Ingenieur April-June 2016 Ingenieur Apr-June 2016 | Page 60

INGENIEUR Integrated Float-over Essentially, there are two ways of installing an IC topsides using the float-over method; the conventional method by navigating the float-over barge in between the jacket legs, or alternatively by the sides of the jacket legs. The latter method involves two barges carrying the integrated topsides at both ends. Instead of sailing through the opening between the jacket legs, the barges have to steer around the jacket on each side and, at the same time, de-ballast to place the topside onto the jacket. The use of this method is very rare, although it can be used for both small and heavy-sized structures (estimated to be around 20,000 MT), depending on the matched pair of barges used. Another technique is using a special type of float-over barge which has a ‘fork-like’ stern that wraps around the jacket prior to mating. There are two ways of mating the topsides to the jacket legs. The conventional method is by barge ballasting whereby the IC topside legs are mated onto the jacket simply by ballasting and de-ballasting of the float-over barge. The most recent method is by using the dynamic/hydraulic jacking system. As a structure with a relatively high VCG, the height of the IC deck needs to be as low as possible during transportation especially in adverse sea conditions. As a result, the jacking system is required to lift the topsides to a sufficient height above the jacket legs to enable successful mating. However, the use of this jacking system can be costly and would defeat the cost advantage of the IC method. Hook-Up and Commissioning (HUC) The MC concept requires more HUC interfaces compared to the IC deck, hence it requires a longer time for offshore hook-up work. Transferring work offshore increases both cost and duration of a project as work carried out offshore is: ●● Much less efficient compared with work done onshore; ●● Requires additional expense for marine spread; and ●● Utilises manpower at a higher unit rate compared with onshore. In comparison, a single integrated topside design limits the work required for an offshore 6 58 VOL - JUNE 2016 VOL66 55APRIL JUNE 2013 hook-up and commissioning. The complete integration of facilities in the fabrication yard allows production and utility systems to be commissioned much more efficiently than is possible with the offshore hook-up of a MC topside. This leads to a significant reduction in offshore HUC duration and cost, a lower risk of schedule delay and a smoother start-up. Project Planning Figure 4 depicts the project planning for both IC and MC execution stage. Figure 4: Front-End Loading of both IC and MC methods The IC method requires heavy front-end loading (FEL) activities to be carried out. Detailed planning must be undertaken very early in the project stage as the transportation and installation barge needs to be identified and locked in upfront. In contrast, the barge selection and load out arrangement decision for a MC can be delayed until the fabrication stage. For an IC, the design of the LSF and LMU must be completed as early as the conceptual stage, to determine the barge capacity requirement. This enables the barge selection process and load out arrangements to take place during the Front-End Engineering Design (FEED) stage. A thorough analysis of the load out details would then be undertaken during the Detailed Design stage which includes yard bulkhead strengthening, dredging, barge installation requirements, structure design, weather window