Abstract
There are clear cost and schedule benefits in maintaining offshore structures within lift-installed parameters for both Jacket structures and Topside modules. The paper sets out initially the context that drives the decision making process for determination of the various available options including lift-installed, launched and float-over.
The benefits in maximising the available lift capacity of heavy lift vessels (HLVs) include; avoiding the need to develop solutions such as barge-launch jackets that result in significantly longer fabrication schedule and costly mechanical systems, as well as the extra weight and associated costs for buoyancy tanks and launch rails. There is thus a significant benefit in maintaining weight and centre-of-gravity within the liftable envelope of the HLV.
This paper describes a case-history of Valemon jacket, installed in July 2012, which at 9200t has a lift weight close to the lifting limit of the Thialf, the world’s largest offshore crane vessel. This was achieved via the implementation of several weight saving measures during post-FEED and detail engineering stages ensuring the weight kept within the target throughout. These measures included the use of high strength steels, a fully-coated jacket, detailed and repeated analyses, the use of more sophisticated methods for ship impact, using structural details such as cones to minimise brace diameters, the use of vortex shedding restraints, minimising pile offsets relative to leg, post-installing caissons and flooding braces to reduce wall thickness.
The result was that the weight was maintained within the limit of 9500t set at the outset, with the design independently checked by the clients’ third party verifier. The application of the measures listed herein may assist in maintaining future jackets in water depths of up to 135m, as at Valemon, within the lifting envelope, with clear associated cost and schedule benefits.
