips are actuated to the biased locking position said platform arrester will allow only downward movement of the support structure and platform relative to the hoist line and restrict upward movement thereof until relative movement of between the platform and the hoist line is diminished and platform trim is within predetermined limits.

A method of installing and tensioning mooring tethers between a subsea foundation and a tension leg platform at an offshore location above a subsea foundation on the ocean floor comprising the steps of:

• positioning a tension leg platform hull above said foundation;
• said tension leg platform having a work deck supported on columns above a buoyant hull, outwardly extending tether support members secured on the columns near the hull, hoist means on the platform deck near each column which are extensible outwardly from the work deck above each said tether support member, and platform arrester means operatively connected with each said hoist means through which a hoist line passes, said platform arrester means moving with said platform during vertical motions thereof and selectively operable to engage said hoist line for allowing only downward movement of said platform arrester means relative to said hoist line,
• securing the upper end of a tether tensioning tool having a tether connector at its lower end to each said hoist line and connecting each said tensioning tool to the upper end of a plurality of tethers, each said tether connector being vertically movable on each said tether for engaging said tether support member;
• positioning the upper ends of said tethers on said tether support members and engaging their lower ends in the foundation;
• applying and maintaining tension in said plurality of tethers;
• and
• while maintaining constant tension in said tethers actuating said platform arrester means to allow only downward movement relative to said hoist line until no further relative movement between said platform arrester means and said hoist line is observed and movement between the upper ends of said tethers and said tether support members has been dampened and platform trim is within predetermined limits;
• and
• upon determination that platform trim is within predetermined limits, activating said tether tensioning tools and said tether connectors to engage said tether support member and apply tension in said tether between its connection in said tether support member and the foundation;
• and thereafter
• removing said tether tensioning tools from said tethers and deballasting the platform hull to a predetermined in-service trim, whereby tension loading in each said tether is transmitted to the platform by the engagement of said tether connectors on said tether support members such that the total platform tension load is equally distributed between the whole tether complement.

A method for restraining the vertical motion of a tension leg platform during installation and tensioning of mooring tethers between a subsea foundation and the tension leg platform, the platform having a work deck supported on columns above a buoyant hull and a hoist system including support structure and a hoist line connected to a winch, comprising the steps of;

• securing a platform arrester apparatus to the hoist support structure to move with the support structure and platform during dynamic vertical motions of the platform, said platform arrester having a housing with slip means movably mounted therein and actuating means operatively connected with said slip means for selectively moving said slip means between a retracted position allowing a hoist line to slide therethrough and a biased locking position engaged with the hoist line to allow only downward movement of said platform arrester relative to the hoist line;
• passing the free end of the hoist line through said slip means and connecting it to the upper end of a tether secured at its lower end to the foundation;
• applying constant tension to the tether through the hoist line;
• while applying constant tension, actuating said slip means to the biased locking position engaged with the hoist line to allow only downward movement of said platform arrester relative to the hoist line, whereby
• said platform arrester will allow only downward movement of the support structure and platform relative to the hoist line and restrict upward movement thereof until relative movement of between the platform and the hoist line is diminished and platform trim is within predetermined limits.