Flexible Riser Technology for Deep Water Applications
Authors: lshii K. , Makino Y. , Fuku T. , Kagoura T. , Yoshizawa M. , Wada H. 
Published: October 2008

With advances in deep offshore oil development, the weight reduction of the flexible risers becomes most important. Furukawa Electric and the Japan National Oil Corporation have been working on thedevelopment of lightweight flexible risers.

One way to reduce their weight is to strengthen the reinforcing steel, and high-tensile-strength steel was developed that was 50% greater than existing steels. As another way of reducing the weight, the reinforcing steel was replaced with the composite materials such as fiber reinforced plastic (FRP).

Using newly developed materials for the reinforcing, the prototypes of lightweight flexible riser weremanufactured and their basic characteristics and fatigue properties were evaluated. It was confirmedthat each prototypes were good enough to apply at the water depth of 1000m and more.

INTRODUCTION

Deep water offshore oil development has progressed in recent years that now oil is being produced at depth in the 500-1000m. (1) (2) And the depth exceeding 1000m are eyed for development until the turn of the 21st century. Floating production systems tend to be the mainstream in deep offshore oil field, and the needs of flexible risers become to increase.

The depth at which flexible risers are able to use is imited by the allowable tension and the laying side pressure, which depend on the weight of the riser itself. (3) (4).

The limit of the depth of existing flexible risers varies from the pipe size, service pressure, and ocean conditions, anyhow it is considered within 1000m. Thus the attempt was produced to developelightweight flexible risers. This paper discussed the design of two types of risers newly developed and theresults of manufacturing and evaluation tests.

APPROACH TO REDUCE THE WEIGHT

Figure 1 shows the structure of the existing flexible risers used in offshore oil production systems. The function of each parts is as follows.Interlock conduit: Stainless steel carcass which protects the inner pipe from collapse due to external pressure and abrasion due to TFL tools.Inner pipe: Thermo plastic pipe ensures that the pipe remains requid-proofing during survice.which sustain the radial load of the pipe caused by internal pressure. Spairalled steel tapes of C- or Z-shaped crosssection are used.Outer reinforcing layer: The reinforcing layers which provide a resistance for the axial load caused by internal pressure and dead weight. Double layers of steel tapes are contrahelically wound to provide a torsional strength.Outer sheath: Thermo plastic sheath forprotecting the reinforcing layer against external corrosion and abrasion. Nylon or polyethylene is extruded over the outer reinforcing layer.

Of above listed layers that make up a flexible riser, the inner and the outer reinforcing layers account for 80% of the total weght, and reducing their weight was thought as the most important goal. Theinterlock conduit makes up 10% of the total weight at most, and because the strength to resist external pressure must be greater in deep water, interlock conduit was made an exception of the object of the weight reduction.