Experimental Investigation of Deep Sea Riser Interaction

Abstract

Summary In future deep sea field developments [he drag force and corresponding static deflections of the risers due [o current can become quite large. The prevention of mechanical contact (collision) between the risers will need more careful evaluation than in moderate water depths. The paper describes a series of model experiments in a Norwegian fjord to determine criteria for on-set of collisions between [he risers of a deep sea TLP. The current was modeled using the natural tidal current in the fjord. Results from the tests are summarized and used for verification of numerical calculations of collision criteria. Introduction When designing future offshore oil and gas production platforms for very deep waters, 1000 [o 2000 meters, the riser system will be one of the most critical parts. The risers will become heavy and may require some sort of buoyancy arrangement to keep the tension at an acceptable level. More serious is the effect of current on such long risers. Let us for instance consider traditional rigid, vertical risers of the type used in TLP and spar type platforms. With given current velocity, diameter, and pretension the static deflection of the risers due to current drag is proportional to the square of their length. Thus very large deflections can occur, and if the different risers in the array deflect somewhat differently, they may start clashing into each other or collide. Objective The objective of this paper is to present the results from a research project, in which the criteria for on-set of riser collisions were determined by model tests in a deep Norwegian fjord. Wake Method for Prediction of Riser Interaction The physics of wake interaction, and possibly collisions, in deep sea riser arrays is as follows: A riser sitting in the wake of an upstream riser will, due to the shielding effect, be subjected to a lower inflow velocity than the upstream riser. Other relevant parameters being equal, its static deflection in the current will be correspondingly smaller, and there is the possibility of mechanical contact. In moderate ocean depths there is normally no collision problem since the total deflections are typically of the same magnitude or smaller than the riser spacings at top and bottom ends. In deep waters, however, the deflections may become so large (proportional to the square of the length) that even a relatively small difference in deflection may cause collision between adjacent risers. In Refs. 1 and 2 is described a simple numerical procedure for predicting static deflection and possible collisions between individual risers due to current. The method is based on the following formulation for the wake field behind a cylinder in homogeneous inflow (see for instance Refs, 1 and 3): (MATHEMATICAL EQUATION AVALABLE IN FULL PAPER) For definition of symbols see Fig. 1. The constants k, and k2 are basically empirical constants, but related to each other through the requirement of momentum conservation. In [he present calculations the values k, = 0.25 and k1 = 1.0 have been used.