Multiple Riser System for Shallow Water
Authors: De Leeneer Yves , Eik Geir 
Published: October 2008

Shallow water and high wave induced ship motions create a challenging and harsh environment for flexible risers and umbilicals. Two new riser systems are presented and analysed. Results of computer simulations for different load cases are presented and compared with the results of model tests.

Introduction

In shallow water riser systems (risers in this text means flexible unbonded flexible risers, subsea hose risers and umbilicals) are subject to a number of environmental and spatial parameters that increase the difficulty to find a suitable riser configuration and as well create high loads on the riser system. The main parameters influencing the riser behaviours in shallow water are the following:Due to the proximity of the wave zone, high drag and inertia loads are induced on the systems, which lead to high motions of the risers.The ship motions also induce high motions to the system which are not able to be damped out by the water and the weight of a vertically hanging riser as in deeper water systemsDue to the short space between the vessel attachment point of the riser and the seabed attachment points the induced motions at the vessel connection point is transmitted quite far down the riser towards the seabed connection point of the riser.The proximity of the seabed and the proximity of the vessel's hull make it difficult for the risers to find the required space to move freely through the water.Vessel offsets together with installation tolerances can reach up to a radial distance (+- from turret center point) of up to 30 % of the water depth. The above parameters create a situation where keeping the risers clear from the seabed and the hull of the vessel together with limiting the angles and loads at the riser-FPSO connection become a very difficult task. Hence finding a suitable riser configuration and riser design might become one of the main criteria for the feasibility of the development of a shallow water oil field using an FPSO or FSO.

Several riser configurations are usually being used in shallow water developments:Steep wave systems.Lazy S systems.Pliant wave systems.Hybrid systems with lazy S and lazy wave configurations. Each system will have some advantages and disadvantages on the others, and a case by case analysis will have to be performed in order to find the most suitable configuration for acertain field development. For more on the use of the above systems in shallow water see [Ref.1].

However on all systems the lack of space between the vessel attachment point and the seabed attachment is one of the most limiting factor when it comes to riser motions and hence vessel motions (offsets as well as wave induced motions).

The lack of this degree of movement of the riser through the water might have some influence on the designs:An external extended turret might be chosen instead of an internal bottom mounted in order to provide additional space for the riser.The exit angles and the loads on bottom mounted turrets are large due to restriction in distance between the bottom of the turret and the seabed, which often leads to having the riser pulled at large angles and heavy loads, which in it's turn leads to requiring huge bend stiffeners.