Abstract

During subsea lifting/lowering operations, offshore operators typically utilize published regulations or recommended practice (e.g. Ref 1 & 2) for calculating key parameters such as maximum and minimum crane wire tension during deployment through the splash zone and through the water column. First hand observations by the authors has revealed shortcomings of accepted practice in key areas such as:

  1. Over-prediction of wire tension and snap loads, leading to lesser weight being lifted

  2. Under-prediction of underwater heave motion of module while being lowered

These issues are discussed in light of the fact that added mass of the module changes with submergence. Model tests and numerical simulations using DNV's software SESAM are used to evaluate the added mass of cylindrical and rectangular modules and to gain an understanding of heave motions experienced by the modules in moderate wave conditions. Sample calculations are used to show areas where existing regulations are inadequate.

Introduction

Offshore drilling and production operations often require heavy objects to be lifted out of, or lowered into the water. Examples include lifting/lowering of subsea pipeline manifold for well fluid marshalling and control, or lowering of suction anchors or anchor piles for offshore structure foundation. A limited capacity crane vessel (also called derrick barge) is utilized to conduct these operations, normally in favorable weather conditions. A schematic of a derrick barge used to conduct this operation is shown in Figure 1. Ref. 3 describesthe background study and the installation process of a subsea production manifold in 620 m water depth at the Albacora field in Campos Basin, offshore Brazil. The manifold dimensions were 19 m × 14.5 m × 7.5 m, and weighed close to 500 tonnes. One of the major considerations of the study was to determine the safest and least time consuming way to lower the manifold in 620 m water, using a conventional crane vessel. Considering that the end result of the operation has limited tolerance in terms of positioning the manifold, and the fact that the environment can create sudden unpredictable challenges on the load dangling off the crane wire, it is obvious that such offshore operations are quite risky and challenging. Thus a very careful in-depth study incorporating numerical and experimental simulations would be a prerequisite to the actual operation (Ref. 4). This would also shed light on weather downtime for derrick barges and ensure safety during operations.

For a crane vessel the ability to work is primarily a function of the relative motions between the package being lifted and the foundation on which it is to be set down. The best way to set the load down is to deposit it gently once, and leave it there. However, this invariably does not happen. A suspended package, when lifted by a floating crane vessel, will experience motions due to its own suspension (such as natural frequency effects), along with induced motions due to the floating vessel as it responds to waves.

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