Mid-Water Arches (MWAs) have been used as alternative to distributed buoyancy systems in flexible riser designs worldwide. Those structures usually involve complex lifting requirements for installation, as the Buoyancy Tank Assemblies (BTAs) are lightweight compared to its volume and footprint area. Handling those odd-sized structures in the deck of a construction vessel and crossing the splash-zone are frequently challenging engineering tasks.
This paper will present a case study of MWA installation performed by GE/WSC as part of the OGX Tubarão Martelo field development, offshore Brazil. The location is prone to swell from South Atlantic and hence a flat sea is a rare event, even when no local wind generated are present. This makes any delicate and weather sensitive operation a critical schedule issue. Still, 8 MWA were installed with minor Waiting on Weather thanks to an extensive analysis envelope and to an engineered installation method aiming to minimize the time the operation was limited by weather condition.
Also, as the scope involved a large number of MWAs, a “mass production” method was developed for the operations, not only covering the offshore installation but also the mobilization and logistic aspects of handling large and heavy items ashore. To allow road transportation by trucks, the BTAs and Gravity Bases (GBs) were shipped to the mobilization port in sub-components for site integration.