Feasibility of Steel and HDPE Sea Water Intake Risers for Deepwater Applications

Abstract

Abstract There is a growing desire in the offshore oil and gas industry to lower CO2 emissions from operations, especially in FPSOs and FLNG units. One method to lower CO2 emission is to reduce power consumption by improving the cooling system so that it utilizes colder water from deep sea. Traditionally, Sea Water Intake Risers (SWIRs), which are usually made of reinforced rubber hoses, are used to supply water from water depths up to 150m to the topside, resulting in an in-take temperature of at least 15°C. Nowadays, many operators have been trying to reach colder water (5-7°C) with target SWIR depths up to 700 m in order to support emission reduction efforts. However, there are many design and material restrictions to achieve this goal. Therefore, this paper focuses on the feasibility for steel and HDPE SWIR configurations for deep sea applications (up to 700m). Conducting a site-specific feasibility assessment at early design stage plays an important role in selecting an optimum SWIR configuration. The main pipe of the SWIRs is generally made of reinforced rubber, steel, or high-density polyethylene (HDPE). The SWIRs are installed in a free hanging configuration at the keel or on the sides of the FPSO. Their design can be challenging for deep water applications due to harsh environmental effects (i.e., wave and current). Interference with adjacent structures such as production risers or mooring lines can make their design even more complicated. In order to investigate the feasibility of 350 m and 700 m SWIRs, nonlinear finite element simulations are performed using HDPE and steel pipes for a conceptual field. The steel SWIRs may provide more confidence in long term reliability as steel is a conventional material for dynamic pipelines (e.g., production risers), but can lead to weight and fabrication constraints for deep water applications. HDPE is neutrally buoyant and eliminates weight restrictions but is also prone to creep, large displacements, and connection challenges. Today, there are many SWIRs in service, operating up to 150 m water depth and their design and operational requirements are well known by the oil and gas industry. Pumping sea water at depths greater than 150 m is a significant advancement for the industry as deeper SWIRs provide colder water and further reduction in the CO2 emission of FPSO's cooling systems. This paper focuses on the challenges and design limits for deep SWIRs (350 m and 700m) made of steel or HDPE.