An Innovative Self Installing Platform (SIP) Concept

Abstract

Centrica Energy (formerly Venture) selected a self installing and re-usable platform concept (SIP) to develop their F3FA gas field in the Northern Part of the Dutch sector of the North Sea. On 6 February 2009 and following an initial FEED study, the contract was awarded to Heerema Fabrication Group (Main Contractor and Fabricator), in partnership with Iv-Oil & Gas (Designer and Procurer of process equipment) and SPT Offshore (Installation Contractor and Foundation Designer). The platform weighed in total 8,800 ton, including a 4,000 ton deck and 4,800 ton substructure including the suction pile foundation. The water depth at the F3FA site is 41m and the bottom of the cellar deck is located at 20m above LAT. The platform has an overall dimension of 63m × 45m footprint and protrudes some 90m above the seabed (excluding vent stack). The platform concept consists basically of a deck of 5 levels supported by 4 unbraced legs (3.25m outside diameter × 77m long) and founded on 4 huge suction piles (15m outside diameter × 13m deep). The platform is transported and installed using a large flat top barge (BOA 35 measuring 124m long × 31.5m wide × 8m high) and 3 tug boats (75, 90 and 183 ton bollard pull) for the tow from HFG’s yard in Vlissingen to the F3FA site in the Dutch sector of the North Sea. The leg lowering and deck lifting was achieved using 12× 900 ton strand jacks. The suction pile foundation was installed using 4× suction pumps. During the detailed design stage of the project, the concept design was subject to several elementary changes in the structural design. This had to do with the severe environment, the increased deck weight and water depth compared to the earlier utilisation of the concept. Fatigue and transport and installation issues had a significant impact on several key structural design elements. This complicated the deck-leg connections and several members in the truss deck. The deck leg connection was amended from a simple clamp connection in to a superbolt and sleeve connection at the top deck and a clamped and grouted connection at the cellar deck. In order to improve the stiffness of the legs, leg stiffening frames were placed on top of the suction piles. Due to the limited and slow roll & pitch response of the platform whilst on the BOA barge during tow, high seafastening forces were predicted. These high seafastening forces were transferred in to the barge using pinned seafastening arms. Water depth limitations and the additional leg stiffening frames necessitated to use a fairly high grillage structure of 6.5m high. All these items changed the deck construction towards a method obstructing completion of non-structural disciplines and changed the inshore mating operation of the substructure with that topside towards a complex lifting and fitting campaign. Despite these elementary design changes and some rigorous changes in the planning of the various activities, the contract schedule was achieved in time. The above efforts had an obvious effect on the managerial issues towards the project. It required a doubling of the manning levels and engineering issues of in some cases R&D nature. Despite all these challenges and complications, the platform left Vlissingen well ahead of time, on 18 August 2010, to set sail for the F3FA site. Upon arrival in the field, the weather deteriorated severely and the tow was forced to seek shelter in Rotterdam. Just before arrival in Rotterdam severe waves were encountered up to 6–7m maximum. Upon inspection in Rotterdam, the design proved to be resilient as no signs of damage were observed. Following 10 days waiting on weather the platform was towed to the field for the second time. Installation and lifting the deck to the top elevation was achieved within a record time of 2 days and 4 hours. Thereafter the platform hook-up team continued to work on the platform including completing the deck leg connections, such as installation of superbolts at the top deck level and clamp shells, rubber pads and grout at the cellar deck level. Further hook-up work consisted of hooking up the life support systems before arrival of the drill rig on 14th October 2010. The drilling of the first well was completed early January 2011 and first gas was introduced in to the pipeline system since week 4 of 2011.