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Hybrid Protective Device for Pipelines Against Dropped Object - Presented during First Asia-Pacific Conference on Offshore System ( April 23-26, 2001)

Abstract

Production from marginal offshore hydrocarbon fields using safe and cost effective production techniques has led to an increase in the number of offshore pipelines which transport fluids between satellite facilities and existing platforms. Risk assessments have identified that these pipelines require additional protective devices to provide protection against dropped objects. The paper summarises the typical impact energy capacity of the usual protective devices.

The paper further discusses the salient features of the Hybrid Protective Cover recently installed at one of the existing platforms in Malaysia. Based on the risk assessment study recommendations, the protective cover was designed for impact energy of 200kJ. The covers are provided with a top plate supported by tubular edge beams and concrete footings.

Introduction

The design of the pipeline protective covers involves two major steps. First will be a risk-based analysis to establish various possible dropped object scenarios and possible impact energies associated for each scenarios. Second will be to select a suitable protective cover material for the pipeline for the length determined in the risk analysis.

Risk Analysis

An object dropped onto the sea surface will continue to sink towards the sea floor. Its terminal velocity will, after a short period of acceleration/deceleration, become stable when the gravity plus buoyancy is equal to the drag force (considering only vertical motion). The terminal velocity may be expressed as:

where v = terminal object velocity
  m = object mass (dry weight)
  = mass density of water
  = mass density of steel
  V = object volume
  g = gravitational acceleration
  A = cross-sectional of the object - normal to flow
  = drag coefficient (2 for vertical pile and vary from 1.6 to 2.0 for square items)

The impact energy includes the kinetic energy due to the terminal velocity and the energy of added hydrodynamic  mass.  The added  mass  may  become  significant  for  large volume  objects  such  as containers. The total impact energy for which the protective cover has to be designed is given as



Where = the added mass coefficient
Since ail the dropped object may not hit the pipeline on the seabed, the impact energy for the protective device is based on risk analysis. The risk analysis methodology to establish the impact to pipeline from dropped object includes

  • Assessing the probability of dropping an object to the sea and the dropped object hitting the pipeline
  • Assessment of how the object will fall towards the sea bed
  • Establishing the terminal velocity of the objects and impact to the pipeline in the sea.
Pipeline Protective Devices

The pipeline protective cover measures range from concrete coating (which is often required to prevent floatation) to very extensive pre-fabricated tunnels. The choice of protective measure demands a weighing of costs, risks and benefits and must also include any characteristic, which leads to economic or other loss.