An Investigation into the Feasibility of Using Non-Metallic Materials for Construction Elements for Subsea Production Systems
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Installation of seabed equipment represents approximately one third of the capital expenditures (CAPEX) for subsea developments. This report is based on investigating into new designs of subsea production system (SPS) structures that can appear to be cost-effective concerning seabed installation. Firstly, the principle of modularization of subsea production structures is considered. Secondly, an investigation into the use of non-metallic materials for construction elements in the subsea structures is carried out. Thus, in addition to the already existing integrated template structure (ITS), a new modularized tight cluster design, called X-Cat, is regarded through the report. Due to the high weight and large footprint of an ITS, only large and costly vessels can be mobilized for the installation of the structure. However, the X-Cat contains smaller modules, which increases the potential of the possibility of using smaller and lighter installation vessels. If the unit-weight of the limiting X-Cat module is reduced accordingly, it will increase the potential of using even smaller and lighter installation vessels. The first approach in the investigation was to suggest three new designs with lower unit-weights for the limiting X-Cat module. The suggested designs were developed by changing the material of selected construction elements from steel to fiberglass, which would reduce the unit-weight significantly. Furthermore, installation cost estimations were performed for the installation of the suggested designs and, subsequently, the effect of reducing vessel size was demonstrated by cost comparisons. In addition, as the alternative structures contained suction anchor technology, a further sensitivity analysis on suction anchor installations with regards to critical pressures were carried out. From the results, installation duration estimates demonstrated that a single ITS could be installed almost twice as fast as a single X-Cat. However, it was observed that if SPS construction elements could be designed in glass-reinforced plastic (GRP) instead of steel, the structures could have a positive impact on the installation costs, as smaller and lighter vessels from a much larger vessel pool could potentially be mobilized for the operations. Concerning the sensitivity analysis on suction anchor installations, the results demonstrated that the suction caisson belonging to the X-Cat should be designed with significantly larger skirt thickness relative to the ITS suction anchors.This report concludes that a material change from steel to fiberglass in construction elements for a modularized SPS design will reduce the unit weight of the structure, and further reduce the installation design load; by reducing the weight of the structure by using GRP as construction material, the corresponding installation cost reduction of the X-Cat could potentially exceed 50%.Further work should involve looking into the manufacturing costs of SPS construction elements made in GRP. It should also include an evaluation and assessment concerning the seastate of large fiberglass structures during subsea installations.