Predicting Motions Of Long Towed Pipe String

ABSTRACT A technique for predicting the motions during maneuvering of long towed pipe strings is presented. The numerical procedure is validated by comparing numerical results with closed form solutions. Scale model experiments demonstrate the ability of the mathematical model to simulate the maneuvering of such pipe strings. INTRODUCTION Offshore pipeline construction is one of the few offshore construction techniques where fabrication is usually carried out offshore. The notable exceptions to offshore fabrication of pipelines are the reel barge method and the pipe towing method. Pipe towing has the greatest advantage for large diameter pipe where the size of the reel and the weight of the pipe become major drawbacks to the reel method. Offshore pipeline installations by towing methods have paralleled and in some cases have exceeded the development of other pipeline construction methods, in all depths and in all parts of the world (Table 1). Some outstanding examples of pipe installation by towing include:Longestl Single tow of a 19-mile pipe string, Arabian Gulf, 1960.Deepest2 Laying and recovery of several test strings in 2600 to 6600 ft depths by Gas de France in 1960 to 1963.Longest Tow.3 Surface tow of a l-km test string along a 560-mile course by CFP in 1975.First Long Distance Bottom Tow.4 Pipe string was towed 240 miles Norwegian trench and back in 1975.First Arctic Offshore Line.5 A 1200-meter flowline bundle was installed off Melville Island for Pan Arctic Ltd. in 1978.First Mid-depth Tow.6 Three encased flowline bundles were towed 264 miles and connected to subsea wells in Conoco's Murchison field in 1980. The primary economic parameters for the installation phase of a pipe towing application will include (1) size and number of tugs required, (2) number and length of pipe segments, (3) speed and weather sensitivity of towing and maneuvering, (4) methods of buoyancy installation and removal, (5) methods of connecting pipe segments, and (6) methods of placing pipe strings on the seafloor. The purpose of this paper is to describe maneuvering simulation techniques suitable for evaluating critical economic factors such as these, as well as aiding field decision making when maneuvering such long strings. Various techniques of pipe towing have been proposed or used ranging from bottom towing to surface towing. While the methods described in this paper specifically address surface towing, they are generally applicable to other techniques as well. For the surface towing technique, a simulation procedure should be capable of simulating a uniform elastic flexural member with tension, undergoing large deformations, interacting with temporally and spatially varying current fields, responding to specified forces, and velocities, and accounting for inertia and hydrodynamic effects. The basic type of numerical procedure utilized is a finite difference scheme for inextensible rods reported 7 by Nordgren.