Allowable compression load in the sagbend area during J-lay

Abstract

Offshore pipelay is the operation of laying a pipe from a surface vessel to the seabed. As the suspended pipe approaches the seabed, it bends downwards forming the sagbend. The sagbend is controlled by the vessel position and a tensile load is applied to counteract the weight of the pipe. During the engineering phase of previous HMC projects, compression was observed in the sagbend, for both shallow and deep water projects. Clients and regulatoratory authorities are hesitant to allow compression, limiting workability. DNV-GL implies that compression can be accepted if it does not induce other failure modes, like local buckling. A global stability assessment should be performed to identify if the resultant deformations are acceptable. HMC is intrigued to discover what the constraints are regarding global buckling during pipe-laying. The objective is to develop a methodology to calculate an allowable compressive load, increasing the operational window.

During pipe-laying, the vessels motions are transferred to the pipe, resulting in a time varying loading that can exceed the applied tension in the sagbend area, leading to compression. This can induce two different mechanisms of stability loss. Either the lateral vibrations can be excited, amplifying the amplitudes of the lateral motion, or tension can change into compression in a segment of the pipe for a part of the wave cycle, leading to a pulsating compressive loading. Focus is given on the instability onset due to the compressive pulsating loading, named as impulse dynamic buckling, as dangerously high strains can occur in that case. The system can exhibit higher load bearing capacity than the quasi-static Euler buckling load, when the loading is dynamic, due to inertia and drag.

Depending on the project requirements, different installation vessels are utilized. The vessels response under wave excitation should be taken into account for the pipe-laying analyses. For that reason, a specialized finite element software, Flexcom, is used. The ability of Flexcom to accurately predict the onset of impulse dynamic buckling is validated by comparison with a simplified analytical model. A curved simply supported Euler-Bernoulli beam is utilized for validation, based on the theory of moderately large displacements. A dynamic buckling criterion and an applicable methodology, capable of predicting the onset of impulse dynamic buckling based on Flexcom’s post-processing results, is proposed. Constant velocity loading is applied to demonstrate the effect of loading velocity in the dynamic buckling load. Harmonic displacement loading is applied to validate the proposed dynamic buckling criterion.

Two previous HMC projects are utilized as base cases to evaluate the proposed methodology. Finite element analyses in Flexcom is performed and the critical parameters regarding dynamic compression generation are identified. Based on the proposed dynamic buckling criterion, global stability assessment is performed for the base cases, against regular and irregular wave excitation. The critical vessel motions for the onset of impulse dynamic buckling, are identified. A sensitivity study on the driving parameters is performed. Following a limit state design approach for the base cases, results demonstrate that compression could be tolerated during pipelay operations, providing that other failure modes are covered. Hence, the limit of the local buckling criteria can be fully exploited despite the occurrence of compression.