Burial of Vertically Loaded Offshore Pipelines in Weak Sediments

Abstract

ABSTRACT Cyclic vertical loads were applied to a 5' long, OS diameter model pipe, in order to simulate the effect on pipeline stability of various lift forces including waves, in addition to self weight. Experiments were carried out in a large sediment -filled tank, applying controlled vertical loads or displacements to the pipe section with a servo-hydraulic system. The cyclic and incremental movement and forces were electronically recorded, to provide data for the behavior of similar full scale structures, and for analysis of the problem. INTRODUCTION Marine pipelines can be subjected to a variety of environmental loading conditions from waves, currents and even nearby platform movements. At many locations where pipelines support the offshore petroleum industry, the pipelines rest on or are embedded in very soft clay-rich sediments. Repeated loading may cause the pipelines to bury in these sediments. In some instances this may be desirable, but if a buried pipeline is subsequently subjected to transverse loads or vertical pullout, large bending stresses and pipe distress may result. In an earlier paper (Morris et aI., 1988), the effect of repeated transverse loading on pipeline behavior was addressed. The present paper discusses vertical pipeline loading and can be considered a companion to the earlier paper. The investigative approach was the same for both studies: experiments were conducted using a model pipe subjected to loading in a soft sediment. By the use of appropriate non-dimensional relationships, results from the model test can be scaled to predict prototype pipe behavior. EOUIPMENT AND TEST PROCEDURE All tests for this study were conducted in a sediment filled tank which was 6' × 6' in plan view and 4' deep. A false bottom was placed in the tank to limit the sediment depth to 30". The tank was filled with a calcium betonies mixture which had 53% clay (0.002mm), 42% silt and 5% very fine sand. The liquid limit was 101 and the plasticity index was 62, which classified the sediment as a CH (Unified Soil Classification System). Water content of the sediment was continually monitored during the test program and adjusted to maintain an untrained shear strength range of 20-30 lbs/sq ft. Vertical loading and/or displacement of the pipe was controlled with a closed loop, servo controlled hydraulic actuator. The actuator was mounted on a frame positioned alongside the test tank; a lever arm which pivoted on the edge of the tank transmitted motion from the actuator to the pipe. A load transducer mounted between the pipe and lever arm measured forces on the pipe and a linear variable differential transducer (LVDT) mounted on the pipe measured the vertical pipe movement. To ensure that the pipe moved only in the vertical direction, guide rods were attached to the ends of the pipe; these rods ran through linear ball bushings which were mounted on a beam spanning the test tank (Fig. 1).