Criteria for the Operation of Lowering a Structure to the seabed based on the Installation Vessel Motion
Authors: Legras Jean-Luc , Wang Jue 
Published: May 2011

The paper is related to the installation on the seabed of heavy structures like manifolds or subsea processing modules and their foundation. The traditionally used criteria for this type of offshore operation are often implemented offshore with difficulty and inaccuracy.

A new method to determine criteria for lowering operations to be applied just before the operation is based on real time monitoring of the vessel motion and time domain simulation by appropriate software of the dynamic behavior of the lowering system under the imposed motion. This help for the decision on the initiation of the operation. This may be complemented by the real time monitoring of the structure vertical motion during the lowering operation using a subsea MRU (e.g. Octans). The method is further improved by including a prediction of the maximum motion of the installation vessel, for the period of time wave forecast information is available; this allows predicting potential stand-by periods in advance. The methodology is in accordance with DNV RP H103.

The paper includes:–a detailed description of the method and of the instrumentation–a report of the application on an installation vessels for lowering operations in West Africa; this allows verification of the stiffness of the cable system and of the water added mass.–comparisons of the acceleration of the structure calculated onboard just prior to the operation and the one measured during the operation.–lessons learned and conclusions from the first projects during which the system has been used.

Introduction

For the development of offshore fields in always increasing water depths, offshore installation contractors are requested to place on the seabed structures that are heavier and heavier, e.g. suction piles, manifolds or subsea processing modules. This raises new challenges because the natural period of the lowering system, at the end of the operation reaches the lower range of waves periods; there is therefore a risk of resonance which may have a number of undesired consequences:–large vertical motion at the time of landing on the seabed, leading to unacceptable impact for fragile pieces of equipment and, for suction piles, potential degradation of the first layer of the seabed,–large dynamic tension in the cables, with the risk of overloading the lifting equipment when the tension is maximum or after the cable get slack (after an upward part of the vertical motion cycle).

The traditionally used criteria for this type of offshore operation are based on environmental characteristics (e.g. maximum significant wave height Hs, peak period of the sea state); they are determined from analyses that are inaccurate by essence because they use idealised wave conditions (single peak wave spectrum) and vessel motion Response Amplitude Operators (RAOs) that may be under or overestimated (roll RAOs are difficult to calibrate because of the non-linear damping). They are often used offshore with difficulty and inaccuracy particularly when the sea condition is multidirectional (sea and swell), which is often the case offshore West Africa and Brazil. Indeed, the motion of the structure as well as the tension in the cable depend on the frequency content of the heave of the hang-off point (crane tip or sheave of the A&R winch) and this cannot be translated in term of simple accessible parameters describing the sea state conditions.